De Novo Lipogenesis and Stearoyl-CoA Desaturase Are Coordinately Regulated in the Human Adipocyte and Protect against Palmitate-induced Cell Injury*

De novo lipogenesis (DNL) is paradoxically up-regulated by its end product, saturated fatty acids (SAFAs). We tested the hypothesis that SAFA-induced up-regulation of DNL reflects coordinate up-regulation of elongation and desaturation pathways for disposal of SAFAs and production of monounsaturated fatty acids to protect cells from SAFA toxicity. Human preadipocytes were differentiated in vitro for 14 days with [U-13C]palmitate (0–200 μm) to distinguish exogenous fatty acids from those synthesized by DNL. Exogenous palmitate up-regulated DNL (p < 0.001) concomitantly with SCD and elongation (each p < 0.001). Adipocytes from some donors were intolerant to high palmitate concentrations (400 μm). Palmitate-intolerant cells showed lower TG accumulation. They had lower expression of SCD mRNA and less monounsaturated fatty acids in TG, emphasizing the importance of desaturation for dealing with exogenous SAFAs. There was greater [U-13C]palmitate incorporation in phospholipids. SCD knockdown with small interfering RNA caused down-regulation of DNL and of expression of DNL-related genes, with reduced membrane fluidity (p < 0.02) and insulin sensitivity (p < 0.01), compared with scrambled small interfering RNA controls. There was preferential channeling of DNL-derived versus exogenous palmitate into elongation and of DNL-derived versus exogenous stearate into desaturation. DNL may not act primarily to increase fat stores but may serve as a key regulator, in tandem with elongation and desaturation, to maintain cell membrane fluidity and insulin sensitivity within the human adipocyte

Ncb5or Deficiency Increases Fatty Acid Catabolism and Oxidative Stress*

The endoplasmic reticulum-associated NADH cytochrome b5 oxidoreductase (Ncb5or) is widely distributed in animal tissues. Ncb5or−/− mice develop diabetes at age 7 weeks and have increased susceptibility to the diabetogenic oxidant 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 and 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). Increased fatty acid catabolism and oxidative stress are evident in Ncb5or−/− hepatocytes and reflect increased mitochondrial content, peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) expression, fatty acid oxidation rates, oxidative stress response gene expression, and oxidized glutathione content. Ncb5or−/− hepatocytes readily incorporate exogenous fatty acids into TAG but accumulate more free fatty acids (FFA) and have greater palmitate-induced oxidative stress responses and cell death than WT, all of which are alleviated by co-incubation with oleate via TAG channeling. 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 lower in Ncb5or−/− than in WT mice, although Ncb5or−/− liver has a greater increase in Scd1 mRNA and protein levels. Together, these findings suggest that increased FFA accumulation and catabolism and oxidative stress are major consequences of Ncb5or deficiency in liver