The mechanisms of the impairment in hepatic glucose metabolism induced by free fatty acids (FFAs) and the importance of FFA oxidation in these mechanisms remain unclear. FFA-induced peripheral insulin resistance has been linked to membrane translocation of novel protein kinase C (PKC) isoforms, but the role of PKC in hepatic insulin resistance has not been assessed. To investigate the biochemical pathways that are induced by FFA in the liver and their relation to glucose metabolism in vivo, we determined endogenous glucose production (EGP), the hepatic content of citrate (product of acetyl-CoA derived from FFA oxidation and oxaloacetate), and hepatic PKC isoform translocation after 2 and 7 h Intralipid + heparin (IH) or SAL in rats. Experiments were performed in the basal state and during hyperinsulinemic clamps (insulin infusion rate, 5 mU. kg(-1). min(-1)). IH increased EGP in the basal state (P < 0.001) and during hyperinsulinemia (P < 0.001) at 2 and 7 h. Also, 7-h infusion of IH induced resistance to the suppressive effect of insulin on EGP (P < 0.05). Glycerol infusion (resulting in plasma glycerol levels similar to IH infusion) did not have any effect on EGP. IH increased hepatic citrate content by twofold, independent of the insulin levels and the duration of IH infusion. IH induced hepatic PKC-delta translocation from the cytosolic to membrane fraction in all groups. PKC-delta translocation was greater at 7 compared with 2 h (P < 0.05). In conclusion, 1) increased FFA oxidation may contribute to the FFA-induced increase in EGP in the basal state and during hyperinsulinemia but is not associated with FFA-induced hepatic insulin resistance, and 2) the progressive insulin resistance induced by FFA in the liver is associated with a progressive increase in hepatic PKC-delta translocation.

-Exposure to high concentrations of glucose and insulin results in insulin resistance of metabolic target tissues, a characteristic feature of type 2 diabetes. High glucose has also been associated with oxidative stress, and increased levels of reactive oxygen species have been proposed to cause insulin resistance. To determine whether oxidative stress contributes to insulin resistance induced by hyperglycemia in vivo, nondiabetic rats were infused with glucose for 6 h to maintain a circulating glucose concentration of 15 mM with and without coinfusion of the antioxidant N-acetylcysteine (NAC), followed by a 2-h hyperinsulinemic-euglycemic clamp. High glucose (HG) induced a significant decrease in insulin-stimulated glucose uptake [tracer-determined disappearance rate (R d), control 41.2 Ϯ 1.7 vs. HG 32.4 Ϯ 1.9 mg ⅐ kg Ϫ1 ⅐ min Ϫ1 , P Ͻ 0.05], which was prevented by NAC (HG ϩ NAC 45.9 Ϯ 3.5 mg ⅐ kg Ϫ1 ⅐ min Ϫ1 ). Similar results were obtained with the antioxidant taurine. Neither NAC nor taurine alone altered Rd. HG caused a significant (5-fold) increase in soleus muscle protein carbonyl content, a marker of oxidative stress that was blocked by NAC, as well as elevated levels of malondialdehyde and 4-hydroxynonenal, markers of lipid peroxidation, which were reduced by taurine. In contrast to findings after long-term hyperglycemia, there was no membrane translocation of novel isoforms of protein kinase C in skeletal muscle after 6 h. These data support the concept that oxidative stress contributes to the pathogenesis of hyperglycemia-induced insulin resistance. euglycemic clamp; insulin resistance; protein carbonyls; protein kinase C; antioxidants INSULIN RESISTANCE is one of the earliest detectable predictors of type 2 diabetes (47, 51, 39) and, along with relative insulin deficiency (39, 56), strongly contributes to the development of overt hyperglycemia. Hyperglycemia is in large part responsible for a host of complications found in diabetic subjects (15,83,86) and can worsen insulin resistance (11,38,72,73,75). The effect of hyperglycemia per se to induce insulin resistance in vivo was first demonstrated by Rossetti et al. (75) in the partially pancreatectomized rat model, which is characterized by moderate fasting hyperglycemia, glucose intolerance, and normal fasting insulin levels. In that study, phlorizin was used to normalize plasma glucose without affecting insulin secretion. Use of a hyperinsulinemic-euglycemic clamp revealed that the decreased insulin-stimulated glucose utilization was completely normalized in the phlorizin-treated rats, indicating a direct role of glucose in the induction of insulin resistance. Several mechanisms have been proposed to mediate hyperglycemia-induced insulin resistance, including the hexosamine biosynthetic pathway (4,31,67,74,87) and protein kinase C (55,68,78

