PM-20, a novel inhibitor of Cdc25A, induces extracellular signal-regulated kinase 1/2 phosphorylation and inhibits hepatocellular carcinoma growth in vitro and in vivo

We have synthesized several new phenyl maleimide compounds, which are potent growth inhibitors of several human tumor cell lines. Among these, PM-20 was the most potent with an IC50 of 700 nmol/L for Hep3B human hepatoma cell growth. Two other derivatives, PM-26 and PM-38, did not inhibit Hep3B cell growth even at 100 [mu mol/L. Interestingly, under identical experimental conditions, PM-20 inhibited DNA synthesis of primary cultures of normal hepatocytes at a 10-fold higher concentration than that needed to inhibit the DNA synthesis of the Hep3B hepatoma cells. PM-20 affected two cellular signaling pathways in Hep3B cells: Cdc25 phosphatase and extracellular signal-regulated kinase (ERK) 1/2. It competitively inhibited the activity of Cdc25 (preferentially Cdc25A) by binding to the active site, likely through the catalytic cysteine, but did not inhibit PTP1B, CD45, or MKP-1 phosphatases. As a result of its action, tyrosine phosphorylation of the cellular Cdc25A substrates Cdk2 and Cdk4 was induced. It also induced strong and persistent phosphorylation of the Cdc25A substrate ERK 1/2. Hep3B cell lysates were found to contain ERK2 phosphatase(s) activity, which was inhibited by the actions of PM-20. However, activity of exogenous dual-specificity ERK2 phosphatase MKP1 was not inhibited. Induction of ERK1/2 phosphorylation correlated with the potency of growth inhibition in tumor cell lines and inhibition of ERK1/2 phosphorylation by the mitogen-activated protein kinase (MAPK)/ERK kinase 1/2 inhibitor U0126 or overexpression of the cdc25A gene in Hep3B cells antagonized the growth inhibitory actions of PM-20. Growth of transplantable rat hepatoma cells in vivo was also inhibited by PM-20 action with a concomitant induction of pERK in the tumors. The mechanism(s) of growth inhibition of Hep3B hepatoma cells by the phenyl maleimide PM-20 involves prolonged ERK1/2 phosphorylation, likely resulting from inhibition of the ERK phosphatase Cdc25A. PM-20 thus represents a novel class of tumor growth inhibitor that inhibits mainly Cdc25A, is dependent on ERK activation, and has a considerable margin of selectivity for tumor cells compared with normal cells.