Parathyroid hormone-stimulated resorption in calvaria cultured in serum-free medium is enhanced by the calcium-mobilizing activity of 1,25-dihydroxyvitamin D(3)

Examining Mechanisms for Voltage-Sensitive Calcium Channel-Mediated Secretion Events in Bone Cells

In addition to their well-described functions in cell excitability, voltage-sensitive calcium channels (VSCCs) serve a critical role in calcium (Ca2+)-mediated secretion of pleiotropic paracrine and endocrine factors, including those produced in bone. Influx of Ca2+ through VSCCs activates intracellular signaling pathways to modulate a variety of cellular processes that include cell proliferation, differentiation, and bone adaptation in response to mechanical stimuli. Less well understood is the role of VSCCs in the control of bone and calcium homeostasis mediated through secreted factors. In this review, we discuss the various functions of VSCCs in skeletal cells as regulators of Ca2+ dynamics and detail how these channels might control the release of bioactive factors from bone cells. Because VSCCs are druggable, a better understanding of the multiple functions of these channels in the skeleton offers the opportunity for developing new therapies to enhance and maintain bone and to improve systemic health.

Redefining the impact of nutrition on breast cancer incidence: is epigenetics involved?

Breast cancer incidence is rising worldwide with an increase in aggressive neoplasias in young women. Possible factors involved include lifestyle changes, notably diet that is known to make an impact on gene transcription. However, among dietary factors, there is sufficient support for only greater body weight and alcohol consumption whereas numerous studies revealing an impact of specific diets and nutrients on breast cancer risk show conflicting results. Also, little information is available from middle- and low-income countries. The diversity of gene expression profiles found in breast cancers indicates that transcription control is critical for the outcome of the disease. This suggests the need for studies on nutrients that affect epigenetic mechanisms of transcription, such as DNA methylation and post-translational modifications of histones. In the present review, a new examination of the relationship between diet and breast cancer based on transcription control is proposed in light of epidemiological, animal and clinical studies. The mechanisms underlying the impact of diets on breast cancer development and factors that impede reaching clear conclusions are discussed. Understanding the interaction between nutrition and epigenetics (gene expression control via chromatin structure) is critical in light of the influence of diet during early stages of mammary gland development on breast cancer risk, suggesting a persistent effect on gene expression as shown by the influence of certain nutrients on DNA methylation. Successful development of breast cancer prevention strategies will require appropriate models, identification of biological markers for rapid assessment of preventive interventions, and coordinated worldwide research to discern the effects of diet.

The vitamin D receptor agonist elocalcitol upregulates L-type calcium channel activity in human and rat bladder

Human bladder contraction mainly depends on Ca2+ influx via L-type voltage-gated Ca2+ channels and on RhoA/Rho kinase contractile signaling, which is upregulated in overactive bladder (OAB). Elocalcitol is a vitamin D receptor agonist inhibiting RhoA/Rho kinase signaling in rat and human bladder. Since in the normal bladder from Sprague-Dawley rats elocalcitol treatment delayed the carbachol-induced contraction without changing maximal responsiveness and increased sensitivity to the L-type Ca2+ channel antagonist isradipine, we investigated whether elocalcitol upregulated L-type Ca2+ channels in human bladder smooth muscle cells (hBCs). In hBCs, elocalcitol induced a rapid increase in intracellular [Ca2+], which was abrogated by the L-type Ca2+ channel antagonist verapamil. Moreover, hBCs exhibited L-type voltage-activated Ca2+ currents (I Ca), which were selectively blocked by isradipine and verapamil and enhanced by the selective L-type agonist BAY K 8644. Addition of elocalcitol (10(-7) M) increased L-type I Ca size and specific conductance by inducing faster activation and inactivation kinetics than control and BAY K 8644, while determining a significant negative shift of the activation and inactivation curves, comparable to BAY K 8644. These effects were strengthened in long-term treated hBCs with elocalcitol (10(-8) M, 48 h), which also showed increased mRNA and protein expression of pore-forming L-type alpha(1C)-subunit. In the bladder from Sprague-Dawley rats, BAY K 8644 induced a dose-dependent increase in tension, which was significantly enhanced by elocalcitol treatment (30 microg.kg(-1).day(-1), 2 wk). In conclusion, elocalcitol upregulated Ca2+ entry through L-type Ca2+ channels in hBCs, thus balancing its inhibitory effect on RhoA/Rho kinase signaling and suggesting its possible efficacy for the modulation of bladder contractile mechanisms.

