The transmission of extracellular signals into intracellular responses is a complex process that often involves the activity of mitogen-activated protein kinases (MAPKs). The MAPK pathway is a three kinase cascade consisting of a MAPK kinase (MAPKKK or MEKK) that activates a MAP/ERK kinase (MEK or MAPKK). This stimulates a phosphorylation-dependent increase in the activity of the MAP kinase. Upon activation, MAPKs phosphorylate a variety of intracellular targets including transcription factors, transcription adaptor proteins, membrane and cytoplasmic substrates as well as other protein kinases.

At least three parallel MAPK pathways exist in humans. The extracellular signal-regulated protein kinase (ERK) pathway primarily transmits mitogenic and differentiation stimuli, while the c-Jun N-terminal kinase (JNK) and p38 pathways predominantly transmit stress and cytokine stimuli. c-Myc, an ERK substrate, is a transcription factor that regulates cell growth and differentiation, glycolysis and apoptosis. Deregulation of c-Myc has been implicated in the origin of diverse human cancers. Elk-1 is a member of the ternary complex factor (TCF) sub-family of the ETS domain family. Elk-1 can be stimulated by all three MAPK pathways, and its main function is the regulation of the activity of the c-Fos promoter in response to extracellular stimuli. MEF2, a member of the MADS box family, is mainly involved in muscle differentiation, but also plays roles in muscle hypertrophy, neuronal survival and T-cell apoptosis. MEF2 is activated by both the p38 and ERK5 pathways.1 STAT1 is involved in activation of IFNα and γ genes, and is activated by p38 and JNK pathways. c-Jun is a member of the activator protein-1 (AP-1) family and is activated by both ERK1/2 and JNK pathways.2 AP-1 members play roles in the expression of genes involved in proliferation and cell cycle progression. ATF-2 is a member of the ATF/CREB family that binds to the cAMP response element (CRE). ATF-2 is activated by ERK1/2, JNK and p38.