Hormone replacement therapy improves distal radius bone structure by endocortical mineral deposition

Hormone replacement therapy (HRT) produces a small increase in bone mineral density (BMD) when measured by dual energy X-ray absorptiometry (DXA). The corresponding decrease in fracture risk is more impressive, implying that other factors that contribute to bone strength are favourably modified by HRT. We investigated, using peripheral quantitated computed tomography (pQCT), the changes produced by HRT in both the distribution of mineral between cortical and trabecular bone and the changes produced by HRT in the apparent structure of trabecular bone, expressed as average hole area and apparent connectivity. Twenty-one postmenopausal women starting HRT and 32 control women were followed for 2 years, with distal radius pQCT measurements every 6 months. HRT prevented the loss of total bone mass seen in controls (p < 0.02). HRT also produced an apparent rapid loss of trabecular bone mass within the first 6 months of the study (p < 0.02), with an associated rapid loss in the apparent connectivity (p = 0.034). Average hole area also increased but not to a statistically significant extent. Exogenous estrogen apparently fills small marrow pores close to the endocortical surface, such that the pQCT-defined boundary between trabecular and cortical bone is shifted in favour of cortical bone. Trabecular bone structure indices are adversely affected, as the central, poorly interconnected trabecular bone with greater than average marrow spaces constitutes a greater fraction of the remaining trabecular bone. This study suggests that the improvements in fracture risk resulting from HRT are explained by a reversal of net endocortical resorption of bone.     

Change of bone mass in postmenopausal Caucasian women with and without hormone replacement therapy is associated with vitamin D receptor and estrogen receptor genotypes

Our purpose is to assess whether genotypes of the vitamin D receptor (VDR) and estrogen receptor (ER) and their interaction influence changes in bone mass in postmenopausal Caucasian women with and without hormone replacement therapy (HRT). A population of 108 US Mid-West women who participated in a study of low-dose continuous estrogen/progestin was genotyped at the VDR BsmI site and the ER XbaI and PvuII sites. Adequate vitamin D and calcium nutritional intakes were assured in all the study subjects. For the 3.5-year duration of the study, we analyzed changes in bone mineral density (BMD) at the spine, femoral neck, distal radius, and the total body (total body bone mineral content, tbBMC). We adjusted for confounding factors, such as age and weight, in the analysis. We found that VDR and/or ER genotypes and/or their interaction generally had significant effects on the changes in the bone mass measurements in both the placebo and HRT groups. When a significant gene-by-gene interaction exists between VDR and ER genotypes, failure to account for them in analyses may yield nonsignificant results, even if significant genotypic effects exist. The amount of variation in changes in bone mass measurements explained by the total genotypic effects of the VDR and ER loci varies from ∼1.0% (for the tbBMC changes in combined placebo and HRT groups) to ∼18.7% (for the spine BMD changes in the HRT group). These results suggest that individual genotypes are important factors in determining changes in bone mass in the elderly with and without HRT and thus may need to be considered with respect to the treatment to preserve bone mass in elderly Caucasian women. Received: 31 July 1998 / Accepted: 11 September 1998     

Antiestrogenic activities of Ginkgo biloba extracts

Most climacteric and postmenopausal women appear to have vasomotor symptoms as well as a high risk of osteoporosis and cardiovascular disease. Although exogenous estrogens can reduce these symptoms, women are reluctant to use hormone replacement therapy (HRT) due to its undesirable side effects, such as irregular bleeding and an increased risk of breast cancer. A previous study suggested that Ginkgo biloba extracts (GBE) have estrogenic activity and might be suitable as an alternative to HRT. However, there are no reports of the preventive effect of GBE on breast cancer, which is the side effect of classical HRT. In this study, it was confirmed that GBE exhibits estrogenic and antiestrogenic activity depending on the E₂ and GBE concentration, via estrogen receptor (ER)-dependent and ER-independent pathways. In addition, GBE reduced the E₂ levels by stimulating the E₂ metabolism and inhibiting E₂ synthesis, which indicates that GBE can induce antiestrogenic activity via the depletion of E₂. Furthermore, GBE might have similar action to selective arylhydrocarbon receptor modulators (SAhRMs), which induce antiestrogenic activity through cross-talk between the arylhydrocarbon receptor (AhR) and ER. In conclusion, GBE has a biphasic effect on estrogen, and can be considered as a potential alternative to HRT with chemopreventive effects on breast cancer. However, further studies on animals and humans will be required.     

