Beta-1 integrins mediate substrate dependent effects of 1alpha,25(OH)2D3 on osteoblasts

Myosin heavy chain-derived peptide of Gadus morhua promotes proliferation and differentiation in osteoblasts and bone formation and maintains bone homeostasis in ovariectomized mice

Gadus morhua is an important commercial fish rich in nutrients required for daily metabolism. However, the regulation of G. morhua peptides (GMP) on osteoblast growth remains unclear. In order to clarify the regulatory effects of GMP on osteoblasts, the effects of GMP on the growth of MC3T3-E1 cells were investigated, and the osteogenic peptides were identified and screened. The results showed that GMP promoted the proliferation and differentiation of osteoblasts by regulating the BMP/WNT signaling pathway at concentrations of 1-100 μg mL^-1. Molecular docking studies showed that a decapeptide, MNKKREAEFQ (P-GM-1), had a high affinity for integrins 3VI4 and 1L5G (-CDOCKER interaction energy: 161.30, 212.27 kcal mol^-1). Additionally, the proliferation rate of MC3T3-E1 cells was increased by 27%, and ALP activity was significantly increased under P-GM-1 treatment (100 μg mL^-1). Moreover, P-GM-1 promotes bone formation, maintains bone homeostasis, and prevents osteoporosis in ovariectomized mice by regulating the BMP/Smad signaling pathway. This study confirmed the potential of GMP in the regulation of bone mineral density and provided a certain theoretical basis for the development of anti-osteoporosis active factors from GMP.

Osteoblasts grown on microroughened titanium surfaces regulate angiogenic growth factor production through specific integrin receptors

Cellular attachment and response to biomaterials are mediated by integrin receptor binding to extracellular matrix proteins adsorbed onto the material surface. Osteoblasts interact with their substrates via several integrin complexes including fibronectin-binding α5β1 and collagen-binding α1β1 and α2β1. Knockdown of α2 or β1 integrin subunits inhibits the production of factors that promote an osteogenic microenvironment, including osteocalcin, osteoprotegerin, and TGFβ1. Osteoblasts also secrete several angiogenic growth factors such as VEGF-A (VEGF¹⁶⁵), FGF-2, and angiopoietin 1, which are regulated by titanium surface topography and surface energy. Here, we examined whether signaling through integrin receptor complexes regulates production and secretion of angiogenic factors during osteoblast differentiation on microtextured Ti surfaces. To do this, integrin subunits α1, α2, α5, and β1 were stably silenced in MG63 osteoblast-like cells cultured on grit-blasted/acid-etched hydrophobic Ti (SLA) or on hydrophilic SLA (modSLA). VEGF-A production increased in response to Ti surface topography and energy in integrin α2, α5, and β1 silenced cells but decreased in α1-silenced cells. FGF-2 decreased on modSLA substrates in both α1 and α2-silenced cells but was unchanged in response to silencing of either α5 or β1. In integrin α1, α2, and β1-silenced cells, Ang-1 increased on modSLA but α5-silencing did not affect Ang-1 production during surface mediated differentiation. These results suggest that signaling through specific integrin receptor complexes during osteoblast differentiation on microstructured Ti substrates, regulates the production of angiogenic factors by those cells, and this is differentially regulated by surface hydrophilicity. STATEMENT OF SIGNIFICANCE: Successful implantation of synthetic biomaterials into bone depends on the biological process known as osseointegration. Osseointegration is a highly regulated communication of cells that orchestrates the migration of progenitor cells towards the implant site and promotes the deposition and mineralization of extracellular matrix proteins within the implant microenvironment, to tightly join the implant to native bone. In this process, angiogenesis functions as the initiation site of progenitor cell migration and is necessary for matrix deposition by providing the necessary nutrients for bone formation. In the present study, we show a novel regulation of specific angiogenic growth factors by integrin receptor complexes. This research is important to develop biomaterials that promote and maintain osseointegration through proper vascularization and prevent implant failure in patients lacking sufficient angiogenesis.

