Abstract #2538: Identification of microRNAs miR-203 and miR-335 forming a network of regulation in breast cancer development

The expression of microRNAs is altered in various cancer types, leading to a nomenclature of onco- and tumor-suppressor microRNAs. In the present study, we investigated the role of two independent microRNAs, miR-203 and miR-335, for the formation of sporadic human breast cancer and their involvement in the network of proteins regulating the cancer susceptibility gene BRCA1. We performed protein quantification of the ligand dependent receptors ER\#945;, AhR, IGF1R and the transcription factors SP1 and ID4 following overexpression of the specific microRNAs. All these proteins are involved in the regulation of BRCA1 and were previously associated with sporadic breast cancer. The significance of the microRNA regulation was determined in various functional assays including cell viability, apoptosis, cell cycle status and determination of BRCA1 expression following overexpression of the microRNAs in a cell culture system. The importance for cancer formation was further validated by expression analysis of primary breast cancer specimens compared to healthy control tissues and association to clinical features. MicroRNA miR-335 is suspected to act as a tumor-suppressor microRNA. In the present study, we found a reduced expression of miR-335 in primary sporadic breast cancer specimens. The expression level of miR-335 correlates positively to the transcript level of BRCA1. Overexpression of miR-335 led to decreased cell viability, increasing rates of apoptosis and downregulation of the hormone receptors (ER\#945;, AhR) and transcription factors (SP1). MicroRNA miR-203 also influences the regulatory pathway of BRCA1 by regulating the expression of ER\#945;, AhR and IGF1R. Increasing levels of miR-203 correlate to BRCA1 downregulation. Functionally, overexpression of miR-203 resulted in a decreased cell viability associated with an increase in apoptosis and cell cycle arrest. Expression analysis of breast cancer specimens revealed two subgroups. Tumors expressing low levels of miR-203 revealed an enhanced formation of lymph node metastasis. Taking the results together, both microRNAs affect the same targets in the signaling pathways of breast cancer cells involving the regulation of the breast cancer susceptibility gene BRCA1. Functionally, they are connected with cell proliferation, apoptosis and cell cycle regulation. A misregulation during cancer development and progression might promote the tumorigenic potential on the one hand and suppress the tumor-suppressing signals on the other. Affecting several overlapping molecules at once, the microRNAs form a superior network of regulation of the breast tissue homeostasis. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2538.