Maziyar Saberi

Hematopoietic Cell-Specific Deletion of Toll-like Receptor 4 Ameliorates Hepatic and Adipose Tissue Insulin Resistance in High-Fat-Fed Mice

Chronic low-grade inflammation, particularly in adipose tissue, is an important modulator of obesity-induced insulin resistance. The Toll-like receptor 4 (Tlr4) is a key initiator of inflammatory responses in macrophages. We performed bone marrow transplantation (BMT) of Tlr4lps-del or control C57Bl/10J donor cells into irradiated wild-type C57Bl6 recipient mice to generate hematopoietic cell-specific Tlr4 deletion mutant (BMT-Tlr4(-/-)) and control (BMT-WT) mice. After 16 weeks of a high-fat diet (HFD), BMT-WT mice developed obesity, hyperinsulinemia, glucose intolerance, and insulin resistance. In contrast, BMT-Tlr4(-/-) mice became obese but did not develop fasting hyperinsulinemia and had improved hepatic and adipose insulin sensitivity during euglycemic clamp studies, compared to HFD BMT-WT controls. HFD BMT-Tlr4(-/-) mice also showed markedly reduced adipose tissue inflammatory markers and macrophage content. In summary, our results indicate that Tlr4 signaling in hematopoietic-derived cells is important for the development of hepatic and adipose tissue insulin resistance in obese mice.

PPARγ activation in adipocytes is sufficient for systemic insulin sensitization

Although peroxisome proliferator-activated receptor gamma (PPARγ) agonists such as thiazolidinediones (TZDs) are widely used to treat type 2 diabetes, how its activation in individual tissues contributes to TZDs therapeutic action remains controversial. As TZDs are known to have receptor-independent effects, we sought to establish gain-of-function animal models to delineate the receptors insulin-sensitizing actions. Unexpectedly, we find that selective activation of PPARγ in adipocytes, but not in macrophages, is sufficient for whole-body insulin sensitization equivalent to systemic TZD treatment. In addition to improved adipokine, inflammatory, and lipid profiles, PPARγ activation in mature adipocytes normalizes serum insulin without increased adipogenesis. Co-culture studies indicated that PPARγ-activated adipocytes broadly suppress induction of inflammatory cytokines and C-X-C family chemokines in macrophages. Collectively, these data describe an “adipocentric” model in which adipose activation of PPARγ is sufficient for complete insulin sensitization and suggest a specific application for fat selective PPARγ modulators in diabetic therapy.

Adipocyte CREB Promotes Insulin Resistance in Obesity

Increases in adiposity trigger metabolic and inflammatory changes that interfere with insulin action in peripheral tissues, culminating in beta cell failure and overt diabetes. We found that the cAMP Response Element Binding protein (CREB) is activated in adipose cells under obese conditions, where it promotes insulin resistance by triggering expression of the transcriptional repressor ATF3 and thereby downregulating expression of the adipokine hormone adiponectin as well as the insulin-sensitive glucose transporter 4 (GLUT4). Transgenic mice expressing a dominant-negative CREB transgene in adipocytes displayed increased whole-body insulin sensitivity in the contexts of diet-induced and genetic obesity, and they were protected from the development of hepatic steatosis and adipose tissue inflammation. These results indicate that adipocyte CREB provides an early signal in the progression to type 2 diabetes.

A Novel Pathway of Insulin Sensitivity in Chromogranin A Null Mice A CRUCIAL ROLE FOR PANCREASTATIN IN GLUCOSE HOMEOSTASIS

