MLK-3 activates the SAPK/JNK and p38/RK pathways via SEK1 and MKK3/6.

Abstract

Mixed lineage kinase‐3 (MLK‐3) is a 97 kDa serine/threonine kinase with multiple interaction domains, including a Cdc42 binding motif, but unknown function. Cdc42 and the related small GTP binding protein Rac1 can activate the SAPK/JNK and p38/RK stress‐responsive kinase cascades, suggesting that MLK‐3 may have a role in upstream regulation of these pathways. In support of this role, we demonstrate that MLK‐3 can specifically activate the SAPK/JNK and p38/RK pathways, but has no effect on the activation of ERKs. Immunoprecipitated MLK‐3 catalyzed the phosphorylation of SEK1 in vitro, and co‐transfected MLK‐3 induced phosphorylation of SEK1 and MKK3 at sites required for activation, suggesting direct regulation of these protein kinases. Furthermore, interactions between MLK‐3 and SEK and MLK‐3 and MKK6 were observed in co‐precipitation experiments. Finally, kinase‐dead mutants of MLK‐3 blocked activation of the SAPK pathway by a newly identified mammalian analog of Ste20, germinal center kinase, but not by MEKK, suggesting that MLK‐3 functions to activate the SAPK/JNK and p38/RK cascades in response to stimuli transduced by Ste20‐like kinases.

Two distinct signaling pathways regulate peroxynitrite-induced apoptosis in PC12 cells

“…Indeed, inhibition of either kinase prevented apoptosis. MLK is an upstream activator of p38 and JNK 22 which regulates the intrinsic apoptotic pathway in trophic factor-deprived sympathetic neurons. 23 Since the inhibition of MLK also attenuated peroxynitrite-induced apoptosis ( Figures 1-3 and 5), it is presumed that peroxynitrite initiates apoptotic signaling upstream of p38 and JNK.…”

Activation and signaling of the p38 MAP kinase pathway

“…Rac1 can bind to MEKK1 or MLK1 while Cdc42 can only bind to MLK1 and both result in activation of p38 via MAP3Ks [35,42]. p21-activated kinases (PAKs) are yet another group of p38 activators.…”

Signaling by dual specificity kinases

“…Similarly, The small GTPases such as Rac and CDC 42 fail to activate MKK3 (Han et al, 1996). However, the recently identi®ed MKKKs such as apoptosis signal regulating kinase or ASK1 (Ichijo et al, 1997), MEKK4/MTK1 (Takekawa et al, 1997), TGFbactivated kinase-1 or TAK1 (Shirakabe et al, 1997) and mixed lineage kinase-3 or MLK3 (Rana et al, 1996;Tibbles et al, 1996) appear to activate MKK3. The identity of the upstream small GTPases and kinase(s) involved in the activation of MKK3 is yet to be de®ned.…”