Proteoglycan 4: From Mere Lubricant to Regulator of Tissue Homeostasis and Inflammation

While Proteoglycan 4 (PRG4) is a well known lubricating molecule, it is hypothesized that it is a potent inflammatory mediator. It is proposed that PRG4 (blue fire) acts to regulate inflammation (red fire). Under pro‐inflammatory conditions, it is suggested that proteases (circular sector shape) generate PRG4 bioactive fragments insufficient to maintain inflammatory homeostasis.

Proteoglycan 4 (PRG4), first identified in synovial fluid, is an extracellular matrix structural protein in the joint implicated in reducing shear at the cartilage surface as well as controlling adhesion‐dependent synovial growth and regulating bulk protein deposition onto the cartilage. However, recent evidence suggests that it can bind to and effect downstream signaling of a number of cell surface receptors implicated in regulating the inflammatory response. Therefore, we pose the hypothesis: Does PRG4 regulate the inflammatory response and maintain tissue homeostasis? Based on these novel findings implicating PRG4 as an inflammatory signaling molecule, we will present and discuss several hypotheses regarding potential mechanisms by which PRG4 may be able to regulate inflammation. If future studies can demonstrate that PRG4 is a potent inflammatory mediator, this will change current paradigms in the musculoskeletal and ophthalmological fields regarding the how the inflammatory microenvironment is regulated in these tissues and potentially others.

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