The Research Of 11β-HSD1, PPARγ And GR In The Mechanism Of Osteoblastic Differentiation

Osteoporosis is a common and preventable disorder of the older adultskeleton that predisposes an individual to an increased risk of fracture, a majorcause of disability in older adults. Osteoporosis, as a public health problemshave exploded in prevalence over the past decade, and the prevalence ofosteoporosis increases with aging society. Moreover, it has brought the heavyburden to the aging society. Now research of osteoporosis has become themajor topic of geriatris.Glucocorticoids exert diverse effects on bone metabolism. In human andrat bone marrow stromal cells, glucocorticoids are crucial for the induction ofosteoblast differentiation and formation of a mineralized extracellular matrix.Paradoxically, glucocorticoids in pharmacological doses cause osteoporosis.11 beta-HSD1 is critical in the regulation of glucocorticoid activity at theprereceptor level, which can convert cortisone into the biologically activecortisol in osteoblasts. So it is an amplifier of glucocorticoid action inosteoblasts. As a consequence, the regulation of 11beta-HSD1 activity inosteoblasts is likely to be an important autocrine determinant of osteoblastproliferation, differentiation, and function.Thiazolidinediones (TZDs), including pioglitazone, are synthetic agonistsof PPARγ. Although normally prescribed as antidiabetes drugs, an unexpected side effect of these compounds seems to be that they also promote osteoporosis.Previous findings suggest the mechanism is that PPARs may inhibitosteoblastic differentiation by promoting an adipocyte differentiation programin bone marrow. Previous studies raise the possibility of an interaction betweenGR and PPAR signaling pathways that could potentially influence osteoblasticdifferentiation. However, the relationship between the GR and PPARγpathways is not clear.In the first part of our study, we investigated the relationship between11β-hydroxysteroid dehydrogenase 1 and osteoblastic differentiation. Theprimary osteoblasts were obtained by sequential enzymatic digestion of fetalrat calvaria and the models of drug-stimulated by Dex (1×10-8mol/L) and thecontrol were built. We detected the expression of the osteoblast related genesand 11β-HSD1 mRNA and protein by RT-PCR and Western blot. As a results,11β-HSD1 was up-regulated in the time course of primary culture. In theDex-stimulated models, ALP and COLⅠwere obviously up-regulatedcompared to the unstimulated group. The mRNA and protein levels of 11β-HSD1 were down-regulated in the Dex-stimulated models. Based on thesefindings, we conclude that the expression of 11β-HSD1 is closely related toosteoblastic differentiation. During the differentiation process, the expressionof 11β-HSD1 decreased. In osteoblasts, there is a negative relation between11β-HSD1 expression and cell differentiation.In the second part of our study, we investigated the effects of different concentrations of the GC hormone Dexamethasone and the PPAR agonistpioglitazone on osteoblastic differentiation. We used primary osteoblastic cellsisolated from rat calvaria as a model, and examined levels of bone markergenes, including Alkaline Phosphatase, related genes 11β-HSD1, GR byreal-time PCR. While expression of ALP and GR increased significantly duringDex-induced differentiation, expression of 11β-HSD1 decreased. Theexpression levels of these genes were unchanged in mature osteoblasts treatedwith pioglitazone only, but treatment with Dex and pioglitazone together led toincreased expression of GR and 11β-HSD1, decreased expression of ALP. Andthis effects also depended on the concentration of Dex. Based on these findings,we conclude that Dex induces osteoblastic differentiation in a concentration-dependent manner. These results also suggest that PPARγagonists can regulateordinarily refractory primary osteoblastic cells differentiation in presence ofGR agonists, suggesting a novel link between these two pathways.