Chromogranin A (CHGA/Chga), a proprotein, widely distributed in endocrine and neuroendocrine tissues (not expressed in muscle, liver, and adipose tissues), generates at least four bioactive peptides. One of those peptides, pancreastatin (PST), has been reported to interfere with insulin action. We generated a Chga knock-out (KO) mouse by the targeted deletion of the Chga gene in neuroendocrine tissues. KO mice displayed hypertension, higher plasma catecholamine, and adipokine levels and lower IL-6 and lipid levels compared with wild type mice. Liver glycogen content was elevated, but the nitric oxide (NO) level was diminished. Glucose, insulin, and pyruvate tolerance tests and hyperinsulinemic-euglycemic clamp studies established increased insulin sensitivity in liver but decreased glucose disposal in muscle. Despite higher catecholamine and ketone body levels and muscle insulin resistance, KO mice maintained euglycemia due to increased liver insulin sensitivity. Suppressed mRNA abundance of phosphoenolpyruvate carboxykinase and glucose-6-phosphatase (G6Pase) in KO mice further support this conclusion. PST administration in KO mice stimulated phosphoenolpyruvate carboxykinase and G6Pase mRNA abundance and raised the blood glucose level. In liver cells transfected with G6Pase promoter, PST caused transcriptional activation in a protein kinase C (PKC)- and NO synthase-dependent manner. Thus, PST action may be mediated by suppressing IRS1/2-phosphatidylinositol 3-kinase-Akt-FOXO-1 signaling and insulin-induced maturation of SREBP1c by PKC and a high level of NO. The combined effects of conventional PKC and endothelial NO synthase activation by PST can suppress insulin signaling. The rise in blood PST level with age and in diabetes suggests that PST is a negative regulator of insulin sensitivity and glucose homeostasis.

The Locus for Hypoglycemic Detection Shifts With the Rate of Fall in Glycemia: The Role of Portal–Superior Mesenteric Vein Glucose Sensing

OBJECTIVE—To ascertain whether portal glucose sensing extends beyond the portal vein to the superior mesenteric vein and then test whether the role of portal–superior mesenteric glucose sensors varies with the rate of fall in glycemia. RESEARCH DESIGN AND METHODS—Chronically cannulated rats underwent afferent ablation of the portal vein (PV) or portal and superior mesenteric veins (PMV) or sham operation (control). One week later, animals underwent hyperinsulinemic-hypoglycemic clamps in which the hypoglycemic nadir, 2.48 ± 0.06 mmol/l, was reached at a rate of decline in glucose of −0.09 or −0.21 mmol · l−1 · min−1 (PMV and control only). Additional PMV and control animals received an intravenous injection of the glucopenic agent 2-deoxyglucose. RESULTS—Inducing hypoglycemia slowly, at a rate of −0.09 mmol · l−1 · min−1, resulted in a 26-fold increase in epinephrine (23.39 ± 0.62 nmol/l) and 12-fold increase in norepinephrine (11.42 ± 0.92 nmol/l) for controls (P < 0.001). The epinephrine response to hypoglycemia was suppressed by 91% in PMV (2.09 ± 0.07 nmol/l) vs. 61% in PV (9.05 ± 1.59 nmol/l) (P < 0.001). The norepinephrine response to hypoglycemia was suppressed by 94 and 80% in PMV and PV, respectively, compared with that in controls. In contrast, when arterial glucose was lowered to 2.49 ± 0.06 mmol/l within 20 min, no significant differences were observed in the catecholamine responses for PMV and controls over the first 45 min of hypoglycemia (20–65 min). Only at min 105 were catecholamines significantly lower for PMV vs. controls. Injection of 2-deoxyglucose induced a very rapid sympathoadrenal response with no significant differences between PMV and controls. CONCLUSIONS—The critical locus for hypoglycemic detection shifts away from the portal-mesenteric vein to some other loci (e.g., the brain) when hypoglycemia develops rapidly.

Novel liver-specific TORC2 siRNA corrects hyperglycemia in rodent models of type 2 diabetes

