EFFECT OF SELENIUM-COMPOUNDS ON MURINE B16 MELANOMA-CELLS AND PIGMENTED CLONED PB16 CELLS

The effects of selenium compounds such as sodium selenite, sodium sele nate, seleno-DL-cystine and seleno-DL-methionine (100 mu M and 10 mu M ) on B16 and pigmented cloned pB16 murine melanoma cells were investig ated in vitro. At the tested concentrations, B16 cells showed a greate r sensitivity to the toxic effects of sodium selenite and seleno-DL-cy stine than pB16 cells, whereas no decrease of B16 and pB16 cell number was observed after incubation with sodium selenate or seleno-DL-methi onine. Glutathione (GSH) percentages were strongly decreased only by s elenite and seleno-DL-cystine; it was marked more in B16 than in pB16 cells. The pretreatment of B16 cells with a GSH depleting agent (10 mu M buthionine-[S,R]-sulfoximine) did not significantly influence the c ytotoxic effects of selenite and seleno-DL-cystine. On both cell popul ations, GSH preincubation (50 mu M) enhanced the cytotoxicity of selen ite whereas the survival of seleno-DL-cystine treated cells was increa sed. Glutathione peroxidase (GSH-Px) activity in B16 cells was more se nsitive than in pB16 cells to the activating effect of selenite, and p articularly of seleno-DL-cystine; however, cell-free controls indicate d that activation was mainly due to glutathione reductase. The rate of Se-75 (as sodium selenite) uptake in both cell populations was maxima l within the first hour of incubation, with a preferential accumulatio n in the cytosol; after 24 h of incubation, the amount of Se-75 in cyt osol and pellet was approximately the same. Gel filtration chromatogra phy of lysed cells after incubation for 6 h with 10 mu M Se-75-selenit e showed that the radioactivity was eluted as two peaks corresponding to low (4-9 kDa) and high (280-320 kDa) molecular weights. Possible to xicological mechanisms are discussed at molecular level. For selenite, a major involvement of GSH is proposed, with production of selenodigl utathione and selenopersulfide, which should be directly responsible o f the decrease in cell number, thiol oxidation and protein synthesis i nhibition. For selenocystine, an active selenol species (Cy-Se-) is al so hypothesized as being responsible for thiol oxidation and mutagenic effects. For both compounds oxygen active species could also be forme d; however, a relevant role of GSH-Px was not apparent. The minor sens itivity of pB16 cells to the toxic effects of selenite and seleno-DL-c ystine could be explained by the smaller depletion of GSH induced by t hose compounds in pB16 cells, a minor formation of selenium active spe cies, the larger amount present of the oxyradical scavenger melanin, t he secretion of some mitogenic factor by pB16 cells and/or a greater r esistance to autocrine cytotoxic factors.