In the cellular garden of forking paths: how p38 MAPKs signal for downstream assistance.

Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved enzymes which connect cell-surface receptors to regulatory targets within cells and convert receptor signals into various outputs. In mammalian cells, four distinct MAPKs have been identified: the extracellular signal-related kinases (ERK)-1/2, the c-jun N-terminal kinases or stress-activated protein kinases 1 (JNK1/2/3, or SAPK1s), the p38 MAPKs (p38 alpha/beta/gamma/delta, or SAPK2s), and the ERK5 or big MAP kinase 1 (BMK1). The p38 MAPK cascade is activated by stress or cytokines and leads to phosphorylation of its central elements, the p38 MAPKs. Downstream of p38 MAPKs there is a diversification and extensive branching of signalling pathways. For that reason, we will focus in this review on the different signalling events that are triggered by p38 activity, and analyse how these events contribute to specific gene expression and cellular responses.