Abstract
Cancer stem-like cells (CSCs) and the differentiated bulk cells mainly contribute to cellular heterogeneity within a tumor. The dichotomous division pattern of the CSCs sitting atop the tumor hierarchy allow them to self-renew, and differentiate to initiate and repopulate aggressive tumors. These CSC-differentiation-derived bulk tumor cells reprogram to stem-like states, accounting for high plasticity within a tumor; a phenomenon governed by the distinct epigenetic profiles of the two cell populations. Reciprocal signaling between the tumor and its complex microenvironment is a vital driver of plasticity and heterogeneity in cancer. The indispensable role of CSCs in cellular cross-talk in the tumor niche as critical drivers of tumorigenesis is well-studied. However, the massive outnumbering of the CSCs by the differentiated cells raises several vital questions regarding the role of the latter in tumor growth; whether they contribute to the complex signaling within the tumor microenvironment (TME) to drive tumor growth remains unexplored. In this study, we aimed to mechanistically delineate the distinct roles of the cancer stem-like and differentiated cells by investigating their interactions with other stromal cells in the TME.In our model system, Glioblastoma (GBM), a highly aggressive brain cancer, we adopted a quantitative, label-free, high-throughput proteomics-based approach to identify the differentially abundant secreted proteins between the glioma stem-like cell (GSC) and differentiated glioma cell (DGC). This analysis showed a significantly higher abundance of the Fibromodulin (FMOD), a collagen-binding proteoglycan, in the conditioned medium of the DGCs compared with that of the GSCs. Our investigation revealed that an activated TGF-β signaling leads to p-SMAD2 occupancy on the FMOD promoter region, leading to increased FMOD secretion by the DGCs. DGCs silenced for FMOD fail to cooperate with co-implanted GSCs to promote tumor growth in a subcutaneous mouse model. We also observed that FMOD downregulation neither affects GSC growth and differentiation nor DGC growth and reprogramming in vitro. Moreover, DGC-secreted FMOD activates integrin-dependent Notch signaling in endothelial cells, promoting angiogenesis. FMOD-silencing also inhibits the capability of the transdifferentiated endothelial cells (derived from the DGCs) to form angiogenic networks. Conditional silencing of FMOD in the de novo formed DGCs in an orthotopic intracranial C57BL/6 mouse model inhibited the growth of GSC-initiated tumors due to poorly developed vasculature, thereby increasing mouse survival. The same observation was recapitulated using patient-derived GSCs in an immunocompromised intracranial mouse model, thereby highlighting the human relevance of our findings. Collectively, our findings demonstrate how paracrine signaling between DGC-secreted FMOD and the stromal cells promotes tumor growth by enhancing angiogenesis, thereby highlighting a previously less studied supportive role of DGCs in tumor growth, albeit the GSCs retaining the tumor-initiating capacity. Thus our study identifies DGC-specific FMOD as a potential therapeutic target and emphasizes that targeting both cancer stem cells and differentiated cancer bulk cells is vital to achieving a durable response in GBM.