Follistatin-like 1 suppresses osteoblast differentiation of bone marrow mesenchymal cells during inflammation

Objective: The current study aimed to explore the effect of Follistatin-like 1 (FSTL1) on osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) in an inflammatory environment.Design: Animal models of FSTL1-deficiency and wild-type mice were used, and the micro-CT images of the femoral head were evaluated. Mouse bone marrow mesenchymal stem cells were treated with various concentrations of recombinant FSTL1 (rFSTL1) in an inflammatory environment in vitro. Meanwhile, overexpression or knockdown of FSTL1 through lentiviral transfection was performed. Alkaline phosphatase (ALP) activity was tested, and Alizarin Red staining (ARS) was performed to evaluate osteogenic differentiation ability. The mRNA expression level of osteogenesis-related genes was detected by RT-qPCR.Results: In vivo experiments revealed a higher number of femoral skulls, higher trabecular thickness, smaller trabecular space, and less osteoporosis in FSTL1-knockdown mice than in the wild-type mice. The BMSCs with overexpression of FSTL1 or those treated with recombinant FSTL1 (rFSTL1) showed suppression of ALP activity, calcium nodule formation, and expression of osteogenesis-related genes osteopontin (OPN), osteocalcin (OCN), collagen type I alpha 1 (Col1 alpha 1), and more importantly, rFSTL1 functions in a dose-dependent manner. In contrast, FSTL1 knockdown promoted the osteogenesis activity and the expression of these osteogenesis-related genes in vitro.Conclusions: FSTL1 is an osteogenic suppressor that inhibits the osteogenic differentiation of BMSCs during inflammation and it can be a new target for bone regeneration.