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Excessive production of triglyceride-rich VLDL is attributable to hypertriglyceridemia. VLDL production is facilitated by microsomal triglyceride transfer protein (MTP) in a rate-limiting step that is regulated by insulin. To characterize the underlying mechanism, we studied hepatic MTP regulation by forkhead box O1 (FoxO1), a transcription factor that plays a key role in hepatic insulin signaling. In HepG2 cells, MTP expression was induced by FoxO1 and inhibited by exposure to insulin. This effect correlated with the ability of FoxO1 to bind and stimulate MTP promoter activity. Deletion or mutation of the FoxO1 target site within the MTP promoter disabled FoxO1 binding and resulted in abolition of insulin-dependent regulation of MTP expression. We generated mice that expressed a constitutively active FoxO1 transgene and found that increased FoxO1 activity was associated with enhanced MTP expression, augmented VLDL production, and elevated plasma triglyceride levels. In contrast, RNAi-mediated silencing of hepatic FoxO1 was associated with reduced MTP and VLDL production in adult mice. Furthermore, we found that hepatic FoxO1 abundance and MTP production were increased in mice with abnormal triglyceride metabolism. These data suggest that FoxO1 mediates insulin regulation of MTP production and that augmented MTP levels may be a causative factor for VLDL overproduction and hypertriglyceridemia in diabetes., Introduction Hypertriglyceridemia is a prominent pathological feature of metabolic syndrome and is characterized by augmented production and/or retarded clearance of triglyceride-rich (TG-rich) particles, such as VLDL and chylomicrons (1), (2). [...]

Nonalcoholic fatty liver disease (NAFLD), hypertriglyceridemia, and elevated free fatty acids are present in the majority of patients with metabolic syndrome and type 2 diabetes mellitus and are strongly associated with hepatic insulin resistance. In the current study, we tested the hypothesis that an increased rate of fatty acid oxidation in liver would prevent the potentially harmful effects of fatty acid elevation, including hepatic triglyceride (TG) accumulation and elevated TG secretion. Primary rat hepatocytes were transduced with adenovirus encoding carnitine palmitoyltransferase 1a (Adv-CPT-1a) or control adenoviruses encoding either [beta]-galactosidase (Adv-[beta]-gal) or carnitine palmitoyltransferase 2 (Adv-CPT-2). Overexpression of CPT-1a increased the rate of [beta]-oxidation and ketogenesis by ~70%, whereas esterification of exogenous fatty acids and de novo lipogenesis were unchanged. Importantly, CPT-1a overexpression was accompanied by a 35% reduction in TG accumulation and a 60% decrease in TG secretion by hepatocytes. There were no changes in secretion of apolipoprotein B (apoB), suggesting the synthesis of smaller, less atherogenic VLDL particles. To evaluate the effect of increasing hepatic CPT-1a activity in vivo, we injected lean or obese male rats with Adv-CPT-1a, Adv-[beta]-gal, or Adv-CPT-2. Hepatic CPT-1a activity was increased by ~46%, and the rate of fatty acid oxidation was increased by ~44% in lean and ~36% in obese CPT-1a-overexpressing animals compared with Adv-CPT-2- or Adv[beta]-gal-treated rats. Similar to observations in vitro, liver TG content was reduced by ~37% (lean) and ~69% (obese) by this in vivo intervention. We conclude that a moderate stimulation of fatty acid oxidation achieved by an increase in CPT-1a activity is sufficient to substantially reduce hepatic TG accumulation both in vitro and in vivo. Therefore, interventions that increase CPT-1a activity could have potential benefits in the treatment of NAFLD. fatty liver

