Regulation of gene expression in the hypothalamus for energy homeostasis
The objectives of the present study were: firstly, to determine whether leptin could influence the melanocortin receptor (MC-R) associated signalling; and secondly, to obtain further evidence of a role in energy balance for the transcription factors, neurological helix-loop-helix-2 (Nhlh-2), neurogenic differentiation-1 (NeuroD-1), and single minded-1 (Sim-1) as well as the precursor-protein convertase (PC) proteins, PC1, PC2, paired basic amino acid cleaving enzyme 4 (PACE4) and PC2 associated neuroendocrine protein 7B2. To investigate the role of leptin in the MC-R associated signalling cascade, the pancreatic cell line CRI-G1 was stably transfected with the human MC4-R DNA. The stimulation of the cells with a receptor agonist caused an increase in cAMP level and c-fos gene expression, which were unaffected by leptin stimulation. The data suggest that the leptin signalling, through its receptor, does not influence either the cAMP level or the c-fos gene expression induced by MC4-R in CRI-G1 cells. To obtain evidence of the involvement of the transcription factors, PCs and 7B2 in the regulation of energy balance, the gene expressions of these factors were analysed by in situ hybridisation in the adult mouse hypothalamus in relation to obese (ob) or diabetes (db) gene mutation, energy deficit and administration of leptin. The results demonstrate that Nhlh-2 and NeuroD-1 are involved in energy balance regulation, since both genes appeared to be indirectly regulated by leptin and since a potential functional relationship was obtained between Nhlh-2 and pro-opiomelanocortin in the arcuate nucleus. Although PC1 gene expression, by contrast to other related candidates, was differentially altered in distinct hypothalamic nuclei by the experimental manipulations, there was no strong and consistent link between PC1 gene expression and energy balance, suggesting that PC1 gene expression is not regulated for energy homeostasis. A potential functional relationship between PC1 and prepro-orexin in the lateral hypothalamus was also demonstrated.