Positive feedback regulation of PLCγ1/Ca²⁺ signaling by PKCθ in restimulated T cells via a Tec kinase-dependent pathway

PKCθ plays an essential role in activation of mature T cells. Here, we report that the TCR/CD28-induced tyrosine phosphorylation and activation of PLCγ1 was significantly impaired in PKCθ ^-/- primary, restimulated T cells. Consistent with this finding, receptor-induced Ca²⁺ mobilization, NF-AT DNA-binding activity and the membrane translocation of PKCα, a PLCγ1-dependent conventional PKC, were also markedly reduced in the same cells. Moreover, a dominant-negative PLCγ1 mutant blocked the PKCθ-induced activation of an AP-1 reporter gene in Jurkat and primary cells. Regulation of PLCγ1 signaling by PKCθ required the tyrosine kinase Tec since a dominant-negative Tec mutant blocked PKCθ-induced AP-1 (but not NF-κB) activation. In addition, wild-type Tec, but not Itk or Rlk, potently activated AP-1. Furthermore, Tec was found to constitutively associate with PKCθ, an interaction that like AP-1 activation required the pleckstrin-homology domain of Tec. These findings define a novel PKCθ-initiated pathway that regulates Ca²⁺ signaling and AP-1 activation via Tec and PLCγ1. Moreover, they identify Tec as a key point downstream of PKCθ, where TCR- and PKCθ-induced signaling pathways, leading to AP-1 versus NF-κB activation, diverge in T cells.