Egr1 deficiency disrupts dynamic equilibrium of chondrocyte extracellular matrix through PPARγ/RUNX2 signaling pathways.

Background: This study is to investigate the effect of Egr1 on the mineralization and accumulation of chondrocyte extracellular matrix.

Methods: The femoral heads of patients of various heights were collected. Egr1 knockout mice were used. Their limb lengtha nd body weight were assessed. The bone characteristics were detected by micro-CT scan and histological staining. Immature murine articular chondrocytes (iMACs) were isolated. Gross morphology was observed by histological staining. Relevant mRNA and protein expression were detected by qRT-PCR and Western blot, respectively. the related proteins were observed by immunohistochemical staining and immunofluorescence assay. Chromatin immunoprecipitation and reporter gene assay were also used. TUNEL was used to detect apoptosis.

Results: It was found that shorter patients had reduced Egr1 expression levels in the hypertrophic cartilage zone of the femoral head. In addition, Egr1 knockout mice exhibited reduced body size. Micro-CT analysis showed that these mice also had reduced bone volume. Safranin-O staining showed that the extracellular matrix of these mice exhibited a relatively limited degree of mineralization, and TUNEL staining showed reduced cell apoptosis levels. After transfecting the iMACs with dominant-negative Egr1 adenoviruses to inhibit Egr1, the enzymes of Adamst4, Adamst5, Mmp3 and Mmp13 were significantly upregulated. ChIP and luciferase assays revealed that Egr1 might regulate the chondrocyte extracellular matrix by the PPARγ/RUNX2 signaling pathways.

Conclusion: Egr1 has an important regulatory effect on the dynamic equilibrium of the chondrocyte extracellular matrix, which may be achieved through the PPARγ/RUNX2 signaling pathways.