Calcium uptake and membrane trafficking in response to PTH or 25(OH)D3 in polarized intestinal epithelial cells

Cell culture techniques providing retention of the polarized enterocyte morphology has allowed, for the first time, comparison of parathyroid hormone (PTH)- and 25-hydroxyvitamin D(3) [25(OH)D(3)]-induced (45)Ca uptake with membrane trafficking events discerned using confocal microscopy. Treatment of cells with 65 pM bPTH(1-34) promoted enhanced (45)Ca uptake between 1 and 10 min after peptide. The protein kinase A (PKA) antagonist, RpcAMP inhibited hormone-mediated uptake. At the microscopic level, cells labeled with the endocytic tracking dye FM1-43 revealed increased punctate staining 50-550s after hormone. Pretreatment of cells with RpcAMP abolished this pattern of staining. The calcium indicator dye fluo-3 AM revealed faint punctate labeling in controls, with increased bands of punctate labeling in the apical region of the cells after peptide hormone, and ultimately the basal region. Parallel studies conducted with the metabolite 25(OH)D(3) resulted in a slower stimulation of (45)Ca uptake 5-10 min after steroid, which was also inhibited by preincubation with RpcAMP. Cells labeled with FM1-43 and then treated with steroid showed no change in distribution of fluorescence during the 10 min incubation period. Confocal microscopy with fluo-3 revealed intense apical fluorescence--that after steroid --streamed to a perinuclear position, and ultimately the basal area. Uniformly diffuse staining, which would indicate cytoplasmic calcium transport, was observed only in controls. Membrane trafficking and compartmentalized calcium appear to be integral to agonist mediated cation transport.

Regulation of expression of 1,25D3-MARRS/ERp57/PDIA3 in rat IEC-6 cells by TGF beta and 1,25(OH)2D3

We examined the transcriptional regulation of expression of the redox-sensitive Membrane-Associated-Rapid Response, Steroid-binding (1,25D(3)-MARRS) protein specific for 1,25(OH)(2)D(3) in a rat small intestinal cell line, IEC-6, that demonstrates rapid responses to 1,25(OH)(2)D(3). 1,25D(3)-MARRS binds and is activated by 1,25(OH)(2)D(3), but is not itself up-regulated by treatment with 1,25(OH)(2)D(3), nor is there a Vitamin D response element (VDRE) in its proximal promoter. We previously reported that transforming growth factor beta (TGFbeta) increased steady state levels of 1,25D(3)-MARRS transcript and protein approximately two-fold [Rohe B, Safford SE, Nemere I, Farach-Carson, MC. Identification and characterization of 1,25D(3)-membrane-associated rapid response, steroid (1,25D(3)-MARRS)-binding protein in rat IEC-6 cells. Steroids 2005;70:458-63]. To determine if this up-regulation could be attributed to the function of a highly conserved consensus smad 3 binding element present in the proximal promoter of the 1,25D(3)-MARRS gene, we created a promoter-reporter [SEAP] construct that was responsive to TGFbeta (200 pM). Deletion or mutation of the smad 3 element greatly reduced the response of the 1,25D(3)-MARRS promoter to TGFbeta. Subsequent studies found that the smad 3 response element is bound by a protein found in the IEC-6 nuclear extract, most likely smad 3. Interestingly, although 1,25(OH)(2)D(3) alone did not increase expression of the 1,25D(3)-MARRS promoter-reporter, co-treatment of transfected IEC-6 cells with 1,25(OH)(2)D(3) and TGFbeta shifted the dose-response curve to a lower effective concentration (100 pM peptide). We conclude that TGFbeta is a transcriptional regulator of 1,25D(3)-MARRS expression via a functional smad 3 element and that cross-talk with non-classical 1,25(OH)(2)D(3)-stimulated pathways occurs. The findings have broad implications for redox-sensitive signaling phenomena including those that regulate phosphate transport in the intestine.

