Development of specific inhibitors against MAP kinase pathways

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(1) Blockade of the ERK-MAP kinase pathway bytreatment with PD98059, a specific inhibitor of MEK, completely suppressed the growth of tumor cells in which the pathway is constitutively activated. Selective up-regulation of P27^<Klp1> was observed alter PD98059 treatment of these tumorcells. The up-regulation of p27^<Klp1> correlated with increased association of p27^<Klp1> with cyclin E-CDK2 complexes, a concomitant inhibition of cyclin E-CDK2 kinase activity, and consequent decrease in the phosphorylation state of RB, which would culminate in the marked G1 cell cycle arrest observed in these tumor cells. Furthermore, PD98059-treatment induced a modest apoptotic response in several tumor cells in which the ERK-MAP pathway is constitutively activated. In cotrast, although PD98059 inhibited the proliferation of human diploid fibroblasts to a considerable degree, it never caused any apoptotic response in these cells. Moreover, growth-inhibited diploid fibroblasts reinitiated proliferation … More soon after removal of the inhibitor. These results strongly suggest that the the ERK-MAP kinase pathway is a potential therapeutic target in a group of tumor cells in which the pathway is constitutively activated.(2) Several polyphenols isolated from green tea leaves potently inhibited the ERK-MAP kinase pathway. These were (-)-epigallocatechin-3-gallate (EGCG) and its derivates. The molecular target of these polyphenols was suggested not to be MEK. Furthermore, several thiofravone derivativates of PD98059 such as 2-(2-amino-3-methoxyphenyl) thiochromone and 2-(2-amino-3-chrophenyl) thiochromone inhibited MEk activity more strongly than PD98059 in in vitro and in vivo experiments.(3) An anthrapyrazolone derivative (SP600125), which was originally developed as a potent inhibitor against c-Jun N-terminal kinase, was synthesized. This compound inhibited the growth of many tumor cells by arresting them at G2/M phase but not at G1 phase of the cell cycle. Thus, this compound issuggested to provide us with anew type of anti-tumor agent. Less