The Role of HOXA1 in oncogenic transformation of the human mammary epithelial cell

Abstract

Expression of homeobox AI. (HOXAI) results in oncogenic transformation of immortalized human mammary epithelial cells with aggressive tumor formation in vivo. However, the mechanisms by which HOXAl mediates oncogenic transformation is not well defined. The primary aim of this thesis was to delineate the downstream effectors and signal transduction pathways utilized for HOXAl mediated oncogenic transformation of human mammary epithelial cells. To identify molecules that could potentially be involved in HOXAl-mediated oncogenic transformation, microarray analysis was utilized to characterize and compare the gene expression pattern in response to forced expression or depletion of HOXAl in human mammary carcinoma cells. Gene expression profiling identified that genes involved in the p44/42 mitogenactivated protein (MAP) kinase activation pathway (GRB2, MAP kinase kinase (MEKI) and SDFRl) or p44/42 MAP kinase-regulated genes (IER3, EPASl, PCNA and catalase) are downstream expression targets ofHOXAl. Expression of exogenous HOXAl increased GRB2 and MAP kinase kinase (MEKl) mRNA and protein expression and increased p44/42 MAP kinase phosphorylation, activity and Elk-1- mediated transcription. Use of a MEKl inhibitor demonstrated that increased p44/42 MAP kinase activity is required for the HOXAl-mediated increase in cell proliferation, survival, oncogenicity and oncogenic transformation. Thus, modulation of the p44/42 MAP kinase pathway is one mechanism by which HOXAl mediates oncogenic transformation of the human mammary epithelial cell. However, treatment with a MEKl inhibitor did not completely prevent HOXAl stimulated oncogenic transformation, suggestive of the involvement of additional signal transduction pathways. Here I report that forced expression of HOXAl in immortalised human mammary epithelial cells significantly increased levels of STAT3/5A/5B mRNA by transcriptional upregulation. Protein levels of STAT3/5B but not STAT5A, and phosphorylation level of STAT3/5B protein were significantly increased by forced expression of HOXAl. Forced expression of STAT3 or. STATSB was sufficient to oncogenically transform an immortalised human mammary epithelial cell. Antagonism of STAT3 or STATSB activity with dominant negative STAT3 or STATSB accordingly abrogated the ability of HOXAl to stimulate cell proliferation, survival, oncogenic transformation and generation of large disorganized multiacinar structures. These results suggest that modulation of the p44/42 MAP kinase, STAT3 and STAT5B pathways are pivotal down stream mechanisms, through which HOXAl mediates oncogenic transformation of human mammary epithelial cell.