Abstract SY37-03: Lifting the metabolic break on caspase 2 to enhance chemotherapy

Abstract

Abstract Chemotherapeutic elimination of many tumors depends on the induction of programmed cell death (apoptosis) in tumor cells. We and others have found that DNA damaging chemotherapeutics (e.g., cisplatin) and cytotoxic agents (e.g., paclitaxel) commonly used in cancer therapy (for eg. ovarian cancer, breast cancer and others) requires the initiator caspase, caspase 2, for robust induction of apoptosis. However, resistance to chemotherapy-induced apoptosis remains a significant obstacle in the treatment of these cancers. Recently, it has been recognized that the increased metabolic rate found in many cancer cells is a contributing factor in chemoresistance. While the mechanism through which metabolism regulates chemoresistance is not fully elucidated, we have found that caspase 2 activity is subject to metabolic control. Specifically, a highly active pentose phosphate pathway (PPP) leads to high levels of NADPH. This promotes suppressive phosphorylation of caspase 2, thereby inhibiting cancer cell death. Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme for entry of G6P into the PPP. G6PD has been reported to promote oncogenic transformation in vitro and in vivo and G6PD activity is elevated in a variety of cancers (1). Moreover, loss of G6PD in both Chinese hamster ovary cells and human fibroblasts has been reported to enhance radiosensitivity (2, 3). PPP flux is enhanced in cancer cells through a variety of mechanisms: it has been reported that mutation of the tumor suppressor p53 prevents its suppression of G6PD and overexpression of TAp73 increases G6PD expression in tumors (4). Our data suggest that a highly significant consequence of increased G6PD levels is the suppression of caspase 2. This is independent of any effects G6PD may have on redox potential in tumors. We now show that lifting this suppression of caspase 2, through inhibition of G6PD, will markedly sensitize ovarian, breast and pancreatic cancer cells to chemotherapeutics. G6PD is likely to be a good drug target for cancer therapy: loss of G6PD is known to have minimal adverse consequences for humans, with nearly 7.5% of the world's population exhibiting some degree of G6PD enzyme deficiency (5). Thus, inhibition of G6PD in normal cells (particularly in the temporally restricted setting of cancer therapy) is likely to cause little toxicity. On a mechanistic level, we have reported that the metabolic control of caspase 2 is exerted via its inhibitory phosphorylation by activated CaMKII (6). We now demonstrate a novel form of control of CaMKII activation by metabolism that depends, at least in part, on dephosphorylation of CaMKII at previously uncharacterized sites (T393/S395). We show that this dephosphorylation is mediated by metabolic activation of protein phosphatase 2A (PP2A) in complex with the B55β targeting subunit and that metabolic activity controls the association of the B55β/PP2A complex . This represents a novel locus of CaMKII control and also provides a mechanism contributing to metabolic control of apoptosis. These findings may also have implications for metabolic control of the many CaMKII-controlled and PP2A-regulated physiological processes, as both enzymes appear to be responsive to alterations in glucose metabolized via the PPP. Finally, we will discuss our recent findings involving a role for caspase 2 in the induction of lipoapoptosis in the setting of fatty liver disease, the most severe form of which (nonalcoholic steatohepatitis) may predispose patients to development of liver cancer. We have found that fatty liver can promote a dramatic induction of caspase 2 in both mouse models of disease and in human patients and will present data to suggest that this is a critical component of disease etiology.