## 3 Discussion  
PCs constitute a family of nine members. With their irreversible limited proteolysis function, they play key roles in regulating both physiological and pathophysiological conditions by activating or inactivating a wide spectrum of precursor proteins, including growth factors, hormones, receptors, and adhesion molecules, thereby generating cleaved products with novel functions [1; 13; 29]. Despite all accumulating evidence on PCs, the significance, regulation, and specific functions of PC7, their most conserved and ancient family member, have yet to be fully elucidated. Nonetheless, the ubiquitously expressed PC7 has been implicated in iron metabolism [8], anxiety/mood regulation [12], triglyceride metabolism [30; 31], and breast cancer progression [14]. On the other hand, miRNAs have pivotal roles in controlling such physiological processes as development, differentiation, apoptosis, proliferation, and metabolism, and dysregulated miRNAs are associated with various pathological conditions [23]. In the present study, we unveiled miR-125a-5p, miR-143-3p, and miR-409-3p as natural negative regulators of _PCSK7_ mRNA via bioinformatics and _in cellulo_ functional enzymatic activity of human PC7. We found that miR-125a-5p, miR-143-3p, and miR-409-3p directly targeted _PCSK7_ mRNA and downregulated its expression, whereas miR-320a and miR-224 did not target _PCSK7_. Additionally, miR-125a-5p not only suppressed the expression of PC7 protein but also reduced its protease function on hTfR1. Our study is the first investigation to report regulatory effects of miRNAs on _PCSK7_ expression and function. We note, however, that such regulation of _PCSK7_ must be viewed in the context that each miRNA may target multiple mRNAs and thus regulate complex cellular pathways. Research in the last decade has established that miRNAs, with their highly organ- and cell-specific expression patterns, participate in the modulation of the expression of targeted genes through transcription regulation, mRNA degradation, translation inhibition, and translation activation [20; 32; 33; 34]. Our expression analysis validated the differential expression of miRNAs and _PCSK7_ in different cell lines. PC7 has been proposed to play critical roles in liver triglyceride regulation, including non-alcoholic fatty liver disease (NAFLD) and hepatocellular carcinoma (HCC) [13; 17; 35]. Interestingly, miR-125a-5p, miR-143-3p, and miR-409-3p participate in a variety of diseases, including liver diseases and hepatic cancer [36; 37; 38; 39]. Previous studies have demonstrated that miR-125a-5p binds to the viral transcript encoding the surface antigen of the hepatitis B virus and represses viral replication by interfering with the expression of this antigen [40; 41]. Likewise, miR-125a-5p in the liver is associated with the replication and progression of the hepatitis B virus in patients with chronic hepatitis B [41]. In addition, serum levels of miR-125 could predict the progression of liver diseases [40]. The differential expression of miR-125-a-5p in the progression of high-fat-diet-induced non-alcoholic fatty liver disease-non-alcoholic steatohepatitis-hepatocellular carcinoma (NAFLD-NASH-HCC) was investigated by [PERSON] et al. [42], who reported that miR-125a-5p and miR-182 exhibited early and significant dysregulation in the sequential hepatic damage process [42]. Therefore, the expression pattern of miR-125a-5p is a suitable marker of liver disease. Another miRNA, miR-143-3p, suppresses the proliferation and invasion of HCC cells by regulating the _FGF1_ gene [43]. Further, the differential expression of miR-143 is associated with HCC, downregulated the expression levels of the Toll-like receptor 2 (TLR2), nuclear factor-kappa B (NF-kB), and matrix metallopeptidase 2 and 9 (MMP-2 and MMP-9) [44]. [PERSON] et al.