Leptin Mediates a Glucose-Fatty Acid Cycle to Maintain Glucose Homeostasis in Starvation.

The transition from the fed to the fasted state necessitates a shift from carbohydrate to fat metabolism that is thought to be mostly orchestrated by reductions in plasma insulin concentrations. Here, we show in awake rats that insulinopenia per se does not cause this transition but that both hypoleptinemia and insulinopenia are necessary. Furthermore, we show that hypoleptinemia mediates a glucose-fatty acid cycle through activation of the hypothalamic-pituitary-adrenal axis, resulting in increased white adipose tissue (WAT) lipolysis rates and increased hepatic acetyl-coenzyme A (CoA) content, which are essential to maintain gluconeogenesis during starvation. We also show that in prolonged starvation, substrate limitation due to reduced rates of glucose-alanine cycling lowers rates of hepatic mitochondrial anaplerosis, oxidation, and gluconeogenesis. Taken together, these data identify a leptin-mediated glucose-fatty acid cycle that integrates responses of the muscle, WAT, and liver to promote a shift from carbohydrate to fat oxidation and maintain glucose homeostasis during starvation.

Glucagon stimulates gluconeogenesis by INSP3R1-mediated hepatic lipolysis.

Although it is well-established that reductions in the ratio of insulin to glucagon in the portal vein have a major role in the dysregulation of hepatic glucose metabolism in type-2 diabetes1-3, the mechanisms by which glucagon affects hepatic glucose production and mitochondrial oxidation are poorly understood. Here we show that glucagon stimulates hepatic gluconeogenesis by increasing the activity of hepatic adipose triglyceride lipase, intrahepatic lipolysis, hepatic acetyl-CoA content and pyruvate carboxylase flux, while also increasing mitochondrial fat oxidation-all of which are mediated by stimulation of the inositol triphosphate receptor 1 (INSP3R1). In rats and mice, chronic physiological increases in plasma glucagon concentrations increased mitochondrial oxidation of fat in the liver and reversed diet-induced hepatic steatosis and insulin resistance. However, these effects of chronic glucagon treatment-reversing hepatic steatosis and glucose intolerance-were abrogated in Insp3r1 (also known as Itpr1)-knockout mice. These results provide insights into glucagon biology and suggest that INSP3R1 may represent a target for therapies that aim to reverse nonalcoholic fatty liver disease and type-2 diabetes.

Leptin's hunger-suppressing effects are mediated by the hypothalamic-pituitary-adrenocortical axis in rodents.

Leptin informs the brain about sufficiency of fuel stores. When insufficient, leptin levels fall, triggering compensatory increases in appetite. Falling leptin is first sensed by hypothalamic neurons, which then initiate adaptive responses. With regard to hunger, it is thought that leptin-sensing neurons work entirely via circuits within the central nervous system (CNS). Very unexpectedly, however, we now show this is not the case. Instead, stimulation of hunger requires an intervening endocrine step, namely activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Increased corticosterone then activates AgRP neurons to fully increase hunger. Importantly, this is true for 2 forms of low leptin-induced hunger, fasting and poorly controlled type 1 diabetes. Hypoglycemia, which also stimulates hunger by activating CNS neurons, albeit independently of leptin, similarly recruits and requires this pathway by which HPA axis activity stimulates AgRP neurons. Thus, HPA axis regulation of AgRP neurons is a previously underappreciated step in homeostatic regulation of hunger.

Leptin mediates postprandial increases in body temperature through hypothalamus-adrenal medulla-adipose tissue crosstalk.

Meal ingestion increases body temperature in multiple species, an effect that is blunted by obesity. However, the mechanisms responsible for these phenomena remain incompletely understood. Here we show that refeeding increases plasma leptin concentrations approximately 8-fold in 48-hour-fasted lean rats, and this normalization of plasma leptin concentrations stimulates adrenomedullary catecholamine secretion. Increased adrenal medulla-derived plasma catecholamines were necessary and sufficient to increase body temperature postprandially, a process that required both fatty acids generated from adipose tissue lipolysis and ß-adrenergic activation of brown adipose tissue (BAT). Diet-induced obese rats, which remained relatively hyperleptinemic while fasting, did not exhibit fasting-induced reductions in temperature. To examine the impact of feeding-induced increases in body temperature on energy balance, we compared rats fed chronically by either 2 carbohydrate-rich boluses daily or a continuous isocaloric intragastric infusion. Bolus feeding increased body temperature and reduced weight gain compared with continuous feeding, an effect abrogated by treatment with atenolol. In summary, these data demonstrate that leptin stimulates a hypothalamus-adrenal medulla-BAT axis, which is necessary and sufficient to induce lipolysis and, as a result, increase body temperature after refeeding.

