In vitro growth inhibition of neoplastically transformed cells by non-transformed cells: requirement for gap junctional intercellular communication

Connexin43 Suppresses Lung Cancer Stem Cells

Alterations in gap junctions and their protein components, connexins, have been associated with neoplastic transformation and drug resistance, and more recently have been shown to play important roles in cancer stem cells (CSCs). However, there is less knowledge of connexins and gap junctions in lung CSCs. To address this, Connexin43 (Cx43), the major human lung epithelial gap junction protein, was expressed ectopically in poorly expressing National Cancer Institute-125 (NCI-H125) metastatic human lung adenocarcinoma cells, and phenotypic characteristics of malignant cells and abundance of CSCs were evaluated. The ectopic expression of Cx43 resulted in the formation of functional gap junctions; a more epithelial morphology; reduced proliferation, invasion, colony formation, tumorsphere formation, pluripotency marker expression, and percentage of aldehyde dehydrogenase (ALDH)-positive cells; and increased cisplatin sensitivity. Similarly, in NCI-H522 (human lung adenocarcinoma) and NCI-H661 (human lung large cell carcinoma) cell lines, which express Cx43 and functional gap junctions endogenously, the Cx43 content was lower in tumorspheres and ALDH-positive cells than in bulk cells. These results demonstrate that Cx43 can reverse several neoplastic characteristics and reduce the abundance of human lung CSCs.

Transcriptional regulation by normal epithelium of premalignant to malignant progression in Barrett's esophagus

In carcinogenesis, intercellular interactions within and between cell types are critical but remain poorly understood. We present a study on intercellular interactions between normal and premalignant epithelial cells and their functional relevance in the context of premalignant to malignant progression in Barrett's esophagus. Using whole transcriptome profiling we found that in the presence of normal epithelial cells, dysplastic cells but not normal cells, exhibit marked down-regulation of a number of key signaling pathways, including the transforming growth factor beta (TGFβ) and epithelial growth factor (EGF). Functional assays revealed both cell types showed repressed proliferation and significant changes in motility (speed, displacement and directionality) as a result of interactions between the two cell types. Cellular interactions appear to be mediated through both direct cell-cell contact and secreted ligands. The findings of this study are important in that they reveal, for the first time, the effects of cellular communication on gene expression and cellular function between premalignant (dysplastic) epithelial cells and their normal counterparts.

Extracellular movement of signaling molecules

Extracellular signaling molecules have crucial roles in development and homeostasis, and their incorrect deployment can lead to developmental defects and disease states. Signaling molecules are released from sending cells, travel to target cells, and act over length scales of several orders of magnitude, from morphogen-mediated patterning of small developmental fields to hormonal signaling throughout the organism. We discuss how signals are modified and assembled for transport, which routes they take to reach their targets, and how their range is affected by mobility and stability.

Regulation of miRNA expression by Src and contact normalization: effects on nonanchored cell growth and migration

Transformation by the Src tyrosine kinase (Src) promotes nonanchored cell growth and migration. However, nontransformed cells can force Src-transformed cells to assume a normal morphology and phenotype by a process called 'contact normalization'. It has become clear that microRNA (miRNA) can affect tumorigenesis by targeting gene products that direct cell growth and migration. However, the roles of miRNA in Src transformation or contact normalization have not yet been reported. We examined the expression of 95 miRNAs and found 9 of them significantly affected by Src. In this study, we report that miR-218 and miR-224 were most significantly induced by Src, but not affected by contact normalization. In contrast, miR-126 was most significantly suppressed by Src and was induced by contact normalization in transformed cells. Mir-126 targets Crk, a component of the focal adhesion network that participates in events required for tumor cell migration. Accordingly, we show that miR-126 expression correlates inversely with Crk levels, motility and the invasive potential of human mammary carcinoma cells. Moreover, we show that miR-224 expression promotes nonanchored growth of nontransformed cells. These data reveal novel insights into how Src regulates miRNA expression to promote hallmarks of tumor cell growth and invasion, and how nontransformed cells can affect miRNA expression in adjacent tumor cells to inhibit this process.

