Mechanical strain promotes osteoblastic differentiation through integrin-ß1-mediated ß-catenin signaling.

As integrins are mechanoresponsive, there exists an intimate relationship between integrins and mechanical strain. Integrin-ß1 mediates the impact of mechanical strain on bone. Mechanical strain induces bone formation through the activation of ß-catenin pathways, which suggests that integrin-ß1 mediates ß-catenin signaling in osteoblasts in response to mechanical strain. In the present study, we examined the role of integrin-ß1 in Wnt/ß-catenin signal transduction in mechanically strained osteoblasts. MC3T3-E1 osteoblastic cells were transfected with integrin-ß1 small interfering RNA (si-Itgß1), and exposed to mechanical tensile strain of 2,500 microstrain (µÎµ) using a four-point bending device. The mechanical strain enhanced the mRNA expression of integrin-ß1, the protein levels of phosphorylated (p-) glycogen synthase kinase-3ß (GSK­3ß) and ß-catenin, simultaneously increased the mRNA levels of runt-related transcriptional factor 2 (Runx2) and osteocalcin (OCN), the protein levels of bone morphogenetic protein (BMP)-2 and -4 and enhanced the alkaline phosphatase (ALP) activity of the ME3T3-E1 cells. The elevations were inhibited by si-Itgß1. Additionally, the mechanical strain induced the nuclear translocation of ß-catenin into the nucleus, which was also inhibited by si-Itgß1. These findings indicated that mechanical strain promoted osteoblastic differentiation through integrin­ß1­mediated ß-catenin signaling.

Differential regulation of stiffness, topography, and dimension of substrates in rat mesenchymal stem cells.

The physiological microenvironment of the stem cell niche, including the three factors of stiffness, topography, and dimension, is crucial to stem cell proliferation and differentiation. Although a growing body of evidence is present to elucidate the importance of these factors individually, the interaction of the biophysical parameters of the factors remains insufficiently characterized, particularly for stem cells. To address this issue fully, we applied a micro-fabricated polyacrylamide hydrogel substrate with two elasticities, two topographies, and three dimensions to systematically test proliferation, morphology and spreading, differentiation, and cytoskeletal re-organization of rat bone marrow mesenchymal stem cells (rBMSCs) on twelve cases. An isolated but not combinatory impact of the factors was found regarding the specific functions. Substrate stiffness or dimension is predominant in regulating cell proliferation by fostering cell growth on stiff, unevenly dimensioned substrate. Topography is a key factor for manipulating cell morphology and spreading via the formation of a large spherical shape in a pillar substrate but not in a grooved substrate. Although stiffness leads to osteogenic or neuronal differentiation of rBMSCs on a stiff or soft substrate, respectively, topography or dimension also plays a lesser role in directing cell differentiation. Neither an isolated effect nor a combinatory effect was found for actin or tubulin expression, whereas a seemingly combinatory effect of topography and dimension was found in manipulating vimentin expression. These results further the understandings of stem cell proliferation, morphology, and differentiation in a physiologically mimicking microenvironment.

Mechanical strain regulates osteoblast proliferation through integrin-mediated ERK activation.

Mechanical strain plays a critical role in the proliferation, differentiation and maturation of bone cells. As mechanical receptor cells, osteoblasts perceive and respond to stress force, such as those associated with compression, strain and shear stress. However, the underlying molecular mechanisms of this process remain unclear. Using a four-point bending device, mouse MC3T3-E1 cells was exposed to mechanical tensile strain. Cell proliferation was determined to be most efficient when stimulated once a day by mechanical strain at a frequency of 0.5 Hz and intensities of 2500 µÎµ with once a day, and a periodicity of 1 h/day for 3 days. The applied mechanical strain resulted in the altered expression of 1992 genes, 41 of which are involved in the mitogen-activated protein kinase (MAPK) signaling pathway. Activation of ERK by mechanical strain promoted cell proliferation and inactivation of ERK by PD98059 suppressed proliferation, confirming that ERK plays an important role in the response to mechanical strain. Furthermore, the membrane-associated receptors integrin ß1 and integrin ß5 were determined to regulate ERK activity and the proliferation of mechanical strain-treated MC3T3-E1 cells in opposite ways. The knockdown of integrin ß1 led to the inhibition of ERK activity and cell proliferation, whereas the knockdown of integrin ß5 led to the enhancement of both processes. This study proposes a novel mechanism by which mechanical strain regulates bone growth and remodeling.