Effect of biological toxins on gap-junctionai intercellular communication in Chinese hamster V79 cells

Since chemical modulation of gap junctional intercellular communication has been implicated in several toxicological endpoints, a study to examine the ability of several biological toxins to inhibit this process was undertaken. Eight biological toxins were tested for their ability to inhibit metabolic cooperation, a measure of gap-junctional intercellular communication, in the Chinese V79 cell system. Aplysiatoxin, anhydrodebromoaplysiatoxin and debromoaplysiatoxin showed the strongest ability to inhibit metabolic cooperation while T2-toxin and vomitoxin inhibited metabolic cooperation to a lesser degree. Afatoxin B₁, afatoxin B₂ and palytoxin were inactive in the Chinese V79 system. Palytoxin, which was extremely cytotoxic, might act as a tumor promoter if it induces compensatory hyperplasia in vivo.Abbreviations 6-TG 6-thioguanine - TPA 12-0-tetradecanoylphorbol-13-acetate     

Cell culture assays for chemicals with tumor-promoting or tumor-inhibiting activity based on the modulation of intercellular communication

The ability of chemicals with tumor-promoting or tumor-inhibiting activity to modulate gap junctional intercellular communication is reviewed. The two most extensively used types of assays for screening tests are (1) metabolic cooperation assays involving exchange between cells of precursors of nucleic acid synthesis and (2) dye-transfer assays that measure exchange of fluorescent dye from loaded cells to adjacent cells. About 300 substances of different biological activities have been studied using various assays. For tumor promoters/epigenetic carcinogens, metabolic cooperation assays have a sensitivity of 62% and dye-transfer assays 60%. Thirty percent of DNA-reactive carcinogens also possess the ability to uncouple cells. The complete estimation of the predictive power of these assays could not be made because the majority of the substances studied for intercellular communication effectsin vitro have not yet been studied for promoting activityin vivo. Both metabolic cooperation assays and dye transfer assays respond well to the following classes of substances: phorbol esters, organochlorine pesticides, polybrominated biphenyls, promoters for urinary bladder, some biological toxins, peroxisome proliferators, and some complex mixtures. Results ofin vitro assays for such tumor promoters/nongenotoxic carcinogens, such as some bile acids, some peroxides, alkanes, some hormones, mineral dusts, ascorbic acid, okadaic acid, and benz(e)pyrene, do not correlated with the data ofin vivo two-stage or complete carcinogenesis. Enhancement of intercellular communication was found for 18 chemicals. Among these, cAMP, retinoids, and carotenoids have demonstrated inhibition of carcinogenesis. We examine a number of factors that are important for routine screening, including the requirement for biotransformation for some agents to exert effects on gap junction. We also discuss the mechanisms of tumor promoter and tumor inhibitor effects on gap junctional permeability, including influences of protein kinase activation, changes in proton and Ca²⁺ intracellular concentrations, and effects of oxy radical production.Abbreviations DMBA 7,12-dimethylbenz(a)anthracene - DT dye transfer - DDT 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane - HGPRT hypoxanthine-guanine phosphoribosyl-transferase - MC metabolic cooperation - PAH polycyclic aromatic hydrocarbons - PKA protein kinase A - PKC protein kinase C - TG 6-thioguanine - TPA 12-O-tetradecanoylphorbol-13-acetate     

Synergistic inhibition of metabolic cooperation by oleic acid or 12-0-tetradecanoylphorbol-13-acetate and dichlorodiphenyltrichlorethane (DDT) in Chinese hamster V79 cells: Implication of a role for protein kinase C in the regulation of gap junctional intercellular communication

The effects of TPA and/or DDT and oleic acid and/or DDT on gap junction-mediated intercellular communication (i.e. metabolic cooperation) between Chinese hamster V79 cells was examined. Addition of TPA, DDT or oleic acid alone to cocultures of 6t-hioguanine-resistant (6-TG ^R ) and 6-thioguanine-sensitive (6-TG ^S ) V79 cells significantly increased the recovery of 6-TG ^R cells indicating inhibition of metabolic cooperation. In the presence of TPA and DDT or oleic acid and DDT the observed recovery of 6-TG ^R cells was significantly greater than the expected (calculated) additive 6-TG ^R cell recovery. No synergistic increases in 6-TG ^R cell recovery were observed when co-cultures of V79 cells were exposed to dieldrin and DDT. These results indicate that TPA and DDT or oleic acid and DDT can act synergistically to inhibit metabolic cooperation. These data suggest a role for protein kinase C in the regulation of gap junction-mediated intercellular communication.Abbreviations DDT dichlorodiphenyltrichlorethane - MC metabolic cooperation defective - 6-TG 6thioguanine - TPA 12-0-tetradecanoylphorbol-13-acetate     

