Transforming growth factor-β (TGF-β) represents an evolutionarily conserved family of secreted polypeptide factors that regulate many aspects of physiological embryogenesis and adult tissue homeostasis. The TGF-β family members are also involved in pathophysiological mechanisms that underlie many diseases. Although the family comprises many factors, which exhibit cell type-specific and developmental stage-dependent biological actions, they all signal via conserved signaling pathways. The signaling mechanisms of the TGF-β family are controlled at the extracellular level, where ligand secretion, deposition to the extracellular matrix and activation prior to signaling play important roles. At the plasma membrane level, TGF-βs associate with receptor kinases that mediate phosphorylation-dependent signaling to downstream mediators, mainly the SMAD proteins, and mediate oligomerization-dependent signaling to ubiquitin ligases and intracellular protein kinases. The interplay between SMADs and other signaling proteins mediate regulatory signals that control expression of target genes, RNA processing at multiple levels, mRNA translation and nuclear or cytoplasmic protein regulation. This article emphasizes signaling mechanisms and the importance of biochemical control in executing biological functions by the prototype member of the family, TGF-β.

Glioblastoma (GBM) is the most aggressive and common glioma subtype with a median survival of 15 months after diagnosis. Current treatments have limited therapeutic efficacy, thus more effective approaches are needed. The glioblastoma tumoral mass is characterized by a small cellular subpopulation, the Glioblastoma stem cells (GSCs), which has been held accountant for initiation, invasion, proliferation, relapse and resistance to chemo- and radiotherapy. Targeted therapies against GSCs are crucial, and so is the understanding of the molecular mechanisms that govern the GSCs. Transforming growth factor β (TGFβ), platelet growth factor (PDGF) signalling and Reactive Oxygen Species (ROS) production govern and regulate cancer-stem cell biology. In this work, we focus on the role of the NADPH oxidase 4 (NOX4) downstream of TGFβ signalling in the GSCs. NOX4 utilises NADPH to generate ROS; TGFβ induces NOX4 expression, thus increasing ROS production. Interestingly, NOX4 itself regulates GSC self-renewal and modulates Since TGFβ regulates PDGFB in GSC, we analysed how PDGFB modulates NOX4 expression and increases ROS production. Both TGFβ and PDGF signalling regulate GSC proliferation in a NOX4/ROS-dependent manner. The transcription factor NRF2, involved in the transcriptional regulation of antioxidant and metabolic responses, is regulated by both TGFβ and NOX4. This results in an antioxidant response, which positively contributes to GSC self-renewal and proliferation. In conclusion, this work functionally establishes NOX4 as a key mediator of GSC biology.