Inhibition of mammary tumor growth by estrogens: is there a specific role for estrogen receptors alpha and beta?

Estrogen therapy induces an unfolded protein response to drive cell death in ER+ breast cancer

Estrogens have been shown to elicit anticancer effects against estrogen receptor α (ER)-positive breast cancer. We sought to determine the mechanism underlying the therapeutic response. Response to 17β-estradiol was assessed in ER+ breast cancer models with resistance to estrogen deprivation: WHIM16 patient-derived xenografts, C7-2-HI and C4-HI murine mammary adenocarcinomas, and long-term estrogen-deprived MCF-7 cells. As another means to reactivate ER, the anti-estrogen fulvestrant was withdrawn from fulvestrant-resistant MCF-7 cells. Transcriptional, growth, apoptosis, and molecular alterations in response to ER reactivation were measured. 17β-estradiol treatment and fulvestrant withdrawal induced transcriptional activation of ER, and cells adapted to estrogen deprivation or fulvestrant were hypersensitive to 17β-estradiol. ER transcriptional response was followed by an unfolded protein response and apoptosis. Such apoptosis was dependent upon the unfolded protein response, p53, and JNK signaling. Anticancer effects were most pronounced in models exhibiting genomic amplification of the gene encoding ER (ESR1), suggesting that engagement of ER at high levels is cytotoxic. These data indicate that long-term adaptation to estrogen deprivation or ER inhibition alters sensitivity to ER reactivation. In such adapted cells, 17β-estradiol treatment and anti-estrogen withdrawal hyperactivate ER, which drives an unfolded protein response and subsequent growth inhibition and apoptosis. 17β-estradiol treatment should be considered as a therapeutic option for anti-estrogen-resistant disease, particularly in patients with tumors harboring ESR1 amplification or ER overexpression. Furthermore, therapeutic strategies that enhance an unfolded protein response may increase the therapeutic effects of ER reactivation.

The effectiveness of nano chemotherapeutic particles combined with mifepristone depends on the PR isoform ratio in preclinical models of breast cancer

There is clinical and experimental evidence suggesting that antiprogestins might be used for the treatment of selected breast cancer patients. Our aim was to evaluate the effect of albumin-bound paclitaxel (Nab-paclitaxel) and pegylated doxorubicin liposomes (PEG-LD) in combination with mifepristone (MFP) in experimental breast cancer models expressing different ratios of progesterone receptor (PR) isoforms A and B. We used two antiprogestin-responsive (PRA>PRB) and two resistant (PRA<PRB) murine mammary carcinomas growing in BALB/c, GFP-BALB/c or nude mice, along with human T47D-YA and T47D-YB xenografts growing in immunocompromised NSG mice. MFP improved the therapeutic effects of suboptimal doses of Nab-paclitaxel or PEG-LD in murine and human carcinomas with higher levels of PRA than PRB. MFP induced tissue remodeling in PRA-overexpressing tumors, increasing the stromal/tumor cell ratio and the number of functional vessels. Accordingly, an increase in nanoparticles and drug accumulation was observed in stromal and tumor cells in MFP-treated tumors. We conclude that MFP induces an increase in vessels during tissue remodeling, favoring the selective accumulation of nanoparticles inside the tumors. We propose that antiprogestins have the potential to enhance the efficacy of chemotherapy in breast tumors with a high PRA/PRB ratio.

ERα-targeted therapy in ovarian cancer cells by a novel estradiol-platinum(II) hybrid

