Editors' ChoiceCancer

Releasing glutamate to stabilize HIF-1α

Sci. Signal.  12 Jul 2016:
Vol. 9, Issue 436, pp. ec158
DOI: 10.1126/scisignal.aah5199

The transcription factor HIF-1α mediates adaptive responses to low oxygen conditions. The presence of oxygen triggers a posttranslational modification of HIF-1α called prolyl hydroxylation that is mediated by the EglN family of enzymes. Prolyl hydroxylation leads to the degradation of HIF-1α, keeping its abundance low under normal oxygen conditions. Aberrantly stabilized HIF-1α promotes the progression of different types of solid tumors, including triple-negative breast cancer (TNBC). Briggs et al. found by immunohistochemical analysis that HIF-1α abundance was increased in TNBC tissue microarrays. Accordingly, the expression of HIF-1α target genes was increased in TNBCs compared with other breast cancer types. In freshly plated TNBC cell lines cultured under normal oxygen conditions, HIF-1α abundance was initially low and was increased by hypoxia exposure, suggesting that the pathway that targets HIF-1α for degradation under normal oxygen conditions was operational. HIF-1α accumulation increased with time in a TNBC cell line cultured under normal oxygen conditions, but not in a cell line derived from another breast cancer type. Experiments using media conditioned by a TNBC cell line indicated that a secreted factor was required for the aberrant stabilization of HIF-1α and that this factor decreased the prolyl hydroxylation of HIF-1α. Analysis of TNBC cell-conditioned media revealed that TNBCs secreted L-glutamate, and application of L-glutamate increased HIF-1α abundance in multiple cancer cell types in addition to TNBCs. L-glutamate was released into the media by the xCT cystine-glutamate antiporter, and accumulation of extracellular L-glutamate inhibited the activity of the xCT antiporter, leading to decreased intracellular cystine and cysteine concentrations and increased intracellular glutamate concentrations. EglN1 is a member of an enzyme family that tends to self-inactivate through reversible oxidation of cysteine residues under oxidizing conditions, which would be present when intracellular cysteine is depleted. In vitro assays indicated that cysteine increased the activity of EglN1; whereas lower cysteine concentrations resulted in oxidation of specific cysteine residues in EglN1 and its inactivation. Thus, glutamate released by TNBCs results in stabilization of HIF-1α under normoxic conditions through the inactivation of EglN enzymes.

K. J. Briggs, P. Koivunen, S. Cao, K. M. Backus, B. A. Olenchock, H. Patel, Q. Zhang, S. Signoretti, G. J. Gerfen, A. L. Richardson, A. K. Witkiewicz, B. F. Cravatt, J. Clardy, W. G. Kaelin Jr., Paracrine induction of HIF by glutamate in breast cancer: EglN1 senses cysteine. Cell 166, 126–139 (2016). [PubMed]