Patent Publication Number: US-2022228110-A1

Title: Expression vectors, methods for screening host cells expressing target proteins, and methods for establishing cell lines stably expressing foreign recombinant genes

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/131,492, filed Dec. 29, 2020, the entirety of which is incorporated by reference herein. 
    
    
     INCORPORATION BY REFERENCE OF SEQUENCE LISTING 
     A sequence listing submitted as a text file via EFS-Web is incorporated herein by reference. The text file containing the sequence listing is named “9044B-A27922_US_Seq_Listing.txt”; its date of creation was Dec. 28, 2021; and its size is 17,676 bytes. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to the construction of an expression vector using FTH1 (Ferritin Heavy Chain 1) gene, and the application of FTH1 gene to a method for screening host cells expressing a target protein, a method for establishing a stable cell line, and a method for preparing a recombinant protein. 
     Description of the Related Art 
     To improve the quality of pharmaceutical-grade recombinant protein drugs, in addition to the optimization of the culture environment of cell-line, and the improvement of purification and process technology, establishing a high-efficiency screening system for a stable cell line to culture high-performance production cell lines is also an important key. 
     Generally, mammalian cells are ideal host cells for the production of complex recombinant protein drugs, mainly because their modifications carried out after translation are compatible with humans in terms of function and pharmacokinetics. In order to produce stable mammalian cell lines expressing the heterologous gene of interest, the heterologous gene, along with a selectable marker gene (e.g., neomycin phosphotransferase), is often introduced into the cell line used by transfection, including the expression of the heterologous gene and the selectable marker gene by a single vector or by co-transfected different vectors. In addition, 2 to 3 days after transfection, if the cells are subsequently cultured in a medium containing the selective agent (e.g., medium containing G418 when the neomycin phosphotransferase gene is used) for several weeks, then the drug-resistant cells can be isolated, and the performance of their gene products can be further investigated. However, since the obtained cell populations have different proportions of heterologous gene expression, in order to identify a pure cell line that highly expresses the heterologous gene of interest, a large number of pure cell lines need to be examined and tested, which is time-consuming, labor-intensive and quite expensive. 
     Gene amplification is quite common in animal cell culture for the production of recombinant protein drugs, and it improves the initially relatively low productivity of many mammalian cell lines. Widely used amplification techniques, such as a gene amplification system based on dihydrofolate reductase (DHFR), is often used in DHFR-deficient Chinese hamster ovary (CHO) cells. This technique allows the transfection of DHFR-deficient CHO cells with a vector carrying genes encoding DHFR and the protein of interest, followed by screening of transfected cells in a medium without glycine, hypoxanthine and thymidine, and the high-productivity cell line amplification can be achieved through the increasing addition of methotrexate (MTX), an inhibitor of dihydrofolate reductase. However, in order to establish stable cell lines, subsequent screening of the obtained high-productivity cells is also highly labor-intensive and time-consuming. 
     In view of the foregoing, the development of a novel cell line screening system to establish a high-efficiency stable cell line is still one of the main research goals of the pharmaceutical industry. 
     SUMMARY 
     In accordance with some embodiments of the present disclosure, a method for screening host cells expressing a target protein is provided. The method includes the following steps: providing an expression vector, the expression vector including a promoter, a gene encoding a target protein and an FTH1 gene; transfecting the host cells with the expression vector; culturing the host cells in a medium; and adding iron ions to the medium, and screening the surviving host cells to obtain the host cells expressing the target protein. 
     In accordance with some embodiments of the present disclosure, a method for establishing a stable cell line is provided. The method includes the following steps: providing an expression vector, the expression vector including a promoter, a gene encoding a target protein and an FTH1 gene; transfecting host cells with the expression vector; culturing the host cells in a medium; and adding iron ions to the medium, and continuously culturing the host cells in the medium containing iron ions for a period of time to screen and obtain a cell line that stably expresses the gene encoding the target protein. 
     In accordance with some embodiments of the present disclosure, an expression vector is provided. The expression vector includes a promoter, a gene encoding a target protein and an FTH1 gene, and the promoter is linked to the gene encoding the target protein and the FTH1 gene. 
     In accordance with some embodiments of the present disclosure, a eukaryotic host cell is provided, which is formed by transfection of the aforementioned expression vector. 
     In accordance with some embodiments of the present disclosure, a method for preparing a recombinant protein is provided. The method includes the following steps: culturing the aforementioned eukaryotic host cell in a medium; and isolating the recombinant protein from the medium. 
     In accordance with some embodiments of the present disclosure, a method for establishing a cell line stably expressing an exogenous recombinant gene is provided, in which an FTH1 gene is used as a selection marker to screen out a host cell with an exogenous recombinant gene. 
     In order to make the features or advantages of the present disclosure clear and easy to understand, a detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  show schematic diagrams of the constructed expression vectors in accordance with some embodiments of the present disclosure. 
         FIG. 2  shows a mechanism of cell line screening system in accordance with some embodiments of the present disclosure. 
         FIG. 3A  shows a sequencing result of nucleic acid sequence of the CHOK1 cell line processed by CRISPR/Cas9 gene editing technique (CHOK1_Fth1_KO_F4) in accordance with some embodiments of the present disclosure. 
         FIG. 3B  shows a result of protein expression level of the CHOK1 cell lines analyzed by Western blotting method in accordance with some embodiments of the present disclosure (columns 1 to 4 respectively represent an FTH1 gene-knockout cell line (CHOK1_Fth1_KO_F4), a wild-type cell line (CHOK1_Fth1_WT), an FTH1 gene-edited cell line (CHOK1_Fth1_F20) and an FTH1 gene-edited cell line (CHOK1_Fth1_C16)). 
         FIG. 4A  shows an analysis result of cell viability of the CHOK1 cell lines Fth1_WT and Fth1_KO after treated with different concentrations of iron ions in accordance with some embodiments of the present disclosure. 
         FIG. 4B  shows an analysis result of the effects of the CHOK1 cell lines Fth1_WT and Fth1_KO under the condition of overexpression of FTH1 gene on iron ion-induced ferroptosis in accordance with some embodiments of the present disclosure. 
         FIG. 5  shows an analysis result of the effects of the CHOK1 cell lines Fth1_WT and Fth1_KO treated with different iron ion sources (C 6 H 8 FeNO 7  and FeSO 4 ) and different concentrations of ferroptosis inducer FINO 2  on iron ion-induced ferroptosis in accordance with some embodiments of the present disclosure. 
         FIGS. 6A to 6D  show analysis results of the effects of the CHOK1 cell lines Fth1_WT and Fth1_KO in the screening formulations with different compositions on iron ion-induced ferroptosis in accordance with some embodiments of the present disclosure. 
         FIGS. 7A to 7C  show schematic diagrams of the constructed expression vectors pCDNA3.1-CMV-cgFth1-p2A-EGFP, pCDNA3.1-CMV-mFth1-p2A-EGFP and pCDNA3.1-CMV-hFth1-p2A-EGFP in accordance with some embodiments of the present disclosure. 
         FIG. 8  shows analysis results of fluorescent protein expression of the CHOK1 cell lines Fth1_WT and Fth1_KO transfected with pCDNA3.1-CMV-cgFth1-p2A-EGFP expression vector in accordance with some embodiments of the present disclosure. 
         FIG. 9A  shows analysis results of fluorescent cell counts of the CHOK1 cell lines transfected with the expression vectors pCDNA3.1-CMV-cgFth1-p2A-EGFP, pCDNA3.1-CMV-mFth1-p2A-EGFP and pCDNA3.1-CMV-hFth1-p2A-EGFP under the condition of iron ion-induced ferroptosis in accordance with some embodiments of the present disclosure. 
         FIG. 9B  shows analysis results of fluorescent cell counts of the HEK293T cell lines transfected with the expression vectors pCDNA3.1-CMV-cgFth1-p2A-EGFP, pCDNA3.1-CMV-mFth1-p2A-EGFP and pCDNA3.1-CMV-hFth1-p2A-EGFP under the condition of iron ion-induced ferroptosis in accordance with some embodiments of the present disclosure. 
         FIG. 10  shows analysis results of fluorescent protein expression of the CHOK1 cell lines the CHOK1 cell lines Fth1_WT and Fth1_KO transfected with the expression vector pCDNA3.1-CMV-cgFth1-p2A-EGFP under the condition of iron ion-induced ferroptosis in accordance with some embodiments of the present disclosure. 
         FIG. 11A  and  FIG. 11B  show quantitative data analysis results obtained after the experimental results of  FIG. 10  are processed in accordance with some embodiments of the present disclosure. 
         FIG. 12A  and  FIG. 12B  show quantitative data analysis results obtained after the experimental results of  FIG. 10  are processed in accordance with some embodiments of the present disclosure. 
         FIG. 13  shows analysis results of fluorescent protein expression of a single cell grown as a cell colony in a medium containing iron ions after screening in accordance with some embodiments of the present disclosure. 
         FIG. 14  shows an analysis result of the fluorescent protein expression of the screened CHOK1 cell line, whose FTH1 is WT after 160 days of continuous subculture. 
         FIG. 15  shows an analysis result of the fluorescent protein expression of the screened CHOK1 cell line after 210 days of continuous subculture. 
         FIG. 16A  shows analysis results of fluorescent cell counts of the HEK293T cell lines transfected with the expression vector pCDNA3.1-CMV-cgFth1-p2A-EGFP under the condition of iron ion-induced ferroptosis in accordance with some embodiments of the present disclosure. 
         FIG. 16B  shows analysis results of fluorescent cell counts of the VERO cell lines transfected with the expression vector pCDNA3.1-CMV-cgFth1-p2A-EGFP under the condition of iron ion-induced ferroptosis in accordance with some embodiments of the present disclosure. 
