Prostaglandin-stimulated second messenger signaling in bone-derived endothelial cells is dependent on confluency in culture

Impact of cell cycle dynamics on pathology recognition: Raman imaging study

Confocal Raman imaging combined with fluorescence-activated cell sorting was used for in vitro studies of cell cultures to look at biochemical differences between the cells in different cell phases. To answer the question what is the impact of the cell cycle phase on discrimination of pathological cells, the combination of several factors was checked: a confluency of cell culture, the cell cycle dynamics and development of pathology. Confluency of 70% and 100% results in significant phenotypic cell changes that can be also diverse for different batches. In 100% confluency cultures, cells from various phases become phenotypically very similar and their recognition based on Raman spectra is not possible. For lower confluency, spectroscopic differences can be found between cell cycle phases (G₀ /G₁ , S and G₂ /M) for control cells and cells incubated with tumor necrosis factor alpha (TNF-α), but when the mycotoxin cytochalasin B is used the Raman signatures of cell phases are not separable. Generally, this work shows that heterogeneity between control and inflamed cells can be bigger than heterogeneity between cell cycle phases, but it is related to several factors, and not always can be treated as a rule.

An in vivo model to study and manipulate the hematopoietic stem cell niche

Because the microenvironment that supports hematopoietic stem cell (HSC) proliferation and differentiation is not fully understood, we adapted a heterotopic bone formation model as a new approach for studying the HSC microenvironment in vivo. Endogenous HSCs homed to tissue-engineered ossicles and individually sorted HSCs from ossicles were able to reconstitute lethally irradiated mice. To further explore this model as a system to study the stem cell niche, ossicles were established with or without anabolic parathyroid hormone (PTH) treatment during the 4-week course of bone development. Histology and micro-computed tomography showed higher bone area-to-total area ratios, thicker cortical bone and trabecular bone, significantly higher bone mineral density and bone volume fraction in PTH-treated groups than in controls. By an in vivo competitive long-term reconstitution assay, HSC frequency in the ossicle marrow was 3 times greater in PTH groups than in controls. When whole bone marrow cells were directly injected into the ossicles after lethal irradiation, the PTH-treated groups showed an enhanced reconstitution rate compared with controls. These findings suggest the residence of HSCs in heterotopic bone marrow and support the future use of this ossicle model in elucidating the composition and regulation of the HSC niche.

Nanomolar level of ouabain increases intracellular calcium to produce nitric oxide in rat aortic endothelial cells

Changes in [Ca(2+)](i) across the cell membrane and/or the sarcoplasmic reticulum regulate endothelial nitric oxide (NO) synthase activity. In the present study, we investigated the effect of ouabain, a specific inhibitor of Na(+)/K(+)-ATPase, on NO release and [Ca(2+)](i) movements in cultured rat aortic endothelial cells (RAEC) by monitoring NO production continuously using an NO-specific real-time sensor and by measuring the change in [Ca(2+)](i) using a fluorescence microscopic imaging technique with high-speed wavelength switching. The t((1/2)) (half-time of the decline of [Ca(2+)](i) to basal levels after stimulation with 10 micro mol/L bradykinin) was used as an index of [Ca(2+)](i) extrusion. A very low concentration of ouabain (10 nmol/L) did not increase the peak of NO production, but decreased the decay of NO release and, accordingly, increased integral NO production by the maximal dose-response concentration induced by bradykinin. The same dose of ouabain affected [Ca(2+)](i) movements across the cell membrane and/or sarcoplasmic reticulum induced by bradykinin with a time-course similar to that of NO release. Moreover, the t((1/2)) was significantly increased. Pretreatment of RAEC with Na(+)-free solution, an inhibitor of the Na(+)/Ca(2+) exchanger, and nickel chloride hexahydrate prevented the effects induced by bradykinin and ouabain. These observations using real-time recording indicate that a small amount of ouabain contributes to the bradykinin-stimulated increase of NO production through inhibition of plasma membrane Na(+)/K(+)-ATPase activity and an increase in intracellular Na(+) concentrations. The membrane was then depolarized, leading to a decline in the bradykinin-stimulated increase in [Ca(2+)](i) by forward mode Na(+)/Ca(2+) exchange to prolong the Ca(2+) signal time. From these results, we suggest that nanomolar levels of ouabain modulate [Ca(2+)](i) movements and NO production in RAEC.