Biology and pathobiology of gap junctional channels in hepatocytes

The present review provides the state of the art of the current knowledge concerning gap junctional channels and their roles in liver functioning. In the first part, we summarize some relevant biochemical properties of hepatic gap junctional channels, including their structure and regulation. In the second part, we discuss the involvement of gap junctional channels in the occurrence of liver cell growth, liver cell differentiation, and liver cell death. We further exemplify their relevance in hepatic pathophysiology. Finally, a number of directions for future liver gap junctional channel research are proposed, and the up-regulation of gap junctional channel activity as a novel strategy in (liver) cancer therapy is illustrated.

Inhibition of Balb/c 3T3 cell transformation by synthetic acyclic retinoid NIK-333; possible involvement of enhanced gap junctional intercellular communication

BACKGROUND

In an attempt to develop effective and non-cytotoxic cancer chemopreventive derivatives of retinoids, a novel acyclic retinoid has previously been synthesized. This acyclic retinoid, NIK-333, had been reported to suppress recurrence of primary hepatocellular carcinomas. We explored the molecular mechanisms by which NIK-333 exerts anti-proliferative effects.

METHODS

We employed Balb/c 3T3 cells, since they can be used as a quantitative transformation assay and since we can study a possible involvement of gap-junctional intercellular communication (GJIC) in their growth control. We included all-trans-retinoic acid (ATRA) for comparison.

RESULTS

We first confirmed that these cells express the retinoid receptors, RARalpha, gamma and RXRalpha, suggesting that they respond to NIK-333 and ATRA. When NIK-333 or ATRA was added to Balb/c 3T3 cells during the induction of cell transformation by a standard (3-methylcholanthrene (MCA) alone) or two-stage (low dose of MCA plus 12-O-tetradecanoylphorbol-13-acetate (TPA)) protocol, there was a significant decrease in the number of transformed foci. The extent of inhibition of transformation by NIK-333 was similar to that exerted by ATRA. Employing the microinjection dye-transfer assay, we found that both retinoids increase GJIC level when measured 24h after treatment. The extent of GJIC increase by NIK-333 was similar to that of ATRA. These retinoids also increased the amount of connexin 43 (Cx43) on the plasma membrane as revealed by immunostaining.

CONCLUSION

These data indicate that NIK-333 suppresses chemical carcinogenesis in vitro and support the hypothesis that enhancement of GJIC is involved in this process.

Detrimental and protective bystander effects: a model approach

This work integrates two important cellular responses to low doses, detrimental bystander effects and apoptosis-mediated protective bystander effects, into a multistage model for chromosome aberrations and in vitro neoplastic transformation: the State-Vector Model. The new models were tested on representative data sets that show supralinear or U-shaped dose responses. The original model without the new low-dose features was also tested for consistency with LNT-shaped dose responses. Reductions of in vitro neoplastic transformation frequencies below the spontaneous level have been reported after exposure of cells to low doses of low-LET radiation. In the current study, this protective effect is explained with bystander-induced apoptosis. An important data set that shows a low-dose detrimental bystander effect for chromosome aberrations was successfully fitted by additional terms within the cell initiation stage. It was found that this approach is equivalent to bystander-induced clonal expansion of initiated cells. This study is an important step toward a comprehensive model that contains all essential biological mechanisms that can influence dose-response curves at low doses.

