The Mitogenic Effect of Parathyroid Hormone Is Associated with E2F-Dependent Activation of Cyclin-Dependent Kinase 1 (cdc2) in Osteoblast Precursors

Abstract

Injections of parathyroid hormone (PTH) have been reported to stimulate skeletal accretion through increased bone formation in several species, and osteoblast proliferation is a critical component of bone formation. However, the biological mechanisms of PTH-stimulated bone cell proliferation are largely unknown. In this study, we demonstrated that PTH stimulates proliferation of the osteoblast precursor cell line, TE-85, in association with increasing cdc2 protein levels and its kinase activity. cdc2 antisense oligonucleotides blocked PTH-induced DNA synthesis and cell cycle progression. Analysis of the time course of PTH-stimulated cdc2 message levels demonstrated that cdc2 mRNA levels were increased 1.5-to 4-fold between 3-18 h following release from cell synchronization. Transfections of TE-85 cells with a series of cdc2 promoter-luciferase deletion constructs revealed PTH stimulation of the cdc2 promoter. Promoter constructs containing a mutation in the E2F binding site were not stimulated by PTH. Gel mobility shift assays demonstrated increased free E2F levels in TE-85 nuclear extracts in response to PTH. Furthermore, the ratios of hyperphosphorylated to hypophosphorylated forms of Rb protein were increased by PTH treatment. These results demonstrate that PTH-stimulated cdc2 expression was associated with TE-85 cell proliferation and that the mechanism of stimulating cdc2 gene expression involved increasing the levels of free E2F. (J Bone Miner Res 1997;12:1596-1605)

Successful Treatment of an Adynamic Bone Disorder with Bone Morphogenetic Protein-7 in a Renal Ablation Model

“…Uremic bone is resistant to the actions of PTH (13), defined as release of calcium from the skeleton after a dose of PTH, and a certain amount of hyperparathyroidism is required to maintain bone turnover. However, PTH is a poor director of osteoblast differentiation in vitro (17,21,69), and excess PTH ultimately leads to accumulation of fibrous cells in trabecular bone (21) and elevated rates of bone formation but excess bone resorption (i.e. high turnover renal osteodystrophy).…”

T Antigens of Simian Virus 40: Molecular Chaperones for Viral Replication and Tumorigenesis

“…Since T antigen is a powerful mitogen with multiple growth-inducing activities, it is possible that the J domain is required to drive the cells to cycle in a manner that indirectly disrupts pRB-E2F complexes. For example, several different mitogens will induce free E2F and transcriptional activity even though they are not known to directly bind to the pRB family (83,92,169,258). Biochemical evidence, however, clearly demonstrates that T antigen has the capability to disrupt pRB-E2F family complexes in vitro (233).…”

Increased bone formation by prevention of osteoblast apoptosis with parathyroid hormone

“…Previous in vivo and in vitro studies have suggested that PTH can stimulate the replication of osteoblast progenitors (41)(42)(43). However, the equivalent anabolic response of normal SAMR1 and SAMP6 mice with defective osteoblastogenesis to daily injections of hPTH , together with the failure to observe a stimulatory effect of the hormone on the number of osteoblast progenitors obtained from the marrow of either strain, strongly suggests that the anabolic effect of PTH is not dependent on increased osteoblastogenesis.…”