Synergistic action of protein kinase C θ and calcineurin is sufficient for Fas ligand expression and induction of a crmA-sensitive apoptosis pathway in Jurkat T cells

Deletion of activated peripheral T cell clones by apoptosis requires the regulated expression of Fas ligand (FasL) and sensitization of these cells to CD95-mediated signaling. To investigate the signaling pathways responsible for Fast expression in T cells, we tested-besides subfamily-selective protein kinase C (PKC) inhibitors - the effect of constitutively active mutants of representatives of ail PKC subfamilies, i.e. PKC alpha,epsilon,theta,iota, On Fast luciferase promoter reporter constructs. In synergy with a constitutively active form of protein phosphatase 2B calcineurin (CaN), only PKC theta, but not PKC alpha,epsilon,iota, preferentially induced Fast promoter reporter activity and, consequently, Fast protein expression in Jurkat T cells. Activation of an inducible PKC theta AE-estrogen receptor fusion mutant led to a CaN-dependent and rapid Fast reporter activity detected as early as 4 h after addition of 4-hydroxytamoxifen, incidating a direct effect of PKC theta action on Fast expression. Consistently, in Jurkat T cells, expression of PKC theta AE/CaN significantly enhanced Fast protein expression and apoptosis in a CD95-dependent manner since cell death was not observed in T cells cc-expressing the caspase-8 inhibitor crmA. Taken together, our results support the notion that PKC theta and CaN are sufficient to regulate apoptosis through Fast expression.