Modified in vivo assay for MSH

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.

Sensitive new in vitro bioassay for melanocyte-stimulating activity using the skin of Anolis carolinensis

An accurate, highly reproducible and sensitive bioassay for melanocyte-stimulating hormone (MSH) using the skin of Anolis carolinensis in vitro is described. The time taken for green Anolis skin fragments to change to a specific, visually assessed, green-brown color is dose-related, and this forms the basis of the new assay. The method is simple to perform, and 1 person may assay 20 samples in a day using the dorsal skin from a single adult lizard. The mean dose-response ranges between 48 X 10(-12) and 375 X 12(-12) M (38 to 625 pg/ml). Using the assay, alpha-MSH, beta-MSH, ACTH (4-10), and ACTH (1-10) were equipotent on a molar basis. For repeated bioassay of rat pituitary extracts, the dose-response curves were highly significant, and only 1 of the 9 pituitary dose-response curves deviated significantly from the slope of the standard alpha-MSH curve. The index of precision, lambda, for the 9 pituitary bioassays ranged between 0.037 and 0.081, while the mean 95% fiducial limits were -6.6 and 7.1% on either side of the estimated potency. The new rate method is compared with an earlier quantal method which also uses the isolated skin of Anolis carolinensis. The quantal method does not have dose-response characteristics and is therefore less accurate and reproducible than the new method; the coefficient of variation for repeated bioassay of the same pituitary extracts ranged from between 12 to 20% for the quantal method and between 2.9 to 5.7% for the new rate method.

Biodegradation of alpha-MSH and derived peptides by rat brain extracts, and by rat and human serum

The peptides alpha-MSH and MSH/ACTH 4-10 were degraded by rat brain extracts and serum to yield free amino acids among the end-products. Breakdown of these two peptides was double that of a related synthetic hexapeptide Met (0)-Glu-His-Phe-D-Lys-Phe. No significant breakdown of the hexapeptide occurred after incubation with human serum; it also had almost negligible pigmentary effects in vivo and in vitro when compared to alpha-MSH. The patterns of amino acid release indicate possible endopeptidase cleavage at Phe-Arg in alpha-MSH followed by secondary exopeptidase action to release free amino acids. For the hexapeptide, the primary cleavage point occurred at the -His3-Phe4 bond. The stability of this analog in human sera, coupled with its lower rate of degradation in the CNS, may contribute to its more potent behavioral actions in vivo.