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, Thr184. 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.