Transcriptional Regulation of Estrogen Receptor Alpha Target Genes by Hexamethylene Bisacetamide-Inducible Gene 1 (HEXIM1) and Its Role in Mammary Gland Development and Breast Cancer

Breast cancer is the second leading cause of death in women in the United States, making the search for more therapeutic targets in breast cancer etiology very significant. This aim of this thesis was to further elucidate the function of hexamethylene bisacetamide-inducible protein 1 (HEXIM1) in estrogen (E2)-mediated signaling and mammary gland development and tumorigenesis. In previous work, our laboratory has shown that HEXIM1 inhibits breast cell growth and estrogen receptor-alpha (ERα) transcriptional activity. Other studies have shown that HEXIM1 inhibits the activity of positive transcription elongation factor b (P-TEFb) with functional consequences for gene expression. However, it is not clear whether HEXIM1 regulation of all genes is P-TEFb-dependent and how the P-TEFb-inhibitory function of HEXIM1 ties into mammary gland development or tumorigenesis—which is the main focus of the work described therein.We demonstrate that HEXIM1 regulates E2-driven transcriptional activity of ERα and P-TEFb within the context of some ERα target genes. P-TEFb inhibition correlated with a decrease in expression of ERα target genes, cyclin D1 and pS2. Cyclin D1 plays a major role in the cell cycle and tumorigenesis. In the mammary gland, we found that increased HEXIM1 expression decreased ductal branching, an E2/ERα-driven process and decreased cyclin D1 expression.Additionally, recent work in our laboratory uncovered a novel role for HEXIM1 during heart and vascular development via the regulation of the vascular endothelial growth factor (VEGF). VEGF is a cytokine that regulates the formation of blood vessels and it represents a major target for tumor therapy. In this thesis, we demonstrate that HEXIM1 regulates VEGF expression in a P-TEFb-independent manner in breast epithelial cells by decreasing E2/ERα-induced expression of hypoxia-inducible factor 1 alpha (HIF-1α), a potent inducer of VEGF. In the mammary gland and during tumorigenesis, HEXIM1 plays a role in regulating HIF-1α and VEGF expression and related-angiogenesis.Overall, the studies provide insight into the role of HEXIM1 in regulating E2/ERα gene expression inbreast cancer cells and in the mammary gland. Ultimately, we hope that the collective of this work demonstrates therapeutic potential for HEXIM1 given its anti-proliferative and anti-angiogenic functions in vivo.