Sympathetic response to insulin is mediated by melanocortin 3/4 receptors in the hypothalamic paraventricular nucleus

NOX4 pathway as a source of selective insulin resistance and responsiveness

OBJECTIVE

Type 2 diabetes mellitus and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis; hyperglycemia, which causes microvascular disease; and hypertension. These features share a common, but unexplained, origin-namely, pathway-selective insulin resistance and responsiveness. Here, we undertook a comprehensive characterization of pathway-selective insulin resistance and responsiveness in liver and hepatocytes by examining 18 downstream targets of the insulin receptor, surveying the AKT, ERK, and NAD(P)H oxidase 4 pathways.

METHODS AND RESULTS

Injection of insulin into hyperphagic, obese type 2 diabetic db/db mice failed to inactivate hepatic protein tyrosine phosphatase gene family members, a crucial action of NAD(P)H oxidase 4 previously thought to be required for all signaling through AKT and ERK. Insulin-stimulated type 2 diabetic livers unexpectedly produced an unusual form of AKT that was phosphorylated at Thr308 (pT308), with only weak insulin-stimulated phosphorylation at Ser473. Remarkably, knockdown or inhibition of NAD(P)H oxidase 4 in cultured hepatocytes recapitulated the entire complicated pattern of pathway-selective insulin resistance and responsiveness seen in vivo-namely, monophosphorylated pT308-AKT, impaired insulin-stimulated pathways for lowering plasma lipids and glucose, but continued lipogenic pathways and robust ERK activation.

CONCLUSIONS

Functional disturbance of a single molecule, NAD(P)H oxidase 4, is sufficient to induce the key harmful features of deranged insulin signaling in type 2 diabetes mellitus, obesity, and other conditions associated with hyperinsulinemia and pathway-selective insulin resistance and responsiveness.

Ablation of LMO4 in glutamatergic neurons impairs leptin control of fat metabolism

The LIM domain only 4 (LMO4) protein is expressed in the hypothalamus, but its function there is not known. Using mice with LMO4 ablated in postnatal glutamatergic neurons, including most neurons of the paraventricular (PVN) and ventromedial (VMH) hypothalamic nuclei where LMO4 is expressed, we asked whether LMO4 is required for metabolic homeostasis. LMO4 mutant mice exhibited early onset adiposity. These mice had reduced energy expenditure and impaired thermogenesis together with reduced sympathetic outflow to adipose tissues. The peptide hormone leptin, produced from adipocytes, activates Jak/Stat3 signaling at the hypothalamus to control food intake, energy expenditure, and fat metabolism. Intracerebroventricular infusion of leptin suppressed feeding similarly in LMO4 mutant and control mice. However, leptin-induced fat loss was impaired and activation of Stat3 in the VMH was blunted in these mice. Thus, our study identifies LMO4 as a novel modulator of leptin function in selective hypothalamic nuclei to regulate fat metabolism.

Role of brainstem GABAergic signaling in central cannabinoid receptor evoked sympathoexcitation and pressor responses in conscious rats

The mechanisms implicated in the sympathoexcitation and pressor responses elicited by central CB₁R activation are not fully understood. Further, the few reported mechanistic studies on this endeavor were conducted in anesthetized rats. Therefore, it was important to identify the dose-related cardiovascular responses elicited by central administration of the cannabinoid receptor (CB₁R) agonist WIN55,212-2 in conscious rats. The second and main objective of the study was to test the hypothesis that brainstem GABAergic transmission is implicated in the CB₁R-evoked sympathoexcitation/pressor response. In conscious rats, intracisternal (i.c) WIN55,212-2 (3, 10, 30 μg/rat) elicited dose-dependent increases in mean arterial pressure (MAP) and plasma norepinephrine (NE; index of sympathoexcitation), and reduced heart rate (HR). Subsequent neurochemical studies showed that i.c WIN55,212-2 (15 μg/rat) significantly increased the number and percentage of neurons that exhibited dual immunostaining for tyrosine hydroxylase (catecholaminergic neurons) and c-Fos (marker of neuronal activity) within the rostral ventrolateral medulla, which suggests enhanced central sympathetic tone. These neurochemical responses along with the increases in MAP and plasma NE were drastically attenuated by prior: (i) blockade of central CB₁R by i.c AM251 (30 μg/rat) or (ii) activation of central GABA(A)R by i.c muscimol (0.1 μg/rat). Collectively, these neurochemical and cardiovascular findings are the first to suggest a pivotal role for the inhibition of brainstem GABAergic transmission in the central CB₁R-evoked sympathoexcitation/pressor response in conscious rats.