THE EFFECTS OF TEMPERATURE ON THE RESPONSE TO MELANOCYTE-STIMULATING HORMONE

Use of an alpha-melanocyte-stimulating hormone analogue to improve alpha-melanocyte-stimulating hormone receptor binding assay in human melanoma

In order to optimize the detection and measurement of alpha-melanocyte-stimulating hormone (alpha-MSH) receptivity in human melanoma cells, and the authors replaced the natural hormone by [Nle4,D-Phe7]-alpha-MSH, a more stable and potent analogue in the receptor binding assay commonly performed with alpha-MSH. The following parameters were investigated: temperature, incubation time, number of cells, and ratio of labelled to unlabelled hormone. Optimal conditions for each assay were determined. The results demonstrate that the analogue has identical binding sites to alpha-MSH, as similar reciprocal displacements of each labelled (125I) hormone by serial dilutions of unlabelled alpha-MSH or [Nle4,D-Phe7]-alpha-MSH (10(-12) M to 10(-6) M) were obtained. To further compare the two hormones, we performed a screening of various human cell lines: ten melanomas and five nonmelanomas. The assay with [Nle4,D-Phe7]-alpha-MSH yielded more receptor expression on six of ten melanoma lines against only four of ten with the natural hormone. In conclusion, the use of radiolabelled [Nle4,D-Phe7]-alpha-MSH analogue instead of labelled alpha-MSH improved both sensitivity and reproducibility in this receptor binding assay on human melanoma lines.

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

The presence of alpha-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 alpha-MSH on human melanoma cells, using a high-specific-activity 125I-labelled hormone (1.5 to 2 mCi/micrograms) with consistent receptor binding (usually exceeding 2 pg/10(6) cells) and stable for 3 weeks. Asynchronized cells in suspension were incubated for 15 min at 37 degrees C with the tracer and various concentrations of unlabelled hormones. Synthetic alpha-MSH was compared to beta-MSH, ACTH1-24, ACTH4-10, beta-LPH, CLIP, CRF, MIF I, A8VP and beta-endorphin. Out of a panel of 8 human melanoma cell lines, 3 showed specific and reproducible alpha-MSH binding curves. No significant binding to human fibroblast and human carcinoma cells was seen. alpha-MSH, beta-MSH and, to a lesser extent ACTH4-10 (a part of the alpha-MSH sequence) were the only peptides able to displace labelled alpha-MSH from its binding sites, indicating the high specificity of the MSH receptor. Affinity constants (Ka) ranged from 10(8) to 10(9) l/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.