Abstract 1501 The reduction of adiposity and hepatic lipid accumulation by bilirubin is dependent on its hormonal function

The global prevalence of obesity and its related comorbidities, including diabetes and metabolic dysfunction associated fatty liver disease (MAFLD), are on the rise. For unknown reasons, serum bilirubin levels are negatively correlated with obesity, diabetes, and MAFLD. We have previously generated pegylated bilirubin nanoparticles (PEGBR) and found they have therapeutic potential in preclinical models of diet-induced obesity and MAFLD. Bilirubin has long been known to function as an antioxidant. However, our lab has previously identified its hormonal function by binding to and activating the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARa). It is unknown whether the beneficial effects of PEGBR are due to its antioxidant or hormonal function. To determine this, we induced obesity and MAFLD in female and male hepatocyte-specific PPARa knockout mice (PparaHepKO) and floxed (Pparaflox) control mice by a high-fat diet for 30 weeks. The mice were then treated with vehicle or PEGBR (30 mg/kg) for six weeks while remaining on the high-fat diet. In female Pparaflox mice, PEGBR significantly lowered body weight, but not in the female PparaHepKOmice. A similar trend was seen in the male Pparaflox mice but was not statistically significant. We found that PEGBR significantly reduced hepatic fat mass and triglycerides in the Pparaflox mice but not in the PparaHepKO mice in both sexes. We then performed mass-spectroscopy to analyze the hepatic lipid composition. In the male mice there was an 93.6% reduction in the number of lipid species that were significantly lowered by PEGBR in the PparaHepKO mice compared to the Pparaflox mice. While in the female PparaHepKO mice, there was a 2.7% reduction in the number of lipid species significantly decreased by PEGBR compared to Pparaflox controls. Plasma lipids and metabolites were quantified using NMR and we found that male Pparaflox mice had an increase in plasma high-density lipoprotein cholesterol and a decrease in very-low-density particle number with PEGBR, but this was not seen in the male PparaHepKO or female mice. PEGBR also significantly reduced fasting blood glucose and insulin levels only in the Pparaflox mice of both female and male mice. Overall, the data indicates bilirubin's hormonal function through PPARa is responsible for the protective effects of bilirubin on obesity, MAFLD, and glucose tolerance. This study also confirms the therapeutic potential of bilirubin nanoparticles for obesity and its associated comorbidities. This work was supported by the National Institutes of Health F31HL170972 (Z.A.K.) and R01DK121797 (T.D.H.J.) and R01DA058933 (T.D.H.J.).