Genetics of osteoporosis: role of steroid hormone receptor gene polymorphisms

Osteoporosis is a common skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue with a consequent increase in bone fragility and susceptibility to fracture. In the past years, twin and family study have shown that this disease recognizes a strong genetic component and that genetic factors play an important role in regulating bone mineral density (BMD). While in few isolate conditions osteoporosis can be inherited in a simple Mendelian pattern, due to single gene mutations, in the majority of cases has to be considered a multifactorial polygenic disease in which genetic determinants are modulated by hormonal, environmental and nutritional factors. Given the important role that steroid hormones play in bone cell development and in the maintenance of normal bone architecture, polymorphisms at receptor of the steroid/thyroid hormone receptor superfamily, such as estrogen receptor alpha (ERalpha) and Vitamin D receptor (VDR) have been thoroughly investigated in the last years and appeared to represent important candidate genes. The individual contribution of these genetic polymorphisms to the pathogenesis of osteoporosis remains to be universally confirmed and an important aim in future work will be to define their functional molecular consequences and how these polymorphisms interact with each other and with the environment to cause the osteoporotic phenotype. A further promising application of genetic studies in osteoporosis comes from their pharmacogenomic implications, with the possibility to give a better guidance for therapeutic agents commonly used to treat this invalidating disorder or to identify target molecules for new therapeutic agents.     

The risks and benefits of HRT

For many years hormone replacement therapy (HRT) was regarded as the gold standard for treatment of osteoporosis. In recent years this status has been challenged, because of the lack of a robust evidence base for anti-fracture efficacy, emerging evidence of adverse extraskeletal effects and the demonstrated efficacy of a number of alternative options in the prevention of osteoporotic fractures. The current consensus is that HRT is no longer regarded as a front-line option for prevention of osteoporotic fractures and that its use for this purpose should be restricted to women with osteoporosis who have menopausal symptoms and to older women who are intolerant of other therapies and/or express a strong preference for HRT despite being informed about potential adverse effects. Nevertheless, the mechanisms by which estrogen exerts its beneficial skeletal effects remain a major area of research that has important implications for the development of novel therapies.     

Characterization of the Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin: use of chemical crosslinking and a monoclonal antibody directed against a 59-kDa protein associated with steroid receptors

The Ah receptor regulates induction of cytochrome P450IA1 (aryl hydrocarbon hydroxylase) by "3-methylcholanthrene-type" compounds and mediates the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons. Hepatic Ah receptor from untreated rodents is localized in the cytosol and has an apparent molecular mass of 250 to 300 kDa. This large form can be dissociated into a smaller ligand-binding subunit upon exposure to high ionic strength. The Ah receptor displays many structural similarities to the receptors for steroid hormones. Two non-ligand-binding proteins have been identified to be associated with the cytosolic forms of the steroid hormone receptors. The first is a 90-kDa heat shock protein (hsp 90); the second is a 59-kDa protein (p59) of unknown function. The cytosolic Ah receptor ligand-binding subunit previously has been shown to be associated with hsp 90. In the present study, we used a monoclonal antibody, KN 382/EC1, generated against the 59-kDa protein which is associated with rabbit steroid receptors to determine if p59 also is a component of the large cytosolic Ah receptor complex. Cytosolic forms of rabbit progesterone receptor, glucocorticoid receptor, and Ah receptor were analyzed by velocity sedimentation on sucrose gradients under low-ionic-strength conditions and in the presence of molybdate. Progesterone receptor from rabbit uterine cytosol and glucocorticoid receptor from rabbit liver each had a sedimentation coefficient of approximately 9 S. In the presence of KN 382/EC1 antibody the progesterone receptor and the glucocorticoid receptor both underwent a shift in sedimentation to a value of approximately 11 S. The increase in sedimentation velocity is an indication that the receptor-protein complexes are interacting with the antibody. Under low-ionic-strength conditions the Ah receptors from rabbit uterine cytosol and liver cytosol had a sedimentation coefficient of approximately 9 S. However, in contrast to the steroid receptors, the Ah receptor showed no change in its sedimentation properties in either tissue in the presence of KN 382/EC1, indicating that the antibody is not interacting with the Ah receptor. Multimeric Ah receptor complexes that were chemically crosslinked still did not show any interaction with KN 382/EC1. These data indicate that the 59-kDa protein either is not associated with the Ah receptor or is present in an altered form which the antibody cannot recognize.     

