BackgroundBone fracture initiates a series of cellular and molecular events including the expression of hypoxia-inducible factor (HIF)-1. HIF-1 is known to facilitate recruitment and differentiation of multipotent human mesenchymal stromal cells (hMSC). Therefore, we analyzed the impact of hypoxia and HIF-1 on the competitive differentiation potential of hMSCs towards adipogenic and osteogenic lineages.Methodology/Principal FindingsBone marrow derived primary hMSCs cultured for 2 weeks either under normoxic (app. 18% O2) or hypoxic (less than 2% O2) conditions were analyzed for the expression of MSC surface markers and for expression of the genes HIF1A, VEGFA, LDHA, PGK1, and GLUT1. Using conditioned medium, adipogenic or osteogenic differentiation as verified by Oil-Red-O or von-Kossa staining was induced in hMSCs under either normoxic or hypoxic conditions. The expression of HIF1A and VEGFA was measured by qPCR. A knockdown of HIF-1α by lentiviral transduction was performed, and the ability of the transduced hMSCs to differentiate into adipogenic and osteogenic lineages was analyzed. Hypoxia induced HIF-1α and HIF-1 target gene expression, but did not alter MSC phenotype or surface marker expression. Hypoxia (i) suppressed adipogenesis and associated HIF1A and PPARG gene expression in hMSCs and (ii) enhanced osteogenesis and associated HIF1A and RUNX2 gene expression. shRNA-mediated knockdown of HIF-1α enhanced adipogenesis under both normoxia and hypoxia, and suppressed hypoxia-induced osteogenesis.Conclusions/SignificanceHypoxia promotes osteogenesis but suppresses adipogenesis of human MSCs in a competitive and HIF-1-dependent manner. We therefore conclude that the effects of hypoxia are crucial for effective bone healing, which may potentially lead to the development of novel therapeutic approaches.

Objective. Glucocorticoids (GCs) exert their antiinflammatory and immunosuppressive effects in humans primarily via the cytosolic GC receptor (cGR) but also via rapid, nongenomic mechanisms. Most likely, membrane-bound GRs (mGR) are involved in nongenomic GC signaling. The aim of this study was to investigate the origin and functional activity of mGR.Methods. We analyzed the origin of mGR using mGR-expressing HEK 293T cells, by transient and stable RNA interference-mediated GR reduction. GR messenger RNA (mRNA) and cGR and mGR protein levels were analyzed by real-time quantitative polymerase chain reaction, immunoblotting, and highsensitivity immunofluorescence staining. Furthermore, we analyzed the functional activity of mGR, using membrane-impermeable bovine serum albumin (BSA)-bound dexamethasone (DEX-BSA) in human monocytes. Membrane-bound GR-expressing monocytes were treated with DEX, DEX-BSA, or BSA. Cell lysates were analyzed using PepChip arrays in order to identify kinases triggered by DEX-BSA, with validation using Bio-Plex assays and immunoblotting.Results. Our data showed that transient reduction of GR mRNA in HEK 293T cells decreased cGR protein levels but not mGR protein levels. However, stably transfected cells showed reduced cGR protein expression and significantly reduced mGR protein expression. Furthermore, 51 kinase substrates were identified for which phosphorylation was either reduced or increased. We observed p38 MAP kinase (MAPK) as one possible upstream kinase. Validation of these data by Bio-Plex phosphoprotein assay and immunoblotting showed increased phosphorylation of p38 MAPK after treatment with DEX-BSA.Conclusion. Our data demonstrate that the human GR gene encodes for both cGR and mGR. Membrane-bound GR retains functional activity, as indicated by induced phosphorylation of p38 MAPK due to DEX-BSA treatment. Membrane-bound GRmediated cellular signaling needs to be investigated further in order to clarify its therapeutic potential.Glucocorticoids (GCs) are the most commonly used drugs in the treatment of a wide range of rheumatic diseases. These substances exert their immunosuppressive, antiinflammatory, and antiallergic effects on primary and secondary immune cells, tissues, and organs (1). Unfortunately, GCs have the potential to exert unwanted effects on metabolism, bone tissue, muscles, eyes, and skin, especially when the dose and duration of treatment are increased (2-5). These disadvantages drive the need for an improved risk/benefit ratio for GCs, for instance by understanding the mechanisms of GC action in greater detail. The therapeutic effects of GCs are primarily mediated by cytosolic GC receptor ␣ (cGR␣), the classic genomic mechanism of GC action Dr. Buttgereit's work was supported by grants from the DFG (Bu 1015/7-1) and the Deutsche Sparkassenstiftung Medizin. Mr. Hahne's work was supported by the DFG Graduate School GSC 203 Project at the Berlin-Brandenburg School for Regenerative Therapies.

Hypoxia is an important feature of inflamed tissue, such as the RA joint. Activated monocytes/macrophages and endothelial cells play a pivotal role in the pathogenesis of RA, implicated in the mechanism of inflammation and erosion. During development, myeloid progenitor cells sequentially give rise to monoblasts, promonocytes, and monocytes that are released from the bone marrow into the bloodstream. After extravasation, monocytes differentiate into long-lived, tissue-specific macrophages or DCs. The effect of different oxygen concentrations experienced by these cells during maturation represents a novel aspect of this developmental process. In inflamed joint tissue, the microvascular architecture is highly dysregulated; thus, efficiency of oxygen supply to the synovium is poor. Therefore, invading cells must adapt instantaneously to changes in the oxygen level of the microenvironment. Angiogenesis is an early event in the inflammatory joint, which is important in enabling activated monocytes to enter via endothelial cells by active recruitment to expand the synovium into a "pannus", resulting in cartilage degradation and bone destruction. The increased metabolic turnover of the expanding synovial pannus outpaces the dysfunctional vascular supply, resulting in hypoxia. The abnormal bioenergetics of the microenvironment further promotes synovial cell invasiveness. In RA, joint hypoxia represents a potential threat to cell function and survival. Notably, oxygen availability is a crucial parameter in the cellular energy metabolism, itself an important factor in determining the function of immune cells.

Hypoxia, a feature of inflammation and tumors, is a potent inducer of the proinflammatory cytokine macrophage migration inhibitory factor (MIF). In transformed cells, MIF was shown to modulate and to be modulated via the oxygen-sensitive transcription factor hypoxia-inducible factor (HIF)-1. Furthermore, anti-inflammatory glucocorticoids (GCs) were described to regulate MIF action. However, in-depth studies of the interaction between MIF and HIF-1 and GC action in nontransformed primary human CD4+ T cells under hypoxia are missing. Therefore, we investigated the functional relationship between MIF and HIF and the impact of the GC dexamethasone (DEX) on these key players of inflammation in human CD4+ T cells. In this article, we show that hypoxia, and specifically HIF-1, is a potent and rapid inducer of MIF expression in primary human CD4+ T cells, as well as in Jurkat T cells. MIF signaling via CD74, in turn, is essential for hypoxia-mediated HIF-1α expression and HIF-1 target gene induction involving ERK/mammalian target of rapamycin activity complemented by PI3K activation upon mitogen stimulation. Furthermore, MIF signaling enhances T cell proliferation under normoxia but not hypoxia. MIF also counterregulates DEX-mediated suppression of MIF and HIF-1α expression. Based on these data, we suggest that hypoxia significantly affects the expression of HIF-1α in a MIF-dependent manner leading to a positive-feedback loop in primary human CD4+ T cells, thus influencing the lymphoproliferative response and DEX action via the GC receptor. Therefore, we suggest that HIF and/or MIF could be useful targets to optimize GC therapy when treating inflammation.