Direct Hepatocyte Insulin Signaling Is Required for Lipogenesis but Is Dispensable for the Suppression of Glucose Production

Abstract

Summary Insulin-resistant syndromes such as type II diabetes mellitus (T2DM) involve disrupted temporal coordination of hepatic metabolism such that synthesis and secretion of lipid and glucose are inappropriately engaged concurrently. Here we test the hypothesis that a combination of direct and indirect actions of insulin on liver can lead to the metabolic phenotype exhibited in T2DM without a defect in proximal hepatic insulin signaling. First, we show that the insulin-dependent inhibition of Foxo1 and activation of mTorc1 by Akt is both necessary and sufficient for the induction of lipogenesis and the lipogenic gene program. In marked contrast, insulin, acting in vivo independent of hepatocyte insulin signaling can suppress glucose production by reducing serum free fatty acids. These studies support the hypothesis that under conditions of obesity and diabetes, intact hepatic insulin signaling can maintain lipogenesis while excess circulating FFAs become a dominant positive regulator of HGP.

Role for hepatic CEACAM1 in regulating fatty acid metabolism along the adipocyte-hepatocyte axis

“…Moreover, chronic hyperinsulinemia can cause insulin resistance to the suppression of plasma FFA levels and increasing de novo lipogenesis but not with regard to hepatic gluconeogenesis (56). Hepatic insulin resistance and increased gluconeogenesis appear to be regulated by increased mobilization of FFA and adipokines from WAT (57,58). Consistently, inhibiting lipolysis and inducing fatty acid oxidation restored insulin action in L-SACC1 transgenics (59), pointing to the role of altered fat metabolism in their sustained insulin resistance that was secondary normal lipid metabolism.…”

mTORC1 stimulates phosphatidylcholine synthesis to promote triglyceride secretion

“…Protein lysates were prepared from frozen livers or hepatocytes in a modified RIPA buffer as described previously (14). The following antibodies were used for immunoblotting: p-Akt (catalog 4060), Akt (catalog 2964), p-S6 (catalog 2215), S6 (catalog 2217), Tsc1 (catalog 6935), Raptor (catalog 2280), and CCT (catalog 6931).…”

“…mTORC1 signaling is required, but not sufficient, for postprandial SREBP1c activation and de novo lipogenesis (14)(15)(16). The inactivation of mTORC1 upon fasting is critical to the induction of ketogenesis (17).…”

“…Under normal conditions, the rate-limiting enzyme for PC synthesis is phosphocholine cytidylyltransferase α (CCTα) ( Figure 5A); inactivation of this protein genetically in mice or humans is associated with reduced hepatic triglyceride secretion and hepatosteatosis (26,27). To assess the flux through CCTα, we incubated hepatocytes from ad libitum-fed mice in the presence of 14 C-choline chloride and measured the amount of radioactivity in the product of this enzyme, CDP-choline. Incorporation of 14 C-choline into CDP-choline was reduced significantly in the absence of raptor ( Figure 5B).…”

“…To assess the flux through CCTα, we incubated hepatocytes from ad libitum-fed mice in the presence of 14 C-choline chloride and measured the amount of radioactivity in the product of this enzyme, CDP-choline. Incorporation of 14 C-choline into CDP-choline was reduced significantly in the absence of raptor ( Figure 5B). Because mTORC1 is known to affect pyrimidine synthesis (28) and potentially the levels of cytidinetriphosphate (CTP), which is a substrate of CCTα, we measured the activity of CCTα in cell lysates supplemented with exogenous p-choline and CTP.…”