Measuring bone mass in children: can we really do it?

Bone densitometry is used to assess skeletal health in clinical and research settings, with the goal of achieving reproducible measurements of bone mass that help to identify individuals predisposed to fracture. The search is now on for better methods of capturing additional factors that contribute to bone strength, including bone size, geometry, microarchitecture, and turnover rates. This has proved particularly challenging in growing children, whose bones continually change in size, shape, and mass. Dual energy X-ray absorptiometry is the preferred method for measuring bone mass in children, but the technique has several limitations, and interpreting the findings can be problematic. Peripheral quantitative computed tomography is a promising method for assessing bone mass and other indices correlating with bone strength, but a lack of precision and paediatric norms currently restricts its clinical utility. Although bone mineral density is predictive of future fracture risk in adults, the evidence in children is less conclusive, and a diagnosis of osteoporosis in a child should not be made on densitometric findings alone. Developing a clearer understanding of how measures of bone mass and strength correlate with bone fracture in children will help target preventive strategies for those in greatest need.     

Adrenergic control of bone remodeling and its implications for the treatment of osteoporosis

Evidence that leptin regulates bone turnover in part through a central nervous system (CNS)/beta-adrenergic system relay has driven attention towards the potential therapeutic benefits of beta-adrenergic blockade to improve bone mass and strength. beta2- adrenergic receptor-mediated signaling in osteoblasts inhibits bone formation and triggers RANKL-mediated osteoclastogenesis and bone resorption. Mouse models of adrenergic-deficiency, particularly the mouse lacking the beta2-adrenergic receptor, have increased bone mass, more specifically increased trabecular bone volume. In turn, beta-blockers, such as propranolol, were reported to inhibit ovariectomy-induced bone loss. In contrast, a number of experiments in mice and rats suggest that inhibition of beta-adrenergic receptor-mediated signaling does not improve, and could actually be detrimental, for bone mass and microstructure. In humans, epidemiological observations suggested that users of beta-blockers have higher bone mineral density (BMD) and/or a reduced risk of fractures, yet not all studies were concordant. Here we review the evidence for a role of the adrenergic system in the regulation of bone metabolism in vitro and in vivo and provide some new evidence for a dual role of beta-adrenergic receptors 1 and 2 on bone turnover. Furthermore, we will examine the similarities and disparities that may exist in the effects of beta-adrenergic and PTH stimulation on bone metabolism.     

New advances in osteoporosis nutritional prevention

With the human race experiencing a progressive increase in life expectancy, we are facing a growing prevalence of chronic age-related conditions, among which osteoporosis is a generalised condition of bone, whose hallmark is increased bone fragility. Based on the overwhelming body of evidence emphasising that gonadal failure at the time of menopause causes osteopenia and the administration of estrogens in postmenopausal women prevents this loss, hormone replacement therapy (HRT) has been widely recommended. However HRT should be limited due to its potential adverse effects. In this light, an effort should be made to integrate alternative therapies of proven values to provide new options for women in midlife. Research in nutrition over the past 30 years has led to exciting and significant progress. Although the primary role of diet is to provide sufficient nutrients to fulfill the metabolic requirements of an individual, there is an emerging rationale to support the hypothesis that, by modulating specific target functions in the body, diet can help to achieve optimal health and also play an important role in reducing the risk of disease. Specifically, it has been recognized that human diet contains, in addition to essential macro- and micronutrients, a complex array of naturally occurring bioactive molecules, the phytochemicals, that may confer significant long-term health benefits. Indeed, besides calcium, micronutrients such as vitamins, polyphenols, phytoestrogens, trace elements or minerals remain a source for putative new and innovative dietary health intervention in the nutritional prevention of osteoporosis.     

