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

Chondrogenesis of mesenchymal cells during in vitro micromass culture requi res the generation of cyclic adenosine monophosphate (cAMP) and subsequent activation of cAMP-dependent protein kinase A (PKA). In this study, we inve stigated 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. In hibition of PKA blocked chondrogenesis. It did not affect precartilage cond ensation, but it blocked the progression from the precartilage condensation stage to cartilage nodule formation. The PKA inhibition-induced blockage o f chondrogenesis was accompanied by an altered expression of N-cadherin. Al though expression of N-cadherin was detected during the early period of cho ndrogenesis, it became reduced as chondrogenesis proceeded. Still, inhibiti on of PKA maintained expression of N-cadherin throughout the micromass cult ure period. The inhibition of PKA did not affect expression of protein kina se C-alpha (PKC alpha), PKC epsilon, PKC zeta, and PKC lambda/iota, which a re the isoforms expressed in differentiating mesenchymal cells. However, PK A inhibition completely blocked activation of PKC alpha. Because PKC activi ty regulates N-cadherin expression and chondrogenesis, the PKA-mediated reg ulation 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.