         FIG. 17A  and  FIG. 17B  respectively show schematic diagrams of the constructed expression vectors pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 and pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 in accordance with some embodiments of the present disclosure. 
         FIG. 18A  shows a result of protein expression level of the CHOK1 cell lines analyzed by Western blotting method in accordance with some embodiments of the present disclosure (columns 1 to 6 respectively represent the cell lines transfected with the expression vectors pCDNA3.1-CMV-cgFth1-p2A-EGFP, pCDNA3.1-CMV-hFth1-p2A-EGF, pCDNA3.1-CMV-mFth1-p2A-EGF, pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1, pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 and WT). 
         FIG. 18B  shows a result of protein expression level of the HEK293T cell lines analyzed by Western blotting method in accordance with some embodiments of the present disclosure (columns 1 to 6 respectively represent the cell lines transfected with the expression vectors pCDNA3.1-CMV-cgFth1-p2A-EGFP, pCDNA3.1-CMV-hFth1-p2A-EGF, pCDNA3.1-CMV-mFth1-p2A-EGF, pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1, pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 and WT). 
         FIG. 19A  shows analysis results of the mRNA expression level of FTH1 gene of the CHOK1 cell lines transfected with the expression vector pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 in accordance with some embodiments of the present disclosure. 
         FIG. 19B  shows analysis results of the mRNA expression level of the heavy chain (VH) and light chain (VL) of Anti-HER2 IgG1 (h4D5) antibody obtained from the CHOK1 cell line transfected with the expression vector pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 in accordance with some embodiments of the present disclosure. 
         FIG. 19C  shows an ELISA analysis result of the expression level of Anti-HER2 IgG1(h4D5) antibody obtained from the CHOK1 cell line transfected with the expression vector pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 in accordance with some embodiments of the present disclosure. 
         FIG. 20A  shows analysis results of the mRNA expression level of the heavy chain (VH) of Anti-HER2 IgG1(h4D5) antibody obtained from the CHOK1 cell lines transfected with the expression vector pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 in accordance with some embodiments of the present disclosure. 
         FIG. 20B  shows analysis results of the mRNA expression level of the light chain (VL) of Anti-HER2 IgG1(h4D5) antibody obtained from the CHOK1 cell lines transfected with the expression vector pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 in accordance with some embodiments of the present disclosure. 
         FIG. 20C  shows analysis results of the mRNA expression of FTH1 gene of the CHOK1 cell lines transfected with the expression vector pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 in accordance with some embodiments of the present disclosure. 
         FIG. 20D  shows ELISA analysis results of the expression level of Anti-HER2 IgG1(h4D5) antibody obtained from the CHOK1 cell line transfected with the expression vector pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 in accordance with some embodiments of the present disclosure. 
         FIG. 21  shows an analysis result of the conformation of Anti-HER2 IgG1 (h4D5) recombinant protein by SDS-PAGE protein electrophoresis in accordance with some embodiments of the present disclosure (columns 1 to 6 respectively represent Anti-HER2 IgG1 recombinant protein_unreduced form, Anti-HER2 IgG1 recombinant protein_reduced form, Anti-HER2 IgG1 recombinant protein_FAC+unreduced form, Anti-HER2 IgG1 recombinant protein_FAC+reduced form, Anti-HER2 IgG standard_unreduced form, and Anti-HER2 IgG1 purified protein standard_reduced form). 
         FIG. 22  shows results of the ADCC biological activity assay of Anti-HER2 IgG1 (h4D5) produced in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The expression vector, the method for screening host cells expressing a target protein, the method for establishing a stable cell line, and the method for preparing a recombinant protein of the present disclosure are described in detail in the following description. It should be understood that in the following detailed description, for purposes of explanation, numerous specific details and embodiments are set forth in order to provide a thorough understanding of the present disclosure. The specific elements and configurations described in the following detailed description are set forth in order to clearly describe the present disclosure. It will be apparent that the exemplary embodiments set forth herein are used merely for the purpose of illustration and not the limitation of the present disclosure. 
     In the following description, the terms “about” and “substantially” typically mean +/−10% of the stated value, or typically +/−5% of the stated value, or typically +/−3% of the stated value, or typically +/−2% of the stated value, or typically +/−1% of the stated value or typically +/−0.5% of the stated value. The stated value of the present disclosure is an approximate value. When there is no specific description, the stated value includes the meaning of “about” and “substantially”. In addition, the term “between the first value and the second value” or “in a range from the first value to the second value” means that the range includes the first value, the second value, and other values in between. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It should be appreciated that, in each case, the term, which is defined in a commonly used dictionary, should be interpreted as having a meaning that conforms to the relative skills of the present disclosure and the background or the context of the present disclosure, and should not be interpreted in an idealized or overly formal manner unless so defined. 
     The term “FTH1 gene” refers to the ferritin heavy chain 1 (Ferritin Heavy Chain 1) gene, which encodes the ferritin heavy chain, which is a ferroxidase enzyme. FTH1 gene is involved in the regulation of iron metabolism in cells, and plays an important role in iron-dependent cell death mechanism (ferroptosis). Ferritin is the main protein used to store iron ions in prokaryotes and eukaryotes. The alteration in the composition of ferritin subunits may affect the absorption rate and the release rate of irons in different tissues. The main function of ferritin is to store irons in a soluble and non-toxic state. The deficiency of ferritin is associated with a variety of neurodegenerative diseases. 
     The term “ferroptosis” refers to a form of cell death involving the production of reactive oxidative substances mediated by irons and resulting in lipid peroxidation. The term “ferroptosis inducer” refers to an agent that induces, promotes or activates ferroptosis. 
     In accordance with the embodiments of the present disclosure, the “target gene (gene of interest)” contained in the expression vector includes a nucleotide sequence of any length encoding the target product. The “gene product” or “product of interest” is usually a protein, polypeptide, peptide, or a fragment or derivative thereof. However, the “gene product” or “product of interest” may also include RNA or antisense RNA. The target gene can exist in its full-length, shortened form, or exist in a form of fusion gene or labeled gene. The gene of interest may be genomic DNA, cDNA or an equivalent fusion fragment. The gene of interest may be a native gene sequence, or a mutated or modified sequence. The modification may include codon optimization and humanization for a particular host cell. 
     The term “nucleotide sequence” or “nucleic acid sequence” refers to oligonucleotides, nucleotides, polynucleotides and fragments thereof, as well as DNA or RNA of genomic or synthetic origin, which can exist in a single-stranded or double-stranded form. Nucleotides may be deoxyribonucleotides, ribonucleotides, or modified nucleotides. Nucleosides consist of purine (adenine (A) or guanine (G) or their derivatives) or pyrimidine (thymine (T), cytosine (C) or uracil (U) or their derivatives) bases and sugars bonds. The four nucleoside units (or bases) in DNA are called deoxyadenosine, deoxyguanosine, deoxythymidine and deoxycytidine. The four nucleoside units (or bases) in RNA are called adenosine, guanosine, uridine and cytidine. Nucleotides are phosphate esters of nucleosides. 
     The term “encode” refers to the properties or functions of a specific sequence of nucleotides in nucleic acids, such as genes in chromosomes or mRNAs, that act as substrates for the synthesis of other polymers and macromolecules in organisms, for example, rRNA, tRNA, mRNA, other RNA molecules, cDNA or polypeptides. Therefore, if a protein is produced in a cell or other biological system by mRNA transcription and subsequent translation, it is called a “gene-encoded protein”. 
     The term “host cell” refers to a cell into which exogenous nucleic acid is introduced, including the progeny of such cells. Host cells include “transformants” and “transformed cells”, which include the originally transformed cell and progeny derived therefrom, regardless of the number of passages. The nucleic acid content of the progeny may not be identical to the parental cell, and may contain mutations. In accordance with the embodiments of the present disclosure, host cells include mutant progeny that have the same function or biological activity as screened or selected in the originally transformed cell. 
     The term “vector” refers to a nucleic acid molecule capable of propagating another nucleic acid to which it is linked. In accordance with the embodiments of the present disclosure, the vector includes a vector that is an autonomously replicating nucleic acid construct, as well as a vector that is incorporated into the genome of the host cell into which it is introduced. Certain vectors are capable of indicating the expression of nucleic acids to which they are operably linked, and such vectors are referred to herein as “expression vectors”. 
     The term “transfect” refers to the delivery of nucleic acids, proteins, or other macromolecules to a target cell for the purpose of expressing the nucleic acid, protein, or other macromolecules within the cell or rendering it biologically functional. 
     The term “culture” refers to the in vitro proliferation of cells or organisms in or on various types of media. It should be understood that the progeny of cells grown in culture may not be completely identical (morphologically, genetically, or phenotypically) to the parent. 
     The term “promoter” or “promoter region” refers to a nucleic acid sequence located upstream (5′ end) of an expressed nucleic acid sequence and controls the expression of the sequence by providing the recognition and binding sites for RNA polymerase. The promoter region may contain other recognition and binding sites of other factors involved in regulation of gene transcription. Promoters can control the transcription of prokaryotic or eukaryotic genes. Furthermore, a promoter can be an inducible promoter and can initiate transcription in response to an inducer, or can be a constitutive promoter where transcription is not under the control of an inducer. In the absence of the inducer, the gene under the control of the inducible promoter is not expressed or is expressed only slightly, whereas in the presence of the inducer, the gene initiates transcription or increases the amount of transcription, which is generally regulated by the binding of specific transcription factors. 
     The term “protein” can be used interchangeably with the terms “peptide” and “polypeptide”. The term “recombinant protein” refers to a protein produced by recombinant DNA techniques, in which the DNA encoding the expressed protein or RNA is typically inserted into an appropriate expression vector, and the expression vector can be used to transfect host cells to produce exogenous protein or RNA. Proteins or polypeptides of biopharmaceutical importance, for example, may include antibodies, enzymes, cytokines, lymphokines, receptors and derivatives or fragments thereof, but they are not limited thereto. 