Fat-induced hepatic insulin resistance plays a key role in the pathogenesis of type 2 diabetes in obese individuals. Although PKC and inflammatory pathways have been implicated in fat-induced hepatic insulin resistance, the sequence of events leading to impaired insulin signaling is unknown. We used Wistar rats to investigate whether PKCδ and oxidative stress play causal roles in this process and whether this occurs via IKKβ- and JNK-dependent pathways. Rats received a 7-h infusion of Intralipid plus heparin (IH) to elevate circulating free fatty acids (FFA). During the last 2 h of the infusion, a hyperinsulinemic-euglycemic clamp with tracer was performed to assess hepatic and peripheral insulin sensitivity. An antioxidant, N-acetyl-L-cysteine (NAC), prevented IH-induced hepatic insulin resistance in parallel with prevention of decreased IκBα content, increased JNK phosphorylation (markers of IKKβ and JNK activation, respectively), increased serine phosphorylation of IRS-1 and IRS-2, and impaired insulin signaling in the liver without affecting IH-induced hepatic PKCδ activation. Furthermore, an antisense oligonucleotide against PKCδ prevented IH-induced phosphorylation of p47(phox) (marker of NADPH oxidase activation) and hepatic insulin resistance. Apocynin, an NADPH oxidase inhibitor, prevented IH-induced hepatic and peripheral insulin resistance similarly to NAC. These results demonstrate that PKCδ, NADPH oxidase, and oxidative stress play a causal role in FFA-induced hepatic insulin resistance in vivo and suggest that the pathway of FFA-induced hepatic insulin resistance is FFA → PKCδ → NADPH oxidase and oxidative stress → IKKβ/JNK → impaired hepatic insulin signaling.

. Effects of portal free fatty acid elevation on insulin clearance and hepatic glucose flux. Am J Physiol Endocrinol Metab 290: E1089 -E1097, 2006. First published January 3, 2006 doi:10.1152/ajpendo.00306.2005.-We tested the hypothesis that, due to greater hepatic free fatty acid (FFA) load, portal delivery of FFAs, as in visceral obesity, induces hyperinsulinemia and increases endogenous glucose production to a greater extent than peripheral FFA delivery. For 5 h, 10 eq ⅐ kg Ϫ1 ⅐ min Ϫ1 portal oleate (n ϭ 6), equidose peripheral oleate (n ϭ 5), or saline (n ϭ 6) were given intravenously to conscious dogs infused with a combination of portal and peripheral insulin to enable calculation of hepatic insulin clearance during a pancreatic euglycemic clamp. Peripheral FFAs were similar with both oleate treatments and were threefold greater than in controls. Portal FFAs were 1.5-to 2-fold greater with portal than with peripheral oleate. Peripheral insulin concentrations were greatest with portal oleate, intermediate with peripheral oleate (P Ͻ 0.001 vs. portal oleate or controls), and lowest in controls, consistent with corresponding reductions in plasma insulin clearance and hepatic insulin clearance. Although endogenous glucose production did not differ between the two routes of oleate delivery, total glucose output (endogenous glucose production plus glucose cycling) was greater with portal than with peripheral oleate (P Ͻ 0.001) despite the higher insulin levels. In conclusion, during euglycemic clamps in dogs, the main effect of short-term elevation in portal FFA is to generate peripheral hyperinsulinemia. This may, in the long term, contribute to the metabolic and cardiovascular risk of visceral obesity. insulin resistance; hepatic glucose production; visceral obesity NUMEROUS STUDIES HAVE SHOWN an association between obesity and cardiovascular disease (39), and recent studies have also shown associations between obesity and some types of cancer (18). A crucial factor that has been implicated in these associations (7, 11) is insulin resistance and the concomitant hyperinsulinemia (20). In obesity, peripheral hyperinsulinemia is a consequence of both insulin hypersecretion (48), usually secondary to insulin resistance, and decreased insulin clearance (47, 48). One factor that can account for impaired insulin clearance in obesity, particularly abdominal obesity (34), is the elevated plasma level of free fatty acids (FFAs) (34). In Balent et. al. (2), decreased insulin clearance was completely responsible for the hyperinsulinemia induced by FFA elevation.There is an undisputed relationship between "central" fat distribution (i.e., fat in the visceral and subcutaneous abdominal region) and features of the metabolic syndrome (20), although the causal nature of this relationship (15), as well as the role of FFAs vs. adipokines (25) and of visceral vs. subcutaneous abdominal fat (35, 41) in the pathogenesis of insulin resistance, remains a matter of debate. There are important metabolic differences between fat stores....

