Identification of a PTH regulated gene selectively induced in vivo during PTH-mediated bone formation

Levels of endocrine-disrupting chemicals are associated with changes in the peri-pubertal epigenome

Puberty marks a transition period, which leads to the attainment of adult sexual maturity. Timing of puberty is a strongly heritable trait. However, large genetic association studies can only explain a fraction of the observed variability and striking secular trends suggest that lifestyle and/or environmental factors are important. Using liquid-chromatography tandem-mass-spectrometry, we measured endocrine-disrupting chemicals (EDCs; triclosan, bisphenol A, benzophenone-3, 2,4-dichlorophenol, 11 metabolites from 5 phthalates) in longitudinal urine samples obtained biannually from peri-pubertal children included in the COPENHAGEN puberty cohort. EDC levels were associated with blood DNA methylation profiles from 31 boys and 20 girls measured both pre- and post-pubertally. We found little evidence of single methylation sites that on their own showed association with urinary excretion levels of EDCs obtained either the same-day or measured as the yearly mean of dichotomized EDC levels. In contrast, methylation of several promoter regions was found to be associated with two or more EDCs, overlap with known gene-chemical interactions, and form a core network with genes known to be important for puberty. Furthermore, children with the highest yearly mean of dichotomized urinary phthalate metabolite levels were associated with higher promoter methylation of the thyroid hormone receptor interactor 6 gene (TRIP6), which again was mirrored by lower circulating TRIP6 protein levels. In general, the mean TRIP6 promoter methylation was mirrored by circulating TRIP6 protein levels. Our results provide a potential molecular mode of action of how exposure to environmental chemicals may modify pubertal development.

Complex dynamics of transcription regulation

Transcription is a tightly regulated cellular function which can be triggered by endogenous (intrinsic) or exogenous (extrinsic) signals. The development of novel techniques to examine the dynamic behavior of transcription factors and the analysis of transcriptional activity at the single cell level with increased temporal resolution has revealed unexpected elements of stochasticity and dynamics of this process. Emerging research reveals a complex picture, wherein a wide range of time scales and temporal transcription patterns overlap to generate transcriptional programs. The challenge now is to develop a perspective that can guide us to common underlying mechanisms, and consolidate these findings. Here we review the recent literature on temporal dynamics and stochastic gene regulation patterns governed by intrinsic or extrinsic signals, utilizing the glucocorticoid receptor (GR)-mediated transcriptional model to illustrate commonality of these emerging concepts. This article is part of a Special Issue entitled: Chromatin in time and space.

Intermittent PTH(1-34) signals through protein kinase A to regulate osteoprotegerin production in human periodontal ligament cells in vitro

Periodontal ligament (PDL) cells have been associated with the regulation of periodontal repair processes by the differential expression of osteoprotegerin and RANKL in response to intermittent parathyroid hormone (PTH) resulting in a modified activity of bone-resorbing osteoclasts. Here, we examined the intracellular signaling pathways that PDL cells use to mediate the PTH(1-34) effect on osteoprotegerin production and hypothesized that those would be dependent on the cellular maturation stage. Two stages of confluence served as a model for cellular maturation of 5th passage human PDL cells from six donors. Intermittent PTH(1-34) (10(-12) M) and PTH(1-31), the latter lacking the protein kinase C (PKC) activating domain, induced a significant decrease of osteoprotegerin production in confluent cultures, whereas the signal-specific fragments PTH(3-34) and PTH(7-34), which both are unable to activate protein kinase A (PKA), had no effect. The addition of the PKA inhibitor H8 antagonized the PTH(1-34) effect, whereas the PKC inhibitor RO-32-0432 did not. In pre-confluent, less mature cultures, intermittent PTH(1-34) resulted in a significant increase of osteoprotegerin. Similar results were obtained when PTH(1-31) substituted for PTH(1-34) as opposed to a lack of an effect of PTH(3-34) and PTH(7-34). Likewise, in confluent cultures, H8 inhibited the PTH(1-34) effect in pre-confluent cultures contrasted by RO-32-0432 which had no effect. These findings indicate that PTH(1-34) signaling targeting osteoprotegerin production in PDL cells involves a PKA-dependent pathway. The PTH(1-34) effect is dependent on cell status, whereas intracellular signal transduction is not. Clinical trials will have to prove whether those in vitro data are of physiological relevance for interference strategies.

Parathyroid hormone synergizes with non-cyclic AMP pathways to activate the cyclic AMP response element

Parathyroid hormone (PTH) activates multiple signaling pathways following binding to the PTH1 receptor in osteoblasts. Previous work revealed a discrepancy between cAMP stimulation and CRE reporter activation of truncated PTH peptides, suggesting that additional signaling pathways contribute to activation of the CRE. Using a CRE-Luciferase reporter containing multiple copies of the CRE stably transfected into the osteoblastic cell line Saos-2, we tested the ability of modulators of alternative pathways to activate the CRE or block the PTH-induced activation of the CRE. Activators of non-cyclic AMP pathways, that is, EGF (Akt, MAPK, JAK/STAT pathways); thapsigargin (intracellular calcium pathway); phorbol myristate acetate (protein kinase C, PKC pathway) induced minor increases in CRE-luciferase activity alone but induced dramatic synergistic effects in combination with PTH. The protein kinase A (PKA) inhibitor H-89 (10 microM) almost completely blocked PTH-induced activation of the CRE-reporter. Adenylate cyclase inhibitors SQ 22536 and DDA had profound and time-dependent biphasic effects on the CRE response. The MAPK inhibitor PD 98059 partially inhibited basal and PTH-induced CRE activity to the same degree, while the PKC inhibitor bisindolylmaleimide (BIS) had variable effects. The calmodulin kinase II inhibitor KN-93 had no significant effect on the response to PTH. We conclude that non-cAMP pathways (EGF pathway, calcium pathway, PKC pathway) converge on, and have synergistic effects on, the response of a CRE reporter to PTH.

Targeting the stem cell niche: squeezing blood from bones

During human development, stem cells establish themselves in specific anatomic locations or niches. The niche harbors the stem cells, and regulates how stem cells proliferate. The interaction between stem cells and their niche affects stem cell function, and offers an opportunity to improve the marrow microenvironment. Osteoblasts produce hematopoietic growth factors and are activated by parathyroid hormone (PTH). A calcium sensing receptor, expressed by hematopoietic stem cells, regulates the niche and can be targeted to increase stem cell numbers. Therefore, drugs that affect osteoblast function or target calcium receptors may be useful for stem cell mobilization and engraftment. In this review, the biology of the stem cell niche and the potential therapeutic manipulations of the stem cell niche are reviewed. PTH is in clinical trials for patients who have not mobilized autologous stem cells well. The limiting cell numbers for adult cord blood transplantation increase the risk of infection, and PTH is currently in a clinical trial following cord blood transplantation in an effort to improve engraftment and immune reconstitution.