Bone and hypopituitarism: not only a mass issue
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EDITORIAL
Bone and hypopituitarism: not only a mass issue Urszula T. Iwaniec
Published online: 29 June 2012 Ó Springer Science+Business Media, LLC 2012
Growth hormone (GH) is an important physiological regulator of bone growth and turnover [1]. Osteoblasts and chondrocytes express receptors for GH, and GH has direct actions on differentiation and proliferation of these cells in primary cell culture. Congenital GH deficiency in humans and laboratory animals results in decreased bone growth and osteopenia. GH levels decrease with age, and this decline may contribute to common metabolic bone diseases, including postmenopausal and senile osteoporosis. Hypophysectomy (hypox) in growing rats results in growth failure, hypogonadism, decreased white adipose tissue mass, and hypoleptinemia. Serum IGF-I levels plummet, as do mRNA levels for IGF-I in liver and bone. Longitudinal bone growth, periosteal bone apposition, and cancellous bone turnover are also drastically decreased. The volume fraction of cancellous bone falls, indicating that the magnitude of bone resorption exceeds formation. Concurrently, there is marked infiltration of bone marrow with fat. Hypox in rats also leads to low circulating levels of important gonadal, thyroid, adipocyte-derived, and adrenal hormones that influence energy metabolism and bone metabolism. GH replacement reinitiates weight gain, normalizes bone marrow adiposity, and restores near normal bone growth and turnover in the hypox animal. In contrast, administration of estradiol, IGF-I, thyroxine or cortisol to female hypox rats fails to reverse the effects of hypox on body weight gain and bone growth and turnover [2, 3]. Taken together, these findings emphasize the
U. T. Iwaniec (&) Skeletal Biology Laboratory, School of Biological and Population Health Sciences, Oregon State University, 108 Milam Hall, Corvallis, OR 97331, USA e-mail: [email protected]
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importance of GH deficiency in mediating the skeletal abnormalities associated with pituitary ablation. While the actions of pituitary hormones on bone growth and turnover have been extensively investigated, less is known regarding their actions on bone quality. In this issue of Endocrine, Bozzini et al. [4] report on the effects of pituitary ablation on biomechanical properties of cortical bone in rats. To circumvent differences in body size associated with GH deficiency, Bozzini et al. evaluated bone in rats of similar size but differing in age under the premise that animals of similar size would be exposed to similar load-induced strains allowing for determination of pituitary hormone deficiency on bone quality. Dramatic differences in extrinsic and intrinsic femoral biomechanical properties were observed between the control and hypox animals. Based on their results, the authors conclude: (1) that femurs from 1-year-old hypox rats (139 ± 8 g body weight) are ‘‘overdesigned,’’ being stronger and stiffer than femurs from the much younger control rats of similar body weight (138 ± 7 g), and (2) that the observed differen
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