Inhibition of methionine adenosyltransferase II induces FasL expression, Fas-DISC formation and caspase-8-dependent apoptotic death in T leukemic cells.

Methionine adenosyltransferase II (MAT II) is a key enzyme in cellular metabolism and catalyzes the formation of S-adenosylmethionine (SAMe) from L-methionine and ATP. Normal resting T lymphocytes have minimal MAT II activity, whereas activated proliferating T lymphocytes and transformed T leukemic cells show significantly enhanced MAT II activity. This work was carried out to examine the role of MAT II activity and SAMe biosynthesis in the survival of leukemic T cells. Inhibition of MAT II and the resultant decrease in SAMe levels enhanced expression of FasL mRNA and protein, and induced DISC (Death Inducing Signaling Complex) formation with FADD (Fas-associated Death Domain) and procaspase-8 recruitment, as well as concomitant increase in caspase-8 activation and decrease in c-FLIP(s) levels. Fas-initiated signaling induced by MAT II inhibition was observed to link to the mitochondrial pathway via Bid cleavage and to ultimately lead to increased caspase-3 activation and DNA fragmentation in these cells. Furthermore, blocking MAT 2A mRNA expression, which encodes the catalytic subunits of MAT II, using a small-interfering RNA approach enhanced FasL expression and cell death, validating the essential nature of MAT II activity in the survival of T leukemic cells.

Ethanol inhibits methionine adenosyltransferase II activity and S-adenosylmethionine biosynthesis and enhances caspase-3-dependent cell death in T lymphocytes: relevance to alcohol-induced immunosuppression.

An important aspect in alcohol abuse-associated immune suppression is the loss of T helper CD4(+) lymphocytes, leading to impairment of multiple immune functions. Our work has shown that ethanol can sensitize CD4(+) T lymphocytes to caspase-3-dependent activation-induced cell death (AICD). It has been demonstrated that the formation of S-adenosylmethionine (SAMe) catalyzed by methionine adenosyltransferase (MAT) II is essential for CD4(+) T-cell activation and proliferation. Since ethanol is known to affect SAMe metabolism in hepatocytes, we investigated the effect of ethanol on MAT II activity/expression, SAMe biosynthesis and cell survival in CD4(+) T lymphocytes. We demonstrate for the first time that ethanol at a physiologically relevant concentration (25 mM) substantially decreased the enzymatic activity of MAT II in T lymphocytes. Ethanol was observed to decrease the transcription of MAT2A, which encodes the catalytic subunit of MAT II and is vital for MAT II activity and SAMe biosynthesis. Furthermore, correspondent to its effect on MAT II, ethanol decreased intracellular SAMe levels and enhanced caspase-3-dependent AICD. Importantly, restoration of intracellular SAMe levels by exogenous SAMe supplementation considerably decreased both caspase-3 activity and apoptotic death in T lymphocytes. In conclusion, our data show that MAT II and SAMe are critical molecular components essential for CD4(+) T-cell survival that are affected by ethanol, leading to enhanced AICD. Furthermore, these studies provide a clinical paradigm for the development of much needed therapy using SAMe supplementation in the treatment of immune dysfunction induced by alcohol abuse.