Abstract P158: Hepatic Trib1 Induces Proteosome-dependent Degradation Of C/ebpα In A Cop1- And Stk40- Dependent Manner

The gene tribbles pseudokinase 1 ( TRIB1 ) has been repeatedly linked to multiple human cardiometabolic traits through genome-wide association studies, including coronary artery disease, plasma cholesterol and triglycerides, and circulating liver transaminases, signaling that TRIB1 is a key regulator of liver metabolism and health. Studies in liver specific Trib1 KO mice have shown that hepatic TRIB1 regulates de novo lipogenesis and steatosis through the regulation of protein levels of the transcription factor C/EBPα, yet the mechanism governing this relationship in hepatocytes has not been investigated. We demonstrate here that human TRIB1 promotes the degradation of C/EBPα in both a COP1- and proteasome-dependent manner in human hepatoma cells. We also observe rapid degradation of TRIB1 protein in hepatoma cells, and find that this is also COP1- and proteasome-dependent. To identify hepatocyte-specific interacting partners of TRIB1 that regulate these processes, we performed tandem-affinity purification of TRIB1 in Huh7 cells and subsequent mass-spec analysis, and identified multiple novel TRIB1 binding partners including the pseudokinases serine/threonine kinase 38 (STK38) and 40 (STK40). We confirmed these interactions in vitro and found that STK40, but not STK38, is required for TRIB1-mediated proteasomal degradation of C/EBPα. Together, our results reveal that TRIB1 induces proteasomal degradation of CEBP/α in a COP1- and proteasome-dependent manner in human hepatocytes. Further, we identify the pseudokinase STK40 as a novel regulator of hepatic TRIB1 function, and ongoing work aims to investigate the role of STK40 in hepatic lipid metabolism. Finally, we show that TRIB1 is itself degraded by the proteosome in a COP1-dependent manner. As increased hepatic Trib1 confers a beneficial metabolic profile in mice, these findings could provide a novel avenue for therapeutic targeting of TRIB1 in the treatment of cardiometabolic disease. Overall, our findings add greater detail to the molecular mechanisms governing the regulation of metabolism by hepatic TRIB1, a gene which human genetics highlights as a crucial regulator of cardiometabolic traits in humans.