Androgen Receptor Roles in Insulin Resistance and Obesity
Androgen Receptor Roles in Insulin Resistance and Obesity
The development of insulin resistance is a complicated process involving the impaired action of insulin in various target tissues. Although underlying mechanisms of impaired insulin signaling may differ among tissues and under various circumstances, it is established that there are complex interorgan communication among various insulin target tissues. Examination of tissue-specific insulin signaling and selective insulin resistance in various tissues have advanced our understanding of the complex pathophysiology of insulin action.
The liver functions as a major metabolic tissue to control glucose and lipid homeostasis. Oxidation of fatty acids by the hepatocytes supplies the substrate for glucose production. Dysregulated fatty acid oxidation and synthesis can lead to the accumulation of fat or hepatic steatosis.
Using albumin-Cre–transgenic mice, Lin et al. directly addressed the role of AR in the liver by specifically deleting AR in hepatocytes (LARKO). After 8 weeks of high-fat diet (HFD) feeding, male LARKO mice developed obesity and significant hepatic steatosis. Hepatic steatosis is known to negatively impact insulin sensitivity and lead to insulin resistance, which is manifested by the reduced ability of insulin to suppress hepatic glucose production. HFD-fed male LARKO mice exhibited fasting hyperglycemia and insulin resistance, indicating impairments in the regulation of glucose homeostasis and insulin sensitivity. At a molecular level, decreased activation of phosphatidylinositide-3 kinase in response to insulin and increased expression of PEPCK in male LARKO liver and isolated hepatocytes were consistent with increased hepatic glucose production and development of insulin resistance.
Mechanistic studies in male LARKO mice showed that activation and upregulation of SREBP1c and acetyl CoA carboxylase produced more malonyl CoA, the substrate for de novo fatty acid synthesis. The transport of free fatty acids from the cytosol to mitochondria is required for their β-oxidation and is mediated by carnitine palmitoyltransferase I, located at the outer membrane of mitochondria. Malonyl CoA is an inhibitor of carnitine palmitoyltransferase I, and increased levels of malonyl CoA reduces transport of fatty acids into mitochondria, resulting in the reduction of fatty-acid oxidation. In a parallel cascade, reduction of peroxisome proliferator–activated receptor-α and malonyl CoA decarboxylase in livers derived from male LARKO mice further increased the production of malonyl CoA. In corresponding hepatocytes, expression of peroxisome proliferator–activated receptor-α was shown to be mediated by DHT-dependent activation of AR. These findings suggest that impeding the entry of free fatty acids into mitochondria, impairing β-oxidation of fatty acids, and promotion of de novo fatty acid synthesis could account for the development of hepatic steatosis in male LARKO mice.
These mechanistic studies of HFD-fed male LARKO mice demonstrate a pivotal role of hepatic AR in regulating insulin sensitivity and lipid homeostasis. Hepatic insulin resistance is shown to be sufficient to produce dyslipidemia and increase the susceptibility to atherosclerosis in mice. In human clinical studies, GnRH agonists to suppress androgen/AR signaling cause increases in total cholesterol and triglycerides. It is therefore likely that functional deficiency of AR in the liver caused by GnRH agonists leads to these lipid alterations. As large cohort studies demonstrate strong correlations between blood cholesterol levels and cardiovascular mortality independent of other coronary risk factors, awareness of altered hepatic AR signaling and lipid metabolism during ADT should prompt the appropriate management of cardiovascular complications.
Compelling data derived from animal studies are mounting that brain insulin resistance may be a critical element in the pathophysiology of obesity, type 2 diabetes, and related metabolic disorders. Defective hypothalamic insulin signaling is able to promote hepatic insulin resistance as demonstrated by the brain-specific insulin receptor knockout mouse model. Partial restoration of liver insulin signaling in the insulin receptor knockout mice fails to normalize insulin action on hepatic glucose production, supporting the importance of hypothalamic insulin signaling in glucose homeostasis.
Male neuronal-specific ARKO mice, generated by selectively targeting AR in neurons, displayed increased body weight and visceral adiposity, as well as increased levels of fasting blood glucose and insulin. Neuronal AR deficiency led to impaired insulin signaling in the hypothalamus, which in turn resulted in reduced suppression of hepatic gluconeogenic genes.
Hypothalamic insulin resistance is reported to act as an early event in the development of systemic insulin resistance due to prolonged exposure to excessive nutrition. Activation of hypothalamic nuclear factor-κB (NF-κB) signaling is critical to induction of insulin resistance following chronic overnutrition.At the neuronal level, loss of suppression by AR resulted in increased activation of hypothalamic NF-κB signaling within a short-term HFD-feeding period. Hence, functional deficiency of AR in neurons directly interferes with insulin signaling and leads to hypothalamic insulin resistance.