Effects of tumor promoters,genotoxic carcinogens and hepatocytotoxins on mouse hepatocyte intercellular communication

Intercellular communication via gap junctions may be an important mechanism of cellular growth control. Tumor promoters can inhibit intercellular communication between cultured cells, while genotoxic carcinogens apparently lack this capability. The inhibition of intercellular communication by tumor promoters may be an essential mechanism by which tumor promotion occurs in vivo. In this study, the liver tumor promoters phenobarbital, lindane (1,2,3,4,5,6-hexachlorocyclohexane, -isomer), DDT (1,1-Bis[4-chlorophenyl],-2,2,2-trichloroethane), Aroclor 1254 (a polychlorinated biphenyl mixture) and dieldrin inhibited intercellular communication between male B6C3F1 mouse hepatocytes in primary culture. Intercellular communication was detected as the passage of [5-³H]uridine nucleotides from pre-labelled donor hepatocytes to non-labelled recipient heptocytes. Mouse hepatocyte intercellular communication was also inhibited by the skin tumor promoter TPA (12-0-tetradecanoyl phorbol-13-acetate), but not by the bladder tumor promoter saccharin. The genotoxic hepatocarcinogens dimethylnitrosamine, diethylnitrosamine, benzo[a]pyrene and 2-acetylaminofluorene, and the hepatocytotoxins bromobenzene, acetaminophen, carbon tetrachloride, chloroform and methotrexate had no effect on mouse hepatocyte intercellular communication at non-cytotoxic levels. These results suggest that the ability to inhibit mouse hepatocyte intercellular communication is an effect specific to tumor promoters.Abbreviations DDT 1,1-Bis[4-chlorophenyl]-2,2,2-trichloroethane - FBS fetal bovine serum - LDH lactate dehydrogenase - TCA trichloroacetic acid - TPA 12-0-tetradecanoyl-phorbol-13-acetate     

Prevention of organochlorine-induced inhibition of gap junctional communication by chaetoglobosin K in astrocytes

Innumerable toxic substances present in the environment inhibit gap junctions, intercellular membrane channels that play fundamental roles in coordinated function of cells and tissues. Included are persistent organochlorine compounds, which pose health risks to humans and animals owing to their widespread use, bioaccumulation, and ability to inhibit gap junction channel-mediated intercellular communication in liver, lung, skin, heart, and brain cells. In this study, the organochlorine xenobiotics dieldrin and endosulfan, at micromolar concentrations, were found to inhibit gap junction-mediated intercellular communication and induce hypophosphorylation of connexin 43 in cultured rat astrocytes, the predominant cell type in the brain coupled through gap junctions. This inhibition of gap junctional communication was substantially reduced by preincubation with chaetoglobosin K (ChK), a bioactive natural produce previously shown to have ras tumor suppressor activity. Chaetoglobosin K also prevented dieldrin and endosulfan-induced hypophosphorylation of connexin 43 and prevented dieldrin-induced connexin 43 plaque dissolution in both astrocytes and cultured liver epithelial cells. The results suggest that stabilization of the native, phosphorylated form of connexin 43 by ChK may contribute to its ability to prevent organochlorine-induced inhibition of gap junction-mediated communication and dissolution of gap junction plaques within the plasma membrane. This revised version was published online in July 2006 with corrections to the Cover Date.     

SRC utilizes Cas to block gap junctional communication mediated by connexin43

The Src tyrosine kinase phosphorylates Cas (Crk-associated substrate) to confer anchorage independence and invasive growth potential to transformed cells. Gap junctional communication is often lower between aggressive tumor cells compared with normal or benign precursors. The gap junction protein connexin43 (Cx43) is a tumor suppressor that can inhibit tumor cell growth. Src can phosphorylate Cx43 to block gap junctional communication between transformed cells. However, mechanisms by which this event actually closes intercellular channels have not been clearly defined. Here, we report that Src and Cas associate with each other at intercellular junctions. In addition, Cas is required for Src to reduce dye transfer and electrical coupling between cells expressing Cx43. Thus, Src utilizes Cas to inhibit gap junctional communication mediated by Cx43. This finding introduces a novel role of the Cas focal adhesion linker protein in the gap junction complex. This observation may help explain how gap junctional communication can be suppressed between malignant and metastatic tumor cells.     

