Intracellular 58-kd selenoprotein levels correlate with inhibition of DNA synthesis in mammary epithelial cells.

Five mouse mammary epithelial cell lines (MMEC) with different growth rates were used to examine the relationship between selenoprotein levels and selenite-mediated inhibition of DNA synthesis. The inhibition of DNA synthesis preceded and was significantly greater than inhibition of protein synthesis. Cycloheximide, a protein synthesis inhibitor, caused a coordinate inhibition of both DNA and protein synthesis over a 50-fold dose range. Of the selenoproteins detected by one-dimensional SDS-PAGE, the 58-, 26- and 23-kd proteins were the only major selenoproteins observed in common among the five MMEC lines before and during inhibition of DNA synthesis. Other selenoproteins were present in some cell lines or after inhibition of DNA synthesis. The level of the 58-kd selenoprotein was most closely correlated with the degree of inhibition of DNA synthesis (r2 = 0.85), whereas the 26- and 23-kd proteins most closely correlated with selenite retention (r2 = 0.78). Upon selenite withdrawal from the growth medium, the decrease in 58-kd, but not in the 26- and 23-kd proteins correlated with resumption of DNA synthesis. Similarly, dose-response studies indicated that the 58-kd protein increased greater than 20-fold, whereas the 26- and 23-kd proteins increased only 5-fold. At high doses of selenite, other selenoproteins of mol. wts 30-101 kd were present but these proteins seemed to appear after inhibition of DNA synthesis. The possibility exists that these proteins may be product/precursors of the major selenoproteins. Since these experiments attempted to quantitate selenoproteins by densitometry of one-dimensional SDS-PAGE autoradiographs, the quality controls used for the experiments are discussed. The results are discussed in terms of the hypothesis that the 58-kd selenoproteins may mediate the effects of selenite on DNA synthesis.