Herpes simplex virus-type 2 infectivity and agents that block gap junctional intercellular communication

In rat liver epithelial (WB) cells, the protein kinase C inhibitor H7 blocked gap junctional intercellular communication (GJIC) and reduced virus infectivity. Octanol, 18-beta-glycyrrhetinic acid, and staurosporine, agents that reduce GJIC, had no effect upon virus infectivity. Previous studies demonstrated that herpes simplex virus-type 2 (HSV-2) infection was accompanied by attenuated GJIC. Of agents tested, only H7 reduced plaque forming unit (pfu) ability in a dose-dependent manner with 100% plaque reduction at 40 microM without evidence of cytotoxicity. Dye transfer indicated that H7 decreased GJIC, although Western blotting revealed that it did not alter phosphorylation of the gap junction protein, connexin 43 (Cx43). Using indirect immunofluorescence, Cx43 was found to localize in membrane plaques in uninfected cells and H7 did not alter this distribution. However, Cx43 was lost from the membrane at 24h in both H7-treated and untreated cells infected with HSV-2. Viral infection increased serine phosphorylation, particularly in the nuclear region, and this effect was reduced following H7 treatment. Thus, H7 attenuated both GJIC and infectivity of HSV-2 in WB cells but the anti-viral effects were due to reduced nuclear protein phosphorylation rather than alterations in phosphorylation or localization of Cx43.

H-Ras selectively up-regulates MMP-9 and COX-2 through activation of ERK1/2 and NF-kappaB: an implication for invasive phenotype in rat liver epithelial cells

One of the most frequent events in carcinogenesis is uncontrolled activation of Ras signaling pathway. A previous study demonstrated that the introduction of H-Ras into the normal WB-F344 rat liver epithelial (WB) cell line and adult male F344 rats resulted in tumorigenicity. The present study investigated whether H-Ras induced the invasive and migrative phenotypes in WB cells, and subsequently aimed at characterizing the underlying mechanisms. H-Ras induced the invasive and migrative phenotypes of WB cells with a selective up-regulation of matrix metalloproteinase (MMP)-9, but not MMP-2. Cyclooxygenase (COX)-2 and the subsequent production of prostaglandin E2 (PGE2) were also induced by H-Ras. Treatment of H-Ras WB cells with GM6001 and NS398, the inhibitors of MMPs and COX-2, respectively, significantly inhibited the H-Ras-induced invasive and migrative phenotypes. DNA binding activity of nuclear factor (NF)-kappaB, but not that of activator protein (AP)-1, was increased by H-Ras. Caffeic acid phenethyl ester and Bay 11-7082, specific inhibitors of NF-kappaB and IKK, respectively, significantly inhibited the expression of MMP-9 and COX-2, invasion and migration of H-Ras WB cells, revealing NF-kappaB as a transcriptional factor responsible for H-Ras-induced malignant phenotypic conversion of WB cells. Activation of ERKs pathway was critical for H-Ras-induced invasive and migrative phenotypes, up-regulation of MMP-9 and COX-2 as well as enhanced DNA binding activity of NF-kappaB in WB cells. Taken together, these results demonstrate that H-Ras up-regulates MMP-9 and COX-2 through activation of ERKs and IKK-IkappaBalpha-NF-kappaB signal pathway which may contribute to the malignant progression of WB rat liver epithelial cells.

Sequential molecular and cellular events during neoplastic progression: a mouse syngeneic ovarian cancer model

Studies performed to identify early events of ovarian cancer and to establish molecular markers to support of early detection and the development of chemopreventive regimens have been hindered by a lack of adequate cell models. Taking advantage of the spontaneous transformation of mouse ovarian surface epithelial (MOSE) cells in culture, we isolated and characterized distinct transitional stages of ovarian cancer as the cells progressed from a premalignant nontumorigenic phenotype to a highly aggressive malignant phenotype. Transitional stages were concurrent with progressive increases in proliferation, anchorage-independent growth capacity, in vivo tumor formation, and aneuploidy. During neoplastic progression, our ovarian cancer model underwent distinct remodeling of the actin cytoskeleton and focal adhesion complexes, concomitant with downregulation and/or aberrant subcellular localization of two tumor-suppressor proteins E-cadherin and connexin-43. In addition, we demonstrate that epigenetic silencing of E-cadherin through promoter methylation is associated with neoplastic progression of our ovarian cancer model. These results establish critical interactions between cellular cytoskeletal remodeling and epigenetic silencing events in the progression of ovarian cancer. Thus, our MOSE model provides an excellent tool to identify both cellular and molecular changes in the early and late stages of ovarian cancer, to evaluate their regulation, and to determine their significance in an immunocompetent in vivo environment.