High-fructose consumption is associated with insulin resistance and diabetic dyslipidemia, but the underlying mechanism is unclear. We show in hamsters that high-fructose feeding stimulated forkhead box O1 (FoxO1) production and promoted its nuclear redistribution in liver, correlating with augmented apolipoprotein C-III (apoC-III) production and impaired triglyceride metabolism. High-fructose feeding upregulated peroxisome proliferator-activated receptor-[gamma] coactivator-l[beta] and sterol regulatory element binding protein-1c expression, accounting for increased fat infiltration in liver. High-fructose-fed hamsters developed hypertriglyceridemia, accompanied by hyperinsulinemia and glucose intolerance. These metabolic aberrations were reversible by fenofibrate, a commonly used anti-hypertriglyceridemia agent that is known to bind and activate peroxisome proliferator-activated receptor-[alpha] (PPAR[alpha]). PPAR[alpha] physically interacted with, but functionally antagonized, FoxO1 in hepatic apoC-III expression. These data underscore the importance of FoxO1 deregulation in the pathogenesis of hypertriglyceridemia in high-fructose-fed hamsters. Counterregulation of hepatic FoxO1 activity by PPAR[alpha] constitutes an important mechanism by which fibrates act to curb apoC-III overproduction and ameliorate hypertriglyceridemia. hypertriglyceridemia; forkhead box O1; peroxisome proliferator-activated receptor-[alpha]; apolipoprotein C-III; very-low-density lipoprotein-triglyceride; microsomal triglyceride transfer protein

Nonalcoholic fatty liver disease (NAFLD), hypertriglyceridemia, and elevated free fatty acids are present in the majority of patients with metabolic syndrome and type 2 diabetes mellitus and are strongly associated with hepatic insulin resistance. In the current study, we tested the hypothesis that an increased rate of fatty acid oxidation in liver would prevent the potentially harmful effects of fatty acid elevation, including hepatic triglyceride (TO) accumulation and elevated TG secretion. Primary rat hepatocytes were transduced with adenovirus encoding carnitine palmitoyltransferase la (Adv-CPT-la) or control adenoviruses encoding either ~3-galactosidase (Adv-~3-gal) or carnitine palmitoyltransferase 2 (Adv-CPT-2). Overexpression of CPT- I a increased the rate of 13-oxidation and ketogenesis by --70%, whereas esterification of exogenous fatty acids and de novo lipogenesis were unchanged. Importantly, CPT-la overexpression was ac- companied by a 35% reduction in TG accumulation and a 60% decrease in TO secretion by hepatocytes. There were no changes in secretion of apolipoprotein B (apoB), suggesting the synthesis of smaller, less atherogenic VLDL particles. To evaluate the effect of increasing hepatic CPT-la activity in vivo, we injected lean or obese male rats with Adv-CPT-la, Adv-13-gal, or Adv-CPT-2. Hepatic CPT-la activity was increased by -46%, and the rate of fatty acid oxidation was increased by --44% in lean and --36% in obese CPT-la-overexpressing animals compared with Adv-CPT-2- or Adv- 13-gal-treated rats. Similar to observations in vitro, liver TG content was reduced by --37% (lean) and -69% (obese) by this in vivo intervention. We conclude that a moderate stimulation of fatty acid oxidation achieved by an increase in CPT-la activity is sufficient to substantially reduce hepatic TO accumulation both in vitro and in vivo. Therefore, interventions that increase CPT-la activity could have potential benefits in the treatment of NAFLD. [ABSTRACT FROM AUTHOR]

High-fructose consumption is associated with insulin resistance and diabetic dyslipidemia, but the underlying mechanism is unclear. We show in hamsters that high-fructose feeding stimulated forkhead box O1 (FoxO1) production and promoted its nuclear redistribution in liver, correlating with augmented apolipoprotein C-Ill (apoC-Ill) production and impaired triglyceride metabolism. High-fructose feeding upregulated peroxisome proliferator-activated receptor-γ coactivator-1β and sterol regulatory element binding protein-1c expression, accounting for increased fat infiltration in liver. High-fructose-fed hamsters developed hypertriglyceridemia, accompanied by hyperinsulinemia and glucose intolerance. These metabolic aberrations were reversible by fenofibrate, a commonly used anti-hypertriglyceridemia agent that is known to bind and activate peroxisome proliferator-activated receptor-α (PPARα). PPARα physically interacted with, but functionally antagonized, FoxO1 in hepatic apoC-Ill expression. These data underscore the importance of FoxO1 deregulation in the pathogenesis of hypertriglyceridemia in high-fructose-fed hamsters. Counterregulation of hepatic FoxO1 activity by PPARα constitutes an important mechanism by which fibrates act to curb apoC-Ill overproduction and ameliorate hypertriglyceridemia. [ABSTRACT FROM AUTHOR]

