Connexin43 gene expression in the rabbit arterial wall: effects of hypercholesterolemia, balloon injury and their combination

The specialized functions of endothelium require intercellular communication between endothelial cells within the monolayer, and between endothelium and other cells present in the vessel wall. This is accomplished by a combination of paracrine soluble mediators and direct gap-junctional intercellular communication (GJIC) mediated by a family of connexin proteins. A prominent connexin expressed by vascular cells in vivo and in vitro is connexin 43 (Cx43). We have investigated the in vivo gene regulation of Cx43 in the context of vascular pathology, as a result of mechanical injury, hypercholesterolemia or both. The aortoiliac bifurcation in the rabbit was examined following three types of insult: (1) diet-induced hypercholesterolemia resulting in macrophage-rich fatty streak lesions, (2) mechanical, stretch-denudation injury resulting in intimal smooth muscle cell (SMC) proliferation and (3) mechanical injury superimposed on hypercholesterolemia resulting in a complex vascular lesion having characteristics of both interventions. The normal rabbit iliac artery expressed approximately equal levels of Cx43 mRNA in the medial SMC layers and in the endothelium. In hypercholesterolemia-induced atherosclerosis, Cx43 expression was most prominent in macrophage foam cells even though normocholesterolemic precursor monocytes did not express Cx43 mRNA. Antibodies directed specifically to Cx43 protein confirmed the expression of macrophage gap junction protein in these cells. Medial SMC in hypercholesterolemia exhibited less Cx43 than their normal counterparts in control animals. Mechanical injury in the absence of hypercholesterolemia resulted in intimal thickening in which Cx43 expression in the intimal SMC was equivalent to that in the subjacent medial SMC, both being approximately equivalent to normal uninjured rabbit medial SMC expression. Cell-specific expression of Cx43 in combined mechanical injury/hypercholesterolemia was similar to that observed in hypercholesterolemia alone: Cx43 upregulation in macrophages, while medial SMC were downregulated. Normo- and hypercholesterolemic alveolar macrophages of the lung and Kupffer cells of the liver did not exhibit induction of Cx43 mRNA, nor did macrophages isolated from peritoneal or bronchial lavage fluid of the same animals. This work extends our previous finding of Cx43 upregulation in human atherectomy tissue and demonstrates that atherosclerotic lesions in situ, in a controlled animal model of atherosclerosis, exhibit cell-specific changes in Cx43 gene expression. Changes in medial SMC migration, proliferation and phenotype, as well as enhanced interactions between adherent/infiltrating monocytes and endothelium may be related to modified GJIC pathways in the vessel wall.

Spatial relationships in early signaling events of flow-mediated endothelial mechanotransduction

Blood flow interactions with the vascular endothelium represent a specialized example of mechanical regulation of cell function that has important physiological and pathological cardiovascular consequences. The endothelial monolayer in vivo acts as a signal transduction interface for forces associated with flowing blood (hemodynamic forces) in the acute regulation of artery tone and chronic structural remodeling of arteries, including the pathology of atherosclerosis. Mechanisms related to spatial relationships at the cell surfaces and throughout the cell that influence flow-mediated endothelial mechanotransduction are discussed. In particular, flow-mediated ion channel activation and cytoskeletal dynamics are considered in relation to topographic analyses of the luminal and abluminal surfaces of living endothelial cells.

Conformationally specific enhancement of receptor-mediated LDL binding and internalization by peptide models of a conserved anionic N-terminal domain of human apolipoprotein E

