Bistable Epigenetic States Explain Age-Dependent Decline in Mesenchymal Stem Cell Heterogeneity.

The molecular mechanisms by which heterogeneity, a major characteristic of stem cells, is achieved are yet unclear. We here study the expression of the membrane stem cell antigen-1 (Sca-1) in mouse bone marrow mesenchymal stem cell (MSC) clones. We show that subpopulations with varying Sca-1 expression profiles regenerate the Sca-1 profile of the mother population within a few days. However, after extensive replication in vitro, the expression profiles shift to lower values and the regeneration time increases. Study of the promoter of Ly6a unravels that the expression level of Sca-1 is related to the promoter occupancy by the activating histone mark H3K4me3. We demonstrate that these findings can be consistently explained by a computational model that considers positive feedback between promoter H3K4me3 modification and gene transcription. This feedback implicates bistable epigenetic states which the cells occupy with an age-dependent frequency due to persistent histone (de-)modification. Our results provide evidence that MSC heterogeneity, and presumably that of other stem cells, is associated with bistable epigenetic states and suggest that MSCs are subject to permanent state fluctuations. Stem Cells 2017;35:694-704.

Priming integrin alpha5 promotes human mesenchymal stromal cell osteoblast differentiation and osteogenesis.

Adult human mesenchymal stromal cells (hMSCs) have the potential to differentiate into chondrogenic, adipogenic, or osteogenic lineages, providing a potential source for tissue regeneration. An important issue for efficient bone regeneration is to identify factors that can be targeted to promote the osteogenic potential of hMSCs. Using transcriptome analysis, we found that integrin alpha5 (ITGA5) expression is up-regulated during dexamethasone-induced osteoblast differentiation of hMSCs. Gain-of-function studies showed that ITGA5 promotes the expression of osteoblast phenotypic markers and in vitro osteogenesis of hMSCs. Down-regulation of endogenous ITGA5 using specific shRNAs blunted osteoblast marker gene expression and osteogenic differentiation. Molecular analyses showed that the enhanced osteoblast differentiation induced by ITGA5 was mediated by activation of focal adhesion kinase/ERK1/2-MAPKs and PI3K signaling pathways. Remarkably, activation of endogenous ITGA5 using agonists such as a specific antibody that primes the integrin or a peptide that specifically activates ITGA5 was sufficient to enhance ERK1/2-MAPKs and PI3K signaling and to promote osteoblast differentiation and osteogenic capacity of hMSCs. Importantly, we demonstrated that hMSCs engineered to overexpress ITGA5 exhibited a marked increase in their osteogenic potential in vivo. Taken together, these findings not only reveal that ITGA5 is required for osteoblast differentiation of adult hMSCs but also provide a targeted strategy using ITGA5 agonists to promote the osteogenic capacity of hMSCs. This may be used for tissue regeneration in bone disorders where the recruitment or capacity of hMSCs is compromised.

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.

Crosstalks between integrin alpha 5 and IGF2/IGFBP2 signalling trigger human bone marrow-derived mesenchymal stromal osteogenic differentiation.

BACKGROUND: The potential of mesenchymal stromal cells (MSCs) to differentiate into functional bone forming cells provides an important tool for bone regeneration. The identification of factors that trigger osteoblast differentiation in MSCs is therefore critical to promote the osteogenic potential of human MSCs. In this study, we used microarray analysis to identify signalling molecules that promote osteogenic differentiation in human bone marrow stroma derived MSCs. RESULTS: Microarray analysis and validation experiments showed that the expression of IGF2 and IGFBP2 was increased together with integrin alpha5 (ITGA5) during dexamethasone-induced osteoblast differentiation in human MSCs. This effect was functional since we found that IGF2 and IGFBP2 enhanced the expression of osteoblast phenotypic markers and in vitro osteogenic capacity of hMSCs. Interestingly, we showed that downregulation of endogenous ITGA5 using specific shRNA decreased IGF2 and IGFBP2 expression in hMSCs. Conversely, ITGA5 overexpression upregulated IGF2 and IGFBP2 expression in hMSCs, which indicates tight crosstalks between these molecules. Consistent with this concept, activation of endogenous ITGA5 using a specific antibody that primes the integrin, or a peptide that specifically activates ITGA5 increased IGF2 and IGFBP2 expression in hMSCs. Finally, we showed that pharmacological inhibition of FAK/ERK1/2-MAPKs or PI3K signalling pathways that are enhanced by ITGA5 activation, blunted IGF2 and IGFBP2 expression in hMSCs. CONCLUSION: The results show that ITGA5 is a key mediator of IGF2 and IGFBP2 expression that promotes osteoblast differentiation in human MSCs, and reveal that crosstalks between ITGA5 and IGF2/IGFBP2 signalling are important mechanisms that trigger osteogenic differentiation in human bone marrow derived mesenchymal stromal cells.

