Riggs, B. Lawrence

Abstract The NOF cost-effectiveness model, based on clinical risk factors and femoral neck aBMD, predicted overall fracture risk in a cohort of postmenopausal women followed for up to 22 years. Introduction: To assess the ability of a statistical model to predict long-term fracture risk for a population of postmenopausal women, we compared observed fractures to those predicted by the National Osteoporosis Foundation's (NOF) cost-effectiveness model. Materials and Methods: In this population-based study, 393 postmenopausal Rochester, MN, women had baseline measurements of femoral neck areal BMD (aBMD) and assessment of the clinical risk factors (personal fracture history, family history of osteoporotic fracture, low body weight, and smoking status) that were included in the NOF model. They were then followed prospectively for up to 22 years. Fractures were ascertained by periodic interview and review of community medical records. Standardized incidence ratios (SIRs) compared observed fractures to predicted numbers. Results: During 4782 person-years of follow-up, 212 women experienced 503 fractures, two-thirds of which were caused by moderate trauma. When undiagnosed (incidentally noted) vertebral and rib fractures were excluded, there was general concordance between observed and predicted fractures of the hip (SIR, 0.78; 95% CI, 0.56-1.01), distal forearm (SIR, 1.22; 95% CI, 0.86-1.68), spine (SIR, 0.76; 95% CI, 0.50-1.11), and all other sites combined (SIR, 1.18; 95% CI, 0.97-1.42). Fracture prediction by the NOF model was about as good after 10 years as it was earlier during follow-up. Conclusions: This study validates the ability of a statistical model based on femoral neck aBMD and common clinical risk factors to predict the actual occurrence of fractures in a cohort of postmenopausal white women.

Abstract Osteoporosis is widely viewed as a major public health concern, but the exact magnitude of the problem is uncertain and likely to depend on how the condition is defined. Noninvasive bone mineral measurements can be used to define a state of heightened fracture risk (osteopenia), or the ultimate clinical manifestation of fracture can be assessed (established osteoporosis). If bone mineral measurements more than 2 standard deviations below the mean of young normal women represent osteopenia, then 45% of white women aged 50 years and over have the condition at one or more sites in the hip, spine, or forearm on the basis of population-based data from Rochester, Minnesota. A smaller proportion is affected at each specific skeletal site: 32% have bone mineral values this low in the lumbar spine, 29% in either of two regions in the proximal femur, and 26% in the midradius. Although this overall estimate is substantial, some other serious chronic diseases are almost as common. More importantly, low bone mass is associated with adverse health outcomes, especially fractures. The lifetime risk of any fracture of the hip, spine, or distal forearm is almost 40% in white women and 13% in white men from age 50 years onward. If the enormous costs associated with these fractures are to be reduced, increased attention must be given to the design and implementation of control programs directed at this major health problem.

Abstract In a population-based, cross-sectional study, we related age-associated changes in vBMD and in bone structural parameters to circulating bioavailable estradiol and testosterone levels in men. Associations between these bone mass/structural parameters and sex steroid levels were progressively stronger with age. Our previously postulated “threshold” for skeletal estrogen deficiency was most evident at cortical sites. Introduction: Serum sex steroids, particularly estrogen levels, are associated with bone mass in men, and previous work has suggested that there may be a “threshold” bioavailable estradiol (bio E2) level below which the male skeleton becomes estrogen deficient. However, previous studies addressing this issue have exclusively used DXA, which cannot separate trabecular from cortical bone or provide information on bone geometry or structure. Materials and Methods: In an age-stratified population sample of 314 men (age, 22-91 years), we assessed volumetric BMD (vBMD) and bone geometry by QCT at the lumbar spine, femoral neck, distal radius, and distal tibia and related these to circulating bio E2 and bio testosterone (T) levels. Results: Compared with young men (age, 20-39 years), middle-aged men (age, 40-59 years) had significantly lower bio T (−26%, p < 0.001) and bio E2 (−9%, p = 0.038) levels, and these decreases were even greater in the elderly men (age ≥ 60 years, −60% and −38% for bio T and bio E2, respectively, p < 0.001 for both). Reflecting their intact gonadal status, vBMD/structural parameters were not related to sex steroid levels in young men, whereas bio E2 levels were associated consistently with vBMD and variably with bone geometric parameters in the elderly men; middle-aged men showed associations with bio E2 and bio T at some sites. At all cortical sites, vBMD was associated with bio E2 at low (<30 pM, R = 0.27-0.41, p < 0.05-0.001) but not high (≥30 pM, R = −0.003 to 0.12, p = not significant) levels; no such differences were evident at trabecular sites. Conclusions: In men, bio E2 is the most consistent predictor of vBMD and some bone geometric variables as assessed by QCT. We also extend our previous findings on a possible “threshold” for skeletal estrogen deficiency by showing that this is most evident for cortical sites.

