Abstract:
Provided is a composition for inducing tumor cell senescence and a method for inducing tumor cell senescence using the same. The composition can inhibit a PAPSS2 gene in a tumor cell to thereby induce senescence of the tumor cell, and if the composition is used concurrently with irradiation of the tumor cell, the composition can improve the sensitivity of the tumor cell to radiation.

Description:
INCORPORATION BY REFERENCE OF SEQUENCE LISTING PROVIDED ELECTRONICALLY 
     An electronic version of the Sequence Listing is filed herewith, the contents of which are incorporated by reference in their entirety. The electronic file was created on Feb. 6, 2015, is 22 kilobytes in size, and titled 433SEQUS1.txt. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a composition for inducing senescence in tumor cells comprising an inhibitor against a 3′-phosphoadenosine 5′-phosphosulfate synthase 2 (PAPSS2) gene or protein encoded by the gene and a method for inducing senescence in tumor cells using the same. 
     Description of Related Art 
     Premature senescence, also known as stress-induced premature senescence, in tumor cells means senescence caused in tumor cells by various stimuli. In contrast to normal cells that undergo senescence after a particular number of cell division, tumor cells divide indefinitely due to changes in characteristics of replicative senescence during tumorigenesis, and it has long been known that tumor cells may not undergo cellular senescence. In recent years, however, various stimuli have been known to rapidly induce senescence in tumor cells, which is called stress-induced premature senescence (Sugrue et al., Proc. Natl. Acad. Sci. USA, 94:9648-9653, 1997; Mason et al., Oncogene, 23; 9238-9246, 2004). It has been reported that stress sources capable of inducing senescence in tumor cells are genotoxic chemicals (e.g., etoposide, cyclophosphamide, etc.), radiation, and UV light (Hemann and Narita, Genes &amp; Dev., 21:1-5, 2007; Chang et al., Proc. Natl. Acad. Sci. USA, 99:389-394, 2002). 
     Suppression of tumor cell proliferation by inducing senescence in tumor cells through stress-induced premature senescence has recently been suggested as an effective mechanism for cancer therapy (Roninson et al., Drug Resist Updates, 4:303-313, 2001; Campisi, Science, 309:886-887, 2005), and studies on the mechanism of cellular senescence contributed to an improvement in the efficiency of cancer therapy (Narita and Lowe, Nature Medicine, 11:920-922, 2005). Moreover, a histological analysis of cancer patients with inhibited malignant progression of tumor reported that senescence was effectively induced in tumor cells (Collado et al., Nature, 436:642, 2005). In addition, the tumor suppressor protein p53 was reported to be implicated in the removal of tumor tissues through cellular senescence, as proven in an animal test (Xue et al., Nature, 445:656-660, 2007). This indicates that cellular senescence can be effectively applied to cancer therapy. In practice, the activation of the senescence mechanism in tumor cells makes it possible to treat cancer with lower doses of anticancer agents or radiation than does the activation of cell death mechanisms, thus improving the side effects associated with conventional cancer therapy and overcoming the resistance to cancer therapy of cancer cells which acquire the resistance to the cell death (Rebbaa, Cancer Lett, 219:1-13, 2005). 
     Meanwhile, 3′-phosphoadenosine 5′-phosphosulfate synthase 2 (PAPSS2) is a gene with National Center for Biotechnology Information (NCBI) Access No. NM_004670 (SEQ ID NO: 1) or NM_001015880 (SEQ ID NO: 2) and has not been known to be associated with premature senescence in tumor cells in the prior art. 
     Technical Problem 
     Accordingly, the present inventors have found that the inhibition of PAPSS2 gene expression in tumor cells induces senescence in tumor cells and the inhibition of PAPSS2 gene expression combined with radiation treatment in tumor cells enhances the sensitivity of tumor cells to radiation and completed the present invention. 
     Therefore, an object of the present invention is to provide a composition for inducing senescence in tumor cells. Moreover, another object of the present invention is to provide a method for inducing senescence in tumor cells using the composition for inducing senescence in tumor cells. 
     BRIEF SUMMARY OF THE INVENTION 
     To achieve the above objects, the present invention provides a composition for inducing senescence in tumor cells comprising an inhibitor against a 3′-phosphoadenosine 5′-phosphosulfate synthase 2 (PAPSS2) gene or protein encoded by the gene. 
     In the present invention, the gene may be a messenger ribonucleic acid (mRNA), and the inhibitor may be a small interfering RNA (siRNA) that can inhibit the mRNA. 
     In the present invention, the PAPSS2 gene comprises a nucleotide sequence of SEQ ID NO: 1 (NCBI Access No. NM_004670) or a nucleotide sequence of SEQ ID NO: 2 (NCBI Access No. NM_001015880). Moreover, the PAPSS2 gene comprises the nucleotide sequence of SEQ ID NO: 1 or the nucleotide sequence of SEQ ID NO: 2 in which one or more nucleotides are substituted, deleted, or inserted. 
