Session Item

Glioblastomas (GBM) are routinely treated with high doses of ionizing radiation (IR), yet these tumors inevitably recur, and the recurrent tumors are highly therapy resistant. Understanding mechanisms driving tumor recurrence after radiotherapy is an unmet need in the field of GBM therapy. During GBM therapy, both the tumor and the normal brain tissue surrounding the tumor are irradiated with up to 60 Gy of IR. IR is a potent inducer of senescence, and senescent stromal cells are known to promote the growth of neighboring tumor cells by secreting proteases, cytokines and growth factors that create a senescence-associated secretory phenotype (SASP). We have reported recently that IR-induced senescence of astrocytes in the brain leads to the secretion of SASP factors that promote the growth and invasiveness of tumor cells in mouse models of GBM. Briefly, we found that cranial irradiation of C57BL/6J mice triggered p21-dependent senescence of astrocytes. Senescent astrocytes showed upregulation of several SASP genes including HGF. HGF secreted by senescent astrocytes activated the receptor tyrosine kinase Met in orthotopically implanted glioma cells thereby promoting their growth and invasiveness. Elimination of senescent astrocytes using the senolytic ABT-263 (Navitoclax) thwarted tumor growth, underscoring the usefulness of senolytics for improving GBM therapy. Following up on this study, we next irradiated a panel of GBM cell lines and found that a significant fraction of these cells senesced within 10 days, as evaluated by analyzing senescence markers like SA-beta-galactosidase staining, induction of p21 and loss of nuclear Lamin B1.  Senescent glioma cells exhibited upregulation of a number of bona fide SASP genes in a NF-kB-dependent manner. The interleukin IL6, which is an activator of the JAK-STAT pathway, was most prominently upregulated in these cells. Conditioned media from senescent GBM cells activated the JAK-STAT pathway in non-senescent GBM cells and promoted tumor cell proliferation and radiation resistance. These effects could be neutralized by antibodies targeting IL6 or its receptor IL6R, as well as by JAK 1/2 inhibitors. Interestingly, conditioned media from senescent GBM cells could also activate the NF-kB pathway in non-senescent cells resulting in induction of SASP genes in the recipient cells. These results indicate that senescent GBM cells can activate pro-tumorigenic pathways in their non-senescent counterparts and SASP can spread from senescent to non-senescent cells, thereby promoting tumor recurrence. Bioinformatic analyses of the transcriptomic profiles of naïve and irradiated GBM cell lines and the TCGA database revealed that the inhibitor of apoptosis protein cIAP2 is a critical survival factor for senescent glioma cells. We find that targeting cIAP2 using Smac mimetics triggers cell death specifically in senescent GBM cells with minimal toxicity towards normal brain cells such as astrocytes. Upregulation of cIAP2 in irradiated tumor cells was also seen in PDX models of GBM. Using these PDX models, we are currently validating novel senolytic-based therapeutic approaches to prevent tumor recurrence after radiotherapy. In sum, our findings illustrate how senescence in both stromal and tumor cells promote GBM recurrence via different mechanisms and underscore the potential utility of adjuvant senolytic therapy for blunting GBM recurrence after radiotherapy.