Inhibition of gap junction intercellular communication by extremely low-frequency electromagnetic fields in osteoblast-like models is dependent on cell differentiation

Electromagnetic fields have been used to augment the healing of fractures because of its ability to increase new bone formation. The mechanism of how electromagnetic fields can promote new bone formation is unknown, although the interaction of electromagnetic fields with components of the plasma membrane of cells has been hypothesized to occur in bone cells. Gap junctions occur among bone forming cells, the osteoblasts, and have been hypothesized to play a role in new bone formation. Thus it was investigated whether extremely low-frequency (ELF) magnetic fields alter gap junction intercellular communication in the pre-osteoblastic model, MC3T3-E1, and the well-differentiated osteoblastic model, ROS 17/2.8. ELF magnetic field exposure systems were designed to be used for an inverted microscope stage and for a tissue culture incubator. Using these systems, it was found that magnetic fields over a frequency range from 30 to 120 Hz and field intensities up to 12.5 G dose dependently decreased gap junction intercellular communication in MC3T3-E1 cells during their proliferative phase of development. The total amount of connexin 43 protein and the distribution of connexin 43 gap junction protein between cytoplasmic and plasma membrane pools were unaltered by treatment with ELF magnetic fields. Cytosolic calcium ([Ca(2+)](i)) which can inhibit gap junction communication, was not altered by magnetic field exposure. Identical exposure conditions did not affect gap junction communication in the ROS 17/2.8 cell line and when MC3T3-E1 cells were more differentiated. Thus ELF magnetic fields may affect only less differentiated or pre-osteoblasts and not fully differentiated osteoblasts. Consequently, electromagnetic fields may aid in the repair of bone by effects exerted only on osteoprogenitor or pre-osteoblasts.

PGE(2) is essential for gap junction-mediated intercellular communication between osteocyte-like MLO-Y4 cells in response to mechanical strain

We have observed, in our previous studies, that fluid flow increases gap junction-mediated intercellular coupling and the expression of a gap junction protein, connexin 43, in osteocyte-like MLO-Y4 cells. Interestingly, this stimulation is further enhanced during the poststress period, indicating that a released factor(s) is likely to be involved. Here, we report that the conditioned medium obtained from the fluid flow-treated MLO-Y4 cells increased the number of functional gap junctions and connexin 43 protein. These changes are similar to those observed in MLO-Y4 cells directly exposed to fluid flow. Fluid flow was found to induce PGE(2) release and increase cyclooxygenase 2 expression. Treatment of the cells with PGE(2) had the same effect as fluid flow, suggesting that PGE(2) could be responsible for these autocrine effects. When PGE(2) was depleted from the fluid flow-conditioned medium, the stimulatory effect on gap junctions was partially, but significantly, decreased. Addition of the cyclooxygenase inhibitor, indomethacin, partially blocked the stimulatory effects of mechanical strain on gap junctions. Taken together, these studies suggest that the stimulatory effect of fluid flow on gap junctions is mediated, in part, by the release of PGE(2). Hence, PGE(2) is an essential mediator between mechanical strain and gap junctions in osteocyte-like cells.

Selectivity of endotoxin-induced defect in endothelial calcium mobilization

BACKGROUND

We hypothesized that endotoxin (LPS) would impair bradykinin (BK)-induced calcium (Ca2+) mobilization in aortic endothelial cells, perhaps due to cytotoxicity or via stimulation of nitric oxide (NO) synthesis. As well, we sought to define contributions of LPS-stimulated Ca2+ mobilization to these effects.

METHODS

LPS- or BK-induced increments of intracellular Ca2+ were assessed by microspectrofluorimetry with fura-2 in passaged bovine aortic endothelial cells. Time- and dose-dependent effects of LPS exposure (+/- inhibitors of NO or prostaglandin synthesis) on subsequent BK-induced Ca2+ mobilization and on attached cell counts were determined.

RESULTS

LPS (0.1 to 1.0 mg/ml) led to rapid increments of Ca2+, while Ca2+ responses were delayed following LPS (1 to 10 microg/ml) and lower doses were without effect. By contrast, LPS more potently (1.0 pg to 1.0 microg/ml) led to dose- and time-dependent impairment of subsequent BK-induced Ca2+ mobilization, with peak effect at four to six hours, persisting for at least 18 hours. This delayed effect on BK-response was unaltered by inhibition of either NO synthase or cyclooxygenase. The effect of LPS on BK-responsivity depended importantly on cell confluence, as it was not observed in subconfluent cells. By contrast, LPS-induced cell detachment, which was observed only at doses > or = 1.0 microg/ml, did not depend on confluence.

