Effect of insulin on triacylglycerol synthesis and secretion by chicken hepatocytes in primary culture

Insulin stimulation of hepatic triacylglycerol secretion in the insulin-replete state: implications for the etiology of peripheral insulin resistance

Observations on humans, on rats in vivo, and on isolated perfused rat livers indicate that insulin stimulates hepatic very-low-density lipoprotein (VLDL)-TAG secretion when the liver is chronically exposed to the hormone. They suggest that frequent stimulation of insulin secretion throughout the diurnal cycle may result in a chronic stimulation of VLDL secretion and increased delivery of acyl moieties to the periphery, particularly to muscle, the most important site of insulin-sensitive glucose disposal. If acyl groups are provided in excess of the oxidative needs of the tissue, this may lead to induction of insulin resistance, irrespective of whether obesity is established concomitantly. Dietary factors that stimulate hepatic VLDL secretion may have the same effect and contribute to the induction of a vicious spiral leading to the development of the full-blown Metabolic Syndrome and its pathological consequences, including type-2 diabetes, stroke, and cardiovascular disease.

Involvement of PI 3-kinase and activated ERK in facilitating insulin-stimulated triacylglycerol synthesis in hepatocytes

Triacylglycerol synthesis was studied in hepatocytes isolated from fasted/refed rats by EDTA perfusion. Insulin induced a 1.5-fold increase in glucose incorporation into triacylglycerol. Insulin-stimulated triacylglycerol synthesis and insulin-stimulated protein kinase B/Akt activity were inhibited by the phosphatidylinositol 3-kinase inhibitors wortmannin and LY 294002, and the mitogen-activated protein kinase kinase inhibitor PD 98059. Inhibition of p70 ribosomal protein-S6 kinase with rapamycin was without effect. Insulin-stimulated pyruvate dehydrogenase activity was abolished by phosphatidylinositol 3-kinase inhibitors. No effect of insulin on acetyl CoA carboxylase activity was observed.

Inhibiting delta9-desaturase activity impairs triacylglycerol secretion in cultured chicken hepatocytes

The relationship between endogenous oleic acid produced by hepatic delta9-desaturase and the secretion of VLDL-triglycerides was investigated in a primary culture of chicken hepatocytes. When the fatty acid compositions of the secreted and intracellular triglycerides (TG) (or triacylglycerols) were compared, an imbalance between monoenes and saturated fatty acids was observed, with the secreted TG being significantly more unsaturated than the intracellular TG. The addition of a mixture of cyclopropenic fatty acids (specific inhibitors of fatty acid desaturation) to the culture medium of cells 24 h before measurement of their delta9-desaturase activity and TG secretion rate caused a significant impairment of both desaturase activity and TG secretion, without affecting total TG synthesis. However, the addition of oleic acid to the culture medium of cells treated with cyclopropenic fatty acids restored the TG secretion rate. Palmitic acid did not restore the TG secretion rate and linoleic acid partly restored the TG secretion rate. Finally, even in the presence of oleic acid in the culture medium of secreting cells, those which had been treated with cyclopropenic fatty acids had a significantly lower TG secretion rate than nontreated cells. Taken together, these results show that TG secretion is highly dependent on the delta9-desaturase activity and suggest that oversecretion of VLDL-TG in chickens and subsequent fattening could originate in a high hepatic delta9-desaturation of saturated fatty acids.