The transcription factor TORC2 [transducer of regulated cAMP-responsive element-binding protein (CREB) activity 2] is a major regulator of hepatic gluconeogenesis and is increased in hyperglycemic rodent models. Because chronic hyperglycemia and increased hepatic glucose production, via increased gluconeogenesis, is a key feature of type 2 diabetes, an effective in vivo method to efficiently knock down TORC2 could provide a potential therapy for treating hyperglycemia and type 2 diabetes. To assess this, primary mouse hepatocytes, high-fat diet (HFD)-fed mice, and Zucker diabetic fatty (ZDF) rats were treated with a siRNA against TORC2 (siTORC2), which was delivered via a novel lipid nanoparticle system, or control siRNA (siCON). Compared with siCON, administration of siTORC2 resulted in highly efficient, sustained (1-3 wk) knockdown of TORC2 and its gluconeogenic target genes phosphoenolpyruvate carboxykinase and glucose-6-phophatase in primary mouse hepatocytes and in the livers of HFD-fed mice. In mice, this knockdown was specific to the liver and did not occur in kidney, skeletal muscle, or adipose tissue. In HFD-fed mice, siTORC2 reduced in vivo gluconeogenic capacity, fasting hepatic glucose production, and hyperglycemia, and led to improved hepatic and skeletal muscle insulin sensitivity. siTORC2 treatment also improved systemic hyperglycemia in ZDF rats. In conclusion, these results demonstrate the importance of TORC2 in modulating HGP in vivo and highlight a novel, liver-specific siRNA approach for the potential treatment of hyperglycemia and type 2 diabetes.

Restoration of Euglycemia After Duodenal Bypass Surgery Is Reliant on Central and Peripheral Inputs in Zucker fa/fa Rats

Gastrointestinal bypass surgeries that result in rerouting and subsequent exclusion of nutrients from the duodenum appear to rapidly alleviate hyperglycemia and hyperinsulinemia independent of weight loss. While the mechanism(s) responsible for normalization of glucose homeostasis remains to be fully elucidated, this rapid normalization coupled with the well-known effects of vagal inputs into glucose homeostasis suggests a neurohormonally mediated mechanism. Our results show that duodenal bypass surgery on obese, insulin-resistant Zucker fa/fa rats restored insulin sensitivity in both liver and peripheral tissues independent of body weight. Restoration of normoglycemia was attributable to an enhancement in key insulin-signaling molecules, including insulin receptor substrate-2, and substrate metabolism through a multifaceted mechanism involving activation of AMP-activated protein kinase and downregulation of key regulatory genes involved in both lipid and glucose metabolism. Importantly, while central nervous system–derived vagal nerves were not essential for restoration of insulin sensitivity, rapid normalization in hepatic gluconeogenic capacity and basal hepatic glucose production required intact vagal innervation. Lastly, duodenal bypass surgery selectively altered the tissue concentration of intestinally derived glucoregulatory hormone peptides in a segment-specific manner. The present data highlight and support the significance of vagal inputs and intestinal hormone peptides toward normalization of glucose and lipid homeostasis after duodenal bypass surgery.

Activation of Hindbrain Neurons Is Mediated by Portal-Mesenteric Vein Glucosensors During Slow-Onset Hypoglycemia

Hypoglycemic detection at the portal-mesenteric vein (PMV) appears mediated by spinal afferents and is critical for the counter-regulatory response (CRR) to slow-onset, but not rapid-onset, hypoglycemia. Since rapid-onset hypoglycemia induces Fos protein expression in discrete brain regions, we hypothesized that denervation of the PMV or lesioning spinal afferents would suppress Fos expression in the dorsal medulla during slow-onset hypoglycemia, revealing a central nervous system reliance on PMV glucosensors. Rats undergoing PMV deafferentation via capsaicin, celiac-superior mesenteric ganglionectomy (CSMG), or total subdiaphragmatic vagotomy (TSV) were exposed to hyperinsulinemic–hypoglycemic clamps where glycemia was lowered slowly over 60–75 min. In response to hypoglycemia, control animals demonstrated a robust CRR along with marked Fos expression in the area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus. Fos expression was suppressed by 65–92% in capsaicin-treated animals, as was epinephrine (74%), norepinephrine (33%), and glucagon (47%). CSMG also suppressed Fos expression and CRR during slow-onset hypoglycemia, whereas TSV failed to impact either. In contrast, CSMG failed to impact upon Fos expression or the CRR during rapid-onset hypoglycemia. Peripheral glucosensory input from the PMV is therefore required for activation of hindbrain neurons and the full CRR during slow-onset hypoglycemia.