Effects of growth hormone on bone tissue in transgenic mice

Abstract

Growth hormone (GH) affect bone-tissue in both humans and rodents. Also sex steroids are important for growth and the maintenance of normal bone physiology. The specific aims of this study were to investigate how high levels of GH affects bone tissue in mice and to determine whether these effects are dependent on an intact gonadal function.The effects of sex steroids on growth and bone tissues not mediated by an altered GH secretion was studied. Bone metabolism is an interaction between bone-formation by osteoblasts (ob) and bone-resorption by osteoclasts (oc). Immature oc can be activated by IL-6. In vitro studies have shown that GH induces IL-6 release from ob. In vivo studies aiming to investigate whether the effects of GH on bone metabolism are dependent on endogenous presence of IL-6 were done. Furthermore, studies to reveal if elevated GH levels can enhance bone-formation to stimulate integration of titanium-implants were performed.The studies are based on transgenic mice models. Metallothionein promoter regulated bovine-GH (bGH) transgenic mice have a high expression of bGH not regulated by endogenous sex steroids. When gonadectomizing these mice effects not induced by altered GH secretion could be studied. Detailed bone analyzes were performed in vivo on tibial and vertebral bone using X-ray technique. Bone compartment measurements combined with volume determinations, dual energy X-ray absorptiometry and peripheral quantitative computer tomography were done ex vivo. The same methods were used in IL-6 knockout mice (IL-6 -/-) treated with GH. Titanium implants were inserted in the forehead of bGH transgenic mice and after four months they were cut out and analyzed using histomorphometry.Elevated levels of GH result in an increased and proportional skeletal growth. GH increases the amount of bone in tibia (females) and in vertebra (males and females). An intact gonadal function was needed to achieve full expression of the GH effects on bone mass. In gonadectomized peripubertal male mice the lack of testis resulted in reduced weight-gain and a disproportional growth with stunted lumbal vertebral length increase. During the early adult period, the lack of testis function increases tibial growth and altered the epiphyseal growth-plate width. These effects were not due to altered GH secretion shown by similar effects in the transgenic animals. The growth of GH treated IL-6 -/- mice were unaltered compared to treated control littermates. The size and mineralization of vertebrae were also increased by GH treatment independent of endogenous IL-6. High systemic levels of GH increased the bone-metal contact of titanium-implants independent of bone thickness.Conclusions: In mice, GH stimulates a proportional bone growth and enhances the amount of tibial and vertebral bone. To achieve full stimulatory effect of elevated levels of GH intact gonadal function is needed in both male and female mice. Testes enhance peripubertal growth of lumbar spine and inhibit tibial growth of young adult mice in a GH-secretion independent manner. GH does not require IL-6 to promote bone growth. Elevated GH levels can enhance titanium osseous-integration.