Role of cyclin D1 as an estrogen receptor cofactor and the influence of hypoxia on estrogen receptor regulation, with focus on prognositic and treatment predictive features in breast cancer

Abstract: Estrogen receptor (ER) status can define breast cancer patients who would benefit from adjuvant tamoxifen therapy. However, resistance to tamoxifen is often observed and possible mechanisms may be loss or reduction of ER, dysfunctional ER- signaling and ligand independent activation of the receptor. Hypoxia and hypoxia inducible factor-1? (HIF-1) expression has been correlated to loss of ER in breast tumors. Cyclin D1, initially described as a cell cycle regulator, might also function as a cofactor to ER inducing ligand independent activation of the receptor. We therefore determined the relation between ER, cyclin D1 and HIF-1 expression in primary breast tumors and cell lines. Further, the prognostic and treatment predictive value of cyclin D1 and HIF-1 was analyzed in breast cancer patients receiving two years of tamoxifen versus no adjuvant treatment.

The results indicated that ER heterogeneity in primary breast tumors was associated with cyclin D1 and HIF-1 expression. Further, breast cancer patients with cyclin D1 high tumors did not benefit from tamoxifen treatment. The survival for untreated patients with cyclin D1 high tumors was nevertheless slightly better than for patients with cyclin D1 low tumors. Hypoxia was also strongly linked to ER downregulation in DCIS and invasive breast cancer and caused ER downregulation in breast cancer cell lines. Interestingly, hypoxic cells were less differentiated, showing changes in morphology, proliferation and cytokeratin 19 expression. The hypoxia induced ER reduction was due to both proteasomal degradation and decreased transcription and active extracellular regulated kinase (ERK1/2)was involved in the transcriptional regulation of ER. Consequently, tamoxifen treatment did not affect proliferation as efficiently in hypoxia as in normoxia, but ERK1/2 inhibitors efficiently increased the tamoxifen effect in hypoxia. Unexpectedly, tumor specific HIF-1 expression was not a predictive marker for tamoxifen response in premenopausal breast cancer patients but associated with a worse recurrence free survival. These results suggest that cyclin D1 is a predictive marker for tamoxifen resistance and HIF 1 a marker of poor prognosis in breast cancer. Targeting cyclin D1 and/or ERK1/2 in conjunction with tamoxifen represent new treatment strategies for improving the tamoxifen response.