Protein Kinase C-associated Kinase Can Activate NFκB in Both a Kinase-dependent and a Kinase-independent Manner

Protein kinase C-associated kinase (PKK, also known as RIP4/DIK) activates NFκB when overexpressed in cell lines and is required for keratinocyte differentiation in vivo . However, very little is understood about the factors upstream of PKK or how PKK activates NFκB. Here we show that certain catalytically inactive mutants of PKK can activate NFκB, although to a lesser degree than wild type PKK. The deletion of specific domains of wild type PKK diminishes the ability of this enzyme to activate NFκB; the same deletions made on a catalytically inactive PKK background completely ablate NFκB activation. PKK may be phosphorylated by two specific mitogen-activated protein kinase kinase kinases, MEKK2 and MEKK3, and this interaction may in part be mediated through a critical activation loop residue, Thr 184 . Catalytically inactive PKK mutants that block phorbol ester-induced NFκB activation do not interfere with, but unexpectedly enhance, the activation of NFκB by these two mitogen-activated protein kinase kinase kinases. Taken together, these data indicate that PKK may function in both a kinase-dependent as well as a kinase-independent manner to activate NFκB.