Rho/ROCK and MEK/ERK activation by transforming growth factor-α induces articular cartilage degradation

Identification and characterization of therapeutic targets for joint conditions, such as osteoarthritis (OA), is exceedingly important for addressing the increasing burden of disease. Transforming growth factor-alpha (TGF alpha) is upregulated by articular chondrocytes in experimentally induced and human OA. To test the potential involvement of TGF alpha, which is an activator of epidermal growth factor receptor (EGFR) signaling, in joint degeneration and to identify signaling mechanisms mediating articular chondrocyte responses to TGF alpha, rat chondrocytes and osteochondral explants were treated with TGF alpha and various inhibitors of intracellular signaling pathways. Stimulation of EGFR signaling in articular chondrocytes by TGF alpha resulted in the activation of RhoA/ROCK (Rho kinase), MEK (MAPK/ERK kinase)/ERK (extracellular-signal-regulated kinase), PI3K (phosphoinositide 3-kinase) and p38 MAPK (mitogen-activated protein kinase) pathways. Modification of the chondrocyte actin cytoskeleton was stimulated by TGF alpha, but inhibition of only Rho or ROCK activation prevented morphological changes. TGF alpha suppressed expression of anabolic genes including Sox9, type II collagen and aggrecan, which were rescued only by inhibiting MEK/ERK activation. Furthermore, catabolic factor upregulation by TGF alpha was prevented by ROCK and p38 MAPK inhibition, including matrix metalloproteinase-13 and tumor necrosis factor-alpha, which are well known to contribute to cartilage digestion in OA. To assess the ability of TGFa to stimulate degradation of mature articular cartilage, type II collagen and aggrecan cleavage fragments were analyzed in rat osteochondral explants exposed to exogenous TGF alpha. Normal articular cartilage contained low levels of both cleavage fragments, but high levels were observed in the cartilage treated with TGF alpha. Selective inhibition of MEK/ERK and Rho/ROCK activation greatly reduced or completely prevented excess type II collagen and aggrecan degradation in response to TGF alpha. These data suggest that TGF alpha is a strong stimulator of cartilage degradation and that Rho/ROCK and MEK/ERK signaling have critical roles in mediating these effects. Laboratory Investigation (2010) 90, 20-30; doi:10.1038/labinvest.2009.111; published online 12 October 2009