Selective potentiation of paclitaxel (Taxol)-induced cell death by mitogen-activated protein kinase kinase inhibition in human cancer cell lines

Activation of the mitogen-activated protein kinase (MAPK) pathway in HeLa a nd Chinese hamster ovary cells after treatment with paclitaxel (Taxol) and other microtubule interacting agents has been investigated. Using a trans-r eporting system, the phosphorylation of the nuclear transcription factors E lk-1 and c-jun was measured. Concentration- and time-dependent activation o f the Elk-1 pathway, mediated primarily by the extracellular signal-regulat ed kinase (ERK) component of the MAPK family, was observed. Inactive drug a nalogs and other cytotoxic compounds that do not target microtubules failed to induce similar levels of activation, thereby indicating that an interac tion between these drugs and the microtubule is essential for the activatio n of MAPKs. Evaluation of the endogenous levels of MAPK expression revealed cell-dependent expression of the ERK, c-jun N-terminal kinase, and p38 pat hways. In the case of HeLa cells, time-dependent activation of ERK coincide d with increased poly(ADP-ribose) polymerase (PARP) cleavage, phosphatidyls erine externalization, and increased accumulation of cells in GA In both ce ll lines, inhibition of ERK activity potentiated paclitaxel-incluced PARP c leavage and phosphatidylserine externalization, suggesting that ERK activit y coincided with, but did not mediate, the cytotoxic effects of paclitaxel. We evaluated the nature of the interaction between paclitaxel and the MAPK kinase inhibitor U0126 in three cell lines, on the basis of a potential ch emotherapeutic advantage of paclitaxel plus ERK inhibition. Our data confir med additivity in those cells lines that undergo paclitaxel-induced ERK act ivation, and antagonism in cells with low ERK activity, suggesting that in tumors with high ERK activity, there may be an application for this strateg y in therapy.