Use of a cell transformation assay with established cell lines, and a metabolic cooperation assay with V79 cells for the detection of tumour promoters: a review

Extensive studies on the safety evaluation of chemicals have indicated that a considerable number of non-genotoxic chemicals are carcinogenic. Tumour promoters are likely to be among these non-genotoxic carcinogens, and their detection is considered to be an important approach to the prevention of cancer. In this review, the results are summarised for in vitro transformation assays involving established cell lines, and for an assay for inhibition of gap junctional intercellular communication for the detection of tumour promoters, which involves V79 cells. Although the number of chemicals examined is still too small to permit a full evaluation of the correlation between in vitro cell transformation and in vivo carcinogenicity, it is clear that the sensitivity of the focus formation assay is very high. In the case of the metabolic cooperation assay, the sensitivity appears to be rather poor, but the assay can be considered to be useful because of its simple procedure and its considerable database. These in vitro assays for tumour promoters are recommended as useful tools for the detection of non-genotoxic carcinogens.     

Radiation Response of Connexin43-Transfected Cells in Relation to the “Contact Effect”

Some cell lines grown for only two cell doublings as multicell spheroids develop a form of resistance to killing by ionizing radiation that has been called the “contact” effect. While our previous results have implicated a role for higher order chromatin structure in the contact effect, another possible explanation is the presence of intercellular gap junctions that might facilitate communication between cells grown as spheroids and thereby enhance the ability of cells to resist or recover from radiation damage. To examine the role of gap junctions in the contact effect, rat glioma C6 and mouse EMT6 cell lines were transfected with a gene encoding the gap junctional protein connexin43. While C6 glioma cells are deficient in gap junctional communication, cells from spheroids were nonetheless more resistant than monolayers to killing by ionizing radiation, and the contact effect was present to a similar extent in the three transfected clones. For mouse EMT6 cells, radiosensitivity was similar whether cells were grown as monolayers or spheroids. Transfection of EMT6 cells with connexin43 increased gap junctional communication but did not promote development of a contact effect. Tumor volume doubling time in SCID mice increased significantly for one transfected clone; however, doubling timein vitrowas also increased relative to the EMT6 parent. We conclude that extensive gap junctional communication is not a requirement for the increased radiation resistance observed when some cell lines are grown as spheroids.     

Modulation of intercellular communication during radiation and chemical carcinogenesis

Carcinogenesis is a multistep process, involving the irreversible conversion of a stem cell to a terminal-differentiation-resistant cell ("initiation"), followed by the clonal expansion of this cell ("promotion") and by the acquisition of other genetic alterations leading to malignancy ("progression"). The initiation and progression steps seem to be facilitated by mutagenesis. Promotion has been associated with agents and conditions that cause mitogenesis. Gap junctional intercellular communication, a fundamental biological process regulating cell growth and differentiation, has been postulated to play a major role in carcinogenesis. The hypothesis is supported by the fact that many cancer cells have some dysfunction in gap junctional intercellular communication, many tumor-promoting chemicals and several oncogenes (i.e., ras, src, mos, neu, but not myc) reduce gap junctional intercellular communication, and several growth factors (i.e., EGF, TGF-beta, bovine pituitary extract) inhibit gap junction function. This integrative concept postulates that chemical promoters, oncogenes coding for growth factors, receptors, or transmembrane signaling elements, and growth factors can isolate an initiated cell from the suppressing influence of surrounding normal cells by down-regulating the transfer of ions and small molecules through gap junctions.     

Effects of 12-O-tetradecanoylphorbol-13-acetate (TPA), retinoic acid and diazepam on intercellular communication in a monolayer of rat liver epithelial cells

We have studied the influence of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), the vitamin A derivative retinoic acid and the benzodiazepine diazepam on intercellular communication via established gap junctions in a monolayer of rat liver epithelial cells (RLB) at various times of incubation. Intercellular communication was measured as the transfer of [3H]hypoxanthine-derived nucleotides between RLB hypoxanthine guanine phosphoribosyl transferase+ (HPRT+) and RLB HPRT- cells. TPA only showed transient inhibition of metabolic cooperation: after 4 h of treatment, intercellular communication was reduced to about 40% of the control and longer treatments showed progressively less effect until 24 h of treatment, when no difference was seen between TPA-treated and control preparations. Retinoic acid was a more effective inhibitor: both 3 X 10(-6) M applied for 24 h and 10(-4) M applied for 6.5 h, caused a 50% inhibition of label transfer. The junctional communication could only be blocked at very high concentrations (5 X 10(-4) M) in short-exposure experiments, but this is possibly a consequence of non-specific effects on the cell membrane. When the incubation time was 24 h, a considerable portion of the gap junctions appeared to persist in the 'open' state. Diazepam showed no significant inhibitory effect in the experiments performed.     

