Breast cancer and microRNAs: therapeutic impact

Summary Despite advances in detection and therapies, breast cancer is still the leading cause of cancer death in women worldwide. The etiology of this neoplasm is complex, and both genetic and environmental factors contribute to the complicate scenario. Gene profiling studies have been extensively used over the last decades as a powerful tool to define the signature of different cancers and to predict outcome and response to therapies. More recently, a new class of small (19-25 nucleotides) non-coding RNAs, microRNAs (miRs or miRNAs) has been linked to several human diseases, included cancer. MicroRNAs are involved in temporal and tissue-specific eukaryotic gene regulation, 1 either by translational inhibition or exonucleolytic mRNA decay, targeted through imperfect complementarity between the microRNA and the 3′ untranslated region (3′UTR) of the mRNA. 2 Since their ability to potentially target any human mRNA, it is likely that microRNAs are involved in almost every biological process, including cell cycle regulation, cell growth, apoptosis, cell differentiation and stress response. 3 The involvement of microRNAs in the biology of human cancer is supported by an increasing body of experimental evidence, that has gradually switched from profiling studies, as the first breast cancer specific signature reported in 2005 by our group 4 describing an aberrant microRNA expression in different tumor types, to biological demonstrations of the causal role of these small molecules in the tumorigenic process, and the possible implications as biomarkers or therapeutic tools. 5 These more recent studies have widely demonstrated that microRNAs can modulate oncogenic or tumor suppressor pathways, and that, at the same time, their expression can be regulated by oncogenes or tumor suppressor genes. The possibility to modulate microRNA expression both in vitro and in vivo by developing synthetic pre-microRNA molecules or antisense oligonucletides has at the same time provided a powerful tool to a deeper comprehension of the molecular mechanisms regulated by these molecules, and suggested the intriguing and promising perspective of a possible use in therapy. Here we review our current knowledge about the involvement of microRNAs in cancer, focusing particularly on breast cancer, and their potential as diagnostic, prognostic and therapeutic tools

Breast cancer-secreted miR-939 downregulates VE-cadherin and destroys the barrier function of endothelial monolayers.

Abstract Exosomes-secreted microRNAs play an important role in metastatic spread. During this process breast cancer cells acquire the ability to transmigrate through blood vessels by inducing changes in the endothelial barrier. We focused on miR-939 that is predicted to target VE-cadherin, a component of adherens junction involved in vessel permeability. By in silico analysis miR-939 was found highly expressed in the basal-like tumor subtypes and in our cohort of 63 triple-negative breast cancers (TNBCs) its expression significantly interacted with lymph node status in predicting disease-free survival probability. We demonstrated, in vitro , that miR-939 directly targets VE-cadherin leading to an increase in HUVECs monolayer permeability. MDA-MB-231 cells transfected with a miR-939 mimic, released miR-939 in exosomes that, once internalized in endothelial cells, favored trans-endothelial migration of MDA-MB-231-GFP cells by the disruption of the endothelial barrier. Notably, when up taken in endothelial cells exosomes caused VE-cadherin down-regulation specifically through miR-939 as we demonstrated by inhibiting miR-939 expression in exosomes-releasing TNBC cells. Together, our data indentify an extracellular pro-tumorigenic role for tumor-derived, exosome-associated miR-939 that can explain its association with worse prognosis in TNBCs

Relationship between p53 and p27 expression following HER2 signaling

HER2, frequently associated with low p27 expression in breast tumors, when activated has been found to upmodulate p53 in tumor cells. The aim of this work was to investigate the role of p53 in the connection between HER2 and p27. Fifty-two breast tumor specimens, characterized for p53 mutations, were analyzed immunohistochemically (IHC) for HER2, p53 and p27 expression. p27, inversely associated with HER2, was found in 29% of tumors with IHC-negative mutated p53 versus 93% of tumors with accumulation of p53 protein and 59% with wild-type p53 (p=0.001), indicating a direct association between p53 and p27 expression. HER2-overexpressing cell lines carrying wild-type or null p53 protein, and treated with heregulin beta1 (HRG), were analyzed for expression and subcellular localization of p53 and p27. In HER2-overexpressing cells stimulated with HRG, p27 protein expression increased in parallel with p53 with no corresponding increase in p27 transcript. No p27 increase was observed in p53-null cells. Transfection with wild-type p53 restored p27 upmodulation in HRG-stimulated cells, indicating a crucial role of p53 in determining p27 upmodulation following HER2 activation. Together, our data demonstrate the crucial role of p53 in determining p27 upmodulation following HER2 activation. This could have implications in the response to Transtuzumab therapy

Genetic Loss of miR-205 Causes Increased Mammary Gland Development

MiRNAs play crucial roles in a broad spectrum of biological processes, both physiological and pathological. Different reports implicate miR-205 in the control of breast stem cell properties. Differential miR-205 expression has been observed in different stages of mammary gland development and maturation. However, a functional role in this process has not been clearly demonstrated. We generated an miR-205 knockout in the FVB/N mouse strain, which is viable and characterized by enhanced mammary gland development. Indeed, mammary glands of miR-205−/− female mice at different ages (1.5 and 5.5 months) show increased outgrowth and branching. This evidence is consistent with our previously reported data demonstrating the direct miR-205-mediated targeting of HER3, a master regulator of mammary gland development, and the oncosuppressive activity of this microRNA in different types of breast cancer

Estrogen mediated-activation of miR-191/425 cluster modulates tumorigenicity of breast cancer cells depending on estrogen receptor status.

MicroRNAs (miRNAs), single-stranded non-coding RNAs, influence myriad biological processes that can contribute to cancer. Although tumor-suppressive and oncogenic functions have been characterized for some miRNAs, the majority of microRNAs have not been investigated for their ability to promote and modulate tumorigenesis. Here, we established that the miR-191/425 cluster is transcriptionally dependent on the host gene, DALRD3, and that the hormone 17β-estradiol (estrogen or E2) controls expression of both miR-191/425 and DALRD3. MiR-191/425 locus characterization revealed that the recruitment of estrogen receptor α (ERα) to the regulatory region of the miR-191/425-DALRD3 unit resulted in the accumulation of miR-191 and miR-425 and subsequent decrease in DALRD3 expression levels. We demonstrated that miR-191 protects ERα positive breast cancer cells from hormone starvation-induced apoptosis through the suppression of tumor-suppressor EGR1. Furthermore, enforced expression of the miR-191/425 cluster in aggressive breast cancer cells altered global gene expression profiles and enabled us to identify important tumor promoting genes, including SATB1, CCND2, and FSCN1, as targets of miR-191 and miR-425. Finally, in vitro and in vivo experiments demonstrated that miR-191 and miR-425 reduced proliferation, impaired tumorigenesis and metastasis, and increased expression of epithelial markers in aggressive breast cancer cells. Our data provide compelling evidence for the transcriptional regulation of the miR-191/425 cluster and for its context-specific biological determinants in breast cancers. Importantly, we demonstrated that the miR-191/425 cluster, by reducing the expression of an extensive network of genes, has a fundamental impact on cancer initiation and progression of breast cancer cells

Editorial to the Special Issue “MicroRNA Dysregulation in Tumor Occurrence, Progression and Response to Therapy”

In the last 20 years, the involvement of microRNAs in the biology of human tumors has been clearly demonstrated, and the scientific community has switched from an initial skepticism to an increasing interest toward what was called the “dark side” of DNA [...