Liver-derived IGF-I is permissive for ovariectomy-induced trabecular bone loss.

INTRODUCTION: Estrogen deficiency results in trabecular bone loss, associated with T-cell proliferation in the bone marrow. Insulin-like growth factor I (IGF-I) is involved in the regulation of both bone metabolism and lymphopoiesis. A major part of serum IGF-I is derived from the liver. The aim of the present study was to investigate the role of liver-derived IGF-I for ovariectomy (ovx)-induced trabecular bone loss. MATERIALS AND METHODS: Mice with adult liver-specific IGF-I inactivation (LI-IGF-I-/-) and wild type mice (WT) were either ovx or sham operated. After 5 weeks, the skeletal phenotype was analyzed by pQCT and microCT. The bone marrow cellularity was analyzed using FACS technique, and mRNA levels were quantified using real-time PCR. RESULTS: Ovx resulted in a pronounced reduction in trabecular bone mineral density (-52%, P < 0.001), number (-45%, P < 0.01) and thickness (-13%, P < 0.01) in WT mice while these bone parameters were unaffected by ovx in LI-IGF-I-/- mice. Furthermore, ovx increased the number of T-cells in the bone marrow of the femur in WT but not in LI-IGF-I-/- mice. Interleukin 7 (IL-7) has been reported to stimulate the formation and function of osteoclasts by inducing the expression of receptor activator of NF-kappaB ligand (RANKL) on T-cells. IL-7 mRNA levels and the RANKL/osteoprotegerin ratio in bone were increased by ovx in WT but not in LI-IGF-I-/- mice. CONCLUSIONS: Liver-derived IGF-I is permissive for ovx-induced trabecular bone loss. Our studies indicate that IGF-I might exert this permissive action by modulation of the number of T-cells and the expression of IL-7, which in turn is of importance for the RANKL/OPG ratio and consequently osteoclastogenesis in the bone marrow.

Androgens and the skeleton.

Loss of estrogens or androgens causes bone loss by increasing the rate of bone remodeling, and also causes an imbalance between resorption and formation by prolonging the lifespan of osteoclasts and shortening the lifespan of osteoblasts. Conversely, treatment with androgens, as well as estrogens, maintains cancellous bone mass and integrity, regardless of age or sex. Both androgens, via the androgen receptor (AR), and estrogens, via the estrogen receptors (ERs) can exert these effects, but the relative contribution of these 2 pathways remains uncertain. Androgens, like estrogens, stimulate endochondral bone formation at the start of puberty, whereas they induce epiphyseal closure at the end of puberty, thus, they have a biphasic effect. Androgen action on the growth plate is, however, clearly mediated via aromatization into estrogens and interaction with ER alpha. Androgens increase, while estrogens decrease radial growth. This differential effect of the sex steroids may be important because bone strength in males seems to be determined by higher periosteal bone formation and, therefore, greater bone dimensions. Experiments in mice suggest that both the AR and ER alpha pathways are involved in androgen action on radial bone growth. ER beta may mediate growth-limiting effects of estrogens in the female but does not seem to be involved in the regulation of bone size in males. In conclusion, androgens may protect men against osteoporosis via maintenance of cancellous bone mass and expansion of cortical bone. This androgen action on bone is mediated by the AR and ER alpha.

Estrogen receptor-beta inhibits skeletal growth and has the capacity to mediate growth plate fusion in female mice.

