Stimulation of cloudman melanoma tyrosinase activity occurs predominantly in G 2 phase of the cell cycle

Abstract

A widely accepted notion is that an increasing cellular cyclic AMP (cAMP) concentration is prerequisite for increasing tyrosinase activity and melanin synthesis and for regulating proliferation of pigment cells. α-Melanocyte stimulating hormone (α-MSH) increases cAMP and tyrosinase activity in Cloudman melanoma cells. Prostaglandins (PGs) E 1 and E 2 increase melanoma cell tyrosinase activity and inhibit proliferation. Both PGs, but not α-MSH, block the progression of Cloudman melanoma cells from G 2 phase of the cell cycle into M or G 1. Only PGE 1 and not PGE 2 causes an elevation of cellular cAMP concentrations. The adenylate cyclase inhibitor 2′,5′-dideoxyadenosine (DDA) at 5 × 10 −4 M effectively blocks the increased cAMP synthesis by cells treated with 10 μg/ml PGE 1. The addition of DDA, however, enhances the melanogenic response of melanoma cells to 10 μg/ml PGE 1 or PGE 2, 10 −7 M α-MSH, 10 −4 M isobutylmethylxanthine, 10 −4 M dibutyryl cyclic AMP. DDA also augments the effects of PGE 1 or PGE 2 on the melanoma cell cycle. Moreover, when DDA is added concomitantly with α-MSH, more cells are recruited into G 2 than observed in untreated controls. Neither α-MSH nor DDA alone has any effect on the cell cycle. These findings undermine the role of cAMP in the melanogenic process and suggest that blocking melanoma cells in G 2 may be required for the remarkable stimulation of tyrosinase activity observed with PGE 1 or PGE 2 alone or in combination with DDA. The observed block in G 2 may be essential for the synthesis of sufficient mRNA, which is required for stimulation of tyrosinase activity.