Disruption of peroxisome proliferator-activated receptor γ coactivator (PGC)-1α reverts key features of the neoplastic phenotype of glioma cells.

The peroxisome proliferator-activated receptor γ coactivator (PGC)-1α is a master regulator of mitochondrial biogenesis and controls metabolism by coordinating transcriptional events. Here, we interrogated whether PGC-1α is involved in tumor growth and the metabolic flexibility of glioblastoma cells. PGC-1α was expressed in a subset of established glioma cell lines and primary glioblastoma cell cultures. Furthermore, a higher PGC-1α expression was associated with an adverse outcome in the TCGA glioblastoma dataset. Suppression of PGC-1α expression by shRNA in the PGC-1α-positive U343MG glioblastoma line suppressed mitochondrial gene expression, reduced mitochondrial membrane potential, and diminished oxygen as well as glucose consumption, and lactate production. Compatible with the known PGC-1α functions in reactive oxygen species (ROS) metabolism, glioblastoma cells deficient in PGC-1α displayed ROS accumulation, had reduced RNA levels of proteins involved in ROS detoxification, and were more susceptible to death induction by H2O2 compared with control cells. PGC-1αsh cells also had impaired proliferation and migration rates in vitro and displayed less stem cell characteristics. Complementary effects were observed in PGC-1α-low LNT-229 cells engineered to overexpress PGC-1α. In an in vivo xenograft experiment, tumors formed by U343MG PGC-1αsh glioblastoma cells grew much slower than control tumors and were less invasive. Interestingly, the PGC-1α knockdown conferred protection against hypoxia-induced cell death, probably as a result of less active anabolic pathways, and this effect was associated with reduced epidermal growth factor expression and mammalian target of rapamycin signaling. In summary, PGC-1α modifies the neoplastic phenotype of glioblastoma cells toward more aggressive behavior and therefore makes PGC-1α a potential target for anti-glioblastoma therapies.

Suppression of oxidative phosphorylation confers resistance against bevacizumab in experimental glioma.

Although bevacizumab initially shows high response rates in gliomas and other tumours, therapy resistance usually develops later. Because anti-angiogenic agents are supposed to induce hypoxia, we asked whether rendering glioma cells independent of oxidative phosphorylation modulates their sensitivity against hypoxia and bevacizumab. LNT-229 glioma cells without functional mitochondria (rho0 ) and control (rho+ ) cells were generated. LNT-229 rho0 -cells displayed reduced expression of oxidative phosphorylation-related genes and diminished oxygen consumption. Conversely, glycolysis was up-regulated in these cells, as shown by increased lactate production and stronger expression of glucose transporter-1 and lactate dehydrogenase-A. However, hypoxia-induced cell death in vitro was nearly completely abolished in the LNT-229 rho0 -cells, these cells were more sensitive towards glucose restriction and the treatment with the glycolysis inhibitor 2-deoxy-D-glucose. In an orthotopic mouse xenograft experiment, bevacizumab induced hypoxia as reflected by elevated Hypoxia-inducible factor 1-alpha staining in both, rho+ - and rho0 -tumours. However, it prolonged survival only in the mice bearing rho+ -tumours (74 days vs. 105 days, p = 0.024 log-rank test) and had no effect on survival in mice carrying LNT-229 rho0 -tumours (75 days vs. 70 days, p = 0.52 log-rank test). Interestingly, inhibition of glycolysis in vivo with 2-deoxy-D-glucose re-established sensitivity of rho0 -tumours against bevacizumab (98 days vs. 80 days, p = 0.0001). In summary, ablation of oxidative phosphorylation in glioma cells leads to a more glycolytic and hypoxia-resistant phenotype and is sufficient to induce bevacizumab-refractory tumours. These results add to increasing evidence that a switch towards glycolysis is one mechanism how tumour cells may evade anti-angiogenic treatments and suggest anti-glycolytic strategies as promising approaches to overcome bevacizumab resistance.