Human osteoblasts exhibit sexual dimorphism in their response to estrogen on microstructured titanium surfaces

BACKGROUND

Osseointegration is dependent on the implant surface, surrounding bone quality, and the systemic host environment, which can differ in male and female patients. Titanium (Ti) implants with microstructured surfaces exhibit greater pullout strength when compared to smooth-surfaced implants and exhibit enhanced osteogenic cellular responses in vitro. Previous studies showed that 1α,25-dihydroxyvitamin D3 [1α,25(OH)₂D₃] has a greater effect on rat osteoblast differentiation on microstructured Ti compared to smooth Ti surfaces and tissue culture polystyrene (TCPS). The stimulatory effect of 17β-estradiol (E₂) on differentiation is observed in female osteoblasts on micro-rough Ti, but it is not known if male osteoblasts behave similarly in response to E₂ and microtopography. This study assessed whether human male and female osteoblasts exhibit sex-specific differences in response to E₂ and 1α,25(OH)₂D₃ when cultured on microstructured Ti surfaces.

METHODS

Osteoblasts from three male and three female human donors were cultured on Ti discs with varying surface profiles: a smooth pretreatment (PT), a coarse grit-blasted/acid-etched (SLA), and an SLA surface having undergone modification in a nitrogen environment and stored in saline to maintain hydrophilicity (modSLA). Cells cultured on these surfaces were treated with E₂ or 1α,25(OH)₂D₃.

RESULTS

Male and female human osteoblasts responded similarly to microstructure although there were donor-specific differences; cell number decreased, and osteocalcin (OCN), osteoprotegerin (OPG), and latent and active transforming growth factor 1 increased on SLA and modSLA compared to TCPS. Female osteoblasts had higher alkaline phosphatase activity and OCN production than male counterparts but produced less OPG. Both sexes responded similarly to 1α,25(OH)₂D₃. E₂ treatment reduced cell number and increased osteoblast differentiation and factor production only in female cells.

CONCLUSIONS

Male and female human osteoblasts respond similarly to microstructure and 1α,25(OH)₂D₃ but exhibit sexual dimorphism in substrate-dependent responses to E₂. E₂ affected female osteoblasts, suggesting that signaling is sex-specific and surface-dependent. Donor osteoblasts varied in response, demonstrating the need to test multiple donors when examining human samples. Understanding how male and female cells respond to orthopedic biomaterials will enable greater predictability post-implantation as well as therapies that are more patient-specific.

Vitamin D Promotes MSC Osteogenic Differentiation Stimulating Cell Adhesion and αVβ3 Expression

Vitamin D (Vit D) by means of its biological active form, 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), has a protective effect on the skeleton by acting on calcium homeostasis and bone formation. Furthermore, Vit D has a direct effect on mesenchymal stem cells (MSCs) in stimulating their osteogenic differentiation. In this work, we present for the first time the effect of 1,25(OH)₂D₃ on MSC adhesion. Considering that cell adhesion to the substrate is fundamental for cell commitment and differentiation, we focused on the expression of α_Vβ₃ integrin, which has a key role in the commitment of MSCs to the osteoblastic lineage. Our data indicate that Vit D increases α_Vβ₃ integrin expression inducing the formation of focal adhesions (FAs). Moreover, we assayed MSC commitment in the presence of the extracellular matrix (ECM) glycoprotein fibronectin (FN), which is able to favor cell adhesion on surfaces and also to induce osteopontin (OPN) expression: this suggests that Vit D and FN synergize in supporting cell adhesion. Taken together, our findings provide evidence that Vit D can promote osteogenic differentiation of MSCs through the modulation of α_Vβ₃ integrin expression and its subcellular organization, thus favoring binding with the matrix protein (FN).

Regulation of Osteoblast Differentiation by Acid-Etched and/or Grit-Blasted Titanium Substrate Topography Is Enhanced by 1,25(OH)2D3 in a Sex-Dependent Manner

This study assessed contributions of micron-scale topography on clinically relevant titanium (Ti) to differentiation of osteoprogenitor cells and osteoblasts; the interaction of this effect with 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃); and if the effects are sex-dependent. Male and female rat bone marrow cells (BMCs) were cultured on acid-etched (A, R_a = 0.87 μm), grit-blasted (GB, R_a = 3.90 μm), or grit-blasted/acid-etched (SLA, R_a = 3.22 μm) Ti. BMCs were sensitive to surface topography and underwent osteoblast differentiation. This was greatest on SLA; acid etching and grit blasting contributed additively. Primary osteoblasts were also sensitive to SLA, with less effect from individual structural components, demonstrated by enhanced local factor production. Sex-dependent responses of BMCs to topography varied with parameter whereas male and female osteoblasts responded similarly to surface treatment. 1α,25(OH)₂D₃ enhanced cell responses on all surfaces similarly. Effects were sex-dependent and male cells grown on a complex microstructured surface were much more sensitive than female cells. These results indicate that effects of the complex SLA topography are greater than acid etching or grit blasting alone on multipotent BMCs and committed osteoblasts and that individual parameters are sex-specific. The effect of 1α,25(OH)₂D₃ was sex dependent. The results also suggest that levels of 1α,25(OH)₂D₃ in the patient may be important in osseointegration.

