Acquired resistance to anti-VEGF therapy in glioblastoma is associated with a mesenchymal transition

PURPOSE

Antiangiogenic therapy reduces vascular permeability and delays progression but may ultimately promote an aggressive treatment-resistant phenotype. The aim of the present study was to identify mechanisms responsible for glioblastoma resistance to antiangiogenic therapy.

EXPERIMENTAL DESIGN

Glioma stem cell (GSC) NSC11 and U87 cell lines with acquired resistance to bevacizumab were developed from orthotopic xenografts in nude mice treated with bevacizumab. Genome-wide analyses were used to identify changes in tumor subtype and specific factors associated with resistance.

RESULTS

Mice with established parental NSC11 and U87 cells responded to bevacizumab, whereas glioma cell lines derived at the time of acquired resistance to anti-VEGF therapy were resistant to bevacizumab and did not have prolongation of survival compared with untreated controls. Gene expression profiling comparing anti-VEGF therapy-resistant cell lines to untreated controls showed an increase in genes associated with a mesenchymal origin, cellular migration/invasion, and inflammation. Gene-set enrichment analysis showed that bevacizumab-treated tumors showed a highly significant correlation to published mesenchymal gene signatures. Mice bearing resistant tumors showed significantly greater infiltration of myeloid cells in NSC11- and U87-resistant tumors. Invasion-related genes were also upregulated in both NSC11 and U87 resistant cells which had higher invasion rates in vitro compared with their respective parental cell lines.

CONCLUSIONS

Our studies identify multiple proinflammatory factors associated with resistance and identify a proneural to mesenchymal transition in tumors resistant to antiangiogenic therapy.

Phase II trial of erlotinib plus concurrent whole-brain radiation therapy for patients with brain metastases from non-small-cell lung cancer

PURPOSE

Brain metastasis (BM) is a leading cause of death from non-small-cell lung cancer (NSCLC). Reasoning that activation of the epidermal growth factor receptor (EGFR) contributes to radiation resistance, we undertook a phase II trial of the EGFR inhibitor erlotinib with whole-brain radiation therapy (WBRT) in an attempt to extend survival time for patients with BM from NSCLC. Additional end points were radiologic response and safety.

PATIENTS AND METHODS

Eligible patients had BM from NSCLC, regardless of EGFR status. Erlotinib was given at 150 mg orally once per day for 1 week, then concurrently with WBRT (2.5 Gy per day 5 days per week, to 35 Gy), followed by maintenance. EGFR mutation status was tested by DNA sequencing at an accredited core facility.

RESULTS

Forty patients were enrolled and completed erlotinib plus WBRT (median age, 59 years; median diagnosis-specific graded prognostic assessment score, 1.5). The overall response rate was 86% (n = 36). No increase in neurotoxicity was detected, and no patient experienced grade ≥ 4 toxicity, but three patients required dose reduction for grade 3 rash. At a median follow-up of 28.5 months (for living patients), median survival time was 11.8 months (95% CI, 7.4 to 19.1 months). Of 17 patients with known EGFR status, median survival time was 9.3 months for those with wild-type EGFR and 19.1 months for those with EGFR mutations.

CONCLUSION

Erlotinib was well tolerated in combination with WBRT, with a favorable objective response rate. The higher-than-expected rate of EGFR mutations in these unselected patients raises the possibility that EGFR-mutated tumors are prone to brain dissemination.

Mesenchymal high-grade glioma is maintained by the ID-RAP1 axis

High-grade gliomas (HGGs) are incurable brain tumors that are characterized by the presence of glioma-initiating cells (GICs). GICs are essential to tumor aggressiveness and retain the capacity for self-renewal and multilineage differentiation as long as they reside in the perivascular niche. ID proteins are master regulators of stemness and anchorage to the extracellular niche microenvironment, suggesting that they may play a role in maintaining GICs. Here, we modeled the probable therapeutic impact of ID inactivation in HGG by selective ablation of Id in tumor cells and after tumor initiation in a new mouse model of human mesenchymal HGG. Deletion of 3 Id genes induced rapid release of GICs from the perivascular niche, followed by tumor regression. GIC displacement was mediated by derepression of Rap1gap and subsequent inhibition of RAP1, a master regulator of cell adhesion. We identified a signature module of 5 genes in the ID pathway, including RAP1GAP, which segregated 2 subgroups of glioma patients with markedly different clinical outcomes. The model-informed survival analysis together with genetic and functional studies establish that ID activity is required for the maintenance of mesenchymal HGG and suggest that pharmacological inactivation of ID proteins could serve as a therapeutic strategy.

Abstract 3314: Phenotypic plasticity in glioma stem cells mediated by NFκB signaling

Here we report the identification of two major subtypes of glioma stem cells (GSCs) that closely mirror previously described glioblastoma (GBM) groups, namely proneural (PN) and mesenchymal (MES), based on gene expression patterns. Using flow cytometry, we found that PN GSCs express both CD15/SSEA1, a marker for neural stem cells, as well as CD44, a MES marker. In contrast MES GSCs were CD15low/CD44high. MES GSCs and CD44high subpopulations in PN GSCs showed increased expression of transcription factors previously reported to induce the MES phenotype (STAT3, C/EBP-α, and TAZ), and were -resistant to α-irradiation. By distilling cytokines that can induce MES differentiation, we found that TNF-α found in patient-derived microglial conditioned media or conditioned media from MES GSCs caused a PN-to-MES transition through activation of NFκB resulting in radio-resistance. We found that NFκB mediates MES transition indirectly by inducing the transcriptional circuitry of STAT3, C/EBP-α, and TAZ. Blockade of NFκB activation using minocycline abrogated the MES phenotype (both induced and constitutive), reduced tumor growth, and rendered GSCs radio-sensitive. Taken together, our findings reveal that phenotypic and surface marker plasticity is common in GSCs, and that the NFκB driven PN-to-MES transition provides an escape mechanism for GSCs to evade cytotoxic treatment. Based on our studies, we propose that minocycline, a commonly used antibiotic, could target GSCs and should be further evaluated in preclinical studies.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3314. doi:1538-7445.AM2012-3314

Molecular biology of brain tumors

Much progress has been made in understanding the molecular genetics of brain tumors, especially gliomas.The development and use of high-throughput platforms that can interrogate molecular lesions on a variety of platforms will increase our ability to identify molecular subclasses of these tumors. Future challenges will include the development of methods to integrate these data among different platforms in order to identify optimal biomarkers and robust subclasses. The ultimate challenge, however, remains the translation of this biological knowledge into improved therapies for patients.