As mentioned, glutamine is the obligate nitrogen donor for nucleotide synthesis. Five enzymatic steps in the synthesis of purines and pyrimidines use glutamine as a source of nitrogen. Recent studies using quantitative RT-PCR and chromatin-immunoprecipitation (ChIP) in multiple cell systems have suggested that c-MYC (Myc) binds and transactivates eleven genes involved in nucleotide biosynthesis [35
]. Myc is a basic helix–loop–helix zipper (bHLHZ) protein that heterodimerizes with the small bHLHZ protein MAX and exerts both activating and repressing transcriptional effects [36
]. Of the five enzymatic steps utilizing glutamine, three (PRPP amidotransferase, carbamoyl phosphate synthetase II, CTP synthetase) are directly regulated by Myc at the transcriptional level [37
]. Carbamoyl phosphate synthetase II, a rate-limiting glutamine-dependent enzyme involved in pyrimidine synthesis, has also been found to be regulated via epidermal growth factor receptor (EGFR)-dependent mitogen-activated protein kinas (MAPK) phosphorylation [38
] as well as caspase-dependent degradation [39
Cancer cells take up and metabolize glucose and glutamine to a degree that far exceeds their needs for these molecules in anabolic macromolecular synthesis. Commonly occurring oncogenic signal transduction pathways initiated by receptor tyrosine kinases or Ras engage PI3K-Akt signaling to directly stimulate glycolytic metabolism [1
]. Oncogenic levels of Myc have recently been linked to increased glutaminolysis through a coordinated transcriptional program [26
]. Myc-activation/amplification is one of the most common oncogenic events observed in cancer and is known to drive the progression of human lymphomas [42
], neuroblastoma [44
] and small cell lung cancer [45
]. Quantitative RT-PCR and ChIP experiments support Myc’s binding and transcriptional activation of two high affinity glutamine transporters: SLC38A5 (also called SN2) and SLC1A5 (ASCT2), the transporter required for glutamine-dependent mTORC1 activation [24
]. In addition to facilitating glutamine uptake, Myc promotes the metabolism of imported glutamine into glutamic acid and ultimately into lactic acid [26
]. Whether the tendency of Myc to complement Ras and PI3K-Akt [46
] is related to the interdependence of glutamine and glucose metabolism in support of cell growth remains an open question.
The importance of glutamine metabolism for the MYC
-amplified cell has been highlighted by a recent demonstration that Myc also influences post-transcriptional regulation of glutamine catabolism. In a screen for Myc-regulated mitochondrial proteins, Gao et al. [41
] found that glutaminase protein levels were significantly upregulated in Myc-overexpressing cells. Yet the glutaminase mRNA expression level did not correlate with its increased protein levels, leading the authors to hypothesize that Myc regulates glutaminase through a post-transcriptional mechanism. Using an algorithm-based approach, the authors found that the microRNA miR-23a/b repressed the translation of glutaminase through binding its 3′ untranslated region (UTR). Notably, the authors had previously identified miR-23a/b as a strong target of Myc transcriptional repression. This study further links Myc overexpression to the cellular ability to catabolize glutamine into glutamic acid, thereby providing cells with a large pool of carbon for anaplerosis and NADPH production. Whether other oncogenic signaling pathways also contribute to the regulation of glutamine metabolism remains to be determined.
Myc-induced metabolic reprogramming triggers cellular dependency on exogenous glutamine as a source of carbon for the maintenance of the mitochondrial membrane potential and macromolecular synthesis. Indeed, glutamine depletion kills transformed cells in a Myc-dependent manner [26
]. The cell death induced by glutamine starvation can be rescued by the overexpression of Bcl-2, Bcl-xL
, or a dominant negative form of caspase-9, implicating the intrinsic apoptotic pathway as the mechanism of cell death [26
]. Substitution of glutamine with a membrane-permeable form of α-ketoglutarate, pyruvate, or oxaloacetate also rescues this death [26
]. However, consistent with the importance of glutamine as an obligate source of nitrogen, neither overexpression of anti-apoptotic proteins nor addition of TCA cycle intermediates can rescue the proliferation defect observed in glutamine-starved cells [26
]. Nevertheless, glutamine depletion of Myc-transformed cells leads to a profound reduction in the levels of TCA cycle metabolites despite abundant extracellular availability of glucose, supporting the importance of glutamine in the maintenance of mitochondrial anaplerosis [48
]. These findings suggest Myc transformation might also suppress the ability of tumor cells to use glucose as an anapleurotic substrate perhaps through upregulation of LDH-A.