Jun activation domain-binding protein 1 binds Smad5 and inhibits bone morphogenetic protein signaling.
OBJECTIVE
Bone morphogenetic proteins (BMPs) play an important role in the development and the homeostasis and pathology of cartilage tissue, particularly in the differentiation and anabolic activity of chondrocytes. The present study was undertaken to identify binding partners of the Smad proteins, the intracellular mediators of BMP activity, which might actively modify BMP signaling in chondrocytes.


METHODS
Yeast 2-hybrid technology was used to screen a complementary DNA library, constructed from human adult articular cartilage, for molecular binding partners of Smad5, a major intracellular mediator of BMP signaling. Primary interaction partners were verified by coimmunoprecipitation, and the relevance of the interactions to BMP signaling was evaluated by transcriptional reporter assay. Additionally, messenger RNA expression analysis (conventional and quantitative polymerase chain reaction) and immunostaining were performed in adult normal and osteoarthritic articular cartilage.


RESULTS
We identified a novel Smad5 interactor, Jun activation domain-binding protein 1 (Jab1), expressed in adult cartilage. The interaction was confirmed in coimmunoprecipitation experiments. Overexpression of Jab1 resulted in an attenuation of BMP-dependent transcriptional responses, suggesting that Jab1 acts as an inhibitor of BMP signaling.


CONCLUSIONS
Jab1 is a newly identified intracellular (negative) modulator of BMP signaling in chondrocytes and other cells. Jab1 represents an interesting molecule for understanding anabolic signaling in chondrocytes, as well as a potential therapeutic target for anabolic activation. Most interestingly, Jab1 appears to crosslink the BMP and interleukin-1 pathways.
Haag
Jochen
J
University of Leipzig, Leipzig, Germany.
Aigner
Thomas
T
D000368:Aged; D019485:Bone Morphogenetic Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D000075686:COP9 Signalosome Complex; D002358:Cartilage, Articular / Q000378:metabolism* / Q000473:pathology; D002460:Cell Line; D019902:Chondrocytes / Q000378:metabolism* / Q000473:pathology; D015870:Gene Expression; D015723:Gene Library; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000378:metabolism / Q000473:pathology; D010447:Peptide Hydrolases / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D051902:Smad5 Protein / Q000235:genetics / Q000378:metabolism*; D020798:Two-Hybrid System Techniques
D019485:Bone Morphogenetic Proteins; D047908:Intracellular Signaling Peptides and Proteins; D012333:RNA, Messenger; C497797:SMAD5 protein, human; D051902:Smad5 Protein; D010447:Peptide Hydrolases; C501488:COPS5 protein, human; D000075686:COP9 Signalosome Complex