The effect of exercise and nutrition on the mechanostat

In this review, we discuss the effect of increased and decreased loading and nutrition deficiency on muscle and bone mass and strength (and bone length and architecture) independently and combined. Both exercise and nutrition are integral components of the mechanostat model but both have distinctly different roles. Mechanical strain imparted by muscle action is responsible for the development of the external size and shape of the bone and subsequently the bone strength. In contrast, immobilization during growth results in reduced growth in bone length and a loss of bone strength due to large losses in bone mass (a result of endosteal resorption in cortical bone and trabecular thinning) and changes in geometry (bone shafts do not develop their characteristic shape but rather develop a rounded default shape). The use of surrogate measures for peak muscle forces acting on bone (muscle strength, size, or mass) limits our ability to confirm a cause-and-effect relationship between peak muscle force acting on bone and changes in bone strength. However, the examples presented in this review support the notion that under adequate nutrition, exercise has the potential to increase peak muscle forces acting on bone and thus can lead to a proportional increase in bone strength. In contrast, nutrition alone does not influence muscle or bone in a dose-dependent manner. Muscle and bone are only influenced when there is nutritional deficiency--and in this case the effect is profound. Similar to immobilization, the immediate effect of malnutrition is a reduction in longitudinal growth. More specifically, protein and energy malnutrition results in massive bone loss due to endosteal resorption in cortical bone and trabecular thinning. Unlike loading however, there is indirect evidence that severe malnutrition when associated with menstrual dysfunction can shift the mechanostat set point upward, thus leading to less bone accrual for a given amount of bone strain.     

Tissue specificity of 8-prenylnaringenin: protection from ovariectomy induced bone loss with minimal trophic effects on the uterus

Plant secondary metabolites with estrogenic activity (phyto-estrogens) have been studied in the past as a potential alternative to classical hormone-replacement therapy (HRT) in menopausal women. No final verdict on the efficacy of soy or red clover based pharmaceutical preparations has been reached despite numerous clinical studies. We have studied the novel and most potent phyto-estrogen 8-prenylnaringenin (8-PN) in adult ovariectomized rats, an established animal model to mimic hormone dependent osteoporosis in menopausal women. Our results demonstrate that 8-PN can completely protect from ovariectomy induced bone-loss while exhibiting minimal, (dose independent) trophic effects on uterus and endometrium. It is estimated that at equivalent bone protective doses of 17beta-estradiol and 8-PN, the phyto-estrogen has a 10-fold lower stimulatory effect on uterus and endometrium. The bone tissue specific effect of 8-PN was confirmed in a transgenic reporter mouse model (ERE-Luc mice). Here we also found pronounced estrogenic activity in prostate. Present results add important aspects to the pharmacological profile of 8-PN and position this compound as an interesting alternative new candidate for treatment of peri- and postmenopausal symptoms.     

Can exercise prevent osteoporosis?

Commonly used definitions of osteoporosis rely upon the measurement of bone mass or bone mineral density and regard the difference between osteopenia and osteoporosis as gradual. An alternative definition has been proposed by Harold Frost, suggesting that osteopenia is the bone's physiological response to disuse. On the contrary, true osteoporoses imply the bone's inability to adapt to the loads imposed on them by their habitual mechanical usage. As a consequence, fractures occur with no or very little trauma in osteoporotic, but not in osteopenic bones. There is now ample evidence that mechanical stimuli can increase strength. Accordingly, exercise, in particular some new forms of it that involve high strain rates, seems to be preventing bone loss and possibly also induces increases in bone mass even at older ages. Hence, exercise may ameliorate osteopenia in the sense of Frost's definition. However, exercise must be feared to facilitate rather than to ameliorate the occurrence of true osteoporoses, e.g., due to microdamage accumulation. This is in sharp contrast to the general 'understanding'.     

