Sojin Lee

FoxO6 Integrates Insulin Signaling With Gluconeogenesis in the Liver

OBJECTIVE Excessive endogenous glucose production contributes to fasting hyperglycemia in diabetes. This effect stems from inept insulin suppression of hepatic gluconeogenesis. To understand the underlying mechanisms, we studied the ability of forkhead box O6 (FoxO6) to mediate insulin action on hepatic gluconeogenesis and its contribution to glucose metabolism. RESEARCH DESIGN AND METHODS We characterized FoxO6 in glucose metabolism in cultured hepatocytes and in rodent models of dietary obesity, insulin resistance, or insulin-deficient diabetes. We determined the effect of FoxO6 on hepatic gluconeogenesis in genetically modified mice with FoxO6 gain- versus loss-of-function and in diabetic db/db mice with selective FoxO6 ablation in the liver. RESULTS FoxO6 integrates insulin signaling to hepatic gluconeogenesis. In mice, elevated FoxO6 activity in the liver augments gluconeogenesis, raising fasting blood glucose levels, and hepatic FoxO6 depletion suppresses gluconeogenesis, resulting in fasting hypoglycemia. FoxO6 stimulates gluconeogenesis, which is counteracted by insulin. Insulin inhibits FoxO6 activity via a distinct mechanism by inducing its phosphorylation and disabling its transcriptional activity, without altering its subcellular distribution in hepatocytes. FoxO6 becomes deregulated in the insulin-resistant liver, accounting for its unbridled activity in promoting gluconeogenesis and correlating with the pathogenesis of fasting hyperglycemia in diabetes. These metabolic abnormalities, along with fasting hyperglycemia, are reversible by selective inhibition of hepatic FoxO6 activity in diabetic mice. CONCLUSIONS Our data uncover a FoxO6-dependent pathway by which the liver orchestrates insulin regulation of gluconeogenesis, providing the proof-of-concept that selective FoxO6 inhibition is beneficial for curbing excessive hepatic glucose production and improving glycemic control in diabetes.

ATF4 Protein Deficiency Protects against High Fructose-induced Hypertriglyceridemia in Mice

Background: Hypertriglyceridemia is the most common lipid disorder with incompletely understood mechanisms. Results: ATF4 deficiency attenuates lipogenesis in the liver and protects against high fructose-induced hypertriglyceridemia in mice. Conclusion: ATF4 plays a pivotal role in regulating hepatic lipid metabolism. Significance: ATF4 is a contributing factor for the pathogenesis of hypertriglyceridemia. Hypertriglyceridemia is the most common lipid disorder in obesity and type 2 diabetes. It results from increased production and/or decreased clearance of triglyceride-rich lipoproteins. To better understand the pathophysiology of hypertriglyceridemia, we studied hepatic regulation of triglyceride metabolism by the activating transcription factor 4 (ATF4), a member of the basic leucine zipper-containing protein subfamily. We determined the effect of ATF4 on hepatic lipid metabolism in Atf4−/− mice fed regular chow or provided with free access to fructose drinking water. ATF4 depletion preferentially attenuated hepatic lipogenesis without affecting hepatic triglyceride production and fatty acid oxidation. This effect prevented excessive fat accumulation in the liver of Atf4−/− mice, when compared with wild-type littermates. To gain insight into the underlying mechanism, we showed that ATF4 depletion resulted in a significant reduction in hepatic expression of peroxisome proliferator-activated receptor-γ, a nuclear receptor that acts to promote lipogenesis in the liver. This effect was accompanied by a significant reduction in hepatic expression of sterol regulatory element-binding protein 1c (SREBP-1c), acetyl-CoA carboxylase, and fatty-acid synthase, three key functions in the lipogenic pathway in Atf4−/− mice. Of particular significance, we found that Atf4−/− mice, as opposed to wild-type littermates, were protected against the development of steatosis and hypertriglyceridemia in response to high fructose feeding. These data demonstrate that ATF4 plays a critical role in regulating hepatic lipid metabolism in response to nutritional cues.

FOXO1 mediates the autocrine effect of endothelin-1 on endothelial cell survival.