FHL2 mediates dexamethasone-induced mesenchymal cell differentiation into osteoblasts by activating Wnt/beta-catenin signaling-dependent Runx2 expression.

The differentiation of bone marrow mesenchymal stem cells (MSCs) into osteoblasts is a crucial step in bone formation. However, the mechanisms involved in the early stages of osteogenic differentiation are not well understood. In this study, we identified FHL2, a member of the LIM-only subclass of the LIM protein superfamily, that is up-regulated during early osteoblast differentiation induced by dexamethasone in murine and human MSCs. Gain-of-function studies showed that FHL2 promotes the expression of the osteoblast transcription factor Runx2, alkaline phosphatase, type I collagen, as well as in vitro extracellular matrix mineralization in murine and human mesenchymal cells. Knocking down FHL2 using sh-RNA reduces basal and dexamethasone-induced osteoblast marker gene expression in MSCs. We demonstrate that FHL2 interacts with beta-catenin, a key player involved in bone formation induced by Wnt signaling. FHL2-beta-catenin interaction potentiates beta-catenin nuclear translocation and TCF/LEF transcription, resulting in increased Runx2 and alkaline phosphatase expression, which was inhibited by the Wnt inhibitor DKK1. Reduction of Runx2 transcriptional activity using a mutant Runx2 results in inhibition of FHL2-induced alkaline phosphatase expression in MSCs. These findings reveal that FHL2 acts as an endogenous activator of mesenchymal cell differentiation into osteoblasts and mediates osteogenic differentiation induced by dexamethasone in MSCs through activation of Wnt/beta-catenin signaling- dependent Runx2 expression.

Distinct osteoblastic differentiation potential of murine fetal liver and bone marrow stroma-derived mesenchymal stem cells.

Bone marrow-derived mesenchymal stem cells (MSC) are able to differentiate into osteoblasts under appropriate induction. Although MSC-derived osteoblasts are part of the hematopoietic niche, the nature of the stromal component in fetal liver remains elusive. Here, we determined the in vitro osteoblastic differentiation potential of murine clonal fetal liver-derived cells (AFT024, BFC012, 2012) in comparison with bone marrow-derived cell lines (BMC9, BMC10). Bone morphogenetic protein-2 (BMP2) increased alkaline phosphatase (ALP) activity, an early osteoblastic marker, in AFT024 and 2012 cells, whereas dexamethasone had little or no effect. BMP2, but not dexamethasone, increased ALP activity in BMC9 cells, and both inducers increased ALP activity in BMC10 cells. BMP2 increased ALP mRNA in AFT024, 2012 and BMC9 cells. By contrast, ALP was not detected in BMC10 and BFC012 cells. BMP2 and dexamethasone increased osteopontin and osteocalcin mRNA expression in 2012 cells. Furthermore, bone marrow-derived cells showed extensive matrix mineralization, whereas fetal liver-derived cell lines showed no or very limited matrix mineralization capacity. These results indicate that the osteoblast differentiation potential differs in bone marrow and fetal liver-derived cell lines, which may be due to a distinct developmental program or different microenvironment in the two hematopoietic sites.

Statin-induced inhibition of 3-hydroxy-3-methyl glutaryl coenzyme a reductase sensitizes human osteosarcoma cells to anticancer drugs.

Osteosarcoma is the most common primary bone tumor in children and young adults. Resistance to chemotherapeutic drugs is a major problem that is responsible for the failure of treatment. This points to the need for increasing the responsiveness to cytotoxic drugs. We previously showed that lipophilic statins induce apoptosis in human osteosarcoma cells. In this study, we investigated the effects of atorvastatin [(3R,5R)-7[2-(4-fluorophenyl)-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H-pyrrol-1-yl]-3,5-dihydroxyheptanoïc acid] in combination with chemotherapeutic drugs on human osteosarcoma cell apoptosis, invasion, and migration. We report here that atorvastatin enhances the reduced cell viability induced by the anticancer drugs doxorubicin (Adriamycin; (1S,3S)-amino-3 tridesoxy-2,3,6 alpha-l-lyxo-hexopyranoside glycoloyl-3 trihydroxy-3,5,12 methoxy-10 dioxo-6,11 naphtacenyl-1) and cisplatin in human osteosarcoma cells. In particular, we found that atorvastatin enhances the induction of osteosarcoma cell apoptosis by anticancer drugs. In addition, we show that atorvastatin enhances the inhibitory effect of anticancer drugs on osteosarcoma cell migration. Moreover, atorvastatin and chemotherapeutic drugs had additive inhibitory effects on osteosarcoma cell invasion. In consistent tests, atorvastatin further augmented the reduction of matrix metalloprotease 2 activity induced by doxorubicin or cisplatin in osteosarcoma cells. The results show for the first time that atorvastatin sensitizes osteosarcoma cells to anticancer drugs, resulting in reduced cell viability, migration, and invasion, which suggest a strategy to improve the response to chemotherapy and reduce tumorigenesis in human osteosarcoma.