In this paper, we test the hypothesis that peptide models of a highly conserved domain of apolipoprotein E (amino acids 41-60 in human apo E) modulate the binding and internalization of LDL to cell surface receptors in a conformationally specific manner. Three peptides were compared: peptide I containing the natural sequence of amino acids 41-60 of human apo E; peptide III containing side-chain lactam cross-links designed to enhance alpha-helical structure; and peptide II containing cross-links designed to prevent formation of alpha-helices. Peptide III was shown previously to consist of two short alpha-helical domains linked by a turn and to have more alpha-helical content than peptide I, while peptide II was shown to have less helical content than either peptide III or I(Luo et al., 1994). Peptide III induced a 30-fold increase in the specific binding of 125I-LDL to normal human skin fibroblasts and a 60-fold increase in the binding to fibroblasts lacking the LDL-R. This same peptide also restored the binding to normal fibroblasts of 125I-LDL from a patient with familial defective apolipoprotein B, the R3500-->Q mutation. Analysis of binding indicated an increase in the apparent number of binding sites, with little effect on the affinity of 125I-LDL for the cell surface. Heparinase treatment of the cells did not abrogate this effect, suggesting that the increased binding is not mediated by cell surface glycans. LDL internalization but not degradation was also increased by peptide III. Similar but smaller effects were also induced by peptide I. Peptide II was much less active than peptide I or III. Thus, the order of biological activity was the same as the order of alpha-helical content, i.e., peptide III > peptide I > peptide II. These results suggest a hitherto unknown biological function for a highly conserved domain of apolipoprotein E, and this bioactivity was shown by peptide models to be specific to the alpha-helical conformation.

A mechanism for heterogeneous endothelial responses to flow in vivo and in vitro

Exposure of endothelium to a nominally uniform flow field in vivo and in vitro frequently results in a heterogeneous distribution of individual cell responses. Extremes in response levels are often noted in neighboring cells. Such variations are important for the spatial interpretation of vascular responses to flow and for an understanding of mechanotransduction mechanisms at the level of single cells. We propose that variations of local forces defined by the cell surface geometry contribute to these differences. Atomic force microscopy measurements of cell surface topography in living endothelium both in vitro and in situ combined with computational fluid dynamics demonstrated large cell-to-cell variations in the distribution of flow-generated shear stresses at the endothelial luminal surface. The distribution of forces throughout the surface of individual cells of the monolayer was also found to vary considerably and to be defined by the surface geometry. We conclude that the endothelial three-dimensional surface geometry defines the detailed distribution of shear stresses and gradients at the single cell level, and that there are large variations in force magnitude and distribution between neighboring cells. The measurements support a topographic basis for differential endothelial responses to flow observed in vivo and in vitro. Included in these studies are the first preliminary measurements of the living endothelial cell surface in an intact artery.

Effects of wall shear stress and fluid recirculation on the localization of circulating monocytes in a three-dimensional flow model

There is a correlation between the location of early atherosclerotic lesions and the hemodynamic characteristics at those sites. Circulating monocytes are key cells in the pathogenesis of atherosclerotic plaques and localize at sites of atherogenesis. The hypothesis that the distribution of monocyte adhesion to the vascular wall is determined in part by hemodynamic factors was addressed by studying monocyte adhesion in an in vitro flow model in the absence of any biological activity in the model wall. Suspensions of U937 cells were perfused (Re = 200) through an axisymmetric silicone flow model with a stenosis followed by a reverse step. The model provided spatially varying wall shear stress, flow separation and reattachment, and a three-dimensional flow pattern. The cell rolling velocity and adhesion rates were determined by analysis of videomicrographs. Wall shear stress was obtained by numerical solution of the equations of fluid motion. Cell adhesion patterns were also studied in the presence of chemotactic peptide gradients. The cell rolling velocity varied linearly with wall shear stress. The adhesion rate tended to decrease with increasing local wall shear stress, but was also affected by the radial component of velocity and the dynamics of the recirculation region and flow reattachment. Adhesion was increased in the vicinity of chemotactic peptide sources downstream of the expansion site. Results with human monocytes were qualitatively similar to the U937 experiments. Differences in the adhesion rates of U937 cells occurring solely as a function of the fluid dynamic properties of the flow field were clearly demonstrated in the absence of any biological activity in the model wall.

Shear activated channels in cell-attached patches of cultured bovine aortic endothelial cells

We investigated the response of inward rectifier K+ (IRK) currents in bovine aortic endothelial cells (BAECs) to shear stress. Shear evoked reversible hyperpolarization in current clamped BAECs. Voltage clamped BAECs exhibited large inward and small outward whole cell K+ currents blocked by cesium and increased in amplitude by exposure to shear stress. The open state probability of IRK channels in cell-attached membrane patches was increased within minutes of exposure to shear stress. IRK channels in inside-out patches were activated by increases in [Ca2+]i from 10(-7) to 10(-6) mM. We demonstrate that shear stress induces hyperpolarization and gating of single channel and whole cell IRK currents in BAECs.

