The regulation of cancer cell glutamine metabolism

We are grateful to Dorai, Pinto, and Cooper for their insightful Commentary on our manuscript ‘The oncogenic transcription factor c-Jun regulates glutaminase expression and sensitizes cells to glutaminase-targeted therapy’ (1,2). As summarized in the Commentary, glutaminase (GLS) catalyzes the first reaction of mitochondrial glutamine catabolism, generating glutamate and ammonia, and is overexpressed in a number of malignancies. Studies using small-molecule inhibitors or genetic approaches to suppress GLS have found that its activity can be important for cancer cell proliferation in vitro and for tumor growth in vivo. Indeed, the GLS inhibitor CB-839 is currently being evaluated in clinical trials for treatment of hematological cancers and triple-negative breast cancer (TNBC) (3,4). In our study, we found that c-Jun, which is up-regulated in TNBC as well as in a number of other human cancers, drives expression of the GLS gene and ultimately causes cancer cells to become dependent on the GLS reaction.

Dorai et al. raise a number of intriguing points and propose some attractive hypotheses. They describe the striking differences that have been reported between GLS and the homologous isozyme GLS2. Numerous studies have demonstrated a pro-oncogenic role for GLS, whereas the function of GLS2 in tumorigenesis is less well understood. Notably, GLS expression is positively regulated by drivers of cell-cycle progression, including c-Myc and c-Jun, whereas GLS2 expression is up-regulated by the tumor suppressor p53 in response to redox stress (2,5). Thus we are tempted to speculate that the down-regulation of GLS2 expression that occurs in some tumors, including those of the liver, might reflect disruption of p53 function rather than direct selection against GLS2 itself. Indeed, in some tumors GLS2 is reported to be pro-oncogenic, such as in neuroblastoma where its expression is up-regulated downstream of N-Myc (5). Nevertheless, in some contexts ectopic overexpression of GLS2 has been found to inhibit tumorigenesis (5). We agree with the authors of the Commentary that it is important for the roles of GLS and GLS2 to be better defined, as glutaminase-targeted therapies are developed.

Dorai et al. propose a hypothesis that links the metastasis suppressor protein SSeCKS/AKAP12 (encoded by the gene AKAP12) with cellular glutamine metabolism. As described in the Commentary, the down-regulation of SSeCKS/AKAP12 is required for v-Jun (and presumably also for c-Jun) mediated cellular transformation. We note a report by Lee et al., who found that SSeCKS/AKAP12 markedly reduced the activating phosphorylation of JNK but not of ERK (6). This led to decreased phosphorylation of c-Jun and c-Fos, and consequently to strongly suppressed activity of AP-1 transcription factors and the down-regulated expression of their target genes (6). We agree with Dorai et al. that it would be of great interest to assess whether SSeCKS/AKAP12 suppresses expression of GLS. Interestingly, data in The Cancer Genome Atlas (TCGA) indicate that loss/down-regulation of AKAP12 expression occurs more frequently in TNBCs than in ER+ breast cancers, which is consistent with the findings that both JNK activation and GLS expression are elevated in TNBC (2,4,7).

A second hypothesis proposed by Dorai et al. is that glutamine regulates its own mitochondrial catabolism in some contexts, based on literature reports that glutamine supplementation stimulates JNK phosphorylation and downstream activation of c-Jun, and also activation of mTORC1. Suppression of GLS expression when cellular glutamine is depleted would reserve glutamine for cytosolic amidotransferases, which conserve the amide nitrogen of glutamine in biosynthetic reactions rather than releasing it as ammonia. We are unaware of any reports of glutamine regulating GLS expression, and it will be of interest to test this hypothesis. Notably, a related regulatory mechanism has been described for glutamine synthetase, whose expression is down-regulated following glutamine supplementation (8).

Finally, Dorai et al. raise an important point that has been highlighted by recent publications, namely the impact that the microenvironment has on cellular metabolism. The metabolic phenotype of KRAS-driven lung tumors differs from tumor-derived cell lines grown in monolayer culture, and highly perfused regions of lung tumors exhibit altered metabolic profiles relative to poorly diffused regions (9,10). As noted in the Commentary, a thorough evaluation of tumor metabolism in vivo will be critical for further developing therapeutic strategies that target GLS.

Acknowledgments

Funding: This work was supported by NIH (GM040654, GM047458, CA201402).

Footnote

Provenance and Peer Review: This article was commissioned and reviewed by the Section Editor Decai Yu (Hepatobiliary Institute of Nanjing University, Department of Hepatobiliary Surgery, Drum Tower Hospital, The affiliated Medical School of Nanjing University, Nanjing, China).

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/tcr.2016.11.39). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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