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

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Abstract
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 FasL expression in T cells, we tested‐besides subfamily‐selective protein kinase C (PKC) inhibitors – the effect of constitutively active mutants of representatives of all PKC subfamilies, i.e. PKCα,ϵ,θ,ι, on FasL luciferase promoter reporter constructs. In synergy with a constitutively active form of protein phosphatase 2B calcineurin (CaN), only PKCθ, but not PKCα,ϵ,ι, preferentially induced FasL promoter reporter activity and, consequently, FasL protein expression in Jurkat T cells. Activation of an inducible PKCθ AE‐estrogen receptor fusion mutant led to a CaN‐dependent and rapid FasL reporter activity detected as early as 4 h after addition of 4‐hydroxytamoxifen, incidating a direct effect of PKCθ action on FasL expression. Consistently, in Jurkat T cells, expression of PKCθ AE / CaN significantly enhanced FasL protein expression and apoptosis in a CD95‐dependent manner since cell death was not observed in T cells co‐expressing the caspase‐8 inhibitor crmA. Taken together, our results support the notion that PKCθ and CaN are sufficient to regulate apoptosis through FasL expression.