Regulation of matrix remodelling phenotype in gingival fibroblasts by substratum topography

Gingival connective tissue often has a composition resembling that of scar surrounding dental implant abutments. Increased cell adhesion, α-smooth muscle actin (α-SMA) expression and increased extracellular matrix deposition are a hallmark of fibrotic cells, but how topographic features influence gingival fibroblast adhesion and adoption of the α-SMA positive myofibroblast phenotype associated with scarring is unknown. The purpose of the present study was to demonstrate whether implant topographies that limit adhesion formation would reduce myofibroblast differentiation and extracellular matrix deposition. Human gingival fibroblasts were cultured on PT (smooth) and SLA (roughened) titanium discs for varying time-points. At 1 and 2 weeks after seeding, incorporation of α-SMA into stress-fibre bundles and fibronectin deposition was significantly higher on PT than SLA surfaces indicating differentiation of the cells towards a myofibroblast phenotype. Analysis of adhesion formation demonstrated that cells formed larger adhesions and more stable adhesions on PT, with more nascent adhesions observed on SLA. Gene expression analysis identified up-regulation of 15 genes at 24 hrs on SLA versus PT associated with matrix remodelling. Pharmacological inhibition of Src/FAK signalling in gingival fibroblasts on PT reduced fibronectin deposition and CCN2 expression. We conclude that topographical features that reduce focal adhesion stability could be applied to inhibit myofibroblast differentiation in gingival fibroblasts.

Role of integrin subunits in mesenchymal stem cell differentiation and osteoblast maturation on graphitic carbon-coated microstructured surfaces

Surface roughness, topography, chemistry, and energy promote osteoblast differentiation and increase osteogenic local factor production in vitro and bone-to-implant contact in vivo, but the mechanisms involved are not well understood. Knockdown of integrin heterodimer alpha2beta1 (α2β1) blocks the osteogenic effects of the surface, suggesting signaling by this integrin homodimer is required. The purpose of the present study was to separate effects of surface chemistry and surface structure on integrin expression by coating smooth or rough titanium (Ti) substrates with graphitic carbon, retaining surface morphology but altering surface chemistry. Ti surfaces (smooth [Ra < 0.4 μm], rough [Ra ≥ 3.4 μm]) were sputter-coated using a magnetron sputtering system with an ultrapure graphite target, producing a graphitic carbon thin film. Human mesenchymal stem cells and MG63 osteoblast-like cells had higher mRNA for integrin subunits α1, α2, αv, and β1 on rough surfaces in comparison to smooth, and integrin αv on graphitic-carbon-coated rough surfaces in comparison to Ti. Osteogenic differentiation was greater on rough surfaces in comparison to smooth, regardless of chemistry. Silencing integrins β1, α1, or α2 decreased osteoblast maturation on rough surfaces independent of surface chemistry. Silencing integrin αv decreased maturation only on graphitic carbon-coated surfaces, not on Ti. These results suggest a major role of the integrin β1 subunit in roughness recognition, and that integrin alpha subunits play a major role in surface chemistry recognition.

Effects of IL-1 receptor-associated kinase-4 gene silencing on human osteoblast-like cells

The aim of this study is to identify the effects of interleukin-1 receptor-associated kinase-4 (IRAK-4) gene silencing on human osteoblast-like cells. The siRNA sequences of the target gene, IRAK-4, were constructed and transferred into MG63 cells (control group = MG63 cells; SC group = MG63 cells transfected with scrambled IRAK-4 siRNA; KD group = MG63 cells transfected with 75 nM IRAK-4 siRNA). The morphological changes, cell growth, cell-cycle progression, apoptosis, and the expression of various cytokines and proteins were compared. Compared with the control and SC groups, IRAK-4 gene silencing in MG63 cells caused morphological changes, inhibited growth, altered the cell-cycle distribution, increased apoptosis (p < 0.05), decreased bone alkaline phosphatase and osteocalcin levels (p < 0.05), and decreased protein expression of Bcl-2/Bax and Bcl-2, p-JNK1/2, p-ERK1/2, and p-p38MAPK (p < 0.05). The results indicated that IRAK-4 gene silencing in MG63 cells inhibited cell proliferation and function and increase apoptosis, which may be related to the decreased Bcl-2/Bax ratio and inhibition of the protein expression of various components of the mitogen-activated protein kinase pathways. The results of this study may help improve the understanding of the relationship between IRAK-4 and osteoblast-like cells and the interactions between various cytokines in the periprosthetic inflammatory environment.

