Regulation of palmitoyl-CoA chain elongation by clofibric acid in the liver of Zucker fa/fa rats

Modulation of palmitate-induced endoplasmic reticulum stress and apoptosis in pancreatic β-cells by stearoyl-CoA desaturase and Elovl6

Induction of endoplasmic reticulum (ER) stress and apoptosis by elevated exogenous saturated fatty acids (FAs) plays a role in the pathogenesis of β-cell dysfunction and loss of islet mass in type 2 diabetes. Regulation of monounsaturated FA (MUFA) synthesis through FA desaturases and elongases may alter the susceptibility of β-cells to saturated FA-induced ER stress and apoptosis. Herein, stearoyl-CoA desaturase (SCD)1 and SCD2 mRNA expression were shown to be induced in islets from prediabetic hyperinsulinemic Zucker diabetic fatty (ZDF) rats, whereas SCD1, SCD2, and fatty acid elongase 6 (Elovl6) mRNA levels were markedly reduced in diabetic ZDF rat islets. Knockdown of SCD in INS-1 β-cells decreased desaturation of palmitate to MUFA, lowered FA partitioning into complex neutral lipids, and increased palmitate-induced ER stress and apoptosis. Overexpression of SCD2 increased desaturation of palmitate to MUFA and attenuated palmitate-induced ER stress and apoptosis. Knockdown of Elovl6 limited palmitate elongation to stearate, increasing palmitoleate production and attenuating palmitate-induced ER stress and apoptosis, whereas overexpression of Elovl6 increased palmitate elongation to stearate and palmitate-induced ER stress and apoptosis. Overall, these data support the hypothesis that enhanced MUFA synthesis via upregulation of SCD2 activity can protect β-cells from elevated saturated FAs, as occurs in prediabetic states. Overt type 2 diabetes is associated with diminished islet expression of SCD and Elovl6, and this can disrupt desaturation of saturated FAs to MUFAs, rendering β-cells more susceptible to saturated FA-induced ER stress and apoptosis.

Effects of pioglitazone on stearoyl-CoA desaturase in obese Zucker fa/fa rats

The effects of a peroxisome proliferator activated receptor gamma (PPARgamma) agonist on hepatic stearoyl-CoA desaturase (SCD) in insulin-resistant and obese Zucker fa/fa rats were studied. The administration of pioglitazone, a PPARgamma agonist, to Zucker obese rats greatly improved their insulin sensitivity. The treatment of Zucker obese rats with pioglitazone did not affect the index of fatty acid desaturation of either serum or liver. Hepatic SCD activity and the mRNA level of SCD1 were not changed by treatment of the rats with pioglitazone. The activity of palmitoly-CoA chain elongase, which is involved in the biosynthesis of oleic acid in concert with SCD, was not significantly altered when Zucker obese rats received pioglitazone. Although neither the activity nor mRNA expression of acyl-CoA oxidase was changed by treatment of Zucker obese rats with pioglitazone, the mRNA expressions of both sterol regulatory element-binding protein-1c and acetyl-CoA carboxylase sensitively responded to the challenge by pioglitazone. These results suggest that the insulin sensitivity of insulin-resistant and obese Zucker fa/fa rats is improved by pioglitazone independently of SCD activity.

Stearoyl-CoA desaturase activity is elevated by the suppression of its degradation by clofibric acid in the liver of rats

A mechanism by which fibrates control stearoyl-CoA desaturase (SCD) in the liver was studied. Treatment of rats with 2-(4-chlorophenoxy)-2-methylpropionic acid (clofibric acid) or feeding of a fat-free diet markedly elevated hepatic activity of SCD. Both the treatment with clofibric acid and the feeding of the fat-free diet caused an increase in the steady-state level of SCD1 mRNA and enhanced transcriptional rate. The half-lives of SCD for control rats, rats treated with clofibric acid rats, and rats fed the fat-free diet were estimated to be 2.0, 3.9, and 1.9 h, respectively. Activity of palmitoyl-CoA chain elongase (PCE) was increased by both clofibric acid treatment and feeding of the fat-free diet as was observed with SCD. Steady-state level of rat fatty acid elongase 2 mRNA was increased by the treatment with clofibric acid or feeding of fat-free diet, although the transcriptional rate was not altered. Different from SCD, PCE was highly stable and its half-life was not changed by either clofibric acid or fat-free diet. These results strongly suggest that the decreased degradation of SCD is responsible for the increase in its activity in addition to increased transcription of SCD1 in the rats treated with clofibric acid.