Effect of angiotensin II and ethanol on the expression of connexin 43 in WB rat liver epithelial cells

Applicability of Scrape Loading-Dye Transfer Assay for Non-Genotoxic Carcinogen Testing

Dysregulation of gap junction intercellular communication (GJIC) is recognized as one of the key hallmarks for identifying non-genotoxic carcinogens (NGTxC). Currently, there is a demand for in vitro assays addressing the gap junction hallmark, which would have the potential to eventually become an integral part of an integrated approach to the testing and assessment (IATA) of NGTxC. The scrape loading-dye transfer (SL-DT) technique is a simple assay for the functional evaluation of GJIC in various in vitro cultured mammalian cells and represents an interesting candidate assay. Out of the various techniques for evaluating GJIC, the SL-DT assay has been used frequently to assess the effects of various chemicals on GJIC in toxicological and tumor promotion research. In this review, we systematically searched the existing literature to gather papers assessing GJIC using the SL-DT assay in a rat liver epithelial cell line, WB-F344, after treating with chemicals, especially environmental and food toxicants, drugs, reproductive-, cardio- and neuro-toxicants and chemical tumor promoters. We discuss findings derived from the SL-DT assay with the known knowledge about the tumor-promoting activity and carcinogenicity of the assessed chemicals to evaluate the predictive capacity of the SL-DT assay in terms of its sensitivity, specificity and accuracy for identifying carcinogens. These data represent important information with respect to the applicability of the SL-DT assay for the testing of NGTxC within the IATA framework.

Alcohol and the Alveolar Epithelium

The distal airways are covered with a heterogeneous layer of cells known as the alveolar epithelium. Alveolar epithelial cells provide the major barrier between the airspace and fluid filled tissue compartments. As such, regulation of the alveolar epithelium is critical to maintain a healthy lung and for optimal gas exchange. In this chapter, we discuss functional roles for alveolar epithelial cells with particular emphasis on intercellular junctions and communication. As a thin layer of cells directly exposed to atmospheric oxygen, alveoli are particularly sensitive to oxidant insults. Alcohol significantly diminishes the normal antioxidant reserves of the alveolar epithelium, thereby rendering it sensitized for an exaggerated damage response to acute and chronic injuries. The effects of alcohol on alveolar epithelia are discussed along with open questions and potential therapeutic targets to prevent the pathophysiology of alcoholic lung disease.

13-cis-Retinoic acid specific down-regulation of angiotensin type 1 receptor in rat liver epithelial and aortic smooth muscle cells

Transcriptional repression through cis- and trans-acting factors enabling an alternate approach to control angiotensin type 1 receptor (AT1 or AGTR1 as listed in the MGI database) expression has not been studied. In previous investigations, treatment with retinoic acid was found to be associated with enhanced insulin sensitivity. In our previous study, expression of AT1 was found to be inversely correlated with intracellular glucose concentrations. Therefore, we hypothesized that 13-cis-retinoic acid (13cRA), an antioxidant, enhances insulin-sensitive glucose-mediated down-regulation of the AT1. In this study, we used continuously passaged rat liver epithelial cells. Our study shows that cells exposed to 13cRA specifically down-regulated the AT1 protein in a dose- and time-dependent manner, independently of any change in receptor affinity. Down-regulation of the AT1 expression leads to reduced AngII-mediated intracellular calcium release, a hallmark of receptor-mediated intracellular signaling. Similarly with receptor down-regulation, we observed a significant reduction in AT1 mRNA; however, the AT1 down-regulation was independent of insulin-sensitive glucose uptake and retinoic acid receptor activation (RAR/RXR). Treatment with 13cRA resulted in phosphorylation of p42/p44 MAP kinases in these cells. Subsequent studies using MEK inhibitor PD98059 prevented 13cRA-mediated AT1 down-regulation and restored AngII-mediated intracellular calcium response. Furthermore, 13cRA-mediated inhibitory effects on AT1 were validated in primary rat aortic smooth muscle cells. In summary, our results demonstrate for the first time that 13cRA has a glucose- and RAR/RXR-independent mechanism for transcriptional inhibition of AT1, suggesting its therapeutic potential in systems in which AT1 expression is deregulated in insulin-sensitive and -insensitive tissues.

