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

Abstract

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-a (TGFa) is upregulated by articular chondrocytes in experimentally induced and human OA. To test the potential involvement of TGFa, which is an activator of epidermal growth factor receptor (EGFR) signaling, in joint degeneration and to identify signaling mechanisms mediating articular chondrocyte responses to TGFa, rat chondrocytes and osteochondral explants were treated with TGFa and various inhibitors of intracellular signaling pathways. Stimulation of EGFR signaling in articular chondrocytes by TGFa resulted in the activation of RhoA/ROCK (Rho kinase), MEK (MAPK/ERK kinase)/ERK (extracellularsignal-regulated kinase), PI3K (phosphoinositide 3-kinase) and p38 MAPK (mitogen-activated protein kinase) pathways. Modification of the chondrocyte actin cytoskeleton was stimulated by TGFa, but inhibition of only Rho or ROCK activation prevented morphological changes. TGFa 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 TGFa was prevented by ROCK and p38 MAPK inhibition, including matrix metalloproteinase-13 and tumor necrosis factor-a, 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 TGFa. Normal articular cartilage contained low levels of both cleavage fragments, but high levels were observed in the cartilage treated with TGFa. Selective inhibition of MEK/ERK and Rho/ROCK activation greatly reduced or completely prevented excess type II collagen and aggrecan degradation in response to TGFa. These data suggest that TGFa is a strong stimulator of cartilage degradation and that Rho/ROCK and MEK/ERK signaling have critical roles in mediating these effects. KEYWORDS: articular cartilage; chondrocyte; epidermal growth factor receptor; osteoarthritis; transforming growth factor alpha Osteoarthritis (OA) is a chronic, degenerative, synovial joint condition, which affects the knees, hips and other smaller joints of B10% of the North American population. The societal burden of OA is increasing and OA already accounts for 25% of visits to primary care physicians and for 50% of nonsteroidal anti-inflammatory drug prescriptions.1,2 Approximately 80% of individuals have radiographic evidence of OA by age 65 years, of which 60% are symptomatic.1 Furthermore, progressive joint degeneration combined with an inherently low capacity for articular cartilage regeneration makes treatment very difficult. As current therapies are strictly palliative, identification and characterization of therapeutic targets is exceedingly important to meet the increasing burden of disease. Althou...

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