Mitogen-activated Protein Kinase p38 Mediates Regulation of Chondrocyte Differentiation by Parathyroid Hormone

Xuechu Zhen,Lei Wei,Qiuqian Wu,Yue Zhang,Qian Chen

doi:10.1074/jbc.m004990200

Abstract

Parathyroid hormone (PTH) and its related peptide regulate endochondral ossification by inhibiting chondrocyte differentiation toward hypertrophy. However, the intracellular pathway for transducing PTH/PTH-related peptide signals in chondrocytes remains unclear. Here, we show that this pathway is mediated by mitogen-activated protein kinase (MAPK) p38. Incubation of hypertrophic chondrocytes with PTH (1-34) induces an inhibition of p38 kinase activity in a time- and dose-dependent manner. Inhibition of protein kinase C prevents PTH-induced p38 MAPK inhibition, whereas inhibition of protein kinase A has no effect. Thus, protein kinase C, but not protein kinase A, is required for the inhibition of p38 MAPK by PTH. Treatment of hypertrophic chondrocytes by PTH or by p38 MAPK inhibitor SB203580 up-regulates Bcl-2, suggesting that Bcl-2 lies downstream of p38 MAPK in the PTH signaling pathway. Inhibition of p38 MAPK in hypertrophic chondrocytes by either PTH, SB303580, or both together leads to a decrease of hypertrophic marker type X collagen mRNA and an increase of the expression of prehypertrophic marker cartilage matrix protein. Therefore, inhibition of p38 converts a hypertrophic cell phenotype to a prehypertrophic one, thereby preventing precocious chondrocyte hypertrophy. Taken together, these data suggest a major role for p38 MAPK in transmitting PTH signals to regulate chondrocyte differentiation.

Highlights

During endochondral ossification, chondrocytes undergo a differentiation process including proliferation, maturation, hypertrophy, and apoptosis [1]
Parathyroid hormone (PTH) Inhibits p38 mitogen-activated protein kinase (MAPK) Activity and Tyrosine Phosphorylation—To determine whether p38 MAPK activity correlates with chondrocyte hypertrophy, we examined the basal level of p38 MAPK activity in both prehypertrophic and hypertrophic chondrocytes
Tyrosine phosphorylation of p38 MAPK was decreased after 10 min of PTH treatment and returned to the basal level after 60 min. This was in agreement with the time course of the PTH inhibition of p38 MAPK activity in hypertrophic chondrocytes

Summary

Introduction

Chondrocytes undergo a differentiation process including proliferation, maturation, hypertrophy, and apoptosis [1]. PTH receptors are expressed at specific stages during chondrocyte differentiation, with the highest level at the prehypertrophic to hypertrophic transition [5]. The PTH/PTHrP receptor plays a fundamental role in the control of endochondral bone formation by transducing signals inhibiting chondrocyte hypertrophy. We will determine the involvement of PKA and PKC pathways in transducing PTH/ PTHrP signals to regulate chondrocyte differentiation. The signaling molecules downstream of PKA or PKC that mediate PTH/PTHrP regulation of chondrocyte differentiation will be defined. We demonstrate that inhibition of p38 MAPK activity is a signaling mechanism responsible for regulation of chondrocyte terminal differentiation by PTH/PTHrP

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