The Paradoxical Role of Uric Acid in Osteoporosis

Because of its high prevalence worldwide, osteoporosis is considered a serious public health concern. Many known risk factors for developing osteoporosis have been identified and are crucial if planning health care needs. Recently, an association between uric acid (UA) and bone fractures had been explored. Extracellular UA exhibits antioxidant properties by effectively scavenging free radicals in human plasma, but this benefit might be disturbed by the hydrophobic lipid layer of the cell membrane. In contrast, intracellular free oxygen radicals are produced during UA degradation, and superoxide is further enhanced by interacting with NADPH oxidase. This intracellular oxidative stress, together with inflammatory cytokines induced by UA, stimulates osteoclast bone resorption and inhibits osteoblast bone formation. UA also inhibits vitamin D production and thereby results in hyper-parathyroidism, which causes less UA excretion in the intestines and renal proximal tubules by inhibiting the urate transporter ATP-binding cassette subfamily G member 2 (ABCG2). At normal or high levels, UA is associated with a reduction in bone mineral density and protects against bone fracture. However, in hyperuricemia or gout arthritis, UA increases bone fracture risk because oxidative stress and inflammatory cytokines can increase bone resorption and decrease bone formation. Vitamin D deficiency, and consequent secondary hyperparathyroidism, can further increase bone resorption and aggravated bone loss in UA-induced osteoporosis.

Bone loss in inflammatory disorders

Chronic inflammatory diseases of almost any cause are associated with bone loss. Bone loss is due to direct effects of inflammation, poor nutrition, reduced lean body mass, immobility and the effects of treatments, especially glucocorticoids. These mechanisms are complex and interrelated but are ultimately mediated through effects on the bone remodelling cycle. Inflammatory disease can increase bone resorption, decrease bone formation but most commonly impacts on both of these processes resulting in an uncoupling of bone formation from resorption in favour of excess resorption. This review will illustrate these interactions between inflammation and bone metabolism and discuss how these are, and might be, manipulated as therapies for inflammation related bone loss.

Glucocorticoid-Induced osteoporosis: clinical and therapeutic aspects

Glucocorticoid-induced osteoporosis (GIO) is the most common form of secondary osteoporosis. Fractures, which are often asymptomatic, may occur in as many as 30_50% of patients receiving chronic glucocorticoid therapy. Vertebral fractures occur early after exposure to glucocorticoids, at a time when bone mineral density (BMD) declines rapidly. Fractures tend to occur at higher BMD levels than in women with postmenopausal osteoporosis. Glucocorticoids have direct and indirect effects on the skeleton. They impair the replication, differentiation, and function of osteoblasts and induce the apoptosis of mature osteoblasts and osteocytes. These effects lead to a suppression of bone formation, a central feature in the pathogenesis of GIO. Glucocorticoids also favor osteoclastogenesis and as a consequence increase bone resorption. Bisphosphonates are the most effective of the various therapies that have been assessed for the management of GIO. Anabolic therapeutic strategies are under investigation. Teriparatide seems to be also efficacious for the treatment of patients with GIO.

Histology, Pathogenesis, and Treatment of Cholesteatoma

Factors present in cholesteatoma connective tissue induce bone resorption in chronic otitis media. The presence of skin and its products appears to exacerbate the destructive influence of connective tissue. One exacerbating factor is pressure, shown in an animal model to increase bone resorption by influencing the subepithelial connective tissue. Consideration of pathogenic factors provides a rationale for modifications in the treatment of middle ear cholesteatoma.