As we previously showed, we have synthesized a new family of 17β-estradiol-platinum(II) hybrids. Earlier studies revealed the VP-128 hybrid to show high efficiency compared with cisplatin toward hormone-dependent breast cancer cells. In the present research, we have studied the antitumor activity of VP-128 in vitro and in vivo against ovarian cancer. In nude mice with ovarian xenografts, VP-128 displayed selective activity toward hormone-dependent tumors and showed higher efficiency than cisplatin to inhibit tumor growth. Similarly, in vitro, transient transfection of estrogen receptor (ER)-α in ERα-negative A2780 cells increased their sensitivity to VP-128-induced apoptosis, confirming the selectivity of VP-128 toward hormone-dependent tumor cells. In agreement, Western blot analysis revealed that VP-128 induced higher caspase-9, caspase-3, and poly (ADP-ribose) polymerase cleavage compared with cisplatin. The activation of caspase-independent apoptosis was also observed in ERα-negative A2780 cells, in which VP-128 rapidly induced the translocation of apoptosis-inducing factor to the nucleus. Conversely, subcellular localization of apoptosis-inducing factor was not modified in ERα-positive Ovcar-3 cells. We also discovered that VP-128 induces autophagy in ovarian cancer cells because of the formation of acidic vesicular organelles (AVOs) and increase of Light Chain 3B-II protein responsible for the formation of autophagosomes; pathways related to autophagy (AKT and mammalian target of rapamycin) were also down-regulated, supporting this mechanism. Finally, the inhibition of autophagy using chloroquine increased VP-128 efficiency, indicating a possible combination therapy. Altogether these results highlight the beneficial value of VP-128 for the treatment of hormone-dependent ovarian cancers and provide preliminary proof of concept for the efficient targeting of ERα- by 17β-estradiol-Pt(II)-linked chemotherapeutic hybrids in these tumors.

Estrogen receptor beta growth-inhibitory effects are repressed through activation of MAPK and PI3K signalling in mammary epithelial and breast cancer cells

Two thirds of breast cancers express estrogen receptors (ER). ER alpha (ERα) mediates breast cancer cell proliferation, and expression of ERα is the standard choice to indicate adjuvant endocrine therapy. ERbeta (ERβ) inhibits growth in vitro; its effects in vivo have been incompletely investigated and its role in breast cancer and potential as alternative target in endocrine therapy needs further study. In this work, mammary epithelial (EpH4 and HC11) and breast cancer (MC4-L2) cells with endogenous ERα and ERβ expression and T47-D human breast cancer cells with recombinant ERβ (T47-DERβ) were used to explore effects exerted in vitro and in vivo by the ERβ agonists 2,3-bis (4-hydroxy-phenyl)-propionitrile (DPN) and 7-bromo-2-(4-hydroxyphenyl)-1,3-benzoxazol-5-ol (WAY). In vivo, ERβ agonists induced mammary gland hyperplasia and MC4-L2 tumour growth to a similar extent as the ERα agonist 4,4',4''-(4-propyl-(1H)-pyrazole-1,3,5-triyl) trisphenol (PPT) or 17β-estradiol (E2) and correlated with higher number of mitotic and lower number of apoptotic features. In vitro, in MC4-L2, EpH4 or HC11 cells incubated under basal conditions, ERβ agonists induced apoptosis measured as upregulation of p53 and apoptosis-inducible factor protein levels and increased caspase 3 activity, whereas PPT and E2 stimulated proliferation. However, when extracellular signal-regulated kinase 1 and 2 (ERK ½) were activated by co-incubation with basement membrane extract or epidermal growth factor, induction of apoptosis by ERβ agonists was repressed and DPN induced proliferation in a similar way as E2 or PPT. In a context of active ERK ½, phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/RAC-alpha serine/threonine-protein kinase (AKT) signalling was necessary to allow proliferation stimulated by ER agonists. Inhibition of MEK ½ with UO126 completely restored ERβ growth-inhibitory effects, whereas inhibition of PI3K by LY294002 inhibited ERβ-induced proliferation. These results show that the cellular context modulates ERβ growth-inhibitory effects and should be taken into consideration upon assessment of ERβ as target for endocrine treatment.

PI3K/AKT pathway regulates phosphorylation of steroid receptors, hormone independence and tumor differentiation in breast cancer

Using a model of medroxyprogesterone acetate (MPA)-induced mouse mammary tumors that transit through different stages of hormone dependence, we previously reported that the activation of the phosphatidylinositol 3-kinase (PI3K)/AKT (protein kinase B) pathway is critical for the growth of hormone-independent (HI) mammary carcinomas but not for the growth of hormone-dependent (HD) mammary carcinomas. The objective of this work was to explore whether the activation of the PI3K/AKT pathway is responsible for the changes in tumor phenotype and for the transition to autonomous growth. We found that the inhibition of the PI3K/AKT/mTOR (mammalian target of rapamycin) pathway suppresses HI tumor growth. In addition, we were able to induce mammary tumors in mice in the absence of MPA by inoculating HD tumor cells expressing a constitutively active form of AKT1, myristoylated AKT1 (myrAKT1). These tumors were highly differentiated and displayed a ductal phenotype with laminin-1 and cytokeratin 8 expression patterns typical of HI tumors. Furthermore, myrAKT1 increased the tumor growth of IBH-6 and IBH-7 human breast cancer cell lines. In the estrogen-dependent IBH-7 cell line, myrAKT1 induced estrogen-independent growth accompanied by the expression of the adhesion markers focal adhesion kinase and E-cadherin. Finally, we found that cells expressing myrAKT1 exhibited increased phosphorylation of the progesterone receptor at Ser190 and Ser294 and of the estrogen receptor α at Ser118 and Ser167, independently of exogenous MPA or estrogen supply. Our results indicate that the activation of the PI3K/AKT/mTOR pathway promotes tissue architecture remodeling and the activation of steroid receptors.

