Sounding the Alarm: Protein Kinase Cascades Activated by Stress and Inflammation

The ultraviolet (UV) response of mammalian cells is characterized by a rapid and selective increase in gene expression mediated by AP-1 and NF-kappa B. The effect on AP-1 transcriptional activity results, in part, from enhanced phosphorylation of the c-Jun NH2-terminal activation domain. Here, we describe the molecular cloning and characterization of JNK1, a distant relative of the MAP kinase group that is activated by dual phosphorylation at Thr and Tyr during the UV response. Significantly, Ha-Ras partially activates JNK1 and potentiates the activation caused by UV. JNK1 binds to the c-Jun transactivation domain and phosphorylates it on Ser-63 and Ser-73. Thus, JNK1 is a component of a novel signal transduction pathway that is activated by oncoproteins and UV irradiation. These properties indicate that JNK1 activation may play an important role in tumor promotion.

The mitogen-activated protein (MAP) kinases Erk-1 and Erk-2 are proline-directed kinases that are themselves activated through concomitant phosphorylation of tyrosine and threonine residues. The kinase p54 (M(r) 54,000), which was first isolated from cycloheximide-treated rats, is proline-directed like Erks-1/2, and requires both Tyr and Ser/Thr phosphorylation for activity. p54 is, however, distinct from Erks-1/2 in its substrate specificity, being unable to phosphorylate pp90rsk but more active in phosphorylating the c-Jun transactivation domain. Molecular cloning of p54 reveals a unique subfamily of extracellularly regulated kinases. Although they are 40-45% identical in sequence to Erks-1/2, unlike Erks-1/2 the p54s are only poorly activated in most cells by mitogens or phorbol esters. However, p54s are the principal c-Jun N-terminal kinases activated by cellular stress and tumour necrosis factor (TNF)-alpha, hence they are designated stress-activated protein kinases, or SAPKs. SAPKs are also activated by sphingomyelinase, which elicits a subset of cellular responses to TNF-alpha (ref. 9). SAPKs therefore define a new TNF-alpha and stress-activated signalling pathway, possibly initiated by sphingomyelin-based second messengers, which regulates the activity of c-Jun.

A new brain serine/threonine protein kinase may be a target for the p21ras-related proteins Cdc42 and Rac1. The kinase sequence is related to that of the yeast protein STE20, implicated in pheromone-response pathways. The kinase complexes specifically with activated (GTP-bound) p21, inhibiting p21 GTPase activity and leading to kinase autophosphorylation and activation. Autophosphorylated kinase has a decreased affinity for Cdc42/Rac, freeing the p21 for further stimulatory activities or downregulation by GTPase-activating proteins. This bimolecular interaction provides a model for studying p21 regulation of mammalian phosphorylation signalling pathways.