Editors' ChoiceCancer

Shutting down oxygen use

Sci. Signal.  19 Jul 2016:
Vol. 9, Issue 437, pp. ec168
DOI: 10.1126/scisignal.aah5723

Mitochondrial oxidative phosphorylation is not the only process that consumes oxygen in cells. Indeed, oxygen-consuming processes include protein folding, lipid and collagen synthesis, demethylation reactions, and hydroxylation reactions. Banh et al. discovered a connection between the oxygen-requiring enzymes α-ketoglutarate–dependent dioxygenase (α-KGDD); protein tyrosine phosphatase 1B (PTP1B), which has oncogenic effects in some breast cancers; and the E3 ubiquitin ligase RNF213, which is encoded by a gene that causes a vascular disorder called Moyamoya disease. When xenografted into mice, HER2+ breast cancer cells in which PTP1B had been knocked down produced smaller tumors that were more necrotic with more areas of hypoxia than tumors from the control HER2+ cells. Inducible knockdown of PTP1B after tumor establishment resulted in increased hypoxia in the tumor. Indeed, knocking down PTP1B in culture had no detectable effect on HER2+ breast cancer cells unless the cells were grown under severely hypoxic conditions, then PTP1B deficiency reduced cell survival despite activation of the known adaptive hypoxia response pathways. Oxygen consumption analysis revealed that, under hypoxic conditions, mitochondrial oxygen consumption was reduced in PTP1B knockdown cells but that nonmitochondrial oxygen consumption was higher than that of the control cells exposed to hypoxia. To identify processes of nonmitochondrial oxygen consumption that may be regulated by PTP1B, the authors performed metabolite analysis and identified α-ketoglutarate as a molecule that was significantly reduced in the PTP1B-deficient cells compared with its abundance in control cells. The α-KGDD enzymes mediate proline hydroxylation, and PTP1B-deficient cells had increased hydroxyproline by Western blotting with an antibody to detect this modified amino acid. Hydroxyproline abundance was reduced by exposing the cells to a nonselective α-KGDD inhibitor. Pretreating the PTP1B-deficient HER2+ cells with this inhibitor protected the cells from hypoxia-induced cell death and reduced nonmitochondrial oxygen consumption. A substrate-trapping assay identified the E3 ubiquitin ligase RNF213 as a substrate of PTP1B, and knocking down RNF213 protected HER2+, PTP1B-deficient cells from hypoxia-induced cell death and resulted in increased growth and reduced the regions of hypoxia in tumors from these cells in mice. Mass spectrometry analysis of the ubiquitylated proteome indicated that PTP1B enhanced the ubiquitylation of ~40% of the detected proteome and a majority of this PTP1B-mediated ubiquitylation involved RNF213. These results have implications for treating HER2+, PTP1B+ cancers and provide insight into a possible mechanism underlying Moyamoya disease.

R. S. Banh, C. Iorio, R. Marcotte, Y. Xu, D. Cojocari, A. A. Rahman, J. Pawling, W. Zhang, A. Sinha, C. M. Rose, M. Isasa, S. Zhang, R. Wu, C. Virtanen, T. Hitomi, T. Habu, S. S. Sidhu, A. Koizumi, S. E. Wilkins, T. Kislinger, S. P. Gygi, C. J. Schofield, J. W. Dennis, B. G. Wouters, B. G. Neel, PTP1B controls non-mitochondrial oxygen consumption by regulating RNF213 to promote tumour survival during hypoxia. Nat. Cell Biol. 18, 803–813 (2016). [PubMed]