The worldwide epidemics of obesity and diabetes have been linked to increased sugar consumption in humans. Here, we review fructose and glucose metabolism, as well as potential molecular mechanisms by which excessive sugar consumption is associated to metabolic diseases and insulin resistance in humans. To this end, we focus on understanding molecular and cellular mechanisms of fructose and glucose transport and sensing in the intestine, the intracellular signaling effects of dietary sugar metabolism, and its impact on glucose homeostasis in health and disease. Finally, the peripheral and central effects of dietary sugars on the gut–brain axis will be reviewed. [ABSTRACT FROM AUTHOR]

Apolipoprotein D (apoD) is a plasma glycoprotein that has a high degree of homology to plasma retinol-binding protein and other members in the lipocalin family. Unlike other apolipoproteins that axe expressed mainly in liver and intestine, apoD is produced in liver, adipocytes, skeletal muscle and brain. Such a diverse tissue distribution presages a broad spectrum of function for apoD. We show that plasma apoD levels are downregulated, but are increased in response to diet/exercise-mediated weight loss in subjects with obesity and type 2 diabetes. These observations lead to the hypothesis that apoD may play a role in glucose metabolism. To address this hypothesis, we deliver ApoD cDNA by adenovirus-mediated gene transfer into liver of normal mice (n=8 per group). This approach resulted in 2-3 folds of elevation in plasma apoD levels. Hepatic apoD production resulted in significant reduction in both fasting and non-fasting plasma glucose levels, accompanied by enhanced glucose tolerance. Using the same approach, we show in high fat-induced obese mice (n=10 per group) that elevated plasma apoD levels suppressed food uptake, resulting in body weight loss over a period of 2 weeks. This effect correlated with the reduction of fat pad mass. As a result, apoD vector-treated obese mice exhibited significantly improved glucose tolerance. In contrast, glucose intolerance persisted along with excessive body weight gain in control vector-treated obese mice. Our studies elucidate that apoD plays a significant role in glucose homeostasis. [ABSTRACT FROM AUTHOR]

Apolipoprotein D (apoD) is an atypical apolipoprotein belonging to the lipocalin family and is mainly present in high-density lipoproteins (HDL). Its physiological role in cholesterol and triglyceride metabolism remains unknown. We demonstrate that plasma apoD levels are downregulated in obesity/diabetes and upregulated in atherosclerosis. To characterize apoD function in triglyceride and cholesterol metabolism, we transferred APOD eDNA into liver by adenovirus-mediated delivery to elevate plasma apoD levels by 2-3 folds in normal and high fat diet-induced obese mice. Hepatic apoD production resulted in significant reduction in plasma triglyceride and cholesterol levels, correlating with decreased plasma HDL and very-low density lipoproteins (VLDL) levels in apoD vector-treated mice. To investigate the mechanism of apoD-mediated reduction in VLDL-triglyceride and HDL-cholesterol levels, we studied key regulatory steps in cholesterol and triglyceride homeostasis. We show that hepatic apoD production resulted in enhanced plasma VLDL clearance, whereas hepatic VLDL production remained unchanged. In contrast, apoD regulates HDL production by a mechanism that involves increased lecithin-cholesterol acyltransferase (LCAT) activity and decreased pre-beta-HDL particles in plasma, without altering hepatic levels of HMG-CoA reductase and scavenger receptor class BI. In conclusion, we show for the first time that apoD plays important roles in regulating triglyceride and cholesterol metabolism. [ABSTRACT FROM AUTHOR]

Excessive production of triglyceride-rich very low-density lipoproteins (VLDL) is a causative factor for hypertriglyceridemia. A rate limiting-step in VLDL production is controlled by microsomal triglyceride transfer protein (MTP), which is regulated by insulin, but with poorly defined mechanisms. To characterize the mechanism underlying insulin-dependent regulation of MTP and VLDL production, we studied the effect of FoxO1 on MTP expression in response to insulin. FoxO1 is a transcription factor that plays a key role in hepatic insulin signaling. We show in cultured hepatocytes that MTP expression is induced by FoxO1 and inhibited by insulin. MTP expression is also stimulated by a dominant-active FoxO1-ADA allele, but this stimulatory effect is refractory to insulin inhibition, correlating with the inability of FoxO1-ADA to undergo insulin-dependent phosphorylation. FoxO1 binds and stimulates the MTP promoter activity. This effect is counteracted by insulin. Elevated hepatic FoxO1 activity is associated with significantly increased MTP expression, correlating with augmented hepatic VLDL production and elevated plasma triglyceride levels in FoxO1 transgenic mice. Furthermore, hepatic MTP expression, along with FoxO1 abundance in liver, is increased in obese mice with altered triglyceride metabolism and in high fructose-induced hyperlipidemic hamsters. These data illustrate a novel mechanism by which FoxO1 mediates insulin action on hepatic MTP and VLDL production, and provide insight into the pathogenesis of hypertriglyceridemia. Augmented MTP production, resulting from the inability of insulin to check hepatic FoxO1 activity, is a causative factor for hepatic VLDL overproduction and development of dyslipidemia in diabetes. ADA-Funded Research [ABSTRACT FROM AUTHOR]

