Resistance to endocrine therapy of breast cancer: recent advances and tomorrow's challenges

Estrogen Regulates Mitochondrial Morphology through Phosphorylation of Dynamin-related Protein 1 in MCF7 Human Breast Cancer Cells

Estrogen affects mitochondrial function in various tissues, but the precise mechanism remains unclear. We, therefore investigated the effect on estrogen-regulated mitochondrial morphology by dynamin-related protein 1 (Drp1) and its Ser616-phosphorylated derivative (pDrp1^Ser616) are involved in mitochondrial fission. MCF7 human breast cancer cells were treated with 17β-estradiol (E₂), an estrogen receptor (ER) α and β antagonist (ICI 182, 780), an ERα antagonist (MPP), and an ERβ antagonist (PHTPP) for 24 hr. The expression of Drp1 and pDrp1^Ser616 was analyzed by western blotting and immunohistochemistry. Mitochondrial morphology was analyzed by transmission electron microscopy (TEM). In control cells, Drp1 was detected in the cytoplasm of all cells while pDrp1 was observed in the cytoplasm of 3.4 ± 1.0% of the total population. After E₂ treatment, pDrp1^Ser616-positive cells comprised 30.6 ± 5.6% of the total population, 10.5 ± 1.7% after E₂ + ICI treatment, 12.4 ± 4.2% after E₂ + MPP treatment, and 24.0 ± 2.2% after E₂ + PHTPP treatment. In ERα knockdown MCF7 cells, pDrp1 expression was decreased after E₂ treatment compared to E₂-treated wild type cells. Tubular pattern mitochondria were found in the control cells but the number of short and small pattern mitochondria (< 0.5 μm²) was significantly increased after E₂ treatment (as observed by TEM). We, therefore concluded that the phosphorylation of Drp1 is important for E₂-dependent mitochondrial morphological changes through ERα.

Understanding the mechanisms of aromatase inhibitor resistance

Aromatase inhibitors (AIs) have a central role in the treatment of breast cancer; however, resistance is a major obstacle to optimal management. Evidence from endocrine, molecular and pathological measurements in clinical material taken before and after therapy with AIs and data from clinical trials in which AIs have been given as treatment either alone or in combination with other targeted agents suggest diverse causes for resistance. These include inherent tumour insensitivity to oestrogen, ineffective inhibition of aromatase, sources of oestrogenic hormones independent of aromatase, activation of signalling by non-endocrine pathways, enhanced cell survival and selection of hormone-insensitive cellular clones during treatment.

Markers of sensitivity, dependence and resistance to endocrine therapy for breast cancer

Owing to its efficacy and relative lack of toxicities, endocrine therapy is a major treatment modality for breast cancer. However, resistance and the inability to accurately predict response are obstacles to optimal management. There is a need to identify markers of clinical response and elucidate mechanisms of resistance. In this article, evidence will be presented demonstrating that: discovery of predictive markers is dependent upon the approach employed and the application required; and mechanisms of resistance are diverse and not simply mirror images of response. Different information may be obtained according to assessment read outs and type of analysis. Tumors respond to endocrine therapy in a variety of ways and a range of end points can be used to monitor hormone dependence, sensitivity and resistance to treatment. Different forms of endocrine therapies may have differing mechanisms of action - hence, markers of sensitivity/response can vary between treatments and there may be correspondingly differing mechanisms of resistance.

Aromatase inhibitors: prediction of response and nature of resistance

IMPORTANCE OF THE FIELD

Aromatase inhibitors (AIs) are recommended for and central to endocrine management of breast cancer patients. Response rates can be high, but resistance is a major obstacle. Optimal management therefore requires accurate prediction of response and an understanding of the nature by which resistance occurs. These are the subjects of this review.

