We have been investigating the relationship between hypoxia-inducible factor-1 (HIF-1), which is the major regulator of the transcriptional response of mammalian cells to low oxygen (hypoxia), and the mitogenic and stress-responsive transcription factor AP-1. While studying the bioenergetics of hypoxic cells, we found that adenosine 5'-monophosphate-activated protein kinase (AMPK) was strongly activated by hypoxia combined with glucose deprivation, independently of HIF-1 activity. AMPK has been termed the """"""""fuel sensor"""""""" of mammalian cells because it directly responds to the depletion of the fuel molecule ATP. We also obtained evidence that AMPK activity was activated by the same low oxygen conditions in the presence of glucose, as well as by less severe hypoxia (physiological hypoxia). These findings are directly relevant for understanding how cells adapt to ATP depletion (energy stress) within solid tumors, which naturally develop gradients of oxygen and glucose as they grow. Because our preliminary research showed that experimental tumors made from cells engineered to lack AMPK activity grew very poorly as mouse xenografts, we propose that AMPK is a novel target for the development of therapy. Since AMPK activity seems to be critical for the adaptation of normal cells to energy stress, we propose that AMPK regulates a general mechanism to conserve total ATP in solid tumors, and thus maintain the viability of energetically stressed tumor cells. To investigate this mechanism, Aim 1 of our research design is to perform biochemical and molecular biology studies to identify AMPK complexes-AMPK consists of catalytic and regulatory subunits-induced by low oxygen in the presence and absence of glucose, and by glucose deprivation. Identifying the exact AMPK activity will be a significant contribution to the fundamental understanding of how energy homeostasis is maintained within solid tumor microenvironments. Increasing our understanding of how tumor cells adapt to hypoxia and glucose deprivation is also clinically relevant because these environmental stresses not only favor malignant progression, but also promote resistance to therapy. Accordingly, our research design for Aim 2 is to investigate the contribution of AMPK to the growth of tumor xenografts and their response to experimental chemotherapy.
Aim 3 will focus on investigating potential mechanisms by which AMPK allows tumor cells to adapt to hypoxia and other stressful microenvironments commonly found in solid tumors. To accomplish these three Aims, we will use both knockout mouse cell lines lacking or defective for AMPK activity and human breast carcinoma cells. Ultimately, we intend to determine whether the AMPK """"""""emergency stress response"""""""" can be exploited for the therapy or prognosis of human cancer.
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