Effects of tissue transglutaminase on retinoic acid-induced cellular differentiation and protection against apoptosis

Retinoic acid (RA) and its various synthetic analogs affect mammalian cell growth, differentiation, and apoptosis. Whereas treatment of the human leuk emia cell line HL60 with RA results in cellular differentiation, addition o f the synthetic retinoid, N-(4-hydroxyphenyl) retinamide (HPR), induces HL6 0 cells to undergo apoptosis. Moreover, pretreatment of HL60 cells as well as other cell lines (i.e. NIH3T3 cells) with RA blocks HPR-induced cell dea th. In attempting to discover the underlying biochemical activities that mi ght account for these cellular effects, we found that monodansylcadaverine (MDC), which binds to the enzyme (transamidase) active site of tissue trans glutaminase (TGase), eliminated RA protection against cell death and in fac t caused RA to become an apoptotic factor, suggesting that the ability of R A to protect against apoptosis is linked to the expression of active TGase. Furthermore, it was determined that expression of exogenous TGase in cells exhibited enhanced GTP binding and transamidation activities and mimicked the survival advantage imparted by RA. We tested whether the ability of thi s dual function enzyme to limit HPR-mediated apoptosis was a result of the ability of TGase to bind GTP and/or catalyze transamidation and found that GTP binding was suffficient for the protective effect. Moreover, excessive transamidation activity did not appear to be detrimental to cell viability. These findings, taken together with observations that the TGase is frequen tly up-regulated by environmental stresses, suggest that TGase may function to ensure cell survival under conditions of differentiation and cell stres s.