Association between polymorphisms of apolipoprotein E, bone mineral density of the lower forearm, quantitative ultrasound of the calcaneus and osteoporotic fractures in postmenopausal women with hip or lower forearm fracture

Role of estrogen in women's Alzheimer's disease risk as modified by APOE

Alzheimer's disease (AD) is characterized by numerous sexual dimorphisms that impact the development, progression, and probably the strategies to prevent and treat the most common form of dementia. In this review, we consider this topic from a female perspective with a specific focus on how women's vulnerability to the disease is affected by the individual and interactive effects of estrogens and apolipoprotein E (APOE) genotype. Importantly, APOE appears to modulate systemic and neural outcomes of both menopause and estrogen-based hormone therapy. In the brain, dementia risk is greater in APOE4 carriers, and the impacts of hormone therapy on cognitive decline and dementia risk vary according to both outcome measure and APOE genotype. Beyond the CNS, estrogen and APOE genotype affect vulnerability to menopause-associated bone loss, dyslipidemia and cardiovascular disease risk. An emerging concept that may link these relationships is the possibility that the effects of APOE in women interact with estrogen status by mechanisms that may include modulation of estrogen responsiveness. This review highlights the need to consider the key AD risk factors of advancing age in a sex-specific manner to optimize development of therapeutic approaches for AD, a view aligned with the principle of personalized medicine.

Role of APOE Gene in Bone Mineral Density and Incidence of Bone Fractures in Brazilian Postmenopausal Women

Osteoporosis is one of the major diseases that affects mostly postmenopausal women. Despite being a multifactorial disease, some genes have been shown to play an important role in osteoporosis. Bone mineral density (BMD) is still largely used to diagnose it, although many other biomarkers are used to better follow the disease onset. It has been shown that the apolipoprotein E (APOE) gene could be a biomarker for risk of fractures as well as to predict lower BMD in patients with osteoporosis. The human APOE gene encodes 3 protein isoforms called ApoE2, ApoE3, and ApoE4, resulting in 4 possible genotypes, because they are a product of a single nucleotide polymorphism found in this gene. So far, the APOE4 allele has been associated with low BMD in postmenopausal women and to incidence of bone breaking in older women. This study aimed to investigate the role of ApoE isoforms in a cohort of 413 postmenopausal Brazilian women. These patients were randomly recruited, clinically examined, and subjected to dual-energy X-ray absorptiometry to measure their BMD. Patients were further grouped as normal BMD (T-score < 0.5) or low BMD (T-score > 1.0, osteopenic or osteoporotic). Patients with osteopenia or osteoporosis were further genotyped for APOE alleles as well as tested for many serum bone turnover biomarkers. Our data showed that presence of the APOE3 allele was associated with both higher BMDs and higher serum concentrations of osteocalcin and alkaline phosphatase, biomarkers for bone formation. On the other hand, the APOE2 and APOE4 alleles were associated with lower BMD as well as higher levels of serum C-terminus collagen peptide and urinary deoxipyridinolines, biomarkers for bone resorption. However, these effects on lower BMD and bone resorption biomarkers observed in either APOE2 or APOE4 alleles were eliminated when patients' genotype carried the APOE3 allele. Codominance of the APOE3 allele was also associated with lesser cases of bone fractures in these patients within a 5-year follow-up. In conclusion, our data show that APOE4 may be associated with lower bone formation as well as increased risk of osteoporosis and bone fractures, whereas APOE3 seems to decrease lowering BMD in postmenopausal women, and its presence seemed to lower the incidence of bone breaking in patients with osteoporosis.

Apolipoprotein E gene E2/E2 genotype is a genetic risk factor for vertebral fractures in humans: a large-scale study

PURPOSE

Although many studies have been performed to evaluate whether or not apolipoprotein E gene (APOE) polymorphisms are differentially associated with bone mineral density (BMD) and fractures, the results have been conflicting. This large-scale study was performed to investigate whether a relationship exists between APOE polymorphisms and risk of fracture.

METHODS

A hospital-based case-control study was conducted in 3,000 patients with fractures and 3,000 age- and gender-matched healthy controls. Polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) assay was applied to assess the APOE gene polymorphisms.

RESULTS

Patients with fractures had a significantly higher frequency of APOE E2/E2 genotype [odds ratio (OR) = 2.02, 95% confidence interval (CI) = 1.30, 3.14; P = 0.002] than healthy controls. When stratifying by fracture type, it was found that patients with vertebral fractures had a significantly higher frequency of APOE E2/E2 genotype (OR = 2.86, 95% CI = 1.73, 4.73; P < 0.001). No significant differences were found in nonvertebral (hip or wrist or other) fractures.

CONCLUSIONS

Our study suggests that APOE E2/E2 genotype is a potential genetic risk factor for vertebral fractures in humans.

