The overall goal of the research in Dr. Shacter's laboratory is to find ways to understand and improve cancer treatment by applying our knowledge of basic biochemical and immunological pathways to the development of novel therapeutic approaches for eliminating tumor cells. Solid tumors such as lymphoma and breast cancer are often infiltrated by inflammatory phagocytes (neutrophils and macrophages) which can generate reactive oxygen species within the tumor tissue. The macrophages play a key role in removing dying tumor cells. Dr. Shacter's laboratory investigates how the oxidants (such as hydrogen peroxide and hypochlorous acid) may influence the efficacy of cancer chemotherapy drugs. Most antineoplastic drugs kill tumor cells by inducing a form of cell death called apoptosis. This mechanism of cell death is thought to be physiologically advantageous because the dying cells are removed from the tissue by phagocytosis prior to lysis and induction of a potentially adverse immune response. Dr. Shacter has found from in vitro studies that hydrogen peroxide, the main oxidant secreted by macrophages, interferes with the ability of chemotherapy drugs to induce apoptosis in human lymphoma cells. Moreover, exposure of apoptotic cells to hydrogen peroxide inhibits the phagocytosis of the dying cells by macrophages. Hence, this oxidant interferes with the ability of the immune system to quietly remove dying cancer cells. Her research shows further that addition of antioxidants to the treatment regimen restores chemotherapy-induced cell killing. These findings raise the possibility that administration of antioxidants together with cancer chemotherapy may reduce the negative, oxidant-related side effects of the chemotherapy drugs (e.g., adriamycin-induced cardiotoxicity) while maintaining full tumor cell killing. This theory is supported by studies in Dr. Shacter's laboratory using a pre-clinical mouse model for breast cancer treatment. Molecular studies being carried out in the laboratory examine mechanisms of chemotherapy-induced apoptosis and the involvement of signal transduction pathways in controlling cell survival. Finally, significant effort is directed towards identifying factors that control uptake and clearance of the dying cells by macrophages. Dr. Shacter's group recently discovered that immune system elimination of apoptotic cells requires the presence of serum protein S, which is a co-factor for activated protein C, a therapeutic protein recently approved for marketing under the tradename Xigris. The research suggests that protein S could serve as a potential therapeutic protein in the treatment of autoimmunity and sepsis. Oxidants and Cell Death. We are investigating how inflammatory oxidants such as hydrogen peroxide (H2O2) kill tumor cells and how they may influence tumor cell recognition and elimination by the immune system. Most chemotherapeutic agents kill tumor cells by inducing apoptosis. Solid tumors are often infiltrated by inflammatory phagocytes which can generate oxidative stress within the tumor tissue. Previously, we found that in the presence of H2O2, human Burkitt's lymphoma (BL) cells are unable to undergo apoptosis in response to cancer chemotherapy drugs and die instead by a form of necrosis. One of the most important consequences of the interaction between H2O2 and the chemotherapy drugs is that the cells do not become phagocytosed by co-cultured macrophages until after their membranes have lysed. This can lead to an undesirable inflammatory reaction to the dying cells, which can further complicate tumor cell depletion. Our recent research has been aimed at identifying the molecular mechanism whereby H2O2 inhibits uptake of dying tumor cells by macrophages and at identifying endogenous cofactors for the phagocytic process. H2O2 inhibits the protein S-stimulated phagocytosis of BL cells even when they express phosphatidylserine (PS) on the exofacial surface of the plasma membrane. These results indicate that PS is necessary, but is not sufficient for recognition and uptake of apoptotic cells by macrophages. Further, they suggest that H2O2 acts by modifying a separate, as yet unidentified phagocytic marker on the surface of apoptotic cells. The molecular target for H2O2 action is being sought so that we may identify additional mechanisms of controlling cell death. Recently, we discovered that phagocytosis of apoptotic lymphoma cells requires the presence of protein S, a serum protein that regulates the activity of activated protein C. This finding identifies a novel link between the coagulation and immune systems and suggests a possible role for protein S in both autoimmunity and in the disseminated intravascular coagulation associated with sepsis. The Role of Small GTPases in Control of Tumor Cell Apoptosis. The goal of this research project is to understand the molecular mechanisms through which Rac GTPase and its associated regulatory proteins control apoptosis in response to cancer chemotherapy drugs. Rac GTPases are members of the Rho family of the Ras superfamily of small GTP-binding proteins. These proteins are key regulators of many aspects of cell function, including cytoskeletal organization, transmission of growth signals from intracellular oxidants, gene transcription, and cell transformation. Rac proteins regulate cell signalling by binding to and activating downstream effector molecules, such as protein kinases, NADPH oxidases, and other regulatory proteins. Rac and other Rho GTPases are overexpressed in many cancers. We study how the Rac GTPases are regulated in response to apoptotic stimuli, and how the perturbation of Rac-related signaling pathways contributes to cell death. Greater understanding of how Rac controls of cancer cell growth and death can lead to the identification of drugs that aid in the selective killing of tumor cells by targeting aberrant Rac signaling pathways. In initial studies, we evaluated the possibility that cellular Rac GTPases serve as caspase substrates. Caspases catalyze the cleavage of intracellular substrates and cause the biochemical and morphological changes that are characteristic of apoptotic death. We found that endogenous Rac1 is, in fact, cleaved in human lymphoma cells in response to chemotherapy drugs during the course of apoptosis. The proteolysis occurs at to non-canonical caspase-3 sites and results in inactivation of Rac1 GTPase and effector-binding activities. Expression of caspase-3-resistant Rac1 mutants in the cells suppresses drug-induced apoptosis. Overall, the results suggest that native Rac1 activity interferes with the apoptotic process and needs to be diminished in order to maximize cell killing by chemotherapy drugs. More recent studies have looked for the pathway through which Rac1 regulates cell survival, focusing specifically on the bcl-2-family member Bad. This protein is known to undergo a reversible phosphorylation cycle that regulates cell survival and apoptosis. Our recent research examines which cellular protein kinases control phosphorylation of Bad in response to Rac1 activity.