Recent studies have implicated inhibitor of kappaB kinase (IKK) in mediating fatty acid (FA)-induced insulin resistance. How IKK causes these effects is unknown. The present study addressed the role of nuclear factor kappaB (NFkappaB), the distal target of IKK activity, in FA-induced insulin resistance in L6 myotubes, an in vitro skeletal muscle model. A 6-h exposure of myotubes to the saturated FA palmitate reduced insulin-stimulated glucose uptake by approximately 30%, phosphatidylinositol-3 kinase and protein kinase B phosphorylation by approximately 40%, and stimulated inhibitor of kappaBalpha degradation and the nuclear translocation of NFkappaB. On the other hand, the Omega-3 polyunsaturated FA linolenate neither induced insulin resistance nor promoted nuclear localization of NFkappaB. Supporting the hypothesis that IKK acts through NFkappaB to cause insulin resistance, the IKK inhibitors acetylsalicylate and parthenolide prevented FA-induced reductions in insulin-stimulated glucose uptake and NFkappaB nuclear translocation. Most importantly, NFkappaB SN50, a cell-permeable peptide that inhibits NFkappaB nuclear translocation downstream of IKK, was sufficient to prevent palmitate-induced reductions in insulin-stimulated glucose uptake. Acetylsalicylate, but not NFkappaB SN50, prevented FA effects on phosphatidylinositol-3 kinase activity and protein kinase B phosphorylation. We conclude that FAs induce insulin resistance and activates NFkappaB in L6 cells. Furthermore, inhibition of NFkappaB activation, indirectly by preventing IKK activation or directly by inhibiting NFkappaB nuclear translocation, prevents the detrimental effects of palmitate on the metabolic actions of insulin in L6 myotubes.

Skeletal muscle insulin resistance may be aggravated by intramyocellular accumulation of fatty acid-derived metabolites that inhibit insulin signaling. We tested the hypothesis that enhanced fatty acid oxidation in myocytes should protect against fatty acid-induced insulin resistance by limiting lipid accumulation. L6 myotubes were transduced with adenoviruses encoding carnitine palmitoyltransferase I (CPT I) isoforms or beta-galactosidase (control). Two to 3-fold overexpression of L-CPT I, the endogenous isoform in L6 cells, proportionally increased oxidation of the long-chain fatty acids palmitate and oleate and increased insulin stimulation of [(14)C]glucose incorporation into glycogen by 60% while enhancing insulin-stimulated phosphorylation of p38MAPK. Incubation of control cells with 0.2 mm palmitate for 18 h caused accumulation of triacylglycerol, diacylglycerol, and ceramide (but not long-chain acyl-CoA) and decreased insulin-stimulated [(14)C]glucose incorporation into glycogen (60%), [(3)H]deoxyglucose uptake (60%), and protein kinase B phosphorylation (20%). In the context of L-CPT I overexpression, palmitate preincubation produced a relative decrease in insulin-stimulated incorporation of [(14)C]glucose into glycogen (60%) and [(3)H]deoxyglucose uptake (40%) but did not inhibit phosphorylation of protein kinase B. Due to the enhancement of insulin-stimulated glucose metabolism induced by L-CPT I overexpression itself, net insulin-stimulated incorporation of [(14)C]glucose into glycogen and [(3)H]deoxyglucose uptake in L-CPT I-transduced, palmitate-treated cells were significantly greater than in palmitate-treated control cells (71 and 75% greater, respectively). However, L-CPT I overexpression failed to decrease intracellular triacylglycerol, diacylglycerol, ceramide, or long-chain acyl-CoA. We propose that accelerated beta-oxidation in muscle cells exerts an insulin-sensitizing effect independently of changes in intracellular lipid content.