Pharmacologic effects of 2-methoxyestradiol on angiotensin type 1 receptor down-regulation in rat liver epithelial and aortic smooth muscle cells

BACKGROUND

Delayed onset of cardiovascular disease (CVD) in female patients is not well understood, but could be due in part to the protective effect of estrogen before menopause. Experimental studies have identified the angiotensin type 1 receptor (AT1R) as a key factor in the progression of CVD.

OBJECTIVE

We examined the effects of the estrogen metabolite 2-methoxyestradiol (2ME2) on AT1R expression.

METHODS

Rat liver cells were exposed to 2ME2 for 24 hours, and angiotensin II (AngII) binding and AT1R mRNA expressions were assessed.

RESULTS

In the presence of 2ME2, cells exhibited significant down-regulation of AngII binding that was both dose and time dependent, independent of estrogen receptors (ERα/ERβ). Down-regulation of AngII binding was AT1R specific, with no change in receptor affinity. Under similar conditions, we observed lower expression of AT1R mRNA, significant inhibition of AngII-mediated increase in intracellular Ca(2+), and increased phosphorylation of ERK1/2. Pretreatment of cells with the MEK inhibitor PD98059 prevented 2ME2-induced ERK1/2 phosphorylation and down-regulation of AT1R expression, which suggests that the observed inhibitory effect is mediated through ERK1/2 signaling intermediates. Similar analyses in stably transfected CHO (Chinese hamster ovary) cell lines with a constitutively active cytomegalovirus promoter showed no change in AT1R expression, which suggests that 2ME2-mediated effects are through transcriptional regulation. The effects of 2ME2 on AT1R down-regulation through ERK1/2 were consistently reproduced in primary rat aortic smooth muscle cells.

CONCLUSIONS

Because AT1R has a critical role in the control of CVD, 2ME2-induced changes in receptor expression may provide beneficial effects to the cardiovascular and other systems.

Omega-3 fatty acids and metabolic syndrome: effects and emerging mechanisms of action

Epidemiological, human, animal, and cell culture studies show that n-3 fatty acids, especially α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), reduce the risk factors of cardiovascular diseases. EPA and DHA, rather than ALA, have been the focus of research on the n-3 fatty acids, probably due to the relatively inefficient conversion of ALA to EPA and DHA in rodents and humans. This review will assess our current understanding of the effects and potential mechanisms of actions of individual n-3 fatty acids on multiple risk factors of metabolic syndrome. Evidence for pharmacological responses and the mechanism of action of each of the n-3 fatty acid trio will be discussed for the major risk factors of metabolic syndrome, especially adiposity, dyslipidemia, insulin resistance and diabetes, hypertension, oxidative stress, and inflammation. Metabolism of n-3 and n-6 fatty acids as well as the interactions of n-3 fatty acids with nutrients, gene expression, and disease states will be addressed to provide a rationale for the use of n-3 fatty acids to reduce the risk factors of metabolic syndrome.

Effects of Losartan on expression of connexins at the early stage of atherosclerosis in rabbits

AIM

to investigate effects of Losartan on expression of connexin 40 and 43 (Cx40 and Cx43), in arteries at the early stage of atherosclerosis in a rabbit model.

METHODS

A total of 28 male New Zealand white rabbits were divided into following groups: control group, high fat diet group, and Losartan group (10 mg/kg/day). Losartan was administrated in food for two weeks. Iliac arteries were obtained for immunohistochemistry, transmission electron microscopy, Western blot, and reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

Transmission electron microscopy revealed abundant gap junctions between neointimal smooth muscle cells (SMCs), which were markedly reduced by treatment. RT-PCR and Western blot assay showed that the mRNA and protein expression of Cx40 and Cx43 were elevated in the neointimal area at the early stage of atherosclerosis. The mRNA and protein expression of Cx43 were significantly down-regulated by losartan treatment but those of Cx40 were not markedly changed.

