Cyclooxygenase-2 protein reduces tamoxifen and N-(4-hydroxyphenyl) retinamide inhibitory effects in breast cancer cells

Approximately 30-40% of estrogen receptor α (ERα)-positive breast tumors express high levels of the cyclooxygenase-2 (COX-2) protein, and these high levels have been associated with a poorer prognosis in breast cancer patients. We speculate that high levels of COX-2 induce drug resistance in ERα-positive breast tumors, thus reducing the survival rate of patients with such tumors. Human breast cancer cell lines that express high levels of COX-2 are generally ERα negative. To determine whether COX-2 induces drug resistance, plasmids encoding the COX-2 gene were stably transfected into ERα-positive MCF-7 human breast cancer cells (MCF-7/COX-2). MCF-7/COX-2 cells were resistant to the selective estrogen receptor modulator tamoxifen but not to its analog, raloxifene. MCF-7/COX-2 cells were also resistant to the retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) but not to its analog, all-trans retinoic acid. In contrast, the sensitivities of MCF-7/COX-2 cells to doxorubicin and paclitaxel were similar to those of the parental MCF-7 cells. We then determined which COX-2 product, prostaglandin E₂ (PGE ₂) or prostaglandin F_2α is involved in the COX-2-mediated drug resistance. PGE₂, but not PGF_2α, blocked the antiproliferative effects of tamoxifen and 4-HPR. Agonists that activate PGE₂receptors and their downstream kinase effectors, protein kinases A and C, also blocked the growth inhibitory effects of these drugs. Increased levels of Bcl-2 and Bcl-X_L proteins have been reported in mammary tumors of COX-2 transgenic mice and in human colon cancer cell lines that have high levels of COX-2. However, we did not observe any changes in Bcl-2, Bcl-X_L, or Bax expression induced by COX-2 or PGE₂. Here we report the novel findings that COX-2 uses PGE₂ to stimulate the activities of protein kinases A and C to induce selectively tamoxifen and 4-HPR resistance in ERα-positive breast cancer cells.