FGFR1 signaling stimulates proliferation of human mesenchymal stem cells by inhibiting the cyclin-dependent kinase inhibitors p21(Waf1) and p27(Kip1).

Signaling through fibroblast growth factor receptor one (FGFR1) is a known inducer of proliferation in both embryonic and human adult mesenchymal stem cells (hMSCs) and positively regulates maintenance of stem cell viability. Leveraging the mitogenic potential of FGF2/FGFR1 signaling in stem cells for therapeutic applications necessitates a mechanistic understanding of how this receptor stimulates cell cycle progression. Using small interfering RNA (siRNA) depletion, antibody-inhibition, and small molecule inhibition, we establish that FGFR1 activity is rate limiting for self-renewal of hMSCs. We show that FGFR1 promotes stem cell proliferation through multiple mechanisms that unite to antagonize cyclin-dependent kinase (CDK) inhibitors. FGFR1 not only stimulates c-Myc to suppress transcription of the CDK inhibitors p21(Waf1) and p27(Kip1), thus promoting cell cycle progression but also increases the activity of protein kinase B (AKT) and the level of S-phase kinase-associated protein 2 (Skp2), resulting in the nuclear exclusion and reduction of p21(Waf1). The in vivo importance of FGFR1 signaling for the control of proliferation in mesenchymal progenitor populations is underscored by defects in ventral mesoderm formation during development upon inhibition of its signaling. Collectively, these studies demonstrate that FGFR1 signaling mediates the continuation of MSC growth and establishes a receptor target for enhancing the expansion of mesenchymal progenitors while maintaining their multilineage potential.

Isolation of a native osteoblast matrix with a specific affinity for BMP2.

During their commitment and differentiation toward the osteoblast lineage, mesenchymal stem cells secrete a unique extracellular matrix (ECM) that contains large quantities of glycosaminoglycans (GAGs). Proteoglycans (PGs) are major structural and functional components of the ECM and are composed of a core protein to which one or more glycosaminoglycan sugar chains (GAGs) attach. The association of BMP2, a member of the TGF-beta super-family of growth factors, and a known heparin-binding protein, with GAGs has been implicated as playing a significant role in modulating the growth factor's in vitro bioactivity. Here we have characterised an osteoblast-derived matrix (MX) obtained from decellularised MC3T3-E1 cell monolayers for its structural attributes, using SEM and histology, and for its functional ability to maintain cell growth and viability. Using a combination of histology and anion exchange chromatography, we first confirmed the retention of GAGs within MX following the decellularisation process. Then the binding specificity of the retained GAG species within the MX for BMP2 was examined using a BMP2-HBP/EGFP (BMP2 Heparin-Binding Peptide/Enhanced Green Fluorescent Protein) fusion protein. The results of this study provide further evidence for a central role of the ECM in the regulation of BMP2 bioactivity, hence on mesenchymal stem cell commitment to the osteoblast lineage.

Cyclin D1 gene amplification is highly homogeneous in breast cancer.

BACKGROUND: Cyclin D1 (CCND1) gene amplification is a molecular key alteration in breast cancer and was suggested to predict resistance to antihormonal therapy. As tissue heterogeneity may affect diagnostic accuracy of predictive biomarkers, CCND1 genetic heterogeneity was assessed in this study. A novel tissue microarray (TMA) platform was manufactured for this purpose. METHODS: Primary breast carcinomas from 147 patients were sampled in a "heterogeneity-TMA" by taking eight different tissue cores from 4 to 8 tumor-containing blocks per case. Additional tissue samples were taken from 1 to 4 corresponding nodal metastases in 35 of these patients. CCND1 amplification was assessed by fluorescence in situ hybridization (FISH). RESULTS: CCND1 amplification was seen in 28 of 133 (21.05 %) informative patients. Amplification was significantly associated with high tumor grade (p = 0.042), but unrelated to tumor type (p = 0.307), stage (p = 0.540) and ER (p = 0.061) or PR (p = 0.871) expression. A discordant Cyclin D1 amplification status was detected in 6 out of 28 (21.43 %) amplified tumors by heterogeneity-TMA analysis. Re-testing on large sections revealed three patients with true heterogeneity of high-level CCND1 amplification and another three patients with variable interpretation of borderline FISH ratios ranging between 1.7 and 2.3. No discrepancies were detected between 22 primary tumors and their matched lymph node metastases. CONCLUSIONS: The high degree of homogeneity seen for CCND1 amplification suggests that this alteration is an early event in the development of a small subset of breast cancers.

