Relationships of hormones of adipose tissue and ghrelin to bone metabolism

Body adipose tissue influences bone metabolism through mechanical load, as well as via hormones released into circulation. Such hormones are adipocytokines - leptin, adiponectin, TNF-α, IL-6, resistin and visfatin. Some of them exert an osteoanabolic effect, while the others activate bone resorption. An increasingly discussed adipocytokine is leptin, which fundamental role is regulation of food intake ensuring survival of the organism during starvation. Leptin also stimulates osteoblasts and activates bone formation. The direct osteotropic effect of leptin is modulated by interaction with hypothalamic centers and neurohormones. Apparently, the most important leptin sensitive pathway involved in bone regulation is the beta-adrenergic system. While activation of β-1-adrenergic receptors by leptin enhances bone formation, activation of β-2-adrenergic receptors in hypothalamus and in the skeleton increases bone resorption. In humans, an anabolic effect on the skeleton prevails. In pubertal girls, leptin extensively released into circulation at the moment when adipose tissue reaches a critical volume, stimulates synthesis of GnRH and induces puberty, which is followed by striking increases in bone mass. Low leptin levels in anorexia nervosa are associated with amenorrhoea, which slows down increase of bone mass and may induce osteopenia. Important adipocytokine with an unambiguous negative effect on bone is adiponectin. Decreased production of this hormone explains in part the lower prevalence of osteoporosis in obese persons. In this article, the osteotropic importance of leptin-sensitive neurohormonal mechanisms and other hormones related to adipose tissue are discussed. Clinical importance of the above mentioned hormones to integrity of the skeleton has not yet been verified.

Keywords: adipose tissue; bone; leptin; adiponectin; β-adrenergic system; ghrelin; pubertal bone