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Extracellular signal-regulated kinase 1/2 (ERK1/2) is known to have bi-functional role in cell death and survival. Cisplatin activates ERK pathway but the exact role of ERK signaling in cisplatin-induced apoptotic cell death is still controversial. We studied the role of nuclear localization of phospho-ERK in cisplatin-induced apoptosis using sensitive OVCAR-3 cells and resistant OVCAR-3/CDDP cells. Western blot analysis showed that considerable amount of phospho-ERK2 existed in the nuclei of OVCAR-3/CDDP cells in the absence of cisplatin whereas it was not detected in OVCAR-3 cells. Nuclear levels of phospho-ERK2 showed biphasic pattern when treated with cisplatin. The levels in OVCAR-3/CDDP cells were increased up to 3 hr, decreased thereafter and then increased again after 24 hr. Similar pattern was observed in OVCAR-3 cells but the levels was significantly lower compared to those in OVCAR-3/CDDP cells. Pre-localization of phospho-ERK2 in nuclei of OVCAR-3/CDDP cells was observed when cisplatin was treated after serum starvation. Treatment of PD98059 and U0126 sensitized OVCAR-3 cells to cisplatin but it did not affect the sensitivity of OVCAR-3/CDDP cells to cisplatin, suggesting that pre-existed phospho-ERK2 in the nuclei is involved in cisplatin resistance. To further demonstrate the role of nuclear localization of phospho- ERK2, cisplatin-induced apoptosis was examined after pretreatment of OVCAR-3 cells with PMA (phorbol 12-myristate 13-actate). Pretreatment of PMA resulted in decrease of cisplatin-induced apoptosis in OVCAR-3 cells. These results collectively suggest that pre-localized phospho-ERK2 in the nuclei inhibits the initiation of cisplatin-induced apoptosis in OVCAR-3/CDDP cells.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]