AP-1 transcription factor complex is a target of signals from both WnT-7a and N-cadherin-dependent cell-cell adhesion complex during the regulation of limb mesenchymal chondrogenesis

Abstract

Wnt signaling has been implicated in the regulation of limb mesenchymal chondrogenesis. In this study, we have analyzed the molecular mechanism of Wnt-7a inhibition of chondrogenic differentiation by examining the involvement of mitogen-activated protein kinase (MAPK) pathways, i.e., Erk and p38. The combination of Wnt-7a misexpression and Erk inhibition partially recovers Wnt-7a inhibition of chondrogenic differentiation, whereas the combination of Wnt-7a misexpression and p38 inhibition acts in a synergistic chondro-inhibitory fashion. Although Wnt-7a misexpression has no direct effect on Erk signaling, it increases the activity of one of the ultimate targets of the MAPK pathway, c-jun, a major component of the activator protein-1 (AP-1) transcription factor complex. In addition, Wnt-7a misexpression enhances the activity of an AP-1 promoter-luciferase reporter construct by approximately 2.3-fold in vitro. Interestingly, misexpression of wild-type N-cadherin in these micromass cultures suppresses the activity of the same AP-1 promoter by approximately 40%, whereas misexpression of an extracellular 390-amino-acid N-terminal deletion mutant of N-cadherin has a stimulatory effect on the AP-1 promoter activity by approximately 2.6-fold. Thus, our results suggest that at least a part of the chondro-inhibitory effect of Wnt-7a misexpression may involve AP-1 transcription factor stimulation. Furthermore, a very tightly regulated level of AP-1 activity is necessary for the process of limb mesenchymal chondrogenesis, and signals from Wnt-ligands (e.g., Wnt-7a), cell adhesion molecules (e.g., N-cadherin), and MAPK pathways (e.g., Erk and p38) are interactively involved in this regulation.