Up-regulation of extracellular-regulated kinases 1/2 (ERK1/2) has been implicated in tumor progression and metastasis in many types of cancer. We have previously shown that ERK1/2 is necessary for invasiveness of Dunning rat prostatic adenocarcinoma cell lines in which levels of activated ERK1/2 correlate with the metastatic potential. Here, we further examined the biological effects of elevated ERK1/2 in the highly metastatic Dunning cell line, MLL, in which the abilities to invade and metastasize are enhanced relative to its progenitor strain. Inhibition of ERK1/2 activation by the MEK1 inhibitor, PD98059, dose-dependently reduced MLL cell invasiveness and motility with similar IC50 values. On the other hand, the abilities of MLL cells to adhere to the extracellular matrix, phosphorylate myosin regulatory light chain and secrete matrix-degrading enzymes, matrix metalloproteinase (MMP)-2 and urokinase plasminogen activator (uPA) were marginally, if at all, affected by PD98059 treatment. These data indicated that the inhibitory effect of PD98059 on the invasiveness of MLL cells was primarily due to the suppression of cell motility, and the up-regulation of ERK1/2 is, at least in part, responsible for the enhanced cellular motility and invasiveness of the MLL cells.

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