Role of Agouti-related protein in adrenal steroidogenesis.

The levels of Agouti-related protein (AgRP) mRNA in the adrenal are second only to those in the hypothalamus, raising questions regarding its target binding sites and its specific role in adrenal steroidogenesis. We and others demonstrated the presence of a population of melanocortin receptor-4 (MC4R) positively coupled to steroidogenesis in adrenal cells. Moreover, AgRP inhibited both the acute and long-term steroidogenic responses of these cells to NDP-alphaMSH through its antagonistic properties towards MC4R. Although AgRP had no antagonistic properties towards the MC2R and did not modify the acute steroidogenic response to ACTH, it exerted a biphasic sustained inhibitory effect on the long-term response to ACTH through an undefined alternate mechanism. Since adrenal cells release a relatively large amount of AgRP, this protein likely exerts a local paracrine/autocrine control on adrenal steroidogenesis.