Michael P. Osber

Phosphorylated isomers of L-dopa stimulate MSH binding capacity and responsiveness to MSH in cultured melanoma cells

L-dopa is a key metabolite in the process of melanogenesis. However, it is difficult to use in biological experiments because it is subject to auto-oxidation and relatively insoluble at neutral pH. Dopa phosphates contain phosphate ester linkages at positions 3 and/or 4 of the phenylalanine ring of L-dopa, rendering them highly soluble and stable to auto-oxidation when compared to L-dopa. Dopa phosphates are readily taken up by melanoma cells in culture and converted to L-dopa and inorganic phosphate by cellular phosphatases, making them useful for studying L-dopa effects in vivo. Here we investigated the effects of dopa phosphates on receptors for MSH in cultured melanoma cells. We found that dopa phosphates caused a 3-fold stimulation of MSH binding capacity by the cells which probably occurred through an increase in the number of receptors for MSH with no apparent change in affinity of the receptors. The increased binding capacity for MSH was followed by increased cellular tyrosinase activity and melanogenesis. Thus dopa phosphates and/or L-dopa can act as regulators of the MSH receptor system. The observations suggest a novel mechanism for regulation of hormonal responsiveness: hormonal signal amplification by a metabolite in the target pathway.

Enhancement of the depigmenting effect of hydroquinone by cystamine and buthionine sulfoximine

Glutathione (GSH) performs several important biological functions, including quenching of reactive oxygen species, and protection of cells from toxic compounds such as quinones. The first step in the synthesis of GSH is catalysed by γ‐glutamylcysteine synthetase, an enzyme which is inhibited by cystamine and buthionine sulfoximine (BSO). In this study, we examined the possibility that the effect of hydroquinone (HQ) on pigmentation could be potentiated by inhibiting the production of GSH. In vitro studies using melanoma cell lines demonstrated that both cystamine and BSO could potentiate the inhibitory effects of HQ on tyrosinase activity and melanin content. A synergistic decrease in hair pigmentation was observed when a combination of HQ (2 or 4%) and BSO (5%) was applied to the dorsal skin of C57BL mice. In black hairless guinea‐pigs, the application of HQ plus either BSO or cystamine resulted in a significant decrease in epidermal pigmentation when compared with any of the agents alone. The possibility exists that in the future a combination of HQ plus cystamine or BSO could be used to treat disorders such as melasma and post‐Inflammatory hyperpigmentation.

The influence of retinoids on cultivated human keratinocytes

Cultured keratinocytes afford an excellent opportunity to study the influence of retinoids on the behavior of a stratified squamous epithelium and the interaction of keratinocytes with substrate. We have found that all-trans-retinoic acid retards the formation of colonies, dose not influence attachment, and causes increased shedding of cells from the cultures. Retinoids do not influence the relative abundance of the keratin polypeptides. Our observations are on human neonatal foreskin-derived keratinocytes grown in Dulbeccos modified Eagle medium containing 20% fetal bovine serum. Because fetal bovine serum contains vitamin A, our findings represent differences between low and high levels of vitamin A.

Keratins in Cultivated Human Keratinocytes Are Stable

Cultures of human keratinocytes were established according to the technique of Rheinwald and Green. When cultures were exposed to [14C] leucine, the uptake of leucine and increase in the specific activity of 6 urea-extractable polypeptides was prompt-each keratin achieved 50% of its peak specific activity in 3-6 hours. Cultures were exposed to [14C] leucine for 6 hours and then permitted to grow in unlabeled medium for 10 days. These confluent cultures shed cells into the medium; the amount or protein shed daily was 22.2 microgram or roughly 0.9% of the protein of the attached cells. Thus, protein shed into the medium over a 10-day period of the pulse-chase experiment was 9% of the total extractable protein. The specific activity of individual polypeptides extracted by urea fell an average of 25% during the 10-day chase. Polypeptides extracted by buffer A showed a fall in specific activity of 55% over this period. The relative amounts of individual urea-extractable polypeptides and individual buffer A-extractable polypeptides remained constant over a 10-day period. The rapid labelling of all urea-extractable polypeptides and the relative stability in the specific activity of these polypeptides is evidence that one keratin is not modified to form another keratin and that, once synthesized, the molecules are stable.

Synthesis of 125I-Labeled β-Melanotropin and Assay of Melanotropin Receptors

Publisher Summary This chapter focuses on the process of synthesis of 125 I-labeled β-melanotropin and assay of melanotropin receptors. The melanotropins exist in three forms: (1) α, (2) β, and (3) γ. They are all small peptide hormones, ranging in size from 12 to 18 amino acids, and in vertebrates are products primarily of the intermediate lobe of the pituitary gland. Melanocyte-stimulating hormones (MSH) receptors are stimulated by the same levels of ultraviolet light that stimulate mammalian melanogenesis, and MSH acts synergistically with UV light to stimulate melanogenesis in mice and guinea pigs. These observations suggest that a primary effect of UV light on mammalian skin may be to stimulate the MSH receptor system that, in turn, enhances cellular responsiveness to NSH as a mechanism for increasing melanin formation. Cultured melanoma cells express internal binding sites and cell surface binding sites for MSH. The internal sites are also regulated by UV light and their expression is greatly reduced in mutant cell lines that exhibit poor responsiveness to MSH.