In vitro testing and the carcinogenic potential of several nitrosated indole compounds

4-chloro-methoxyindole is a naturally occurring compound in Vicia faba which can easily react with nitrite to form a N-nitroso compound. In this in vitro study, the potential genotoxic effects of nitrosated 4-chloro-6-methoxyindole and its structural analogue 4-chloroindole were evaluated for the first time by using both Salmonella and Chinese hamster V79 cells. Additionally, the inhibition of gap junctional intercellular communication in V79 cells by these compounds was determined; this is a validated parameter for tumor-promoting activity. Most assays were also performed with nitrosated indole-3-acetonitrile, a naturally occurring compound in brassicas. Both nitrosated chloroindoles were highly mutagenic to Salmonella typhimurium TA100 without the need of exogenous metabolic activation and were potent inducers of Sister Chromatid Exchanges. Nitrosated indole-3-acetonitrile generated the same effects, although at much higher concentrations. Equivocal results were obtained for the nitrosated chloroindoles in a forward mutation assay using the hypoxanthine guaninephosphoribosyltransferase locus. All nitrosated indole compounds significantly inhibited gap junctional intercellular communication. These results indicate that nitrosated chloroindoles and nitrosated indole-3-acetonitrile should be considered as mutagens and agents with potential tumor-promoting capacity.Abbreviations BrdU 5-bromo-2-deoxyuridine - 4Cl 4-chloroindole - 4C6MI 4-chloro-6-methoxy-indole - DMSO dimethyl sulfoxide - EBSS Earle's balanced salt solution - EMS ethyl methanesulfonate - GJIC gap junctional intercellular communication - HBSS Hanks balanced salt solution - HGPRT hypoxanthine guaninephosphoribosyl transferase - I₃A indole-3-acetonitrile - MNNG 1-methyl-1-nitroso-3-nitroguanidine - NOC N-nitroso compounds - NQO 4-nitroquinolone-N-oxide - SCE sister chromatid exchange - 6TG 6-thioguanine - TPA 12-O-tetradecanoylphorbol-13-acetate     

Quantitative Determination of Gap Junction Intercellular Communication by Scrape Loading and Image Analysis

Gap junction intercellular communication (GJIC) consists of intercellular exchange of low molecular weight molecules. Chemically induced alterations of this communication have been suggested to result in abnormal cell growth and tumour promotion. Several in vitro assays have been developed to determine the effect of chemicals on gap junction communication in cultured cells. The scrape loading dye transfer technique is based on studying the transfer of the fluorescent dye Lucifer Yellow in cells where the dye is loaded through a cut in the cell monolayer. This technique is rapid and relatively uncomplicated, but has only been used to qualitatively demonstrate communication, due to lack of an appropriate method for quantification of the dye spreading. We show here that analysis of digital fluorescence images of cells scrape loaded with Lucifer Yellow can be used for quantitative determination of GJIC. We have analysed the images both by means of distance of diffusion of the dye in the cell monolayer, as well as by area of dye-coupled cells. The results are consistent with that obtained using microinjection of Lucifer Yellow and the method offers a simple way for quantitative determination of GJIC.     

Loss of Gap Junction Plaques and Inhibition of Intercellular Communication in Ilimaquinone-treated BICR-M1Rk and NRK Cells

