MicroRNA miR‐335 is crucial for the BRCA1 regulatory cascade in breast cancer development

Abstract

The expression of microRNAs is altered in various cancer types, leading to their definition as onco-and tumor-suppressor microRNAs. In our study, we investigated the role of miR-335 in the formation of sporadic human breast cancer and its involvement in the regulatory network of the breast cancer susceptibility gene BRCA1. To validate single components of the BRCA1 cascade, microRNA overexpression was performed in a cell culture model with subsequent protein analysis and luciferase reporter assays. Here, we were able to identify miR-335 as simultaneously regulating the known BRCA1 activators ERa, IGF1R, SP1 and the repressor ID4, including a feedback regulation of miR-335 expression by estrogens. Overexpression of miR-335 resulted in an upregulation of BRCA1 mRNA expression, suggesting a functional dominance of ID4 signaling. The relevance of the miR-335 regulation for human breast cancer was confirmed in primary sporadic breast cancer specimens with significantly decreased miR-335 levels (p < 0.05) in comparison to normal controls. Interestingly, the microRNA expression level correlated positively to the BRCA1 transcript level, supporting the hypothesis of a miR-335-mediated regulation of the tumor suppressor gene. Functionally, overexpression of miR-335 led to decreased cell viability and an increase in apoptosis, supporting its tumor-suppressive function. In summary, our data indicate that miR-335 affects different targets in the upstream BRCA1-regulatory cascade with impact on key cellular functions such as proliferation and apoptosis. Deregulation of the microRNA during breast cancer development and progression may thereby lead to an increased tumorigenic potential by inactivating crucial tumor-suppressive signals.In Western countries, breast cancer is the most frequent tumor found in women, with a low incidence in men. The tumor develops in a multistep process from ductal epithelial cells by triggers as yet poorly defined. Human breast cancers can be divided into two major subtypes, the inherited (5-10%) and the sporadic form, representing the greatest part. 1 The inherited subtype is predominantly characterized by germline mutations in one of the cancer susceptibility genes, known as BRCA1 and BRCA2, favoring a high predisposition to develop breast and ovarian cancer. 2 Although BRCA1 mutations have been found only rarely in the sporadic form, decreased expression levels of the gene were identified in these tumors. 3,4 As BRCA1 expression is embedded in a tightly controlled network, involving various activating and repressing factors, an altered transcriptional regulation may cause deregulation of the gene. Here, the expression of BRCA1 was determined to be hormone-dependent via direct activation through the estrogen (ERa), the aryl hydrocarbon (AhR) and the insulinlike growth factor 1 (IGF1R) receptors. [5][6][7] These are supported by hormone-independent factors such as the specific protein 1 (SP1) and inhibited via the inhibitor of DNA binding 4 (ID4). 8,9 These factors were identified as potent regula...

DNA damage and breast cancer

“…Some microRNA is also suggested by initial studies to confer resistance to commonly used chemotherapy drugs such as Fulvestrant [15] . This complements studies suggesting that microRNAs can regulate BRCA1 cascades and thus DNA damage programs in cancer cells during cancer development [20] . For a brief review of breast cancer classification, screening tests, current treatment and expected outcomes (Table 1).…”

MicroRNAs in breast cancer: oncogene and tumor suppressors with clinical potential

“…Tavazoie et al (2008) reported that miR-335 can repress breast cancer migration and invasion through targeting SRY-related HMG-box 4 (SOX4) and tenascin C. By simultaneously regulating the known breast cancer 1 (BRCA1) activators, ER-α, IGF1 and Sp1, and the repressor ID4, miR-335 exerts its tumor-suppressive function by decreasing cell viability and inducing apoptosis (Heyn et al, 2011).…”

Dual roles for Id4 in the regulation of estrogen signaling in the mammary gland and ovary

“…In the male reproductive system, Id4 loss results in significantly reduced seminal vesicle size, prostatic intraepithelial neoplasia (PIN) lesions and sterility (Sharma et al, 2013), while a recent study in the mammary gland has shown that Id4 governs ductal elongation and branching via the Id4 target p38MAPK (Dong et al, 2011). In breast epithelial cells, Id4 negatively controls expression of the key tumor suppressor Brca1 (Beger et al, 2001) and also suppresses miR335, a positive regulator of Brca1 (Heyn et al, 2011). In the context of breast cancer, overexpression of Id4 tightly correlates with the 'triple-negative' subtype (de Candia et al, 2006;Wen et al, 2012) and inversely correlates with expression of BRCA1 and estrogen receptor α (ERα; Esr1) (Molyneux et al, 2010;Roldán et al, 2006).…”