Bone marrow stromal cells express two distinct splice variants of ER-alpha that are regulated by estrogen

Bone Remodeling in Post-menopausal Osteoporosis

Bone mass in the skeleton is dependent on the coordinated activities of bone-forming osteoblasts and bone-resorbing osteoclasts in discrete bone multi-cellular units. Remodeling of bone in these units is important not only for maintaining bone mass, but also to repair microdamage, to prevent accumulation of too much old bone, and for mineral homeostasis. The activities of osteoblasts and osteoclasts are controlled by a variety of hormones and cytokines, as well as by mechanical loading. Most importantly, sex hormones are very crucial for keeping bone mass in balance, and the lack of either estrogen or testosterone leads to decreased bone mass and increased risk for osteoporosis. The prevalence of osteoporotic fractures is increasing dramatically in the Western part of the world and is a major health problem in many countries. In the present review, the cellular and molecular mechanisms controlling bone remodeling and the influence of sex hormones on these processes are summarized. In a separate paper in this issue, the pathogenesis of post-menopausal osteoporosis will be compared with that of inflammation-induced bone remodeling, including the evidence for and against the hypothesis that concomitant post-menopausal osteoporotic disease influences the progression of periodontal disease.

Complex organization of the 5'-untranslated region of the mouse estrogen receptor α gene: identification of numerous mRNA transcripts with distinct 5'-ends

The 5'-untranslated region (5'-UTR) of the estrogen receptor α (ERα) gene plays an important role in determining its tissue-specific expression. We examined the 5'-UTRs of the mouse ERα mRNA variants in depth using the Basic Local Alignment Search Tool (BLAST), rapid amplification of 5'-cDNA ends (5'-RACE) and RT-PCR. We demonstrated the presence of multiple variants containing unique 5'-UTRs. We mapped the cDNA sequences onto the mouse genome, and found that both alternative splicing from four different leader exons (A, C, F1, and H) to exon 1, and combinations of 12 internal exons (X1, X2, X3, X4, F2/X5, X6, X7, X8, X9, X10, X11, and B) generate multiple ERα transcripts. Mouse exon B, that has homologies with human exon B and rat exon 0T, was used as an internal exon, not as a leader exon. RT-PCR analysis revealed distinct expression patterns of the variants, suggesting that the alternative promoter usage and alternative splicing are regulated in a tissue-specific manner. Our results indicate that the genomic organization of the mouse ERα gene is complicated as previously shown in the rat ERα gene. In addition, both alternative promoter usage and alternative splicing contribute to the remarkable mRNA diversity of the mouse ERα gene.

Identification of C-terminally and N-terminally truncated estrogen receptor α variants in the mouse

We re-examined mouse ERα mRNA variants using rapid amplification of cDNA ends (RACE) and RT-PCR. Our analysis showed the presence of several mRNA variants containing unique 5'- or 3'-nucleotide sequences. We mapped the cDNA sequences on the mouse genome, and identified four novel 3'-terminal and 5'-leader exons in the intronic region between exons 4 and 5. RT-PCR analysis revealed that the expression patterns of the C-terminally truncated ERα products (CTERPs) were similar to that of Wild-type ERα and that the N-terminally truncated ERα products (NTERPs) appeared to have different expression profiles. Moreover, we constructed expression vectors and analyzed the subcellular localization and the transcriptional activation abilities of the variant proteins in transfected HEK293 cells using immunocytochemistry and luciferase reporter assay. The CTERP variants localized in the nuclei and constitutively activated estrogen response element (ERE)-driven promoters, while the NTERP variant was located in the extra-nuclear regions and had no ability to activate the ERE promoters in the presence or absence of 10 nM estradiol. Our results indicate that the mouse ERα gene is more complex than previously thought in terms of genomic organization and that alternative splicing and alternative usage of intronic promoters contribute to the remarkable diversity of ERα mRNAs and proteins.

Alternative promoter usage and alternative splicing of the rat estrogen receptor alpha gene generate numerous mRNA variants with distinct 5'-ends

The 5'-untranslated region (UTR) of the estrogen receptor alpha (ERalpha) gene plays an important role in determining tissue-specific expression. To elucidate the regulatory mechanisms of rat ERalpha gene expression, the genomic organization must be investigated. We therefore analyzed the structure of the rat ERalpha mRNA 5'-UTR using rapid amplification of 5'-cDNA ends (5'-RACE) and RT-PCR. The analysis showed the presence of multiple variants containing unique 5'-UTRs. We mapped the cDNA sequences on the rat genome, and newly identified one leader exon (exon 0U) and ten untranslated internal exons (exons I1-10). Both splicing from four different leader exons (exons 0S, 0N, 0U, and 0/B) onto exon 1 and alternative splicing in combination with eleven internal exons (exons I1-10, and 0T) produce multiple transcripts. RT-PCR analysis revealed that each variant had preferred expression sites, suggesting that promoter usage and splicing are regulated in tissue-specific manners. Moreover, we determined a splicing event to yield Deltaexon 1 variants (0S-2-3-4-5-6-7-8), which are translated into rat 46 kDa ERalpha proteins. Our results indicate that the rat ERalpha gene is more complex than previously thought in terms of genomic organization and that both alternative promoter usage and alternative splicing contribute to the remarkable diversity of ERalpha mRNAs.