Estrogen receptor-β activation in combination with letrozole blocks the growth of breast cancer tumors resistant to letrozole therapy

Treatment with anti-estrogens or aromatase inhibitors (AI) is the main therapeutic strategy used against estrogen receptor ERα-positive breast cancer. Resistance to these therapies presents a major challenge in the management of breast cancer. Little is known about ERβ in breast carcinogenesis. Our aim in this study is to examine potential novel strategies utilizing ERβ activity to overcome AI resistance. We provide evidence that ERβ agonist can reduce the growth of AI-resistant breast cancer cells. Our data further confirm that therapeutic activation of ERβ by DPN, an ERβ agonist, blocks letrozole-resistant tumor growth in a xenograft model. Interestingly, DPN exerted tumor growth inhibition only in the presence of the AI letrozole, suggesting that combination therapy including ERβ activators and AI may be used in the clinical setting treating AI resistant breast cancer. An increase in ERβ levels, with diminished ERα/ERβ ratio, was observed in the tumors from mice treated with DPN/letrozole combination compared to single agents and control. Decreased Cyclin D1 and increased CyclinD1/CDK inhibitors p21 and p27 levels in DPN/letrozole treated tumors were observed, suggesting that the combination treatment may inhibit tumor growth by blocking G1/S phase cell cycle progression. Our data show a decrease in MAPK phosphorylation levels without affecting total levels. In addition to providing evidence suggesting the potential use of ERβ agonists in combination with letrozole in treating AI resistant breast cancer and prolonging sensitivity to AI, we also provide mechanistic evidence supporting the role of ERβ in altering the expression profile associated with resistance.

Responsiveness to PI3K and MEK inhibitors in breast cancer. Use of a 3D culture system to study pathways related to hormone independence in mice

BACKGROUND

A significant proportion of breast cancer patients face failure of endocrine therapy due to the acquisition of endocrine resistance. We have explored mechanisms involved in such disease progression by using a mouse breast cancer model that is induced by medroxyprogesterone acetate (MPA). These tumors transit through different stages of hormone sensitivity. However, when cells from tumor variants were seeded on plastic, all were stimulated by progestins and inhibited by antiprogestins such as RU486. Furthermore, cells from a RU486-resistant tumor variant recovered antiprogestin sensitivity.

HYPOTHESIS

A three-dimensional (3D) culture system, by maintaining differential cellular organization that is typical of each tumor variant, may allow for the maintenance of particular hormone responses and thus be appropriate for the study of the effects of specific inhibitors of signaling pathways associated with disease progression.

METHOD

We compared the behavior of tumors growing in vivo and cancer cells ex vivo (in 3D Matrigel). In this system, we evaluated the effects of kinase inhibitors and hormone antagonists on tumor growth.

PRINCIPAL FINDINGS

LY294002, a PI3K/AKT pathway inhibitor, decreased both tumor growth in vivo and cell survival in Matrigel in MPA-independent tumors with higher AKT activity. Induction of cell death by anti-hormones such as ICI182780 and ZK230211 was more effective in MPA-dependent tumors with lower AKT activity. Inhibition of MEK with PD98059 did not affect tumor growth in any tested variant. Finally, while Matrigel reproduced differential responsiveness of MPA-dependent and -independent breast cancer cells, it was not sufficient to preserve antiprogestin resistance of RU486-resistant tumors.

CONCLUSION

We demonstrated that the PI3K/AKT pathway is relevant for MPA-independent tumor growth. Three-dimensional cultures were useful to test the effects of kinase inhibitors on breast cancer growth and highlight the need for in vivo models to validate experimental tools used for selective therapeutic targeting.