Suberoylanilide Hydroxamic Acid Enhances Gap Junctional Intercellular Communication via Acetylation of Histone Containing Connexin 43 Gene Locus

A histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid (SAHA), induces apoptosis in neoplastic cells, but its effect on gap junctional intercellular communication in relation to apoptosis was unclear. Therefore, we carried out a comparative study of the effects of two HDAC inhibitors, SAHA and trichostatin-A, on gap junctional intercellular communication in nonmalignant human peritoneal mesothelial cells (HPMC) and tumorigenic ras oncogene-transformed rat liver epithelial cells (WB-ras) that showed a significantly lower level of gap junctional intercellular communication than did HPMC. Gap junctional intercellular communication was assessed by recovery rate of fluorescence recovery after photobleaching. Treatment of HPMC with SAHA at nanomolar concentrations caused a dose-dependent increase of recovery rate without inducing apoptosis. This effect was accompanied by enhanced connexin 43 (Cx43) mRNA and protein expression and increased presence of Cx43 protein on cell membrane. Trichostatin-A induced apoptosis in HPMC but was less potent than SAHA in enhancing the recovery rate. In contrast, treatment of WB-ras cells with SAHA or trichostatin-A induced apoptosis at low concentrations, in spite of smaller increases in recovery rate, Cx43 mRNA, and protein than in HPMC. Chromatin immunoprecipitation analysis revealed that SAHA enhanced acetylated histones H3 and H4 in the chromatin fragments associated with Cx43 gene in HPMC. These results indicate that SAHA at low concentrations selectively up-regulates Cx43 expression in normal human cells without induction of apoptosis, as a result of histone acetylation in selective chromatin fragments, in contrast to the apoptotic effect observed in tumorigenic WB-ras cells. These results support a cancer therapeutic and preventive role for specific HDAC inhibitors.

Characterization of cell-cycle progression and growth of WB-F344 normal rat liver epithelial cells following gamma-ray exposure

BACKGROUND

Apparently normal rat liver epithelial cells (WB-F344) have been widely used in studies pertaining to carcinogenesis. Ionizing radiation, a well known carcinogen, is known to perturb cell-cycle progression in a dose-dependent manner, thereby causing delay in cell proliferation. However, for WB-F344 cells, there is a paucity of such data, which are of substantial importance in understanding their radiation response. Here, the distribution of phases in the cell-cycle and the proliferation ability of WB-F344 cells are characterized at various time points after the cells have been irradiated with different doses of gamma-rays.

METHODS

After WB-F344 cells reached 100% confluence, they were trypsinized and suspended at 3.5 x 10(5) cells/ml in culture medium. Cells were irradiated in suspension with (137)Cs gamma-rays at doses from 1-10 Gy. After irradiation, 1 x 10(5) cells were plated into 60 x 15-mm culture dishes and incubated at 37 degrees C, with 2% CO(2) and 98% air. At 12, 24, 36, 48, and 60 h postirradiation, cells were harvested, counted, and subjected to flow cytometric cell-cycle analysis.

RESULTS

Growth curves of WB-F344 cells irradiated with gamma-rays started to separate at 36 h postirradiation. By 60 h postirradiation, the growth curves for each of the 10 absorbed doses were distinctly separated. Drastic redistributions of control and irradiated cells within G(0)/G(1)-, S-, and G(2)/M-phases of the cell cycle were observed during the first 36 h of cell growth. At each time point postirradiation, cell-cycle phase profiles of irradiated cells were altered in a dose-dependent manner. In general, there was a strong correlation between the percentage of G(2)/M-phase cells and absorbed dose, with the exception of 24 h postirradiation. The percentage of G(2)/M-phase cells increased as a function of time postirradiation, suggestive of delays in the passage of cells through the G(2) cell-cycle checkpoint.