The role of BMP-6, IL-6, and BMP-4 in mesenchymal stem cell-dependent bone development: effects on osteoblastic differentiation induced by parathyroid hormone and vitamin D(3)

Human bone marrow-derived mesenchymal stem cells (MSCs) represent an ideal source for cell therapy for inherited and degenerative diseases, bone and cartilage repair, and as target for gene therapy. The role of the combination of human parathyroid hormone (PTH) and vitamin D(3) in bone formation and mineralization has been established in several osteoblast cell culture studies. The aim of the present study was to evaluate the role of this hormonal combination alone and in the presence of bone morphogenetic protein-4 (BMP-4) or-6 (BMP-6) in inducing osteogenic differentiation of human MSC. Human MSC derived from adult normal bone marrow that are positive for CD29, CD44, CD105, and CD166 and negative for CD14, CD34, and CD45, were treated with the PTH and 1,25-dihydroxyvitamin D(3) in the presence and absence of recombinant human BMP-4 or BMP6. PTH and vitamin D(3) induced high levels of expression of two key markers of bone formation: osteocalcin and alkaline phosphatase by MSCs. BMP-6 but not BMP-4 increased osteocalcin expression induced by PTH and vitamin D(3). Both BMPs enhanced calcium formation in MSC cultures and this response was potentiated by PTH and vitamin D(3). The present results revealed a novel potent effect of PTH and vitamin D(3) plus BMPs in inducing bone development by human MSCs. These results may facilitate therapeutic utility of MSCs for bone disease and help clarify mechanisms involved in stem cell-mediated bone development.

Osteoblast Ca(2+) permeability and voltage-sensitive Ca(2+) channel expression is temporally regulated by 1,25-dihydroxyvitamin D(3)

The cardiac subtype of the L-type voltage-sensitive Ca(2+) channel (VSCC) Cav1.2 (alpha(1C)) is the primary voltage-sensitive channel responsible for Ca(2+) influx into actively proliferating osteoblasts. This channel also serves as the major transducer of Ca(2+) signals in growth-phase osteoblasts in response to hormone treatment. In this study, we have demonstrated that 24-h treatment of MC3T3-E1 preosteoblasts with 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], a coupling factor for bone resorption, coordinately downregulates Cav1.2 (alpha(1C)) and uniquely upregulates T-type channel Cav3.2 (alpha(1H)). No other voltage-sensitive channel alpha-subunit of the 10 that were surveyed was upregulated by 1,25(OH)(2)D(3). The shift from predominantly L-type to T-type channel expression has been demonstrated to occur at both mRNA and protein levels detected using quantitative PCR and immunohistochemistry with antibodies specific for each channel type. Functional and pharmacological studies using specific inhibitors have revealed that treatment with 1,25(OH)(2)D(3) also alters the Ca(2+) permeability properties of the osteoblast membrane from a state of primarily L-current sensitivity to T-current sensitivity. We conclude that the L-type channel is likely to support proliferation of osteoblast cells, whereas T-type channels are more likely to be involved in supporting differentiated functions after 1,25(OH)(2)D(3)-mediated reversal of remodeling has occurred. This latter observation is consistent with the unique expression of the T-type VSCC Cav3.2 (alpha(1H)) in terminally differentiated osteocytes as we recently reported.

Influence of brain injury on early posttraumatic bone metabolism

BACKGROUND

Various clinical studies and observations demonstrate enhanced osteogenesis in patients sustaining traumatic brain injury. It is presumed that the induction of this process starts early after trauma. The purpose of our study was to investigate humoral markers of bone metabolism during the early posttraumatic period, with special regard to traumatic brain injury.

METHODS

Serum concentrations of biochemical markers of bone metabolism (calcium, inorganic phosphorus, carboxyl-terminal propeptide of type 1 procollagen, pyridinoline cross-linked telopeptide domain of type 1 collagen, Ostase, osteocalcin, intact parathyroid hormone, and calcitonin) were measured in three different groups of 80 patients during the first posttraumatic week. Patients were categorized into three groups: group I, fractures only; group II, isolated traumatic brain injury; and group III, traumatic brain injury in combination with fractures.

RESULTS

Osteocalcin levels were significantly lower in the presence of traumatic brain injury (p < .05). Elevated pyridinoline cross-linked telopeptide domain of type 1 collagen levels expressed enhanced bone resorption in all groups, but levels were significantly higher in the absence of traumatic brain injury (p < .05). Intact parathyroid hormone levels were significantly higher on days 0 and 1 in the combined presence of traumatic brain injury plus fractures.

CONCLUSION

These results demonstrate an imbalance of bone formation and resorption parameters in patients with traumatic brain injury during the early posttraumatic period, suggesting a central regulation in bone formation. The lower levels of osteocalcin detected in this study may play an important role in patients with brain injury and the later development of posttraumatic heterotopic ossification.