     The term “antibody” is used herein in the broadest sense and encompasses a variety of antibody structures, e.g., which includes monoclonal antibodies, polyclonal antibodies, multispecific antibodies (e.g., bispecific antibodies) or antibody fragments, but is not limited thereto. The term “antibody fragment” refers to a molecule other than an intact antibody that includes a portion of an intact antibody, and the antibody fragment binds to the same antigen as the intact antibody. Antibody fragments may include Fv, Fab, Fab′, Fab′-SH, F(ab′) 2 ; diabodies; linear antibodies; single-chain antibody molecules (e.g., scFv); and multispecific antibodies formed of antibody fragments, but they are not limited thereto. 
     In accordance with the embodiments of the present disclosure, several genes with high potential to enhance the performance or productivity of specific cell lines are identified, from the regulatory strategy of key cell biosynthesis pathway genes, through the analysis of biological information and functional genomics. It was found that FTH1 gene (X11 gene) is highly expressed when cells produce exogenous recombinant proteins. FTH1 gene is mainly involved in the metabolic regulation of iron ions in cells, and plays an important role in the iron-dependent cell death mechanism (ferroptosis). 
     Embodiments of the present disclosure have verified the relevance of FTH1 gene regulation and induction of ferroptosis in cells, and further confirmed that overexpression of FTH1 gene can significantly improve the tolerance of cell lines to the induced ferroptosis under the action of excessive iron ions addition or ferroptosis inducers. In the embodiments of the present disclosure, expression vectors and formulations of the medium are developed based on the aforementioned characteristics, so as to construct a high-efficiency cell line screening system. In this way, a cell line that can stably express exogenous recombinant genes can be established and the expression of exogenous genes in the cell line is promoted to achieve the effect of improving the yield of recombinant protein drugs. 
     In accordance with the embodiment of the present disclosure, a method for screening hosts cell expressing a target protein is provided. The method includes the following steps: (a) providing an expression vector, the expression vector including a promoter, a gene encoding a target protein and an FTH1 gene; (b) transfecting host cells with the expression vector; (c) culturing the host cells in a medium; and (d) adding iron ions to the medium, and screening the surviving host cells to obtain host cells expressing the target protein. 
     The expression vector can be designed and constructed by the techniques known in the art. In the expression vector, the promoter is linked with the gene encoding the target protein and the FTH1 gene. Specifically, as shown in  FIG. 1A , in accordance with some embodiments, the gene encoding the target protein (indicated by GOI) and the FTH1 gene can be driven by different promoters (indicated by arrows). That is, the gene encoding the target protein can be driven by one promoter, and the FTH1 gene can be driven by another promoter, and the type of promoter that drives the gene encoding the target protein can be different from the type of promoter that drives the FTH1 gene. In accordance with some other embodiments, as shown in  FIG. 1B  and  FIG. 1C , the gene encoding the target protein (indicated by GOI) and the FTH1 gene can be driven by the same promoter. As shown in  FIG. 1B , in accordance with some embodiments, the gene encoding the target protein is located downstream of the FTH1 gene. As shown in  FIG. 1C , in accordance with some embodiments, the gene encoding the target protein is located upstream of the FTH1 gene. 
     In accordance with some embodiments, the promoter driving the gene encoding the target protein or the FTH1 gene may include a CMV promoter, a SV40 promoter, an EF1α promoter or another suitable promoter, but it is not limited thereto. In accordance with some embodiments, the expression vector may further include an internal ribosome entry site (IRES) located between the gene encoding the target protein and the FTH1 gene. 
     The expression vector may include a nucleotide sequence of any length that encodes a target protein. In accordance with some embodiments, the target protein may include a recombinant protein, but it is not limited thereto. In accordance with some embodiments, the recombinant protein may include an antibody, but it is not limited thereto. In accordance with different embodiments, any suitable target protein can be selected for the design of the expression vector according to needs. 
     Furthermore, FTH1 gene is involved in the metabolic regulation of iron ions in cells, and plays an important role in the ferroptosis mechanism. The similarity of FTH1 gene among different species is high. For example, the alignment result of the amino acid sequences of FTH1 gene of human ( Homo sapiens ), mouse ( Mus musculus ) and Chinese hamster ( Cricetulus griseus ) shows that the amino acid sequences have more than 90% of similarity. In accordance with some embodiments, the amino acid sequence encoded by the FTH1 gene has at least 85%, for example, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%, of similarity with the sequence shown in any one of SEQ ID NOs: 1 to 3, but it is not limited thereto. In accordance with some embodiments, the FTH1 gene is an FTH1 gene derived from Chinese hamster, which encodes the amino acid sequence of SEQ ID NO: 1. In accordance with some embodiments, the FTH1 gene is an FTH1 gene derived from mouse, which encodes the amino acid sequence of SEQ ID NO: 2. In accordance with some embodiments, the FTH1 gene is an FTH1 gene derived from human, which encodes the amino acid sequence of SEQ ID NO: 3. 
     The transfection of host cell can be carried out by the techniques known in the art. For example, the genetic information carried by the expression vector can be introduced into the host cell by chemical transfection (e.g., liposome transfection or calcium phosphate transfection), viral transfection, electrotransfection, or other suitable methods. In accordance with some embodiments, the host cells may include, but are not limited to, CHO cells, HEK293 cells, Hela cells, VERO cells, NSO cells, or other suitable cell lines. 
     In accordance with some embodiments, the host cells can be first cultured in a suitable medium for a period of time, and then the host cells can be transfected with an expression vector. For example, in accordance with some embodiments, the host cells can be cultured in the medium for about 12 hours to about 36 hours, e.g., about 16 hours, about 20 hours, about 24 hours, about 28 hours, or about 32 hours, before transfection. 
     In accordance with some embodiments, after the transfection, iron ions are added to the medium to increase the iron ion concentration in the medium to induce ferroptosis, so that host cells expressing the target protein can be obtained by screening. For the detailed screening mechanism, refer to  FIG. 2 . Under the stress condition of excess iron ions, the iron metabolism in wild-type (WT) host cells or FTH1 gene-knockout (FTH1−/−) host cells will be abnormal, and at the same time, it will also lead to the accumulation of a large number of cell membrane lipid peroxidation products (e.g., lipid reactive oxygen species (ROS)) in the cell, thereby inducing cell death. On the other hand, the transfected host cell with the expression vector has the FTH1 gene, which can replicate the vector in large quantities and translate into ferritin. Therefore, even under the stress condition of excess iron ions, the transfected host cell can survive. It is worth noting that since the expression vector further carries the gene encoding the target protein (indicated by GOI), the host cell will also translate the target protein. In other words, the addition of iron ions can also screen out the host cells expressing the target protein. Through the effect of iron ions inducing ferroptosis, the aforementioned screening mechanism can have the dual effect of improving the screening efficiency of the gene encoding the target protein and the mass production of the target protein. 
     In accordance with some embodiments, the concentration of iron ions added to the medium may be about 100 μM to about 1.5 mM, or about 125 μM to about 1 mM, for example, about 250 μM, about 500 μM, or about 750 μM, but it is not limited thereto. In accordance with some embodiments, the iron ions may include divalent iron ions or trivalent iron ions. In accordance with some embodiments, the source of iron ions may include ferric sulfate (Fe 2 (SO 4 ) 3 ), ferrous sulfate (FeSO 4 ), ferric ammonium citrate (C 6 H 8 FeNO 7 ), ferric citrate (C 6 H 5 FeO 7 ), ferric chloride (FeCl 3 ), ferric nitrate (Fe(NO 3 ) 3 ), iron oxalate (Fe 2 (C 2 O 4 ) 3 ), ferric phosphate (FePO 4 ), or other suitable sources, but it is not limited thereto. In accordance with different embodiments, the appropriate concentration range of iron ions can be adjusted according to the type of host cells used. 
     In accordance with some embodiments, the surviving host cells can be screened by any method known in the art to obtain the host cells expressing the target protein. 
     Furthermore, in accordance with some embodiments, the step of screening the host cells may further include adding an ferroptosis inducer in the medium. The use of ferroptosis inducer in the screening process can further promote the occurrence of ferroptosis, improve the sensitivity of cells to iron ions, and optimize the screening effect. 
     In accordance with some embodiments, the ferroptosis inducer may be an inducer that can exert dual effects involving iron oxidation and loss of GPX4 enzymatic activity to induce ferroptosis. For example, in accordance with some embodiments, the ferroptosis inducer may include FINO 2  (ferroptosis inducer endoperoxide), Erastin, piperazine Erastin (PE), imidazole ketone Erastin (IKE), sulfasalazine, sorafenib, glutamate, RAS-selective lethal 3 (RSL3), molecular libraries 162 (ML162), diverse pharmacological inhibitor (DPI) compounds 7, 10, 12, 13, 17, 18, 19, ferroptosis inducer 56 (FIN56), caspase-independent lethal 56 (CIL56), or other suitable ferroptosis inducers, but it is not limited thereto. In accordance with some embodiments, the ferroptosis inducer may be added at a concentration of about 0.5 μM to about 2 mM, or about 1 μM to about 1.8 mM, or about 10 μM to about 1.5 mM, or about 20 μM to about 1.2 mM, or about 50 μM to about 1 mM, for example, about 100 μM, about 200 μM, about 300 μM, about 400 μM, about 500 μM, about 600 μM, about 700 μM, about 800 μM, or about 900 μM, but it is not limited thereto. In accordance with different embodiments, the appropriate concentration range of the ferroptosis inducer can be adjusted according to the type of host cells used. 
     Furthermore, in accordance with some embodiments, the step of screening the host cells may further include adding fatty acids in the medium. The addition of fatty acids to the medium can further promote the production of lipid peroxidation products (e.g., ROS), thereby inducing ferroptosis, and optimizing the screening effect. 