CONCLUSIONS

This work provides a general description of cell cycle redistribution and repopulation kinetics of WB-F344 cells at various times postirradiation of quiescent cells that were subsequently allowed to proliferate. In general, growth inhibition and delays in progression through G(2)/M-phase correlated well with radiation dose. These data should be of considerable significance in the design of experiments that examine the radiation response of these cells.

Cross-presentation by intercellular peptide transfer through gap junctions

Major histocompatibility complex (MHC) class I molecules present peptides that are derived from endogenous proteins. These antigens can also be transferred to professional antigen-presenting cells in a process called cross-presentation, which precedes initiation of a proper T-cell response; but exactly how they do this is unclear. We tested whether peptides can be transferred directly from the cytoplasm of one cell into the cytoplasm of its neighbour through gap junctions. Here we show that peptides with a relative molecular mass of up to approximately 1,800 diffuse intercellularly through gap junctions unless a three-dimensional structure is imposed. This intercellular peptide transfer causes cytotoxic T-cell recognition of adjacent, innocent bystander cells as well as activated monocytes. Gap-junction-mediated peptide transfer is restricted to a few coupling cells owing to the high cytosolic peptidase activity. We present a mechanism of antigen acquisition for cross-presentation that couples the antigen presentation system of two adjacent cells and is lost in most tumours: gap-junction-mediated intercellular peptide coupling for presentation by bystander MHC class I molecules and transfer to professional antigen presenting cells for cross-priming.

Phosphorylation of serine 262 in the gap junction protein connexin-43 regulates DNA synthesis in cell-cell contact forming cardiomyocytes

Mitogenic stimulation of cardiomyocytes is associated with decreased gap junction coupling and protein kinase C (PKC)-mediated phosphorylation of the gap junction protein connexin43 (Cx43). Identification of and interference with the amino acid(s) that becomes phosphorylated in response to stimulation are important steps towards defining the relationship between Cx43 phosphorylation and cell cycle. Using immunoblotting and phosphospecific antibodies we were able to show that serine-262 (S262) on Cx43 becomes phosphorylated in response to growth factor or PKC stimulation of cardiomyocytes. To examine the effect of Cx43, S262 phosphorylation and cell-cell contact (and/or coupling) on DNA synthesis, we overexpressed wild-type (wt) or mutant Cx43, carrying a S262-to-alanine (S262A, simulating the unphosphorylated state) or a S262-to-aspartate (S262D, simulating constitutive phosphorylation) substitutions in cultures of cell-cell contact forming or isolated cardiomyocytes. Overexpression of wt-Cx43 caused a significant decrease in DNA synthesis irrespective of the presence of cell-cell contact. In cell-cell contact forming cultures, the S262D mutation reversed while the S262A mutation increased the inhibitory effect of Cx43. In the absence of cell-cell contact, the S262-Cx43 mutations had no significant effect on Cx43 inhibition of DNA synthesis. Dye-coupling, evaluated by scrape-loading, indicated increased gap junction permeability in S262A (compared to wt or S262D) overexpressing myocytes. We conclude that Cx43 inhibits cardiomyocyte DNA synthesis irrespectively of cell-cell contact or coupling. Cell-cell contact, and possibly gap junction-mediated communication is required, however, in order to reverse Cx43 inhibition of DNA synthesis by S262 phosphorylation.

Bone marrow stroma inhibits proliferation and apoptosis in leukemic cells through gap junction-mediated cell communication

