| 背景 |
Four distinct MAP kinase signaling pathways involving 7 MEK enzymes have been identified. MEK3 and MEK6 are functionally similar and encoded by MAP2K3 and MAP2K6 genes, respectively. The genes are both located on chromosome 17q. MEK3 and MEK6 consist of 347 and 334 amino acids residues respectively. Structurally MEK6 differs from MEK3 in terms of C- and N- terminal regions. However, the ATP binding sites, and serine/threonine and tyrosine catalytic sites are conserved. MEK3/6 signaling pathway is activated by growth factor stimulation through RTKs. Additionally, the cascade can also be activated by G-protein coupled receptors, intracellular receptors, and toll-like receptors, in response to numerous stimuli including physical and chemical stresses, hormones, UV irradiation, and cytokines, such as interleukin-1 and tumor necrosis factor. These stimuli activate different MAPK kinase kinases (MAPKKKs), which include TAK1, ASK1/2, DLK, MEKK4, TAO1/2/3 and MLK2/3. Active MAPKKKs phosphorylate and activate MEK3/6, which in turn catalyzes the concomitant phosphorylation of a threonine/serine and a tyrosine residue in the p38 MAPK. MEK6 activates all the four isoforms of p38 MAP kinase (α, β, γ and δ) whereas MEK3 can only activate p38α and p38β isoforms. MEK3/6-p38 MAPK cascade promotes p53-dependent growth arrest by phosphorylating p53 at serine 33 and 46. Together, these targets of MEK3/6-p38 MAPK pathway (cyclin D1, Cdc25, and p53) cooperate to arrest the cell cycle.
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