1. Characterization of Stat3 CKO mice
To achieve the conditional knockout of Stat3 in osteoblast, the transgenic mice of Osx-Cre was used to cross with Stat3f/f mice. In Fig. 2A, the mouse genotypes, including Osx-Cre; Stat3f/f (Stat3 CKO) and Cre negative littermate control, were displayed. The calvaria isolated from 5-week-old Stat3 CKO mice showed positive GFP signal due to Osx-GFP-Cre, whereas it was absent in CON mice (Fig. 2B). To verify the elimination of Stat3 in osteoblasts, the quantitative real-time polymerase chain reaction (qRT-PCR) and western blot were performed to assess the expression of Stat3. Stat3 were found in the osteoblasts in the calvaria of the 5-week-old Con mice, but were hardly expressed in the calvaria of the Stat3 CKO mice (Fig. 2C, D) (**P<0.01).
2. Depletion of Stat3 in osteoblasts led to a decrease of bone mass in calvarial osteolytic lesions
To explore the impact of Stat3-deficiency on bone regeneration in inflammatory microenvironment, the LPS-stimulated calvaria lesion was induced on both Stat3 CKO and control mice. According to the morphological observation of calvaria from day 3 to day 14 (Fig. 3A), the knockout of Stat3 in osteoblasts led to an increase of LPS-induced osteolysis, which was characterized by visible osteolysis damage and bone surface erosion, especially at day 3 and day 7. Micro-CT 3D reconstruction observations indicated an obvious bone loss in the Stat3 CKO mice on calvaria at day 7 (Fig. 3B). Meanwhile, no significant difference of microstructural indices between CON and Stat3 CKO mice was observed at day 3 and 14. On the other hand, a marked reduction of trabecular bone mass in Stat3 CKO mice was found at day 7, assessed by the CKO vs. CON groups of BV/TV, Tb.Th., Tb.N., and Tb.Sp. (Fig. 3C) (*P<0.05, **P<0.01).
3. Stat3 deficiency in osteoblasts caused an increased inflammatory osteolysis
Considering that the obvious lesion difference between CON and Stat3 CKO mice was found at day 7 after LPS-stimulation, these samples were collected for paraffin section preparation. According to HE staining, the inflammatory cell infiltration along with significant bone erosion was observed in the calvaria of Stat3 CKO mice (Fig. 4A). The decrease of mineralized bone matrix in Stat3 CKO mice was confirmed by Masson staining (Fig. 4B) (**P<0.01). Consistent with the increased bone erosion, the osteolytic lesion with abundant TRAP-positive osteoclasts was found on Stat3 CKO mice, whereas the CON group was presented with fewer TRAP-positive osteoclasts (Fig. 4C) (**P<0.01). Moreover, the increased mRNA expression of RANKL in osteolytic lesion was found on Stat3 CKO group (Fig. 4D) (vs. CON, *P<0.05).
4. In the bone destructive lesion, the osteogenic markers were decreased in Stat3 CKO mice
The IHC staining observations revealed that the number of Stat3 positive osteoblasts in Stat3 CKO group was dramatically dropped compared with CON group (***P<0.001) (Fig. 5A). Meanwhile, the numbers of Runx2, OPN and COL1A1 positive osteoblasts per hpf in osteolytic lesion were significantly decreased in Stats3 CKO mice (**P<0.01) (Fig. 5A). A considerably reduction of the mRNA and protein expressions of osteogenic markers Runx2, OPN, and COL1A1 were detected in the calvaria of Stat3 CKO mice, compared with CON group (*P<0.05, **P<0.01) (Fig. 5B-D).
5. Knockdown of Stat3 in osteoblasts led to a reduced mineralization in the inflammatory microenvironment in vitro
To gain insight into the Stat3-mediated bone matrix mineralization in osteoblasts, osteoblasts from primary cell culture of passage 2-3 were seeded into plates followed by mineralization induction for 21 days. Less formation of calcium deposit was observed in the Stat3-dificient cells when compared with the Con, which was visualized by the Alizarin Red staining (***P<0.001) (Fig. 6A).
The expression of total Stat3 and nuclear phosphorylated Stat3 (p-Stat3) was significantly decreased in Stat3 CKO cells in both normal and inflammatory microenvironment, confirming that Stat3 gene was effectively knocked out in osteoblasts of Stat3 CKO mice (*P<0.05, ***P<0.001) (Fig. 6B-C). The lack of Stat3 significantly suppressed the mRNA expression of Runx2, COL1A1, and OPN in osteoblasts (*P<0.05, **P<0.01, ***P<0.001) (Fig. 6 B-C)). The mRNA expression of RANKL in osteoblasts was increased while difference was not found in the expression of OPG, which was consistent with the in vivo data (*P<0.05) (Fig. 6D-E). The protein levels of osteogenic markers were also down-regulated in both normal and inflammatory microenvironment, compared with the control (*P<0.05, **P<0.01) (Fig. 5F-G).
6. The proliferation and migration of Stat3-dificient osteoblasts were inhibited in the inflammatory microenvironment
According to the EdU assay, fewer EdU positive cells were observed in Stat3 CKO group in both normal and inflammatory microenvironment (**P<0.01) (Fig. 7A, B). The number of migrated osteoblasts in the Stat3 CKO group was significantly decreased in normal microenvironment and further less in inflammatory microenvironment, compared with the control (***P<0.001) (Fig. 7C, D).