Normal and leukemic blood cell progenitors depend upon the bone marrow (BM) stroma with which they communicate through soluble and membrane-anchored mediators, adhesive interactions and gap junctions (GJ). Regarding hematopoiesis, it is believed that it can be influenced by connexin expression, but the exact role of GJ in cell death and proliferation is not clear. Using flow cytometry, we monitored the division rate of leukemic cell lines, communicating and not communicating with stromal cell line through GJ. We found that GJ-coupled cells (i) did not proliferate; (ii) were kept in G0; and (iii) were protected from drug-induced apoptosis when compared to either total or uncoupled cell population. We conclude that GJ coupling between stroma and leukemic lymphoblasts prevents proliferation, keeping cells in a quiescent state, thus increasing their resistance to antimitotic drugs. Since GJ are particularly abundant in the sub-endosteal environment, which harbors blood stem cells, we also asked which cells within the normal human BM communicate with the stroma. Using a primary BM stroma cell culture, our results show that 80% of CD34+ progenitors communicate through GJ. We propose that blood cell progenitors might be retained in the low-cycling state by GJ-mediated communication with the hematopoietic stroma.

NBT-II carcinoma behaviour is not dependent on cell-cell communication through gap junctions

To study the mechanism(s) underlying the proliferation of heterogeneous cell populations within a solid tumour, the NBT-II rat bladder carcinoma system was used. It has been first investigated whether the different cell populations are coupled through gap junctions (GJIC). Cells overexpressing the Cx43 were generated to test for any tumour suppressive activity in vivo. To determine whether GJIC is essential for tumour proliferation and the establishment of a cooperative community effect, NBT-II cells that are incompetent for cell coupling were generated. The data report that (i) carcinoma cells expressing or not FGF-1 are coupled through GJIC in vitro and in coculture and express the gap junction protein Cx43, (ii) overexpression of Cx43 in these cells does not affect their in vitro coupling capacities and in vivo tumourigenic growth properties, (iii) inhibition of GJIC through antisense strategy has no in vivo obvious consequence on the tumour growth properties of the carcinoma, and (iv) the community effect between two carcinoma cell populations does not critically involve cell coupling through gap junctions.

Preventive effect of germanium dioxide on the inhibition of gap junctional intercellular communication by TPA

Gap junctional intercellular communication (GJIC) is thought to be essential for maintaining cellular homeostasis and growth control. In order to detect any protective agent against tumor formation, we examined the anticarcinogenic effect of a germanium dioxide (GeO(2)) using a model system of GJIC in F344 rat liver epithelial cells, named WB cells. 12-O-tetradecanoylphorbol-13-acetate (TPA), known as tumor promoters, inhibited GJIC in the epithelial cells as determined by the scrape loading/dye transfer (SL/DT) assay. And GeO(2) recovered this inhibition of GJIC. Immunostaining of connexin 43 (Cx43) protein in WB cells indicated that TPA caused a loss of Cx43 protein from the cell membranes. However, GeO(2) treatment showed re-appearance of Cx43 protein on the membrane. Reverse transcription-polymerase chain reaction (RT-PCR) and Western blots were analyzed to determine whether the test compounds might have altered the steady-state levels of gap junction mRNA and/or connexin protein levels or phosphorylation. The inhibition of GJIC by TPA in WB cells was correlated with the hyperphosphorylation of Cx43 as measured by mobility shifts of the western blot bands of Cx43. TPA induced hyperphosphorylation of Cx43 protein, while GeO(2) appeared to partially block this hyperphosphorylation. Here, we showed that pre- and co-incubation with GeO(2) in TPA-treated WB-cells abolished down-regulation of GJIC by TPA. These data suggest that GeO(2) may inhibit tumor promotion by enhancing GJIC.

Gap-junctional communication between feeder cells and recipient normal epithelial cells correlates with growth stimulation

LA7 rat mammary tumor cells stimulate the proliferation, in culture, of three normal epithelial cell types, namely mouse mammary, rat mammary, and mouse thymic cells. Gap-junctional communication between LA7 feeders and mouse mammary cells was demonstrated by microinjection of lucifer yellow, which traveled from LA7 to the surrounding mouse mammary cells. The amount of 3H-uridine exchange between feeder and recipient mouse mammary, rat mammary, and mouse thymus cells correlated with the growth rate induced by the feeders. Cells of the Madin Darby canine kidney (MDCK) line, which do not appreciably stimulate mouse mammary cell growth when used as feeder cells, also exchange little 3H-uridine with them. Expression of connexins Cx43, 32, and 26 was studied in all these cell lines and strains by immunocytochemistry. Mouse mammary cells expressed Cx26, and a few mouse thymic cells expressed Cx32. LA7, mouse mammary, mouse thymic, and rat mammary cells all expressed easily detectable amounts of the gap-junction protein Cx43, in contrast to MDCK cells, which expressed only a hint of the protein. These results suggest that gap junctions composed of Cx43 are those by which the normal epithelial cells communicate with the LA feeders. Thus, the ability of feeder cells to stimulate proliferation in recipients correlates with the expression of Cx43 in both members of the feeder/recipient pair and the capacity to form functional gap junctions between these cells.