These findings uncover a new mechanism of insulin resistance caused by testosterone deficiency through decreased function of AR in the brain. Suppression of the hypothalamic NF-κB by AR provides a potential way to manage the metabolic complications that develop in patients with PCa undergoing ADT by targeting neuronal AR.
The adipose tissue is a key target of insulin action, important for glucose uptake, and a potential site for testosterone's action in regulating body mass composition.
Selectively targeting AR in adipocytes was used to generate the adipose-specific ARKO (AARKO) mouse model to study the role of AR in fat tissue. In male AARKO mice, body weight and fat pad mass were indistinguishable from that of wild-type littermates. Despite identical levels of adiposity, male AARKO mice exhibited elevated levels of serum leptin, suggesting that loss of the AR affects leptin secretion by adipose tissue. Interestingly, enhanced leptin production in AR-deficient adipose tissue did not result in leptin resistance, as male AARKO mice showed increased sensitivity in response to exogenous leptin challenge. Increased estradiol levels were observed in epididymal adipose tissue, suggesting enhanced estrogen receptor (ER) transactivation contributed to upregulation of leptin gene transcription.
Adipose tissue expressed several steroidogenic enzymes that control tissue steroid concentrations and ligand bioavailability for intracellular receptors. It is possible that altered activities of steroid-converting enzymes due to loss of AR resulted in a larger steroid reservoir in AR-deficient adipocytes. Increased steroid precursors may have provided increased available substrate for aromatases in the adipose tissue, resulting in enhanced estradiol production. In male GARKO mice, the elevation of leptin occurred prior to the onset of obesity, suggesting a similar mechanism of increased intracellular estradiol conversion may have contributed to enhanced leptin production.
Sex steroid hormones are important regulators of metabolism, accumulation of fat, and distribution of adipose tissue. In humans, fat distribution is different between males and females. Sex steroid hormones predispose males to a more central accumulation of fat, whereas in females, a more subcutaneous accumulation of fat is observed. This difference has important metabolic consequences, as visceral obesity is considered a risk factor for cardiovascular diseases, and men have a higher incidence of cardiovascular diseases than women. Menopause in women increases central distribution of fat and incidence of cardiovascular diseases. The mechanism by which sex steroid hormones control the amount and distribution of fat is not clear. One mechanism may be through the transcriptional regulation of key proteins in adipose tissue. Future studies on the local synthesis of sex steroid hormones and the regulation of AR and ER signaling in different types of adipose tissue may increase our understanding of steroid action in the adipocyte.
A critical feature of skeletal muscle in glucose homeostasis is insulin-stimulated glucose uptake and use. Testosterone is an important regulator of lean mass, and anabolic effects of testosterone on skeletal muscles are thought to be mediated predominantly through AR. AR is expressed in various cell types of skeletal muscle in humans and rodents including myocytes, satellite cells, fibroblasts, and mesenchymal stem cells, and all are potential targets of testosterone's action.
One muscle-specific ARKO (MARKO) mouse model, generated by myocyte-specific AR deletion, demonstrates altered fiber composition. Myocyte-specific deletion of AR resulted in an increase of slow-twitch fibers without affecting muscle strength. Unexpectedly, MARKO mice showed a reduction in intra-abdominal fat mass. In another MARKO mouse model, AR ablation in myocytes affected intrinsic contractile functions in fast- and intermediary-twitch muscles. Androgens induced hypertrophy of muscle fibers through AR-dependent pathways in perineal muscles and AR-independent pathways in limb muscles. Discrepancies between the two MARKO mouse models may be related to differences in genetic backgrounds of the mouse lines, Cre recombinase transgenic mouse lines (MCK-Cre vs. HSA-Cre), and experimental protocols used. Whether insulin sensitivity or glucose homeostasis is influenced by myocyte-specific AR ablation is unclear and awaits further investigation.
Insights into AR signaling in muscle and the role of AR in regulating metabolic homeostasis may be facilitated by using transgenic animal models in which AR is selectively overexpressed in various cell types. Overexpression of AR in myoctes increased lean mass and reduced fat mass in transgenic rats. AR signaling in myocytes was sufficient to promote systemic oxidative metabolism through increasing activity of mitochondrial enzymes and oxygen consumption in skeletal muscle. Targeted AR overexpression in mesenchymal stem cells reduced fat mass and reciprocally increased lean mass in male mice. Transgenic AR mice showed improved glucose use in response to exogenous glucose challenge. These studies suggest AR signaling in muscle is involved in mitochondrial respiration and glucose disposal.