CONCLUSION

Cx40 and Cx43 in the neointimal SMCs were up-regulated at the early stage of atherosclerosis. Losartan (an angiotensin-converting enzyme inhibitor) could reduce neointima proliferation and down-regulate the elevated protein expression of Cx43, suggesting the rennin-angiotensin system (RAS) plays an important role in the remodeling of gap junction between ventricular myocytes under pathological conditions.

High glucose enhances angiotensin-II-mediated peroxisome proliferation-activated receptor-gamma inactivation in human coronary artery endothelial cells

Activation of the renin-angiotensin system plays an important role in the pathogenesis of vascular complications of hyperglycemia. Clinical studies have demonstrated that hypoglycemic effects of peroxisome proliferation-activated receptor-gamma (PPAR-gamma) activation is potentially associated with a significant decrease of cardiovascular disease events in diabetes patients. We assessed the effect of high glucose on the angiotensin II (Ang II), which induced the inactivation of PPAR-gamma and its signal pathways in human coronary artery endothelial cells (HCAECs). The expression of angiotensin II receptor I (AT1R) protein was analyzed by Western blot and knocked down using siRNA. PPAR-gamma activation was examined using a luminometer and a Dual Luciferase Reporter Assay System. Adhesion molecule expressions of HCAECs were measured using ELISA. Both high glucose and Ang II induced a progressive increase in AT1R protein expression on the HCAECs. Troglitazone, a PPAR-gamma activator, significantly increased the transcription activity of PPAR-gamma in HCAECs in vitro. However, activation of PPAR-gamma was significantly inhibited by high glucose and Ang II stimulation. Furthermore, silencing of AT1R expression was able to inhibit the inactivation of PPAR-gamma induced by Ang II and high glucose. Meanwhile, expression of proinflammatory adhesion molecules was increased by high glucose and Ang II in HCAECs, which is blocked by troglitazone and silencing of AT1R expression. These data strongly suggest high glucose enhanced Ang-II-mediated peroxisome proliferation-activated receptor-gamma inactivation and expression of proinflammatory adhesion molecules in human coronary artery endothelial cells.

Cross-talk between pulmonary injury, oxidant stress, and gap junctional communication

Gap junction channels interconnect several different types of cells in the lung, ranging from the alveolar epithelium to the pulmonary vasculature, each of which expresses a unique subset of gap junction proteins (connexins). Major lung functions regulated by gap junctional communication include coordination of ciliary beat frequency and inflammation. Gap junctions help enable the alveolus to regulate surfactant secretion as an integrated system, in which type I cells act as mechanical sensors that transmit calcium transients to type II cells. Thus, disruption of epithelial gap junctional communication, particularly during acute lung injury, can interfere with these processes and increase the severity of injury. Consistent with this, connexin expression is altered during lung injury, and connexin-deficiency has a negative impact on the injury response and lung-growth control. It has recently been shown that alcohol abuse is a significant risk factor associated with acute respiratory distress syndrome. Oxidant stress and hormone-signaling cascades in the lung induced by prolonged alcohol ingestion are discussed, as well as the effects of these pathways on connexin expression and function.

