1874-P: A Novel Role for SerpinB1 in the Regulation of Insulin Signaling in Hepatocytes

Compensatory expansion of β cell mass has been pursued as a potential therapeutic strategy to cure diabetes. We previously identified SerpinB1 as a hepatocyte-derived circulating factor that contributes to adaptive beta cell expansion in the liver-specific insulin receptor knockout (LIRKO) mouse. To understand how SerpinB1 is regulated and secreted, we began by first investigating the mechanism(s) and signaling pathways that modulate the gene and protein expression of SerpinB1 in hepatocytes. Hepatic SerpinB1 expression was elevated in insulin resistant mouse models such as db/db and ob/ob, and circulating interleukin-6 (IL-6) levels were higher in these mice. Indeed, IL-6 induced SerpinB1 expression in HepG2 cells and human primary hepatocytes via the JAK/STAT3 pathway, indicating an important role for this cytokine in inducing the expression of the protease inhibitor in insulin resistant states. Conversely, silencing of SERPINB1 gene in hepatocytes decreased INSR gene and protein expression with a consequent alteration of expression of several downstream signaling genes related to gluconeogenesis and cell proliferation in hepatocytes. Ribosomal protein S3 (RPS3), identified as a binding partner of SerpinB1 in HepG2 cells, exhibited reduced expression in db/db and ob/ob mice liver along with reduced insulin receptor (IR) expression. Silencing of RPS3 gene in HepG2 cells, phenocopied the effects of SerpinB1 gene silencing, and decreased IR protein expression with consequent alterations in downstream signaling proteins. Additional studies are required to examine the precise regulation of IR expression by SerpinB1 and how the latter is secreted from insulin resistant hepatocytes. These results indicate that SerpinB1 expression is upregulated in insulin resistance by IL-6 and the SerpinB1/RPS3 complex plays a role in regulating insulin signaling in hepatocytes.