AREAS COVERED IN THIS REVIEW

The complications of assessing response in different clinical settings and the types of response in terms of clinical, pathological, proliferative and molecular endpoints are reviewed. The current status of predictors of response such as estrogen receptors (ERs), progesterone receptors, other markers of estrogen action, ER phosphorylation, ER coregulators and multigene signatures are assessed. Different types of resistance to AIs, their heterogeneity, diversity in mechanisms of resistance and their identification are also considered.

WHAT THE READER WILL GAIN

The review provides fundamental information on response and predictors of response to AIs as well as an understanding of the diversity of resistance mechanisms to such endocrine agents.

TAKE HOME MESSAGES

ER status is the only factor used routinely for treatment selection, but additional markers are needed to predict response. Other markers have some predictive powers, but are of limited utility. The hope is, therefore, that discovery strategies based on genome-wide searches will identify new markers. Assessments may be required both before and after a short period of treatment so that early changes can be used to predict subsequent clinical response. Mechanisms of resistance to AIs are diverse. Knowledge of specific resistance mechanisms in individual cases will be necessary if strategies to circumvent resistance are to be developed rationally. A future can be envisaged in which molecular phenotyping of individual tumors is used to decide not only which patients should be treated with AIs but also whether AIs should be used alone or in combination/sequence with other drug regimes.

Aromatase inhibition: translation into a successful therapeutic approach

The development of the novel third-generation aromatase inhibitors and inactivators for breast cancer treatment is one of the most successful contemporary achievements in cancer therapy. Parallel to studies evaluating toxicity and clinical efficacy in metastatic disease, the endocrine effects of multiple compounds were evaluated, leading to the identification of the highly potent third-generation aromatase inhibitors based on estrogen deprivation and aromatase inhibition in vivo. Thus, translational studies have been of vital importance identifying the unique characteristics of these compounds. Whereas first- and second-generation aromatase inhibitors inhibit estrogen synthesis in vivo by up to 90%, the third-generation compounds anastrozole, exemestane, and letrozole were found to cause > or =98% aromatase inhibition. This article summarizes and discusses the "translational research" that provided the background for the implementation of the third-generation aromatase inhibitors and inactivators into large clinical trials. The need for future translational research exploiting the mechanisms of resistance to these compounds for future improvement of endocrine therapy is emphasized.

Effects of anastrozole on the intratumoral gene expression in locally advanced breast cancer

Intratumoral levels of E1 (oestrone), E1S (oestrone sulphate) and E2 (oestradiol) are significantly reduced by treatment with the aromatase inhibitor anastrozole regardless of treatment response. The purpose of the present pilot study was to look for additional markers of biochemical response to aromatase inhibitors on mRNA expression level. Whole genome expression was studied using microarray analysis of breast cancer tissue from 12 patients with locally advanced tumors, both before and following 15 weeks of treatment with the aromatase inhibitor anastrozole (Arimidex). Intratumoral mRNA levels for a subset of genes coding for steroid metabolizing enzymes, hormone receptors and some growth mediators involved in cell cycle control were analysed by quantitative RT-PCR. There was a correlation between the two methods for some but not all genes. The mRNA expression levels of the different genes were correlated to each other and to the intratumoral levels of E1, E2 and E1S, before and after the treatment. Notably, a correlation of the E1/E2 metabolic ratio to the mRNA levels of CYP19A1 was observed before treatment (r=0.745, p<0.005). Whole genome expression analysis of these 12 breast cancer patients revealed similar tumor classification to previously published larger studies. Tumors with no or low expression of ESR1 (oestrogen receptor) clustered together and were characterized by a strong basal-like signature highly expressing keratins 5/17, cadherin 3, frizzled and apolipoprotein D, among others. The luminal epithelial tumor cluster, on the other hand, highly expressed ESR1, GATA binding protein 3 and N-acetyl transferase. An evident ERBB2 cluster was observed due to the marked over-expression of the ERBB2 gene and GRB7 and PPARBP in this patient material). Using significance analysis of microarrays (SAM), we identified 298 genes significantly differently expressed between the partial response and progressive disease groups.

