Protein kinase A regulates chondrogenesis of mesenchymal cells at the post-precartilage condensation stage via protein kinase C-α signaling

Chondrogenesis of mesenchymal cells during in vitro micromass culture requires the generation of cyclic adenosine monophosphate (cAMP) and subsequent activation of cAMP-dependent protein kinase A (PKA). In this study, we investigated the regulatory activity of PKA during chondrogenesis of chick limb bud mesenchymal cells. PKB activity was high in 1-day and 2-day cultures, which was followed by a slight decrease in 4-day and 5-day old cultures. Inhibition of PKA blocked chondrogenesis. It did not affect precartilage condensation, but it blocked the progression from the precartilage condensation stage to cartilage nodule formation. The PKA inhibition-induced blockage of chondrogenesis was accompanied by an altered expression of N-cadherin. Although expression of N-cadherin was detected during the early period of chondrogenesis, it became reduced as chondrogenesis proceeded. Still, inhibition of PKA maintained expression of N-cadherin throughout the micromass culture period. The inhibition of PKA did not affect expression of protein kinase C-alpha (PKC alpha), PKC epsilon, PKC zeta, and PKC lambda/iota, which are the isoforms expressed in differentiating mesenchymal cells. However, PKA inhibition completely blocked activation of PKC alpha. Because PKC activity regulates N-cadherin expression and chondrogenesis, the PKA-mediated regulation of PKC alpha appears to be responsible for the PKA regulation of N-cadherin expression and chondrogenesis. Taken together, our results suggest that PKA regulates chondrogenesis by activating PKC alpha at the stage of post-precartilage condensation.