Abstract Long-term fracture prediction using bone mineral density remains controversial, as does the additional contribution from assessing bone turnover or clinical risk factors. We measured bone mineral density at various sites, along with biochemical markers of bone turnover, sex steroid levels, and over 100 clinical variables, at baseline on an age-stratified sample of 304 Rochester, MN women in 1980. The 225 postmenopausal women were subsequently followed for 3146 person-years (median, 16.2 years per subject), wherein they experienced 302 new fractures: 81% resulted from minimal or moderate trauma and 60% of these involved the proximal femur, thoracic or lumbar vertebrae, or distal forearm. Accounting for multiple fractures per subject, these osteoporotic fractures together were best predicted by baseline femoral neck bone mineral density (age-adjusted hazard ratio [HR] per SD decrease, 1.37; 95% CI, 1.10–1.70); 19 moderate trauma forearm fractures were best predicted by distal radius bone mineral content, whereas 28 hip fractures and 100 vertebral fractures were best predicted by femoral neck bone mineral density. Femoral neck bone mineral density performed comparably in predicting osteoporotic fracture risk within the first decade of follow-up (HR, 1.38; 95% CI, 1.10–1.74) as well as more than 10 years after baseline (HR, 1.39; 95% CI, 1.05–1.84). The older biochemical markers were not associated with fractures, but serum “free” estradiol index was independently predictive of short- and long-term fracture risk. Consistent clinical risk factors were not identified, but statistical power was limited. Identifying patients at increased long-term risk of fracture is challenging, but it is reassuring that femoral neck bone mineral density can predict osteoporotic fractures up to 20 years later.

Abstract Runx2/Cbfa1 has been identified as a “master gene” controlling osteoblast differentiation. However, its role in inducing the osteoblast phenotype has been characterized primarily in rodent systems. Thus, we examined Runx2/Cbfa1 messenger RNA, protein, and activity levels during osteoblastic differentiation of human bone marrow stromal (BMSC) cells. Semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that the expression of alkaline phosphatase and osteocalcin mRNAs increased in a time-dependent manner with the development of the osteoblast phenotype by these cells (hMS2–15). Type II Runx2/Cbfa1 messenger RNA was found to be constitutively expressed in hMS2–15 cells and not altered during differentiation; there was no detectable expression of the type I Runx2/Cbfa1 transcript. Interestingly, despite the absence of any change in Runx2/Cbfa1 messenger RNA levels during osteoblastic differentiation of these cells, the activity of Runx2/Cbfa1, as assessed by binding to the osteoblast-specific cis-acting element 2 (OSE2), increased markedly at all time-points examined, with the highest activity level seen at day 7. Similar results were observed in primary cultures of less differentiated human marrow-derived mesenchymal stem cells. Immunoprecipitation and Western blot analysis revealed that whereas there was no increase in Runx2/Cbfa1 protein levels with differentiation in hMS2–15 cells, there was an increase in Runx2/Cbfa1 phosphorylation. Thus, in contrast to rodent systems where osteoblast differentiation is associated with increased synthesis of Runx2/Cbfa1, we find that in human BMSC, osteoblastic differentiation is associated primarily with increases in Runx2/Cbfa1 activity, without a change in messenger RNA or protein levels. Our findings also show that the increase in Runx2/Cbfa1 activity occurs through a posttranslational mechanism involving phosphorylation of key residues.

Abstract Estrogen induction of progesterone receptor (PR) expression may be important to bone physiology because progesterone has been implicated in the control of bone formation and resorption. Although PR gene expression can be induced in osteoblasts by estrogen signaling through the estrogen receptor (ER) α isoform, it is unknown whether the ER-β isoform is involved in this regulation. The effect of estrogen on PR expression was examined in human fetal osteoblast (hFOB) cell lines stably transfected with either ER-α or ER-β. Estrogen treatment of hFOB/ER-α cells induced PR messenger RNA (mRNA) steady-state levels after 24 h and protein levels after 48 h, as established by competitive reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting. Interestingly, no induction of PR expression was observed in the hFOB/ER-β cells during this period. However, PR mRNA was induced progressively after 48 h of treatment with estrogen with maximum levels achieved at 12 days posttreatment. ER protein also was increased after 12 days of treatment. Both A and B isoforms of PR (PRA and PRB) were induced by estrogen in the hFOB/ER-α cells as well as much later in hFOB/ER-β cells. The pure antiestrogen ICI 182,780 prevented PR induction by estrogen in both cell lines. An ER-β-selective antagonist R, R-tetrahydrochrysene (THC) abolished the induction of PR mRNA in hFOB/ER-β but not in hFOB/ER-α cells, verifying that the response in the former cell line was ER-β-mediated. Transient cotransfection of hFOB cells with ER-α or ER-β together with either a human PRA or PRB promoter linked to a reporter plasmid revealed that although the PRB promoter was stimulated equally by estrogen activation of either ER isoform, PRA was activated preferentially by ER-α. Together, these results show that although estrogen can up-regulate endogenous PR gene expression in osteoblasts via both ER isoforms, ER-α is the predominant inducer.

Abstract Although median levels of bone turnover are increased in postmenopausal women, it is unclear whether the low circulating levels of endogenous estrogen exert a regulatory role on these levels. This issue was evaluated by assessing the effect of a blockade of estrogen synthesis on bone turnover markers in 42 normal women (mean age ± SD, 69 ± 5 years) randomly assigned to groups receiving the potent aromatase inhibitor letrozole or placebo for 6 months. Letrozole treatment reduced serum estrone (E1) and estradiol (E2) to near undetectable levels (p < 0.0001). This treatment did not affect bone formation markers but, as compared with the placebo group, increased bone resorption markers (urine 24-h pyridinoline [PYD] by 13.3% [p < 0.05] and 24-h urine deoxypyridinoline [DPD] by 14.2% [p < 0.05]) and decreased serum parathyroid hormone (PTH) by 22% (p = 0.002). These data indicate that in late postmenopausal women even the low serum estrogen levels present exert a restraining effect on bone turnover and support the concept that variations in these low levels may contribute to differences in their rate of bone loss.