Effect of alpha-MSH on the tyrosinase activity and the rate of melanin accumulation of melanoma cells in vitro

Aglycone of Rh4 inhibits melanin synthesis in B16 melanoma cells: possible involvement of the protein kinase A pathway

To our knowledge, there is no report that directly shows an inhibitory effect of ginsenoside on melanin synthesis in B16 melanoma cells. Hence, we investigated whether the aglycone of Rh(4) (A-Rh4) inhibits melanin synthesis in B16 melanoma cells, and determined the mechanism of melanin inhibition. We isolated 12 ginsenoside compounds from leaves of Panax ginseng and tested them in B16 melanoma cells. It significantly reduced melanin content and tyrosinase activity under alpha-melanocyte stimulating hormone- and forskolin-stimulated conditions. It significantly reduced the cyclic AMP (cAMP) level in B16 melanoma cells, and this might be responsible for the regulation down of MITF and tyrosinase. Phosphorylation of a downstream molecule, a cAMP response-element binding protein, was significantly decreased according to Western blotting and immunofluorescence assay. These data suggest that A-Rh4 has an anti-melanogenic effect via the protein kinase A pathway.

Tyrosinase activity and abundance in Cloudman melanoma cells

Rabbit anti-tyrosinase antibodies were used to study the abundance, processing, and degradation of tyrosinase in murine (Cloudman) melanoma cells. The polyclonal antibodies precipitated low-molecular-weight (68,000 and 70,000) and high-molecular-weight (78,000 and 80,000) tyrosinases that had a precursor-product relationship. Cells with high basal tyrosinase activity had high levels of newly synthesized tyrosinase. Cells with low tyrosinase activity synthesized less tyrosinase and degraded the enzyme at a faster rate than cells with high tyrosinase activity. Melanotropin (melanocyte stimulating hormone), dibutyryl cyclic adenosine monophosphate, and isobutylmethylxanthine caused an increase in the abundance of newly synthesized tyrosinase that was directly proportional to the increase in enzyme activity. This enzyme was not a phosphoprotein. Other changes in the culture conditions that increased the level of tyrosinase activity increased the abundance of newly synthesized enzyme. It is thus concluded that the level of tyrosinase activity in Cloudman melanoma cells is a direct reflection of the abundance of enzyme protein.

Application of the tyrosinase assay to normal melanocytes in culture

Using a special selection technique, normal guinea-pig melanocytes were maintained in highly purified but sparse cultures (approximately 10(4) cells/25 cm2 culture vessel) which showed little proliferative activity. The applicability of the Pomerantz tyrosinase assay was tested in this in vitro model system using three different approaches, namely crude cell extracts and viable cell cultures either in situ or in suspension. The latter modifications both proved too insensitive, whereas crude cell extracts allowed accurate measurements of the basal tyrosinase activity and its stimulation by various agents. In unstimulated cultures basal tyrosinase activities ranged from 30% to 700% (mean 260%) above the blank values; intra-assay and inter-assay variability were 4.2% and 77.5%, respectively. Stimulation with alpha-MSH (10(-5) M, 10(-6) M), beta-MSH (10(-5) M), choleratoxin (10(-11) M) and cAMP (10(-4) M) plus theophylline (10(-4) M) resulted in an increase of tyrosinase activity 30-65% above basal values. Melanotropin potentiating factor (10(-8) M) enhanced the effects of alpha-MSH (10(-6) M) by 20%. This assay modification provides a sensitive tool for comparative studies of melanogenesis in normal melanocytes, malignant melanocytes and otherwise altered melanocytes.

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.

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

The acute in vitro action of adrenocorticotropin (ACTH) and corticosterone alone and in combination were determined in the Harding-Passey (HP) melanoma grown in vivo. Hormone treated melanoma dice (5--240 min) were analyzed for tyrosinase activity, cyclic AMP (cAMP) and cyclic GMP (cGMP). ACTH elevated cAMP and cGMP levels 20- and 13-fold, respectively, in the HP melanoma. However, these large increases in cyclic nucleotide levels were accompanied by only a 49% increase in tyrosinase activity. Corticosterone elicited a similar response. ACTH plus corticosterone produced an early cAMP and cGMP peak followed by depression. ACTH plus corticosterone stimulated tyrosinase activity coincident with the early cyclic nucleotide peak followed by a drop in tyrosinase activity which was subsequently elevated. The results indicate that neither cAMP nor cGMP are the sole modulators of tyrosinase activity and suggest the interaction of ACTH, corticosterone and cyclic nucleotides in the regulation of melanoma tyrosinase activity.