The expression of proinflammatory cytokine and proinsulin by bone marrow-derived mesenchymal cells for fracture healing in long term diabetic mice

Background: Diabetes mellitus (DM) causes bone dysfunction due to poor bone quality and leads to severe deterioration of quality of life. The mechanisms of bone metabolism in DM remain unclear, although chemical and/or mechanical factors are known to disrupt the homeostasis of osteoblasts and osteoclasts. The purpose of this study was to identify the biochemical characteristics of osteoblasts and osteoclasts, using a mouse fracture model of long-term hyperglycemia (LT-HG). Methods: C57BL/6J mice and green fluorescent protein (GFP)-positive bone marrow transplanted C57BL/6J mice with LT-HG in which hyperglycemia was maintained for 2 months were used in this study. After the experimental fracture, we examined the immunohistochemical expression of proinsulin and tumor necrosis factor (TNF) -α at the fracture site. C57BL/6J fracture model mice without hyperglycemia were used as the control sample. Results: In the LT-HG mice, osteoblasts showed an irregular arrangement at the fracture site. The osteoclasts were scattered with a decrement in the number of nuclei. The positive expression of proinsulin was seen in mesenchymal stem cells (MSCs) with neovascularization 2 and 3 weeks after fracture. Immunopositivity for TNF-α was seen in immature chondrocytes or MSCs with neovascularization at 2 weeks, and the number of positive cells was not decreased at 3 weeks. Examination of GFP-grafted hyperglycemic mice showed that the majority of cells at the fracture site were GFP-positive. Immunohistochemistry showed that the rate of double positives was 15% for GFP and proinsulin and 47% for GFP and TNF-α. Conclusion: LT-HG induced an increase in the number of proinsulin and TNF-α positive cells derived from systemic bone marrow cells. The proinsulin and TNF-α positive cells cause both bone formation and bone resorption, and they suppress inflammatory cytokines and impair glucose metabolism.