Interactions between insulin and the cyclic AMP system of Cloudman S91 mouse melanoma cells

Internal binding sites for MSH: analyses in wild-type and variant Cloudman melanoma cells

Cloudman S91 mouse melanoma cells express both external (plasma membrane) and internal binding sites for MSH. Using 125I-beta melanotropin (beta-MSH) as a probe, we report here an extensive series of studies on the biological relevance of these internal sites. Cells were swollen in a hypotonic buffer and lysed, and a particulate fraction was prepared by high-speed centrifugation. This fraction was incubated with 125I-beta-MSH with or without excess nonradioactive beta-MSH in the cold for 2 hours. The material was then layered onto a step-wise sucrose gradient (8-80%) and centrifuged (156,000g, 60 min); fractions were collected and counted in a gamma counter or assayed for various enzymatic activities. The following points were established: 1) Specific binding sites for MSH were observed sedimenting at an average density of 50% sucrose in amelanotic cells and at higher densities in melanotic cells. 2) These sites were similar in density to those observed when intact cells were labeled externally with 125I-beta-MSH and then warmed to promote internalization of the hormone. 3) Most of the internal binding sites were not as dense as fully melanized melanosomes. 4) In control experiments, the MSH binding sites were not found in cultured hepatoma cells. 5) Variant melanoma cells, which differed from the wild-type in their responses to MSH, had reduced expression of internal binding sites even though their ability to bind MSH to the outer cell surface appeared normal. (MSH-induced responses included changes in tyrosinase, dopa oxidase, and dopachrome conversion factor activities, melanization, proliferation, and morphology.) 6) Isobutylmethylxanthine, which enhanced cellular responsiveness to MSH, also enhanced expression of internal binding sites. The results indicate that expression of internal binding sites for MSH is an important criterion for cellular responsiveness to the hormone.

L-tyrosine stimulates induction of tyrosinase activity by MSH and reduces cooperative interactions between MSH receptors in hamster melanoma cells

L-tyrosine, a precurosr to melanin, has recently been shown to be a regulator of the melanogenic pathway in some cultured melanoma cell lines. In this paper we demonstrated that L-tyrosine, besides increasing binding capacity for MSH, decreased cooperativity between MSH receptors and increased the level of tyrosinase induction by MSH. Apparently, regulation of MSH receptor activity by L-tyrosine involves specific changes in the interactions between the receptors and modification of the cellular responsiveness to MSH.

A defined medium for and the effect of insulin on the growth, amino acid transport, and morphology of Chinese hamster ovary cells, CHO-K1 (CCL 61) and the isolation of insulin "independent" mutants

Insulin, FeSO4, or transferrin are major requirements together with HEPES buffer and selenium for the growth of CHO-K1 (CCL 61) in a modified F12 medium (M-F12). Insulin stimulates growth at 1 ng/ml to 10 micrograms/ml. In the defined medium minus insulin, CHO-K1 grows slowly as elongated, elliptical cells in parallel arrays typical of normal diploid fibroblasts in contrast to round-to-cuboid cells in loosely overlapping arrays in the presence of serum or insulin. During prolonged incubation in the absence of insulin the cells gather up into a large spherical cluster of viable cells. Insulin "independent" mutants have been isolated whose growth rate during exponential phase in the absence of insulin (48 h to 84 or 96 hrs) is 2.7 to 3.6 times that of the parental culture. Insulin stimulates the growth of these variants only during the first 48 h and is inhibitory at 50 to 500 ng/ml during the exponential phase. Insulin induction of the A system of amino acid transport occurs in about 8 h and requires both protein and RNA synthesis.