Regina Ebert

Selenium supplementation restores the antioxidative capacity and prevents cell damage in bone marrow stromal cells in vitro

Bone marrow stromal cells (BMSCs) and other cell populations derived from mesenchymal precursors are developed for cell‐based therapeutic strategies and undergo cellular stress during ex vivo procedures. Reactive oxygen species (ROS) of cellular and environmental origin are involved in redox signaling, cumulative cell damage, senescence, and tumor development. Selenium‐dependent (glutathione peroxidases [GPxs] and thioredoxin reductases [TrxRs]) and selenium‐independent (superoxide dismutases [SODs] and catalase [CAT]) enzyme systems regulate cellular ROS steady state levels. SODs process superoxide anion to hydrogen peroxide, which is subsequently neutralized by GPx and CAT; TrxR neutralizes other ROS, such as peroxinitrite. Primary BMSCs and telomerase‐immortalized human mesenchymal stem cells (hMSC‐TERT) express GPx1–3, TrxR1, TrxR2, SOD1, SOD2, and CAT. We show here that in standard cell cultures (5%–10% fetal calf serum, 5–10 nM selenite), the activity of antioxidative selenoenzymes is impaired in hMSC‐TERT and BMSCs. Under these conditions, the superoxide anion processing enzyme SOD1 is not sufficiently stimulated by an ROS load. Resulting oxidative stress favors generation of micronuclei in BMSCs. Supplementation of selenite (100 nM) restores basal GPx and TrxR activity, rescues basal and ROS‐stimulated SOD1 mRNA expression and activity, and reduces ROS accumulation in hMSC‐TERT and micronuclei generation in BMSCs. In conclusion, BMSCs in routine cell culture have low antioxi‐dative capacity and are subjected to oxidative stress, as indicated by the generation of micronuclei. Selenite supplementation of BMSC cultures appears to be an important countermeasure to restore their antioxidative capacity and to reduce cell damage in the context of tissue engineering and transplantation procedures.

Vitamin D signaling is modulated on multiple levels in health and disease

Vitamin D signaling is dependent on the availability and turnover of the active Vitamin D receptor (VDR) ligand 1,25-dihydroxycholecalciferol and on the efficiency of VDR transactivation. Activating and inactivating secosteroid metabolizing p450 enzymes, e.g. 25-hydroxylases, 1alpha-hydroxylase and 24-hydroxylase, are responsible for ligand availability on the basis of substrate production in the skin and of nutritional intake of precursors. Net availability of active hormone depends on the delivery of substrate and the balance of activating and inactivating enzymes. 1Alpha-hydroxylase is the critical activating enzyme. It is expressed in the kidney for systemic supply and in target tissues for local secosteroid activation. It is upregulated in the kidney by low calcium intake and parathyroid hormone, downregulated by phosphatonins and proinflammatory signal transduction. Transactivation of VDR depends on the correct molecule structure, effective nuclear translocation and the presence of the unliganded heterodimer partner retinoid X-receptor (RXR) and other nuclear cofactors. Rapid Vitamin D-dependent membrane associated effects and consecutive second messenger activation exert an own pattern of gene regulation. A membrane receptor for these effects is hypothesized but not yet identified. Rickets is the long known clinical syndrome of impaired Vitamin D signaling due to Vitamin D3 deficiency. It can be caused by inherited defects of the cascade, nutritional deficits, lack of sunlight exposure, malabsorption and underlying diseases like chronic inflammation. It has been shown during the last decades that many modifiers of Vitamin D signaling are targets of disease in terms of inherited and acquired syndromes and that Vitamin D signaling is modulated at multiple levels and is more complex than mere mechanistic ligand/receptor/DNA interaction.

The transcriptional profile of mesenchymal stem cell populations in primary osteoporosis is distinct and shows overexpression of osteogenic inhibitors.

