Glioblastoma are heterogeneous tumours composed of different subpopulations of cells with different behavioral characteristics. Areas of highly angiogenic cells co-exist with populations of non-angiogenic but highly invasive and infiltrative cells. This heterogeneity is a key clinical challenge in glioma treatment.

We used a large scale RNA sequencing approach to investigate the molecular components which differentiate infiltrative from angiogenic cells, in a Patient Derived Xenograft (PDX) mouse model. Our bioinformatic analysis revealed TGFβ1 to be a master regulator of tumor development, and Thrombospondin-1 (Tsp1) to be up regulated in infiltrative areas as compared to angiogenic area. Thrombospondins are known as anti-angiogenic factors, however the full roles of these multi domain proteins in tumor development remain to be elucidated.

We found Tsp1 expression to be upregulated in grade IV-GBM and in silico in the GBM mesenchymal subclass. It is expressed not only in tumor cells, but also in tumor blood vessel endothelial cells (ECs). TGFβ1 transcriptionally regulated Tsp1 via Smad1 and Smad3. However, contrary to previous results, we found that Tsp1 was not involved in TGFβ1-activation in tumor cells. We showed that Tsp1 regulates cell migration and invasion both in vitro and in vivo. Inhibition of Tsp1 expression in vivo correlated with increased tumor vascularization in both the chick CAM assay and the PDX mouse model.

Anti-angiogenic treatments in the PDX mouse model leads to increased tumor hypoxia and invasive tumor cell behavior, as described in our previous work. In this context, both TGFβ1 and Tsp1 were upregulated in tumor cells. Downregulation of tumor-derived Tsp1 by shRNA in the presence of anti-angiogenic therapy led to reduced tumor growth and invasion in vivo. Finally, peptide-mediated inhibition of Tsp1 activity demonstrated that Tsp1/CD47 interaction is involved in the invasive capacity of GBM cells.

Taken together, our data suggest that Tsp1 inhibition may be a promising therapeutic approach to limit tumor infiltration induced by treatment with anti-angiogenic agents.

Citation Format: Thomas Daubon, Celine Leon, Kim Clarke, Mathilde Poulet, Hrvoje Miletic, Francesco Falciani, Rolf Bjerkvig, Andreas Bikfalvi. Thrombospondin-1 is a master regulator of glioblastoma vascularization and infiltration. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4413.