Flow-mediated endothelial mechanotransduction

Mechanical forces associated with blood flow play important roles in the acute control of vascular tone, the regulation of arterial structure and remodeling, and the localization of atherosclerotic lesions. Major regulation of the blood vessel responses occurs by the action of hemodynamic shear stresses on the endothelium. The transmission of hemodynamic forces throughout the endothelium and the mechanotransduction mechanisms that lead to biophysical, biochemical, and gene regulatory responses of endothelial cells to hemodynamic shear stresses are reviewed.

Subcellular distribution of shear stress at the surface of flow-aligned and nonaligned endothelial monolayers

The stresses acting on the luminal surface of endothelial cells due to shear flow were determined on a subcellular scale. Atomic force microscopy was used to measure the surface topography of confluent endothelial monolayers cultured under no-flow conditions or exposed to steady shear stress (12 dyn/cm2 for 24 h). Flow over these surface geometries was simulated by computational fluid dynamics, and the distribution of shear stress on the cell surface was calculated. Flow perturbations due to the undulating surface produced cell-scale variations of shear stress magnitude and hence large shear stress gradients. Reorganization of the endothelial surface in response to prolonged exposure to steady flow resulted in significant reductions in the peak shear stresses and shear stress gradients. From the relationship between surface geometry and the resulting shear stress distribution, we have defined a hydrodynamic shape factor that characterizes the three-dimensional morphological response of endothelial cells to flow. The analysis provides a complete description of the spatial distribution of stresses on individual endothelial cells within a confluent monolayer on a scale relevant to the study of physical mechanisms of mechanotransduction.

Endothelial Cell Surface Imaging: Insights Into Hemodynamic Force Transduction

Hemodynamic forces elicit a range of endothelial cellular responses that regulate tone and vascular structure. Here we review some new approaches to live-cell real-time imaging of endothelial cell surfaces that reveal the dynamic nature of these structures and their mutability when subjected to defined flow forces.

Stimulation of transcription factors NF kappa B and AP1 in endothelial cells subjected to shear stress

Hemodynamic shear stress forces influence several important endothelial genes associated with arterial relaxation, pro/anti-coagulation, and growth control; however, the regulatory pathways remain unclear. Here we demonstrate stimulation of DNA binding activities of nuclear factor kappa B (NF kappa B), a member of the Rel Family of transcription factors, and nuclear factor activator protein-1 (AP-1) following exposure of endothelial cells to unidirectional shear stress in laminar flow. NF kappa B binding was stimulated within 30 minutes, reaching and maintaining maximal levels at 1 hour. DNA binding activity was inhibited by pre-incubation of nuclear extract with antibody directed against NF kappa B p65 subunit. AP-1 binding activity was biphasic, rising fourfold within 20 minutes and returning to basal levels before steadily increasing by 2 hours to a high level relative to basal values. These protein kinase C-coupled transcriptional factors may modulate endothelial genes that are shear stress-responsive and that possess appropriate binding sites in the promoter region.

Quantitative studies of endothelial cell adhesion. Directional remodeling of focal adhesion sites in response to flow forces

Focal adhesion sites were observed in cultured endothelial cells by tandem scanning confocal microscopy and digitized image analysis, techniques that provide real-time images of adhesion site area and topography in living cells. Image subtraction demonstrated that in the presence of unidirectional steady laminar flow (shear stress [tau] = 10 dyn/cm2) a substantial fraction of focal adhesion sites remodeled in the direction of flow. In contrast, focal adhesions of control (no flow) cells remodeled without preferred direction. In confluent monolayers subjected to shear stresses of 10 dyn/cm2, cells began to realign in the direction of flow after 7-9 h. This was accompanied by redistribution of intracellular stress fibers, alignment of individual focal adhesion sites, and the coalescence of smaller sites resulting in fewer, but larger, focal adhesions per cell. Cell adhesion, repeatedly calculated in the same cells as a function of the areas of focal contact and the separation distances between membrane and substratum, varied by < 10% during both short (30 min), or prolonged (< or = 24 h), periods of exposure to flow. Consistent with these measurements, the gains and losses of focal adhesion area as each site remodeled were approximately equivalent. When the glass substratum was coated with gelatin, rates of remodeling were inhibited by 47% during flow (tau = 10 dyn/cm2). These studies: (a) reveal the dynamic nature of focal adhesion; (b) demonstrate that these sites at the ablumenal endothelial membrane are both acutely and chronically responsive to frictional shear stress forces applied to the opposite (lumenal) cell surface; and (c) suggest that components of the focal adhesion complex may be mechanically responsive elements coupled to the cytoskeleton.