     In the present invention, the protein encoded by the PAPSS2 gene comprises a polypeptide of SEQ ID NO: 3 or a polypeptide of SEQ ID NO: 4. Moreover, the protein encoded by the PAPSS2 gene comprises a polypeptide of SEQ ID NO: 3 or the a polypeptide of SEQ ID NO: 4 in which one or more amino acids are substituted, deleted, or inserted. 
     In the present invention, it is obvious to those skilled in the art that the sequences of the PAPSS2 gene are merely examples, and the present invention is not limited thereto. Sequences having substantial sequence identity or substantial sequence homology to the sequences also fall within the scope of the present invention. As used herein, the term “substantial sequence identify” or “substantial sequence homology” is used to indicate that a sequence exhibits substantial, structural, or functional equivalence with another sequence. These differences are due to inherent variations in codon usage among different species, for example. Structural differences are considered de minimis if there is a significant amount of sequence overlap or similarity between two or more different sequences, even if the sequences differ in length or structure. 
     In the present invention, the inhibitor may be an siRNA that can inhibit the expression of the PAPSS2 gene. That is, the inhibitor may be a siRNA that can inhibit the expression of an mRNA for a 3′-phosphoadenosine 5′-phosphosulfate synthase 2 (PAPSS2) gene. The siRNA may be a double stranded siRNA composed of the sense sequence of SEQ ID NO: 5 (5′-ACA ACC UGU ACU CUU CCA A-3′) and the antisense sequence SEQ ID NO: 6 (5′-UUG GAA GAG UAC AGG UUG U-3′) complementary thereto. The siRNA may have two thymine residues (dTdT) attached at the 3′ end of the sense sequence and/or the antisense sequence. 
     In the present invention, the inhibitor against the protein may be an antibody. The antibody may be a monoclonal antibody, a polyclonal antibody, and/or a recombinant antibody, which specifically binds to a protein encoded by a 3′-phosphoadenosine 5′-phosphosulfate synthase 2 (PAPSS2) gene, and may be a commercially available product or may be directly prepared using a well-known method. 
     In the present invention, the tumor cells may be cancer cells. The cancer may be at least one selected from the group consisting of liver cancer, stomach cancer, breast cancer, colon cancer, bone cancer, pancreatic cancer, head or neck cancer, uterine cancer, colorectal cancer, lung cancer, ovarian cancer, rectal cancer, esophageal cancer, small bowel cancer, anal cancer, fallopian tube carcinoma, endometrial carcinoma, cervical carcinoma, vaginal carcinoma, vulvar carcinoma, Hodgkin&#39;s disease, prostate cancer, bladder cancer, kidney cancer, ureter cancer, renal cell carcinoma, renal pelvis carcinoma, osteosarcoma, and central nervous system tumor. The cells may preferably be breast cancer cells or lung cancer cells, but not limited thereto. 
     The composition for inducing senescence in tumor cells of the present invention may be a pharmaceutical composition and can exhibit anticancer activity due to senescence induced in tumor cells by the inhibitor against the PAPSS2 gene or protein encoded by the gene contained in the composition for inducing senescence in tumor cells of the present invention. 
     The composition for inducing senescence in tumor cells of the present invention may comprise one or more carriers, diluents, excipients, or combinations thereof, which are commonly used in biological preparations. The pharmaceutically acceptable carrier is not particularly limited as long as it is suitable for in vivo delivery of the composition and may include one or more compounds described in Merck Index, 13th ed., Merck &amp; Co. Inc., saline, sterile water, Ringer&#39;s solution, buffered saline, dextrose solution, maltodextrin solution, glycerol, ethanol and combinations thereof. If necessary, other general additives, such as antioxidant, buffer solution, anti-bacterial agent, etc., may be added to the composition. Moreover, the composition of the present invention may be formulated into injections such as aqueous solution, suspension, emulsion, etc., pills, capsules, granules, or tablets by further adding diluent, dispersant, surfactant, binder, and lubricant. Furthermore, the composition of the present invention may be formulated by suitable methods known in the art depending on the disease and/or ingredients. 
     The composition for inducing senescence in tumor cells of the present invention may be administered once a day at a dose of 0.01 ng/kg-100 mg/kg for adults with respect to a gene inhibitor, for example, a siRNA, to induce senescence in tumor cells, may be administered once a day at a dose of 2-10 mg/kg for adults with respect to a gene inhibitor, for example, an antibody, and may be administered in the usual manner via intravenous, intraarterial, intraperitoneal, intramuscular, intraperitoneal, intrasternal, percutaneous, intranasal, inhalation, topical, rectal, oral, intraocular or intradermal routes. 
     A therapeutically effective amount of the composition for inducing senescence in tumor cells of the present invention may vary depending on various factors such as administration route, target site, a patient&#39;s condition, etc. Therefore, the dose to be used in human should be determined as an appropriate amount in terms of both safety and effectiveness. It is possible to extrapolate the dose to be used in human from an effective amount determined in animal tests. 