Hepatocytic Differentiation Potential of Human Fetal Liver Mesenchymal Stem Cells: In Vitro and In Vivo Evaluation.

In line with the search of effective stem cell population that would progress liver cell therapy and because the rate and differentiation potential of mesenchymal stem cells (MSC) decreases with age, the current study investigates the hepatogenic differentiation potential of human fetal liver MSCs (FL-MSCs). After isolation from 11-12 gestational weeks' human fetal livers, FL-MSCs were shown to express characteristic markers such as CD73, CD90, and CD146 and to display adipocytic and osteoblastic differentiation potential. Thereafter, we explored their hepatocytic differentiation potential using the hepatogenic protocol applied for adult human liver mesenchymal cells. FL-MSCs differentiated in this way displayed significant features of hepatocyte-like cells as demonstrated in vitro by the upregulated expression of specific hepatocytic markers and the induction of metabolic functions including CYP3A4 activity, indocyanine green uptake/release, and glucose 6-phosphatase activity. Following transplantation, naive and differentiated FL-MSC were engrafted into the hepatic parenchyma of newborn immunodeficient mice and differentiated in situ. Hence, FL-MSCs appeared to be interesting candidates to investigate the liver development at the mesenchymal compartment level. Standardization of their isolation, expansion, and differentiation may also support their use for liver cell-based therapy development.

CYR61 downregulation reduces osteosarcoma cell invasion, migration, and metastasis.

Osteosarcoma is the most common primary tumor of bone. The rapid development of metastatic lesions and resistance to chemotherapy remain major mechanisms responsible for the failure of treatments and the poor survival rate for patients. We showed previously that the HMGCoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase inhibitor statin exhibits antitumoral effects on osteosarcoma cells. Here, using microarray analysis, we identify Cyr61 as a new target of statins. Transcriptome and molecular analyses revealed that statins downregulate Cyr61 expression in human and murine osteosarcoma cells. Cyr61 silencing in osteosarcoma cell lines enhanced cell death and reduced cell migration and cell invasion compared with parental cells, whereas Cyr61 overexpression had opposite effects. Cyr61 expression was evaluated in 231 tissue cores from osteosarcoma patients. Tissue microarray analysis revealed that Cyr61 protein expression was higher in human osteosarcoma than in normal bone tissue and was further increased in metastatic tissues. Finally, tumor behavior and metastasis occurrence were analyzed by intramuscular injection of modified osteosarcoma cells into BALB/c mice. Cyr61 overexpression enhanced lung metastasis development, whereas cyr61 silencing strongly reduced lung metastases in mice. The results reveal that cyr61 expression increases with tumor grade in human osteosarcoma and demonstrate that cyr61 silencing inhibits in vitro osteosarcoma cell invasion and migration as well as in vivo lung metastases in mice. These data provide a novel molecular target for therapeutic intervention in metastatic osteosarcoma.

Blockade of the RhoA-JNK-c-Jun-MMP2 cascade by atorvastatin reduces osteosarcoma cell invasion.

Osteosarcoma is characterized by a high malignant and metastatic potential, which points to the need for new therapeutic strategies to prevent cell metastasis. In this study, we show that statin-induced HMG-CoA reductase inhibition reduces cell migration and invasion in human and murine osteosarcoma cells, independently of the genotype. The statin-induced reduction of cell migration and invasion was independent of induction of apoptosis and was geranylgeranylpyrophosphate-dependent. The statin reduced matrix metalloproteinase (MMP) 2, 9, and 14 and TIMP2 expression or activity in invading cells. Forced expression of MMP2 and MMP14 overcame the inhibitory effect of the statin on cell invasion, suggesting a role for these MMPs in invasive potential. We also investigated the mechanisms involved in the reduced MMP2 activity and cell invasion. Inhibition of JNK, but not ERK1/2 signaling, reduced MMP2 activity. Pharmacological or constitutive activation of JNK overcame the reduced MMP2 activity and cell invasion induced by the statin. The statin decreased JNK phosphorylation and c-Jun nuclear translocation, suggesting that HMG-CoA reductase inhibition targets the JNK-c-Jun signaling pathway. We showed that mevalonate or geranylgeranylpyrophosphate treatment prevented the statin-induced reduction in JNK phosphorylation, MMP2 activity, and cell invasion. Forced expression of a constitutively active form of RhoA increased JNK phosphorylation and overcame the inhibitory effect of atorvastatin on MMP2 activity and cell invasion. The data establish a link between RhoA, JNK, c-Jun, and MMP2 activity that is functionally involved in the reduction in osteosarcoma cell invasion by the statin. This suggests a novel strategy targeting RhoA-JNK-c-Jun signaling to reduce osteosarcoma cell tumorigenesis.