ABSTRACT
Although GH is known to regulate somatic growth during development, its role in regulating adult body composition is less well defined. The effects of GH on individual body compartments – water, fat, protein and mineral – are achieved both by the action of GH and by a GH-induced hormone, insulin-like growth factor-I (IGF-I). We used a genetic model of GH deficiency, the 'little' (gene symbol lit) mouse, to determine the GH regulation of IGF-I and its insulin-like growth factor-binding proteins (IGFBPs) and to define the interaction between these hormones and each body compartment in adults.
Our results showed that GH-deficient lit/lit mice had reduced levels of serum IGF-I (range 38–130 μg/l) compared with normal lit/+ littermates (range 432–567 μg/l) between 2 and 52 weeks of age. The lit/lit mice did not experience the fivefold increase in IGF-I between 2 and 4 weeks of age that was seen in lit/+ mice. In lit/lit serum, overall binding of 125I-labelled IGF-I to the four IGFBPs was reduced, solely in response to a reduced amount of IGFBP-3. No overall differences were found between lit/lit and lit/+ mice in the binding of 125I-labelled IGF-I to IGFBP-2, -1 or -4. Age-related declines in IGF-I and IGFBPs were seen in lit/lit mice. However, adult levels of IGF-I were maintained in lit/+ mice to at least 52 weeks of age, as were levels of IGFBP-1 and -4, while IGFBP-3 and -2 declined with age.
With respect to body composition, comparison of lit/lit with lit/+ mice showed that the lit/lit mice were characterized by abnormally large adipose tissue stores and reduced body water, protein and mineral from 2 weeks onward. These changes occurred despite normal energy intake in lit/lit mice up to 52 weeks of age, indicating that neither undernutrition nor hyperphagia is characteristic of this GH-induced model of obesity. Furthermore, lit/lit males accrued more body fat beginning at an earlier age than lit/lit females. With advancing age, the per cent body fat increased in both lit/lit and lit/+ mice, while the per cent body water and mineral declined. In lit/lit but not lit/+ mice, per cent protein also declined with age. The changes in body water and fat are attributable to lack of adequate GH in the genetically GH-deficient lit/lit mouse. On the other hand, the changes in body protein are more likely to be effects of IGF-I. Changes in mineral observed in lit/lit mice could be the result of action by GH, IGF-I or both hormones. Therefore, when GH is chronically manipulated by GH deficiency as in lit/lit mice, by GH excess as in acromegaly, or by GH therapy, all four body compartments are affected, suggesting that GH therapy is most valuable when the treatment goal is to alter overall body composition.
Journal of Endocrinology (1993) 136, 91–104
Comment on Association between Insulin-Like Growth Factor-I (IGF-I) and Bone Mineral Density: Further Evidence Linking IGF-I to Breast Cancer Risk