Involvement of connexin 43 in angiotensin II-induced migration and proliferation of saphenous vein smooth muscle cells via the MAPK-AP-1 signaling pathway

Proliferation and migration of vascular smooth muscle cells (VSMCs) lead to intimal thickening and influence the long-term patency of venous graft post coronary arterial bypass graft. There is increasing evidence that connexins are involved in the development of intimal hyperplasia and restenosis. We assessed connexin 43 (Cx43) expression and its role in angiotensin II-induced proliferation and migration of smooth muscle cells and the signal pathways involved in human saphenous vein bypass conduits. Angiotensin II significantly increased gap junctional intercellular communication and induced the expression of Cx43 in human saphenous vein SMCs in a dose- and time-dependent manner through angiotensin II type 1 receptor. The effect of angiotensin II was blocked by siRNA of ERK 1/2, p38 MAPK and JNK, respectively. Overexpression of Cx43 markedly increased the proliferation of saphenous vein SMCs. However, siRNA for Cx43 inhibited angiotensin II-induced proliferation, cyclin E expression and migration of human saphenous vein SMCs. In dual-luciferase reporter assay, angiotensin II markedly activated AP-1 transcription factor, which was significantly attenuated by a dominant-negative AP-1 (A-Fos) with subsequent inhibition of angiotensin II-induced transcriptional expression of Cx43. These data demonstrate the role of Cx43 in the proliferation and migration of human saphenous vein SMCs and angiotensin II-induced Cx43 expression via mitogen-activated protein kinases (MAPK)-AP-1 signaling pathway.

Connexin 43 contributes to differentiation of retinal pigment epithelial cells via cyclic AMP signaling

Retinal pigment epithelium (RPE) cells play important roles in the visual system that supports neurosensory retina homeostasis. Connexin (Cx) 43-mediated gap-junctional intercellular communication (GJIC) participates in the regulation of retinal organogenesis, but much of the function of Cx43 on the differentiation of RPE cells is unclear. Here, we report the involvement of Cx43 in RPE differentiation. Knockdown of Cx43 in RPE cells dramatically inhibited the differentiation, whereas Cx43-overexpression successfully induced RPE cell differentiation under de-differentiation conditions. From the experiments using GJIC inhibitors and C-terminus-truncated mutant of Cx43, it was clearly demonstrated that the regulation of RPE cell differentiation by Cx43 did not result from Cx43-mediated GJIC. The RPE cell differentiation induced by Cx43-overexpression was abolished by a cAMP antagonist. In contrast, the treatment with forskolin and phosphodiesterase inhibitor rolipram induced RPE cell differentiation under de-differentiation conditions. These findings indicate that Cx43 contributes to RPE differentiation via cAMP signaling.

Angiotensin II and IGF-1 regulate connexin43 expression via ERK and p38 signaling pathways in vascular smooth muscle cells of coronary artery bypass conduits

BACKGROUND

Changes in connexin expression have been found in vascular smooth muscle cells (VSMCs) during the progression of atherosclerotic lesion and intimal hyperplasia. It is our hypothesis that increased connexin43 expression following stimulation of VSMCs with Ang II and IGF-1 contributes to more proliferation in saphenous vein (SV) than in the internal mammary artery (IMA).

MATERIALS AND METHOD

Using immunohistochemistry, Western blot, and reverse-transcription polymerase chain reaction, we assessed the effect of Ang II and IGF-1 stimulation on connexin43 expression and the signaling pathways involved in VSMCs of SV and IMA.

RESULTS

Immunostaining demonstrated strong expression of connexin43 in SV compared with IMA after stimulation with Ang II and IGF-1. Ang II up-regulated the expression of connexin43 in VSMCs of SV in a dose- and time-dependent manner. This was inhibited by p38 and ERK MAP kinase inhibitors, SB203580 and PD98059, respectively. In the VSMCs of IMA, the connexin43 expression was markedly low and maintained at a reduced level even after 3 h stimulation. IGF-1 dose-dependently induced mRNA expression of connexin43 in the VSMCs of SV, which was blocked by PD98059. However, in VSMCs of IMA there was no significant effect of IGF-1 on the connexin43 mRNA expression.

CONCLUSION

These data suggest that connexin43 expression can be influenced by Ang II and IGF-1 through ERK and p38 pathways and may contribute to the pathogenesis of vein graft disease following coronary artery bypass grafting.

