Role of the proteasome in the regulation of estrogen receptor alpha turnover and function in MCF-7 breast carcinoma cells

Loss of antagonistic activity of tamoxifen by replacement of one N-methyl of its side chain by fluorinated residues

Efforts to limit the metabolic alteration of the aminoalkyl side chain of tamoxifen by fluorination largely decrease its ER-mediated antagonistic properties in MCF-7 cells (i.e., ability to inhibit growth, to stabilize ER, and to modulate ERE and AP-1 transcriptional activity). This loss is associated with an enhancement of agonistic activity. Loss of interaction between Asp 351 and the nitrogen atom of tamoxifen provoked by the fluorination of its side chain may explain this property.

The catalytic subunit of the proteasome is engaged in the entire process of estrogen receptor-regulated transcription

The ubiquitin-proteasome system plays an important role in a variety of cellular functions by means of its proteolytic activity. Interestingly, recent studies have indicated that the proteasome components are also integral parts of transcription complexes. In genome-wide screening for steroid receptor coactivator (SRC)-interacting proteins using yeast two-hybrid system, we found that the 20S proteasome beta subunit LMP2 (Low Molecular mass Polypeptide 2) interacts directly with the SRC coactivators. We showed that LMP2 is required for estrogen receptor (ER)-mediated gene transcription and for estrogen-stimulated cell cycle progression. We found that LMP2-associated proteasome is recruited to the entire sequence of ER target genes, implicating a role for the proteasome in both transcription initiation and elongation. We demonstrated that the recruitment of LMP2 by SRC coactivators is necessary for cyclic association of ER-regulated transcription complexes on ER targets. These results revealed a mechanism by which the proteasome machinery is recruited in ER-mediated gene transcription. Our experiments also provided evidence implicating SRC coactivators in gene transcription elongation.

Physiologic levels of 2-methoxyestradiol interfere with nongenomic signaling of 17beta-estradiol in human breast cancer cells

PURPOSE

The purpose of this investigation is to determine the effects of physiologic levels (10-50 nmol/L) of 2-methoxyestradiol (2ME) on the growth of estrogen receptor (ER)-positive breast cancer cells and provide insights into its mechanism(s) of action.

EXPERIMENTAL DESIGN

Using the ERalpha-positive breast cancer cells, we studied the effects of 2ME on cell proliferation and cell signaling. Our hypothesis is that 17beta-estradiol (E(2)) and 2ME can affect shared cell signaling pathways, leading to different outcomes in cell proliferation, depending on the absence/presence of E(2).

RESULTS

E(2) stimulated the growth of MCF-7 and T-47 D cells and induced Akt phosphorylation, a nongenomic signaling pathway. In the absence of E(2), 10 to 50 nmol/L of 2ME enhanced cell growth and Akt phosphorylation. However, in the presence of E(2), 2ME inhibited E(2)-induced cell growth and prevented E(2)-induced Akt phosphorylation. Confocal microscopic studies showed that 2ME inhibited subcellular distribution of ERalpha in response to E(2) in MCF-7 and T-47D cells. 2ME also down-regulated E(2)-induced increases in cyclic AMP and ornithine decarboxylase activity. In addition, treatment of MCF-7 cells with 2ME in the presence of E(2) resulted in a decrease in ERalpha level by 72 hours. Accelerated down-regulation of ERalpha may contribute to growth inhibition in the presence of E(2)/2ME combinations. In contrast, a concentration of up to 2.5 mumol/L 2ME had no effect on the growth of ER-negative SK-BR-3 cells, either in the presence or absence of E(2).

CONCLUSIONS

Our results provide evidence for the nongenomic action of 2ME in ER-positive cells. In the presence of E(2), 2ME suppressed E(2)-induced cell growth, Akt signaling, and generation of cyclic AMP, whereas it acted as an estrogen in the absence of E(2). The intriguing growth-stimulatory and growth-inhibitory effects of 2ME on breast cancer cells suggests the need for its selective use in patients.

