PTHrP modulates chondrocyte differentiation through AP-1 and CREB signaling

During the process of differentiation, chondrocytes integrate a complex arr ay of signals from local or systemic factors like parathyroid hormone-relat ed peptide (PTHrP), Indian hedgehog, bone morphogenetic proteins and transf orming growth factor beta, While PTHrP is known to be a critical regulator of chondrocyte proliferation and differentiation, the signaling pathways th rough which this factor acts remain to be elucidated. Here we show that bot h cAMP response element-binding protein (CREB) and AP-1 activation are crit ical to PTHrP signaling in chondrocytes. PTHrP treatment leads to rapid CRE B phosphorylation and activation, while CREB DNA binding activity is consti tutive, In contrast, PTHrP induces AP-1 DNA binding activity through induct ion of c-Fos protein expression. PTHrP activates CRE and TRE reporter const ructs primarily through PKA-mediated signaling events. Both signaling pathw ays were found to be important mediators of PTHrP effects on chondrocyte ph enotype, Alone, PTHrP suppresses maturation and stimulates proliferation of the chondrocyte cultures, However, in the presence of dominant negative in hibitors of CREB and c-Fos, these PTHrP effects were suppressed, and chondr ocyte maturation was accelerated. Moreover, in combination, the effects of dominant negative c-Fos and CREB are synergistic, suggesting interaction be tween these signaling pathways during chondrocyte differentiation.