Chronic hyperglycemia exerts a deleterious effect on endothelium, contributing to endothelial dysfunction and microvascular complications in poorly controlled diabetes. To understand the underlying mechanism, we studied the effect of endothelin-1 (ET-1) on endothelial production of Forkhead box O1 (FOXO1), a forkhead transcription factor that plays an important role in cell survival. ET-1 is a 21-amino acid peptide that is secreted primarily from endothelium. Using adenovirus-mediated gene transfer approach, we delivered FOXO1 cDNA into cultured human aorta endothelial cells. FOXO1 was shown to stimulate B cell leukemia/lymphoma 2-associated death promoter (BAD) production and promote cellular apoptosis. This effect was counteracted by ET-1. In response to ET-1, FOXO1 was phosphorylated and translocated from the nucleus to cytoplasm, resulting in inhibition of BAD production and mitigation of FOXO1-mediated apoptosis. Hyperglycemia stimulated FOXO1 O-glycosylation and promoted its nuclear localization in human aorta endothelial cells. This effect accounted for unbridled FOXO1 activity in the nucleus, contributing to augmented BAD production and endothelial apoptosis under hyperglycemic conditions. FOXO1 expression became deregulated in the aorta of both streptozotocin-induced diabetic mice and diabetic db/db mice. This hyperglycemia-elicited FOXO1 deregulation and its ensuing effect on endothelial cell survival was corrected by ET-1. Likewise, FoxO1 deregulation in the aorta of diabetic mice was reversible after the reduction of hyperglycemia by insulin therapy. These data reveal a mechanism by which FOXO1 mediated the autocrine effect of ET-1 on endothelial cell survival. FOXO1 deregulation, resulting from an impaired ability of ET-1 to control FOXO1 activity in endothelium, may contribute to hyperglycemia-induced endothelial lesion in diabetes.

Forkhead Box O6 (FoxO6) Depletion Attenuates Hepatic Gluconeogenesis and Protects against Fat-induced Glucose Disorder in Mice

Background: Excessive endogenous glucose production is a major contributing factor for fasting hyperglycemia in diabetes. Results: FoxO6 deficiency attenuates hepatic gluconeogenesis and protects against fat-induced glucose disorder in mice. Conclusion: FoxO6 plays a significant role in regulating gluconeogenesis in the liver. Significance: FoxO6 is a potential therapeutic target for improving glucose metabolism in diabetes. Excessive endogenous glucose production contributes to fasting hyperglycemia in diabetes. FoxO6 is a distinct member of the FoxO subfamily. To elucidate the role of FoxO6 in hepatic gluconeogenesis and assess its contribution to the pathogenesis of fasting hyperglycemia in diabetes, we generated FoxO6 knock-out (FoxO6-KO) mice followed by determining the effect of FoxO6 loss-of-function on hepatic gluconeogenesis under physiological and pathological conditions. FoxO6 depletion attenuated hepatic gluconeogenesis and lowered fasting glycemia in FoxO6-KO mice. FoxO6-deficient primary hepatocytes were associated with reduced capacities to produce glucose in response to glucagon. When fed a high fat diet, FoxO6-KO mice exhibited significantly enhanced glucose tolerance and reduced blood glucose levels accompanied by improved insulin sensitivity. These effects correlated with attenuated hepatic gluconeogenesis in FoxO6-KO mice. In contrast, wild-type littermates developed fat-induced glucose intolerance with a concomitant induction of fasting hyperinsulinemia and hyperglycemia. Furthermore, FoxO6-KO mice displayed significantly diminished macrophage infiltration into liver and adipose tissues, correlating with the reduction of macrophage expression of C-C chemokine receptor 2 (CCR2), a factor that is critical for regulating macrophage recruitment in peripheral tissues. Our data indicate that FoxO6 depletion protected against diet-induced glucose intolerance and insulin resistance by attenuating hepatic gluconeogenesis and curbing macrophage infiltration in liver and adipose tissues in mice.

FoxO6 integrates insulin signaling with MTP for regulating VLDL production in the liver.