     To achieve the above objects, the present invention provides a method for inducing senescence in tumor cells, wherein the method comprising treating tumor cells with a composition for inducing senescence in tumor cells comprising an inhibitor against a 3′-phosphoadenosine 5′-phosphosulfate synthase 2 (PAPSS2) gene or protein encoded by the gene. 
     In the present invention, the cells may be tumor cells of a mammal other than a human. The tumor cells may preferably be cancer cells. The cancer may be at least one selected from the group consisting of liver cancer, stomach cancer, breast cancer, colon cancer, bone cancer, pancreatic cancer, head or neck cancer, uterine cancer, colorectal cancer, lung cancer, ovarian cancer, rectal cancer, esophageal cancer, small bowel cancer, anal cancer, fallopian tube carcinoma, endometrial carcinoma, cervical carcinoma, vaginal carcinoma, vulvar carcinoma, Hodgkin&#39;s disease, prostate cancer, bladder cancer, kidney cancer, ureter cancer, renal cell carcinoma, renal pelvis carcinoma, osteosarcoma, and central nervous system tumor. The cells may preferably be breast cancer cells or lung cancer cells, but not limited thereto. 
     In the present invention, the method for inducing senescence in tumor cells may further comprises treating the cells with radiation. 
     In the present invention, the radiation may be gamma rays, and the gamma rays may be irradiated at a dose of 2 to 6 Gy, but not limited thereto. 
     In the method of the present invention, the treatment of the tumor cells with a composition for inducing senescence in tumor cells comprising an inhibitor against a PAPSS2 gene or protein encoded by the gene may be performed simultaneously or sequentially with the treatment of the tumor cells with radiation. 
     To achieve the above objects, the present invention provides a use of an inhibitor against a PAPSS2 gene or protein encoded by the gene for the preparation of a composition for inducing senescence in tumor cells. 
     To achieve the above objects, the present invention provides a use of an inhibitor against a PAPSS2 gene or protein encoded by the gene for the preparation of a composition for inducing senescence in tumor cells, which enhances the sensitivity of tumor cells to radiation. 
     Advantageous Effects 
     The composition for inducing senescence in tumor cells according to the present invention can induce senescence in tumor cells by inhibiting PAPSS2 gene expression in tumor cells and enhance the sensitivity of tumor cells to radiation when used in combination with the treatment of tumor cells with radiation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the results of microscopic observation for each day in the control group and the irradiation group of human breast cancer cell line MCF-7. 
         FIG. 2  shows the results of microscopic observation for each day in the control group and the irradiation group of human lung cancer cell line H460. 
         FIG. 3  shows the results of Western blotting illustrating the increased expression of PAPSS2, p53, and p21 and the decreased phosphorylation Rb for each day up to 4 days after irradiation in human breast cancer cell line MCF-7. 
         FIG. 4  shows the results of Western blotting illustrating the increased expression of PAPSS2, p53, and p21 and the decreased phosphorylation of Rb for each day up to 3 days after irradiation in human lung cancer cell line H460. 
         FIG. 5  shows the results of reverse transcriptase-PCR illustrating the change in the expression level of mRNA in the control group (treated with siControl) and the experimental group (treated with siPAPSS2) in human breast cancer cell line MCF-7. 
         FIG. 6  shows the induction of senescence in human breast cancer cell line MCF-7 by a PAPSS2 gene inhibitor (siRNA) and is a graph illustrating the cell proliferation rate in the control group (treated with siControl) and the experimental group (treated with siPAPSS2). 
         FIG. 7  shows the induction of senescence in human breast cancer cell line MCF-7 by a PAPSS2 gene inhibitor (siRNA) and is an image illustrating the formation of colonies of tumor cells over 8 days after treatment with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2, respectively. 
         FIG. 8  shows images observed under a phase-contrast microscope after treatment of human breast cancer cell line MCF-7 with siControl and siPAPSS2, respectively, followed by staining with a reagent specific to activated senescence-associated beta-galactosidase on the days indicated in  FIG. 8 . 
         FIG. 9  is a graph illustrating the percentage of cells positive for senescence-associated beta-galactosidase after treatment of human breast cancer cell line MCF-7 with siControl and siPAPSS2, respectively, followed by staining with a reagent specific to activated senescence-associated beta-galactosidase on the days indicated in  FIG. 9 . 
         FIG. 10  is a graph illustrating the cell proliferation rate at 3 days after treatment of human breast cancer cell line MCF-7 with siControl and siPAPSS2, respectively, followed by gamma-irradiation at a dose of 2 or 4 Gy after 24 hours. 
         FIG. 11  is an image illustrating the formation of colonies of tumor cells over 8 days after treatment with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2, respectively, in combination with radiation treatment. 
