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

Lab Invest. 2005 Nov;85(11):1357-67. doi: 10.1038/labinvest.3700339.


Approximately 30-40% of estrogen receptor alpha (ERalpha)-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 ERalpha-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 ERalpha negative. To determine whether COX-2 induces drug resistance, plasmids encoding the COX-2 gene were stably transfected into ERalpha-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 (PGE2) or prostaglandin F2alpha is involved in the COX-2-mediated drug resistance. PGE2, but not PGF2alpha, blocked the antiproliferative effects of tamoxifen and 4-HPR. Agonists that activate PGE2 receptors 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-XL 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-XL, 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 ERalpha-positive breast cancer cells.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antineoplastic Agents / metabolism*
  • Breast Neoplasms / pathology
  • Cell Line, Tumor
  • Clone Cells
  • Cyclooxygenase 2 / metabolism*
  • Drug Resistance, Neoplasm
  • Female
  • Fenretinide / antagonists & inhibitors*
  • Humans
  • Inhibitory Concentration 50
  • Membrane Proteins / metabolism*
  • Selective Estrogen Receptor Modulators / metabolism*
  • Tamoxifen / antagonists & inhibitors*


  • Antineoplastic Agents
  • Membrane Proteins
  • Selective Estrogen Receptor Modulators
  • Tamoxifen
  • Fenretinide
  • Cyclooxygenase 2
  • PTGS2 protein, human