UNLABELLED: To determine the long-term role of ER beta in the regulation of longitudinal bone growth, appendicular and axial skeletal growth was followed and compared in female ER beta-/-, ER alpha-/-, and ER alpha-/- beta-/- mice. Our results show that ER beta inhibits appendicular and axial skeletal growth and has the capacity to induce fusion of the growth plates. INTRODUCTION: Estrogen affects skeletal growth and promotes growth plate fusion in humans. In rodents, the growth plates do not fuse after sexual maturation, but prolonged treatment with supraphysiological levels of estradiol has the capacity to fuse the growth plates. It should be emphasized that the estrogen receptor (ER) alpha-/- and the ER alpha-/- beta-/-, but not the ER beta-/-, mouse models have clearly increased serum levels of estradiol. MATERIALS AND METHODS: The skeletal growth was monitored by X-ray and dynamic histomorphometry, and the growth plates were analyzed by quantitative histology, calcein double labeling, bromodeoxyuridine (BrdU) incorporation, and TUNEL assay in 4- and 18-month-old female ER beta-/-, ER alpha-/-, and ER alpha-/- beta-/- mice. RESULTS: Young adult (4-month-old) ER beta-/- mice demonstrated an increased axial- and appendicular-skeletal growth, supporting the notion that ER beta inhibits skeletal growth in young adult female mice. Interestingly, the growth plates were consistently fused in the appendicular skeleton of 18-month-old female ER alpha-/- mice. This fusion of growth plates, caused by a prolonged exposure to supraphysiological levels of estradiol in female ER alpha-/- mice, must be mediated through ER beta because old ER alpah-/- beta-/- mice displayed unchanged, unfused growth plates. CONCLUSIONS: Our results confirm that ER beta is a physiological inhibitor of appendicular- and axial-skeletal growth in young adult female mice. Furthermore, we made the novel observation that ER beta, after prolonged supraphysiological estradiol exposure, has the capacity to mediate growth plate fusion in old female mice.

Raloxifene- and estradiol-mediated effects on uterus, bone and B lymphocytes in mice.

Raloxifene is a selective estrogen receptor modulator approved for the prevention of osteoporosis in postmenopausal women. It is selective by having estrogen-agonistic effects on bone, vessels and blood lipids while it is antagonistic on mammary and uterine tissue. Our aim was to study the agonistic and antagonistic properties of the raloxifene analogue LY117018 (LY) on uterus, bone, B lymphopoiesis and B cell function. Oophorectomized and sham-operated animals were treated with s.c. injections of equipotent anti-osteoporotic doses of 17beta-estradiol (E2) (0.1 mg/kg) or LY (3 mg/kg) or vehicle as controls. Effects on bone mineral density (BMD) were studied using peripheral quantitative computed tomography, uterine weight was examined, B lymphopoiesis was examined using flow cytometry and B cell function in bone marrow and spleen was studied by the use of an ELISPOT assay. E2 and LY had similar effects on BMD and bone marrow B lymphopoiesis, while LY had a clear antagonistic effect on endogenous estrogen in uterine tissue and no stimulating effect on the frequency of Ig-producing B cells in sham-operated animals. Our results are discussed in the context of estrogen receptor biology, relations between the immune system and bone metabolism and also with respect to the estrogen-mediated effects on rheumatic diseases.

Estrogen receptor specificity for the effects of estrogen in ovariectomized mice.

Estrogen exerts a variety of important physiological effects, which have been suggested to be mediated via the two known estrogen receptors (ERs), alpha and beta. Three-month-old ovariectomized mice, lacking one or both of the two estrogen receptors, were given estrogen subcutaneously (2.3 micro g/mouse per day) and the effects on different estrogen-responsive parameters, including skeletal effects, were studied. We found that estrogen increased the cortical bone dimensions in both wild-type (WT) and double ER knockout (DERKO) mice. DNA microarray analysis was performed to characterize this effect on cortical bone and it identified four genes that were regulated by estrogen in both WT and DERKO mice. The effect of estrogen on cortical bone in DERKO mice might either be due to remaining ERalpha activity or represent an ERalpha/ERbeta-independent effect. Other effects of estrogen, such as increased trabecular bone mineral density, thymic atrophy, fat reduction and increased uterine weight, were mainly ERalpha mediated.

Estrogen receptor alpha, but not estrogen receptor beta, is involved in the regulation of the OPG/RANKL (osteoprotegerin/receptor activator of NF-kappa B ligand) ratio and serum interleukin-6 in male mice.