Excessive production of triglyceride-rich very low-density lipoproteins (VLDL-TG) contributes to hypertriglyceridemia in obesity and type 2 diabetes. To understand the underlying mechanism, we studied hepatic regulation of VLDL-TG production by (forkhead box O6) FoxO6, a forkhead transcription factor that integrates insulin signaling to hepatic metabolism. We showed that transgenic mice expressing a constitutively active FoxO6 allele developed hypertriglyceridemia, culminating in elevated VLDL-TG levels and impaired postprandial TG clearance. This effect resulted in part from increased hepatic VLDL-TG production. We recapitulated these findings in cultured HepG2 cells and human primary hepatocytes, demonstrating that FoxO6 promoted hepatic VLDL-TG secretion. This action correlated with the ability of FoxO6 to stimulate hepatic production of microsomal triglyceride transfer protein (MTP), a molecular chaperone that catalyzes the rate-limiting step in VLDL-TG assembly and secretion. FoxO6 was shown to bind to the MTP promoter and stimulate MTP promoter activity in HepG2 cells. This effect was inhibited by insulin, consistent with the ability of insulin to promote FoxO6 phosphorylation and disable FoxO6 DNA-binding activity. Mutations of the FoxO6 target site within the MTP promoter abrogated FoxO6-mediated induction of MTP promoter activity. Hepatic FoxO6 expression became deregulated in insulin-resistant mice with obesity and type 2 diabetes. FoxO6 inhibition in insulin-resistant liver suppressed hepatic MTP expression and curbed VLDL-TG overproduction, contributing to the amelioration of hypertriglyceridemia in obese and diabetic db/db mice. These results characterize FoxO6 as an important signaling molecule upstream of MTP for regulating hepatic VLDL-TG production.

FoxO1 Plays an Important Role in Regulating β-Cell Compensation for Insulin Resistance in Male Mice

β-Cell compensation is an essential mechanism by which β-cells increase insulin secretion for overcoming insulin resistance to maintain euglycemia in obesity. Failure of β-cells to compensate for insulin resistance contributes to insulin insufficiency and overt diabetes. To understand the mechanism of β-cell compensation, we characterized the role of forkhead box O1 (FoxO1) in β-cell compensation in mice under physiological and pathological conditions. FoxO1 is a key transcription factor that serves as a nutrient sensor for integrating insulin signaling to cell metabolism, growth, and proliferation. We showed that FoxO1 improved β-cell compensation via 3 distinct mechanisms by increasing β-cell mass, enhancing β-cell glucose sensing, and augmenting β-cell antioxidative function. These effects accounted for increased glucose-stimulated insulin secretion and enhanced glucose tolerance in β-cell-specific FoxO1-transgenic mice. When fed a high-fat diet, β-cell-specific FoxO1-transgenic mice were protected from developing fat-induced glucose disorder. This effect was attributable to increased β-cell mass and function. Furthermore, we showed that FoxO1 activity was up-regulated in islets, correlating with the induction of physiological β-cell compensation in high-fat-induced obese C57BL/6J mice. These data characterize FoxO1 as a pivotal factor for orchestrating physiological adaptation of β-cell mass and function to overnutrition and obesity.

FoxO6 in glucose metabolism (FoxO6).

The forkhead box O (FoxO) subfamily has four members, namely FoxO1, FoxO3, FoxO4, and FoxO6. Unlike the other three members of the FoxO family, FoxO6 has garnered considerably less attention because of earlier reports that FoxO6 expression was limited to the brain. Recent data indicate that FoxO6 is produced in the liver of both rodents and humans. Hepatic FoxO6 activity, which remains at low basal levels in fed states, is markedly induced in fasted mice. FoxO6 activity becomes abnormally higher in the liver of mice with dietary obesity or type 2 diabetes (T2D). Genetically engineered mice with elevated FoxO6 activity in the liver exhibit prediabetes, culminating in the development of glucose intolerance, fasting hyperglycemia, and hyperinsulinemia. Conversely, inhibition of FoxO6 activity in the insulin‐resistant liver results in a reduction in fasting hyperglycemia, contributing to the amelioration of hyperinsulinemia in T2D mice. These new data suggest that FoxO6 is an important regulator of hepatic glucose metabolism in response to insulin or physiological cues. Insulin inhibits FoxO6 activity by promoting its phosphorylation and disabling its activity in the nucleus without altering its subcellular distribution via a mechanism that is distinct from other members of the FoxO subfamily. In this article, we comprehensively review the role of FoxO6 in glucose metabolism in health and disease. We also address whether FoxO6 dysregulation is a contributing factor for the pathogenesis of fasting hyperglycemia and discuss whether FoxO6 is a potential therapeutic target for improving fasting hyperglycemia in T2D.

