Abstract
Biglycan, a ubiquitous proteoglycan, acts as a danger signal when released from the extracellular matrix. As such, biglycan triggers the synthesis and maturation of interleukin-1β (IL-1β) in a Toll-like receptor (TLR) 2-, TLR4-, and reactive oxygen species (ROS)-dependent manner. Here, we discovered that biglycan autonomously regulates the balance in IL-1β production in vitro and in vivo by modulating expression, activity and stability of NADPH oxidase (NOX) 1, 2 and 4 enzymes via different TLR pathways. In primary murine macrophages, biglycan triggered NOX1/4-mediated ROS generation, thereby enhancing IL-1β expression. Surprisingly, biglycan inhibited IL-1β due to enhancement of NOX2 synthesis and activation, by selectively interacting with TLR4. Synthesis of NOX2 was mediated by adaptor molecule Toll/IL-1R domain-containing adaptor inducing IFN-β (TRIF). Via myeloid differentiation primary response protein (MyD88) as well as Rac1 activation and Erk phosphorylation, biglycan triggered translocation of the cytosolic NOX2 subunit p47(phox) to the plasma membrane, an obligatory step for NOX2 activation. In contrast, by engaging TLR2, soluble biglycan stimulated the expression of heat shock protein (HSP) 70, which bound to NOX2, and consequently impaired the inhibitory function of NOX2 on IL-1β expression. Notably, a genetic background lacking biglycan reduced HSP70 expression, rescued the enhanced renal IL-1β production and improved kidney function of Nox2(-/y) mice in a model of renal ischemia reperfusion injury. Here, we provide a novel mechanism where the danger molecule biglycan influences NOX2 synthesis and activation via different TLR pathways, thereby regulating inflammation severity. Thus, selective inhibition of biglycan-TLR2 or biglycan-TLR4 signaling could be a novel therapeutic approach in ROS-mediated inflammatory diseases.