Functional coupling between the extracellular matrix and nuclear lamina by Wnt signaling in progeria.

The segmental premature aging disease Hutchinson-Gilford Progeria (HGPS) is caused by a truncated and farnesylated form of Lamin A. In a mouse model for HGPS, a similar Lamin A variant causes the proliferative arrest and death of postnatal, but not embryonic, fibroblasts. Arrest is due to an inability to produce a functional extracellular matrix (ECM), because growth on normal ECM rescues proliferation. The defects are associated with inhibition of canonical Wnt signaling, due to reduced nuclear localization and transcriptional activity of Lef1, but not Tcf4, in both mouse and human progeric cells. Defective Wnt signaling, affecting ECM synthesis, may be critical to the etiology of HGPS because mice exhibit skeletal defects and apoptosis in major blood vessels proximal to the heart. These results establish a functional link between the nuclear envelope/lamina and the cell surface/ECM and may provide insights into the role of Wnt signaling and the ECM in aging.

Stem cell fate decisions: the role of heparan sulfate in the control of autocrine and paracrine signals.

The aim of this review is to explore the idea that the glycosaminoglycan sugar heparan sulfate (HS), richly concentrated on the plasma membrane of all animal cells studied so far and a major component of extracellular matrices, is by virtue of its ability to modulate protein gradients and signal transduction, the master regulator of stem cell fate (and thus wound healing). Moreover, the interaction between HS and members of the TGF-beta superfamily is emerging as a central tenet for stem cells. The potential significance of this interaction is best understood by examining both how HS modulates ligand interactions and stability, and how it maintains protein gradients with varying degrees of specificity. Importantly, HS also regulates the activity of numerous antagonists, thus underscoring its importance as a primary regulator of stem cell fate decisions.

Role of growth hormone receptor signaling in osteogenesis from murine bone marrow progenitor cells.

Growth hormone (GH) regulates many of the factors responsible for controlling the development of bone marrow progenitor cells (BMPCs). The aim of this study was to elucidate the role of GH in osteogenic differentiation of BMPCs using GH receptor null mice (GHRKO). BMPCs from GHRKO and their wild-type (WT) littermates were quantified by flow cytometry and their osteogenic differentiation in vitro was determined by cell morphology, real-time RT-PCR, and biochemical analyses. We found that freshly harvested GHRKO marrow contains 3% CD34 (hematopoietic lineage), 43.5% CD45 (monocyte/macrophage lineage), and 2.5% CD106 positive (CFU-F/BMPC) cells compared to 11.2%, 45%, and 3.4% positive cells for (WT) marrow cells, respectively. When cultured for 14 days under conditions suitable for CFU-F expansion, GHRKO marrow cells lost CD34 positivity, and were markedly reduced for CD45, but 3- to 4-fold higher for CD106. While WT marrow cells also lost CD34 expression, they maintained CD45 and increased CD106 levels by 16-fold. When BMPCs from GHRKO mice were cultured under osteogenic conditions, they failed to elongate, in contrast to WT cells. Furthermore, GHRKO cultures expressed less alkaline phosphatase, contained less mineralized calcium, and displayed lower osteocalcin expression than WT cells. However, GHRKO cells displayed similar or higher expression of cbfa-1, collagen I, and osteopontin mRNA compared to WT. In conclusion, we show that GH has an effect on the proportions of hematopoietic and mesenchymal progenitor cells in the bone marrow, and that GH is essential for both the induction and later progression of osteogenesis.