Alteration of the Cloudman melanoma cell cycle by prostaglandins E1 and E2 determined by using a 5‐bromo‐2′‐deoxyuridine method of DNA analysis

Prostaglandins (PGs) E1 and E2 stimulate tyrosinase activity and suppress the proliferation of Cloudman S91 melanoma cells by altering their progression through the cell cycle. Prostaglandin E1 and PGE2 have prolonged or residual effects on melanoma cells. Cells treated for 5 or 24 hours with 10 μg/ml PGE2 demonstrated decreased proliferation and increased tyrosinase activity for 48 hours after removal of the PGs. The effects of PGs on the cell cycle were investigated by determining total DNA content in cells stained with propidium iodide (Pl) and analyzed by a fluorescence activated cell sorter (FACS). Prostaglandin E1 blocked cells in G2 phase after 5 hours of treatment, corresponding to when inhibition of proliferation was first evident. Similarly, after 9 hours of treatment with PGE2, more cells were in late S, early G2 phase and less in G1 than their control counterparts. Also, melanoma cells were pulse‐labeled with 5‐bromo‐2′‐deoxyuridine (BrdUrd) prior to or at the end of PG treatment and then stained with a fluoresceinated monoclonal antibody to BrdUrd, and with Pl. This allows one to observe how BrdUrd‐labeled S‐phase cells cycle with time. Both PGE1 and PGE2 inhibit proliferation by blocking cells in G2 phase of the cell cycle. The PG‐induced block in G2 may be required by melanoma cells to synthesize mRNA and proteins that are essential for stimulation of tyrosinase activity. Ultrastructurally, only a subpopulation of the cells treated with PGE1 or PGE2 contained more mature melanosomes than control cells.