Editors' ChoiceCancer

A stiff path to aggressiveness

Sci. Signal.  20 Dec 2016:
Vol. 9, Issue 459, pp. ec304
DOI: 10.1126/scisignal.aam6149

Diffuse low-grade gliomas (WHO grades II and III) can recur as more aggressive and lethal glioblastomas (WHO grade IV) and are associated with mutations in the gene encoding the metabolic enzyme isocitrate dehydrogenase 1(IDH1). The effect of these mutations on the activity of the hypoxia-inducible factor 1α (HIF1α), a transcription factor associated with more aggressive tumors, has been unclear. Miroshnikova et al. investigated the relationship between mutated IDH1, HIF1α activity, and extracellular matrix (ECM) stiffness in gliomas and glioblastomas. Analysis of human brain autopsies revealed that WHO grade correlated with ECM stiffness and mechanosignaling [as assessed by the phosphorylation of focal adhesion kinase (PAK) and myosin light chain 2 (MLC2)], such that grade IV glioblastomas were associated with increased ECM stiffness and mechanosignaling. However, the gliomas and glioblastomas with the R132H IDH1 mutation were associated with softer ECM and decreased mechanosignaling. Tumors with stiffer ECM were associated with increased production of the ECM glycoprotein tenascin-C, whereas those with the R132H IDH1 mutation were associated with reduced production of tenascin-C. Accordingly, xenografts formed from glioblastoma cells expressing the R132H IDH1 mutant had softer ECM, decreased mechanosignaling, and low tenascin-C production. Furthermore, tumors formed from xenografted glioblastoma cells in which tenascin-C was knocked down also had softer ECM and decreased mechanosignaling, and survival was increased in mice bearing these xenografts compared with mice with xenografts of cells with tenascin-C. Chromatin immunoprecipitation indicated that HIF1α bound to the promoter of the gene encoding tenascin-C. HIF1α abundance was low in R132H IDH1 glioblastoma xenografts, even in areas of hypoxia, and HIF1α abundance in R132H IDH1 glioblastoma cells plated on soft substrates did not increase in response to hypoxia, suggesting that the R132H IDH1 mutant prevented the induction of HIF1α in response to hypoxia. Analysis of human lower-grade gliomas and glioblastomas with the R132H IDH1 mutation indicated that those patients with secondary glioblastomas (tumors that recurred after treatment) had stiffer ECM than patients with primary gliomas or glioblastomas. Plating R132H IDH1 glioblastoma cells on stiff substrates restored the induction of HIF1α to hypoxia, and in R132H IDH1 glioblastoma cells that overexpressed a constitutively active β1 integrin that activates FAK, HIF1α abundance was increased in response to hypoxia regardless of the substrate stiffness. These results suggested that stiffening of the ECM and the associated increase in mechanosignaling overrides the blunting effect of the R132H IDH1 mutation on the hypoxic induction of HIF1α. Xenografts formed from R132H IDH1 glioblastoma cells that overexpressed the constitutively active β1 integrin showed increased mechanosignaling, ECM stiffness, and abundance of HIF1α and tenascin C, and mice bearing these xenografts had decreased survival than those bearing xenografts with wild-type β1 integrin. The microRNA miR-203, which was more abundant in R132H IDH1 cells plated on soft substrates than in those plated on stiff substrates, inhibited HIF1α transcription. Orthotopically injected tumors formed from R132H IDH1 glioblastoma cells with antagomiR-mediated knockdown of miR-203 had stiffer ECM and increased tenascin-C abundance, and survival was decreased in mice bearing these tumors. The abundance of miR-203 was higher in patients with secondary tumors than those with primary gliomas or glioblastomas. Thus, the R132H IDH1 mutant prevents the hypoxic induction of HIF1α in glioblastoma cells on soft substrates, but the increase in ECM stiffness that occurs after glioblastoma treatment overrides this effect in secondary tumors.

Y. A. Miroshnikova, J. K. Mouw, J. M. Barnes, M. W. Pickup, J. N. Lakins, Y. Kim, K. Lobo, A. I. Persson, G. F. Reis, T. R. McKnight, E. C. Holland, J. J. Phillips, V. M. Weaver, Tissue mechanics promote IDH1-dependent HIF1α–tenascin C feedback to regulate glioblastoma aggression. Nat. Cell Biol. 18, 1336–1345 (2016). [PubMed]

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