The study on signal mechanism of protein kinase C zeta-involved NF-kappaB activation in LPS-stimulated TLR4 signaling pathways

NF-kappaB is a key transcription factor mediating a wide spectrum of innate immune responses induced by bacterial lipopolysaccharide (LPS). Although LPS activating NF-kappaB through TLR4-TRAF6-IKK signaling cascade has been extensively studied, the detailed signal transduction pathways induced by LPS that lead to NF-kappaB activation, still need to be defined. Recently we found that atypical PKCzeta is involved in LPS-induced activation of NF-kappaB in macrophages. LPS induces time-dependent activation of PKCzeta, and blocking PKCzeta activity by PKCzeta pseudosubstrate inhibitor or siRNA effectively inhibits LPS-induced NF-kappaB activation. Co-immunoprecipitation with anti-PKCzeta antibody indicates association between PKCzeta, and TRAF6 or TAK1. PKCzeta activity is TRAF6-dependent because inhibition of TRAF6 by transfection with siRNA and dominant-negative mutant attenuates LPS-induced PKCzeta activation. Furthermore, PKCzeta activating NF-kappaB is mediated by TAK1, which is supported by the observation that inhibition of PKCzeta impairs activation of TAK1 and knock-down of TAK1 attenuates the activation of NF-kappaB induced by constitutively active PKCzeta. The results demonstrate a signaling pathway involving PKCzeta, present in LPS/TLR4-induced NF-kappaB activation, and the function of PKCzeta is TRAF6-dependent and mediated by TAK1.