Transforming growth factor α suppression of articular chondrocyte phenotype and Sox9 expression in a rat model of osteoarthritis

Abstract

Objective. To define the roles of transforming growth factor ␣ (TGF␣) in cartilage degradation.Methods. Primary rat articular chondrocytes and articular osteochondral explants were cultured with TGF␣ to assess the effects of TGF␣ on chondrocyte physiology and phenotype.Results. TGF␣ altered chondrocyte morphology through reorganization of the actin cytoskeleton and formation of stress fibers. Expression of anabolic genes, including aggrecan, type II collagen, and cartilage link protein, was reduced in response to TGF␣. Proliferation of chondrocytes and formation of articular chondrocyte clusters was stimulated by TGF␣. Expression of matrix metalloproteinase 13 and cathepsin C was increased by TGF␣. We demonstrated the down-regulation of Sox9 messenger RNA and protein levels by TGF␣. This was associated with reduced levels of phosphorylated and total SOX9 in cartilage explants upon TGF␣ treatment. In contrast, another growth factor identified in our microarrays, Kitl, had no effects on the chondrocyte parameters tested. To examine correlations between the increased levels of TGF␣ in experimental knee osteoarthritis (OA) with the levels of TGF␣ in humans with knee OA, a microarray analysis of mRNA from 13 normal and 12 late-stage OA cartilage samples was performed. Seven OA samples showed TGFA mRNA levels similar to those in the normal controls, but expression was markedly increased in the other 5 OA samples. These data confirm that TGFA transcript levels are increased in a subset of patients with OA.Conclusion. This study adds TGF␣ to the list of dysregulated cytokines present in degrading cartilage in OA. Since TGF␣ inhibits articular chondrocyte anabolic capacity, increases catabolic factors, and contributes to the development of chondrocyte clusters, TGF␣ may be a potential target for therapeutic strategies in the treatment of OA.Articular cartilage degeneration is a defining feature of osteoarthritis (OA) (1). Under normal circumstances, chondrocytes are responsible for maintaining the cartilage extracellular matrix (ECM), which consists largely of type II collagen and proteoglycans. However, degenerative influences stimulate pathologic changes to the chondrocyte phenotype in OA. As a result, catabolic factors (e.g., matrix metalloproteinases [MMPs]) are synthesized and released from chondrocytes, weakening the ECM (2). These events make cartilage susceptible to swelling and mechanical disruption due to compromised structural integrity (3). OA chondrocytes also attempt to repair damaged tissues by chondrocyte proliferation (4), but repair is generally suboptimal.Cytokine release from articular chondrocytes is thought to promote cartilage degradation in OA. Numerous studies of interleukin-1␤ (IL-1␤) and tumor

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

“…We have previously shown that TGFa stimulates stress fiber formation in chondrocytes, 8 a phenotype consistent with activation of the RhoA pathway. 15 Therefore, we assessed RhoA activity in primary chondrocyte cultures to determine the ability of TGFa signaling (through EGFR) to activate the RhoA/ROCK pathway.…”

“…8 A microplate reader (Tecan US, Durham, NC, USA) was used to quantify the colorimetric reaction by 600 nm absorbance readings.…”

“…8 TGFa is a member of the epidermal growth factor (EGF) family of ligands and is well characterized for its roles in inflammatory responses to infection, epithelial maintenance and malignancies. [9][10][11] Our further investigations determined that TGFa inhibits anabolic and promotes catabolic processes in articular cartilage.…”

“…[9][10][11] Our further investigations determined that TGFa inhibits anabolic and promotes catabolic processes in articular cartilage. 8 Thus, interference with TGFa signaling in chondrocytes represents a potential therapeutic strategy for OA.…”

Cartilage-specific deletion of Mig-6 results in osteoarthritis-like disorder with excessive articular chondrocyte proliferation

“…The most well-established role for Mig-6 in cancer cells is as a negative feedback regulator of EGFR (3)(4)(5)(6), and EGFR signaling is known to play an important role in bone development (28)(29)(30)(31). TGF-α, one of the ligands for EGFR, has been reported to be increased in human osteoarthritic joints (32), and it can induce articular cartilage degradation in animal models (33,34). EGFR signaling appears to be activated in the chondrocytes of the articular cartilage and in the osteophyte of the Mig-6 flox/flox ;Col2a1 Cre knee joints (Fig.…”