Insulin-independent regulation of hepatic triglyceride synthesis by fatty acids

Abstract

A central paradox in type 2 diabetes is the apparent selective nature of hepatic insulin resistance--wherein insulin fails to suppress hepatic glucose production yet continues to stimulate lipogenesis, resulting in hyperglycemia, hyperlipidemia, and hepatic steatosis. Although efforts to explain this have focused on finding a branch point in insulin signaling where hepatic glucose and lipid metabolism diverge, we hypothesized that hepatic triglyceride synthesis could be driven by substrate, independent of changes in hepatic insulin signaling. We tested this hypothesis in rats by infusing [U-(13)C] palmitate to measure rates of fatty acid esterification into hepatic triglyceride while varying plasma fatty acid and insulin concentrations independently. These experiments were performed in normal rats, high fat-fed insulin-resistant rats, and insulin receptor 2'-O-methoxyethyl chimeric antisense oligonucleotide-treated rats. Rates of fatty acid esterification into hepatic triglyceride were found to be dependent on plasma fatty acid infusion rates, independent of changes in plasma insulin concentrations and independent of hepatocellular insulin signaling. Taken together, these results obviate a paradox of selective insulin resistance, because the major source of hepatic lipid synthesis, esterification of preformed fatty acids, is primarily dependent on substrate delivery and largely independent of hepatic insulin action.

Keywords: esterification; hepatic insulin resistance; lipogenesis; mass spectrometry; nonalcoholic fatty liver disease.

Fig. 1.

Infusion studies with insulin-sensitive regular chow-fed rats and insulin-resistant fat-fed rats. (A and B) Regular chow-fed rats. (C and D) Fat-fed rats. (A and C) Hepatic Akt2 phosphorylation immunoblots, comparing basal insulin infused (Basal) vs. high insulin infused (Stim) groups. Bar graphs represents quantitation of Western blot bands, ratio phosphorylated Akt2 to total Akt2, normalized to the basal group. (B and D) Esterification rates. Bar graph symbols—Western blot quantitation: , basal group; , high insulin (Stim) group. **P < 0.01 and ****P < 0.0001 comparing insulin stimulated to basal. Esterification rates: , low insulin/saline infusion; , high insulin/saline infusion; , low insulin/Intralipid infusion; , high insulin/Intralipid infusion. Data reported as mean ± SEM.

Fig. 2.

Infusion studies with IRASO-treated rats. (A) Hepatic insulin receptor immunoblots, comparing control ASO vs. IRASO groups. (B) Hepatic Akt2 phosphorylation in control ASO treated vs. IRASO treated rats. Overnight fasted rats (Basal) compared with rats subject to a 20-min hyperinsulinemic clamp (Clamp). Bar graphs represent the ratio of phosphorylated Akt2 to total Akt2. , basal group; , clamp group. **P < 0.01. (C) Esterification rates, saline infusion vs. Intralipid infusion. , saline infusion; , Intralipid infusion. Data reported as mean ± SEM.

Fig. 3.

De novo hepatic lipogenesis in control vs. IRASO-treated rats. (A) Regular chow-fed vs. high fat-fed animals. (B) Control ASO treatment vs. IRASO treatment. *P < 0.04; ****P < 0.0001. Data reported as mean ± SEM.