Enhanced sensitivity of human oral carcinomas to induction of apoptosis byselenium compounds: Involvement of mitogen-activated protein kinase and Fas pathways

Prospective studies and recent intervention trials suggest that the risk of some cancers, including respiratory tract cancers, may be inversely relate d to selenium (SE) intake, and this is supported by strong experimental evi dence with chemical-induced animal cancer models. How this cancer-protectiv e effect is mediated is unclear, but interference with the balance of growt h/apoptosis during tumor outgrowth is one plausible hypothesis. In general, there is a correlation between the effectiveness of SE compounds as chemop reventive agents in vivo and their ability to inhibit cell growth and induc e apoptosis in vitro. This study has investigated the signal transduction p athways affected by SE compounds in biopsies of normal human oral mucosa ce lls and human oral squamous carcinoma cells (SCCs), using a primary culture system. Two SE compounds were tested: selenodiglutathione (SDG), the prima ry metabolite of selenite and the most commonly used cancer-protective SE c ompound in animal models, and the synthetic SE compound, 1,4-phenylenebis(m ethylene)selenocyanate (p-XSC), one of the most potent chemopreventive phar macological SE compounds. Three novel findings are reported: (a) SCCs were found to be significantly more sensitive to induction of apoptosis by SDG t han normal human oral mucosa cells, though the differences were marginal wi th p-XSC; (b) both SE compounds induced the expression of Fas ligand (Fas-L ) in oral cells to a degree that correlated with the extent of apoptosis in duction; and (c) both SDG and p-XSC induced the stress pathway kinases, Jun NH2-terminal kinase (JNK) and p38 kinase, at concentrations causing apopto sis; p-XSC, and to a lesser extent SDG, also activated extracellular regula ted kinases 1&2 (ERKs 1&2) and protein kinase-B or Akt. To test their funct ional involvement, the effect of inhibiting each of these pathways on induc tion of apoptosis: by SDG and p-XSC was determined in SCCs. Inhibiting the ERKs 1&2 or Akt pathways with specific chemical inhibitors (PD98059 or LY29 4002, respectively) did not affect the extent of apoptosis induced by SDG o r p-XSC (with the exception of LY294002, which actually enhanced the level of induction of apoptosis by SDG). The JNK pathway appeared to be most impo rtant for induction of Fas-L and apoptosis because concentrations of SB2021 90 that inhibited activation of both the JNK and p38 kinase (but not ERKs 1 &2) in SCC reduced the extent of induction of Fas-L and apoptosis by SDG an d p-XSC, whereas lower concentrations that inhibited activation only of p38 kinase did not. This was confirmed by the fact that exogenous expression o f a dominant negative deletion mutant of c-Jun (TAM67) reduced the inductio n of both apoptosis and Fas-L by SDG.