Antagonist and agonist activities of chemically synthetized mouse agouti protein fragment (91-131) (AP91-131) at the melanocortin type-1 receptor (MCl-R) were assessed using B16-F1 mouse melanoma cells in vitro and the following assay systems: (i) receptor binding, (ii) adenylate cyclase, (iii) tyrosinase, (iv) melanin production, and (v) cell proliferation. In competition binding studies AP91-131 was about 3-fold less potent than the natural agonist α-melanocyte-stimulating hormone (α-MSH) in displacing the radioligand [125I]-[Nle4, D-Phe7]-α-MSH (Ki 6.5 ± 0.8 nmol/1), α-MSH-induced tyrosinase activation and melanin production were completely inhibited by a 100fold higher concentration of AP91-131; the IC50 values for AP91-131 in the two assay systems were 91 ± 22 nM and 95 ± 15 nM respectively. Basal melanin production and adenylate cyclase activity in the absence of agonist were decreased by AP91-131 with IC50 values of 9.64 ± 1.8 nM and 5.04 ± 2.4 nM, respectively. This indicates inverse agonist activity of AP91-131 similar to that of native AR The presence of 10 nM melanin-concentrating hormone (MCH) slightly potentiated the inhibitory activity of AP91-131 in the adenylate cyclase and melanin assays. On the other hand, AP91-131 inhibited cell growth similar to α-MSH (IC50 11.0 ± 2.1 nM; maximal inhibition 1.8-fold higher than that of α-MSH). Furthermore, MC1-R was down-regulated by AP91-131 with about the same potency and time-course as with α-MSH. These results demonstrate that AP91-131 displays both agonist and antagonist activities at the MC1-R and hence that it is the cysteine-rich region of agouti protein which inhibits and mimics the different α-MSH functions, most likely by simultaneous modulation of different intracellular signalling pathways.
|Number of pages||21|
|Journal||Journal of Receptor and Signal Transduction Research|
|Publication status||Published - Nov 9 2001|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology