The cyclin-dependent kinase inhibitor p57Kip2 mediates proliferative actions of PTHrP in chondrocytes
Helen E. MacLean, … , David Cobrinik, Henry M. Kronenberg
Helen E. MacLean, … , David Cobrinik, Henry M. Kronenberg
Published May 1, 2004
Citation Information: J Clin Invest. 2004;113(9):1334-1343. https://doi.org/10.1172/JCI21252.
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Article Bone biology

The cyclin-dependent kinase inhibitor p57Kip2 mediates proliferative actions of PTHrP in chondrocytes

Abstract

Parathyroid hormone–related peptide (PTHrP) is a positive regulator of chondrocyte proliferation during bone development. In embryonic mice lacking PTHrP, chondrocytes stop proliferating prematurely, with accelerated differentiation. Because the bone phenotype of mice lacking the cyclin-dependent kinase inhibitor p57Kip2 is the opposite of the PTHrP-null phenotype, we hypothesized that PTHrP’s proliferative actions in chondrocytes might be mediated by opposing p57. We generated p57/PTHrP-null embryos, which showed partial rescue of the PTHrP-null phenotype. There was reversal of the loss of proliferative chondrocytes in most bones, with reversal of the accelerated differentiation that occurs in the PTHrP-null phenotype. p57 mRNA and protein were upregulated in proliferative chondrocytes in the absence of PTHrP. Metatarsal culture studies confirmed the action of PTHrP to decrease p57 mRNA and protein levels in a model in which parathyroid hormone (PTH), used as an analog of PTHrP, increased chondrocyte proliferation rate and the length of the proliferative domain. PTH treatment of p57-null metatarsals had no effect on proliferation rate in round proliferative chondrocytes but still stimulated proliferation in columnar chondrocytes. These studies suggest that the effects of PTHrP on both the rate and extent of chondrocyte proliferation are mediated, at least in part, through suppression of p57 expression.

Authors

Helen E. MacLean, Jun Guo, Melissa C. Knight, Pumin Zhang, David Cobrinik, Henry M. Kronenberg

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p57-null metatarsal response to PTH treatment. (A) Chondrocyte prolifera...
p57-null metatarsal response to PTH treatment. (A) Chondrocyte proliferation rate in E16.5 wild-type and p57-null metatarsal cultures. Metatarsals were cultured 48 hours, treated 24 hours with vehicle (white bars) or 10_7 M PTH(1–34) (black bars), and BrdU incorporation rate quantitated. PTH treatment caused a significant increase in the proliferation rate of both round and columnar chondrocytes in the wild-type metatarsals and significantly increased proliferation rate of columnar but not round chondrocytes in the p57-null metatarsals. Bars represent mean ± SEM. *P < 0.05; **P < 0.005 versus no PTH treatment (control). (B) BrdU immunohistochemical detection in sections from E16.5 wild-type and p57-null metatarsals treated with vehicle or 10_7 M PTH(1–34) for 24 hours. In both wild-type and p57-null metatarsals, PTH treatment increases the size of the proliferative pool, as indicated by increased length of BrdU-positive round and columnar proliferative regions; it also delays cell cycle exit, as indicated by the reduction in the region of postproliferative hypertrophic chondrocytes, which do not incorporate BrdU. Original magnification, ∞20.