STEEL participates in fracture healing through upregulating angiogenesis-related genes by recruiting PARP 1

To explore the effect of STEEL on fracture healing and its underlying mechanism.

A total of 31 patients with long bone fracture and who received reoperation because of bone nonunion, delayed union or healing disorder in the Wuxi Nine Hospital Affiliated to Soochow University from July 2016 to February 2018 were selected. The bone callus at the fracture site was collected from each patient during the reoperation. QRT-PCR (Quantitative Real-Time Polymerase Chain Reaction) was used to detect STEEL expression in the callus tissues of the treatment group (bone nonunion or delayed union) and the control group. In addition, we measured the number of blood vessels in the fracture tissues by immunohistochemistry. After the construction of tibial fracture model in mice, STEEL expression and the total number of blood vessels in the treatment group (sawing treatment) and the control group (sham operation) were detected, respectively. For in vitro experiments, CCK-8 (cell counting kit-8) assay was performed to detect cell proliferation after knockdown or overexpression of STEEL in the vascular endothelial cells. The binding condition of STEEL and its interacting proteins were detected by RIP (RNA binding protein immunoprecipitation), and the binding of PARP 1 [poly (ADP-ribose) polymerase 1] with gene promoter was observed by ChIP (chromatin immunoprecipitation assay). Western blot was used to detect the expression level of VEGF (vascular endothelial growth factor).

STEEL expression and the vascular density in the callus tissues of the treatment group were significantly lower than those of the control group. Downregulated STEEL remarkably decreased the proliferation ability of HUVEC cells. Meanwhile, the vascular density was also significantly decreased in mice with a tibial fracture. Overexpressed STEEL obtained the opposite results. STEEL could interact with PARP 1 to regulate expressions of downstream genes. Moreover, STEEL could also promote angiogenesis by elevating VEGF expression.

We showed that STEEL expression could partly represent the angiogenesis of fracture sites. Moreover, it promoted angiogenesis by elevating VEGF expression.