Effects of growth hormone and steroids on human osteoblast-like cells

Abstract

Bone tissue is a metabolic active organ and is continuously remodulated. The regulation of bone resorption, bone formation and interactions between different hormones and cytokines in human osteoblasts is not completely understood. Growth hormone (GH) is important in determining final body height and for normal bone physiology. High levels of cortisol result in osteoporosis, while estrogen has a protective effect on bone mass. Interleukin-6 (IL-6) and interleukin-1 (IL-1) are two cytokines which are believed to be of importance for the local regulation of bone remodeling. This thesis investigates the effects of GH, estrogen and cortisol and their interactions with each other and with interleukins in vitro in primary isolated human osteoblast-like (hOB) cells. The specific aims were to investigate if hOB cells express functional GH receptors, if GH regulates the expression of IL-6, if estrogen and/or cortisol regulate the expression of GH receptors and GH-responsivness and, finally, if cortisol regulates the expression of insulin-like growth factor I (IGF-I), IL-6, or IL-1b in hOB cells.It was demonstrated that hOB cells express the GH receptor transcript. Binding studies showed that the transcript is translated and that the cells have approximately 2000 GH-binding sites/cell. GH-stimulation resulted in increased thymidine incorporation, demonstrating that the GH receptors are functional. High levels of IL-6 were expressed by hOB cells, and the expression was increased with GH. It is speculated whether GH-induced bone resorption is indirectly mediated via increased IL-6 production and/or if GH via autocrine produced IL-6 exerts an anabolic effect on hOB cells. Under specific culture conditions, neither GH nor estrogen alone had any effect on osteoblast proliferation, but they induced cell proliferation if given together. Estrogen increased GH receptor expression in osteoblasts. Thus, it is speculated that estrogen enhances GH-activity via an estrogen-induced increase in GH receptor expression. Furthermore, cortisol increased GH receptor mRNA levels and GH-binding. Thus, the mechanism for cortisol-induced GH-insensitivity in osteoblasts is not via decreased GH receptor expression. It could be speculated that the inhibitory effect of high levels of cortisol on GH-responsivness could be a post-receptor effect. HOB cells express IGF-I and IGF-II mRNA transcripts. Cortisol decreases IGF-I mRNA levels in hOB cells. This finding may explain the mechanism of the negativ effect of cortisol on bone formation. Other effects of cortisol on hOB cells were decreased IL-6 and IL-1 expression. IL-6 and IL-1 are believed to participate in bone resorption. Therefore, it is difficult to explain if, or how cortisol-induced bone resorption could be mediated through reduced, IL-6 and IL-1 expression. However, there are indications that these interleukins exert autocrine anabolic effects on hOB cells and this anabolic effect might be reduced by high levels of cortisol.This thesis contributes to the increased knowledge of the interplay between GH, estrogen, cortisol, IGF-I/II and cytokines in bone physiology. Increased knowledge about basal mechanisms in bone physiology is important for the development of new treatment strategies for patients with osteoporosis and other metabolic bone diseases.