The role of apolipoprotein E in bone metabolism

Apolipoprotein E (apoE) is a major structural apolipoprotein of several lipoprotein classes. Over the last 13 years, numerous studies have focused on the question whether human apoE affects bone phenotypes and, more recently, whether apoE regulates bone metabolism in mice. Here, we first provide a brief background introduction into the structure, established physiological and pathophysiological functions of apoE, and will then discuss the new aspects of the emerging role of apoE in bone.

Intake of vitamin K1 and K2 and risk of hip fractures: The Hordaland Health Study

BACKGROUND

Evidence of the effect of vitamin K on bone health is conflicting. The aim was to investigate the association between intake of vitamins K1 and K2 and subsequent risk of hip fracture in a general population sample, as well as potential effect modification by apolipoprotein E gene (APOE) status by presence of the E4 allele.

METHODS

1238 men and 1569 women 71-75 years of age were included in the community-based Hordaland Health Study 1997-1999 in Western Norway. Information on hip fracture was obtained from hospitalizations in the region from enrolment until 31 December 2009. Information on intake of vitamins K1 and K2 collected at baseline was used as potential predictors of hip fracture in Cox proportional hazards regression analyses.

RESULTS

Participants in the lowest compared to the highest quartile of vitamin K1 intake had increased risk of suffering a hip fracture (hazard ratio (HR)=1.57 [95% CI 1.09, 2.26]). Vitamin K2 intake was not associated with hip fracture. Presence of APOE4-allele did not increase the risk of hip fracture, nor was there any effect modification with vitamin K1 in relation to risk of hip fracture.

CONCLUSIONS

A low intake of vitamin K1, but not K2, was associated with an increased risk of hip fractures.

Association of apolipoprotein E promoter polymorphisms with bone structural traits is modified by dietary saturated fat intake - the Cardiovascular Risk in Young Finns study

Association of apolipoprotein E (APOE) ε4 allele with peripheral quantitative computed tomography (pQCT) bone traits at the distal and shaft sites of the radius and tibia was evaluated in the Young Finns Cohort (n=1777). We also analyzed the interactions of the APOE promoter polymorphisms (-219G/T rs405509 and +113G/C rs440446) and bone traits within the APOE ε3/ε3 genotype (n=1025 and n=1013, respectively), and investigated the gene-environment interactions on bone traits with longitudinal saturated fatty acids (SAFA) intake. Differences between the ε4 allele carriers and noncarriers were modest and mostly nonsignificant. Within the APOE promoter -219G/T polymorphism, cortical strength index (CSI) and compressive bone strength index (BSI) at the distal radius (linear, P=0.003 and P=0.05, respectively) and tibia (linear, P=0.01 and P=0.03, respectively), and CSI at the tibial shaft (linear, P=0.04) decreased towards the -219T/T genotype in women. In men, total cross-sectional areas at the radial site and stress-strain index (SSI) at the radial shaft (linear, P=0.03 and P=0.04 and P=0.05, respectively) increased, and conversely cortical bone density and CSI at the radial shaft (linear, P=0.005 and P=0.05, respectively) and CSI at the tibial shaft (linear, P=0.03) decreased towards the -219T/T genotype. In the highest SAFA tertile, women with the -219T/T genotype had the smallest total area and SSI at the radial shaft (P=0.01 and P=0.02, respectively). Subjects with the APOE +113C/C genotype shared similar bone traits as subjects with the APOE -219T/T genotype. In conclusion, APOE genotypes -219T/T and +113C/C could be genetic markers for cortical bone strength. Furthermore, high longitudinal SAFA intake seems to be more detrimental to bone in women with the -219T/T and +133C/C genotypes than others.

Association between an intronic apolipoprotein E polymorphism and bone mineral density in Singaporean Chinese females

INTRODUCTION

Apolipoprotein E (ApoE) is implicated in the pathogenesis of osteoporosis.

OBJECTIVE

To investigate possible association of the non-classical APOE gene +113C/G (rs440446) intron 1 enhancer polymorphism with bone mineral density (BMD) in a homogeneous Chinese population in Singapore.

METHODS

A total of 655 volunteers, males and females, aged between 31 and 72 years, from the public participated. BMD was measured using dual-energy X-ray absorptiometry and APOE +113C/G (rs440446) genotypes were determined by Sequenom MassARRAY system. To adjust for potential confounders, anthropometric, demographic, and lifestyle determinants were obtained, and serum lipids and E(2) were measured.

