Arachidonic Acid Inhibits the Insulin Induction of Glucose-6-phosphate Dehydrogenase via p38 MAP Kinase

Abstract

Insulin is the central hormone required for the activation of lipogenic genes in the liver. Feeding animals a high carbohydrate diet enhances the expression of the lipogenic genes. This effect involves the stimulatory actions of both dietary glucose and insulin (1, 2). In contrast, dietary polyunsaturated fats attenuate the stimulatory effect of feeding a high carbohydrate diet (3, 4). We have used glucose-6-phosphate dehydrogenase (G6PD), 2 a member of the lipogenic gene family, as a model system for studying the mechanism of action of fatty acids. The advantage of this model is that insulin is the primary inducer of G6PD expression, and fatty acids such as arachidonic acid are the primary inhibitors of G6PD expression; this regulation is independent of other hormonal requirements (5, 6). The intracellular mechanisms by which polyunsaturated fats inhibit G6PD or other lipogenic genes are not completely understood. Inhibition by polyunsaturated fatty acid may represent a direct action of fatty acids on factors involved in gene expression. Alternatively, fatty acids may act indirectly via the inhibition of stimulatory signal transduction pathways of glucose or insulin. We hypothesized that fatty acids inhibit G6PD expression by inhibition of the insulin induction.Insulin transduces its signal upon binding to the insulin receptor. Transduction of this signal in liver involves phosphorylation of two intracellular substrates, insulin receptor substrate (IRS)-1 and IRS-2 (7). These proteins play complementary roles in insulin signaling (8). Activation of phosphoinositide (PI) 3-kinase is associated with the stimulatory effects of insulin on metabolic pathways, including lipogenesis (9 -11).The IRS proteins can be phosphorylated on both tyrosines and serines. A known mechanism for the inhibition of IRS-1 activation is by phosphorylation at serines 307, 612, and 632 (12). These serine residues, when phosphorylated might interfere with the interaction between IRS-1 and the insulin receptor or PI 3-kinase (13,14). Among the factors known to cause serine phosphorylation of IRS-1 are the mitogen-activated protein (MAP) kinases. Activation of the MAP kinases extracellular regulated kinase (ERK) (15, 16), c-Jun NH 2 -terminal kinase (JNK) (17-19), or p38 MAP kinase (MAPK) (17,20) is associated with the development of insulin resistance in muscle and adipose tissue.Known activators of MAP kinases include tumor necrosis factor ␣ (TNF␣) and very high fat diets. TNF␣, a potent mediator of insulin resistance, activates all three of the MAP kinases. Phosphorylation and activation of p38 MAPK by TNF␣ correlates with IRS-1 serine phosphorylation and a decrease in PI 3-kinase activity (17,20). In muscle and adipose tissue, this results in the decrease in glucose uptake associated with insulin resistance. Likewise, diets containing 40% or more of the energy content as fat also decrease PI 3-kinase activation and result in an insulin-resistant phenotype in intact animals (21-23). This may involve activation of MAP kinases (18,19). In c...

Serine Arginine Splicing Factor 3 Is Involved in Enhanced Splicing of Glucose-6-phosphate Dehydrogenase RNA in Response to Nutrients and Hormones in Liver

“…Previous evidence from our lab demonstrates that the increase in splicing of G6PD that we observe with insulin treatment is due to an increase in its signaling through the PI3K pathway, whereas arachidonic acid treatment attenuates insulin signaling by inhibiting PI3K activity (47). Arachidonic acid decreases the amount of phosphorylated SR proteins and causes a coincident decrease in the accumulation of G6PD mRNA (Fig.…”

Chenodeoxycholic acid suppresses the activation of acetyl-coenzyme A carboxylase-α gene transcription by the liver X receptor agonist T0-901317

“…In support of a role of p38 MAPK in mediating the effect of CDCA on hepatic lipogenic gene expression, Xiong et al (67) recently showed that activation of p38 MAPK with a constitutively active form of MKK6 inhibits the expression of FAS and SREBP-1c in rat hepatocytes. Interestingly, glucagon and polyunsaturated fatty acids also inhibit the hepatic expression of glucose-6-phosphate dehydrogenase, FAS, and SREBP-1c via a mechanism that is dependent on the presence of p38 MAPK and/or ERK (67)(68)(69). Thus, p38 MAPK and ERK appear to have a general role in inhibitory pathways controlling lipogenic gene expression.…”

An Exonic Splicing Silencer Is Involved in the Regulated Splicing of Glucose 6-Phosphate Dehydrogenase mRNA

“…Peptides were accepted if they had a cross-correlation (X corr ) score of at least 1.9, 2.2, or 3.7 for the ϩ1, ϩ2, or ϩ3 charge states, respectively. Western analysis was as previously described (40). The hnRNP antibodies (Immuquest) and SR antibodies (Zymed Laboratories Inc.) were purchased from the indicated sources.…”