Preventive effect of epicatechin and ginsenoside Rb(2) on the inhibition of gap junctional intercellular communication by TPA and H(2)O(2)

The anticarcinogenic effects of epicatechin (EC) and ginsenoside Rb(2) (Rb(2)), which are major components of green tea and Korea ginseng, respectively, were investigated using a model system of gap junctional intercellular communication (GJIC) in WB-F344 rat liver epithelial cells. 12-O-tetradecanoylphorbol-13-acetate (TPA) and hydrogen peroxide, known as cancer promoters, inhibited GJIC in the epithelial cells as determined by the scrape loading/dye transfer assay, fluorescence redistribution assay after photobleaching, and immunofluorescent staining of connexin 43 using a laser confocal microscope. The inhibition of GJIC by TPA and H(2)O(2) was prevented with treatment of Rb(2) or EC. The effect of EC on GJIC was stronger in TPA-treated cells than in H(2)O(2)-treated cells, while the effect of Rb(2) was opposite to that of EC. EC, at the concentration of 27.8 microg/ml, prevented the TPA-induced GJIC inhibition by about 60%. Rb(2,) at the concentration of 277 microg/ml, recovered the H(2)O(2)-induced GJIC inhibition by about 60%. These results suggest that Rb(2) and EC may prevent human cancers by preventing the down-regulation of GJIC during the cancer promotion phase and that the anticancer effect of green tea and Korea ginseng may come from the major respective components, EC and Rb(2).

Dynamics of intercellular communication during melanoma development

Melanoma development involves processes determined collectively by various microenvironmental factors, among which intercellular communication has drawn increased attention. Cell-cell crosstalk mediated by cadherins and connexins results in coordinated regulation of cell growth, differentiation, apoptosis and migration. Abnormal expression of adhesion receptors and dysregulated intercellular communication appears to drive tumor development and progression.

Induction of skin papillomas, carcinomas, and sarcomas in mice in which the connexin 43 gene is heterologously deleted

It has been suggested that blocked gap junctional intercellular communication plays a crucial part in multistage carcinogenesis. The mouse skin tumor-promoting phorbol esters are potent inhibitors of gap junctional intercellular communication and this inhibition is considered to be a mechanism by which clonal expansion of "initiated" cells is promoted. We examined whether mice in which the gene for a gap junction protein, connexin 43, is heterozygously deleted are more susceptible to chemical carcinogenesis; connexin 43 is expressed in the basal cell layer and the dermis of the skin. When the back skin was painted with 7,12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol 13-acetate, the incidence and yields of both papillomas and carcinomas were similar in connexin 43+/- and connexin 43+/+ mice; for this experiment, the original mice with C57BL/6 genetic background was crossed with CD1 strain for three generations. Subcutaneous injection of 7, 12-dimethylbenz[a]anthracene resulted in induction of fibrosarcomas in connexin 43+/- and connexin 43+/+ mice to a similar extent. All papillomas and carcinomas induced with 7, 12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol 13-acetate contained the 7,12-dimethylbenz[a] anthracene-specific mutation in the ras gene (A to T transversion at the 61st codon). About 50% of fibrosarcomas also contained this mutation, but in the Ki-ras gene; there was no difference in the prevalence of this mutation in tumors from connexin 43+/- and connexin 43+/+ mice. None of the tumors examined, however, showed any mutation in the connexin 43 gene. These results suggest that the deletion of one allele of the connexin 43 gene does not significantly contribute to, nor alter, the molecular events involved in skin carcinogenesis. These results are compatible with previous observations that nongenetic disruption of function rather than mutations of connexins, commonly occurs in cancer cells.