RESULTS

The +113C/G (rs440446) polymorphism within the APOE gene was associated with BMD in Chinese Singaporean females only. Females with the heterozygous CG genotype were significantly associated with reduced total, lumbar spine, and femoral neck of hip BMD, after multilevel adjustment of confounders. The association was stronger in the spine than in the hip. When females were stratified according to WHO classification for osteoporosis, those with CG and GG genotypes had increased risk (OR 3.50 and 2.22, respectively) of developing osteopenia/osteoporosis in the lumbar spine. Serum lipids did not explain the influence of APOE +113 C/G (rs440446) on BMD.

CONCLUSION

This study demonstrated an association between APOE +113C/G (rs440446) polymorphism with measures of BMD in Singaporean Chinese females.

Searching for genes underlying susceptibility to osteoporotic fracture: current progress and future prospect

INTRODUCTION

Osteoporotic fracture (OF) is a public health problem. It is a common practice in the genetics of osteoporosis that bone mineral density (BMD) was studied as a major surrogate phenotype in gene search for risk of OF (ROF) because of their high phenotypic correlation. However, some studies indicate that the genetic correlation between BMD and ROF is very low. This implies that most genes found important for BMD may not be relevant to ROF. Ideally, employing OF per se as a direct study phenotype can directly find the relevant genes underlying ROF.

EVIDENCE

Here, we summarized some evidence supporting ROF under moderate genetic control, and the current progress of molecular genetic studies employing OF as the direct study phenotype, then give our consideration on the future prospects in the genetics of ROF.

Links between cardiovascular disease and osteoporosis in postmenopausal women: serum lipids or atherosclerosis per se?

INTRODUCTION AND HYPOTHESIS

Epidemiological observations suggest links between osteoporosis and risk of acute cardiovascular events and vice versa. Whether the two clinical conditions are linked by common pathogenic factors or atherosclerosis per se remains incompletely understood. We investigated whether serum lipids and polymorphism in the ApoE gene modifying serum lipids could be a biological linkage.

METHODS

This was an observational study including 1176 elderly women 60-85 years old. Women were genotyped for epsilon (epsilon) allelic variants of the ApoE gene, and data concerning serum lipids (total cholesterol, triglycerides, HDL-C, LDL-C, apoA1, ApoB, Lp(a)), hip and spine BMD, aorta calcification (AC), radiographic vertebral fracture and self-reported wrist and hip fractures, cardiovascular events together with a wide array of demographic and lifestyle characteristics were collected.

RESULTS

Presence of the ApoE epsilon 4 allele had a significant impact on serum lipid profile, yet no association with spine/hip BMD or AC could be established. In multiple regression models, apoA1 was a significant independent contributor to the variation in AC. However, none of the lipid components were independent contributors to the variation in spine or hip BMD. When comparing the women with or without vertebral fractures, serum triglycerides showed significant differences. This finding was however not applicable to hip or wrist fractures. After adjustment for age, severe AC score (>or=6) and/or manifest cardiovascular disease increased the risk of hip but not vertebral or wrist fractures.

CONCLUSION

The contribution of serum lipids to the modulators of BMD does not seem to be direct but rather indirect via promotion of atherosclerosis, which in turn can affect bone metabolism locally, especially when skeletal sites supplied by end-arteries are concerned. Further studies are needed to explore the genetic or environmental risk factors underlying the association of low triglyceride levels to vertebral fractures.

Increased bone formation in mice lacking apolipoprotein E

ApoE is a plasma protein that plays a major role in lipoprotein metabolism. Here we describe that ApoE expression is strongly induced on mineralization of primary osteoblast cultures. ApoE-deficient mice display an increased bone formation rate compared with wildtype controls, thereby showing that ApoE has a physiologic function in bone remodeling.

INTRODUCTION

Apolipoprotein E (ApoE) is a protein component of lipoproteins and facilitates their clearance from the circulation. This is confirmed by the phenotype of ApoE-deficient mice that have high plasma cholesterol levels and spontaneously develop atherosclerotic lesions. The bone phenotype of these mice has not been analyzed to date, although an association between certain ApoE alleles and BMD has been reported.

MATERIALS AND METHODS

Primary osteoblasts were isolated from newborn mouse calvariae and mineralized ex vivo. A genome-wide expression analysis was performed during the course of differentiation using the Affymetrix gene chip system. Bones from ApoE-deficient mice and wildtype controls were analyzed using radiography, micro CT imaging, and undecalcified histology. Cellular activities were assessed using dynamic histomorphometry and by measuring urinary collagen degradation products. Lipoprotein uptake assays were performed with (125)I-labeled triglyceride-rich lipoprotein-remnants (TRL-R) using primary osteoblasts from wildtype and ApoE-deficient mice. Serum concentrations of osteocalcin were determined by radioimmunoassay after hydroxyapatite chromatography.

