Making Bone with Oxysterols

Abstract

Mesenchymal cells can be directed to an osteoblastic lineage (the cells that form bone) by specific oxysterols, which are naturally occurring products of cholesterol oxidation. Dwyer et al . provide evidence that coapplication of 20( S )- and 22( S )-hydroxycholesterol (abbreviated SS) to pluripotent mouse marrow stromal cell line activates the expression of genes implicated in the Hedgehog pathway. Specifically, SS stimulated the genes encoding the transcription factor Gli and the receptor Patched, which are both components and targets of the Hedgehog pathway. Furthermore, SS or the Sonic hedgehog N-terminal signaling domain (ShhN) stimulated the expression of a Gli-responsive reporter gene, and this reporter gene expression was inhibited by the Smoothened inhibitor cyclopamine. Hedgehog acts at the Patched receptor, which then alleviates repression of the Smoothened transmembrane protein to stimulate Hedgehog-responsive gene expression. SS also stimulated various markers of osteoblastic differentiation, and these phenotypic changes were inhibited by cyclopamine. The effects of SS were not mediated by transcriptional stimulation of the expression of ligands of the Hedgehog pathway. SS required Smoothened, and in mouse embryo fibroblasts deficient for Smoothened, SS did not stimulate osteoblastic differentiation, although these cells retained the ability to adopt such a phenotype as bone morphogenic protein 7 did stimulate osteoblastic changes. SS apparently activates Smoothened either through a site separate from that of cyclopamine (it did not compete with cyclopamine for binding to Smoothened) or by acting indirectly to modulate Smoothened activity. Indeed, inhibition of protein kinase C activity, which had been previously implicated in the cellular response to oxysterols, blocked the SS-mediated increase in the expression of the genes encoding Gli and Patched. It remains unclear exactly how all of these pieces fit together to control the activity of the Hedgehog pathway. J. R. Dwyer, N. Sever, M. Carlson. S. F. Nelson, P. A. Beachy, F. Parhami, Oxysterols are novel activators of the Hedgehog signaling pathway in pluripotent mesenchymal cells. J. Biol. Chem. 282 , 8959-8968 (2007). [Abstract] [Full Text]