     The fatty acids may include saturated fatty acids, unsaturated fatty acids, or a combination thereof. In accordance with some embodiments, the fatty acid may include palmitic acid (PA), linoleic acid (LA), arachidonic acid (AA), eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), docosahexaenoic acid (DHA), or other suitable fatty acids, but it is not limited thereto. In accordance with some embodiments, the fatty acid may be added at a concentration of about 50 μM to about 500 μM, or about 60 μM to about 190 μM, or about 70 μM to about 180 μM, for example, about 80 μM, about 90 μM, about 100 μM, about 110 μM, about 120 μM, about 130 μM, about 140 μM, about 150 μM, about 160 μM, or about 170 μM, but it is not limited thereto. In accordance with different embodiments, the appropriate concentration range of fatty acid can be adjusted according to the type of host cells used. 
     Furthermore, in accordance with some embodiments, the host cell may be an FTH1 gene-knockout cell. In accordance with some embodiments, the use of FTH1 gene-knockout host cells can further enhance the difference in tolerance of host cells with or without a vector to ferroptosis, so that the screening effect is more obvious. In accordance with some embodiments, gene knockout cell lines can be constructed by CRISPR/Cas9 technique, but the present disclosure is not limited thereto. 
     As mentioned above, in accordance with the embodiments of the present disclosure, the method for screening host cells expressing the target protein uses FTH1 gene as a selection marker in combination with the medium of a specific formula to screen for the cells including the gene encoding target protein. The screening method thus established can rapidly screen and obtain specific cells stably expressing exogenous recombinant genes, and can promote the expression of exogenous genes in cells. 
     In addition, in accordance with the embodiments of the present disclosure, a method for establishing a stable cell line is also provided. The method includes the following steps: (a) providing an expression vector, the expression vector including a promoter, a gene encoding a target protein and an FTH1 gene; (b) transfecting host cells with the expression vector; (c) culturing the host cells in a medium; and (d) adding iron ions in the medium, and continuously culturing the host cells in the medium containing iron ions for a period of time, to screen and obtain a cell line that stably expresses the gene encoding the target protein. 
     Regarding the detailed descriptions of steps (a) to (c), reference can be made to the aforementioned method for screening host cells expressing the target protein, and will not be repeated herein. In accordance with some embodiments, after adding iron ions in the medium, the host cells are continuously cultured and iron ions are continuously supplemented to maintain the iron ion concentration in the medium, so as to continuously screen the cells to obtain a cell line that stably expresses the gene encoding the target protein. 
     In accordance with some embodiments, the host cells can be continuously screened and cultured for about 5 days to about 30 days, e.g., 6 days, 7 days, 8 days, 9 days, 10 days, 11 days, 12 days, 13 days, 14 days, 15 days, 16 days, 17 days, 18 days, 19 days, 20 days, 21 days, 22 days, 23 days, 24 days, 25 days, 26 days, 27 days, 28 days, or 29 days. Specifically, in accordance with some embodiments, after about 5 days to about 10 days of continuous culture and selection, 100% of the surviving host cells can carry the exogenous gene of the expression vector, i.e. the FTH1 gene and the gene encoding the target protein. Thereafter, the cells can be cultured in a medium containing iron ions to maintain their growth and maintain the expression of the gene encoding the target protein. 
     In accordance with some embodiments, after screening to obtain a cell line that stably expresses the gene encoding the target protein, the cell line can be continuously screened and expanded in an environment containing iron ions to establish a stable cell line. As mentioned above, the step of screening host cells may further include adding the ferroptosis inducer and fatty acids to the medium. In accordance with some embodiments, after establishing a stable cell line, the ferroptosis inducers and/or fatty acids may no longer be added to the medium. 
     As mentioned above, in accordance with some embodiments, the method for establishing a stable cell line uses FTH1 gene as a screening marker in combination with the medium of a specific formula, to obtain the cell line that stably expresses the gene encoded the target protein by screening through ferroptosis. This method can quickly obtain a cell line stably expressing the exogenous recombinant gene, and can promote the expression of the exogenous gene in the cell line. It is worth noting that the screened cell lines can stably express the target protein for a long time. For example, in accordance with some embodiments, nearly 100% of the screened cell lines express the target protein and can continuously express the target protein for about 160 days, about 200 days or even about a year or more. 
     In addition, in accordance with some embodiments of the present disclosure, a eukaryotic host cell is also provided, which is formed by transfection of the expression vector provided in the embodiments of the present disclosure. In accordance with some embodiments, the eukaryotic host cell may be a CHO cell, HEK293 cell, Hela cell, VERO cell, NS0 cell, or other suitable cells, but it is not limited thereto. 
     In accordance with some embodiments, a method for preparing a recombinant protein is also provided. The method includes the following steps: (a) culturing the eukaryotic host cell provided in the embodiments of the present disclosure in a medium; and (b) isolating the recombinant protein from the medium. In accordance with some embodiments, the recombinant protein may include an antibody, but it is not limited thereto. In accordance with some embodiments, the eukaryotic host cell described above can be cultured under conditions suitable for antibody expression, and the antibody can be recovered from the host cell (or medium of the host cell). 
     It is worth noting that the eukaryotic host cells formed by the transfection of the expression vector provided in the embodiments of the present disclosure can stably express the recombinant protein drug, and promote the mass production of the target recombinant protein drug. 
     In order to make the above-mentioned and other purposes, features and advantages of the present disclosure thorough and easy to understand, a number of preparation examples, examples, and comparative examples are given below, and are described in detail as follows, but they are not intended to limit the scope of the present disclosure. 
     Example 1: Relevance Verification Between the Expression of FTH1 Gene and Iron-Induced Ferroptosis 
     In order to verify the relevance between FTH1 gene expression and iron-induced ferroptosis, the FTH1 gene was knocked out (knockout, KO) using a CRISPR/Cas9 technique and the FTH1 gene was overexpressed (Plasmid Overexpression) using a vector transfection technique for functional verification. 
     First, the FTH1 gene-knockout CHOK1 cell line (FTH1-KO) was constructed using a CRISPR/Cas9 gene editing technique, and the guide ribonucleic acid (guide RNA) was designed using the FTH1 gene chromosome of Chinese hamster (CgFth1). Then, the complex CRISPR/Cas9 ribonucleoprotein (RNP) formed by Cas9 protein and the guide RNA was sent into the CHOK1 cell line by electroporation (NEPA21 Electroporator) for gene editing. Afterwards, single-cell screening and culture and gene sequencing of mutant strains were performed to confirm the mutant type, and protein analysis of mutant strains was performed with FTH1 antibody (ab65080, Abcam) using Western blotting to verify the expression of the FTH1 protein of cell lines. The results of nucleic acid sequencing and the Western blotting are shown in  FIG. 3A  and  FIG. 3B , respectively. 
     Refer to  FIG. 3A , the result of nucleic acid sequencing shows that the FTH1 gene-knockout cell line (F4) had a single nucleic acid deletion (single base deletion) on both paired gene chromosomes. Since two types of point mutations of single base deletion were located on the exon (Exon-1) of the FTH1 gene, it is predicted that a stop codon may be generated, causing the protein to fail to complete translation. 
     Referring to  FIG. 3B , columns 1 to 4 respectively represent the results of the FTH1 gene-knockout cell line (CHOK1_Fth1_KO_F4), the wild-type cell line (CHOK1_Fth1_WT), the FTH1 gene-edited cell line (CHOK1_Fth1_F20) and the FTH1 gene-edited cell line (CHOK1_Fth1_C16), and beta-actin serves as a control group. The results of Western blotting analysis showed that the expression of the protein was not detected in the CHOK1_Fth1_KO_F4 mutant cell line, indicating that the FTH1 gene was successfully knocked out in the CHOK1 cell line. That is, the FTH1 gene-knockout cell line was successfully established by CRISPR/Cas9 gene editing technique. 
     Then, two types of CHOK1 cell lines, Fth1_WT (CHOK1, BCRC) and Fth1_KO (Fth1_WT was the wild-type cell line; Fth1_KO was the FTH1 gene-knock out cell line), were respectively seeded in 96-well culture plates with 1×10 4  cells/well. After adherent culture in F12 medium (Ham&#39;s F-12 Nutrient Mix, Gibco) containing 10% FBS for 24 hours, different concentrations of ferrous sulfate (FeSO 4 ) were added as a source of iron ions to induce ferroptosis. On the 5th day (120 hours) after treatment, cell viability was detected with AlamarBlue™ cell viability reagent (Thermo Fisher Scientific Inc.). The percentage of cell viability under the treatment of different concentrations of iron ions was calculated by dividing the fluorescence detection readings (Ex/Em: 560 nm/590 nm) and the fluorescence value of the untreated negative control group. The experimental results are shown in  FIG. 4A . 
     Furthermore, two types of CHOK1 cell lines, Fth1-WT (CHOK1, BCRC) and Fth1-KO, were seeded in 96-well culture plates with 1×10 4  cells/well respectively. After adherent culture in F12 medium (Ham&#39;s F-12 Nutrient Mix, Gibco) containing 10% FBS for 24 hours, the transfection of the vector (pcDNA3.1-CMV-CgFth1-p2A-EGFP) was carried out with Lipofectamine 3000 (Thermo Fisher Scientific Inc.) to promote the overexpression of the FTH1 gene. 24 hours after the DNA transfection of the vector, ferroptosis was induced by adding different concentrations of ferrous sulfate (FeSO 4 ). On the 5th day (120 hours) after treatment, cell viability was detected with AlamarBlue™ cell viability reagent (Thermo Fisher Scientific Inc.). By detecting the fluorescence readings (Ex/Em: 560 nm/590 nm), the effects of two CHOK1 cell lines on iron-induced ferroptosis under the condition of FTH1 overexpression were compared. The experimental results are shown in  FIG. 4B . 
     As shown in  FIG. 4A  and  FIG. 4B , the experimental results confirm that the knockout of the FTH1 gene makes the CHO cell lines more sensitive to iron-induced ferroptosis. In addition, the strategy of overexpressing exogenous FTH1 gene by the vector can significantly slow down iron-induced ferroptosis. According to the above results, it can be seen that the expression of FTH1 gene in cells is highly correlated with iron-induced ferroptosis. 
     Example 2: Tolerance Change of FTH1 Gene Expression in Cells Under the Effects of a Ferroptosis Inducer 
     In order to verify the relevance of the expression level of FTH1 gene and ferroptosis, two different iron ion sources, ferric ammonium citrate (C 6 H 8 FeNO 7 ) and ferrous sulfate (FeSO 4 ), were added to the cell culture broth, and the cells were treated with different concentrations of ferroptosis inducer FINO 2 . 
     Two types of CHOK1 cell lines, Fth1_WT and Fth1_KO, were seeded in 96-well culture plates with 1×10 4  cells/well respectively. After 24 hours of adherent culture, 200 μM of ferric ammonium citrate (C 6 H 8 FeNO 7 ) and ferrous sulfate (FeSO 4 ) were respectively added to F12 medium (Ham&#39;s F-12 Nutrient Mix, Gibco) containing 10% FBS, and at the same time different doses of ferroptosis inducer FINO 2  (CAS #869298-31-7; Cayman Chemical) were added to induce ferroptosis. After 24 hours of action, cell viability was detected with AlamarBlue™ cell viability reagent (Thermo Fisher Scientific Inc.). By detecting the fluorescence readings (Ex/Em: 560 nm/590 nm), the differences of ferroptosis occurring in the two CHOK1 cell lines were compared. The experimental results are shown in  FIG. 5 . 
     As shown in  FIG. 5 , the experimental results show that both the above-mentioned two sources of iron ions can induce ferroptosis under the action of the ferroptosis inducer. The expression of FTH1 gene is negatively correlated with the degree of ferroptosis, and the addition of ferroptosis inducers can improve the cytotoxic effect, indicating that FTH1 gene can protect CHOK1 cells under the action of ferroptosis inducers and inhibit the occurrence of ferroptosis. 
     Example 3: Test of Screening Condition for Induction of Iron-Induced Ferroptosis 
     In order to construct a high-efficiency cell line screening method, the ferroptosis inducer FINO 2  was combined with the addition of iron ions FeSO 4  and the presence or absence of fatty acid palmitic acid (PA) to carry out the cytotoxicity test, to find out a suitable medium formulation for cell line screening. 
     Two types of CHOK1 cell lines, Fth1_WT (CHOK1, BCRC) and Fth1_KO, were respectively seeded in 96-well culture plates with 1×10 4  cells/well. After 24 hours of adherent culture, 125 μM ferrous sulfate (FeSO 4 ) was added to F12 medium (Ham&#39;s F-12 Nutrient Mix, Gibco), and at the same time different doses of ferroptosis inducer FINO 2  and/or fatty acid PA (Palmitic acid, P0500Sigma-Aldrich) were added to induce ferroptosis. After 24 hours of action, cell viability was detected with AlamarBlue™ cell viability reagent (Thermo Fisher Scientific Inc.). By detecting the fluorescence readings (Ex/Em: 560 nm/590 nm), the effects of inducing ferroptosis of the two CHOK1 cell lines on under different screening medium formulations were compared. The experimental results are shown in  FIGS. 6A to 6D . 
     Refer to  FIG. 6A , in the condition that the medium simultaneously contained FINO 2 , 125 μM FeSO 4 , and fatty acid PA, the Fth1_KO cell line caused significant cytotoxic effect in 48 hours under the action of 2 μM FIN04. Referring to  FIG. 6B , in the condition that only FINO 2  and 125 μM FeSO 4  were added, the Fth1_KO cell line caused significant cytotoxic effect under the addition of 5 μM FINO 2 , and there was no obvious difference between the Fth1_WT cell line and the Fth1_KO cell line. Referring to  FIG. 6C , FINO 2  and fatty acid PA did not cause cytotoxicity without the addition of iron ions FeSO 4 . Furthermore, referring to  FIG. 6D , in the condition that only FINO 2  was added, even a dose of 5 μM did not cause significant cytotoxicity. 
     According to the aforementioned experimental results, the occurrence of ferroptosis, in the condition of adding iron ions, FINO 2  and fatty acids, can cause obvious cytotoxicity effect, and the addition of FINO 2  and fatty acids has the effect of promoting cytotoxicity. 
     Example 4: Establish FTH1 Recombinant Protein Expression Vector for Screening of Stable Cell Line 
     The similarity of FTH1 gene among species is high. Among them, the amino acid sequences of the FTH1 genes of human ( Homo sapiens ), mouse ( Mus musculus ) and Chinese hamster ( Cricetulus griseus ) were compared. The results showed that the sequences of the three had more than 90% of similarity, and it can be inferred that their biological functions were similar. Therefore, the FTH1 genes of these three species were selected for the construction of subsequent expression vectors. 
     Referring to  FIGS. 7A to 7C ,  FIGS. 7A to 7C  show schematic diagrams of the expression vectors constructed by inserting Chinese hamster FTH1 gene (cgFth1), mouse FTH1 gene (mFth1) and human FTH1 gene (hFth1) based on pcDNA3.1-CMV vector, pcDNA3.1+P2A-eGFP (GenScript). 
     As shown in  FIG. 7A , the pCDNA3.1-CMV-cgFth1-p2A-EGFP expression vector was based on the pCDNA3.1-CMV vector as a backbone, and the CMV promoter, cgFth1 gene, p2A peptide, EGFP reporter gene and pA terminator were inserted therein. The cgFth1 gene and the EGFP reporter gene was concatenated with the p2A peptide, the CMV promoter had the nucleic acid sequence shown in SEQ ID NO: 7, the cgFth1 gene had the nucleic acid sequence shown in SEQ ID NO: 4, and the p2A peptide had the nucleic acid sequence shown in SEQ ID NO: 8, the EGFP reporter gene had the nucleic acid sequence shown in SEQ ID NO: 9, and the pA terminator had the nucleic acid sequence shown in SEQ ID NO: 10. 
     As shown in  FIG. 7B , the pCDNA3.1-CMV-mFth1-p2A-EGFP expression vector was based on the pCDNA3.1-CMV vector as a backbone, and the CMV promoter, mFth1 gene, p2A peptide, EGFP reporter gene and pA terminator were inserted therein. The mFth1 gene and the EGFP reporter gene were concatenated with the p2A peptide, the CMV promoter had the nucleic acid sequence shown in SEQ ID NO: 7, the mFth1 gene had the nucleic acid sequence shown in SEQ ID NO: 5, and the p2A peptide had the nucleic acid sequence shown in SEQ ID NO: 8, the EGFP reporter gene had the nucleic acid sequence shown in SEQ ID NO: 9, and the pA terminator had the nucleic acid sequence shown in SEQ ID NO: 10. 
     As shown in  FIG. 7C , the pCDNA3.1-CMV-hFth1-p2A-EGFP expression vector was based on the pCDNA3.1-CMV vector as a backbone, and the CMV promoter, hFth1 gene, p2A peptide, EGFP reporter gene and pA terminator were inserted therein. The hFth1 gene and the EGFP reporter gene were concatenated with the p2A peptide, the CMV promoter had the nucleic acid sequence shown in SEQ ID NO: 7, the hFth1 gene had the nucleic acid sequence shown in SEQ ID NO: 6, and the p2A peptide had the nucleic acid sequence shown in SEQ ID NO: 8, the EGFP reporter gene had the nucleic acid sequence shown in SEQ ID NO: 9, and the pA terminator had the nucleic acid sequence shown in SEQ ID NO: 10. 
     In order to verify that the overexpression of FTH1 gene can be used for the screening of stable cell lines, the pCDNA3.1-CMV-cgFth1-p2A-EGFP expression vector was used to transfect to the CHOK1 cell lines Fth1_WT (CHOK1, BCRC) and Fth1_KO respectively. 1×10 6  of cells were taken and electroporated (2 mm cuvette, NEPA21 Electroporator) at 125V for 24 hours to promote the overexpression of the Fth1 gene. In addition, 24 hours after the transfection of the vector, ferroptosis was induced by adding high doses of ferrous sulfate (FeSO 4 , 500 μM). Refer to the fluorescent microscope observation results shown in  FIG. 8 . After 14 days of treatment, the cell lines with EGFP fluorescent expression were gradually screened out. Therefore, it was confirmed that the FTH1 recombinant protein expression vector and the addition of iron ions indeed had the effect of screening stable cell lines, and the Fth1_KO were more dependent on the retention of FTH1 exogenous genes than the Fth1_WT cells. 
     Example 5: Efficiency Comparison of FTH1 Gene of Different Species for Screening System 
     In order to further verify that the FTH1 gene of different species origin can be used for the screening of stable cell line, the CHOK1 cell line and the HEK293T cell line were used as host cells, and the pCDNA3.1-CMV-cgFth1-p2A-EGFP expression vector, pCDNA3.1-CMV-mFth1-p2A-EGFP expression vector and pCDNA3.1-CMV-hFth1-p2A-EGFP expression vector were used to carry out experimental verification. 
     The CHOK1 cell line was seeded in the 96-well culture plate with 1×10 4  cells/well. After 24 hours of adherent culture in F12 medium (Ham&#39;s F-12 Nutrient Mix, Gibco) containing 10% FBS, the transfection of the aforementioned FTH1 gene expression vectors of the three species was carried out with Lipofectamine 3000 (Thermo Fisher Scientific Inc.), to promote the overexpression of the FTH1 gene. 24 hours after the DNA transfection of the vectors, 125 μM ferrous sulfate (FeSO 4 ) was added to the cell medium, and at the same time different doses of ferroptosis inducers (FINO 2 , 0 μM to 5 μM) were added according to cell characteristics to induce ferroptosis. Fluorescent cells were counted with a Celigo Image Cytometer (Nexcelom). After 3 days (72 hours) of action, the fluorescent cells were counted with Celigo Image Cytometer (Nexcelom) and compared with the control group without FINO 2  treatment, thereby estimating the screening efficacy. The experimental results are shown in  FIG. 9A . 
