Overcoming Resistance to Endocrine Therapy in Breast Cancer: New Approaches to a Nagging Problem

Abstract

In the majority of women, breast cancer progresses through increased transcriptional activity due to over-expressed oestrogen receptors (ER). Therapeutic strategies include: (i) reduction of circulating ovarian oestrogens or of peripherally produced oestrogen (in postmenopausal women) with aromatase inhibitors and (ii) application of selective ER modulators for receptor blockade. The success of these interventions is limited by the variable but persistent onset of acquired resistance and by an intrinsic refractiveness which manifests despite adequate levels of ER in about 50% of patients with advanced metastatic disease. Loss of functional ER leads to endocrine insensitivity, loss of cellular adhesion and polarity, and increased migratory potential due to trans-differentiation of the epithelial cancer cells into a mesenchymal-like phenotype (epithelial-mesenchymal transition; EMT). Multiple mechanisms contributing to therapeutic failure have been proposed: (i) loss or modification of ER expression including epigenetic mechanisms, (ii) agonistic actions of selective ER modulators that may be enhanced through an increased expression of co-activators, (iii) attenuation of the tamoxifen metabolism through expression of genetic variants of P450 cytochromes which leads to more or less active metabolites and (iv) increased growth factor signalling particularly through epidermal growth factor receptor activation of pathways involving keratinocyte growth factor, platelet-derived growth factor, and nuclear factor x03BA;B. In addition, the small non-coding microRNAs, recently recognized as critical gene regulators, exhibit differential expression in tamoxifen-sensitive versus resistant cell lines. Several studies suggest the potential of using these either as targets or as therapeutic agents to modulate EMT regulators as a means of reversing the aggressive metastatic phenotype by reversal of the EMT, with the added benefit of re-sensitization to anti-oestrogens.

Fig. 1

Mechanisms of endocrine resistance. Various molecules are integrated in a complex network, each playing a role in the development of hormonal therapy resistance in breast cancer. RTK, with their down-stream signalling molecules (e.g. FAK, Src, Akt and PI3K) promote membrane oestrogen-independent ER phosphorylation, leading to constitutive activation, to induce a more invasive and malignant phenotype. Loss/reduction of ERβ can also lead to increased ERα-promoted activation. Other cell-signalling receptors such as integrins contribute through synergizing of the action of RTK and the activation of FAK. Environmental stress leads to an elevated expression of Hsp and p38, nboth of which enhance resistance by maintaining high levels of RTK and over-expression of ER co-activator molecules which stimulate ER signalling. Endocrine resistance can also be due to loss/mutation of ER by promotion of EMT through mediators such as SNAIL. ERα can also be downregulated by specific miRNA. Tamoxifen metabolism to its more active metabolite endoxifen can be reduced by a lower activity of CYP2D6 enzyme variants, as well as by an increased efflux through an upregulated P-glycoprotein pump. TAM = Tamoxifen; ENDOX = endoxifen; Co-A = co-activator.

Fig. 2

EMT is determined by various molecular events; examples are illustrated. Most commonly, several groups of transcription factors can act to downregulate epithelial markers such as E-cadherin. Regulation of such transcription factors by molecules such as YB-1 can sustain EMT. Conversely, miRNAs can block EMT by targeting its mediators.

Fig. 3

Integrated miRNA network. miRNAs can be involved in a complex network where they can target the same or different mRNAs affecting different processes. miRNA-221/222 can target and suppress the expression of phosphatase and tensin homologue and p27kip1, thus alleviating the repression of MAPK and HER2, respectively, ultimately leading to a high cell proliferation. miRNA-221/222 also directly targets ER, pushing cancer cells toward EMT and endocrine resistance. On the other hand, miRNA-124 and miRNA-145 attenuate EMT and resistance to hormonal therapy by targeting EMT-related transcription factors (SLUG and SNAII), thus preventing their inhibition of E-cadherin. miRNA-124 also targets and degrades ETS, which leads to a lower cell proliferation. miRNA-145 represses the migration capacities of cancer cells by targeting JAM-1 and fascin. miRNA-9 plays a crucial role in the induction of EMT through direct repression of E-cadherin and indirect upregulation of vimentin.

Fig. 4

miRNA manipulation as a treatment modality. miRNA binds to a specific target mRNA to facilitate the downregulation of target proteins. miRNA mimics are synthetic oligonucleotide duplexes that have the same sequence as the endogenous miRNA and function as replacements for the downregulated miRNAs (acting as tumour suppressors). Antagomirs (also called antimiRs) are oligonucleotides that have complementarity with the target miRNA, thereby binding and preventing them from interacting with the target mRNA, allowing a normal translation. An miR mask is complementary to a sequence in the mRNA and binds to it without affecting the mRNA integrity or translation, yet it prevents the endogenous miRNA from binding. A sponge miR mask differs from the miR mask in that it binds to any mRNA with a similar target sequence and is therefore miRNA seed specific and not gene specific.