Metabolic bone disease is a major extra-intestinal manifestation of inflammatory bowel disease (IBD), with over half of patients having evidence of osteoporosis or osteopenia. The etiology of bone disease remains unclear. We have recently identified caspase-1 mediated intestinal epithelial cell (IEC) pyroptosis as a likely candidate for the cause of mucosal barrier dysfunction resulting in chronic intestinal inflammation. The metabolic bone disease in the interleukin-10 deficient (IL-10 KO) rodent model of IBD is well established.Aim:
Examine the role of mucosal barrier dysfunction and IL-1β secretion in bone disease. We hypothesize that IL-1β secretion resulting from IEC pyroptosis plays an important role in the pathogenesis of bone disease in IBD.Methods:
A time-course experiment was performed on IL-10 KO mice with age-matched control C57BL/6 mice (aged 5, 6, and 7 months) to examine the correlation between bone disease, barrier dysfunction (as measured by activated capapse-1 expression), IL-1β activity, and colitis over time. Additionally, WT mice were injected with IL-1β to examine whether bone disease can be induced by IL-1β. Double caspase-1 KO and IL-10 KO mice were generated to determine the absence of capase-1 activity on bone disease in the IL-10 KO mice. Micro-CT of IL-10 KO, Casp1/IL-10 KO, and background C57BL/6 femurs and vertebrae was performed to determine the bone density independently of body size. Intestinal mucosal scrapings were analyzed for caspase-1 and IL-1β expression. Histological scores of colitis were used to assess the degree of mucosal inflammation.Results:
IL-10 KO mice had decreased volumetric density (mg/cm3) of the cancellous bone of the lumbar vertebra and distal femur and of the cortical bone of the femoral midshaft (P < 0.02). The decrease was characterized by reduced cancellous bone volume and cortical thickness (P < 0.001). Increased activated caspase-1 (P < 0.05) and IL-1β (P < 0.001) levels in the intestine were observed in IL-10 KO mice in all age groups. The changes in IL-10 KO bone density of the cancellous bone of the lumbar vertebrae and femur over time correlated with increased IL-1β levels (P < 0.05) and inflammation score. WT mice injected with IL-1β had reduced volumetric bone density of the cancellous bone of the lumbar vertebrae and distal femur and reduced cortical thickness compared to vehicle-treated WT mice. Reduction in cancellous bone volume was observed in the absence of intestinal inflammation. In the Casp1/IL-10 double KO mice, increased bone density of both the lumbar vertebrae and distal femur were observed compared to IL-10 KO mice. IL-1β expression was reduced in Casp1/IL-10 double KO with bone density not fully reverting to WT levels.Conclusions:
Bone disease correlated with increased IL-1β activity and bone loss could be induced by IL-1β in the absence of intestinal inflammation. In the Casp1/IL-10 double KO mice, the changes found in the IL-10 KO animals were ameliorated. These data support the notion that IL-1β secreted by pyroptotic IECs contributes to the development of bone disease in IBD and results in reduced bone density.