Altered glucose homeostasis in proopiomelanocortin-null mouse mutants lacking central and peripheral melanocortin.

Prolonged obesity frequently leads to insulin resistance and, eventually, to diabetes. This relationship reflects the integration of fat stores and carbohydrate metabolism and the coordination of central nervous system functions, e.g. appetite, and peripheral metabolism. Recent work suggests that the melanocortin system is involved in this integration; specifically, central administration of melanocyte-stimulating hormone (MSH) decreases, whereas lack of central MSH signaling increases, peripheral insulin resistance. Here we asked whether MSH acting in the periphery has a complementary role in insulin resistance. We tested this in a mouse model where the proopiomelanocortin (POMC) gene encoding all of the melanocortins has been genetically deleted. The homozygous POMC-null mouse lacks central as well as peripheral MSH signaling; in addition, it lacks adrenal glands and thus is devoid of corticosterone and epinephrine. Here we report that homozygous POMC mutants have normal serum levels of insulin, normal fasting levels of glucose, and normal clearance of glucose in glucose tolerance tests. Thus, insulin production and sensitivity and glucose uptake in peripheral tissues are functioning normally. However, we found a striking inability of the homozygous POMC mutants to recover from insulin-induced hypoglycemia. This defect was in the glucagon-mediated counterregulatory response. Both peripheral administration of an MSH analog and supplementation with corticosterone alleviated the hypoglycemia after insulin challenge, but did not make the obese POMC mutant mice diabetic. We conclude that, similar to the regulation of body weight homeostasis, the regulation of glucose homeostasis requires the integration of both central and peripheral melanocortin signaling systems.

Obliteration of alpha-melanocyte-stimulating hormone derived from POMC in pituitary and brains of PC2-deficient mice.

Alpha-melanocyte-stimulating hormone (alpha-MSH) is a neuropeptide expressed in pituitary and brain that is known to regulate energy balance, appetite control, and neuroimmune functions. The biosynthesis of alpha-MSH requires proteolytic processing of the proopiomelanocortin (POMC) precursor. Therefore, this study investigated the in vivo role of the prohormone convertase 2 (PC2) processing enzyme for production of alpha-MSH in PC2-deficient mice. Specific detection of alpha-MSH utilized radioimmunoassay (RIA) that does not crossreact with the POMC precursor, and which does not crossreact with other adrenocorticotropin hormone (ACTH) and beta-endorphin peptide products derived from POMC. alpha-MSH in PC2-deficient mice was essentially obliterated in pituitary, hypothalamus, cortex, and other brain regions (collectively), compared to wild-type controls. These results demonstrate the critical requirement of PC2 for the production of alpha-MSH. The absence of alpha-MSH was accompanied by accumulation of ACTH, ACTH-containing imtermediates, and POMC precursor. ACTH was increased in pituitary and hypothalamus of PC2-deficient mice, evaluated by RIA and reversed-phase high pressure liquid chromatography (RP-HPLC). Accumulation of ACTH demonstrates its role as a PC2 substrate that can be converted for alpha-MSH production. Further analyses of POMC-derived intermediates in pituitary, conducted by denaturing western blot conditions, showed accumulation of ACTH-containing intermediates in pituitaries of PC2-deficient mice, which implicate participation of such intermediates as PC2 substrates. Moreover, accumulation of POMC was observed in PC2-deficient mice by western blots with anti-ACTH and anti-beta-endorphin. In addition, increased beta-endorphin1-31 was observed in pituitary and hypothalamus of PC2-deficient mice, suggesting beta-endorphin1-31 as a substrate for PC2 in these tissues. Overall, these studies demonstrated that the PC2 processing enzyme is critical for the in vivo production of alpha-MSH in pituitary and brain.

The facilitative effects of alpha-MSH and melanin on learning, thermoregulation, and pain in neonatal MSG-treated rats.

Administration of monosodium glutamate (MSG) in the neonatal period renders the rat to be alpha-MSH deficient later in life. In this study rats received MSG in their neonatal period and were examined at the age of 60 days. alpha-MSH caused hypothermia, potentiated induced hypothermia, blocked paradoxical behavioral thermoregulation, improved performance in the Morris water tank, but had no effect on pain threshold. Melanin only caused an increase in pain threshold. It is suggested that the differential effect of alpha-MSH and melanin is governed by the dopaminergic system.