Primary osteoporosis is an age-related disease characterized by an imbalance in bone homeostasis. While the resorptive aspect of the disease has been studied intensely, less is known about the anabolic part of the syndrome or presumptive deficiencies in bone regeneration. Multipotent mesenchymal stem cells (MSC) are the primary source of osteogenic regeneration. In the present study we aimed to unravel whether MSC biology is directly involved in the pathophysiology of the disease and therefore performed microarray analyses of hMSC of elderly patients (79–94 years old) suffering from osteoporosis (hMSC-OP). In comparison to age-matched controls we detected profound changes in the transcriptome in hMSC-OP, e.g. enhanced mRNA expression of known osteoporosis-associated genes (LRP5, RUNX2, COL1A1) and of genes involved in osteoclastogenesis (CSF1, PTH1R), but most notably of genes coding for inhibitors of WNT and BMP signaling, such as Sclerostin and MAB21L2. These candidate genes indicate intrinsic deficiencies in self-renewal and differentiation potential in osteoporotic stem cells. We also compared both hMSC-OP and non-osteoporotic hMSC-old of elderly donors to hMSC of ∼30 years younger donors and found that the transcriptional changes acquired between the sixth and the ninth decade of life differed widely between osteoporotic and non-osteoporotic stem cells. In addition, we compared the osteoporotic transcriptome to long term-cultivated, senescent hMSC and detected some signs for pre-senescence in hMSC-OP. Our results suggest that in primary osteoporosis the transcriptomes of hMSC populations show distinct signatures and little overlap with non-osteoporotic aging, although we detected some hints for senescence-associated changes. While there are remarkable inter-individual variations as expected for polygenetic diseases, we could identify many susceptibility genes for osteoporosis known from genetic studies. We also found new candidates, e.g. MAB21L2, a novel repressor of BMP-induced transcription. Such transcriptional changes may reflect epigenetic changes, which are part of a specific osteoporosis-associated aging process.

Uncovering the cellular and molecular changes in tendon stem/progenitor cells attributed to tendon aging and degeneration.

Although the link between altered stem cell properties and tissue aging has been recognized, the molecular and cellular processes of tendon aging have not been elucidated. As tendons contain stem/progenitor cells (TSPC), we investigated whether the molecular and cellular attributes of TSPC alter during tendon aging and degeneration. Comparing TSPC derived from young/healthy (Y‐TSPC) and aged/degenerated human Achilles tendon biopsies (A‐TSPC), we observed that A‐TSPC exhibit a profound self‐renewal and clonogenic deficits, while their multipotency was still retained. Senescence analysis showed a premature entry into senescence of the A‐TSPC, a finding accompanied by an upregulation of p16INK4A. To identify age‐related molecular factors, we performed microarray and gene ontology analyses. These analyses revealed an intriguing transcriptomal shift in A‐TSPC, where the most differentially expressed probesets encode for genes regulating cell adhesion, migration, and actin cytoskeleton. Time‐lapse analysis showed that A‐TSPC exhibit decelerated motion and delayed wound closure concomitant to a higher actin stress fiber content and a slower turnover of actin filaments. Lastly, based on the expression analyses of microarray candidates, we suggest that dysregulated cell–matrix interactions and the ROCK kinase pathway might be key players in TSPC aging. Taken together, we propose that during tendon aging and degeneration, the TSPC pool is becoming exhausted in terms of size and functional fitness. Thus, our study provides the first fundamental basis for further exploration into the molecular mechanisms behind tendon aging and degeneration as well as for the selection of novel tendon‐specific therapeutical targets.

Pulse treatment with zoledronic acid causes sustained commitment of bone marrow derived mesenchymal stem cells for osteogenic differentiation