Estrogens are important for the male skeleton. Osteoprotegerin (OPG), receptor activator of NF-kappa B ligand (RANKL), interleukin-6 (IL-6), IL-1 and tumor necrosis factor alpha (TNFalpha) have been suggested to be involved in the skeletal effects of estrogen. We treated orchidectomized mice with estradiol for 2 weeks and observed a 143% increase in the trabecular bone mineral density of the distal metaphysis of femur that was associated with a decreased OPG/RANKL mRNA ratio in vertebral bone. A similar decreased OPG/RANKL ratio was also seen after estrogen treatment of ovariectomized female mice. The effect of estrogen receptor (ER) inactivation on the OPG/RANKL ratio was dissected by using intact male mice lacking ER alpha (ERKO), ER beta (BERKO) or both receptors (DERKO). The expression of OPG was increased in ERKO and DERKO but not in BERKO male mice, resulting in an increased OPG/RANKL ratio. Furthermore, serum levels of IL-6 and tartrate-resistant acid phosphatase 5b (TRAP 5b) were decreased in ERKO and DERKO, but not in BERKO male mice. These results demonstrate that ER alpha, but not ER beta, is involved in the regulation of the vertebral OPG/RANKL ratio, serum levels of IL-6 and TRAP 5b in male mice.

Estrogen receptor specificity in the regulation of the skeleton in female mice.

There are two known estrogen receptors, estrogen receptor-alpha (ER alpha) and estrogen receptor-beta (ER beta), which may mediate the actions of estrogen. The aim of the present study was to compare fat content, skeletal growth and adult bone metabolism in female mice lacking ER alpha (ERKO), ER beta (BERKO) or both ERs (DERKO). We demonstrate that endogenous estrogens decrease the fat content in female mice via ER alpha and not ER beta. Interestingly, the longitudinal bone growth was decreased in ERKO, increased in BERKO, but was intermediate in DERKO females, demonstrating that ER alpha and ER beta exert opposing effects in the regulation of longitudinal bone growth. The effects on longitudinal bone growth were correlated with similar effects on serum levels of IGF-I. A complex regulation of the trabecular bone mineral density (BMD), probably caused by a disturbed feedback regulation of estrogen and testosterone, was observed in female ER-inactivated mice. Nevertheless, a partial functional redundancy for ER alpha and ER beta in the maintenance of the trabecular BMD was observed in the female mice at 60 days of age. Thus, ER alpha and ER beta may have separate effects (regulation of fat), opposing effects (longitudinal bone growth) or partial redundant effects (trabecular BMD at 60 days of age), depending on which parameter is studied.

Obesity and disturbed lipoprotein profile in estrogen receptor-alpha-deficient male mice.

Clinical case reports have documented disturbances of carbohydrate and lipid metabolism in aromatase deficient and estrogen resistant males. The aim of the present study was to explore the metabolic functions of estrogens in male mice and to dissect the estrogen receptor (ER) specificity of such effects. Total body fat content and serum levels of leptin were followed in ERalpha knockout (ERKO), ERbeta knockout (BERKO), and ERalpha/beta double knockout (DERKO) mice. Neither the total body fat nor serum leptin levels were altered in any group before or during sexual maturation. However, after sexual maturation ERKO and DERKO, but not BERKO, demonstrated a clear increase in total body fat and enhanced serum leptin levels. Serum cholesterol was increased and a qualitative change in the lipoprotein profile, including smaller LDL particles, was observed in ERKO and DERKO mice. In conclusion, ERalpha but not ERbeta-inactivated male mice develop obesity after sexual maturation.

Estrogen receptor specificity in the regulation of skeletal growth and maturation in male mice.

Androgens may regulate the male skeleton directly through a stimulation of androgen receptors or indirectly through aromatization of androgens into estrogen and, thereafter, through stimulation of estrogen receptors (ERs). The relative importance of ER subtypes in the regulation of the male skeleton was studied in ERalpha-knockout (ERKO), ERbeta-knockout (BERKO), and double ERalpha/beta-knockout (DERKO) mice. ERKO and DERKO, but not BERKO, demonstrated decreased longitudinal as well as radial skeletal growth associated with decreased serum levels of insulin-like growth factor I. Therefore, ERalpha, but not ERbeta, mediates important effects of estrogen in the skeleton of male mice during growth and maturation.