Osteoprotegerin, s-RANKL, and selected interleukins in the pathology of bone metabolism in patients with Crohn's disease

Signaling pathways associated with bone loss in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder of the gastrointestinal tract characterized in many patients by extraintestinal manifestations. One of the most common comorbidities seen in IBD patients is a significant reduction in their bone mass. The pathogenesis of IBD is mainly attributed to the disrupted immune responses in the gastrointestinal mucosa and putative disruptions in the gut microbiomes. The excessive inflammation of the gastrointestinal tract activates different systems, such as the RANKL/RANK/OPG and the Wnt pathways linked with bone alterations in IBD patients, thereby suggesting a multifactorial etiology. The mechanism responsible for the reduced bone mineral density in IBD patients is thought to be multifactorial, and, so far, the principal pathophysiological pathway has not been well established. However, in recent years, many investigations have increased our understanding of the effect of gut inflammation on the systemic immune response and bone metabolism. Here, we review the main signaling pathways associated with altered bone metabolism in IBD.

Jianpi Qingchang Bushen decoction improves inflammatory response and metabolic bone disorder in inflammatory bowel disease-induced bone loss

BACKGROUND

Bone loss and osteoporosis are commonly described as extra-intestinal manifestations of inflammatory bowel disease (IBD). Jianpi Qingchang Bushen decoction (JQBD) is a prescription used in clinical practice. However, further studies are needed to determine whether JQBD regulates the receptor activator of nuclear factor kappa B (NF-κB) (RANK)/receptor activator of NF-κB ligand (RANKL)/ osteoprotegerin (OPG) pathways and could play a role in treating IBD-induced bone loss.

AIM

To evaluate the therapeutic effect of JQBD in IBD-induced bone loss and explore the underlying mechanisms.

METHODS

An IBD-induced bone loss model was constructed by feeding 12 6-to-8-wk-old interleukin-10 (IL-10)-knockout mice with piroxicam for 10 d. The mice were randomly divided into model and JQBD groups. We used wild-type mice as a control. The JQBD group was administered the JQBD suspension for 2 wk by gavage, while the control and model groups were given normal saline at the corresponding time points. All mice were killed after the intervention. The effect of JQBD on body weight, disease activity index (DAI), and colon length was analyzed. Histopathological examination, colon ultrastructure observation, and micro-computed tomographic scanning of the lumbar vertebrae were performed. The gene expression of NF-κB, tumor necrosis factor-α (TNF-α), IL-1β, IL-6, and IL-8 in the colon was evaluated by real-time polymerase chain reaction. Colon samples were assessed by Western blot for the expression of RANKL, OPG, RANK, and NF-κB proteins.

RESULTS

The model group lost body weight, had a shorter colon, and showed a dramatic increase in DAI score, whereas JQBD had protective and therapeutic effects. Treatment with JQBD significantly improved inflammatory cell infiltration and reduced crypt abscess and ulcer formation. Three-dimensional imaging of the vertebral centrum in the model group revealed a lower bone mass, loose trabeculae, and “rod-shaped” changes in the structure compared to the control group and JQBD groups. The bone volume/total volume ratio and bone mineral density were significantly lower in the model group than in the control group. JQBD intervention downregulated the NF-κB, TNF-α, IL-1β, IL-6, and IL-8 mRNA expression levels. The RANKL and OPG protein levels were also improved.

CONCLUSION

JQBD reduces inflammation of the colonic mucosa and inhibits activation of the RANK/ RANKL/OPG signaling pathway, thereby reducing osteoclast activation and bone resorption and improving bone metabolism.

Bone Fragility in Gastrointestinal Disorders

Osteoporosis is a common systemic disease of the skeleton, characterized by compromised bone mass and strength, consequently leading to an increased risk of fragility fractures. In women, the disease mainly occurs due to the menopausal fall in estrogen levels, leading to an imbalance between bone resorption and bone formation and, consequently, to bone loss and bone fragility. Moreover, osteoporosis may affect men and may occur as a sequela to different diseases or even to their treatments. Despite their wide prevalence in the general population, the skeletal implications of many gastrointestinal diseases have been poorly investigated and their potential contribution to bone fragility is often underestimated in clinical practice. However, proper functioning of the gastrointestinal system appears essential for the skeleton, allowing correct absorption of calcium, vitamins, or other nutrients relevant to bone, preserving the gastrointestinal barrier function, and maintaining an optimal endocrine-metabolic balance, so that it is very likely that most chronic diseases of the gastrointestinal tract, and even gastrointestinal dysbiosis, may have profound implications for bone health. In this manuscript, we provide an updated and critical revision of the role of major gastrointestinal disorders in the pathogenesis of osteoporosis and fragility fractures.

Impact of Cigarette Smoking on the Risk of Osteoporosis in Inflammatory Bowel Diseases

Cigarette smoking constitutes one of the most important modifiable factors of osteoporosis, as well as contributes to an early death, tumors, and numerous chronic diseases. The group with an increased risk of a lower bone mineral density are patients suffering from inflammatory bowel diseases. In fact, tobacco smoke, which contains more than 7000 chemical compounds, affects bone mineral density (BMD) both directly and indirectly, as it has an impact on the RANK-RANKL-OPG pathway, intestinal microbiota composition, and calcium-phosphate balance. Constant cigarette use interferes with the production of protective mucus and inhibits the repair processes in the intestinal mucus. Nicotine as well as the other compounds of the cigarette smoke are important risk factors of the inflammatory bowel disease and osteoporosis. Additionally, cigarette smoking may decrease BMD in the IBD patients. Interestingly, it affects patients with Crohn's disease and ulcerative colitis in different ways-on the one hand it protects against ulcerative colitis, whereas on the other it increases the risk of Crohn's disease development. Nevertheless, all patients should be encouraged to cease smoking in order to decrease the risk of developing other disorders.

Nutrients in the Prevention of Osteoporosis in Patients with Inflammatory Bowel Diseases

The chronic character of inflammatory bowel diseases, such as Crohn's disease and ulcerative colitis, results in various complications. One of them is osteoporosis, manifested by low bone mineral density, which leads to an increased risk of fractures. The aetiology of low bone mineral density is multifactorial and includes both diet and nutritional status. Calcium and vitamin D are the most often discussed nutrients with regard to bone mineral density. Moreover, vitamins A, K, C, B12; folic acid; calcium; phosphorus; magnesium; sodium; zinc; copper; and selenium are also involved in the formation of bone mass. Patients suffering from inflammatory bowel diseases frequently consume inadequate amounts of the aforementioned minerals and vitamins or their absorption is disturbed, resulting innutritional deficiency and an increased risk of osteoporosis. Thus, nutritional guidelines for inflammatory bowel disease patients should comprise information concerning the prevention of osteoporosis.