Glutamine metabolism in cancer cells
Glutamine, the most abundant amino acid in the blood, plays a particularly important role in cell growth and metabolism as a precursor for αKG of the tricarboxylic acid cycle, for nucleotides and for other amino acids. The importance of glutamine as a nutrient is further underscored by the observation that cancer cells are particularly dependent on this amino acid. Most cancer patients develop glutamine depletion which ultimately leads to cachexia. Glutamine is metabolized through glutaminolysis, catalyzed by glutaminase (GLS) and glutamate dehydrogenase (GDH), to produce αKG. GLS is regulated at the expression level by the oncogene c-myc, and GLS activity correlates with tumor growth (Tennant, Duran & Gottlieb, Nature Rev. Cancer 2010). The essential amino acid leucine directly binds and activates GDH to stimulate deamination of glutamate and thereby αKG production. Thus, leucine and glutamine, two of the most important amino acids in energy metabolism and nutrient signaling, cooperate in stimulating glutaminolysis. Our recent findings showed that glutamine in combination with leucine activates mTOR pathway by enhancing glutaminolysis and αKG production (Durán et al., Cell Cycle 2012).
mTOR signaling and glutamine metabolism
mTOR is a serine/threonine kinase highly conserved from yeast to humans. In mammalian cells, mTOR integrates several stimuli to regulate cell growth, metabolism and aging. mTOR forms two functionally and structurally distinct complexes termed mTORC1 and mTORC2. mTORC1 regulates protein synthesis, ribosome biogenesis, nutrient uptake and autophagy in response to growth factors, amino acids, and cellular energy. The Rag GTPases, activate mTORC1 in response to amino acids, especially leucine (Duran & Hall, EMBO Rep. 2012). Addition of amino acids allows the Rag heterodimer to bind and thereby recruit mTORC1 to the lysosome, where mTORC1 is activated. Our results showed that glutaminolysis is a critical step in the Rag-dependent recruitment of mTORC1 to the lysosome. Inhibition of glutaminolysis prevents the Rag-dependent lysosomal translocation and subsequent activation of mTORC1. Conversely, enhanced glutaminolysis or a cell permeable αKG analogue stimulates lysosomal translocation and activation of mTORC1. Finally, cell growth and autophagy, two processes controlled by mTORC1, are regulated by glutaminolysis. Thus, mTORC1 senses and is activated by glutamine and leucine via glutaminolysis and αKG production upstream of Rag (Duran et al., Mol. Cell 2012).
Prolyl hydroxylases as regulators of cell metabolism
Our findings position αKG as an intracellular messenger for nutrients availability and suggest new interesting questions, for instance, how is αKG sensed by the cell? On this lead, our studies also suggest that prolyl hydroxylases (PHD) are the αKG sensors that mediate the activation of mTORC1 by glutaminolysis (Duran et al., Oncogene 2013). PHDs are considered the oxygen sensors of cells. However, under normoxic conditions, the availability of αKG is critical for the activity of PHD (Boulahbel, Duran & Gottlieb, Biochem. Soc. Trans. 2009). Importantly, our work shows that amino acid starvation causes depletion of αKG that leads to PHD inactivation. Importantly, loss of PHD activity during amino acid starvation does not activate HIF, likely due to a lack of HIF expression. In agreement with this result, PHD inhibition blocks the response of mTORC1 to amino acids and induces autophagy independently of HIF (Duran et al., Oncogene 2013). Therefore, PHDs act as general metabolic sensors in the cell, detecting scarcity not only of oxygen, but also of amino acids, and forming a link between glutaminolysis and the mTORC1 pathway (Nguyen & Duran, Int J Biochem Cell Biol 2016) (Figure 1).
Glutamoptosis: a new autophagy-inhibited cell death mechanism during nutritional imbalance
Despite the role of both glutaminolysis and mTORC1 in the promotion of cell growth, we recently observed that the unbalanced activation of the glutaminolysis/mTORC1 pathway in the absence of other amino acids induced apoptotic cell death in human cells. We termed this unusual type of cell death as “glutamoptosis” (Villar & Duran, Autophagy 2017). During glutamoptosis, the long-term production of glutaminolysis-derived αKG in the absence of other amino acids was sufficient to activate mTORC1 and to inhibit autophagy, but concomitantly reduced cell viability and activated apoptosis. Inhibition of mTORC1 or re-activation of autophagy were sufficient to suppress the glutaminolysis-induced apoptosis. We additionally observed that the ability of rapamycin to prevent apoptosis resides in its pro-autophagic potential: re-inhibition of autophagy prevented cell survival in rapamycin-treated cells (Villar et al., Nature Comm 2017) (Figure 2). The surprising role of glutaminolysis as a cell death inducing mechanism during nutrient restriction points at the importance of nutritional imbalance in the control of cell viability and the potential use of this metabolic disequilibrium to identify new metabolic components and targets with potential implication in the treatment of nutrition-related diseases, such as obesity, diabetes, cancer, or cardiovascular diseases. In any case, whether the activation of apoptosis by the unbalanced activation of glutaminolysis and mTORC1 plays an active role in the biochemical basis of those diseases is a question that remains to be elucidated.