To examine the mechanism(s) and pathways of gap junction formation and removal a novel and reversible inhibitor of protein secretion, ilimaquinone (IQ), was employed. IQ has been reported to cause the vesiculation of Golgi membranes, block protein transport at the cis-Golgi and depolymerize cytoplasmic microtubules. Connexin43 (Cx43) immunolabeling and dye microinjection experiments revealed that gap junction plaques were lost and intercellular communication was inhibited following IQ treatment for 1 hr in BICR-M1R_k rat mammary tumor cells and for 2 hr in normal rat kidney (NRK) cells. Gap junction plaques and intercellular communication recovered within 2 hr when IQ was removed. IQ, however, did not affect the distribution of zonula occludens-1, a protein associated with tight junctions. Western blot analysis revealed that the IQ-induced loss of gap junction plaques was accompanied by a limited reduction in the highly phosphorylated form of Cx43, previously shown to be correlated with gap junction plaques. The presence of IQ inhibited the formation of new gap junction plaques in BICR-M1R_k cells under conditions where preexisting gap junctions were downregulated by brefeldin A treatment. Treatment of BICR-M1R_k and NRK cells with other microtubule depolymerization agents did not inhibit plaque formation or promote rapid gap junction removal. These findings suggest that IQ disrupts intercellular communication by inhibiting the events that are involved in plaque formation and/or retention at the cell surface independent of its effects on microtubules. Our results also suggest that additional factors other than phosphorylation are necessary for Cx43 assembly into gap junction plaques. Received: 16 January 1996/Revised: 20 September 1996     

Connexins and their channels in cell growth and cell death

Direct communication between cells, mediated by gap junctions, is nowadays considered as an indispensable mechanism in the maintenance of cellular homeostasis. In fact, gap junctional intercellular communication is actively involved in virtually all aspects of the cellular life cycle, ranging from cell growth to cell death. For a long time, it was believed that this was merely a result of the capacity of gap junctions to control the direct intercellular exchange of essential cellular messengers. However, recent data show that the picture is more complicated than initially thought, as structural precursors of gap junctions, connexins and gap junction hemichannels, can affect the cellular homeostatic balance independently of gap junctional intercellular communication. In this paper, we summarize the current knowledge concerning the roles of connexins and their channels in the control of cellular homeostasis, with the emphasis on cell growth and cell death. We also briefly discuss the role of gap junctional intercellular communication in carcinogenesis and the potential use of connexins as tools for cancer therapy.     

Characterization of a rat liver epithelial cell line to detect inhibitors of metabolic cooperation

Summary A normal rat liver epithelial cell line, with phenotype characteristics of “oval” cells (WB-F344), was examined for its ability to perform gap-junctional intercellular communication as measured by metabolic cooperation. To test for gap-junctional intercellular communication, 6-thioguanine-sensitive cells were cocultivated with 6-thioguanine-resistant cells. It was found that the recovery of 6-thioguanine-resistant cells depended on the densities of the 6-thioguanine-sensitive cells. Higher densities of 6-thioguanine-sensitive cells reduced the recovery of 6-thioguanine-resistant cells. These observations demonstrate that rat liver epithelial cells could metabolically cooperate, implying they could perform gap-junctional intercellular communication. Two tumor-promoting organochlorine pesticides, aldrin and dieldrin, were potent inhibitors of metabolic cooperation for these cells, but 12-0-tetradecanoyl-phorbol-13-acetate and teleocidin, known mouse skin tumor promoters, were not significantly effective in inhibiting metabolic cooperation. The results suggest that these cells might provide the basis for an in vitro assay specifically to study liver tumor promoters. Research was sponsored by a grant from the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under grant AFOSR-86-0084. The U. S. Government is authorized to reproduce and distribute reprints for government purposes notwithstanding any copyright notation thereon.     

CONNEXIN43 GAP JUNCTION LEVELS DURING DEVELOPMENT OF THE THORACIC AORTA ARE TEMPORALLY CORRELATED WITH ELASTIC LAMINAE DEPOSITION AND INCREASED BLOOD PRESSURE

A characteristic property of the vascular smooth muscle cell is its ability to modulate between a contractile phenotype, responsible for control of vascular tone and tension, through to a synthetic phenotype, capable of migration and synthesis of extracellular matrix molecules. Smooth muscle cells are coupled by gap junctions, the membrane structures which permit direct intercellular passage of ions and small molecules, and which play a role both in electrical coupling and intercellular communication during patterning and development. We have previously found that connexin43 type gap junction expression is upregulated in the synthetic phenotype smooth muscle cellin vitroand during atherosclerotic plaque formation in human coronary arteries. On the basis of immunohistochemical labelling, confocal laser scanning microscopy and digital image analysis, we now report that relatively high levels of connexin43 are expressed during development of the rat thoracic aorta, temporally correlating with reported periods of smooth muscle cell proliferation and secretion of elastic laminae. A major peak in expression occurs at seven days post-natal, with a second less pronounced peak at 72 days post-natal. The principal peak in gap junction levels appears to coincide with increased post-natal blood pressure and aorta media thickening. The amount of gap junction labelling falls off to normal adult levels as the smooth muscle cells modulate towards the contractile phenotype and growth is completed. The results indicate an association between direct cell-to-cell communication and synthetic phenotype smooth muscle cell activity during aortic growth and patterning.     

