MELATONIN INTERACTIONS WITH CULTURED MURINE B16 MELANOMA-CELLS

Both in vitro and in vivo observations have suggested that melatonin m odulates malignant cell growth. The present studies aimed to character ize the interactions of melatonin with cultured murine B16 melanoma ce lls. Time- and temperature-dependent specific melatonin accumulation b y B16 murine melanoma cells was observed. B16 cells possessed a high a ffinity binding site (K-D = 1.4 nM) which exhibited structural specifi city in its affinity for analogues of melatonin (melatonin > 6-hydroxy melatonin = N-acetyl-5-hydroxytryptamine > 5-methoxytryptamine >> 5-hy droxytryptamine). Evidence for a lower affinity uptake system without structural specificity was also observed. Ninety-five per cent of the specific cell-associated melatonin in B16 cells was present in the sol uble subcellular fraction of lysed cells; more than 97% of the cell-as sociated radioactivity was authentic melatonin. When the solubilized c ell extracts from the binding assay were analysed by gel filtration Im mediately, all of the bound counts eluted at the void volume. Continuo us exposure to melatonin for 48-120 h did not affect B16 cell prolifer ation as determined by cell counts, [4,5-dimethylthiazol-2-yl]-2,5-dip henyltetrazolium bromide assay or [H-3]thymidine incorporation. After 8-h pulse exposures to melatonin daily for 3 days, a 15% stimulation o f B16 cell proliferation (p < 0.02) was observed at melatonin concentr ations of 0.1 and 1 nM. The anti-oestrogen, tamoxifen, Inhibited B16 c ell growth and increased specific melatonin accumulation by B16 cells at 1 x 10(-6) M (p < 0.02). Cultured 816 murine melanoma cells possess ed a specific, high affinity uptake system for melatonin which appeare d to be altered by anti-oestrogen exposure.