Mifepristone induces growth arrest, caspase activation, and apoptosis of estrogen receptor-expressing, antiestrogen-resistant breast cancer cells

PURPOSE

A major clinical problem in the treatment of breast cancer is the inherent and acquired resistance to antiestrogen therapy. In this study, we sought to determine whether antiprogestin treatment, used as a monotherapy or in combination with antiestrogen therapy, induced growth arrest and active cell death in antiestrogen-resistant breast cancer cells.

EXPERIMENTAL DESIGN

MCF-7 sublines were established from independent clonal isolations performed in the absence of drug selection and tested for their response to the antiestrogens 4-hydroxytamoxifen (4-OHT) and ICI 182,780 (fulvestrant), and the antiprogestin mifepristone (MIF). The cytostatic (growth arrest) effects of the hormones were assessed with proliferation assays, cell counting, flow cytometry, and a determination of the phosphorylation status of the retinoblastoma protein. The cytotoxic (apoptotic) effects were analyzed by assessing increases in caspase activity and cleavage of poly(ADP-ribose) polymerase.

RESULTS

All of the clonally derived MCF-7 sublines expressed estrogen receptor and progesterone receptor but showed a wide range of antiestrogen sensitivity, including resistance to physiological levels of 4-OHT. Importantly, all of the clones were sensitive to the antiprogestin MIF, whether used as a monotherapy or in combination with 4-OHT. MIF induced retinoblastoma activation, G(1) arrest, and apoptosis preceded by caspase activation.

CONCLUSIONS

We demonstrate that: (a) estrogen receptor(+)progesterone receptor(+), 4-OHT-resistant clonal variants can be isolated from an MCF-7 cell line in the absence of antiestrogen selection; and (b) MIF and MIF plus 4-OHT combination therapy induces growth arrest and active cell death of the antiestrogen-resistant breast cancer cells. These preclinical findings show potential for a combined hormonal regimen of an antiestrogen and an antiprogestin to combat the emergence of antiestrogen-resistant breast cancer cells and, ultimately, improve the therapeutic index of antiestrogen therapy.

Oestrogen suppression--lessons from clinical studies

In few, if any, areas of cancer research has "translational research" played a role such as the one it has played in the development of endocrine therapy. Thus, much of the understanding of the mechanisms of action of different therapies has been achieved by developing novel hypotheses based on clinical observations. Ovarian ablation was developed as an empirical therapy long before characterization of oestrogen disposition or detection of the oestrogen receptor. The first-generation aromatase inhibitor, aminoglutethimide, was implemented as a strategy to achieve a 'medical adrenalectomy' and was discovered as an aromatase inhibitor subsequent to clinical observations. Currently, observations such as the lack of cross-resistance between aromatase inhibitors and inactivators and the diverse effects from combined therapy using tamoxifen in concert with hormonal suppression in pre- and post-menopausals are provoking interesting questions for further research.

Biological rationale for endocrine therapy in breast cancer

Oestrogens are heavily implicated in the risk to, and progression of, breast cancer. Therapeutic strategies targeted at the oestrogenic stimulus to the breast and hormone-sensitive breast cancers are extremely attractive measures both to prevent the disease and to treat established tumours. The present review outlines the biological rationale for such endocrine therapy and traces the evolution whereby irreversible surgical procedures have been replaced by potent and specific drugs. In particular, the development of the latest generation of agents which inhibit oestrogen biosynthesis (aromatase inhibitors) is considered by defining the central role of the aromatase enzyme, its regulation and contribution to circulating and tumour endogenous oestrogens. The nature of response and resistance which may be elicited following the use of endocrine therapy is also described as this may determine the optimal use of aromatase inhibitors and, more generally, anti-hormone therapy in the management of women at high risk to, or with, breast cancer.

