Evidence for alpha‐melanocyte‐stimulating hormone (α‐MSH) receptors on human malignant melanoma cells

Abstract

The presence of α-MSH receptors on human melanoma has so far been suggested in the literature but not proved. We describe a reproducible and specific binding assay of α-MSH on human melanoma cells, using a high-specific-activity ¹²⁵I-labelled hormone (1.5 to 2 mCi/μg) with consistent receptor binding (usually exceeding 2 pg/10⁶cells) and stable for 3 weeks. Asynchronized cells in suspension were incubated for 15 min at 37°C with the tracer and various concentrations of unlabelled hormones. Synthetic α-MSH was compared to β-MSH, ACTH^1-24, ACTH^4–10, β-LPH, CLIP, CRF, MIF I, A⁸VP and β-endorphin. Out of a panel of 8 human melanoma cell lines, 3 showed specific and reproducible α-MSH binding curves. No significant binding to human fibroblast and human carcinoma cells was seen. α-MSH, β-MSH and, to a lesser extent ACTH^4–10 (a part of the α-MSH sequence) were the only peptides able to displace labelled α-MSH from its binding sites, indicating the high specificity of the MSH receptor. Affinity constants (Ka) ranged from 10⁸ to 10⁹ I/mole and the estimated receptor number was 1,000 to 2,000 per cell. We conclude that some human melanoma cell lines expressed specific MSH receptors with stable affinity but which are low in number.