     The HEK293T cell line was seeded in the 96-well culture plate with 1×10 4  cells/well. After 24 hours of adherent culture in DMEM medium (Gibco) containing 10% FBS, the transfection of the aforementioned FTH1 gene expression vectors of the three species was carried out with Lipofectamine 3000 (Thermo Fisher Scientific Inc.), to promote the overexpression of the FTH1 gene. 24 hours after the DNA transfection of the vectors, 125 μM ferrous sulfate (FeSO 4 ) was added to the cell medium, and at the same time different doses of ferroptosis inducers (FINO 2 , 0 μM to 5 μM) were added according to cell characteristics to induce ferroptosis. Fluorescent cells were counted with a Celigo Image Cytometer (Nexcelom). After 3 days (72 hours) of action, the fluorescent cells were counted with Celigo Image Cytometer (Nexcelom) and compared with the control group without FINO 2  treatment, thereby estimating the screening efficacy. The experimental results are shown in  FIG. 9B . 
     According to the results of  FIG. 9A  and  FIG. 9B , it can be seen that in the presence of iron ions and ferroptosis inducers, for the expression vectors constructed with the FTH1 genes of Chinese hamster, mouse and human, compared to the Mock vector (blank control group), the expression vectors containing the FTH1 gene can promote the resistance of the transfected cell lines to apoptosis and maintain the expression of the exogenous gene (EGFP reporter gene). The results confirmed that the FTH1 gene derived from the three species all can be used in the screening of exogenous gene expression cell lines. 
     Example 6: Efficacy Evaluation of Screening System for Establishing Stable Cell Lines-1 
     According to the verification experiment results of the previous embodiment, a general flow process of the screening system for establishing the cell line is as follows: the expression vector is sent to the host cells for expression, after culturing the host cells for 24 hours, the screening formula (iron ions, ferroptosis inducers, fatty acids) was added for culture and screening, and after about 8 days of screening, about 100% of the cells can express the exogenous genes (for example, EGFP reporter genes), and then the cells are maintained in the medium formulation containing iron ions (500 μM ferrous sulfate or 1 mM ferric ammonium citrate) to maintain cell growth and expression of exogenous genes. 
     According to the aforementioned screening system, the CHOK1 cell lines Fth1-WT (CHOK1, BCRC) and Fth1-KO (CRISPR/Cas9 mediated knockout) were seeded in 96-well culture plates with 1×10 4  cells/well respectively. After 24 hours of adherent culture in F12 medium (Ham&#39;s F-12 Nutrient Mix, Gibco) containing 10% FBS, the transfection of vector (pCDNA3.1-CMV-cgFth1-p2A-EGFP) was carried out with Lipofectamine 3000 (Thermo Fisher Scientific Inc.) to promote the overexpression of FTH1 gene. 24 hours after the DNA transfection of the vectors, 125 μM ferrous sulfate (FeSO 4 ) was added to the cell culture broth, and at the same time different doses of ferroptosis inducers FINO 2  (0 μM to 20 μM) were added to induce ferroptosis. After 3 days (72 hours) of action, the Celigo Image Cytometer (Nexcelom) was used to analyze the cell morphology, viability, cell count, fluorescent cell count, and fluorescence quantification, thereby estimating the screening efficiency. 
     As shown in  FIG. 10 , the cytotoxic effect showed a dose-dependent change, and with the increase of the dose of the ferroptosis inducer, most cells experienced significant ferroptosis. Among them, cells expressing fluorescent proteins had higher tolerance to screening and had a better survival advantage. It can be seen that the expression of the expression vector constructed in the Examples of the present disclosure in the host cells can enhance the cell&#39;s tolerance to ferroptosis induced by the screening formula. In addition, in the condition of high dose of ferroptosis inducer (&gt;15 μM), the cell screening rate can reach 100%. 
       FIG. 11A  and  FIG. 11B  present the quantitative data of the experimental results of  FIG. 10 . As shown in  FIG. 11A , the increased dose of FINO 2  in the screening formula was negatively correlated with overall cell viability. As shown in  FIG. 11B , the increased dose of FINO 2  in the screening formula was positively correlated with the expression of exogenous genes (EGFP fluorescent protein), indicating that the vector system could endow the cell line with maintaining the expression of the exogenous gene. 
     Refer to  FIG. 12A  and  FIG. 12B .  FIG. 12A  and  FIG. 12B  present the quantitative data of the experimental results of  FIG. 10 , which compares the fluorescent expression level of surviving cells after screening ( FIG. 12A ) and the fluorescent expression level of fluorescent cells ( FIG. 12B ). The results showed that the cells that survived the screening had a strong fluorescent protein expression level, and the CHOK1 cell line Fth1_KO cell line also showed a higher fluorescent protein expression level. The above results confirm that the screening system established in the Examples of the present disclosure can help to maintain the expression of exogenous genes in cells, and has a good effect of recombinant protein expression. 
     Furthermore, in order to verify the ability of the exogenous genes to maintain expression after screening, the screened cell lines in Example 6 were transferred to a 6-well culture plate for culture, and the screening drug FINO 2  was continuously added for 3 days of screening. Thereafter, the cells were maintained for continuous subculture in F12 medium (Ham&#39;s) containing 500 μM ferric sulfate. Refer to  FIG. 13 , which shows the results of the fluorescence expression of a single cell growing into a cell colony in the medium containing iron ions (500 μM ferrous sulfate) after the above-mentioned screening. As shown in  FIG. 13 , the proportion of cells expressing the fluorescent proteins could reach 100%. 
     Example 7: Efficacy Evaluation of Screening System for Establishing Stable Cell Lines-2 
     In order to verify the ability of the exogenous gene to maintain expression, after the screening culture, the cells were maintained in F12 medium (Ham&#39;s) containing 500 μM ferric sulfate for continuous subculture, and the proportion of cells with fluorescent protein expression was assessed by flow cytometer, thereby verifying the efficacy of stable expression of exogenous genes. 
       FIG. 14  and  FIG. 15  show the results of fluorescence performance of the screened CHOK1_WT cell line after continuous subculture for 160 days and 210 days, respectively. As shown in  FIG. 14  and  FIG. 15 , the results show that after 160 days and 210 days of continuous subculture, the proportion of cells expressing fluorescent proteins could still be maintained at 100%. It is shown that cell lines stably expressing exogenous genes can be established through the screening system in the Examples of the present disclosure. 
     Example 8: Efficacy Evaluation of Screening System for Establishing Stable Cell Lines-3 
     In order to verify that the screening system established in the Examples of the present disclosure can be applied to a variety of industrial-grade cell lines, further verification experiments were performed using the HEK293T cell line and VERO cell line. 
     The HEK293T cell line (HEK293T, ATCC) and the VERO cell line (Vero, CCL-81; ATCC) were seeded in 96-well culture plates with 1×10 4  cells/well respectively. After 24 hours of adherent culture in F12 medium (Ham&#39;s F-12 Nutrient Mix, Gibco) containing 10% FBS, the transfection of vectors (pCDNA3.1-CMV-cgFth1-p2A-EGFP and pCDNA3.1-CMV-p2A-EGFP_Mock, which served as a blank control group) were carried out with Lipofectamine 3000 (Thermo Fisher Scientific Inc.) to promote the overexpression of FTH1 gene. 24 hours after the DNA transfection of the vectors, 125 μM ferrous sulfate (FeSO 4 ) was added to the cell culture broth, and at the same time different doses of ferroptosis inducers FINO 2  (0 μM to 20 μM) were added to induce ferroptosis. After 3 days (72 hours) of action, the Celigo Image Cytometer (Nexcelom) was used to analyze the fluorescence cell count and fluorescence quantification, thereby estimating the screening efficiency. The results are shown in  FIG. 16A  and  FIG. 16B . 
       FIG. 16A  and  FIG. 16B  show the experimental results of the HEK293T cell line and the VERO cell line, respectively. The screening performance was evaluated according to the number of fluorescent cells after screening, which showed that increasing the dose of FINO 2  in the screening formula would cause significant death of cells transfected with Mock vector (Fth1 was not overexpressed), while the number of surviving fluorescent cells transfected with the vector with the expression of the Fth1 gene and the exogenous gene was significantly increased. It was shown that the vector system could endow cells with resistance to iron ion apoptosis, and maintain exogenous gene expression. In addition, both cell lines exhibited dose-dependent cytotoxic effects, but HEK293T cells were more sensitive to screening formulations and could produce significant screening effects at 1.25 uM FINO 2 , while VERO cells produced significant screening effects at 5 μM. The experimental results show that the screening system established in the Examples of the present disclosure can also use these two cell lines to screen stable cell lines. 
     Example 9: Establishment of FTH1 Recombinant Protein Expression Vector for the Production of Recombinant Protein 
     In order to verify that the screening system established in the present disclosure can be applied to the production of recombinant proteins, two expression vectors were respectively constructed based on pBudCE4.1 vector (Invitrogen™) and pCDNA3.1 vector (GenScript) to express a recombinant antibody protein Anti-HER2 IgG1, including pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2(V2) and pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1(V3) to express Anti-HER2 IgG1 (h4D5) recombinant monoclonal antibody. 
     Refer to  FIG. 17A  and  FIG. 17B , which respectively show schematic diagrams of the expression vectors pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 and pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1. 
