Date of Graduation
Doctor of Philosophy (PhD)
Gary E. Gallick
Anil K. Sood
David J. McConkey
Mark A. Titus
Prostate cancer (PCa) is the second leading cause of cancer-related deaths in men in the United States, with most deaths occurring from bone metastasis. Several new therapies have been FDA approved for bone-metastatic PCa, but patient survival has only marginally improved due to therapy resistance, which often arises from constitutive activation of compensatory signaling pathways. This dissertation work focused on a mechanistic understanding of how cross talk between tyrosine kinase receptors contributes to therapy resistance, and how this may be overcome by downregulating expression of these receptors. In PCa cell lines and xenograft models, I demonstrated that activation of IGF-1R receptor tyrosine kinase (RTK) through IGF-1 leads to delayed, ligand-independent activation of another RTK, MET, that requires Src activation and transcription, suggesting that downregulation of expression of these kinases may be required for better inhibition of their functions.
I therefore examined the biologic effects of overexpression of miR-34a, a tumor suppressive microRNA that downregulates multiple proteins involved in PCa progression. I demonstrated that miR-34a is downregulated in high metastatic PCa cell lines, concomitant with its targets being overexpressed. Overexpression of miR-34a decreased several properties associated with metastasis, including-migration, invasion, and proliferation. I next demonstrated that miR-34a delivery to xenografts grown in the femurs of immunocompromised mice inhibited prostate tumor growth and preserved bone integrity.
To examine the mechanisms by which miR-34a overexpression inhibited cancer growth, autophagy and apoptosis pathways were studied. I determined the expression of autophagy markers and the requirement of key signaling intermediates in the autophagic pathway upon miR-34a overexpression. I demonstrated that miR-34a overexpression induced apoptosis along with a non-canonical form of autophagy that is independent of ATG5, ATG7 and Beclin-1 expression.
In summary, studies in this dissertation provide evidence for IGF-1/1R induced ligand-independent MET activation, suggesting that cross talk among receptors may be responsible for resistance to targeted therapies. To potentially overcome this problem, I demonstrated that delivery of miR-34a that downregulates proteins involved in PCa progression decreases tumor growth in the bone. Overexpression of miR-34a induces apoptosis and a novel form of autophagy that might contribute to its therapeutic effects.
Prostate cancer, miRNA, miR-34a, bone, chitosan, autophagy