TGF‐β activates pericytes via induction of the epithelial‐to‐mesenchymal transition protein SLUG in glioblastoma

Abstract

Aims In primary central nervous system tumours, epithelial‐to‐mesenchymal transition (EMT) gene expression is associated with increased malignancy. However, it has also been shown that EMT factors in gliomas are almost exclusively expressed by glioma vessel‐associated pericytes (GA‐Peris). In this study, we aimed to identify the mechanism of EMT in GA‐Peris and its impact on angiogenic processes. Methods In glioma patients, vascular density and the expression of the pericytic markers platelet derived growth factor receptor (PDGFR)‐β and smooth muscle actin (αSMA) were examined in relation to the expression of the EMT transcription factor SLUG and were correlated with survival of patients with glioblastoma (GBM). Functional mechanisms of SLUG regulation and the effects on primary human brain vascular pericytes (HBVP) were studied in vitro by measuring proliferation, cell motility and growth characteristics. Results The number of PDGFR‐β‐ and αSMA‐positive pericytes did not change with increased malignancy nor showed an association with the survival of GBM patients. However, SLUG‐expressing pericytes displayed considerable morphological changes in GBM‐associated vessels, and TGF‐β induced SLUG upregulation led to enhanced proliferation, motility and altered growth patterns in HBVP. Downregulation of SLUG or addition of a TGF‐β antagonising antibody abolished these effects. Conclusions We provide evidence that in GA‐Peris, elevated SLUG expression is mediated by TGF‐β, a cytokine secreted by most glioma cells, indicating that the latter actively modulate neovascularisation not only by modulating endothelial cells, but also by influencing pericytes. This process might be responsible for the formation of an unstructured tumour vasculature as well as for the breakdown of the blood–brain barrier in GBM.

SLUG and Truncated TAL1 Reduce Glioblastoma Stem Cell Growth Downstream of Notch1 and Define Distinct Vascular Subpopulations in Glioblastoma Multiforme

“…Moreover, SLUG has been associated with poor prognosis and an aggressive mesenchymal GBM signature in genomic studies [ 24 , 25 , 67 ]. Interestingly, high numbers of tumor-associated pericytes expressing SLUG have recently been associated with increased neovascularization, malignancy, and therapeutic resistance in GBM, underscoring an important function of pericytes in gliomagenesis [ 68 ]. SLUG expression has been shown to be increased in vascular remodeling, where it appears to be associated with morphological changes and the proliferation of smooth muscle cells [ 68 , 69 ].…”

“…Interestingly, high numbers of tumor-associated pericytes expressing SLUG have recently been associated with increased neovascularization, malignancy, and therapeutic resistance in GBM, underscoring an important function of pericytes in gliomagenesis [ 68 ]. SLUG expression has been shown to be increased in vascular remodeling, where it appears to be associated with morphological changes and the proliferation of smooth muscle cells [ 68 , 69 ]. It is thus likely that the specific detection of SLUG observed in GBM vascular cells reflects its role in the vascular reorganization typically observed in high-grade gliomas.…”

TGF-beta Modulates the Integrity of the Blood Brain Barrier in Vitro, and is Associated with Metabolic Alterations in Pericytes

“…However, metabolic changes in pericytes, and especially in GBM associated pericytes have undergone very limited analysis to date. We primarily observed changes in the metabolic activity of pericytes that were treated with TGF-β, this associated with an elevated proliferation and motility, with morphological alteration of these cells [8] as well as with the breakdown of an intact barrier (Figure 2). As it has been described that the activation of peripheral pericytes was associated to changes in energy generation, especially the switch of ATP production by oxidative phosphorylation towards glycolysis [22], we first determined mitochondrial and glycolyic ATP production upon TGF-β treatment.…”

“…We have recently published that TGF-β modulates the function of HBVP. It induces cell motility, proliferation and additionally modulates the pericytic growth pattern and morphology [8]. In this regard, we were interested whether these changes are associated with the breakdown of the BBB.…”

“…We have recently demonstrated that pericytes covering GBM associated vessels (GA-Peris) can be distinguished from "normal" pericytes covering vessels outside the tumor area by their expression profile, especially by the expression of epithelial to mesenchymal transition (EMT) factors like SLUG or TWIST [7]. We have further demonstrated that pericytes are prominently involved in the formation of GBM associated vascular proliferations and that GBM cells, by secretion of TGF-β, drive pericytes to change their growth morphology, induce proliferation and increase cell motility [8]. In consequence, we hypothesized that these functional alterations have an impact on the BBB integrity.…”