Summary Because MYC plays a causal role in many human cancers, including those with hypoxic and nutrient-poor tumor microenvironments, we have determined the metabolic responses of a MYC-inducible human Burkitt lymphoma model P493 cell line to aerobic and hypoxic conditions, and to glucose deprivation, using Stable Isotope Resolved Metabolomics. Using [U-13C]-glucose as the tracer, both glucose consumption and lactate production were increased by MYC expression and hypoxia. Using [U-13C,15N]-glutamine as the tracer, glutamine import and metabolism through the TCA cycle persisted under hypoxia, and glutamine contributed significantly to citrate carbons. Under glucose deprivation, glutamine-derived fumarate, malate, and citrate were significantly increased. Their 13C labeling patterns demonstrate an alternative energy-generating glutaminolysis pathway involving a glucose-independent TCA cycle. The essential role of glutamine metabolism in cell survival and proliferation under hypoxia and glucose deficiency, makes them susceptible to the glutaminase inhibitor BPTES, and hence could be targeted for cancer therapy.

In 1927, Otto Warburg and coworkers reported the increased uptake of glucose and production of lactate by tumors in vivo as compared with normal tissues. This phenomenon, now known as the Warburg effect, was recapitulated in vitro with cancer tissue slices exhibiting excessive lactate production even with adequate oxygen. Warburg's in vivo studies of tumors further suggest that the dependency of tumors in vivo on glucose could be exploited for therapy, because reduction of arterial glucose by half resulted in a four-fold reduction in tumor fermentation. Recent work in cancer metabolism indicates that the Warburg effect or aerobic glycolysis contributes to redox balance and lipid synthesis, but glycolysis is insufficient to sustain a growing and dividing cancer cell. In this regard, glutamine, which contributes its carbons to the tricarboxylic acid (TCA) cycle, has been re-discovered as an essential bioenergetic and anabolic substrate for many cancer cell types. Could alterations in cancer metabolism be exploited for therapy? Here, we address this question by reviewing current concepts of normal metabolism and altered metabolism in cancer cells with specific emphasis on molecular targets involved directly in glycolysis or glutamine metabolism.

Although aerobic glycolysis provides an advantage in the hypoxic tumor microenvironment, some cancer cells can also respire via oxidative phosphorylation. These respiring ("non-Warburg") cells were previously thought not to play a key role in tumorigenesis and thus fell from favor in the literature. We sought to determine whether subpopulations of hypoxic cancer cells have different metabolic phenotypes and gene-expression profiles that could influence tumorigenicity and therapeutic response, and we therefore developed a dual fluorescent protein reporter, HypoxCR, that detects hypoxic [hypoxia-inducible factor (HIF) active] and/ or cycling cells. Using HEK293T cells as a model, we identified four distinct hypoxic cell populations by flow cytometry. The non-HIF/ noncycling cell population expressed a unique set of genes involved in mitochondrial function. Relative to the other subpopulations, these hypoxic "non-Warburg" cells had highest oxygen consumption rates and mitochondrial capacity consistent with increased mitochondrial respiration. We found that these respiring cells were unexpectedly tumorigenic, suggesting that continued respiration under limiting oxygen conditions may be required for tumorigenicity.antiangiogenesis | metabolism | mitochondria C hanges in cancer-cell metabolism have been linked to genetic alterations of oncogenes, tumor suppressors, and metabolic enzymes (1-3). The hypoxic tumor microenvironment further modifies metabolism through activation of hypoxia-inducible factors (HIFs). The HIFs enhance tumorigenesis by stimulating glycolysis, cell motility, and angiogenesis (4, 5). Thus, hypoxia portends poor prognosis in common cancers, such as gastric, lung, ovarian, pancreatic, prostate, and renal carcinomas (5).Although Otto Warburg observed respiration in certain cancer types, his obsession with aerobic glycolysis as a cause of cancer promulgated the prevailing misconception that cancers only exhibit the Warburg effect exclusive of respiration (6). Because the hypoxic tumor microenvironment activates HIFs and diminishes respiration, whether hypoxia enhances tumorigenicity at the expense of respiration is not fully understood (7). We found recently that oxidative and glycolytic metabolism coexist in hypoxic B lymphocytes, such that the shunting of glucose to lactate away from the tricarboxylic acid cycle (TCA) cycle by hypoxia is compensated through glutamine oxidation in the TCA cycle (8). These metabolic aberrations suggest the existence of hypoxic respiring (herein termed non-Warburg) cells capable of continued oxidative metabolism under hypoxic conditions. Further, it is believed that cancer cells within the tumor microenvironment are either aerobic or hypoxic because of oxygen gradients coming from nearby imperfect blood vessels. An intriguing commensal metabolic relationship between hypoxic and aerobic cells has been documented, whereby hypoxic cells produce lactate that is converted to pyruvate for respiration by aerobic cancer cells located nearby the blood vessel (9). We hypothesi...