Obesity-associated, but not obesity-independent, tumors respond to insulin by increasing mitochondrial glucose oxidation.

Obesity is associated with increased incidence and worse prognosis of more than one dozen tumor types; however, the molecular mechanisms for this association remain under debate. We hypothesized that insulin, which is elevated in obesity-driven insulin resistance, would increase tumor glucose oxidation in obesity-associated tumors. To test this hypothesis, we applied and validated a stable isotope method to measure the ratio of pyruvate dehydrogenase flux to citrate synthase flux (VPDH/VCS, i.e. the percent of total mitochondrial oxidation fueled by glucose) in tumor cells. Using this method, we found that three tumor cell lines associated with obesity (colon cancer [MC38], breast cancer [4T1], and prostate cancer [TRAMP-C3] cells) increase VPDH/VCS in response to physiologic concentrations of insulin. In contrast, three tumor cell lines that are not associated with obesity (melanoma [YUMM1.7], B cell lymphoma [BCL1 clone 5B1b], and small cell lung cancer [NCI-H69] cells) exhibited no oxidative response to insulin. The observed increase in glucose oxidation in response to insulin correlated with a dose-dependent increase in cell division in obesity-associated tumor cell lines when grown in insulin, whereas no alteration in cell division was seen in tumor types not associated with obesity. These data reveal that a shift in substrate preference in the setting of physiologic insulin may comprise a metabolic signature of obesity-associated tumors that differs from that of those not associated with obesity.

Dehydration and insulinopenia are necessary and sufficient for euglycemic ketoacidosis in SGLT2 inhibitor-treated rats.

Sodium-glucose transport protein 2 (SGLT2) inhibitors are a class of anti-diabetic agents; however, concerns have been raised about their potential to induce euglycemic ketoacidosis and to increase both glucose production and glucagon secretion. The mechanisms behind these alterations are unknown. Here we show that the SGLT2 inhibitor (SGLT2i) dapagliflozin promotes ketoacidosis in both healthy and type 2 diabetic rats in the setting of insulinopenia through increased plasma catecholamine and corticosterone concentrations secondary to volume depletion. These derangements increase white adipose tissue (WAT) lipolysis and hepatic acetyl-CoA content, rates of hepatic glucose production, and hepatic ketogenesis. Treatment with a loop diuretic, furosemide, under insulinopenic conditions replicates the effect of dapagliflozin and causes ketoacidosis. Furthermore, the effects of SGLT2 inhibition to promote ketoacidosis are independent from hyperglucagonemia. Taken together these data in rats identify the combination of insulinopenia and dehydration as a potential target to prevent euglycemic ketoacidosis associated with SGLT2i.

Uncoupling Hepatic Oxidative Phosphorylation Reduces Tumor Growth in Two Murine Models of Colon Cancer.

Obesity is associated with colon cancer pathogenesis, but the underlying mechanism is actively debated. Here, we confirm that diet-induced obesity promotes tumor growth in two murine colon cancer models and show that this effect is reversed by an orally administered controlled-release mitochondrial protonophore (CRMP) that acts as a liver-specific uncoupler of oxidative phosphorylation. This agent lowered circulating insulin, and the reduction of tumor growth was abrogated by an insulin infusion raising plasma insulin to the level of high-fat-fed mice. We also demonstrate that hyperinsulinemia increases glucose uptake and oxidation in vivo in tumors and that CRMP reverses these effects. This study provides evidence that perturbations of whole-organism energy balance or hepatic energy metabolism can influence neoplastic growth. Furthermore, the data show that glucose uptake and utilization by cancers in vivo are not necessarily constitutively high but rather may vary according to the hormonal milieu.

Mechanisms by which a Very-Low-Calorie Diet Reverses Hyperglycemia in a Rat Model of Type 2 Diabetes.

Caloric restriction rapidly reverses type 2 diabetes (T2D), but the mechanism(s) of this reversal are poorly understood. Here we show that 3 days of a very-low-calorie diet (VLCD, one-quarter their typical intake) lowered plasma glucose and insulin concentrations in a rat model of T2D without altering body weight. The lower plasma glucose was associated with a 30% reduction in hepatic glucose production resulting from suppression of both gluconeogenesis from pyruvate carboxylase (VPC), explained by a reduction in hepatic acetyl-CoA content, and net hepatic glycogenolysis. In addition, VLCD resulted in reductions in hepatic triglyceride and diacylglycerol content and PKCɛ translocation, associated with improved hepatic insulin sensitivity. Taken together, these data show that there are pleotropic mechanisms by which VLCD reverses hyperglycemia in a rat model of T2D, including reduced DAG-PKCɛ-induced hepatic insulin resistance, reduced hepatic glycogenolysis, and reduced hepatic acetyl-CoA content, PC flux, and gluconeogenesis.