Effects of angiotensin II on connexin 43 of VSMCs in arteriosclerosis

OBJECTIVE

To observe the effect of angiotensin II (Ang II) on expression of gap junction channel protein connexin 43 (Cx43) in the proliferation process of vascular smooth muscle cells (VSMCs) during the early stage of arteriosclerosis.

METHODS

Thirty-two adult male rabbits were randomly divided into 4 groups. Rabbits in Group A were fed common diet while others in Groups B, C, and D were fed high-cholesterol diet. Losartan (10 mg/(kg.d)) and ramipril (0.5 mg/(kg.d)) were added in the diet of Groups C and D, respectively. The animals were sacrificed after 8 weeks and abdominal aortas were removed and dissected. The expression of Cx43 was assayed using RT-PCR and Western Blotting analysis.

RESULTS

Cx43 was increased markedly in both protein and mRNA level in Groups B, C, and D fed high-cholesterol diet compared with that in control group (P<0.01). Cx43 level in losartan or ramipril treated groups was higher than that in control group (P<0.01, P<0.05), but lower than that in high-cholesterol diet groups (P<0.05, P<0.01).

CONCLUSION

Cx43 level was upregulated in VSMCs during early atherosclerosis. Losartan and ramipril can inhibit the expression of Cx43.

Ethanol inhibits gap-junctional coupling between P19 cells

BACKGROUND

Gap junctions are plaques of multiple intercellular channels that connect the cytoplasm of adjacent cells. They provide both electrical and metabolic coupling and are an essential element in normal growth, development, and physiology. Little research exists on the relationship between alcohol administration and gap-junctional function or expression. This study looks at the function and expression of gap junctions after incubation and withdrawal of ethanol with P19 cell cultures.

METHODS

Gap-junctional communication was assessed after 24 and 48 hr of exposure to 20 and 40 mM ethanol and after a 24-hr withdrawal period. The seeding technique was used, and diacyl-3,3'-indocarbocyanine iodide/calcein-stained donor cells were seeded on an unstained monolayer and then reviewed by confocal microscope and counted by flow cytometry. Analysis of connexin (Cx) proteins was performed by Western blot, gel electrophoresis, and immunoblots with antibodies for Cx26 and Cx43.

RESULTS

All treatment regimens produced similar results, reducing dye coupling by more than 50% without recovery after a 24-hr withdrawal period. Exposing the cells to 20 mM ethanol for 48 hr did not significantly change the levels of Cx26 protein, but ethanol significantly decreased the levels of Cx43 in cultured P19 cells.

CONCLUSIONS

This study illustrates that ethanol can inhibit gap-junction function in the P19 cell line. Chronic exposure to 20 mM ethanol selectively decreased the levels of Cx43 protein in the membrane fraction of the cell cultures.

Ethanol acutely decreases astroglial gap junction permeability in primary cultures from defined brain regions

The acute effect of hyperosmotic ethanol on gap junction permeability was examined in astroglial cells in primary culture from five different brain regions. Gap junction permeability was analyzed by measuring dye spreading from cell to cell with the low molecular weight dye Lucifer Yellow. Ethanol concentrations 25-300 mM significantly decreased dye spreading in cultures from the cerebral cortex in a dose-dependent but time-independent manner for up to 60 min. Besides cerebral cortex, exposure to 150 mM ethanol decreased dye spreading in astroglial cultures from the hippocampus and from the brain stem, while cultures from the olfactory bulb and from the hypothalamus were not significantly affected. The ethanol-induced decrease in dye spreading in cultures from the cerebral cortex was not mediated through changes in cell volume, osmolarity, protein kinase C (PKC) phosphorylation, intracellular pH, or intracellular calcium concentration ([Ca(2+)](i)). The decrease in dye spreading was abolished upon incubation in sodium-reduced buffer, and after blockage of the Na(+)/K(+)/2Cl(-) cotransporter with furosemide. The results presented here indicate that ethanol-mediated decrease in dye spreading is directly or indirectly dependent on sodium.