Stimulatory effect of genistein and apigenin on the growth of breast cancer cells correlates with their ability to activate ER alpha

Genistein and apigenin are phytoestrogens present in commercial preparations used for the treatment of postmenopausal symptoms. In this study, we assessed the influence of these compounds on mammary tumor growth. Both compounds stimulate the proliferation of MCF-7 and T47D cells [estrogen receptor alpha (ERalpha-positive)], but do not stimulate the proliferation of an ERalpha-negative cell line (MDA-MB-435 cells). Genistein appeared more efficient in this regard due to its higher binding affinity for ERalpha, a property explained by a structural analysis of the binding of these compounds to the ERalpha's ligand binding domain. As previously described for estradiol (E(2)), genistein and apigenin down regulated ERalpha and enhanced estrogen response element (ERE)-dependent gene expression. The additional finding that genistein antagonizes the anti-proliferative effect of hydroxytamoxifen suggests phytoestrogens may be detrimental in women with breast cancer who are being treated with tamoxifen. In addition, because of their ability to stimulate breast cell growth, the widespread use of phytoestrogens in postmenopausal women could be detrimental.

Estrogen and tamoxifen induce cytoskeletal remodeling and migration in endometrial cancer cells

Much research effort has been directed toward understanding how estrogen [17beta-estradiol (E2)] regulates cell proliferation and motility through the rapid, direct activation of cytoplasmic signaling cascades (i.e. nongenomic signaling). Cell migration is critical to cancer cell invasion and metastasis and involves dynamic filamentous actin cytoskeletal remodeling and disassembly of focal adhesion sites. Although estrogen is recognized to induce cell migration in some model systems, very little information is available regarding the underlying pathways and potential influence of selective estrogen receptor modulators such as 4-hydroxytamoxifen on these processes. Using the human endometrial cancer cell lines Hec 1A and Hec 1B as model systems, we have investigated the effects of E2 and Tam on endometrial nongenomic signaling, cytoskeletal remodeling, and cell motility. Results indicate that both E2 and Tam triggered rapid activation of ERK1/2, c-Src, and focal adhesion kinase signaling pathways and filamentous actin cytoskeletal changes. These changes included dissolution of stress fibers, dynamic actin accumulation at the cell periphery, and formation of lamellipodia, filopodia, and membrane spikes. Longer treatments with either agent induced cell migration in wound healing and Boyden chamber assays. Agent-induced cytoskeletal remodeling and cell migration were blocked by a Src inhibitor. These findings define cytoskeletal remodeling and cell migration as processes regulated by E2 and 4-hydroxytamoxifen nongenomic signaling in endometrial cancer. This new information may serve as the foundation for the development of new clinical therapeutic strategies.

Additive growth inhibitory effects of ibandronate and antiestrogens in estrogen receptor-positive breast cancer cell lines

INTRODUCTION

Bisphosphonates are inhibitors of osteoclast-mediated tumor-stimulated osteolysis, and they have become standard therapy for the management of bone metastases from breast cancer. These drugs can also directly induce growth inhibition and apoptosis of osteotropic cancer cells, including estrogen receptor-positive (ER+) breast cancer cells.

METHODS

We examined the anti-proliferative properties of ibandronate on two ER+ breast cancer cell lines (MCF-7 and IBEP-2), and on one ER negative (ER-) cell line (MDA-MB-231). Experiments were performed in steroid-free medium to assess ER regulation and the effect of ibandronate in combination with estrogen or antiestrogens.

RESULTS

Ibandronate inhibited cancer cell growth in a dose- and time-dependent manner (approximate IC50: 10(-4) M for MCF-7 and IBEP-2 cells; 3 x 10(-4) M for MDA-MB-231 cells), partly through apoptosis induction. It completely abolished the mitogenic effect induced by 17beta-estradiol in ER+ breast cancer cells, but affected neither ER regulation nor estrogen-induced progesterone receptor expression, as documented in MCF-7 cells. Moreover, ibandronate enhanced the growth inhibitory action of partial (4-hydroxytamoxifen) and pure (ICI 182,780, now called fluvestrant or Faslodex) antiestrogens in estrogen-sensitive breast cancer cells. Combination analysis identified additive interactions between ibandronate and ER antagonists.

CONCLUSION

These data constitute the first in vitro evidence for additive effects between ibandronate and antiestrogens, supporting their combined use for the treatment of bone metastases from breast cancer.