Involvement of BMPs/Smad signaling pathway in mechanical response in osteoblasts

Mechanical strain plays an important role in osteoblasts differentiation and bone formation but the underlying mechanism remains unclear. The aim of this study was to determine whether Bone Morphogenetic Proteins (BMPs)/Smad signaling pathway is involved in mechanical response in osteoblasts.

MC3T3-E1 cells were exposed to mechanical strain via a four-point bending system. mRNA levels and protein levels of BMP-2, BMP-4, Smad1, Smad5, Smurf1, and Smurf2 were assessed using RT-PCR and immunoblotting. Protein levels of BMP-2 and BMP-4 in the culture medium were also determined using Enzyme-linked Immunosorbent Assay (ELISA). Pretreatment with Noggin and transfection with Smad4 siRNA were carried out to block the BMPs/Smad signaling pathway and MG132 was used to inhibit the proteasome pathway.

We found that mechanical strain enhanced alkaline phosphatase (ALP) expression and activated BMPs/Smad signaling pathway. Mechanical strain induced expression of ALP was attenuated by Noggin and by Smad4 siRNA. The protein levels of Smad1 and Smad5, but not their mRNA levels, were up-regulated by mechanical strain. This finding could be explained by the down-regulation of Smurf1. The protein degradation of Smad might be inhibited by mechanical strain through down-regulation of Smuf1 expression. The addition of MG132 further enhanced the mechanical strain induced activation of Smad proteins and the increased expression of ALP.

Mechanical strain might promote osteoblasts differentiation through BMPs/Smad signaling pathway. The strain causes a drop in Smurf1 levels, leading to accumulation of Smad proteins and, subsequently, to enhanced BMPs/Smad signaling.