The aminobisphosphonate zoledronic acid (ZA) is a bone seeking specific inhibitor of protein farnesylation and geranylgeranylation, which causes inhibition of osteoclast function and apoptosis. It is widely used as an osteoclast targeted antiresorptive treatment of metastatic bone disease, Pagets disease and osteoporosis. Mesenchymal stem cells (MSC) and osteoblast precursors can also be targets of bisphosphonates, but the clinical relevance of these effects is under debate. We show here that ZA in vitro causes inhibition of proliferation and induction of apoptosis in hMSC, when applied in concentrations of 20 and 50 microM for more than 24 h which can be rescued by treatment with 10 microM geranylgeranyl pyrophosphate (GGPP). However, pulse stimulation for 3 and 6 h with these concentrations and subsequent culture for up to 2 weeks under osteogenic conditions exerts sustained regulation of osteogenic marker genes in hMSC. The effect on gene regulation translates into marked enhancement of mineralization, as shown by alizarin red and alkaline phosphatase staining after 4 weeks of osteogenic culture. ZA, when applied as a pulse stimulus, might therefore also stimulate osteogenic differentiation in vivo, since muM plasma concentrations can be achieved by intravenous application of 5 mg in patients. These data set the stage for the future dissection of the effects of ZA and other aminobisphosphonates on cells beyond osteoclasts, with respect to cell differentiation in benign metabolic and to antitumor efficacy in metastatic bone diseases, as well as adverse events due to putative substance accumulation in bone during long-term treatment.

Estrogen receptor and Wnt signaling interact to regulate early gene expression in response to mechanical strain in osteoblastic cells

Bone mass homeostasis is regulated by an interaction of various factors, including growth factors, systemic hormones and mechanical loading. Two signal transduction pathways, the estrogen receptor (ER) and the Wnt/beta-catenin signal transduction pathway, have been shown to have an important role in regulating osteoblast and osteoclast function and to be involved in mechanotransduction. Therefore, dysfunction of these pathways can lead to osteoporotic bone loss. However, less is known about the modulation of gene expression by the interaction of these pathways in response to mechanical strain. We performed in vitro stretch experiments using osteoblastic MC3T3-E1 cells to study the effect of both pathways and mechanical strain on the expression of cyclooxygenase-2 (Cox-2), which is involved in the synthesis of prostaglandins, modulators of bone formation and resorption. Using specific agonists and antagonists, we demonstrated a regulation by an interaction of these pathways in mechantransduction. Estradiol (E2) had a sensitizing effect on mechanically induced Cox-2 expression, which seemed to be ligand-specific as it could be abolished using the antiestrogen ICI182,780. However, mechanical strain in the presence of Wnt signaling activators diminished both the E2 sensitizing effect and the stimulatory effect of Wnt signaling in the absence of strain. This interaction might be one regulatory mechanism by which mechanical loading exerts its role in bone mass homeostasis.

Zoledronic acid induces apoptosis and changes the TRAIL/OPG ratio in breast cancer cells

Breast cancer has a propensity to metastasize to bone, thus causing pathological fractures. Bisphosphonates are established drugs in the treatment of bone metastasis that inhibit osteoclast activity and interrupt the vicious cycle of osteoclast-tumor cell interactions. We evaluated the direct effects of zoledronic acid on estrogen receptor (ER)-negative MDA-MB-231 and ER-positive MCF-7 breast cancer cells. While zoledronic acid (100 microM) inhibited MDA-MB-231 cell proliferation after 72 h, and induced apoptosis via activation of caspase-3 and -7, it had only minor effects on MCF-7 cells. In addition, zoledronic acid induced apoptosis by up-regulating TNF-related apoptosis-inducing ligand (TRAIL) in MDA-MB-231 cells (p<0.01), but had no effect on the expression of its decoy receptor osteoprotegerin (OPG). In MCF-7 cells, both cytokines were suppressed by zoledronic acid. In conclusion, zoledronic acid enhanced the TRAIL-to-OPG ratio in TRAIL-sensitive MDA-MB-231 cells, indicating that the TRAIL/OPG cytokine system is a bisphosphonate-responsive target in breast cancer.

