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Sox9 is a high-mobility-group domain-containing transcription factor required for chondrocyte differentiation and cartilage formation. We used a yeast two-hybrid method based on Son of Sevenless (SOS) recruitment to screen a chondrocyte cDNA library and found that the catalytic subunit of cyclic AMP (cAMP)-dependent protein kinase A (PKA-Calpha) interacted specifically with SOX9. Next we found that two consensus PKA phosphorylation sites within SOX9 could be phosphorylated by PKA in vitro and that SOX9 could be phosphorylated by PKA-Calpha in vivo. In COS-7 cells cotransfected with PKA-Calpha and SOX9 expression plasmids, PKA enhanced the phosphorylation of wild-type SOX9 but did not affect phosphorylation of a SOX9 protein in which the two PKA phosphorylation sites (S(64) and S(211)) were mutated. Using a phosphospecific antibody that specifically recognized SOX9 phosphorylated at serine 211, one of the two PKA phosphorylation sites, we demonstrated that addition of cAMP to chondrocytes strongly increased the phosphorylation of endogenous Sox9. In addition, immunohistochemistry of mouse embryo hind legs showed that Sox9 phosphorylated at serine 211 was principally localized in the prehypertrophic zone of the growth plate, corresponding to the major site of expression of the parathyroid hormone-related peptide (PTHrP) receptor. Since cAMP has previously been shown to effectively increase the mRNA levels of Col2a1 and other specific markers of chondrocyte differentiation in culture, we then asked whether PKA phosphorylation could modulate the activity of SOX9. Addition of 8-bromo-cAMP to chondrocytes in culture increased the activity of a transiently transfected SOX9-dependent 48-bp Col2a1 chondrocyte-specific enhancer; similarly, cotransfection of PKA-Calpha increased the activity of this enhancer. Mutations of the two PKA phosphorylation consensus sites of SOX9 markedly decreased the PKA-Calpha activation of this enhancer by SOX9. PKA phosphorylation and the mutations in the consensus PKA phosphorylation sites of SOX9 did not alter its nuclear localization. In vitro phosphorylation of SOX9 by PKA resulted in more efficient DNA binding. We conclude that SOX9 is a target of cAMP signaling and that phosphorylation of SOX9 by PKA enhances its transcriptional and DNA-binding activity. Because PTHrP signaling is mediated by cAMP, our results support the hypothesis that Sox9 is a target of PTHrP signaling in the growth plate and that the increased activity of Sox9 might mediate the effect of PTHrP in maintaining the cells as nonhypertrophic chondrocytes.

Phosphorylation of SOX9 by Cyclic AMP-Dependent Protein Kinase A Enhances SOX9's Ability To Transactivate aCol2a1 Chondrocyte-Specific Enhancer