RESULTS

ApoE expression is strongly induced on mineralization of primary osteoblast cultures ex vivo. Mice lacking ApoE display a high bone mass phenotype that is caused by an increased bone formation rate, whereas bone resorption is not affected. This phenotype may be explained by a decreased uptake of triglyceride-rich lipoproteins by osteoblasts, resulting in elevated levels of undercarboxylated osteocalcin in the serum of ApoE-deficient mice.

CONCLUSION

The specific induction of ApoE gene expression during osteoblast differentiation along with the increased bone formation rate observed in ApoE-deficient mice shows that ApoE has a physiologic role as a regulator of osteoblast function.

Expression of LRP1 by human osteoblasts: a mechanism for the delivery of lipoproteins and vitamin K1 to bone

Accumulating clinical and experimental data show the importance of dietary lipids and lipophilic vitamins, such as vitamin K1, for bone formation. The molecular mechanism of how they enter the osteoblast is unknown. Here we describe the expression of the multifunctional LRP1 by human osteoblasts in vitro and in vivo. We provide evidence that LRP1 plays an important role in the uptake of postprandial lipoproteins and vitamin K1 by human osteoblasts.

INTRODUCTION

Chylomicrons (CM) and their remnants (CR) represent the postprandial plasma carriers of dietary lipids. Dietary vitamin K1 is known to be transported in the circulation as part of CM/CR and is required by osteoblasts as an essential co-factor for the gamma-carboxylation of bone matrix proteins. The molecular mechanisms underlying the delivery of lipophilic substances to bone are not understood. In this study, the expression and function of CM/CR receptors was examined in human osteoblasts.

MATERIALS AND METHODS

Four human osteoblast-like cell lines were analyzed: two osteosarcoma lines (MG63, SaOS-2) and two telomerase-immortalized human bone marrow stromal cell lines (hMSC-TERT [4] and [20]) after 1,25(OH)2 vitamin D3 induction of osteoblastic differentiation (hMSC-TERT-OB). Receptor expression was examined by Western blotting and immunohistochemistry of normal human bone sections. Endocytotic receptor function was analyzed by cellular uptake assays using fluorescent and radiolabeled human CR. Vitamin K1-enriched CR (CR-K1) were generated in vivo after oral vitamin administration and vitamin K1 uptake by osteoblasts was measured by HPLC. The effect of CR-K1 uptake on osteocalcin carboxylation was measured by ELISA.

RESULTS

Osteoblasts exhibit high levels of protein expression of the CR receptors LRP1 and LDLR. VLDLR is expressed to a lower degree. Immunohistochemistry of normal human bone sections showed strong LRP1 expression by osteoblasts and marrow stromal cells. Uptake of fluorescent CR by osteoblasts resulted in the typical pattern of receptor-mediated endocytosis. CR uptake was stimulated by the exogenous addition of the lipoprotein receptor ligands apolipoprotein E and lipoprotein lipase. Uptake was reduced by the known LRP1 inhibitors RAP, lactoferrin, and suramin, but not by LDL, which exclusively binds to the LDLR. Vitamin K1 uptake by hMSC-TERT-OB after incubation with CR-K1 was also shown to be sensitive to LPL stimulation and the LRP1 specific inhibitor lactoferrin. CR-K1 uptake into osteoblasts stimulated the gamma-carboxylation of osteocalcin.

CONCLUSION

Human osteoblasts express receptors of the LDLR family with a capacity for vitamin K1 uptake through CR endocytosis, a novel mechanism for the delivery of dietary lipids and lipophilic vitamins to human bone. The current data suggest that, among the expressed receptors, LRP1 plays a predominant role.

APOE haplotypes influence bone mineral density in Caucasian males but not females

Low bone mineral density (BMD) is one of the most important risk factors for osteoporosis. Apolipoprotein E (APOE) has been considered as a candidate gene for osteoporosis because of its influence on osteoblast uptake of lipoprotein-borne vitamin K. Using the quantitative transmission disequilibrium test QTDT, we examined linkage and/or association of APOE and BMD at the lumbar spine and the total hip in a sample of 387 Caucasian nuclear families with 715 parents and 953 children. The children were aged 20-50 years and female offspring were premenopausal as well. Four single nucleotide polymorphisms (SNP1-4) in the APOE gene, 4-locus haplotypes and 2-locus haplotypes (epsilon1, epsilon2, epsilon3, epsilon4 isoforms, reconstructed by SNP3 and SNP4) were analyzed. In the whole sample and the female offspring families we found no evidence of linkage or association for either single SNP or haplotype with BMD at the two studied skeletal sites. In the male offspring families, within-family associations were observed at the haplotypes CGTC (P = 0.001), GGTT (P = 0.002), and GATC (P = 0.006) for the lumbar spine BMD, and GATC (P = 0.008) for the total hip BMD. These data suggested that in our studied Caucasian population, APOE may have effects on BMD variation in males but not females. Further studies with a larger sample size are required to confirm such results.