RESUMO
Glucocorticoids are an effective therapy for a variety of severe inflammatory and autoimmune disorders; however, the therapeutic use of glucocorticoids is severely limited by their negative side effects, particularly on osteogenesis. Glucocorticoids regulate transcription by binding to the glucocorticoid receptor (GR), which then binds the promoters of target genes to induce either activation or repression. The gene activation effects of nuclear hormone receptors broadly require the cooperation of the chromatin remodeling complex known as SWI/SNF, which is powered by an ATPase core. The well-studied SWI/SNF ATPase, BRG1, is required for gene activation by a spectrum of nuclear hormone receptors including GR. However, glucocorticoid-induced side effects specifically related to impaired osteogenesis are mostly linked with GR-mediated repression. We have considered whether cis-repression of osteogenic genes by GR may be mediated by a distinct subclass of SWI/SNF powered by the alternative ATPase, BRM. BRM does not have an essential role in mammalian development, but plays a repressor role in osteoblast differentiation and favors adipogenic lineage selection over osteoblast commitment, effects that mirror the repressor effects of GR. The studies reported here examine three key GR cis-repression gene targets, and show that GR association with these promoters is sharply reduced in BRM deficient cells. Each of these GR-targeted genes act in a different way. Bglap encodes osteocalcin, which contributes to normal maturation of osteoblasts from committed pre-osteoblasts. The Per3 gene product acts in uncommitted mesenchymal stem cells to influence the osteoblast/adipocyte lineage selection point. Fas ligand, encoded by FasL, is a means by which osteoblasts can modulate bone degradation by osteoclasts. Repression of each of these genes by glucocorticoid favors bone loss. The essential role of BRM in cooperation with GR at each of these control points offers a novel mechanistic understanding of the role of GR in bone loss.
RESUMO
Redirecting the adipogenic potential of bone marrow-derived mesenchymal stem cells to other lineages, particularly osteoblasts, is a key goal in regenerative medicine. Controlling lineage selection through chromatin remodeling complexes such as SWI/SNF, which act coordinately to establish new patterns of gene expression, would be a desirable intervention point, but the requirement for the complex in essentially every lineage pathway has generally precluded selectivity. However, a novel approach now appears possible by targeting the subset of SWI/SNF powered by the alternative ATPase, mammalian brahma (BRM). BRM is not required for development, which has hindered understanding of its contributions, but knockdown genetics here, designed to explore the hypothesis that BRM-SWI/SNF has different regulatory roles in different mesenchymal stem cell lineages, shows that depleting BRM from mesenchymal stem cells has a dramatic effect on the balance of lineage selection between osteoblasts and adipocytes. BRM depletion enhances the proportion of cells expressing markers of osteoblast precursors at the expense of cells able to differentiate along the adipocyte lineage. This effect is evident in primary bone marrow stromal cells as well as in established cell culture models. The altered precursor balance has major physiological significance, which becomes apparent as protection against age-related osteoporosis and as reduced bone marrow adiposity in adult BRM-null mice.
