Journal
BONE
Volume 40, Issue 6, Pages 1434-1446Publisher
ELSEVIER SCIENCE INC
DOI: 10.1016/j.bone.2007.03.017
Keywords
parathyroid hormone; bone formation; osteoblasts; differentiation; apoptosis
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Funding
- NIAMS NIH HHS [R01 AR046823, R01 AR46823, R01 AR046823-05] Funding Source: Medline
- NIA NIH HHS [P01 AG13918, P01 AG013918, P01 AG013918-110007] Funding Source: Medline
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Intermittent administration of parathyroid hormone (PTH) stimulates bone formation by increasing osteoblast number, but the molecular and cellular mechanisms underlying this effect are not completely understood. In vitro and in vivo studies have shown that PTH directly activates survival signaling in osteoblasts; and that delay of osteoblast apoptosis is a major contributor to the increased osteoblast number, at least in mice. This effect requires Runx2-dependent expression of anti-apoptotic genes like Bcl-2. PTH also causes exit of replicating progenitors from the cell cycle by decreasing expression of cyclin D and increasing expression of several cyclin-dependent kinase inhibitors. Exit from the cell cycle may set the stage for pro-differentiating and pro-survival effects of locally produced growth factors and cytokines, the level and/or activity of which are known to be influenced by PTH. Observations from genetically modified mice suggest that the anabolic effect of intermittent PTH requires insulin-like growth factor-I (IGF-I), fibroblast growth factor-2 (FGF-2), and perhaps Writs. Attenuation of the negative effects of PPAR gamma may also lead to increased osteoblast number. Daily injections of PTH may add to the pro-differentiating and pro-survival effects of locally produced PTH related protein (PTHrP). As a result, osteoblast number increases beyond that needed to replace the bone removed by osteoclasts during bone remodeling. The pleiotropic effects of intermittent PTH, each of which alone may increase osteoblast number, may explain why this therapy reverses bone loss in most osteoporotic individuals regardless of the underlying pathophysiology. Published by Elsevier Inc.
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