During hormone depletion or tamoxifen treatment of breast cancer cells the estrogen receptor apoprotein supports cell cycling through the retinoic acid receptor α1 apoprotein

Abstract

IntroductionCurrent hormonal adjuvant therapies for breast cancer including tamoxifen treatment and estrogen depletion are overall tumoristatic and are severely limited by the frequent recurrence of the tumors. Regardless of the resistance mechanism, development and progression of the resistant tumors requires the persistence of a basal level of cycling cells during the treatment for which the underlying causes are unclear.MethodsIn estrogen-sensitive breast cancer cells the effects of hormone depletion and treatment with estrogen, tamoxifen, all-trans retinoic acid (ATRA), fulvestrant, estrogen receptor α (ER) siRNA or retinoic acid receptor α (RARα) siRNA were studied by examining cell growth and cycling, apoptosis, various mRNA and protein expression levels, mRNA profiles and known chromatin associations of RAR. RARα subtype expression was also examined in breast cancer cell lines and tumors by competitive PCR.ResultsBasal proliferation persisted in estrogen-sensitive breast cancer cells grown in hormone depleted conditioned media without or with 4-hydroxytamoxifen (OH-Tam). Downregulating ER using either siRNA or fulvestrant inhibited basal proliferation by promoting cell cycle arrest, without enrichment for ErbB2/3+ overexpressing cells. The basal expression of RARα1, the only RARα isoform that was expressed in breast cancer cell lines and in most breast tumors, was supported by apo-ER but was unaffected by OH-Tam; RAR-β and -γ were not regulated by apo-ER. Depleting basal RARα1 reproduced the antiproliferative effect of depleting ER whereas its restoration in the ER depleted cells partially rescued the basal cycling. The overlapping tamoxifen-insensitive gene regulation by apo-ER and apo-RARα1 comprised activation of mainly genes promoting cell cycle and mitosis and suppression of genes involved in growth inhibition; these target genes were generally insensitive to ATRA but were enriched in RAR binding sites in associated chromatin regions.ConclusionsIn hormone-sensitive breast cancer, ER can support a basal fraction of S-phase cells (i) without obvious association with ErbB2/3 expression, (ii) by mechanisms unaffected by hormone depletion or OH-Tam and (iii) through maintenance of the basal expression of apo-RARα1 to regulate a set of ATRA-insensitive genes. Since isoform 1 of RARα is genetically redundant, its targeted inactivation or downregulation should be further investigated as a potential means of enhancing hormonal adjuvant therapy.

Metabolomics and Transcriptional Responses in Estrogen Receptor Positive Breast Cancer Cells

“…Like breast tumors, breast cancer cell lines can also yield clonal populations of tamoxifen resistant cells that have variable ER dependence (Coser et al, 2009). This in vitro evidence supports the fact that the basal population of hormone sensitive cells may be a prerequisite condition for eventual occurrence of hormone insensitive tumors where current therapies such as tamoxifen are no more effective (Salazar et al, 2011). Development of resistance to tamoxifen or aromatase inhibitors (AIs) is a result of adaptive changes which lead to the activation of alternate signaling pathways.…”

“…However, a major obstacle to breast cancer treatment is the development of drug resistance. Resistance is commonly associated with an increased expression of Erb proteins including EGFR (epidermal growth factor receptor); in addition, there is evidence of non-hormonal role of ER in hormone sensitive MCF-7 cells to maintain basal proliferation (Salazar et al, 2011). Interestingly, there was no expression of Erb2 and 3, but ER supported a population of basal fraction of actively dividing S phase cells which were unaffected by hormone depletion or hydroxy tamoxifen treatment (Salazar et al, 2011).…”

“…Resistance is commonly associated with an increased expression of Erb proteins including EGFR (epidermal growth factor receptor); in addition, there is evidence of non-hormonal role of ER in hormone sensitive MCF-7 cells to maintain basal proliferation (Salazar et al, 2011). Interestingly, there was no expression of Erb2 and 3, but ER supported a population of basal fraction of actively dividing S phase cells which were unaffected by hormone depletion or hydroxy tamoxifen treatment (Salazar et al, 2011). Like breast tumors, breast cancer cell lines can also yield clonal populations of tamoxifen resistant cells that have variable ER dependence (Coser et al, 2009).…”

Relaxant effect of all-trans-retinoic acid via NO-sGC-cGMP pathway and calcium-activated potassium channels in rat mesenteric artery

“…The vasorelaxant effect ATRA is physiologically relevant because the intravenous infusion of ATRA decreased blood pressure in normotensive rats. We conclude that ATRA relaxes resistance vessels via both RARs and RXRs receptors that are mediated by the endothelium-dependent NO-cGMP pathway, which may participate in the control of blood pressure.all-trans-retinoic acid; mesenteric artery; vasorelaxation; nitric oxide synthase; blood pressure A BIOLOGICALLY ACTIVE metabolite of vitamin A, all-transretinoic acid (ATRA) has anti-inflammatory, anticancer, and immunomodulatory actions (20,24,31). ATRA exerts its biological effects by modulating gene transcription through distinct intracellular proteins, including the retinoic acid receptor (RAR) and retinoic X receptor (RXR) (2, 18), and activating some key transcription factors, such as nuclear factor-B (20).…”

“…all-trans-retinoic acid; mesenteric artery; vasorelaxation; nitric oxide synthase; blood pressure A BIOLOGICALLY ACTIVE metabolite of vitamin A, all-transretinoic acid (ATRA) has anti-inflammatory, anticancer, and immunomodulatory actions (20,24,31). ATRA exerts its biological effects by modulating gene transcription through distinct intracellular proteins, including the retinoic acid receptor (RAR) and retinoic X receptor (RXR) (2, 18), and activating some key transcription factors, such as nuclear factor-B (20).…”