Inhibition of gap-junctional intercellular communication between Chinese hamster lung fibroblasts by di(2-ethylhexyl) phthalate (DEHP) and trisodium nitrilotriacetate monohydrate (NTA)

Di(2-ethylhexyl)phthalate and trisodium nitrilotriacetate monohydrate, two apparently nongenotoxic carcinogens, were tested for effects on gap-junctional communication between Chinese hamster V79 lung fibroblasts. Both compounds inhibited gap-junctional communication in a concentration-dependent manner. The inhibiting effects of these chemicals on gap-junctional communication in vitro correlate with their tumor-promoting activity. Such results further support the hypothesis that inhibition of gap-junctional communication is an in vitro biomarker for some tumor-promoting chemicals.Abbreviations CAS Chemical Abstracts Service - DEHP di(2-ethylhexyl)phthalate - GJIC gap-junctional intercellular communication - NTA trisodium nitrilotriacetate monohydrate     

Effects of pyrethroid insecticides on gap junctional intercellular communications in Balb/c3T3 cells by dye-transfer assay

The effects of fenvalerate, esfenvalerate, permethrin, cypermethrin, deltamethrin, p-chlorophenylisovaleric acid (CPIA, major metabolite of fenvalerate) and DDT, a liver tumor promoter, on gap junctional intercellular communication (GJIC) were examined in Balb/c3T3 cells by dye-transfer assay. Separate groups of Balb/c3T3 cells were exposed to the chemicals for 1 day. On the following day, GJIC was measured by counting the number of dye-transferring cells per injection of Lucifer Yellow under a fluorescent microscope. Fenvalerate, esfenvalerate, permethrin, cypermethrin, deltamethrin and DDT inhibited GJIC at noncytotoxic concentrations, while CPIA did not inhibit GJIC even, at a cytotoxic concentration. It is concluded that the examined pyrethroid insecticides, but not a metabolite, have inhibitory effects on GJIC in Balb/c3T3 cells.Abbreviations DDT 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane - DMSO dimethyl sulfoxide     

Physiological levels of melatonin enhance gap junction communication in primary cultures of mouse hepatocytes

Gap junction communication is known to be involved in controlling cell proliferation and differentiation, and seems to play a crucial role in suppression of tumor promotion. Melatonin, a hormone secreted by the pineal gland, has putative oncostatic properties. Intercellular communication through gap junctions was assessed by microinjecting Lucifer yellow fluorescent dye into primary hepatocytes and visualizing the spread of the dye to adjacent neighboring cells using phase contrast/fluorescent microscopy. Treatment of primary hepatocyte cultures with a physiological range of melatonin concentrations for 24 h prior to microinjection resulted in significant enhancement in intercellular communication at 0.2 and 0.4 nmol/L but not at lower (0.1 nmol/L) or higher (0.8 or 1.0 nmol/L) concentrations. A time-dependent study showed that the changes in intercellular communication began 10 h after melatonin treatment and reached a maximum at 12 h of treatment. This nonlinear, functional gap junction response to melatonin occurred in the physiological concentration range detected in blood of mammals during nightly releases of the hormone by the pineal gland. These melatonin levels may affect the ability of gap junction communication to exert cell growth control in vivo. The uneven decline between individuals in nocturnal release of melatonin that occurs with age could identify potentially sensitive subpopulations susceptible to developing pathologies involving alterations in biological processes dependent on gap junction communication. This revised version was published online in July 2006 with corrections to the Cover Date.     

Connexin expression systems: To what extent do they reflect the situation in the animal?

Intercellular communication is mediated by specialized cell-cell contact areas known as gap junctions. Connexins are the constitutive proteins of gap junction intercellular channels. Various cell expression systems are used to express connexins and, in turn, these expression systems can then be tested for their ability to form functional cell-cell channels. In this review, expression of murine endogenous connexins in primary cells and established cell lines is compared with results obtained by expression of exogenous connexins inXenopus oocytes and cultured mammalian cells. In addition, first reports on characterization of connexin-deficient mice are discussed.