Interaction of ACTH, corticosterone and cyclic nucleotides in Harding-Passey melanoma melanogenesis

Epitope mapping of the melanosomal matrix protein gp100 (PMEL17): rapid processing in the endoplasmic reticulum and glycosylation in the early Golgi compartment

Melanosomes, specific organelles produced only by melanocytes, undergo a unique maturation process that involves their transition form amorphous rounded vesicles to fibrillar ellipsoid organelles, during which they move from the perinuclear to the distal areas of the cells. This depends upon the trafficking and processing of gp100 (also known as Pmel17 and the silver protein), a protein of great interest, because it elicits immune responses in melanoma patients but in which specific function(s) remains elusive. In this study, we have used biochemical and immunochemical approaches to more critically assess the synthesis, processing, glycosylation, and trafficking of gp100. We now report that gp100 is processed and sorted in a manner distinct from other melanosomal proteins (such as tyrosinase, Tyrp1 and Dct) and is predominantly delivered directly to immature melanosomes following its rapid processing in the endoplasmic reticulum and cis-Golgi. Following its arrival, gp100 is cleaved at the amino and at the carboxyl termini in a series of specific steps that result in the reorganization of immature melanosomes to the fibrillar mature melanosomes. Once this structural reorganization occurs, melanogenic enzymes begin to be targeted to the melanosomes, which are then competent to synthesize melanin pigment.

Human melanocytes as a target tissue for hormones: in vitro studies with 1 alpha-25, dihydroxyvitamin D3, alpha-melanocyte stimulating hormone, and beta-estradiol

Clinical evidence exists which suggests that normal pigment cell (melanocyte) function is subject to hormonal influences, but the nature of these interactions at a cellular level is poorly understood. We have investigated the effects of the vitamin D-derived secosteroid hormone 1 alpha-25, dihydroxyvitamin D3 (1,25(OH)2D3), the pituitary-derived peptide alpha-melanocyte stimulating hormone (alpha-MSH), and the sex steroid beta-estradiol on melanocytes cultured from normal human foreskin. Human melanocytes specifically internalized 1,25(OH)2D3 with high affinity (Kd 0.5-0.8 nM). Incubation with 1,25(OH)2D3 (10(-9) M) for 48 h resulted in a 100% increase in 25-hydroxyvitamin D3-24-hydroxylase activity and a 50% increase in tyrosinase activity. There was no significant effect of 1,25(OH)2D3 on intracellular cyclic adenosine monophosphate (cAMP). In contrast to 1,25(OH)2D3, alpha-MSH at a concentration of 5 X 10(-7) M caused a sevenfold increase in intracellular cAMP after 12 min but only a modest increase (less than 20%) in melanocyte tyrosinase activity after 48 h. Incubation with beta-estradiol for 24 h caused a dose-dependent increase in tyrosinase activity. The maximal response varied from 145%-213% of basal activity depending on the donor source. These results indicate that melanocytes from normal human foreskin in culture have the capacity to respond directly to several hormones. They also suggest that these cells form a useful model to study the effect of various hormones on pigment cell function.

Acute effects of two melanocytolytic agents, hydroquinone and beta-mercaptoethanolamine, upon tyrosinase activity and cyclic nucleotide levels in murine melanomas

The acute in vitro actions of two potent melanocytolytic agents, hydroquinone (HQ) and beta-mercaptoethanolamine (MEA), were determined in the B-16, Cloudman S-91 and Harding-Passey (HP) murine melanomas grown in vivo. Drug treated melanoma dice (5--480 min) were analyzed for tyrosinase activity and cyclic nucleotide levels (cAMP, cGMP). HQ and MEA effects on tyrosinase activity are complex and vary with tumor type, duration of treatment and agent tested. MEA or HQ inhibited B-16 tyrosinase activity. With combined drug therapy, low concentrations of MEA plus HQ stimulate B-16 tyrosinase activity while high concentrations of the drugs have little effect on enzymatic activity. MEA depresses tyrosinase activity while HQ elevates enzymatic activity in the S-19 melanoma. Both high and low concentrations of the combined drugs (MEA plus HQ) elicit the same response, stimulation at 10 min followed by continued depression of tyrosinase activity for the remainder of the 4 h study period. MEA initially stimulates HP tyrosinase activity followed by depression of enzymic activity. In contrast, HQ initially depresses HP tyrosinase activity followed by stimulation of enzyme activity. In combination the drugs inhibit HP tyrosinase activity. The effects of MEA and/or HQ on murine melanoma cyclic nucleotide levels are equally complex. MEA or HQ elevate cAMP and cGMP levels in all three tumors with the exception of S-91 cGMP levels which are not altered. In combination the drugs increase cyclic nucleotide levels in each of the three tumor types but at different times. No correlation is present between cyclic nucleotide levels and tyrosinase activity. Thus, the action of increased cyclic nucleotide levels in melanogenesis can not be separated from the direct actions of MEA and HQ upon melanogenesis. The divergent effects of MEA and/or HQ on tyrosinase activity and cyclic nucleotide levels in these melanomas are not correlated with the known in vivo melanocytolytic activity of these drugs. Thus, these parameters appear to be inadequate indicators of melanoma cell viability in chemotherapeutic screening of drugs effective in destroying malignant melanoma.

Effect of ACTH and corticosterone on melanogenesis and cyclic nucleotide levels in the B-16 melanoma

The acute in vitro action of ACTH and corticosterone individually and in combination were determined in the B-16 melanoma grown in vivo. ACTH elevated levels 10-fold while tyrosinase activity generally was depressed. Corticosterone depressed cAMP levels and tyrosinase activity. ACTH in the presence of corticosterone produced a coincident peak in tyrosinase activity and cAMP levels followed by a depression of enzymatic activity. The results demonstrate that cAMP is not the sole modulator of tyrosinase activity and that ACTH, corticosterone and cAMP interact in the regulation of B-16 melanoma tyrosinase activity.