Receptor Activator of Nuclear Factor κB Ligand (RANKL) Protein Expression by B Lymphocytes Contributes to Ovariectomy-induced Bone Loss

Production of the cytokine receptor activator of NFκB ligand (RANKL) by lymphocytes has been proposed as a mechanism by which sex steroid deficiency causes bone loss. However, there have been no studies that functionally link RANKL expression in lymphocytes with bone loss in this condition. Herein, we examined whether RANKL expression in either B or T lymphocytes contributes to ovariectomy-induced bone loss in mice. Mice harboring a conditional RANKL allele were crossed with CD19-Cre or Lck-Cre mice to delete RANKL in B or T lymphocytes, respectively. Deletion of RANKL from either cell type had no impact on bone mass in estrogen-replete mice up to 7 months of age. However, mice lacking RANKL in B lymphocytes were partially protected from the bone loss caused by ovariectomy. This protection occurred in cancellous, but not cortical, bone and was associated with a failure to increase osteoclast numbers in the conditional knock-out mice. Deletion of RANKL from T lymphocytes had no impact on ovariectomy-induced bone loss. These results demonstrate that lymphocyte RANKL is not involved in basal bone remodeling, but B cell RANKL does contribute to the increase in osteoclasts and cancellous bone loss that occurs after loss of estrogen.     

Disordered osteoclast formation in RAGE-deficient mouse establishes an essential role for RAGE in diabetes related bone loss

The mechanisms underlying diabetes-mediated bone loss are not well defined. It has been reported that the advanced glycation endproducts (AGEs) and receptor for AGEs (RAGEs) are involved in diabetic complications. Here, mice deficient in RAGE were used as a model for investigating the effects of RAGE on bone mass. We found that RAGE-/- mice have a significantly increased bone mass and bone biomechanical strength and a decreased number of osteoclasts compared to wild-type mice. The serum levels of IL-6 and bone breakdown marker pyridinoline were significantly decreased in RAGE-/- mice. RAGE-/- mice maintain bone mass following ovariectomy, whereas wild-type mice lose bone mass. Furthermore, osteoclast-like cells do express RAGE mRNA. Our data therefore indicate that RAGE serves as a positive factor to regulate the osteoclast formation, directly implicates a role for RAGE in diabetes-promoted bone destruction, and documents that the AGE-RAGE interaction may account for diabetes associated bone loss.     

Phytoestrogens oestrogen synthesis and breast cancer

Phytoestrogens are used as ‘natural’ alternatives to HRT and, although epidemiological evidence implies that diets rich in phytoestrogens reduce the incidence of breast cancer, their weak oestrogenicity is also known to stimulate growth in experimental models of breast cancer. This review addresses the question as to how phytoestrogens may protect against breast cancer through their ability to bind preferentially to oestrogen receptor β, inhibit enzymes that convert circulating steroid precursors into oestradiol and inhibit cell signalling pathways of growth factors.     

The benefit of combining non-mechanical agents with mechanical loading: a perspective based on the Utah Paradigm of Skeletal Physiology

The Utah Paradigm of Skeletal Physiology with its key component, the mechanostat hypothesis, suggest plausible explanations of some of the tissue-level changes occurring from combining selected non-mechanical agents (anabolic and anti-resorptive/( re)modeling agents) with mechanical loading (osteogenic exercise) to increase bone mass and strength. The evidence for combining selected anabolic agents like parathyroid hormone, prostaglandin E(2), growth hormone, etc. with mechanical loading can increase bone mass is strong. Anabolic agents influence loading-related bone formation changes in a permissive manner and modulate (increase) the responsiveness of bone tissue to mechanical loading by changing thresholds for bone formation and resorption. However, any beneficial effect of combining selected anti-resorptive/(re)modeling agents like estrogen with loading is marginal, especially in adult skeletons. Postulated changes in modeling and remodeling thresholds (set points) and known direct effects on bone cells by non-mechanical agents may explain the observed tissue-level changes associated with large and minor increases in bone mass. Although the pharmaceutical industry has avoided considering osteogenic loading in the treatment of osteoporosis, a methodical dose-response study of anabolic agents combined with loading should: (1) provide opportunities for therapeutic intervention to imitate or enhance the osteogenic response to loading in order to correct osteopenias; (2) provide the potential to diminish the dosage of drugs required to induce bone formation in ways that enhanced efficacy and reduced any side effects; and (3) improve the quality of life and reduce the risk of falls by improving balance, gait speed and muscle strength with a non-mechanical agent like GH that could improve both muscle and bone mass and strength. Lastly, more studies are needed which determine bone strength instead of only "mass" in aged skeletons so one can assess how effective such treatments would reduce the risk of fracture in the clinic.     

