Preventing Osteoarthritis by Targeting the Transforming Growth Factor Beta Signaling Pathway

Osteoarthritis (OA) is characterized by degenerated cartilage. In age-related OA, the molecular mechanisms that initiate cartilage erosion are not well defined. However, transforming growth factor beta (TGF-β) signaling is attenuated in the cartilage of patients with age-related OA and therefore may play a role in the disease. Degenerated cartilage has a rough surface and exhibits changes in stiffness. Proteoglycan 4 (PRG4), also known as lubricin, helps lubricate the surface of cartilage, and 3’-phosphoadenosine 5’-phosphosulfate synthase 2 (PAPSS2) helps maintain the stiffness of cartilage. Both PRG4 and PAPSS2 are regulated by TGF-β and down-regulated in OA cartilage. The present research elucidated how TGF-β regulates PRG4 and PAPSS2 and assessed whether parts of the TGF-β signaling pathway could be targeted to prevent OA.

First, cell signaling experiments were performed to elucidate how TGF-β regulates PRG4 and PAPSS2. These experiments showed that TGF-β cooperated with SMAD3 to regulate PRG4 mRNA and that TGF-β cooperated with SOX9 to regulate PAPSS2 mRNA. Next, a mouse model with OA induced by a dominant-negative mutant of the TGF-β type II receptor (DNIIR) was used to determine whether Prg4 could prevent DNIIR-induced OA. Prg4 was shown to prevent foot misplacement, cartilage fibrillation, cartilage thinning, and loss of superficial zone cartilage in DNIIR mice. However, Prg4 did not restore TGF-β signaling in DNIIR mice. Together with previously published studies on Prg4, the present research supports the idea that Prg4 can prevent multiple kinds of OA regardless of the etiology. Finally, an in vitro 3D OA cartilage model was developed by attenuating TGF-β signaling in chondrocytes using an inhibitor of the TGF-β type I receptor, SB431542, and by culturing the cells in Transwell inserts on an orbital shaker. After 21 days of cultivation, cartilage-like tissues formed. The OA tissue model exhibited reduced proteoglycan staining, reduced collagen staining, thinning, and reduced stiffness compared to a control tissue. Preliminary data shows that the model can be used to test whether TGF-β-related molecules may be potential targets for OA treatments. Future work will involve further development of the in vitro 3D OA cartilage model and identification of additional potential targets for OA treatments.