         FIG. 12  shows images observed under a phase-contrast microscope after treatment of human breast cancer cell line MCF-7 with siControl and siPAPSS2, respectively, and gamma-irradiation at a dose of 0, 2 or 4 Gy, followed by staining with a reagent specific to activated senescence-associated beta-galactosidase after 3 days. 
         FIG. 13  is a graph illustrating the percentage of cells positive for senescence-associated beta-galactosidase after treatment of human breast cancer cell line MCF-7 with siControl and siPAPSS2, respectively, and gamma-irradiation at a dose of 0, 2 or 4 Gy, followed by staining with a reagent specific to activated senescence-associated beta-galactosidase after 3 days. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described in more detail with reference to the following Examples. However, it should be understood that the following Examples are not intended to limit the scope of the present invention, but are intended to facilitate understanding the present invention. 
     Example 1 
     Identification of Genes with Expression Increased by Radiation-Induced Senescence in Tumor Cells 
     1-1 Cell Culture 
     Human breast cancer cell line MCF-7 (ATCC, USA) was cultured in Dulbeco&#39;s Modified Eagle&#39;s Medium (DMEM) containing 10% fatal bovine serum (FBS; Welgene, Korea), 100 μg/ml streptomycin, and 100 units/ml penicillin (Gibco BRL) in an incubator maintained at 5% CO2 and 37° C. 
     Human lung cancer cell line H460 (ATCC, USA) was cultured in Dulbeco&#39;s Modified Eagle&#39;s Medium (DMEM) containing 10% fatal bovine serum (FBS; Welgene, Korea), 100 μg/ml streptomycin, and 100 units/ml penicillin (Gibco BRL) in an incubator maintained at 5% CO 2  and 37° C. 
     The human breast cancer cell line MCF-7 was exposed to 6 Gy gamma-rays from  137 Cs gamma-ray source (Atomic Energy of Canada Ltd., Mississauga, Ontario, Canada) at a dose rate of 3.81 Gy/min and then cultured in an incubator with 5% CO 2  at 37° C. for 1 to 4 days. 
     The human lung cancer cell line H460 was exposed to 6 Gy gamma-rays from  137 Cs gamma-ray source (Atomic Energy of Canada Ltd., Mississauga, Ontario, Canada) at a dose rate of 3.81 Gy/min and then cultured in an incubator with 5% CO 2  at 37° C. for 1 to 4 days. 
     Moreover, cells of the human breast cancer cell line MCF-7, which were not exposed to gamma-irradiation, were used as a control. Also, cells of the human lung cancer cell line H460, which were not exposed to gamma-irradiation, were used as a control. 
     1-2 Identification of Radiation-Induced Senescence 
     To identify the induction of cellular senescence, characteristic changes in cell morphology of the control group of Example 1-1 and the human breast cancer cell line MCF-7 at 4 days after irradiation were observed under a microscope (ECLIPSE TE300, Nikon), and the results are shown in  FIG. 1  (observed under a microscope for each day indicated in  FIG. 1 ). As a result, it can be seen that the human breast cancer cell line MCF-7 at 4 days after irradiation became large in size and flat, which are characteristics of senescent cells, unlike the control group. 
     Moreover, to identify the induction of cellular senescence, characteristic changes in cell morphology of the control group of Example 1-1 and the human lung cancer cell line H460 at 3 days after irradiation were observed under a microscope (ECLIPSE TE300, Nikon), and the results are shown in  FIG. 2  (observed under a microscope for each day indicated in  FIG. 2 ). As a result, it can be seen that the human lung cancer cell line H460 at 3 days after irradiation H450 became large in size and flat, which are characteristics of senescent cells, unlike the control group. 
     1-3 Identification of Genes with Expression Changed by Radiation-Induced Senescence Using Microarray Analysis 
     The human breast cancer cell line MCF-7 at 4 days after irradiation, prepared in Example 1-1, was washed with phosphate buffered saline (PBS) and subjected to RNA isolation using TRI Reagent (MRC, Inc. Cat #TR-118). The total RNA thus isolated was quantified using a UV spectrophotometer (Ultrospec 3100 PRO, Amersham Bioscience). cDNA was synthesized from 500 ng of the total RNA using reverse transcriptase, followed by in vitro amplification/transcription (Illumina TotalPrep RNA Amplification kit, Ambion Inc.) to synthesize biotinylated cRNA. 1.5 μg of the amplified biotinylated cRNA was hybridized to the BeadChip (Illumina Human-6 BeadChip Illumina, Inc.) and visualized with Cy3 fluorescence dye (Amersham Fluorolink streptavidin-Cy3, GE Healthcare Bio-Sciences). The chip was scanned on a confocal scanner (BeadStation 500GXDW; Illumina, Inc.) to detect hybridized signals which were then analyzed using Illumina BeadStudio software. 
     The microarray analysis performed in the above manner identified genes whose mRNA levels were changed in the radiation-induced senescent cells as follows (see Table 1). 
     Moreover, the identification of genes with expression changed by radiation-induced senescence using microarray analysis was also performed on the human lung cancer cell line H460, instead of the breast cancer cell line MCF-7, in the same manner as in Example 1-3, and the results are shown together in the following Table 1. 
     
