Abstract:
Glioblastoma (GBM) is the most common malignant primary brain tumor in adults. Despite various modern therapeutic strategies, GBM remains an incurable and highly lethal disease with a very poor prognosis. Among others, one of the defining pathological characteristics of GBM is its high angiogenesis ability and the resultant excessive and dysfunctional vascularization being a main contributor to the devastating prognosis of GBM patients. Recently, pericytes have been identified to mainly constitute the irregular vessel architecture in GBM. In vitro data suggest that TGF-β secreted by glioma cells stimulates the development of a mesenchymal phenotype in pericytes via an epithelial-to-mesenchymal transition (EMT)-like activation that results in pericyte proliferation, migration and morphological changes favoring the formation of GBMs´ chaotic vasculature. Thus, this study aimed to investigate if TGF-β modulates VAMC (vessel-associated mural cell) functions via the induction of the EMT factor SLUG and its potential influences on the development of vascular abnormalities associated with GBM in vivo.
Therefore, in a syngeneic mouse GBM model, we assessed the activation status of glioma-associated VAMCs by disrupting the TGF-β- and SLUG-mediated EMT- induction either by preventing TGF-β secretion from GBM cells or by specifically knocking out the EMT regulator SLUG in VAMCs before tumor cell implantation. The VAMC activation state was evaluated by measuring the levels of PDGFRβ and αSMA proteins. Stainings for the endothelial cell marker CD31 were performed to investigate the corresponding intratumoral vessel structure.
Blocking EMT-mediated activation of glioma-associated VAMCs significantly reduced the number of PDGFRβ- and αSMA-positive VAMCs, regardless of whether TGF-β secretion by GBM cells was inhibited or SLUG was specifically knocked out in VAMCs prior to tumor cell implantation. The decrease in PDGFR+ or αSMA+ VAMCs we observed under those circumstances corresponded with diminished vessel density and fewer vascular abnormalities and was even more pronounced after SLUG KO in VAMCs compared to mice that harbor TGF-β-KO GBM.
Our findings suggest that at least in experimental mouse GBMs the SLUG-mediated modulation of VAMC activity is induced by GBM-secreted TGF-β¬ and that activated VAMCs play a crucial role in neo-angiogenic processes within this tumor. We propose that dysregulated activation of VAMCs in the tumor microenvironment is responsible for the disorganized tumor vasculature.
There is emerging evidence that vessel normalization can mitigate tumor hypoxia, reduce tumor-associated edema, and enhance drug delivery. Thus, preventing the formation of unstructured and dysfunctional tumor vasculature during tumor recurrence holds promise as a therapeutic strategy for GBM and other highly angiogenic tumors. The involvement of VAMCs in tumor angiogenesis we demonstrated in this study potentially identifies these cells as new therapeutic targets. However, further research is required to evaluate whether pericytes are suitable as target structures for vascular-normalizing therapeutic approaches.
Epithelial-to-mesenchymal transition