Reduced transforming growth factor-beta signaling in cartilage of old mice: role in impaired repair capacity

Osteoarthritis (OA) is a common joint disease, mainly effecting the elderly population. The cause of OA seems to be an imbalance in catabolic and anabolic factors that develops with age. IL-1 is a catabolic factor known to induce cartilage damage, and transforming growth factor (TGF)-beta is an anabolic factor that can counteract many IL-1-induced effects. In old mice, we observed reduced responsiveness to TGF-beta-induced IL-1 counteraction. We investigated whether expression of TGF- beta and its signaling molecules altered with age. To mimic the TGFbeta deprived conditions in aged mice, we assessed the functional consequence of TGF- beta blocking. We isolated knee joints of mice aged 5 months or 2 years, half of which were exposed to IL-1 by intra-articular injection 24 h prior to knee joint isolation. Immunohistochemistry was performed, staining for TGF- beta1, -2 or -3, TGF- betaRI or -RII, Smad2, -3, -4, -6 and 7 and Smad-2P. The percentage of cells staining positive was determined in tibial cartilage. To mimic the lack of TGF- beta signaling in old mice, young mice were injected with IL-1 and after 2 days Ad-LAP (TGF- beta inhibitor) or a control virus were injected. Proteoglycan (PG) synthesis ((35)S-sulfate incorporation) and PG content of the cartilage were determined. Our experiments revealed that TGF-beta2 and -3 expression decreased with age, as did the TGF- beta receptors. Although the number of cells positive for the Smad proteins was not altered, the number of cells expressing Smad2P strongly dropped in old mice. IL-1 did not alter the expression patterns. We mimicked the lack of TGF- beta signaling in old mice by TGFbeta inhibition with LAP. This resulted in a reduced level of PG synthesis and aggravation of PG depletion. The limited response of old mice to TGF- beta induced-IL-1 counteraction is not due to a diminished level of intracellular signaling molecules or an upregulation of intracellular inhibitors, but is likely due to an intrinsic absence of sufficient TGF- beta receptor expression. Blocking TGF- beta distorted the natural repair response after IL-1 injection. In conclusion, TGF- beta appears to play an important role in repair of cartilage and a lack of TGF- beta responsiveness in old mice might be at the root of OA development.