Role of autophagy in apoptotic regulation by Akt in pancreatic cancer.

Pancreatic cancer is the most lethal of all malignancies due both to the typically late presentation following the establishment of metastatic disease as well as the profound resistance to the cytotoxic effect of traditional chemotherapy. Apoptosis is considered the primary mechanism of chemotherapy-induced cell death. However, apoptosis is just one mechanism regulating the balance of cell survival and cell death following exposure to noxious extracellular stimuli. Autophagy is a broadly-conserved catabolic process in which bulk cytoplasmic proteins as well as organelles are degraded by the formation of autophagosomes with subsequent fusion to lysosomes for recycling of non-essential macromolecules to preserve cellular energy stores (1). Although originally described as a cell survival response induced by metabolic starvation, it has recently been recognized that the autophagy pathway can also mediate cell death (2). There is further emerging evidence to suggest a central role of autophagy in cancer, both as a survival mechanism triggered by hypoxia but also potentially as a mediator of stress-induced cell death (3, 4). The specific mechanisms that regulate this dichotomous effect of autophagy are unclear, though length and magnitude of cell stress have been postulated (5, 6). While the signaling pathways for autophagosome assembly are well-established, the pathways that sense and initiate autophagy are unclear. The investigation of the biochemical mechanisms responsible for the inherent apoptotic resistance of pancreatic cancer has identified Akt, a serine-threonine kinase, as a potential cause. Our group as well as others have shown that Akt is constitutively activated in human pancreatic cancer cell lines as well as human tumor specimens (7-9). This activation is associated with resistance to chemotherapy-induced apoptosis; inhibiting Akt has been shown to increase the apoptotic effect of chemotherapy in pancreatic cancer (10-12). mTOR, a downstream target of autophagy, has been shown to regulate autophagy, however it remains unclear whether the autophagy initiates or inhibits cell death (13, 14). Fujiwara et al. have suggested that induction of autophagy by mTOR inhibition in malignant glioma augments cell death event as the cytotoxic effect of radiation therapy was enhanced (15). However, Takeuchi et al. demonstrated that while directly inhibiting either Akt or mTOR was sufficient to induce autophagy, only Akt inhibition led to cell death in malignant glioma cells (16). Conversely, Aoki et al. inhibited the Akt/mTOR pathways in glioma cells and noted the induction of autophagy but without any concomitant cell death (17). These studies suggest the involvement of divergent pathways that independently regulate autophagy and apoptosis may be both cell-specific as well as dependent on additional inducers of apoptosis (such as chemotherapy or radiation therapy) or simultaneous activation of other signaling pathways, such as ERK. We, therefore, tested the hypothesis that inhibition of both Akt and mTOR would initiate autophagy in pancreatic cancer, and that furthermore, these events would subsequently lead to the induction of cell death independently of apoptosis.