       
         
               
             
               
               
             
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Gene with expression of mRNA changed by 
               
               
                 radiation-induced senescence in tumor cells 
               
             
          
           
               
                   
                 Rate of increase in expression 1   
               
             
          
           
               
                   
                 MCF-7 
                 H460 
               
             
          
           
               
                 Gene 
                 1 day 2   
                 2 days 
                 3 days 
                 4 days 
                 4 days 
               
               
                   
               
               
                 PAPSS2 
                 1.1 
                 1.4 
                 1.8 
                 3 
                 2.2 
               
               
                   
               
               
                   1 Rate compared to the control without irradiation 
               
               
                   2 days after irradiation (6 Gy) 
               
             
          
         
       
     
     As shown in Table 1, it can be seen that the expression of the PAPSS2 gene increased in the senescence-induced human breast cancer cell line MCF-7 or human lung cancer cell line H460 after irradiation, compared to the control group. 
     1-4 Identification of Genes with Expression Changed by Radiation-Induced Senescence Using Western Blotting Analysis 
     The human breast cancer cell line MCF-7 at 1 to 4 days after irradiation, prepared in the same manner as in Example 1-1, was washed with PBS and lysed with a cell lysis buffer (50 mM Tri-HCl, 1% NP-40, 0.25% sodium deoxycholate, 150 mM NaCl, 1 mM PMSF, 50 mM NaF, 0.2 mM Na 3 VO 4 , 10 μg/ml aprotinin, 2 μg/ml leupeptin) to extract proteins, and the extracted proteins were centrifuged at 11,000 rpm for 10 minutes. Then, the supernatant was collected, and the proteins were quantified using the Bradford method (Bradford, M., Anal. Biochem. 72:248-254 (1976)). 20 μg of protein was added to 2×SDS loading buffer (60 mM Tris-Cl (pH 6.8), 25% glycerol, 2% SDS, 14.4 mM mercaptoethanol, 0.1% bromophenol blue), and the mixture was heated at 95° C. for 5 minutes, followed by electrophoresis at 80 V for 2 hours on a 8% to 10% SDS polyacrylamide gel. 
     The proteins separated by electrophoresis were transferred onto a nitrocellulose membrane (Whatman), which was then blocked in PBS containing 5% skim milk for 1 hour at room temperature, and then primary antibodies diluted to 1:500 to 1:1000 were added to the membrane and reacted at 4° C. for 16 hours. The primary antibodies used were a polyclonal anti-PAPSS2 antibody (polyclonal anti-PAPSS2 Ab, Santa Cruz), a polyclonal anti-p21 antibody (polyclonal anti-p21 Ab, Santa Cruz), a polyclonal anti-actin antibody (polyclonal anti-actin Ab, Santa Cruz), a polyclonal anti-p53 antibody (polyclonal anti-p53 Ab, Leica), and an anti-phospho-pRb (p-pRb) antibody specifically recognizing only the Ser 807/811 phosphorylated form of pRb (anti-phospho-pRb Ab, Cell Signaling), and a horseradish peroxidase-conjugated anti-rabbit or anti-mouse antibody (HRP conjugated goat anti-rabbit IgG, HRP conjugated goat anti-mouse IgG, Santa Cruz) was used as a secondary antibody to detect the protein expression with enhanced chemiluminescence (ECL) reagent (Amersham). The results of the expression of PAPSS2 protein are shown in  FIG. 3 . 
     Moreover, the human lung cancer cell line H460 at 1 to 3 days after irradiation, prepared in the same manner as in Example 1-1, was washed with PBS and lysed with a cell lysis buffer (50 mM Tri-HCl, 1% NP-40, 0.25% sodium deoxycholate, 150 mM NaCl, 1 mM PMSF, 50 mM NaF, 0.2 mM Na 3 VO 4 , 10 g/ml aprotinin, 2 g/ml leupeptin) to extract proteins, and the extracted proteins were centrifuged at 11,000 rpm for 10 minutes. Then, the supernatant was collected, and the proteins were quantified using the Bradford method (Bradford, M., Anal. Biochem. 72:248-254 (1976)). 20 μg of protein was added to 2×SDS loading buffer (60 mm Tris-Cl (pH 6.8), 25% glycerol, 2% SDS, 14.4 mM mercaptoethanol, 0.1% bromophenol blue), and the mixture was heated at 95° C. for 5 minutes, followed by electrophoresis at 80 V for 2 hours on a 8% to 10% SDS polyacrylamide gel. 
     The proteins separated by electrophoresis were transferred onto a nitrocellulose membrane (Whatman), which was then blocked in PBS containing 5% skim milk for 1 hour at room temperature, and then primary antibodies diluted to 1:500 to 1:1000 were added to the membrane and reacted at 4° C. for 16 hours. The primary antibodies used were a polyclonal anti-PAPSS2 antibody (polyclonal anti-PAPSS2 Ab, Santa Cruz), a polyclonal anti-p21 antibody (polyclonal anti-p21 Ab, Santa Cruz), a polyclonal anti-actin antibody (polyclonal anti-actin Ab, Santa Cruz), a polyclonal anti-p53 antibody (polyclonal anti-p53 Ab, Leica), and an anti-phospho-pRb (p-pRb) antibody specifically recognizing only the Ser 807/811 phosphorylated form of pRb (anti-phospho-pRb Ab, Cell Signaling), and a horseradish peroxidase-conjugated anti-rabbit or anti-mouse antibody (HRP conjugated goat anti-rabbit IgG, HRP conjugated goat anti-mouse IgG, Santa Cruz) was used as a secondary antibody to detect the protein expression with enhanced chemiluminescence (ECL) reagent (Amersham). The results of the expression of PAPSS2 protein are shown in  FIG. 4 . 
     In  FIGS. 3 and 4 , actin, a housekeeping gene which is expressed at constant levels in all cells, was used as a reference to determine whether the same amount of proteins was analyzed. 