Insulin inhibits glucose production by a direct effect in diabetic depancreatized dogs during euglycemia. Am J Physiol Endocrinol Metab 283: E1002-E1007, 2002; 10.1152/ ajpendo.00091.2002.-In our previous studies in nondiabetic dogs and humans, insulin suppressed glucose production (GP) by both an indirect extrahepatic and a direct hepatic effect. However, insulin had no direct effect on GP in diabetic depancreatized dogs under conditions of moderate hyperglycemia. The present study was designed to investigate whether insulin can inhibit GP by a direct effect in this model under conditions of euglycemia. Depancreatized dogs were made euglycemic (ϳ6 mmol/l), rather than moderately hyperglycemic (ϳ10 mmol/l) as in our previous studies, by basal portal insulin infusion. After ϳ100 min of euglycemia, a hyperinsulinemic euglycemic clamp was performed by giving an additional infusion of insulin either portally (POR) or peripherally at about one-half the rate (½ PER) to match the peripheral venous insulin concentrations. The greater hepatic insulin load in POR resulted in greater suppression of GP (from 16.5 Ϯ 1.8 to 12.2 Ϯ 1.6 mol ⅐ kg Ϫ1 ⅐ min Ϫ1 ) than ½ PER (from 17.8 Ϯ 1.9 to 15.6 Ϯ 2.0 mol ⅐ kg Ϫ1 ⅐ min Ϫ1 , P Ͻ 0.001 vs. POR), consistent with insulin having a direct hepatic effect in suppressing GP. We conclude that the direct effect of insulin to inhibit GP is present in diabetic depancreatized dogs under conditions of acutely induced euglycemia. These results suggest that, in diabetes, the prevailing glycemic level is a determinant of the balance between insulin's direct and indirect effects on GP.peripheral and hepatic effects of insulin; direct and indirect effects of insulin; hyperglycemia; free fatty acids INSULIN HAS A STRONG INHIBITORY EFFECT on glucose production (GP). This inhibition is in part direct, i.e., due to hepatic sinusoidal insulin's interaction with the hepatocyte insulin receptor (18,19,28,29,31,32), and in part indirect, due to peripheral insulin's actions on extrahepatic tissues (1,18,19,23,28,29,31,32). These actions consist mainly of the antilipolytic effect of insulin in the adipose tissue (16,17,24,25,30). In nondiabetic animals and humans, the importance of either the direct or the indirect regulation of GP by insulin has been differently emphasized (2, 4). Undoubtedly, however, under normal physiological conditions, the impact on GP of even a small direct effect of insulin is magnified by the greater hepatic than peripheral insulinization.Individuals with type 1 diabetes are treated with subcutaneous injections of insulin, which result in peripheral absorption of insulin and thus a level of hepatic insulinization that is not greater than peripheral insulinization. To the extent that the direct effect of insulin plays a role in the suppression of GP, peripheral hyperinsulinemia should be required to elevate the hepatic sinusoidal levels to adequately suppress GP. Because hyperinsulinemia has been associated with atherosclerosis (27) and recently also with some types of cancer (11), ...