ERK inhibitor PD98059 inactivate ERK1/2 in untreated and gemcitabine-treated breast cancer cells. A, BxPC-3 and MIAPaCa-2 cells were exposed to the indicated concentrations of gemcitabine for 18 hr. The cells were then lysed and subjected to WB analysis to monitor pERK1/2 (Thr42/Tyr44) expression as described in Materials and Methods. B, BxPC-3 and MIAPaCa-2 cells were exposed (18 hours) to either 5 μM PD98059, 0.5-1.0 μM of gemcitabine, or the combination, after which proteins were prepared and subjected to WB as described above to monitor pERK1/2 expression. For (A) and (B), lanes were loaded with 25 μg of protein; blots were then stripped and re-probed with GAPDH to ensure equivalent loading and transfer. Representative results are shown; two additional studies yielded equivalent results.