Determine the Function of RORa in Breast Tissue Polarization and Malignancy

Loss of tissue polarity is often associated with enhanced cell invasion in breast cancer, and both characteristics are hallmarks of cancer progression from preinvasive to invasive disease. Therefore, identification of factors and pathways modulating tissue polarization and cell invasion is crucial not only to our understanding of breast cancer development and progression, but also to the discovery of potential targets for development of novel therapeutic strategies. By analyzing expression profiles of polarized and disorganized human mammary epithelial cells (HMECs) in physiologically relevant three-dimensional (3D) culture, we have identified retinoid orphan nuclear receptor alpha (RORa) as a potential transcription regulator of the polarization genes in HMECs. We have shown that RORa is suppressed by activated EGFR pathway, and the protein levels of RORa are associated with both tissue structure and malignant status of HMECs. Introduction of exogenous RORa reprograms the breast cancer cells to form polarized andlor non-invasive structures in 3D cultures, accompanied with activation of the tumor suppressor gene SEMA3F. Overexpression of SEMA3F also reverses the malignant phenotypes of breast cancer cells, suggesting that the function of RORa is at least partially mediated by SEMA3F. Building upon these findings, we hypothesize that aberrant activation of EGFR down-regulates RORa in human breast cancer, which promotes tumor development and progression through suppressing SEMA3F transcription. The objective of this proposal is to characterize the function of RORa in breast cancer progression and delineate the molecular mechanisms of signal transduction in the EGFR-RORa-SEMA3Faxis. To test our central hypothesis and achieve the objective of this proposal, we have designed experiments with the following specific aims. Aim 1: Define the signaling pathways by which EGFR signals modulate RORa activities. Aim 2: Determine the molecular mechanisms by which RORa regulates SEMA3F transcription. Aim 3: Characterize the function of RORa in mammary tumor development and progression in vivo.