Signaling from the Small GTP-binding Proteins Rac1 and Cdc42 to the c-Jun N-terminal Kinase/Stress-activated Protein Kinase Pathway

Abstract

Certain small GTP-binding proteins control the enzymatic activity of a family of closely related serine-threonine kinases known as mitogen-activated protein kinases (MAPKs). In turn, these MAPKs, such as p44 mapk and p42 mapk , referred to herein as MAPKs, and stressactivated protein kinases, also termed c-Jun N-terminal kinases (JNKs), phosphorylate and regulate the activity of key molecules that ultimately control the expression of genes essential for many cellular processes. Whereas Ras controls the activation of MAPK, we and others have recently observed that two members of the Rho family of small GTP-binding proteins, Rac1 and Cdc42, regulate the activity of JNKs. The identity of molecules communicating Rac1 and Cdc42 to JNK is still poorly understood. It has been suggested that Pak1 is the most upstream kinase connecting these GTPases to JNK; however, we have observed that coexpression of Pak1 with activated forms of Cdc42 or Rac1 diminishes rather than enhances JNK activation. This prompted us to explore the possibility that kinases other than Pak might participate in signaling from GTP-binding proteins to JNK. In this regard, a computer-assisted search for proteins containing areas of homology to that in Pak1 that is involved in binding to Rac1 and Cdc42 led to the identification of mixed lineage kinase 3 (MLK3), also known as protein-tyrosine kinase 1, as a potential candidate for this function. In this study, we found that MLK3 overexpression is sufficient to activate JNK potently without affecting the phosphorylating activity of MAPK or p38. Furthermore, we present evidence that MLK3 binds the GTP-binding proteins Cdc42 and Rac1 in vivo and that MLK3 mediates activation of MEKK-SEK-JNK kinase cascade by Rac1 and Cdc42. Taken together, these findings strongly suggest that members of the novel MLK family of highly related kinases link small GTP-binding proteins to the JNK signaling pathway.

Signalling of the Ret receptor tyrosine kinase through the c-Jun NH2-terminal protein kinases (JNKs): evidence for a divergence of the ERKs and JNKs pathways induced by Ret

“…To interfere with JNK activation, we have used a dominant negative construct for SEK (pEBG SEK KR). SEK, also referred to as MKK4 or JNKK1, is the kinase responsible for JNK activation, by acting downstream MLK3 and MEKK (Sanchez et al, 1994;Teramoto et al, 1996). In preliminary experiments we (7) with heat shock, and NIH3T3-RET/ MEN2A, or from untransfected PC Cl 3 stimulated (+) or not (7) with heat shock and PC Cl 3-RET/PTC1 were immunoprecipitated with a JNK1-speci®c polyclonal antibody.…”

Statins induce apoptosis in ovarian cancer cells through activation of JNK and enhancement of Bim expression

“…Both Rac1 and Cdc42 mediate apoptosis induced by diverse stimuli [31]. It seems that the contribution of Rac1 and Cdc42 to apoptosis is to regulate mainly activa tion of the JNK pathway [32][33][34].…”

Evidence for a Role of Mixed Lineage Kinases in Neuronal Apoptosis

“…The small GTPbinding proteins act via various, multiple downstream mitogenactivated protein (MAP) kinase kinase kinases, depending on the cell type, which bind the active GTP-bound form of the small G-proteins and induce JNK activity and transcriptional activation (Lim et al, 1996;Tapon and Hall, 1997;Van Aelst and D'Souza-Schorey, 1997) (for review, see Johnson, 1999). There are several likely intermediates in the signaling cascades linking Cdc42 to the JNK pathway, including the p21-activated kinases (rat p65PAK/PAK1/PAK ␣, PAK-2/PAK ␥, PAK-3/PAK ␤, and PAK4; Bagrodia et al, 1995;Knaus et al, 1995;Manser et al, 1995;Martin et al, 1995;Brown et al, 1996;Abo et al, 1998), the germinal center kinase (GCK; Pombo et al, 1995), the nickinteracting protein (NIK; Su et al, 1997), MEKK1 and MEKK4 (Fanger et al, 1997;Gerwins et al, 1997), plenty of SH3 (POSH) (Tapon et al, 1998), and the mixed lineage kinase family (MLK1, MLK2, MLK3, DLK, LZK) (Dorow et al, 1993;Holzman et al, 1994;Reddy and Pleasure, 1994;Fan et al, 1996;Hirai et al, 1996;Rana et al, 1996;Teramoto et al, 1996;Tibbles et al, 1996;Hirai et al, 1997;Sakuma et al, 1997;Nagata et al, 1998).…”