Shear stress-induced reorganization of the surface topography of living endothelial cells imaged by atomic force microscopy

We report the first topographical data of the surface of living endothelial cells at sub-light-microscopic resolution, measurements essential for a detailed understanding of force distribution in the endothelium subjected to flow. Atomic force microscopy was used to observe the surface topography of living endothelial cells in confluent monolayers maintained in static culture or subjected to unidirectional shear stress in laminar flow (12 dyne/cm2 for 24 hours). The surface of polygonal unsheared cells was smooth, with mean excursion of surface undulation between peak height (over the nucleus) and minima (at intercellular junctions) of 3.4 +/- 0.7 microns (mean +/- SD); the mean height to length ratio was 0.11 +/- 0.02. In cells that were aligned in the direction of flow after a 24-hour exposure to laminar shear stress, height differentials were significantly reduced (mean, 1.8 +/- 0.5 micron), and the mean height to length ratio was 0.045 +/- 0.009. Calculation of maximum shear stress and maximum gradient of shear stress (delta tau/delta x, where tau is shear stress at the cell surface) at constant flow velocity revealed substantial streamling of aligned cells that reduced delta tau/delta x by more than 50% at a nominal shear stress of 10 dyne/cm2. Aligned cells exhibited ridges extending in the direction of flow that represented imprints of submembranous F-actin stress-fiber bundles mechanically coupled to the cell membrane. The surface ridges, approximately 50 nm in height and 200 to 1000 nm in width, were particularly prominent in the periphery of the aligned cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple endocrine neoplasm, type 1. Gastrinomas, pancreatic neoplasms, microcarcinoids, the Zollinger-Ellison syndrome, lymph nodes, and hepatic metastases

OBJECTIVE

We reviewed the age of presentation, malignant potential, and outcome of gastrinomas and pancreatic tumors in patients with multiple endocrine neoplasm, type 1.

DESIGN

Screening of one very large and one smaller, possibly related family on an island, including serum gastrin estimations and, when indicated, pancreatic ultrasound.

SETTING AND PATIENTS

Over 2000 family members and their family physicians were advised on screening procedures.

INTERVENTION

Data were collected and reviewed retrospectively and prospectively for all medical records, investigations, surgical procedures, and available tissue samples.

OUTCOME MEASUREMENTS

Criteria for diagnosis were established for radiological, biochemical, and histological studies.

RESULTS

Sixty-two patients had evidence of gastrinoma or pancreatic neoplasm. In 19 patients the diagnosis was based on demonstration of a tumor. In 21 patients the diagnosis was based on elevated serum gastrin concentration in the absence of demonstrable tumor. None of these patients required gastric surgery if they first underwent parathyroidectomy. In 18 patients the diagnosis was based on the combination of demonstrated pancreatic tumor plus elevated glucagon (two patients), gastrin (11 patients), or insulin (five patients) concentration. Peptic ulcer was difficult to control in seven of the 11 patients with elevated gastrin concentrations plus demonstrated tumor. Four patients had liver metastases that appeared to be secondary to the pancreatic gastrinoma. In patients with insulinomas, the first symptoms occurred before age 20 years. Elevated serum gastrin concentrations were not seen before age 24 years and were observed to occur for the first time in two patients after age 50 years.