     As shown in  FIG. 17A , the pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 expression vector was based on the pBudCE4.1 vector as a backbone, and the CMV promoter, GOI1 gene (Anti-HER2 IgG1 (h4D5) heavy chain variable region), pA terminator, SV40 promoter, cgFth1 gene, EF1a promoter, G012 gene (Anti-HER2 IgG1 (h4D5) light chain variable region), and pA terminator were inserted therein. The cgFth1 gene and the Anti-HER2 IgG1(h4D5) gene were driven by two different promoters. The CMV promoter had the nucleotide sequence shown in SEQ ID NO: 7, the GOI1 gene (Anti-HER2 IgG1(h4D5) heavy chain) had the nucleotide sequence shown in SEQ ID NO: 11, the G012 gene (Anti-HER2 IgG1(h4D5) light chain) had the nucleic acid sequence shown in SEQ ID NO: 12, the cgFth1 gene had the nucleic acid sequence shown in SEQ ID NO: 4, the EF1α promoter had the nucleic acid sequence shown in SEQ ID NO: 13, and the pA terminator had the nucleic acid sequence shown in SEQ ID NO: 10. 
     As shown in  FIG. 17B , the expression vector pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 was based on the pcDNA3.1 vector as the backbone, and the CMV promoter, GOI1 gene (Anti-HER2 IgG1 (h4D5) heavy chain variable region), furin gene, p2A gene, G012 gene (Anti-HER2 IgG1 (h4D5) light chain variable region), IRES gene, cgFth1 gene, and pA terminator were inserted therein. The IRES gene was located between the cgFth1 gene and the Anti-HER2 IgG1 (h4D5) gene. The CMV promoter had the nucleotide sequence shown in SEQ ID NO: 7, the GOI1 gene (Anti-HER2 IgG1 (h4D5) heavy chain) had the nucleotide sequence shown in SEQ ID NO: 11, and the G012 gene (Anti-HER2 IgG1 (h4D5) light chain) had the nucleotide sequence shown in SEQ ID NO: 12, the furin gene had the nucleic acid sequence shown in SEQ ID NO: 14, the p2A peptide had the nucleic acid sequence shown in SEQ ID NO: 8, and the IRES gene had the nucleic acid sequence shown in SEQ ID NO: 15, the cgFth1 gene had the nucleic acid sequence shown in SEQ ID NO: 4, and the pA terminator had the nucleic acid sequence shown in SEQ ID NO: 10. 
     In order to further verify that FTH1 gene can be expressed by exogenous vector to cause the overexpression of recombinant protein in the cell lines, the expression vectors established in the foregoing Examples were used to transfect the CHOK1 cell line and the HEK293T cell line to promote the overexpression of the FTH1 gene for protein expression analysis. 
     Specifically, two types of cell lines, CHOK1 and HEK293T, were seeded in 6-well culture plates with 1×10 5  cells/well respectively. After 24 hours of adherent culture, the transfection of vectors (pCDNA3.1-CMV-cgFth1-p2A-EGFP, pCDNA3.1-CMV-mFth1-p2A-EGFP, pCDNA3.1-CMV-hFth1-p2A-EGFP, pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 and pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1) were carried out with Lipofectamine 3000 (Thermo Fisher Scientific Inc.) to promote the overexpression of FTH1 gene. 72 hours after the DNA transfection of the vectors, cell lysate was prepared with RIPA cell lysate (containing 1× protease inhibitor), and protein was quantified by BCA protein quantification method (Pierce BCA Protein Assay Kit, Thermo Fisher Scientific Inc.), and 8 μg of total protein was taken for protein electrophoresis. In addition, the expression of FTH1 protein was analyzed by Western blotting using anti-FTH1 antibody (ab65080, Abcam Inc.). Furthermore, endogenous protein analysis was performed with GAPDH antibody (ab8245, Abcam Inc.) as a control for the amount of protein. Subsequently, Goat anti-rabbit HRP and Goat anti-mouse HRP were used as secondary antibodies for reaction, and ECL (SuperSignal™ West Pico PLUS Chemiluminescent Substrate, Thermo Fisher Scientific Inc.) was used for color development, and the protein expression level was analyzed with luminescence image analyzer (Fujifilm LAS-4000). The experimental results are shown in  FIG. 18A  and  FIG. 18B . 
     Referring to  FIG. 18A  and  FIG. 18B , columns 1 to 6 respectively the results of the expression vectors pCDNA3.1-CMV-cgFth1-p2A-EGFP, pCDNA3.1-CMV-hFth1-p2A-EGF, pCDNA3.1-CMV-mFth1-p2A-EGF, pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1, pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 and untransfected vector CHOK1 cell line (WT). It can be seen from the above experimental results that both the CHOK1 and HEK293T cell lines can achieve overexpression of FTH1 recombinant protein in the cell lines by means of exogenous vector expression. 
     Example 10: Production of Recombinant Antibody Using FTH1 Recombinant Protein Expression Vector-1 
     Next, it was further verified that the expression vector pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 and the cell line screening method provided in the Examples of the present disclosure can be used for the production of recombinant proteins. After the aforementioned vector was delivered to CHOK1 cell line, the Chinese hamster FTH1 gene (cgFth1) and Anti-HER2 recombinant antibody gene were used as targets to analyze the mRNA expression and protein expression. 
     First, 1×10 6  of CHOK1 cells (CHOK1, BCRC) were taken and the electroporation (2 mm cuvette, NEPA21 Electroporator) was carried out under the condition of 125V, and 20 μg of the expression vector pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 was delivered to the cells. 72 hours after the vector was delivered to the cells, the mRNA and protein expression levels were analyzed using the Chinese hamster Fth1 gene and the Anti-HER2 IgG1 (h4D5) recombinant antibody gene as the targets. 
     Furthermore, RNA extraction and mRNA qPCR analysis of the FTH1 and the heavy and light chain of Anti-HER2 IgG1 (h4D5) were performed. 1×10 6  of cells of a single cell line was extracted with 500 μl TRIsure reagent (Bioline Inc.) for RNA extraction and purified with Direct-zol™ RNA MiniPrep (Zymogen Inc. cat #R2052). After RNA quantification, 200 ng of total RNA, 1 μg of Oligo dT and SuperScript™ IV Reverse Transcriptase were taken for mRNA reverse-transcription to obtain cDNA. The primers for the Anit-Her2 heavy chain transcript, light chain transcript and Chinese hamster FTH1 gene were designed from cDNA using KAPA SYBR® FAST Universal 2×qPCR Master Mix (1 mL) (cat #KK4600, Kapasystems Inc.), for further qPCR analysis. The primer pair of Anti-HER2 IgG1 (h4D5) heavy chain was shown in SEQ ID NOs: 16 and 17, the primer pair of Anti-HER2 IgG1 (h4D5) light chain was shown in SEQ ID NOs: 18 and 19, the primer pair of beta-actin (Cg_Actin-beta) of Chinese hamster, which served as a control group, was shown in SEQ ID NOs: 20 and 21, and the primer pair of Chinese hamster FTH1 (Cg_Fth1) was shown in SEQ ID NOs: 22 and 23. 
     The quantitative analysis of Anti-HER2 IgG1 (h4D5) recombinant antibody was carried out by ELISA. 1 μg/ml HER2 (HER2-ECD-hFc) recombinant protein was prepared with 0.1 M sodium bicarbonate buffer, and 100 μl of which was added to the 96-well culture plate and placed at 4° C. for coating for 24 hours. Subsequent blocking with Superblock™ (PBS) and washing with PBST buffer at each stage were performed. The cell medium sample was diluted 50 times with StartingBlock blocking solution, and then added to the 96-well culture plate that had been coated with HER2 at 37° C. for 1 hour. After that, it was washed with 200 μl PBST, and the secondary antibody Goat Anti-Human Ig κ chain Antibody (HRP conjugate (Millipore, AP502P) 1:10000 in Superblock™ (PBS)) was added and reacted at 37° C. for 1 hour. After washing with 200 μl of PBST, 100 μl of Ne-blue TMB solution was added for color development, and then the reaction was stopped with 2N HCl, and the assay value was read at OD450 with an ELISA reader. 
     The experimental results using pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 as the expression vector are shown in  FIGS. 19A to 19C . As shown in  FIG. 19A , the analysis results of FTH1 mRNA expression showed that in three sets of vector transfection repeatability experiments (T-1, T-2, T-3), the mRNA expression of the FTH1 gene of the CHOK1 cell line in the experimental groups were all significantly higher than that in the control group (about 2.5-4 times). As shown in  FIG. 19B , mRNAs of the heavy chain (VH) and light chain (VL) of Anti-HER2 IgG1 (h4D5) antibody were highly expressed (VL: about 200×/control group, VH: about 500-800×/control group). In addition, as shown in  FIG. 19C , Anti-HER2 IgG1 (h4D5) antibody could be detected in the cell culture broth by ELISA. It showed that with the design of the pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2 expression vector using the FTH1 gene as the selection marker, the cell lines with stable expression could be screened out by the cell line screening method provided in the Examples of the present disclosure, and which can be applied to subsequent protein production. 
     Example 11: Production of Recombinant Antibody Using FTH1 Recombinant Protein Expression Vector-2 
     Next, it was further verified that the expression vector pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 and the cell line screening method provided in the Examples of the present disclosure can be used for the production of recombinant proteins. After the aforementioned vector was delivered to CHOK1 cell line, the Chinese hamster FTH1 gene (cgFth1) and Anti-HER2 recombinant antibody gene were used as targets to analyze the mRNA expression and protein expression. 
     First, 1×10 6  of CHOK1 cells (CHOK1, BCRC) were taken and the electroporation (2 mm cuvette, NEPA21 Electroporator) was carried out under the condition of 125V, and 20 μg of the expression vector pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 was delivered to the cells, and screened with the screening system established in the Examples of the present disclosure. After 8 days of screening, the cells were maintained in the medium formulation containing only iron ions (500 μM ferrous sulfate or 1 mM ferric ammonium citrate) and maintained the gene expression. After screening, the stable cell lines were cultured in suspension with serum-free medium (HyClone CDM4PERMAb medium, 4 mM glutamic acid), and subjected to limiting dilution, and a single cell line derived from a single cell was selected for subsequent analysis of recombinant antibodies. 
