Role of Membrane-Derived Second Messengers and Bmx/Etk in Response to Radiation Treatment of Prostate Cancer

摘要

Radiation-induced activation of the PI3K/Akt signal transduction pathway requires Akt binding to phosphatidyl-inositol phosphates (PIPs) on the cell membrane. The tyrosine kinase, bone marrow X-kinase (Bmx), binds to PIPs in a manner similar to Akt. Since Bmx is known to be involved in cell growth and survival pathways, Bmx could contribute to the radiation response within the vascular endothelium and prostate cancer, which highly express this protein. We therefore are studying Bmx signaling within the vascular endothelium and prostate cancer. Our initial studies have focused on the vascular endothelium. Bmx was activated rapidly in response to clinically relevant doses of ionizing radiation. Bmx inhibition enhanced the efficacy of radiotherapy in endothelial cells (human umbilical vein endothelial cells or HUVEC) within in vitro systems. Retroviral shRNA knockdown of Bmx protein enhanced HUVEC radiosensitization. Furthermore, pretreatment of HUVEC with a pharmacological inhibitor of Bmx, LFM-A13, produced significant radiosensitization of endothelial cells as measured by clonogenic survival analysis and apoptosis as well as functional assays including cell migration and tubule formation. In vivo, LFM-A13, when combined with radiation resulted in significant inhibition of blood flow within prostate tumors as measured by pixel weighted power doppler. However, when tumor growth delay studies were performed in mice using human prostate cancers, the combination of LFM-A13 with radiation was not superior to either treatment alone. So, although LFM-A13 inhibition of Bmx did produce a tumor growth delay effect vs. vehicle control, no radiation enhancement was detected. Bmx, thus, represents a standalone molecular target for prostate cancer treatment and for radiosensitizing tumor vasculature, but how to best combine treatments remains unclear.