Aberrant hypertrophy in Smad3-deficient murine chondrocytes is rescued by restoring transforming growth factor beta-activated kinase 1/activating transcription factor 2 signaling: a potential clinical implication for osteoarthritis.

Abstract

OBJECTIVE:

To investigate the biologic significance of Smad3 in the progression of osteoarthritis (OA), the crosstalk between Smad3 and activating transcription factor 2 (ATF-2) in the transforming growth factor beta (TGFbeta) signaling pathway, and the effects of ATF-2 overexpression and p38 activation in chondrocyte differentiation.

METHODS:

Joint disease in Smad3-knockout (Smad3(-/-)) mice was examined by microfocal computed tomography and histologic analysis. Numerous in vitro methods including immunostaining, real-time polymerase chain reaction, Western blotting, an ATF-2 DNA-binding assay, and a p38 kinase activity assay were used to study the various signaling responses and protein interactions underlying the altered chondrocyte phenotype in Smad3(-/-) mice.

RESULTS:

In Smad3(-/-) mice, an end-stage OA phenotype gradually developed. TGFbeta-activated kinase 1 (TAK1)/ATF-2 signaling was disrupted in Smad3(-/-) mouse chondrocytes at the level of p38 MAP kinase (MAPK) activation, resulting in reduced ATF-2 phosphorylation and transcriptional activity. Reintroduction of Smad3 into Smad3(-/-) cells restored the normal p38 response to TGFbeta. Phosphorylated p38 formed a complex with Smad3 by binding to a portion of Smad3 containing both the MAD homology 1 and linker domains. Additionally, Smad3 inhibited the dephosphorylation of p38 by MAPK phosphatase 1 (MKP-1). Both ATF-2 overexpression and p38 activation repressed type X collagen expression in wild-type and Smad3(-/-) chondrocytes. P38 was detected in articular cartilage and perichondrium; articular and sternal chondrocytes expressed p38 isoforms alpha, beta, and gamma, but not delta.

CONCLUSION:

Smad3 is involved in both the onset and progression of OA. Loss of Smad3 abrogates TAK1/ATF-2 signaling, most likely by disrupting the Smad3-phosphorylated p38 complex, thereby promoting p38 dephosphorylation and inactivation by MKP-1. ATF-2 and p38 activation inhibit chondrocyte hypertrophy. Modulation of p38 isoform activity may provide a new therapeutic approach for OA.

• Authors: Tian-Fang Li, Lin Gao, Tzong-Jen Sheu, Erik R Sampson, Lisa M Flick, Yrjö T Konttinen, Di Chen, Edward M Schwarz, Michael J Zuscik, Jennifer H Jonason, Regis J O'Keefe
• Journal: Arthritis and rheumatism (Arthritis Rheum) Vol. 62 Issue 8 Pg. 2359-69 (Aug 2010) ISSN: 1529-0131 [Electronic] United States
• PMID: 20506210 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, Non-U.S. Gov't)

Chemical References

• Activating Transcription Factor 2
• Smad3 Protein
• Transforming Growth Factor beta1
• p38 Mitogen-Activated Protein Kinases

Topics

• Activating Transcription Factor 2 (genetics, metabolism)
• Animals
• Blotting, Western
• Cell Differentiation (drug effects, physiology)
• Cells, Cultured
• Chondrocytes (cytology, drug effects, metabolism, pathology)
• Disease Progression
• Immunohistochemistry
• Mice
• Mice, Knockout
• Osteoarthritis (metabolism, pathology)
• Phosphorylation (drug effects, physiology)
• Reverse Transcriptase Polymerase Chain Reaction
• Signal Transduction (drug effects, physiology)
• Smad3 Protein (genetics, metabolism)
• Transforming Growth Factor beta1 (metabolism, pharmacology)
• p38 Mitogen-Activated Protein Kinases (metabolism)