Effect of different surface nanoroughness of titanium dioxide films on the growth of human osteoblast-like MG63 cells

Cell behavior depends strongly on the physical and chemical properties of the material surface, for example, its chemistry and topography. The authors have therefore assessed the influence of materials of different chemical composition (i.e., glass substrates with and without TiO(2) films in anatase form) and different surface roughness (R(a) = 0, 40, 100, or 170 nm) on the adhesion, proliferation, and osteogenic differentiation of human osteoblast-like MG63 cells. On day 1 after seeding, the largest cell spreading area was found on flat TiO(2) films (R(a) = 0 nm). On TiO(2) films with R(a) = 170 nm, the cell spreading area was larger and the number of initially adhering cells was higher than the values on the corresponding uncoated glass. On day 3 after seeding, the cell number was higher on the TiO(2) films (R(a) = 0 and 40 nm) than on the corresponding glass substrates and the standard polystyrene dishes. On day 7, all TiO(2) films contained higher cell numbers than the corresponding glass substrates, and the cells on the TiO(2) films with R(a) = 40 and 100 nm also contained a higher concentration of β-actin. These results indicate that TiO(2) coating had a positive influence on the adhesion and subsequent proliferation of MG63 cells. In addition, on all investigated materials, the cell population density achieved on day 7 decreased with increasing surface roughness. The concentration of osteocalcin, measured per mg of protein, was significantly lower in the cells on rougher TiO(2) films (R(a) = 100 and 170 nm) than in the cells on the polystyrene dishes. Thus, it can be concluded that the adhesion, growth, and phenotypic maturation of MG63 cells were controlled by the interplay between the material chemistry and surface topography, and were usually better on smoother and TiO(2)-coated surfaces than on rougher and uncoated glass substrates.

Osteogenic differentiation of stem cells alters vitamin D receptor expression

Pluripotent and multipotent stem cells adopt an osteoblastic phenotype when cultured in environments that enhance their osteogenic potential. Embryonic stem cells differentiated as embryoid bodies (EBs) in osteogenic medium containing β-glycerophosphate exhibit increased expression of bone markers, indicating that cells are osteoblastic. Interestingly, 1α,25-dihydroxyvitaminD3 (1,25D) enhances the osteogenic phenotype not just in EBs but also in multipotent adult mesenchymal stem cells (MSCs). 1,25D acts on osteoblasts via classical vitamin D receptors (VDR) and via a membrane 1,25D-binding protein [protein disulfide isomerase family A, member 3 (PDIA3)], which activates protein kinase C-signaling. The aims of this study were to determine whether these receptors are regulated during osteogenic differentiation of stem cells and if stem cells and differentiated progeny are responsive to 1,25D. mRNA and protein levels for VDR, PDIA3, and osteoblast-associated proteins were measured in undifferentiated cells and in cells treated with osteogenic medium. Mouse EBs expressed both VDR and PDIA3, but VDR increased as cells underwent osteogenic differentiation. Human MSCs expressed Pdia3 at constant levels throughout differentiation, but VDR increased in cells treated with osteogenic medium. These results suggest that both 1,25D signaling mechanisms are important, with PDIA3 playing a greater role during early events and VDR playing a greater role in later stages of differentiation. Understanding these coordinated events provide a powerful tool to control pluripotent and multipotent stem cell differentiation through induction medium.

