A Novel Role for SerpinB1, a Protease Inhibitor, in Hepatic Biology

SerpinB1, a protease inhibitor that is secreted from the liver in response to insulin resistance, was recently reported to promote pancreatic β cell proliferation. We focused on investigating the significance of SerpinB1 in the liver to gain insights into its role in regulating whole body metabolism. We determined SerpinB1 gene expression in multiple metabolic tissues harvested under fasting or fed states from ∼34-week-old male Liver-specific insulin receptor knockout (LIRKO) mice and littermate controls. While in controls SerpinB1 expression was reduced by ∼50% after feeding when compared to the fasting state, (P=0.02, n=4-5), the LIRKOs, in contrast, exhibited a 2.3-fold higher (P=0.01, n=5-6) expression in the fed vs. fasting states and the higher levels were consistent with earlier observations in the LIRKO livers. We next took advantage of the availability of liver samples from 18 to 24-week-old male SerpinB1 KO mice and littermate controls and subjected them to RNA-sequencing. As expected, SerpinB1 gene expression was absent in the KO samples, and interestingly, was associated with a significant decrease in gene expression related to cell cycle, NF-kappa B signaling, and inflammatory pathways. Independent validation of these data by RT-qPCR confirmed a decrease in cell cycle-related genes such as Ccna2 (-60%, P=0.04), Ccnb2 (-54%, P=0.04), Ccnd1 (-46%, P=0.03), and Ccne1 (-51%, P=0.07), and the cell proliferation marker, Mki67 (-60%, P=0.00) (n=5). Furthermore, knockdown of SerpinB1 in the human hepatocyte cell line, HepG2, exhibited significantly decreased expression of Ccnb1 (-36%, P=0.01), Ccnb2 (-44%, P=0.00), Ccne1 (-21%, P=0.00), MKi67 (-30%, P=0.00), and MCM2 (-29%, P=0.00) genes, and increased expression of P21 (+165%, P=0.00) (n=3 for control vs. knockdown groups). These novel data implicate endogenous SerpinB1 in cell proliferation in hepatocytes and warrant further investigation of its role in hepatocyte function and the overall impact on glucose homeostasis.