The irradiated brain microenvironment supports glioma stemness and survival via astrocyte-derived Transglutaminase 2

Abstract

The tumor microenvironment plays an essential role in supporting glioma stemness and radioresistance; however, little is known about how the microenvironment responds to radiation. Here, we found that astrocytes, when pre-irradiated, increased stemness and survival of co-cultured glioma cells. Tumor-naïve brains increased reactive astrocytes in response to radiation, and mice subjected to radiation prior to implantation of glioma cells developed more aggressive tumors. We identified extracellular matrix derived from irradiated astrocytes as a major driver of this phenotype, and astrocyte-derived transglutaminase 2 (TGM2) as a promoter of glioma stemness and radioresistance. TGM2 inhibitors abrogated glioma stemness induced by irradiated astrocytes. TGM2 inhibition reduced survival of glioma cells in in vitro and ex vivo tumor models.These data suggest that irradiation of the brain results in the formation of a tumor-supportive microenvironment. Therapeutic targeting of radiation-induced, astrocyte-derived extracellular matrix proteins may enhance the efficacy of standard of care radiotherapy in glioma.

Hypoxia-Induced Reactivity of Tumor-Associated Astrocytes Affects Glioma Cell Properties

“…These damages include extrinsic factors, such as radiation and chemotherapy treatment, or could also present as intrinsic factors of the tumor microenvironment, such as hypoxia. As we have previously reported [ 33 ], irradiation induced reactive astrogliosis in primary astrocyte cultures in vitro and astrocytes exposed to a single dose of 10 Gy exhibited elevated levels of vimentin ( Figure 1 A) as well as somatic hypertrophy, shown by an increase in both cell area and cell perimeter ( Figure 1 A). Interestingly, astrocytes treated with temozolomide, a chemotherapeutic agent frequently administered after or while patients undergo radiation treatment [ 1 ], also showed an increase in features of reactive astrogliosis ( Figure 1 A), similar to that observed after irradiation.…”

“…We recently identified cell-derived matrix from astrocytes activated by irradiation as a promoter of glioma cell stemness [ 33 ]. Because we found astrocytes throughout the tumor ( Figure 3 B), we set out to examine the effect of hypoxic astrocytes on glioma cell growth by purifying extracellular matrix generated by astrocytes cultured in normoxia (ADM 21 ) and intermediate hypoxia (ADM 1 ) and determining the effect of ADM on the colony forming ability of glioma cells growing on it ( Figure 4 A).…”

“…PIGPC cells cultured on ADM 0.1 showed an increase in SP compared to cells cultured on either ADM 21 or ADM 1 ( Figure 4 D). This suggests that matrix from hypoxic astrocytes, similarly to that from irradiated astrocytes [ 33 ], can promote drug efflux by glioma cells.…”

“…Angiogenin has been implicated in tumor cell proliferation [ 47 , 59 ] and was found to interact with extracellular matrix proteins, such as fibulin-1 [ 60 ]. In a recent study by our lab, we detected fibulin-1 in matrix produced by astrocytes [ 33 ]. An interaction between angiogenin and fibulin-1 could provide one possible mechanism behind the increased proliferation we observed when glioma cells were grown on matrix generated by hypoxic astrocytes.…”

“…Astrocytes respond to brain injury by becoming reactive and entering a process termed reactive astrogliosis [ 32 ]. Recently, our lab showed that astrocytes exposed to radiation become reactive and subsequently promote GBM stemness, potentially contributing to future recurrence [ 33 ]. However, the effect of other microenvironmental stresses on astrocytes, such as hypoxia, remains elusive in the context of brain tumors.…”