     The results of the Western blotting analysis confirm that the phosphorylation of senescence-specific Rb decreases and the expression of p53 and p21 increases by irradiation in the human breast cancer cell line MCF-7 or the human lung cancer cell line H460 after irradiation, indicating the induction of senescence by radiation. Moreover, the results of the Western blotting analysis confirm that the expression of PAPSS2 gene increases in the senescence-induced human breast cancer cell line MCF-7 or human lung cancer cell line H460 after irradiation, i.e., the expression level of PAPSS2 protein increases compared to the control group. 
     Example 2 
     Identification of Induction of Senescence in Tumor Cells by PAPSS2 Gene Inhibitor (siRNA) 
     2-1 Treatment with Small Interfering RNA (siRNA) Against PAPSS2 Gene 
     To examine the effect of a reduction in the expression level of PAPSS2 gene, the human breast cancer cell line MCF-7 (ATCC, USA) was cultured in DMEM containing 10% fatal bovine serum (FBS; Welgene, Korea) and an antibiotic (Gibco BRL) and passaged to 60-mm culture dishes one day before treatment with siRNA. Then, 2 μl of RNAiMAX (Invitrogen, Cat #13778-075) and OptiMEM® I medium (Invitrogen, Cat #31985) were added to the cells, which were then treated with a small interfering RNA against PAPSS2 gene (siPAPSS2; a double stranded siRNA composed of the nucleotide sequence of 5′-ACA ACC UGU ACU CUU CCA A-3′ (SEQ ID NO: 5) and the nucleotide sequence of 5′-UUG GAA GAG UAC AGG UUG U-3′ (SEQ ID NO: 6), in which two thymine residues (dTdT) are attached at the 3′ end of each sequence) at a concentration of 50 nM, followed by incubation for 6 hours. After the medium was replaced by DMEM containing 10% FBS (Welgene), 100 μg/ml streptomycin, and 100 units/ml penicillin (Gibco BRL), the cells were incubated in an incubator with 5% CO 2  at 37° C. for 4 days. 
     Moreover, cells of the human breast cancer cell line MCF-7 treated with non-specific small interfering RNA (siControl; a double stranded siRNA composed of the nucleotide sequence of 5′-CCU ACG CCA CCA AUU UCG U-3′ (SEQ ID NO: 7) and the nucleotide sequence of 5′-ACG AAA UUG GUG GCG UAG G-3′ (SEQ ID NO: 8), in which two thymine residues (dTdT) are attached at the 3′ end of each sequence) were used as a control. 
     All siRNAs were synthesized in Bioneer (Korea). In detail, β-cyanoethyl phosphoramidite was used for the synthesis of siRNAs by linking phosphodiester bonds that form the backbone of DNA (see Sinha et al., Nucleic Acids Research, 12:4539-4557, 1984). That is, using an RNA synthesizer (Perceptive Biosystems 8909, PE Biosystems, USA), a series of processes including deblocking, coupling, oxidation and capping were repeated on a solid support, to which nucleotides were attached, to obtain a reactant containing RNAs of a desired length. Then, RNA was isolated from the reactant using HPLC LC918 (Japan Analytical Industry, Japan) equipped with Daisogel C18 (Daiso, Japan) and its sequence was analyzed by MALDI-TOF mass spectroscopy (Shimadzu, Japan). Then, sense and antisense RNA strands were combined to prepare the desired double stranded siRNA. 
     2-2 Determination of Decrease in Protein Expression by Specific Small Interfering RNA 
       FIG. 5  shows the results of reverse transcriptase-PCR illustrating the reduction in the expression of PAPSS2 mRNA in the human breast cancer cell line MCF-7 after treatment with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2. In detail, the human breast cancer cell line MCF-7 was treated with the specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2, and after 2 days, the cell medium was discarded, the cells were washed twice with PBS. Then, RNA was extracted from the cells with 1 ml Trizol (Molecular research center, MRC), and cDNA was synthesized using a thymine residue (oligo dT), which selectively recognizes an adenosine residue at the 3′ end of RNA, and a reverse transcriptase (Invitrogen). The synthesized cDNA was used for the polymerase chain reaction (PCR) amplification of PAPSS2 gene using a forward primer (5′-GTCTCTCTGGTGCTGGAAAA-3′, SEQ ID NO: 9), and a reverse primer (5′-TGCGAATGGAGAAATAAAGC-3′, SEQ ID NO: 10), followed by electrophoresis at 100 V for 30 minutes on a 1% agarose gel. The agarose gel was stained with ethidium bromide (EtBr, sigma) and observed under an ultraviolet lamp (Bio-Rad). In  FIG. 5 , GAPDH is a gene which is expressed at constant levels in all samples and indicates that the amounts of proteins used in all samples are the same. 
     As shown in  FIG. 5 , it can be seen that the expression of PAPSS2 mRNA in the breast cancer cell line MCF-7 significantly decreased in the experimental group (treated with siPAPSS2) compared to the control group (treated with siControl). 
     2-3 Identification of Cell Proliferation Rate Using Trypan Blue Staining 
       FIG. 6  shows the cell proliferation rate obtained by treating the human breast cancer cell line MCF-7 with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2, culturing the cell line in an incubator with 5% CO 2  at 37° C. for 1 to 4 days, collecting cell supernatants on the days indicated in  FIG. 6 , splitting the cells using Trypsin, washing the cells twice with PBS, mixing the cells with 0.4% Trypan blue (NUNK) at a ratio of 1:1, leaving the cells at room temperature for 5 minutes, counting the number of cells, which were not stained with Trypan blue, on a hemocytometer (MARIENFELD) to calculate the cell proliferation rate as a percentage to the total number of cells. 
       FIG. 6  is a graph illustrating the cell proliferation rate in the control group (treated with siControl) and the experimental group (treated with siPAPSS2). In  FIG. 6 , the x-axis represents the days of cell culture after treatment with siPAPSS2 or siControl and the y-axis represents the cell proliferation rate based on the number of cells at 0 day. As shown in  FIG. 6 , it can be seen that the cell proliferation rate of the tumor cells (i.e., the human breast cancer cell line MCF-7) decreased in the experimental group treated with siPAPSS2 over time compared to the control group. Therefore, it can be seen from these results that the inhibition of the expression of PAPSS2 gene in tumor cells (e.g., the human breast cancer cell line MCF-7) induces senescence in tumor cells, which results in a decrease in cell proliferation rate. 