Breast cancer tissue estrogens and their manipulation with aromatase inhibitors and inactivators

Despite the dramatic fall in plasma estrogen levels at menopause, only minor differences in breast tissue estrogen levels have been reported comparing pre- and postmenopausal women. Thus, postmenopausal breast tissue has the ability to maintain concentrations of estrone (E1) and estradiol (E2) that are 2-10- and 10-20-fold higher than the corresponding plasma estrogen levels. This finding may be explained by uptake of estrogens from the circulation and/or local estrogen production. Local aromatase activity in breast tissue seems to be of crucial importance for the local estrogen production in some patients while uptake from the circulation may be more important in other patients. Beside aromatase, breast tissue expresses estrogen sulfotransferase and sulfatase as well as dehydrogenase activity, allowing estrogen storage and release in the cells as well as conversions between estrone and estradiol. The activity of the enzyme network in breast cancer tissue is modified by a variety of factors like growth factors and cytokines. Aromatase inhibitors have been used for more than two decades in the treatment of postmenopausal metastatic breast cancer and are currently investigated in the adjuvant treatment and even prevention of breast cancer. Novel aromatase inhibitors and inactivators have been shown to suppress plasma estrogen levels effectively in postmenopausal breast cancer patients. However, knowledge about the influence of these drugs on estrogen levels in breast cancer tissue is limited. Using a novel HPLC-RIA method developed for the determination of breast tissue estrogen concentrations, we measured tissue E1, E2 and estrone sulfate (E1S) levels in postmenopausal breast cancer patients before and during treatment with anastrozole. Our findings revealed high breast tumor tissue estrogen concentrations that were effectively decreased by anastrozole. While E1S was the dominating estrogen fraction in the plasma, estradiol was the estrogen fraction with the highest concentration in tumor tissue. Moreover, plasma estrogen levels did not correlate with tissue estrogen concentrations. The overall experience with aromatase inhibitors and inactivators concerning their influences on breast tissue estrogen concentrations is summarized.

Comparison between aromatase inhibitors and sequential use

The biochemical efficacy of aromatase inhibitors and inactivators in vivo may be determined by two types of methods; by measuring plasma or tissue estrogen levels, or assessment of the conversion of the androgen substrate (in practice, androstenedione) into estrogens (estrone) by the use of tracer methods. While methods to determine plasma and tissue estrogens are limited through lack of sensitivity required to measure the very low concentrations recorded in postmenopausal women on treatment with these compounds, measurement of in vivo aromatization is an extensive procedure, applicable to a limited number of patients only. While we may correlate the mean level of aromatase inhibition achieved with different compounds to clinical efficacy, data correlating individual estrogen suppression to clinical outcome among patients treated with a specific compound is limited. The now well-characterized phenomenon of lack of cross-resistance between non-steroidal aromatase inhibitors and steroidal aromatase inactivators are likely due to biochemical effects not related to differences in total body aromatase inhibition.

Aromatase inhibitors and inactivators for breast cancer therapy

Aromatase inhibitors and inactivators are increasingly important to the therapy of advanced breast cancer in postmenopausal women. These compounds are also currently being evaluated in the adjuvant setting and may have potential in breast cancer prevention. In addition to the recent clinical results, experimental research with development of aromatase 'knockout' mice as well as certain clinical observations in individuals lacking this enzyme have deepened our understanding of estrogens outside of the field of reproduction. Such information should help us to further develop this type of therapy in breast cancer and, in particular, extend our understanding of the lack of complete cross-resistance between aromatase inhibitors and inactivators. Clinically, third-generation aromatase inhibitors and inactivators have shown superiority compared with conventional treatment in advanced postmenopausal breast cancer with respect to second-line (tamoxifen failures) as well as first-line therapy. The fact that tamoxifen is noncurative in metastatic disease but improves long-term survival in the adjuvant setting suggests that even modest improvements in therapy of advanced disease may be translated into survival benefits in patients with early disease. In addition, these novel compounds with lack of complete cross-resistance extend the scope of using sequential treatment options to maximise the duration of optimal endocrine therapy in metastatic breast cancer disease.