Endothelial cell adhesion in real time. Measurements in vitro by tandem scanning confocal image analysis

Real time measurements of cell-substratum adhesion in endothelial cells were obtained by tandem scanning confocal microscopy of sites of focal contact (focal adhesions) at the abluminal cell surface. Focal contact sites were sharply defined (low radiance levels) in the living cell such that the images could be enhanced, digitized, and isolated from other cellular detail. Sites of focal contact are the principal determinant of cell-substratum adhesion. Measurements of (a) the focal contact area and (b) the closeness of contact (inverse radiance) were used to nominally define the adhesion of a single cell or field of cells, and to record spontaneous and induced changes of cell adhesion in real time. The topography of focal contacts was estimated by calculating separation distances from radiance values using a calibration technique based on interference ring optics. While slightly closer contact was noted between the cell membrane and substratum at or near the center of each focal contact, separation distances throughout the adhesion regions were always < 50 nm. Subtraction of consecutive images revealed continuous spontaneous remodeling of individual focal adhesions in unperturbed cells during periods of < 1 min. Despite extensive remodeling of focal contact sites, however, cell adhesion calculated for an entire cell over extended periods varied by < 10%. When cytoskeletal stability was impaired by exposure to cytochalasin or when cells were exposed to proteolytic enzyme, endothelial adhesion declined rapidly. Such changes were recorded at the level of single cells, groups of cells, and at single focal adhesions. In both unperturbed and manipulated cells, the dynamics of remodeling and cell adhesion characteristics varied greatly between individual sites within the same cell; disappearance of existing sites and appearance of new ones often occurred within minutes while adjacent sites underwent minimal remodelling. Tandem scanning confocal microscopy image analysis of living cells in real time provides repetitive spatial, temporal, and quantitative information about cell adhesion. Such an approach should allow more precise quantitative analyses to be made of the interactions between extracellular matrix, adhesion proteins, integrins, and the cytoskeleton in the living cell.

Changes in gap junction connexin-43 messenger ribonucleic acid levels associated with rat ovarian follicular development as demonstrated by in situ hybridization

OBJECTIVE

The purpose was to evaluate the changes in gap junction connexin-43 messenger ribonucleic acid levels associated with rat ovarian follicular development. Gap junctions connect the plasma membranes of adjacent cells through cell-to-cell channels, allowing synchronization of cellular events, including ovarian follicular development. Ovarian gap junctions consist of the protein connexin-43.

STUDY DESIGN

We used the hypophysectomized immature rat treated with estrogen or gonadotropins as a model to study the ovarian regulation of connexin-43 messenger ribonucleic acid. In situ hybridization with radiolabeled riboprobes was used to localize and quantitate connexin-43 messenger ribonucleic acid.

RESULTS

We demonstrated that connexin-43 messenger ribonucleic acid was localized to follicular granulosa cells. Estrogen significantly up-regulated connexin-43 messenger ribonucleic acid (91%), whereas gonadotropins that stimulate ovulation and corpus luteum formation completely down-regulated the connexin-43 gene. These results correlate closely with previous immunohistochemical studies of connexin-43 protein.

CONCLUSION

The positive correlation between follicular development and granulosa cell content of connexin-43 messenger ribonucleic acid is caused by transcriptional activation of the gap junction connexin-43 gene, posttranscriptional stability of connexin-43 messenger ribonucleic acid, or both. Future studies will determine the molecular mechanisms of hormonal regulation of the connexin-43 gene.

Ultrasonography of the pancreas in patients with multiple endocrine neoplasia type I

Fifty-seven patients with the multiple endocrine neoplasia type I (MEN-1) syndrome underwent sonographic examinations, and focal pancreatic lesions were demonstrated in 18 (33%). Size ranged from 5 to 65 mm in diameter, and multiple lesions were seen in five patients. Eight patients with pancreatic lesions less than 20 mm have been followed over a period of 1 to 6 years. Of these eight patients, only one had a lesion that increased in size. Ultrasonography was able to detect asymptomatic pancreatic tumors in a higher proportion of MEN-1 patients than previously. Sonography is a useful method of detecting islet cell tumors greater than 5 mm in diameter and is able to follow up these lesions to assess increase in lesion size and number.

