Apoptosis is a highly regulated process of cell death, allowing the deletion of cells that are damaged or otherwise targeted for destruction. Resistance to apoptosis underlies both the development and the survival of tumors. Understanding the sites of resistance in tumors may lead to more effective therapy. Two signaling pathways are known to activate the proteases called caspases that mediate apoptosis: one, the DNA damage pathway which involves a mitochondrial step in order to activate caspases and the other the death ligand pathway which can bypass mitochondria to activate caspases directly. Crosstalk between the pathways may lead to synergistic apoptotic responses.

We study apoptosis in mesothelioma and lung cancer lines, as models for highly resistant solid tumors. A major focus of the laboratory is 1) to identify mechanisms of resistance to apoptosis in these lines and 2) to identify means of amplifying apoptosis. We have now described a synergistic apoptotic response of mesothelioma lines when exposed to both a death ligand, TNF-related apoptosis inducing ligand (TRAIL), and chemotherapeutic agents. The synergy can be shown to involve amplification of mitochondrial depolarization and amplified release of cytochrome c. We are now studying the signaling steps by which these two pathways (death receptor and DNA damage) converge on the mitochondria and amplify apoptotic death. Other synergistic combinations appear to act at different levels within the cell, e.g. by increasing expression of the death receptors. We have recently found that, in our p53 inactive cells, DNA damage sensitizes to apoptosis via the JNK stress activated pathway. Some examples of interest are the use of TRAIL or fas ligand together with proteasome inhibitors, with NFkappa B inhibition, and with stimulators of JNK signals to bypass DNA damage.

In other work, we are developing 3-dimensional models for studying apoptotic resistance in tumor cell lines and in tumors themselves. Tumor cells grown on non-adherent surfaces clump together and remodel to form multicellular tumor spheroids. These tumor cells grown as spheroids acquire a high level of resistance to treatments that are successful against the same cells when grown as a monolayer. We are interested in the mechanisms of resistance in this more relevant 3-dimensional model. In addition, we are growing actual tumors (mesothelioma and lung cancers) in vitro by mincing the tumors and allowing them to grow on non-adherent surfaces. In this environment, the tumor fragments remodel into spheroids and remain viable and representative of the original tumor for weeks. We are studying this in vitro tumor model for mechanisms of apoptotic resistance in human tumors. When compared to 2-dimensional monolayer cell culture, the 3-dimensional multicellular spheroids and tumor fragment spheroids provide models more relevant to actual tumors. As such, we are applying the mechanisms learned in cell monolayers for their applicability to the more relevant and complex systems.