Novel Mediators of Diabetes-Associated Cardiovascular Disease

Abstract

Individuals with diabetes have a two- to four-fold increased risk of cardiovascular disease (CVD) compared to non-diabetics, and over 65% of diabetics die from CVD. Current treatments do not normalize this increased risk, necessitating the investigation of new drug targets for diabetes-associated CVD. Using the Liver Insulin Receptor Knockout (LIRKO) mouse model, we have characterized two novel mediators of diabetes-associated CVD. By examining microarray data from the LIRKO and other models of insulin resistance, we found that the cholesterol biosynthetic pathway is heavily downregulated in insulin resistant states. This decreased cholesterologenic gene expression can be attributed to an increased level of cholic acid in bile, which promotes cholesterol absorption in the gut. Knockdown of Cyp8b1, the rate-limiting enzyme in cholic acid synthesis, normalizes plasma total and LDL cholesterol in LIRKO mice on a Western diet. These data provide a novel role for bile acid metabolism in the pathophysiology of insulin resistance, and they suggest that lowering cholic acid levels could improve the metabolic health of diabetic patients. Previous work by the Biddinger laboratory identified flavin-containing monooxygenase 3 (Fmo3) as an important regulator of lipid metabolism in insulin resistant states. Work from other groups has shown that an enzymatic product of FMO3, TMAO, is correlated with CVD in mice and humans, but its function in lipid metabolism is only now being studied. We dissected the role of FoxO1 downstream of FMO3 using antisense oligonucleotides. Although many of FMO3’s effects on glucose and lipid metabolism are primarily mediated by FoxO1, we identified a novel, hepatic FoxO1-independent role for FMO3 in modulating adiposity. We also created an Fmo3 knockout mouse using CRISPR/Cas9 to examine the non-hepatic effects of FMO3. Studies using this model also suggest a role for Fmo3 in adipose tissue energy balance. This dissertation improves our understanding of the mechanisms by which insulin regulates lipid metabolism. Our results validate CYP8B1 and FMO3 as molecules of great importance downstream of insulin signaling, and as potential drug targets for diabetes-associated CVD and other metabolic diseases.