Autocrine Fibroblast Growth Factor 18 mediates dexamethasone-induced osteogenic differentiation of murine mesenchymal stem cells

The potential of mesenchymal stem cells (MSC) to differentiate into functional bone forming cells provides an important tool for bone regeneration. The identification of factors capable of promoting osteoblast differentiation in MSCs is therefore critical to enhance the osteogenic potential of MSCs. Using microarray analysis combined with biochemical and molecular approach, we found that FGF18, a member of the FGF family, is upregulated during osteoblast differentiation induced by dexamethasone in murine MSCs. We showed that overexpression of FGF18 by lentiviral (LV) infection, or treatment of MSCs with recombinant human (rh)FGF18 increased the expression of the osteoblast specific transcription factor Runx2, and enhanced osteoblast phenotypic marker gene expression and in vitro osteogenesis. Molecular silencing using lentiviral shRNA demonstrated that downregulation of FGFR1 or FGFR2 abrogated osteoblast gene expression induced by either LV‐FGF18 or rhFGF18, indicating that FGF18 enhances osteoblast differentiation in MSCs via activation of FGFR1 or FGFR2 signaling. Biochemical and pharmacological analyses showed that the induction of phenotypic osteoblast markers by LV‐FGF18 is mediated by activation of ERK1/2‐MAPKs and PI3K signaling in MSCs. These results reveal that FGF18 is an essential autocrine positive regulator of the osteogenic differentiation program in murine MSCs and indicate that osteogenic differentiation induced by FGF18 in MSCs is triggered by FGFR1/FGFR2‐mediated ERK1/2‐MAPKs and PI3K signaling. J. Cell. Physiol. 224: 509–515, 2010.

1,25-Dihydroxyvitamin D3 Treatment Delays Cellular Aging in Human Mesenchymal Stem Cells while Maintaining Their Multipotent Capacity

1,25-dihydroxyvitamin D3 (1,25D3) was reported to induce premature organismal aging in fibroblast growth factor-23 (Fgf23) and klotho deficient mice, which is of main interest as 1,25D3 supplementation of its precursor cholecalciferol is used in basic osteoporosis treatment. We wanted to know if 1,25D3 is able to modulate aging processes on a cellular level in human mesenchymal stem cells (hMSC). Effects of 100 nM 1,25D3 on hMSC were analyzed by cell proliferation and apoptosis assay, β-galactosidase staining, VDR and surface marker immunocytochemistry, RT-PCR of 1,25D3-responsive, quiescence- and replicative senescence-associated genes. 1,25D3 treatment significantly inhibited hMSC proliferation and apoptosis after 72 h and delayed the development of replicative senescence in long-term cultures according to β-galactosidase staining and P16 expression. Cell morphology changed from a fibroblast like appearance to broad and rounded shapes. Long term treatment did not induce lineage commitment in terms of osteogenic pathways but maintained their clonogenic capacity, their surface marker characteristics (expression of CD73, CD90, CD105) and their multipotency to develop towards the chondrogenic, adipogenic and osteogenic pathways. In conclusion, 1,25D3 delays replicative senescence in primary hMSC while the pro-aging effects seen in mouse models might mainly be due to elevated systemic phosphate levels, which propagate organismal aging.

FGF23 is a putative marker for bone healing and regeneration.

Besides numerous other factors, fibroblast growth factor receptor (FGFR) signaling is involved in fracture healing and bone remodeling. FGF23 is a phosphatonin produced by osteoblastic cells, which signals via FGFR1, thereby exerting effects in bone and kidney. We analyzed if serum FGF23 levels might be an indicator to predict fracture healing and union. FGF23 (C‐Term) was elevated on day 3 postoperatively in 55 patients sustaining an exchange of total hip implants due to aseptic loosening. A prospective study of 40 patients undergoing primary hip arthroplasty also showed elevated FGF23 (C‐Term) but no change in FGF23 (intact) levels on days 1, 4, and 10 postoperatively. Serum phosphate and phosphate clearance stayed within normal ranges. FGF23 mRNA expression in ovine callus was compared between a standard and delayed course of osteotomy healing. In the standard model, a marked increase in FGF23 mRNA expression compared to the delayed healing situation was observed. Immunohistochemical analysis showed FGF23 production of osteoblasts and granulation tissue in the fracture callus during bone healing. In conclusion, FGF23 is involved in bone healing, can be measured by a sensitive assay in peripheral blood, and is a promising candidate as an indicator for healing processes prone to reunion versus nonunion.