CONCLUSIONS

Different mechanisms lead to endothelial cytotoxicity and to impaired BK-response following LPS. Only the former effect, occurring at higher doses, might depend on initial LPS-induced Ca2+ mobilization.

Expression of parathyroid hormone/parathyroid hormone-related protein receptor in vascular endothelial cells

The parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor has been reported to be expressed in many tissues, including vascular smooth-muscle cells (VSMCs), but it has not been identified in vascular endothelial cells. To determine whether vascular endothelial cells can express the PTH/PTHrP receptor, its gene expression was examined in simian virus 40-transformed rat lung vascular endothelial cells (TRLECs) by the reverse-transcription polymerase chain reaction (RT-PCR). Results in TRLEC, with rat VSMCs and kidney as controls, showed identical 741-bp products. Furthermore, incubation with PTHrP[1-34] reduced the thrombin-stimulated endothelin-1 (ET-1) expression in TRLECs. Our results demonstrate that vascular endothelial cells can express the PTH/PTHrP receptor and therefore are also a target tissue for PTHrP.

HPE cells: a clonal endothelial cell line established from human parathyroid tissue (human parathyroid cell line)

We report the culture and cloning of human endothelial cells derived from parathyroid tissue surgically removed from a patient affected by Multiple Endocrine Neoplasia Type 1 syndrome. These cells, known as HPE, have been isolated and maintained in culture by serial passages for more than 15 months. The clonal cell line grows in a medium containing serum substitutes which favour endothelial cell growth. HPE cells replicate with a mean doubling time of 120 h, showing typical functional and morphological features of endothelial cells, such as uptake of acetylated low density lipoprotein and positive reaction for Factor VIII-Related Antigen. Basic fibroblast growth factor, vascular endothelial growth factor, insulin-like growth factor type I and ascorbic acid stimulate cell proliferation, whereas transforming growth factor beta and heparin act as inhibitory factors. Prostaglandin E2, secretin and epinephrine increased cAMP production, while human parathyroid hormone, histamine and glucagon were inert. Cells were found to express pro-collagen alpha 1 (type I) mRNA. In HPE cells Restriction Fragments Length Polymorphism and PCR analysis did not show allelic loss at chromosome 11q12-13, known to be a typical feature of MEN 1 parathyroid tumors. These cells are the first example of an established normal human clonal cell line with an endothelial phenotype.

Mechanisms of electrolyte transport across the endometrium. I. Regulation by PGF2 alpha and cAMP

The purpose of this study was to characterize the transport mechanisms in endometrial epithelial cells that are responsible for regulation of Na and K concentrations in uterine luminal fluid. Porcine endometrial tissues were mounted in Ussing chambers and bathed in plasmalike Ringer solution. The mean basal short-circuit current (Isc) was 40 microA/cm2, and the mean tissue conductance was 3.6 mS/cm2. Addition of amiloride to the luminal solution inhibited 86% of the basal Isc. Concentration-response experiments using amiloride analogues showed a rank order of potency of benzamil > amiloride > 5-(N-methyl-N-isobutyl)-amiloride in blocking the Isc, with no response to ethylisopropylamiloride. Na channel immunoreactivity was localized to the apical membrane of surface epithelial cells. The Na-to-K selectivity ratio of the amiloride-sensitive Na channel was calculated to be 6.4:1. Prostaglandin (PG) F2 alpha or 8-(chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPT-cAMP) added to the luminal solution stimulated a twofold increase in Isc that was inhibited by pretreatment with amiloride. Experiments using both amphotericin B-permeabilized tissues and intact tissues showed that PGF2 alpha and cAMP increased Na absorption by activation of basolateral K channels. Treatment of the luminal solution with 4-aminopyridine produced an effect on Isc that was consistent with block of K secretion and a subsequent decrease in Na absorption. These experiments showed that Na and K transport are tightly coupled processes occurring under basal conditions in surface endometrial epithelial cells and that these processes are regulated by PGF2 alpha and cAMP.