Activity of Se-allylselenocysteine in the presence of methionine gamma-lyase on cell growth, DNA integrity, apoptosis, and cell-cycle regulatory molecules.

Se-allylselenocysteine (ASC) is effective in inhibiting mammary epithelial cell growth in vitro and mammary carcinogenesis in vivo, but its mechanism is unknown. We recently reported that ASC reduces cell growth in a dose- and time-dependent manner, induces a loss of DNA integrity, and increases apoptosis. However, the level of ASC required for growth inhibition in vitro is 10- to 20-fold higher than that required in vivo. One possible explanation for this difference is that the cells used in in vitro studies have limited lyase activity required to release the allyl Se moiety from selenocysteine, whereas animals have abundant lyase activity in tissues. In the present study, we found that methionine gamma-lyase (MGL) added to culture medium containing ASC produced biological effects with lower levels of ASC, comparable to the selenium levels in plasma achieved during in vivo chemoprevention. The combination of 2.5 microM ASC and MGL inhibited the growth of TM12 cells and increased apoptosis without loss of DNA integrity. Treatment of TM12 cells with ASC and MGL resulted in an elevation of the protein levels of p53, Cip1/p21, and Kip1/p27, concomitant with a decrease in cyclins D1 and E and modest reductions in cyclin-dependent kinase inhibitors 4 and 2. Cells treated with ASC and MGL also showed decreased phosphorylation of retinoblastoma tumor-suppressor protein. Taken together, these results suggest that a physiologically relevant concentration of ASC with MGL exerts an inhibitory effect on cell growth and that this effect is likely to involve modulation of signaling pathways that suppress the phosphorylation of retinoblastoma tumor-suppressor protein.