gms | German Medical Science

Objective: Glioblastoma (GBM) present with an abundant and aberrant neo-vasculature and a high degree of invasion. We have previously established the neuroendocrine hormone apelin as an angiogenic factor during embryonic development and described its upregulation in GBM patient samples. Here, we investigated whether apelin signaling has a role in angiogenesis and invasive GBM growth as well as its therapeutic potential.

Methods: Towards this aim, we investigated expression of apelin and its receptor in GBM stereotactic biopsies using in situ hybridization and qPCR, screened 19 patient-derived GBM stem cell cultures performing RNAseq and compared the expression pattern to the TCGA (the cancer genome atlas) data set. Furthermore, we performed orthotopic implantation of human patient-derived and mouse subtype-specific GBM cells and employed genetic and pharmacological blockade of apelin signaling complemented by in vitro invasion assays. To study the specific contribution of glioma-derived apelin, we implanted human glioma cells expressing lentiviral control- or apelin-shRNA. For vascular apelin expression Apelin-KO mice were used.

Results: In a serial implantation model of invasive human GBM in mice we found that upregulation of apelin and its receptor correlated with induction of the angiogenic switch. Interrogating the TCGA data set for apelin, co-expressed genes showed that angiogenesis signature genes were more than 64 times over-represented. We then screened patient-derived GBM stem cell cultures and detected expression of apelin and its receptor in all described genetic GBM subtypes. We found that depletion of apelin expression (reduction by over 90%) in patient-derived GBM cells massively reduced the glioma vascular network (reduction by 71%). Additionally, blockage of the endothelial apelin signal using Apelin-KO mice led to a complete loss of glioma angiogenesis, a reduction of glioma volume by 69% and a significant increase in survival of the mice by 42%. Moreover, direct infusion of the peptide apelin-13 rescued the vascular loss-of-function phenotype. RNAseq confirmed enrichment for angiogenesis but also for invasion-related human and mouse gene signatures in apelin-depleted xenografts. Consistently, we found that apelin is not only able to attract endothelial cells to the brain tumor site but also enhances invasion of primary GBM cells by more than 2 times in vitro as well as in patient-derived xenografts in vivo. Finally, therapeutic application of an apelin receptor antagonist (Apelin-F13A) blocked tumor angiogenesis to normal levels, reduced GBM cell invasion by 46% and led to an overall attenuation of tumor size by 47%.

Conclusion: In summary, we found that apelin regulates glioma angiogenesis and directly mediates primary GBM cell invasion. Consequently, we propose apelin and its receptor as novel targets for GBM therapy.