Cell Type–Specific ARKO Mouse Models
The development of insulin resistance is a complicated process involving the impaired action of insulin in various target tissues. Although underlying mechanisms of impaired insulin signaling may differ among tissues and under various circumstances, it is established that there are complex interorgan communication among various insulin target tissues. Examination of tissue-specific insulin signaling and selective insulin resistance in various tissues have advanced our understanding of the complex pathophysiology of insulin action.
Liver-specific ARKO Mouse Model
The liver functions as a major metabolic tissue to control glucose and lipid homeostasis. Oxidation of fatty acids by the hepatocytes supplies the substrate for glucose production. Dysregulated fatty acid oxidation and synthesis can lead to the accumulation of fat or hepatic steatosis.
Using albumin-Cre–transgenic mice, Lin et al. directly addressed the role of AR in the liver by specifically deleting AR in hepatocytes (LARKO). After 8 weeks of high-fat diet (HFD) feeding, male LARKO mice developed obesity and significant hepatic steatosis. Hepatic steatosis is known to negatively impact insulin sensitivity and lead to insulin resistance, which is manifested by the reduced ability of insulin to suppress hepatic glucose production. HFD-fed male LARKO mice exhibited fasting hyperglycemia and insulin resistance, indicating impairments in the regulation of glucose homeostasis and insulin sensitivity. At a molecular level, decreased activation of phosphatidylinositide-3 kinase in response to insulin and increased expression of PEPCK in male LARKO liver and isolated hepatocytes were consistent with increased hepatic glucose production and development of insulin resistance.
Mechanistic studies in male LARKO mice showed that activation and upregulation of SREBP1c and acetyl CoA carboxylase produced more malonyl CoA, the substrate for de novo fatty acid synthesis. The transport of free fatty acids from the cytosol to mitochondria is required for their β-oxidation and is mediated by carnitine palmitoyltransferase I, located at the outer membrane of mitochondria. Malonyl CoA is an inhibitor of carnitine palmitoyltransferase I, and increased levels of malonyl CoA reduces transport of fatty acids into mitochondria, resulting in the reduction of fatty-acid oxidation. In a parallel cascade, reduction of peroxisome proliferator–activated receptor-α and malonyl CoA decarboxylase in livers derived from male LARKO mice further increased the production of malonyl CoA. In corresponding hepatocytes, expression of peroxisome proliferator–activated receptor-α was shown to be mediated by DHT-dependent activation of AR. These findings suggest that impeding the entry of free fatty acids into mitochondria, impairing β-oxidation of fatty acids, and promotion of de novo fatty acid synthesis could account for the development of hepatic steatosis in male LARKO mice.
These mechanistic studies of HFD-fed male LARKO mice demonstrate a pivotal role of hepatic AR in regulating insulin sensitivity and lipid homeostasis. Hepatic insulin resistance is shown to be sufficient to produce dyslipidemia and increase the susceptibility to atherosclerosis in mice. In human clinical studies, GnRH agonists to suppress androgen/AR signaling cause increases in total cholesterol and triglycerides. It is therefore likely that functional deficiency of AR in the liver caused by GnRH agonists leads to these lipid alterations. As large cohort studies demonstrate strong correlations between blood cholesterol levels and cardiovascular mortality independent of other coronary risk factors, awareness of altered hepatic AR signaling and lipid metabolism during ADT should prompt the appropriate management of cardiovascular complications.
Neuronal-specific ARKO Mouse Model
Compelling data derived from animal studies are mounting that brain insulin resistance may be a critical element in the pathophysiology of obesity, type 2 diabetes, and related metabolic disorders. Defective hypothalamic insulin signaling is able to promote hepatic insulin resistance as demonstrated by the brain-specific insulin receptor knockout mouse model. Partial restoration of liver insulin signaling in the insulin receptor knockout mice fails to normalize insulin action on hepatic glucose production, supporting the importance of hypothalamic insulin signaling in glucose homeostasis.
Male neuronal-specific ARKO mice, generated by selectively targeting AR in neurons, displayed increased body weight and visceral adiposity, as well as increased levels of fasting blood glucose and insulin. Neuronal AR deficiency led to impaired insulin signaling in the hypothalamus, which in turn resulted in reduced suppression of hepatic gluconeogenic genes.
Hypothalamic insulin resistance is reported to act as an early event in the development of systemic insulin resistance due to prolonged exposure to excessive nutrition. Activation of hypothalamic nuclear factor-κB (NF-κB) signaling is critical to induction of insulin resistance following chronic overnutrition.At the neuronal level, loss of suppression by AR resulted in increased activation of hypothalamic NF-κB signaling within a short-term HFD-feeding period. Hence, functional deficiency of AR in neurons directly interferes with insulin signaling and leads to hypothalamic insulin resistance.