     Next, a small-scale mass production of Anti-HER2 IgG1 (h4D5) recombinant antibody was carried out. The single cell line was seeded in 4.5 ml of serum-free medium (HyClone CDM4PERMAb medium, 4 mM glutamic acid) with 3×10 5  cells/ml, and was subjected to suspension culture in 50 ml of TubeSpin® Bioreactor (225 rpm, 37° C., 5% CO 2 ). After 4 days of culture, the cells were removed by centrifugation and the cell culture broth was collected for enzyme-linked immunosorbent assay (ELISA) to analyze the concentration of antibody. Furthermore, the antibody was subsequently purified using MabSelect SuRe™ LX. 
     Furthermore, RNA extraction and mRNA qPCR analysis of the FTH1 and the heavy and light chain of Anti-HER2 IgG1 (h4D5) were performed. 1×10 6  cells of a single cell line was extracted with 500 μl TRIsure reagent (Bioline Inc.) for RNA extraction and purified with Direct-zol™ RNA MiniPrep (Zymogen Inc. cat #R2052). After RNA quantification, 200 ng of total RNA, 1 μg of Oligo dT and SuperScript™ IV Reverse Transcriptase were taken for mRNA reverse-transcription to obtain cDNA. The primers for the Anit-Her2 heavy chain transcript, light chain transcript and Chinese hamster FTH1 gene were designed from cDNA using KAPA SYBR® FAST Universal 2×qPCR Master Mix (1 mL) (cat #KK4600, Kapasystems Inc.), for further qPCR analysis. The primer pair of Anti-HER2 IgG1 (h4D5) heavy chain was shown in SEQ ID NOs: 16 and 17, the primer pair of Anti-HER2 IgG1 (h4D5) light chain was shown in SEQ ID NOs: 18 and 19, the primer pair of beta-actin (Cg_Actin-beta) of Chinese hamster, which served as a control group, was shown in SEQ ID NOs: 20 and 21, and the primer pair of Chinese hamster FTH1 (Cg_Fth1) was shown in SEQ ID NOs: 22 and 23. 
     The quantitative analysis of Anti-HER2 IgG1 (h4D5) recombinant antibody was carried out by ELISA. 1 μg/ml HER2 (HER2-ECD-hFc) recombinant protein was prepared with 0.1 M sodium bicarbonate buffer, and 100 μl of which was added to the 96-well culture plate and placed at 4° C. for coating for 24 hours. Subsequent blocking with Superblock™ (PBS) and washing with PBST buffer at each stage were performed. The cell medium sample was diluted 50 times with StartingBlock blocking solution, and then added to the 96-well culture plate that had been coated with HER2 at 37° C. for 1 hour. After that, it was washed with 200 μl PBST, and the secondary antibody Goat Anti-Human Ig κ chain Antibody (HRP conjugate (Millipore, AP502P) 1:10000 in Superblock™ (PBS)) was added and reacted at 37° C. for 1 hour. After washing with 200 μl of PBST, 100 μl of Ne-blue TMB solution was added for color development, and then the reaction was stopped with 2N HCl, and the assay value was read at OD450 with an ELISA reader. 
     The experimental results using pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 as the expression vector are shown in  FIGS. 20A to 20D . As shown in  FIG. 20A  and  FIG. 20B , for the four groups of clones (clone-3, clone-4, clone-8, clone-11), both the mRNAs of the heavy chain (VH) and light chain (VL) of Anti-HER2 IgG1 (h4D5) antibody were highly expressed. In addition, as shown in  FIG. 20C , the analysis results of the expression of FTH1 mRNA showed that the mRNA expression levels of FTH1 gene in four groups of CHOK1 cell line clones (clone-3, clone-4, clone-8, clone-11) were significantly higher than those of the control group. In addition, as shown in  FIG. 20D , Anti-HER2 IgG1 (h4D5) antibody could be detected in the cell culture broth by ELISA, and the expression level of clone-8 (cell strain C8) was relatively high. It showed that with the design of the pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 expression vector using the FTH1 gene as the selection marker, the cell lines with stable expression could be screened out by the cell line screening method provided in the Examples of the present disclosure, and which can be applied to subsequent protein production. 
     Example 12: Conformation Verification of Recombinant Antibodies Produced Using FTH1 Recombinant Protein Expression Vector 
     In order to verify that the conformation of the Anti-HER2 antibody (IgG antibody) expressed by the expression vector is correct, protein electrophoresis was used for analysis. 
     The Anti-HER2 IgG1 (h4D5) recombinant protein that was produced by cell line C8 screened in Example 11 was analyzed by SDS-PAGE protein electrophoresis, and Anti-HER2 IgG1 (h4D5) recombinant protein produced by a self-constructed vector (pCMV-GOI1-EF1α-GOI2-SV40-Hygromycin B phosphotransferase) using Hygromycin B as a selection marker was used as a standard for comparison. 1.5 μg of purified recombinant protein samples were taken, and 100 μM DTT reduced and non-reduced proteins were used for SDS gel electrophoresis (NuPAGE™ 4-12%, Bis-Tris, Thermo Fisher Scientific Inc.). The gel electrophoresis analysis was performed with MOPS buffer at 200V voltage, and finally staining analysis was conducted with protein gel stain (InstantBlue™ Ultrafast Protein Stain, Sigma-Aldrich), and the correctness of the produced recombinant antibodies was evaluated by protein molecular weight. The experimental results are shown in  FIG. 21 . 
     Referring to  FIG. 21 , columns 1 to 6 respectively represent the results of Anti-HER2 IgG1 recombinant protein_unreduced form, Anti-HER2 IgG1 recombinant protein_reduced form, Anti-HER2 IgG1 recombinant protein_FAC+unreduced form, Anti-HER2 IgG1 recombinant protein_FAC+reduced form, Anti-HER2 IgG standard_unreduced form, and Anti-HER2 IgG1 standard_reduced form, wherein the protein ladder marker used was: BenchMark Protein Ladder SDS page, and 1.5 μg of recombinant protein was added to each column. 
     As shown in  FIG. 21 , compared with the Anti-HER2 IgG1 (h4D5) protein standard, the cell line C8 obtained by the expression vector and the screening system provided by the present disclosure could express complete IgG1 antibody and had the correct heavy chain (VH) and light chain (VL) molecular weights when the protein was in the unreduced form and the reduced form, or when the medium contained 250 μM ferric ammonium citrate (FAC). It showed that the cell line obtained by the cell line screening method provided in the Examples of the present disclosure can produce the correct recombinant antibody protein. 
     Example 13: Functional Analysis of Recombinant Antibodies Produced Using FTH1 Recombinant Protein Expression Vectors 
     In order to verify that the Anti-HER2 IgG1 (h4D5) antibody produced by the expression vector is a functional antibody, an analysis test was performed with antibody-dependent cell-mediated cytotoxicity (ADCC). 
     Jurkat-hFcγRIIIa-NFAT gene transgenic cells was used as effector cells and SKBR3 breast cancer cells rich in HER2 antigen was used as target cells in ADCC biological activity reporter gene detection system (Promega, #G9790). In addition, the ADCC biological activity detection of anti-HER2 monoclonal antibody, Anti-HER2 IgG1, was carried out by luminescent luciferase detection system (BioGlo™ Luciferase Assay system). 
     For the cell line C8 obtained by screening in Example 11 (the expression vector pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 (V3) was used), the recombinant antibodies produced under the conditions of adding iron ions (250 Anti-HER2 IgG1_V3_C8-FAC-250 μM FAC+) and without adding iron ions (Anti-HER2 IgG1_V3_C8) during the screening process were tested. The recombinant antibody (Anti-HER2 IgG1_V2) produced by the expression vector pBudCE4.1-CMV-GOI1-cgFth1-SV40/EF1α-GOI2(V2) was also evaluated. 
     First, 100 μl of SKBR3 cells with a cell density of 10 5 /ml were added to the 96-well culture plate, placed at 37° C. for overnight culture to allow the cells to adhere and grow, the supernatant was removed the next day, and 25 μl of serially diluted recombinant antibodies prepared in GOIMI-1640 medium containing 0.5% fetal bovine serum were added. Next, 25 μl of lurkat-hFcγRIIIa-NFAT effector cells with a cell density of 6×10 6 /ml were added. After 24 hours of reaction, 75 μl of Bio-Glo™ Luciferase Assay Buffer was added for the reaction, and the luminescence signal was read on the GloMax® Navigator Microplate Luminometer to measure the ADCC effect of the antibody Anti-HER2 IgG1(h4D5). The experimental results are shown in  FIG. 22 . 
     As shown in  FIG. 22 , compared with the Anti-HER2 IgG1 (h4D5) recombinant antibody protein validated after purification, the recombinant antibodies obtained using the expression vectors pcDNA3.1-CMV-GOI1-furin-p2A-GOI2-IRES-cgFth1 (V3) and pBudCE4.1-CMV-GOI1-SV40-cgFth1-EF1α-GOI2(V2) could obviously induce ADCC. In addition, the group in which iron ions were added to the screening process (Anti-HER2 IgG1_V3_C8-FAC-250 μM FAC+) also had the same effect, indicating that the addition of iron ions did not affect the functionality of the recombinant antibody. 
     Although some embodiments of the present disclosure and their advantages have been described as above, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. In addition, each claim constitutes an individual embodiment, and the claimed scope of the present disclosure also includes the combinations of the claims and embodiments. The scope of protection of the present disclosure is subject to the definition of the scope of the appended claims.