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Tamoxifen enhances the anti-proliferative effect of roscovitine, a selective cyclin- 37 dependent kinase inhibitor, on human ER-positive human breast cancer cells
David Gritsch, Margarita Maurer, Nora Zulehner and Józefa Wesierska-Gadek

We reported recently that roscovitine (ROSC), a selective cyclin-dependent kinase (CDK) inhibitor, can arrest human ER-positive MCF-7 breast cancer cells in the G2 phase of the cell cycle and concomitantly induce apoptosis. The observed effects of ROSC were diminished in MCF-7 cells maintained in the presence of estrogen-mimicking compounds. Therefore, we decided to test whether combining ROSC with anti-estrogen therapy would modulate the efficacy of ROSC action. Exposure of MCF-7 cells to tamoxifen (TAM) for 24 h decreased the number of living cells by approximately 10%. This was associated with a ca. 25% increase in the G1 cell population and reduction in the proportion of S-phase cells. Unlike TAM, estrogen had very weak effects on the cell cycle progression of MCF-7 cells within 24 h. The proliferation-promoting effect of estrogen did not become evident until cultivation of cells for 48 h. Addition of estrogen to MCF-7 cells 1 h prior to TAM administration abolished the anti-estrogen-induced G1 arrest. Simultaneous treatment of MCF-7 cells with ROSC and TAM strongly enhanced the anti-proliferative effect of ROSC. This was potentiated after co-treatment with estrogen. These results clearly indicate that the efficacy of treating ER-positive breast cancers by ROSC can be enhanced by combined application of antiestrogens.

Keywords: anti-estrogens; apoptosis, cell cycle arrest; cyclin-dependent kinases; estrogen; inhibitors of cyclindependent kinases; roscovitine

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