N-acetylation of hypothalamic α-melanocyte-stimulating hormone and regulation by leptin

Abstract

The central melanocortin system is critical in the regulation of appetite and body weight, and leptin exerts its anorexigenic actions partly by increasing hypothalamic proopiomelanocortin (POMC) expression. The POMC-derived peptide ␣-melanocytestimulating hormone (␣MSH) is a melanocortin 4 receptor agonist, and its potency in reducing energy intake is strongly increased by N-acetylation. The reason for the higher biological activity of N-acetylated ␣MSH (Act-␣MSH) compared with that of N-desacetylated ␣MSH (Des-␣MSH) is unclear, and regulation of acetylation by leptin has not been investigated. We show here that total hypothalamic ␣MSH levels are decreased in leptin-deficient ob͞ob mice and increased in leptin-treated ob͞ob and C57BL͞6J mice. The increase in total ␣MSH occurred as soon as 3 h after leptin injection and was entirely due to an increase in Act-␣MSH. Consistent with this observation, leptin rapidly induced the enzymatic activity of a N-acetyltransferase in the hypothalamus of mice. In 293T cells expressing the melanocortin 4 receptor, Act-␣MSH is far more potent than Des-␣MSH in stimulating cAMP accumulation, an effect caused by a dramatically increased stability of Act-␣MSH. Moreover, Des-␣MSH is rapidly degraded in the hypothalamus after intracerebroventricular injection in rats and was less potent in inhibiting energy intake. The results suggest that leptin activates a N-acetyltransferase in POMC neurons, leading to increased hypothalamic levels of Act-␣MSH. Due to its increased stability, this posttranslational modification of ␣MSH may play a critical role in leptin action via the central melanocortin pathway.T he adipocyte-derived hormone leptin is expressed in fat tissue and acts on the central nervous system to inform key regulatory centers about energy stores (1-3). Leptin receptors (ObRs) are highly expressed within the hypothalamus, including in proopiomelanocortin (POMC) neurons located in the arcuate nucleus (4, 5). POMC-derived neuropeptides, primarily ␣-melanocyte-stimulating hormone (␣MSH), activate the melanocortin 4 receptor (MC4R) and function downstream of leptin signaling to regulate energy balance, although other pathways are also important (6-10). Supporting the role of the melanocortin pathway in leptin action are data showing that central injection of synthetic melanocortin receptor antagonists inhibits the effect of leptin to reduce food intake (10). Furthermore, mutations in the pomc and mc4r genes result in severe obesity in mice (6, 11) and humans (7,12).Studies investigating the regulation of POMC neurons by leptin have focused mainly on gene expression analyses. It is known that fasting, which is a state of low serum leptin levels, results in reduced amounts of POMC mRNA levels. This reduction is also seen in the hypothalamus of the leptin-deficient ob͞ob mice (13-16). In addition, hypothalamic POMC mRNA is stimulated by the administration of recombinant leptin to rodents (16). However, few studies have examined regulation of hypothalamic POMC-derived peptides, such as ...

PTP1B and SHP2 in POMC neurons reciprocally regulate energy balance in mice

“…Supernatants were collected and protein concentration measured by BCA assay. Samples were acidified and peptides purified over C-18 Sep-columns (200 mg) according to the manufacturer's protocol (Phoenix Pharmaceuticals Inc.) and as described previously (108,109). The elution fraction was dried overnight in a Speed-Vac and reconstituted in 120 μl assay buffer, and αMSH levels were measured by enzyme immunoassay (Phoenix Pharmaceuticals EK-043-01).…”

Prolylcarboxypeptidase regulates food intake by inactivating α-MSH in rodents

“…5). Once produced, α-MSH 1-13 undergoes a maturation process: desacetylated α-MSH 1-13 (Des-α-MSH [1][2][3][4][5][6][7][8][9][10][11][12][13] ) is converted to N-acetylated α-MSH [1][2][3][4][5][6][7][8][9][10][11][12][13] (Act-α-MSH [1][2][3][4][5][6][7][8][9][10][11][12][13] ) by an N-acetyltransferase that has not yet been identified (5,6). The acetylation of α-MSH 1-13 may serve as a protective measure that stabilizes the peptide against degradation to prolong its action on target cells.…”

Peripherally administered desacetyl α-MSH and α-MSH both influence postnatal rat growth and associated rat hypothalamic protein expression

“…Leptin plays a critical neurotrophic role in the hypothalamus during the neonatal period studied here (8,52), and leptin also regulates desacetyl ␣-MSH and ␣-MSH levels (27). It is possible, therefore, that leptin's neurotrophic effects are in part mediated though the MSH peptides.…”

“…It is generally believed today that desacetyl ␣-MSH is less efficacious than ␣-MSH because it is more rapidly degraded compared with ␣-MSH. This reasoning is based primarily on three studies: one in rabbit plasma (57) and two studies where rat brain homogenates were shown to degrade desacetyl ␣-MSH much faster than ␣-MSH (27,50). However, rapid degradation is difficult to reconcile with the fact that desacetyl ␣-MSH exists in the brain and circulation and has been reported to have biological activities in vitro and in vivo at a point when ␣-MSH has little or no response.…”