The forkhead transcription factor Foxc2 promotes osteoblastogenesis via up-regulation of integrin β1 expression

The forkhead box C2 (Foxc2) protein, a member of the forkhead/winged helix transcription factor family, plays an important role in regulation of metabolism, arterial specification, and vascular sprouting. Foxc2-null mutants die prenatally or perinatally, and they exhibit hypoplasia of the vertebrae and insufficient chondrification or ossification of medial structures. However, the role of Foxc2 in osteoblastogenesis is not yet fully understood. According to the degree of differentiation of osteoblasts, we found that Foxc2 expression was gradually increased and dose-dependently up-regulated by well-known bone anabolic agents, such as hPTH(1-34) and BMP2. In ex vivo mouse calvarial organ culture, a significant reduction of the basal expression of Foxc2 induced by siFoxc2 remarkably suppressed cell proliferation and differentiation and induced cell death. Knockdown of Foxc2 expression using siFoxc2 in both MC3T3-E1 and primary mouse calvarial cells also resulted in a significant suppression of proliferation and differentiation, and induced cell death, supporting the ex vivo observations. In addition, the resistance to apoptosis induced by serum deprivation and phosphorylation of both Akt and ERK was significantly reduced after siFoxc2 treatment. Conversely, overexpression of Foxc2 increased the proliferation of MC3T3-E1 and primary mouse calvarial cells. Furthermore, we found that Foxc2 enhanced the expression of integrin β1, an important modulator of osteoblastogenesis, by direct binding to a Forkhead-binding element in its promoter. Taken together, these results indicate that Foxc2 plays an important role in osteoblastogenesis by promoting osteoblast proliferation, survival and differentiation through up-regulation of integrin β1 in response to stimuli which induce bone formation.

Direct and indirect effects of microstructured titanium substrates on the induction of mesenchymal stem cell differentiation towards the osteoblast lineage

Microstructured and high surface energy titanium substrates increase osseointegration in vivo. In vitro, osteoblast differentiation is increased, but effects of the surface directly on multipotent mesenchymal stem cells (MSCs) and consequences for MSCs in the peri-implant environment are not known. We evaluated responses of human MSCs to substrate surface properties and examined the underlying mechanisms involved. MSCs exhibited osteoblast characteristics (alkaline phosphatase, RUNX2, and osteocalcin) when grown on microstructured Ti; this effect was more robust with increased hydrophilicity. Factors produced by osteoblasts grown on microstructured Ti were sufficient to induce co-cultured MSC differentiation to osteoblasts. Silencing studies showed that this was due to signaling via alpha2beta1 integrins in osteoblasts on the substrate surface and paracrine action of secreted Dkk2. Thus, human MSCs are sensitive to substrate properties that induce osteoblastic differentiation; osteoblasts interact with these surface properties via alpha2beta1 and secrete Dkk2, which acts on distal MSCs.

The roles of Wnt signaling modulators Dickkopf-1 (Dkk1) and Dickkopf-2 (Dkk2) and cell maturation state in osteogenesis on microstructured titanium surfaces

Osteoblast differentiation on tissue culture polystyrene (TCPS) requires Wnt/beta-catenin signaling, regulating modulators of the Wnt pathway like Dickkopf-1 (Dkk1) and Dkk2. Osteoblast differentiation is increased on microstructured titanium (Ti) surfaces compared to TCPS; therefore, we hypothesized that surface topography and hydrophilicity affect Dkk1 and Dkk2 expression and that their roles in osteoblast differentiation on Ti differs depending on cell maturation state. Human osteoblast-like MG63 cells, normal human osteoblasts (HOBs), and human mesenchymal stem cells (MSCs), as well as MG63 cells stably silenced for Dkk1 or Dkk2 were grown for 6 days on TCPS and Ti surfaces (PT [Ra<0.2 microm], SLA [Ra=4 microm], modSLA [hydrophilic-SLA]). Dkk1 and Dkk2 mRNA and protein increased on SLA and modSLA for all cell types, but exogenous rhDkk1 and rhDkk2 affected MSCs differently than MG63 cells and HOBs. Silencing Dkk1 reduced MG63 cell number on TCPS and PT, but increased differentiation on these substrates. Silencing Dkk2 reduced stimulatory effects of SLA and modSLA on osteoblast differentiation; Dkk2 but not Dkk1 restored these effects. Antibodies to Dkk1 or Dkk2 specifically blocked substrate-dependent changes caused by the proteins, demonstrating their autocrine action. This indicates major roles for Dkk1 and the canonical Wnt pathway in early-stage differentiation, and for Dkk2 and Wnt/Ca2+-dependent signaling in late-stage differentiation on microstructured and hydrophilic surfaces, during osseointegration.