     2-4 Identification of Induction of Senescence in Tumor Cells by Colony Formation Assay 
     The human breast cancer cell line MCF-7 treated with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2, prepared in the same manner as in Example 2-1, was distributed at a density of 500 cells in 60-mm dishes by passage. After incubation for 8 days, colonies thus formed were stained with a Diff Quick reagent (Sysmex Cat #38721). In detail, the medium was removed and the cells were washed once with PBS. Then, the cells were mixed softly with 0.5 mL of solution A and removed. And then the cells were mixed softly with 0.5 mL of solution B and removed. After that the cells were mixed softly with 0.5 mL of solution C and removed. Thereafter, the cells were rinsed with a sufficient amount of distilled water and dried at room temperature for 30 minutes, followed by colony counting (Image Product International #880). 
       FIG. 7  is an image illustrating the formation of colonies of tumor cells over 8 days after treatment with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2, respectively. As shown in  FIG. 7 , it can be seen that the formation of colonies of tumor cells significantly decreased in the tumor cell line treated with siPAPSS2 (the human breast cancer cell line MCF-7) compared to the control group. Therefore, it can be seen from these results that the inhibition of the expression of PAPSS2 gene in tumor cells (e.g., the human breast cancer cell line MCF-7) induces senescence in tumor cells, which results in a decrease in the formation of colonies of tumor cells. 
     2-5 Identification of Induction of Senescence in Tumor Cells by siPAPSS2 Using Senescence-Associated Beta-Galactosidase Activity Staining 
     To identify the induction of senescence in tumor cells by radiation, senescence-associated beta-galactosidase activity staining was performed on the control group (treated with non-specific small interfering RNA in the human breast cancer cell line MCF-7) and the experimental group (treated with specific small interfering RNA in the human breast cancer cell line MCF-7) of Example 2-1. The control and experimental groups of Example 2-1 were treated with non-specific small interfering RNA (siControl) and specific small interfering RNA (siPAPSS2) against PAPSS2, respectively, followed by senescence-associated beta-galactosidase activity staining on the groups at 1 day, 2 days, 3 days, and 4 days, respectively. This staining was performed according to the Dimri method (Dimri et al., Proc. Natl. Acad. Sci. USA, 92:9363-9367, 1995) as follows. 
     Cells were washed twice with PBS and fixed at room temperature for 3 to 5 minutes in 3% formaldehyde. The fixed cells were washed once again with PBS and incubated with 5 mL of a β-galactosidase staining solution (1 mg/ml X-Gal, 40 mM citric acid/sodium phosphate (pH 6.0), 5 mM potassium ferrocyanide, 5 mM potassium ferricyanide, 150 mM sodium chloride, 2 mM magnesium chloride) in an incubator at 37° C. for 12 to 16 hours while the culture dishes were wrapped with foil so as to keep a dark condition. 
     The beta-galactosidase activity level was measured under a phase-contrast microscope (ECLIPSE TE300, Nikon), and the results are shown in  FIG. 8 . Moreover, the number of stained cells after irradiation was measured using a microscope (ECLIPSE TE300, Nikon), and a graph representing the percentage of cells positive for senescence-associated beta-galactosidase is shown in  FIG. 9 . 
     It can be seen that the number of cells stained in the experimental group, i.e., the number of cells positive for senescence-associated beta-galactosidase increased, compared to the control group. In particular, it can be seen that the senescence occurred significantly in the experimental group treated with specific small interfering RNA (siPAPSS2) against PAPSS2 over time, compared to the control group. 
     Therefore, it can be seen from these results that the inhibition of the expression of PAPSS2 gene in tumor cells (e.g., the human breast cancer cell line MCF-7) induces senescence in tumor cells, thus exhibiting anticancer activity. 
     Example 3 
     Identification of Increase in Sensitivity of Tumor Cells to Radiation-Induced Senescence by Treatment with PAPSS2 Gene Inhibitor (siRNA) 
     3-1 Identification of Cell Proliferation Rate Using Trypan Blue Staining 
     The human breast cancer cell line MCF-7 was treated with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2, and the cell line was cultured in an incubator with 5% CO 2  at 37° C. for 24 hours, exposed to 2 Gy or 4 Gy gamma-rays from  137 Cs gamma-ray source (Atomic Energy of Canada Ltd., Mississauga, Ontario, Canada), and then cultured in an incubator for 3 days. The human breast cancer cell line MCF-7 was treated with non-specific small interfering RNA (siControl) against PAPSS2, and the cell line was cultured in an incubator with 5% CO 2  at 37° C. for 24 hours and then cultured in an incubator for 3 days, without being exposed to gamma-rays. The non-specific small interfering RNA (siControl) and the specific small interfering RNA (siPAPSS2) against PAPSS2 were prepared in the same manner as in Example 2-1 and used. After collecting all cell supernatants, the cells were split using Trypsin, washed twice with PBS, mixed with 0.4% Trypan blue (NUNK) at a ratio of 1:1, and left at room temperature for 5 minutes, and the number of cells, which were not stained with Trypan blue, was counted on a hemocytometer (MARIENFELD) to calculate the cell proliferation rate as a percentage to the total number of cells. 
       FIG. 10  is a graph illustrating the cell proliferation rate at 3 days after gamma-irradiation to the human breast cancer cell line MCF-7 at a dose of 2 or 4 Gy after 24 hours. In  FIG. 10 , the relative cell proliferation rate is shown with respect to the cell proliferation rate of 1 in the group in which the human breast cancer cell line MCF-7 was treated with non-specific small interfering RNA (siControl) without being exposed to gamma-rays. 
