Palmitate Induces Insulin Resistance in H4IIEC3 Hepatocytes through Reactive Oxygen Species Produced by Mitochondria

Abstract

Visceral adiposity in obesity causes excessive free fatty acid (FFA) flux into the liver via the portal vein and may cause fatty liver disease and hepatic insulin resistance. However, because animal models of insulin resistance induced by lipid infusion or a high fat diet are complex and may be accompanied by alterations not restricted to the liver, it is difficult to determine the contribution of FFAs to hepatic insulin resistance. Therefore, we treated H4IIEC3 cells, a rat hepatocyte cell line, with a monounsaturated fatty acid (oleate) and a saturated fatty acid (palmitate) to investigate the direct and initial effects of FFAs on hepatocytes. We show that palmitate, but not oleate, inhibited insulin-stimulated tyrosine phosphorylation of insulin receptor substrate 2 and serine phosphorylation of Akt, through c-Jun NH 2 -terminal kinase (JNK) activation. Among the well established stimuli for JNK activation, reactive oxygen species (ROS) played a causal role in palmitate-induced JNK activation. In addition, etomoxir, an inhibitor of carnitine palmitoyltransferase-1, which is the rate-limiting enzyme in mitochondrial fatty acid ␤-oxidation, as well as inhibitors of the mitochondrial respiratory chain complex (thenoyltrifluoroacetone and carbonyl cyanide m-chlorophenylhydrazone) decreased palmitateinduced ROS production. Together, our findings in hepatocytes indicate that palmitate inhibited insulin signal transduction through JNK activation and that accelerated ␤-oxidation of palmitate caused excess electron flux in the mitochondrial respiratory chain, resulting in increased ROS generation. Thus, mitochondria-derived ROS induced by palmitate may be major contributors to JNK activation and cellular insulin resistance.

Vitamin E prevents the age-dependent and palmitate-induced disturbances of sphingolipid turnover in liver cells

“…Infusion of saturated fatty acids to the mice significantly increased ceramide content in the liver and soleus muscles, while myriocin, an inhibitor of SPT activity, injection to the animals blocked ceramide production in tissues (Holland et al 2007). Palmitic acid addition to the culture media of hepatocytes induced significantly the intracellular accumulation of ceramide (Wei et al 2006;Nakamura et al 2009). Palmitate-induced ceramide accumulation in the liver cells and astrocytes was prevented by the inhibitors of SPT (L-cycloserine) or ceramide synthase (fumonisin B1), respectively, but not by the aSMase inhibitor (disipramine) (Wei et al 2006;Blázquez et al 2000).…”

The effects of ezetimibe on non-alcoholic fatty liver disease and glucose metabolism: a randomised controlled trial

“…Ezetimibe therapy also altered the hepatic profile of fatty acid components by significantly increasing hepatic levels of lauric, myristic, palmitic, palmitoleic, margaric, stearic, oleic and linoleic acids. Experimentally, palmitate induces interleukin-8 [32], endoplasmic reticulum stress, and c-Jun amino-terminal kinase activation and promotes apoptosis in the liver [5,33,34]. Lipid-induced oxidative stress and inflammation are closely related to insulin resistance [3,5], which could be relevant to the ezetimibe-induced deterioration of glucose homeostasis.…”

“…Steatosis of the liver is closely associated with insulin resistance. However, the toxic lipids are not intrahepatic triacylglycerols but, rather, it is non-esterified cholesterol [3,4] and some NEFA [5] that contribute to inflammation and insulin resistance in hepatocytes.…”

Ectopic Fat Accumulation in the Liver and Glucose Homeostasis

“…Rather, free cholesterol and saturated free fatty acid palmitate are regarded as toxic lipids that cause hepatic insulin resistance via oxidative stress (17) 2) In an in vitro fatty liver system using H4IIEC3 hepatocytes, a saturated fatty acid palmitate, but not an unsaturated fatty acid oleate, inhibits insulin-stimulated tyrosine phosphorylation of insulin receptor substrate 2 and serine phosphorylation of Akt, through c-Jun NH 2 -terminal kinase (JNK) activation (5). In this model, mitochondrial β oxidation-derived reactive oxygen species (ROS) play a causal role in the palmitate-induced JNK activation (5). Therefore, toxic lipid-induced mitochondrial ROS may also underlie the link between steatosis and insulin resistance in the liver.…”

“…Although obesity is less common (2), diabetes is a huge and growing problem in Asia (3), suggesting that Asian people may be feasible to obesity-associated metabolic dysregulation. Accumulating evidence suggests that ectopic fat accumulation in insulin-target organs leads to development of insulin resistance in each organ by altering oxidative stress (4,5) and gene expression profiles (6,7). Specifically, the liver functions as a center to maintain whole body energy homeostasis by sensing nutrient stimuli and by producing a variety of nutrients and bioactive substances.…”