The role of Ncb5or in fatty acid metabolism and redox homeostasis

Abstract

The endoplasmic reticulum (ER)-associated NADH cytochrome b5 oxidoreductase (Ncb5or) is widely distributed in animal tissues. It contains two redox domains that are homologous to microsomal cytochrome b5 (Cyb5A) at the N terminus and its cognate reductase (Cyb5R3) at the C terminus. Ncb5or null mice develop diabetes at age 7 weeks and have increased susceptibility to the diabetogenic agent streptozotocin. Ncb5or deficiency also results in lipoatrophy and increased hepatocyte sensitivity to cytotoxic effects of saturated fatty acids. Here we investigate the mechanisms of these phenomena in prediabetic Ncb5or-/- mice. We find that, despite increased rates of fatty acid uptake and synthesis and higher stearoyl-CoA desaturase (SCD) expression, Ncb5or -/- liver accumulates less triacylglycerol (TAG) than wild-type (WT). Ncb5or-/- hepatocytes readily incorporate exogenous fatty acids into TAG but accumulate more free fatty acids (FFA). A high-fat diet rich in palmitate and oleate stimulates both lipogenesis and fatty acid catabolism in Ncb5or -/- liver, resulting in TAG levels similar to WT but increased intracellular FFA accumulation. Hepatic SCD specific activity is 2-fold lower in Ncb5or-/- than in WT mice when up to 8 fold increases in Scd1 mRNA and protein levels are considered. Accumulation of saturated fatty acids in Ncb5or-/- hepatocytes increases mitochondrial content, peroxisome proliferator-activated receptor-ã coactivator 1á (PGC-1á) expression, fatty acid oxidation rates, oxidative stress response gene expression, oxidized glutathione content and cell death. These phenotypes could be alleviated by co-incubation with oleic acid via TAG channeling. Taken together, these findings suggest that increased FFA accumulation and catabolism and oxidative stress are major consequences of Ncb5or deficiency in liver. Our study has provided new insight into fatty acid metabolism and revealed a novel role of Ncb5or in preventing saturated fatty acid induced oxidative stress.