Comparative temporal and dose-dependent morphological and transcriptional uterine effects elicited by tamoxifen and ethynylestradiol in immature, ovariectomized mice

BACKGROUND

Uterine temporal and dose-dependent histopathologic, morphometric and gene expression responses to the selective estrogen receptor modulator tamoxifen (TAM) were comprehensively examined to further elucidate its estrogen receptor-mediated effects. These results were systematically compared to the effects elicited by the potent estrogen receptor ligand 17alpha-ethynylestradiol (EE) to identify pathways similarly and uniquely modified by each compound.

RESULTS

Three daily doses of 100 microg/kg TAM elicited a dose-dependent increase in uterine wet weight (UWW) in immature, ovariectomized C57BL/6 mice at 72 hrs with concurrent increases in luminal epithelial cell height (LECH), luminal circumference and glandular epithelial tubule number. Significant UWW and LECH increases were detected at 24 hrs after a single dose of 100 microg/kg TAM. cDNA microarray analysis identified 2235 differentially expressed genes following a single dose of 100 microg/kg TAM at 2, 4, 8, 12, 18 and 24 hrs, and at 72 hrs after three daily doses (3 x 24 hrs). Functional annotation of differentially expressed genes was associated with cell growth and proliferation, cytoskeletal organization, extracellular matrix modification, nucleotide synthesis, DNA replication, protein synthesis and turnover, lipid metabolism, glycolysis and immunological responses as is expected from the uterotrophic response. Comparative analysis of TAM and EE treatments identified 1209 common, differentially expressed genes, the majority of which exhibited similar profiles despite a temporal delay in TAM elicited responses. However, several conserved and treatment specific responses were identified that are consistent with proliferation (Fos, Cdkn1a, Anapc1), and water imbibition (Slc30a3, Slc30a5) responses elicited by EE.

CONCLUSION

Overall, TAM and EE share similar gene expression profiles. However, TAM responses exhibit lower efficacy, while responses unique to EE are consistent with the physiological differences elicited between compounds.

Influence of hormones on osteogenic differentiation processes of mesenchymal stem cells

Bone development, regeneration and maintenance are governed by osteogenic differentiation processes from mesenchymal stem cells through to mature bone cells, which are directed by local growth and differentiation factors and modulated strongly by hormones. Mesenchymal stem cells develop from both mesoderm and neural crest and can give rise to development, regeneration and maintenance of mesenchymal tissues, such as bone, cartilage, muscle, tendons and discs. There are only limited data regarding the effects of hormones on early events, such as regulation of stemness and maintenance of the mesenchymal stem cell pool. Hormones, such as estrogens, vitamin D-hormone and parathyroid hormone, besides others, are important modulators of osteogenic differentiation processes and bone formation, starting off with fate decision and the development of osteogenic offspring from mesenchymal stem cells, which end up in osteoblasts and osteocytes. Hormones are involved in fetal bone development and regeneration and, in childhood, adolescence and adulthood, they control adaptive needs for growth and reproduction, nutrition, physical power and crisis adaptation. As in other tissues, aging in mesenchymal stem cells and their osteogenic offspring is accompanied by the accumulation of genomic and proteomic damage caused by oxidative burden and insufficient repair. Failsafe programs, such as apoptosis and cellular senescence avoid tumorigenesis. Hormones can influence the pace of such events, thus supporting the quality of tissue regeneration in aging organisms in vivo; for example, by delaying osteoporosis development. The potential for hormones in systemic therapeutic strategies is well appreciated and some concepts are approved for clinical use already. Their potential for cell-based therapeutic strategies for tissue regeneration is probably underestimated and could enhance the quality of tissue-engineering constructs for transplantation and the concept of in situ-guided tissue regeneration.

Association of estrogen receptor alpha gene polymorphisms with bone mineral density in postmenopausal Indian women

Bone mineral density (BMD) is the major determinant of osteoporotic fracture risk with a particular genetic background. However, consensus on the association of BMD with specific gene locus has not been reached. In the present study, we investigated the potential association of estrogen receptor alpha (ER alpha) gene intron I polymorphisms with BMD in 246 postmenopausal Indian women (average age 54.2+/-3.4 years). All the subjects were genotyped for XbaI and PvuII polymorphisms and underwent BMD measurements at spine and hip by dual energy X-ray absorptiometery. The average BMD of subjects with the genotypes XX and PP (absence of restriction sites for XbaI and PvuII, respectively) was 12.7 and 5.4% higher at the spine and 13.1 and 4.6% higher at the hip, respectively, than those with genotypes xx and pp. In age vs. BMD scatterplot, the intercept and slope of regression lines for genotypes xx and pp at spine and hip demonstrated comparatively rapid decrease in BMD across the age. The genotype XX was significantly prevalent (p<0.001) in women with normal bone mass (32%) and genotype xx in women with osteoporotic bone mass (35.3%), within the group. A significantly higher relative risk was associated with xx genotype. The study concludes that genetic variations at ER alpha gene locus, perhaps, are associated with BMD in Indian women and may influence some determinant of bone metabolism resulting in accelerated bone loss with age.