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Effects of recombinant agouti-signaling protein on melanocortin action
Y K Yang1, M M Ollmann, B D Wilson
1Department of Internal Medicine, University of Michigan, Ann Arbor 48109-0682, USA.
Abstract:
Mouse agouti protein is a paracrine signaling molecule that has previously been demonstrated to be an antagonist of melanocortin action at several cloned rodent and human melanocortin receptors. In this study we report the effects of agouti-signaling protein (ASIP), the human homolog of mouse agouti, on the action of alpha-MSH or ACTH at the five known human melanocortin receptor subtypes (hMCR 1-5). When stably expressed in L cells (hMC1R, hMC3R, hMC4R, hMC5R) or in the adrenocortical cell line OS3 (hMC1R, hMC2R, hMC4R), purified recombinant ASIP inhibits the generation of cAMP stimulated by alpha-MSH (hMC1R, hMC3R, hMC4R, hMC5R) or by ACTH (hMC2R). However, dose-response and Schild analysis indicated that the degree of ASIP inhibition varied significantly among the receptor subtypes; ASIP is a potent inhibitor of the hMC1R, hMC2R, and hMC4R, but has relatively weak effects at the hMC3R and hMC5R. These analyses also indicated that the apparent mechanism of ASIP antagonism varied among receptor subtypes, with characteristics consistent with competitive antagonism observed only at the hMC1R, and more complex behavior observed at the other receptors. ASIP inhibition at these latter receptors, nonetheless, can be classified as surmountable (hMC3R, hMC4R and hMC5R) or nonsurmountable (hMC2R). Recombinant ASIP also inhibited binding of radiolabeled melanocortins, [125I-Nle4, D-Phe7] alpha-MSH and [125I-Phe2, Nle4]ACTH 1-24, to the hMCR 1-5 receptors, with a relative efficacy that paralleled the ability of ASIP to inhibit cAMP accumulation at the hMC1R, hMC2R, hMC3R, and hMC4R. These results provide new insight into the biochemical mechanism of ASIP action and suggest that ASIP may play an important role in modulating melanocortin signaling in humans.
Insights
Agouti-signaling protein (ASIP) antagonizes human melanocortin receptors, with potent effects on MC1R, MC2R, and MC4R. Its inhibitory mechanism varies, impacting cAMP signaling and receptor binding.
Area of Science:
- Endocrinology
- Molecular Pharmacology
- Cell Signaling
Background:
- Agouti-signaling protein (ASIP) is the human homolog of mouse agouti, a known antagonist of melanocortin signaling.
- Melanocortins, including alpha-MSH and ACTH, regulate various physiological processes via five human melanocortin receptor (hMCR) subtypes.
Purpose of the Study:
- To investigate the effects of ASIP on the five human melanocortin receptor subtypes (hMCR 1-5).
- To characterize the mechanism and potency of ASIP antagonism across different hMCRs.
Main Methods:
- Recombinant ASIP was used to treat cell lines expressing specific hMCRs (L cells and OS3 cells).
- Inhibition of cAMP generation stimulated by alpha-MSH or ACTH was measured.
- Dose-response and Schild analyses were performed.
- Binding assays with radiolabeled melanocortins were conducted.
Main Results:
- ASIP inhibited cAMP accumulation stimulated by alpha-MSH (hMC1R, hMC3R, hMC4R, hMC5R) and ACTH (hMC2R).
- ASIP was a potent inhibitor of hMC1R, hMC2R, and hMC4R, but weakly affected hMC3R and hMC5R.
- Antagonism mechanisms varied: competitive at hMC1R, surmountable at hMC3R, hMC4R, hMC5R, and nonsurmountable at hMC2R.
- ASIP inhibited radiolabeled melanocortin binding to hMCRs, correlating with its cAMP inhibition efficacy.
Conclusions:
- ASIP exhibits differential antagonism of human melanocortin receptors.
- The mechanism of ASIP action is complex and receptor-subtype specific.
- ASIP plays a significant role in modulating human melanocortin signaling pathways.