Carboxypeptidase Z (CPZ) Links Thyroid Hormone and Wnt Signaling Pathways in Growth Plate Chondrocytes

Carboxypeptidase Z (CPZ) removes carboxyl‐terminal basic amino acid residues, particularly arginine residues, from proteins. CPZ contains a cysteine‐rich domain (CRD) similar to the CRD found in the frizzled family of Wnt receptors. We have previously shown that thyroid hormone regulates terminal differentiation of growth plate chondrocytes through activation of Wnt‐4 expression and Wnt/β‐catenin signaling. The Wnt‐4 protein contains a C‐terminal arginine residue and binds to CPZ through the CRD. The objective of this study was to determine whether CPZ modulates Wnt/β‐catenin signaling and terminal differentiation of growth plate chondrocytes. Our results show that CPZ and Wnt‐4 mRNA are co‐expressed throughout growth plate cartilage. In primary pellet cultures of rat growth plate chondrocytes, thyroid hormone increases both Wnt‐4 and CPZ expression, as well as CPZ enzymatic activity. Knockdown of either Wnt‐4 or CPZ mRNA levels using an RNA interference technique or blocking CPZ enzymatic activity with the carboxypeptidase inhibitor GEMSA reduces the thyroid hormone effect on both alkaline phosphatase activity and Col10a1 mRNA expression. Adenoviral overexpression of CPZ activates Wnt/β‐catenin signaling and promotes the terminal differentiation of growth plate cells. Overexpression of CPZ in growth plate chondrocytes also removes the C‐terminal arginine residue from a synthetic peptide consisting of the carboxyl‐terminal 16 amino acids of the Wnt‐4 protein. Removal of the C‐terminal arginine residue of Wnt‐4 by site‐directed mutagenesis enhances the positive effect of Wnt‐4 on terminal differentiation. These data indicate that thyroid hormone may regulate terminal differentiation of growth plate chondrocytes in part by modulating Wnt signaling pathways through the induction of CPZ and subsequent CPZ‐enhanced activation of Wnt‐4.