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.