Integrins as novel drug targets for overcoming innate drug resistance.

Acquired drug resistance continues to be one of the major obstacles hindering the successful treatment of many forms of cancer. Compounds utilized as antagonists of these cytoprotective mechanisms have, for the most part, proven to be ineffective at overcoming clinical resistance to cytotoxic drugs. Recently, the tumor cell microenvironment has been found to have a significant bearing on the survival of tumor cells following exposure to a wide variety of anti-neoplastic agents, prior to the acquisition of known drug resistance mechanisms. Specifically, interactions between cell surface integrins and extracellular matrix components have been shown to be responsible for this phenomenon of innate drug resistance, which we have termed Cell Adhesion Mediated Drug Resistance, or CAM-DR. Following its discovery using a multiple myeloma cell line model, evidence for CAM-DR has been found in a multitude of other human tumor cell types. In contrast to many other drug resistance mechanisms, integrin-mediated cell signaling is capable of protecting against death induced by an extremely wide variety of structurally and functionally diverse agents from traditional DNA damaging agents to the promising novel kinase inhibitor STI-571. This review examines the role of integrins in regard to their ability to protect tumor cells from drug- and radiation-induced apoptosis through numerous intracellular mechanisms. Current and future antagonists of specific integrin heterodimers may have the potential to sensitize tumor cells when used in combination with standard chemotherapy regimens. Specific signal transduction pathways initiated by integrin ligation will also be discussed as potential bridge points for inhibiting cell survival during cytotoxic drug exposure.