The p38 mitogen-activated protein kinase (MAPK) is activated in vitro by three different protein kinases: MKK3, MKK4, and MKK6. To examine the relative roles of these protein kinases in the mechanism of p38 MAP kinase activation in vivo, we examined the effect of disruption of the murine Mkk3, Mkk4, and Mkk6 genes on the p38 MAPK signaling pathway. We show that MKK3 and MKK6 are essential for tumor necrosis factor-stimulated p38 MAPK activation. In contrast, ultraviolet radiation-stimulated p38 MAPK activation was mediated by MKK3, MKK4, and MKK6. Loss of p38 MAPK activation in the mutant cells was associated with defects in growth arrest and increased tumorigenesis. These data indicate that p38 MAPK is regulated by the coordinated and selective actions of three different protein kinases in response to cytokines and exposure to environmental stress. Several groups of mitogen-activated protein kinase (MAPK) signal transduction pathways have been identified in mammals, including extracellular signal-regulated protein kinase (ERK), c-Jun NH 2 -terminal kinase (JNK), and p38 MAPK. Each of these groups of MAPK is activated by dual phosphorylation on Thr and Tyr within a tripeptide motif (Thr-Xaa-Tyr) located within the activation loop of the MAPK. This phosphorylation is mediated by seven MAPK kinases (MAPKKs) that have specificity for individual MAPK isoforms. Thus, ERK1 and ERK2 are activated by MEK1 and MEK2, ERK5 is activated by MEK5, JNK is activated by MKK4 and MKK7, and p38 MAPK is activated by MKK3 and MKK6 (Schaeffer and Weber 1999; Kyriakis and Avruch 2001). These MAPKKs and MAPKs can create independent signaling modules that may function in parallel.The mechanism that accounts for the specificity of MAPKKs to activate individual MAPK isoforms is mediated, in part, by an interaction between an N-terminal region located on the MAPKK and a docking site located on the MAPK (Bardwell and Thorner 1996; Enslen and Davis 2001). Recently, structural insight into the mechanism of interaction between a MAPKK and a MAPK has been achieved by X-ray crystallography (Chang et al. 2002). This analysis demonstrated that there is a direct interaction of the N-terminal region of the MAPKK with a docking groove present on the surface of the MAPK distant from the catalytic active site (Weston et al. 2002). A second determinant of MAPKK specificity is the structure of the MAPK activation loop that contains the ThrXaa-Tyr dual phosphorylation motif (Enslen et al. 2000). The specificity of these interactions mediate, in part, the ability of an individual MAPKK to activate a particular MAPK selectively.It is interesting that mammalian MAPK signaling modules include more than one MAPKK because in yeast only a single MAPKK appears to activate each MAPK. The role of this pathway complexity in mammals is unclear. However, it may be significant that individual yeast MAPK isoforms are activated by only a limited group of extracellular stimuli, but mammalian MAPK isoforms are activated by a wide array of extracellular stimuli. It is there...

Stem cell function is central for the maintenance of normal tissue homeostasis. Here we show that deletion of p38alpha mitogen-activated protein (MAP) kinase in adult mice results in increased proliferation and defective differentiation of lung stem and progenitor cells both in vivo and in vitro. We found that p38alpha positively regulates factors such as CCAAT/enhancer-binding protein that are required for lung cell differentiation. In addition, p38alpha controls self-renewal of the lung stem and progenitor cell population by inhibiting proliferation-inducing signals, most notably epidermal growth factor receptor. As a consequence, the inactivation of p38alpha leads to an immature and hyperproliferative lung epithelium that is highly sensitized to K-Ras(G12V)-induced tumorigenesis. Our results indicate that by coordinating proliferation and differentiation signals in lung stem and progenitor cells, p38alpha has a key role in the regulation of lung cell renewal and tumorigenesis.

The c-Jun NH 2 -terminal kinase (JNK) has been implicated in both cell death and survival responses to different stimuli. Here we reexamine the function of JNK in tumor necrosis factor (TNF)-stimulated cell death using fibroblasts isolated from wild-type, Mkk4, and Jnk1 −/− Jnk2 −/− mice. We demonstrate that JNK can act to suppress TNF-stimulated apoptosis. However, we find that JNK can also potentiate TNF-stimulated necrosis by increasing the production of reactive oxygen species (ROS). Together, these data indicate that JNK can shift the balance of TNF-stimulated cell death from apoptosis to necrosis. Increased necrosis may represent a contributing factor in stress-induced inflammatory responses mediated by JNK.

Exposure of primary murine embryonic fibroblasts to tumor necrosis factor (TNF) causes biphasic activation of the c-Jun NH(2)-terminal kinase (JNK) signaling pathway. The early phase (30 min) of the response to TNF is a large and transient increase in JNK activity. This response is followed by a second and more sustained phase of JNK activation that lasts many hours. We employed a chemical genetic strategy to dissect the functional consequences of these two phases of JNK activation. We report that both the early and late phases of JNK activation contribute to TNF-induced gene expression. In contrast, the early transient phase of JNK activation (<1 hr) can signal cell survival, while the later and more sustained phase of JNK activation (1-6 hr) can mediate proapoptotic signaling. These data indicate that the time course of JNK signaling can influence the biological response to JNK activation.