Dexamethasone suppresses Smad3 pathway in osteoblastic cells

Abstract

Central in the pathogenesis of glucocorticoid (GC)-induced osteoporosis is the effects of GC on bone formation. However, the mechanism of GC-inhibited bone formation is not well known. Transforming growth factor (TGF)-is most abundant in bone matrix compared with other tissues, and we have recently proposed that Smad3, a TGF-signaling molecule, is important for promoting bone formation. However, no reports have been available about the effects of GC on Smad3 in osteoblasts. In the present study, we investigated whether dexamethasone (Dex), an active GC analog, would affect the expression and activity of Smad3 in mouse osteoblastic MC3T3-E1 and rat osteoblastic UMR-106 cells. Dex significantly suppressed Smad3-stimulated alkaline phosphatase (ALP) activity, although it did not affect TGF--inhibited ALP activity in MC3T3-E1 cells. Moreover, pretreatment with Dex suppressed TGF--enhanced expression of type I collagen in MC3T3-E1 and UMR-106 cells. In the luciferase assay using p3TP-Lux with a Smad3-specific response element, Dex significantly suppressed the transcriptional activity induced by TGF-as well as Smad3. However, Dex did not affect the expression of Smad3 in these cells at both mRNA and protein levels. In conclusion, the present study indicates that Dex inhibits ALP activity and type I collagen expression, presumably by suppressing Smad3-induced transcriptional activity but not by modulating Smad3 expression in osteoblastic cells.

The Effects of Dexamethasone, Ascorbic Acid, and β-Glycerophosphate on Osteoblastic Differentiation by Regulating Estrogen Receptor and Osteopontin Expression

“…DEX and GP seemed to have little influence on cell viability. DEX was reported to decrease the number of osteoblasts and osteocytes through enhanced apoptosis as well as the suppressed turnover of the cell cycle [5]. However, this effect was not observed in MC3T3-E1cells in the short-term application.…”

Mechanism of Glucocorticoid-Induced Osteoporosis: An Update

“…Other evidence supporting the role of GR in glucocorticoid-induced bone loss includes: 1] the glucocorticoid-activated GR binds directly to the negative glucocorticoid response elements (nGREs) in the promoter region of the osteocalcin (Ocn) gene, an osteoblastspecific gene that plays an important role in bone mineralization, and inhibit its transcription [36;108]; 2] GR transcriptionally activates the expression of MAP kinase phosphatase-1 (MKP-1] [109], which inactivates MAP kinase and thus inhibits osteogenic differentiation [64;110;111]; and 3] GR can physically interact with and inhibit the transcriptional functions of Smad3, an intracellular signaling mediator of transforming growth factor-beta (TGF-) [112] (Figure 4]. Glucocorticoids have been known to antagonize TGF- action in bone [113][114][115] and TGF- stimulates osteoprogenitor cell proliferation [116][117][118][119] and attract osteoprogenitor cells to the remodeling sites during bone remodeling [120]. It is important to note that while the catabolic effects of glucocorticoids are often associated with long-term glucocorticoid excess [1-4;7], a short term exposure to glucocorticoid seems beneficial; for example, treatment of bone marrow stromal cells or osteoblasts with dexamethasone enhances, rather than inhibits, alkaline phosphatase (ALP) activity.…”

Glucocorticoid-Induced Osteoporosis: A Review

“…GCs also inhibit osteoblast differentiation through the repression of growth hormone (GH) and bone morphogenic protein 2 (BMP-2) which enhances OB transcription factors [32]. Other less known mechanisms have been identified to explain the impact of GCs on OB differentiation involving Krox 20/ EGR2 [46], MPK-1 [47], Notch [48], and the TGF-b-Smad pathway [49]. GCs have pro-apoptotic effects on OBs and osteocytes due to the activation of caspase-3, a key mediator of multiple apoptotic signaling pathway [50] and Bim whose expression occurs prior to activation of caspase 3 [51].…”