An overview of nomegestrol acetate selective receptor binding and lack of estrogenic action on hormone-dependent cancer cells

The specific pharmacological profile of the 19-norprogestin nomegestrol acetate (NOMAC) is, at least in part, defined by its pattern of binding affinities to the different steroid hormone receptors. In the present study, its affinity to the progesterone receptor (PgR), the androgen receptor (AR) and the estrogen receptor (ER) was re-evaluated and compared to those obtained for progesterone (P) and several progestins. The characteristics of binding to the PgR in rat uterus were determined and Ki were found to be roughly similar with 22.8 and 34.3 nM for NOMAC and P, respectively. The binding characteristics of 3H-NOMAC were also determined and compared to that of 3H-ORG2058 with Kd of 5 and 0.6 nM, respectively for rat uterus and 4 and 3 nM, respectively for human T47-D cells. Structure-affinity and -activity relationships were studied on a variety of compounds related to NOMAC in order to assess its specificity as a progestin. The effects of NOMAC on the binding of androgen to the AR were investigated, using rat ventral prostate as target model. Contrary to what was observed for MPA, the RBA of NOMAC was found to decline with time, showing anti-androgenic rather than androgenic potential, a result that was confirmed in vivo. Regarding the ER, since none of the progestins were able to compete with estrogen for binding in rat uterus as well as in Ishikawa cells, the induction of alkaline phosphatase activity (APase) was used as an estrogen-specific response. It confirmed the intrinsic estrogenicity of progestins derived from 19-nor-testosterone (19NT), norethisterone acetate (NETA), levonorgestrel (LNG) or norgestimate (NGM) and others. In contrast, all P and 19-norP derivatives remained inactive. Finally, to complete this overview of NOMAC at the sex steroid receptor levels, the lack of estrogenic or estrogenic-like activity was checked out in different in vitro models. Data from this study have demonstrated that NOMAC is a progestin that has greater steroid receptor selectivity compared to MPA or some other synthetic progestins. It may provide a better pharmacological profile than those progestins currently in use in HRT and OC.     

Molecular mechanisms of steroid receptor-mediated actions by synthetic progestins used in HRT and contraception

Synthetic progestins are used by millions of women as contraceptives and in hormone replacement therapy (HRT), although their molecular mechanisms of action are not well understood. The importance of investigating these mechanisms, as compared to those of progesterone, has been highlighted by clinical evidence showing that medroxyprogesterone acetate (MPA), a first generation progestin, increases the risk of breast cancer and coronary heart disease in HRT users. A diverse range of later generation progestins with varying structures and pharmacological properties is available for therapeutic use and it is becoming clear that different progestins elicit beneficial and adverse effects to different extents. These differences in biological activity are likely to be due to many factors including variations in dose, metabolism, pharmacokinetics, bioavailability, and regulation of, and/or binding, to serum-binding proteins and steroidogenic enzymes. Since the intracellular effects on gene expression and cell signaling of steroids are mediated via intracellular steroid receptors, differential actions via the progesterone and other steroid receptors and their isoforms, are likely to be the major cause of differential intracellular actions of progestins. Since many progestins bind not only to the progesterone receptor, but also to the glucocorticoid, androgen, mineralocorticoid, and possibly the estrogen receptors, it is plausible that synthetic progestins exert therapeutic actions as well as side-effects via some of these receptors. Here we review the molecular mechanisms of intracellular actions of old (MPA, norethisterone, levonorgestrel, gestodene) vs. new (drospirenone, dienogest, trimegestone) generation progestins, via steroid receptors.     

Aromatase inhibitors: their biochemistry and clinical potential

It has been proposed that one of the endocrinological factors in the pathogenesis of benign prostatic hyperplasia is estrogen stimulation of stromal growth. Current clinical experience with anti-estrogenic compounds indicates that, in the case of mammary carcinoma, aromatase inhibitors provide a viable alternative to estrogen receptor antagonists for treatment of the disease. It is proposed that inhibitors of estrogen biosynthesis could likewise provide a non-invasive therapy for benign prostate disease. Some aspects of the activity of known aromatase inhibitors as substrates for enzymes of steroid metabolism and their potential relevance to the pharmacology of the compounds are discussed.