FoxO6 in Glucose Metabolism

The forkhead box O (FoxO) subfamily has four members, namely FoxO1, FoxO3, FoxO4, and FoxO6. Unlike the other three members of the FoxO family, FoxO6 has garnered considerably less attention because of earlier reports that FoxO6 expression was limited to the brain. Recent data indicate that FoxO6 is produced in the liver of both rodents and humans. Hepatic FoxO6 activity, which remains at low basal levels in fed states, is markedly induced in fasted mice. FoxO6 activity becomes abnormally higher in the liver of mice with dietary obesity or type 2 diabetes (T2D). Genetically engineered mice with elevated FoxO6 activity in the liver exhibit prediabetes, culminating in the development of glucose intolerance, fasting hyperglycemia, and hyperinsulinemia. Conversely, inhibition of FoxO6 activity in the insulin‐resistant liver results in a reduction in fasting hyperglycemia, contributing to the amelioration of hyperinsulinemia in T2D mice. These new data suggest that FoxO6 is an important regulator of hepatic glucose metabolism in response to insulin or physiological cues. Insulin inhibits FoxO6 activity by promoting its phosphorylation and disabling its activity in the nucleus without altering its subcellular distribution via a mechanism that is distinct from other members of the FoxO subfamily. In this article, we comprehensively review the role of FoxO6 in glucose metabolism in health and disease. We also address whether FoxO6 dysregulation is a contributing factor for the pathogenesis of fasting hyperglycemia and discuss whether FoxO6 is a potential therapeutic target for improving fasting hyperglycemia in T2D.

Treatment with a Catalytic Superoxide Dismutase (SOD) Mimetic Improves Liver Steatosis, Insulin Sensitivity, and Inflammation in Obesity-Induced Type 2 Diabetes

Oxidative stress and persistent inflammation are exaggerated through chronic over-nutrition and a sedentary lifestyle, resulting in insulin resistance. In type 2 diabetes (T2D), impaired insulin signaling leads to hyperglycemia and long-term complications, including metabolic liver dysfunction, resulting in non-alcoholic fatty liver disease (NAFLD). The manganese metalloporphyrin superoxide dismustase (SOD) mimetic, manganese (III) meso-tetrakis (N-ethylpyridinium-2-yl) porphyrin (MnP), is an oxidoreductase known to scavenge reactive oxygen species (ROS) and decrease pro-inflammatory cytokine production, by inhibiting nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. We hypothesized that targeting oxidative stress-induced inflammation with MnP would assuage liver complications and enhance insulin sensitivity and glucose tolerance in a high-fat diet (HFD)-induced mouse model of T2D. During 12 weeks of feeding, we saw significant improvements in weight, hepatic steatosis, and biomarkers of liver dysfunction with redox modulation by MnP treatment in HFD-fed mice. Additionally, MnP treatment improved insulin sensitivity and glucose tolerance, while reducing serum insulin and leptin levels. We attribute these effects to redox modulation and inhibition of hepatic NF-κB activation, resulting in diminished ROS and pro-inflammatory cytokine production. This study highlights the importance of controlling oxidative stress and secondary inflammation in obesity-mediated insulin resistance and T2D. Our data confirm the role of NF-κB-mediated inflammation in the development of T2D, and demonstrate the efficacy of MnP in preventing the progression to disease by specifically improving liver pathology and hepatic insulin resistance in obesity.

FoxO6 in glucose metabolism (FoxO6在葡萄糖代谢中的作用)

The forkhead box O (FoxO) subfamily has four members, namely FoxO1, FoxO3, FoxO4, and FoxO6. Unlike the other three members of the FoxO family, FoxO6 has garnered considerably less attention because of earlier reports that FoxO6 expression was limited to the brain. Recent data indicate that FoxO6 is produced in the liver of both rodents and humans. Hepatic FoxO6 activity, which remains at low basal levels in fed states, is markedly induced in fasted mice. FoxO6 activity becomes abnormally higher in the liver of mice with dietary obesity or type 2 diabetes (T2D). Genetically engineered mice with elevated FoxO6 activity in the liver exhibit prediabetes, culminating in the development of glucose intolerance, fasting hyperglycemia, and hyperinsulinemia. Conversely, inhibition of FoxO6 activity in the insulin‐resistant liver results in a reduction in fasting hyperglycemia, contributing to the amelioration of hyperinsulinemia in T2D mice. These new data suggest that FoxO6 is an important regulator of hepatic glucose metabolism in response to insulin or physiological cues. Insulin inhibits FoxO6 activity by promoting its phosphorylation and disabling its activity in the nucleus without altering its subcellular distribution via a mechanism that is distinct from other members of the FoxO subfamily. In this article, we comprehensively review the role of FoxO6 in glucose metabolism in health and disease. We also address whether FoxO6 dysregulation is a contributing factor for the pathogenesis of fasting hyperglycemia and discuss whether FoxO6 is a potential therapeutic target for improving fasting hyperglycemia in T2D.