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Review
. 2012 Jun;5(2):135-42.

Insulin like growth factor-I: a critical mediator of the skeletal response to parathyroid hormone

Daniel D Bikle  1 , Yongmei Wang
Affiliations
Review

Insulin like growth factor-I: a critical mediator of the skeletal response to parathyroid hormone

Daniel D Bikle et al. Curr Mol Pharmacol. 2012 Jun.

Abstract

This review focuses on the mechanisms by which PTH stimulates both osteoblast and osteoclast function, emphasizing the critical role that IGF-I plays in these processes. After reviewing the current literature on the skeletal actions of PTH and the modulation of IGF action on bone by the different IGF-binding proteins, the review then examines studies from mouse models in which IGF-I or its receptor have been selectively deleted in different cells of the skeletal system, in particular osteoprogenitors, mature osteoblasts, and osteoclasts. Mice in which IGF-I production has been deleted from all cells are deficient in both bone formation and bone resorption with few osteoblasts or osteoclasts in bone in vivo, reduced osteoblast colony forming units, and an inability of either the osteoblasts or osteoclast precursors to support osteoclastogenesis in vitro. Mice in which the IGF-I receptor is specifically deleted in mature osteoblasts have a mineralization defect in vivo, and bone marrow stromal cells from these mice fail to mineralize in vitro. Mice in which the IGF-I receptor is deleted in osteoprogenitor cells have a marked reduction in osteoblast proliferation and differentiation leading to osteopenia. Finally mice lacking the IGF-I receptor in their osteoclasts have increased bone and decreased osteoclast formation. PTH fails to stimulate bone formation in the mice lacking IGF-I or its receptor in osteoblasts or enhance the signaling between osteoblasts and osteoclasts through RANKL/RANK and Ephrin B2/Eph B4, emphasizing the role IGF-I signaling plays in cell-communication per se and as stimulated by PTH.

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Conflict of interest statement

CONFLICTS OF INTEREST

None.

Figures

Fig. 1
Fig. 1. The role of IGF-I in the anabolic and catabolic actions of PTH: working model
PTH stimulates IGF-I production in osteoblasts through a cAMP dependent mechanism. The IGF-I so produced enhances the proliferation and differentiation of osteoprogenitors, at least in part by activating both the MAPK and PI3K pathways, and enables the expression of both RANKL and m-CSF which along with IGF-I promote the differentiation of osteoclasts. The osteoclasts in turn facilitate osteoblast differentiation via Ephrin B2:EphB4 signaling, the expression of which is also dependent on IGF-I.
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