Tumor necrosis factor signaling to stress-activated protein kinase (SAPK)/Jun NH2-terminal kinase (JNK) and p38. Germinal center kinase couples TRAF2 to mitogen-activated protein kinase/ERK kinase kinase 1 and SAPK while receptor interacting protein associates with a mitogen-activated protein kinase kinase kinase upstream of MKK6 and p38.
Tumor necrosis factor (TNF) elicits a diverse array of inflammatory responses through engagement of its type-1 receptor (TNFR1). Many of these responses require de novo gene expression mediated by the activator protein-1 (AP-1) transcription factor. We investigated the mechanism by which TNFR1 recruits the stress-activated protein kinases (SAPKs) and the p38s, two mitogen-activated protein kinase (MAPK) families that together regulate AP-1. We show that the human SPS1 homologue germinal center kinase (GCK) can interact in vivo with the TNFR1 signal transducer TNFR-associated factor-2 (TRAF2) and with MAPK/ERK kinase kinase 1 (MEKK1), a MAPK kinase kinase (MAPKKK) upstream of the SAPKs, thereby coupling TRAF2 to the SAPKs. Receptor interacting protein (RIP) is a second TNFR signal transducer which can bind TRAF2. We show that RIP activates both p38 and SAPK; and that TRAF2 activation of p38 requires RIP. We also demonstrate that the RIP noncatalytic intermediate domain associates in vivo with an endogenous MAPKKK that can activate the p38 pathway in vitro. Thus, TRAF2 initiates SAPK and p38 activation by binding two proximal protein kinases: GCK and RIP. GCK and RIP, in turn, signal by binding MAPKKKs upstream of the SAPKs and p38s.