These findings uncover a new mechanism of insulin resistance caused by testosterone deficiency through decreased function of AR in the brain. Suppression of the hypothalamic NF-κB by AR provides a potential way to manage the metabolic complications that develop in patients with PCa undergoing ADT by targeting neuronal AR.
Adipose-specific ARKO Mouse Model
The adipose tissue is a key target of insulin action, important for glucose uptake, and a potential site for testosterone's action in regulating body mass composition.
Selectively targeting AR in adipocytes was used to generate the adipose-specific ARKO (AARKO) mouse model to study the role of AR in fat tissue. In male AARKO mice, body weight and fat pad mass were indistinguishable from that of wild-type littermates. Despite identical levels of adiposity, male AARKO mice exhibited elevated levels of serum leptin, suggesting that loss of the AR affects leptin secretion by adipose tissue. Interestingly, enhanced leptin production in AR-deficient adipose tissue did not result in leptin resistance, as male AARKO mice showed increased sensitivity in response to exogenous leptin challenge. Increased estradiol levels were observed in epididymal adipose tissue, suggesting enhanced estrogen receptor (ER) transactivation contributed to upregulation of leptin gene transcription.
Adipose tissue expressed several steroidogenic enzymes that control tissue steroid concentrations and ligand bioavailability for intracellular receptors. It is possible that altered activities of steroid-converting enzymes due to loss of AR resulted in a larger steroid reservoir in AR-deficient adipocytes. Increased steroid precursors may have provided increased available substrate for aromatases in the adipose tissue, resulting in enhanced estradiol production. In male GARKO mice, the elevation of leptin occurred prior to the onset of obesity, suggesting a similar mechanism of increased intracellular estradiol conversion may have contributed to enhanced leptin production.
Sex steroid hormones are important regulators of metabolism, accumulation of fat, and distribution of adipose tissue. In humans, fat distribution is different between males and females. Sex steroid hormones predispose males to a more central accumulation of fat, whereas in females, a more subcutaneous accumulation of fat is observed. This difference has important metabolic consequences, as visceral obesity is considered a risk factor for cardiovascular diseases, and men have a higher incidence of cardiovascular diseases than women. Menopause in women increases central distribution of fat and incidence of cardiovascular diseases. The mechanism by which sex steroid hormones control the amount and distribution of fat is not clear. One mechanism may be through the transcriptional regulation of key proteins in adipose tissue. Future studies on the local synthesis of sex steroid hormones and the regulation of AR and ER signaling in different types of adipose tissue may increase our understanding of steroid action in the adipocyte.
Muscle-specific ARKO Mouse Model
A critical feature of skeletal muscle in glucose homeostasis is insulin-stimulated glucose uptake and use. Testosterone is an important regulator of lean mass, and anabolic effects of testosterone on skeletal muscles are thought to be mediated predominantly through AR. AR is expressed in various cell types of skeletal muscle in humans and rodents including myocytes, satellite cells, fibroblasts, and mesenchymal stem cells, and all are potential targets of testosterone's action.
One muscle-specific ARKO (MARKO) mouse model, generated by myocyte-specific AR deletion, demonstrates altered fiber composition. Myocyte-specific deletion of AR resulted in an increase of slow-twitch fibers without affecting muscle strength. Unexpectedly, MARKO mice showed a reduction in intra-abdominal fat mass. In another MARKO mouse model, AR ablation in myocytes affected intrinsic contractile functions in fast- and intermediary-twitch muscles. Androgens induced hypertrophy of muscle fibers through AR-dependent pathways in perineal muscles and AR-independent pathways in limb muscles. Discrepancies between the two MARKO mouse models may be related to differences in genetic backgrounds of the mouse lines, Cre recombinase transgenic mouse lines (MCK-Cre vs. HSA-Cre), and experimental protocols used. Whether insulin sensitivity or glucose homeostasis is influenced by myocyte-specific AR ablation is unclear and awaits further investigation.
Insights into AR signaling in muscle and the role of AR in regulating metabolic homeostasis may be facilitated by using transgenic animal models in which AR is selectively overexpressed in various cell types. Overexpression of AR in myoctes increased lean mass and reduced fat mass in transgenic rats. AR signaling in myocytes was sufficient to promote systemic oxidative metabolism through increasing activity of mitochondrial enzymes and oxygen consumption in skeletal muscle. Targeted AR overexpression in mesenchymal stem cells reduced fat mass and reciprocally increased lean mass in male mice. Transgenic AR mice showed improved glucose use in response to exogenous glucose challenge. These studies suggest AR signaling in muscle is involved in mitochondrial respiration and glucose disposal.
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