     As shown in  FIG. 10 , it can be seen that the cell proliferation rate of tumor cells (e.g., the human breast cancer cell line MCF-7) decreased over time in the group treated with siPAPSS2 compared to the control group. Therefore, it can be seen from these results that the inhibition of the expression of PAPSS2 gene in tumor cells (e.g., the human breast cancer cell line MCF-7) induces senescence in tumor cells, which results in a decrease in cell proliferation rate. 
     3-2 Identification of Induction of Senescence in Tumor Cells by Colony Formation Assay 
     The human breast cancer cell line MCF-7 treated with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2 was distributed at a density of 500 cells in 60-mm dishes by passage. The non-specific small interfering RNA (siControl) and the specific small interfering RNA (siControl) against PAPSS2 were prepared in the same manner as in Example 2-1 and used. The cell line was cultured in an incubator with 5% CO 2  at 37° C. for 24 hours and exposed to 2 Gy or 4 Gy gamma-rays from  137 Cs gamma-ray source (Atomic Energy of Canada Ltd., Mississauga, Ontario, Canada), and then cultured in an incubator for 8 days. Then, the thus formed colonies were stained with a Diff Quick reagent (Sysmex Cat #38721). In detail, the medium was removed and the cells were washed once with PBS. Then, the cells were mixed softly with 0.5 mL of solution A and removed. And then the cells were mixed softly with 0.5 mL of solution B and removed. After that the cells were mixed softly with 0.5 mL of solution C and removed. Thereafter, the cells were rinsed with a sufficient amount of distilled water and dried at room temperature for 30 minutes, followed by analysis. 
       FIG. 11  is an image illustrating the formation of colonies of tumor cells over 8 days after treatment with specific small interfering RNA (siPAPSS2) or non-specific small interfering RNA (siControl) against PAPSS2, respectively, in combination with radiation treatment. As shown in  FIG. 11 , upon exposure to the same dose of radiation, the formation of colonies of tumor cells significantly decreased in the tumor cell line treated with siPAPSS2 (the human breast cancer cell line MCF-7) compared to the control group. Therefore, it can be seen that the sensitivity of tumor cells (e.g., the human breast cancer cell line MCF-7) to the induction of senescence by radiation increases in the group treated with the PAPSS2 gene inhibitor (siRNA), which results in a decrease in the formation of colonies of tumor cells, compared to the control group. That is, it can be seen from these results that the inhibition of the expression of PAPSS2 gene in tumor cells (e.g., the human breast cancer cell line MCF-7) increases the sensitivity of tumor cells to the induction of senescence by radiation, thus exhibiting greater anticancer activity. 
     3-3 Identification of Increase in Sensitivity of Tumor Cells to Radiation-Induced Senescence by siPAPSS2 Using Senescence-Associated Beta-Galactosidase Activity Staining 
     The control and experimental groups were treated with non-specific small interfering RNA (siControl) and specific small interfering RNA (siPAPSS2) against PAPSS2, respectively, cultured in an incubator with 5% CO 2  at 37° C. for 24 hours, and exposed to 2 Gy or 4 Gy gamma-rays from  137 Cs gamma-ray source (Atomic Energy of Canada Ltd., Mississauga, Ontario, Canada), and then cultured in an incubator for 3 days, followed by senescence-associated beta-galactosidase activity staining. The non-specific small interfering RNA (siControl) and the specific small interfering RNA (siPAPSS2) against PAPSS2 were prepared in the same manner as in Example 2-1 and used. This staining was performed according to the Dimri method (Dimri et al., Proc. Natl. Acad. Sci. USA, 92:9363-9367, 1995) as follows. 
     Cells were washed twice with PBS and fixed at room temperature for 3 to 5 minutes in 3% formaldehyde. The fixed cells were washed once again with PBS and incubated with 5 mL of a β-galactosidase staining solution (1 mg/ml X-Gal, 40 mM citric acid/sodium phosphate (pH 6.0), 5 mM potassium ferrocyanide, 5 mM potassium ferricyanide, 150 mM sodium chloride, 2 mM magnesium chloride) in an incubator with 5% CO 2  at 37° C. for 12 to 16 hours while the culture dishes were wrapped with foil so as to keep a dark condition. 
     The beta-galactosidase activity level was measured under a phase-contrast microscope (ECLIPSE TE300, Nikon), and the results are shown in  FIG. 12 . Moreover, the number of stained cells was measured using a microscope (ECLIPSE TE300, Nikon) after irradiation, and a graph representing the percentage of cells positive for senescence-associated beta-galactosidase is shown in  FIG. 13 . 
     As shown in  FIG. 12 , upon exposure to the same dose of radiation, the percentage of cells that became large in size and flat, which are characteristics of senescent cells, increased in the group treated with the PAPSS2 gene inhibitor (siRNA) compared to the control group. Moreover, as shown in  FIG. 13 , upon exposure to the same dose of radiation, the percentage of cells positive for senescence-associated beta-galactosidase increased in the group treated with the PAPSS2 gene inhibitor (siRNA) compared to the control group. Therefore, it can be seen that the sensitivity of tumor cells (e.g., the human breast cancer cell line MCF-7) to the induction of senescence by radiation increases in the group treated with the PAPSS2 gene inhibitor (siRNA) compared to the control group. That is, it can be seen from these results that the inhibition of the expression of PAPSS2 gene in tumor cells (e.g., the human breast cancer cell line MCF-7) increases the sensitivity of tumor cells to the induction of senescence by radiation, thus exhibiting greater anticancer activity. 
     As described above, the composition for inducing senescence in tumor cells according to the present invention can induce senescence in tumor cells by inhibiting PAPSS2 gene expression in tumor cells and can be used to enhance the sensitivity of tumor cells to radiation when used in combination with the treatment of tumor cells with radiation.