Gap junctional communication between vascular cells. Induction of connexin43 messenger RNA in macrophage foam cells of atherosclerotic lesions

The structure and function of blood vessels depend on the ability of vascular cells to receive and transduce signals and to communicate with each other. One means by which vascular cells have been shown to communicate is via gap junctions, specifically connexin43. In atherosclerosis, the normal physical patterns of communication are disrupted by the subendothelial infiltration and accumulation of blood monocytes, which in turn can differentiate into resident foam cells. In this paper we report that neither freshly isolated human peripheral blood monocytes nor differentiated monocytes/macrophages exhibit functional gap junctional dye transfer in homo-cellular culture or in co-culture with endothelial cells or smooth muscle cells. By Northern analysis, neither freshly isolated blood monocytes nor pure cultures of differentiated monocyte/macrophages expressed gap junction messenger RNA. However, immunohistochemical staining followed by in situ hybridization on sections of human atherosclerotic carotid arteries revealed strong expression of gap junction connexin43 messenger RNA by macrophage foam cells. These results suggest that tissue-specific conditions present in atherosclerotic arteries induce expression of connexin43 messenger RNA in monocyte/macrophages.

Mechanical stress mechanisms and the cell. An endothelial paradigm

There are important physiological and pathological cardiovascular consequences related to endothelial biomechanical properties. The endothelium, however, is not unique in responding to external forces; virtually all cells accommodate or respond to the mechanical environment.

Hemodynamic forces and vascular cell communication in arteries

As the interface between the blood and the rest of the vessel wall, the endothelium is directly affected by hemodynamic shear stress (frictional) forces that locally regulate vascular tone and are implicated in the localization of atherosclerosis. There are many diverse responses of endothelial cells to hemodynamically related mechanical stresses ranging from ion channel activation to gene regulatory events. The processes of force transmission from the blood to the cell, and force transduction within the endothelium to electrophysiologic, biochemical, and transcriptional responses are poorly understood. This article reviews the principal mechanisms currently thought to be involved and outlines the signal pathways from the endothelium to underlying smooth-muscle cells.

Mechanisms of flow-mediated signal transduction in endothelial cells: kinetics of ATP surface concentrations

Intracellular free calcium ([Ca2+]i) was measured in single cells of a confluent endothelial monolayer subjected to defined flow. Flow medium containing adenosine triphosphate (ATP) was used to study the influence of flow forces upon agonist-response coupling as mediated via the P2y-purinoceptor. [Ca2+]i responses were highly sensitive to the fluid motion at the cell surface; consecutive small increases of flow stimulated large [Ca2+]i transients with the levels returning to baseline at the new flow rate within 250 s. The characteristics of [Ca2+]i transients were also influenced by decreasing flow. Since potent ectonucleotidases at the endothelial cell surface rapidly degrade ATP, we postulated that a combination of flow and degradative enzymes regulates the mass transport of ATP in the boundary layer. The hypothesis predicts that step increases of flow exceed the capacity of the ectonucleotidases and allow ATP to reach the receptor. Experiments were conducted to compare ATP and ADP beta S, a nonhydrolyzable ATP analog that resists degradation by surface ectonucleotidases, and calculations of ATP mass transport to the cell surface were compared to estimates of surface clearance rates. Calculations of mass transport coefficients for ATP in the boundary layer demonstrated that changes of flow which elicited a prominent [Ca2+]i response represented 26-73% changes in the mass transport of ATP from the bulk fluid. When steadystate mass transport coefficients for ATP under various flow conditions were compared with the estimated rate constant for surface degradation of ATP, ratios close to unity were obtained. These results suggest that both boundary layer mass transport and ATP clearance rates can be rate-limiting for flow-mediated activation of the P2y-receptor. The experiments provide evidence for differential signal transduction responses in the endothelium driven by diffusion gradients (derived from both the blood and the vessel wall), which are likely to vary widely in the complex flow fields encountered in vivo.

Vascular endothelium responds to fluid shear stress gradients

In vitro investigations of the responses of vascular endothelium to fluid shear stress have typically been conducted under conditions where the time-mean shear stress is uniform. In contrast, the in vitro experiments reported here have re-created the large gradients in surface fluid shear stress found near arterial branches in vivo; specifically, we have produced a disturbed-flow region that includes both flow separation and reattachment. Near reattachment regions, shear stress is small but its gradient is large. Cells migrate away from this region, predominantly in the downstream direction. Those that remain divide at a rate that is high compared with that of cells subjected to uniform shear. We speculate that large shear stress gradients can induce morphological and functional changes in the endothelium in regions of disturbed flow in vivo and thus may contribute to the formation of atherosclerotic lesions.