Gene expression profiling of fibroblasts resistant toward oncogene-mediated transformation reveals preferential transcription of negative growth regulators

The signal-transducing Ras proteins are important driving forces of diverse cellular processes such as proliferation, neoplastic transformation, differentiation and growth inhibition. As a step toward understanding the complex mechanisms underlying cellular responses, gene expression patterns were examined in two phenotypically normal fibroblast lines which differ in their sensitivity toward oncogene-mediated transformation. Suppression subtractive hybridization (SSH) was used to establish a subtracted cDNA library specific for the REF52 cell line which, like normal diploid fibroblasts, is refractory toward neoplastic transformation induced by mutated HRAS oncogenes. In contrast, rat 208F control cells can be efficiently transformed by HRAS. The nucleotide sequence of 549 subtracted cDNA clones ('REF52 minus 208F') was determined. We identified 93 preferentially expressed gene fragments in resistant REF52 cells as compared to 208F cells. Seventeen of the 52 known genes (32.6%) are capable of inhibiting cell proliferation or of adversely affecting oncogenic signal transduction pathways. These results suggest that the anti-oncogenic properties of resistant REF52 cells are determined by multiple negative growth regulators. The preneoplastic state expressed in 208F cells is characterized by impairment of unexpectedly redundant control mechanisms. Our results also demonstrate that SSH is a powerful method for identifying specific transcriptional patterns in closely related cell types.

Inhibition of focus formation of transformed cloned cells by contact with non-transformed BALB/c 3T3 A31-1-1 cells

When transformed cells were co-cultured with various densities of non-transformed BALB/c 3T3 A31-1-1 cells, the number of transformed cell foci decreased as the density of the A31-1-1 cells was increased. Under the condition of separate co-cultivation in which transformed cloned cells could not make contact with A31-1-1 cells, no inhibitory effect was induced. We examined with a dye-transfer assay the formation of heterologous gap-junctional intercellular communication (GJIC), links between the transformed cells and A31-1-1 cells before and after focus formation. Heterologous GJIC was observed almost always before, but almost never after, focus formation. Using time-lapse photography to record the fate of transformed cloned cells that did not form foci in the co-cultivation, it was noted that most of them were living, but did not proliferate. These results suggested that focus formation of transformed cloned cells was inhibited by contact with non-transformed A31-1-1 cells.

Connexin diversity and gap junction regulation by pHi

The molecular mechanisms controlling pH-sensitivity of gap junctions formed of two different connexins are yet to be determined. We used a proton-sensitive fluorophore and electrophysiological techniques to correlate changes in intracellular pH (pHi) with electrical coupling between connexin-expressing Xenopus oocytes. The pH sensitivities of alpha 3 (connexin46), alpha 2 (connexin38), and alpha 1 (connexin43) were studied when these proteins were expressed as: 1) nonjunctional hemichannels (for alpha 3 and alpha 2), 2) homotypic gap junctions, and 3) heterotypic gap junctions. We found that alpha 3 hemichannels are sensitive to changes in pHi within a physiological range (pKa = 7.13 +/- 0.03; Hill coefficient = 3.25 +/- 1.73; n = 8; mean +/- SEM); an even more alkaline pKa was obtained for alpha 2 hemichannels (pKa = 7.50 +/- 0.03; Hill coefficient = 3.22 +/- 0.66; n = 13). The pH sensitivity curves of alpha 2 and alpha 3 homotypic junctions were indistinguishable from those recorded from hemichannels of the same connexin. Based on a comparison of pKa values, both alpha 3 and alpha 2 gap junctions were more pHi-dependent than alpha 1. The pH sensitivity of alpha 2-containing heterotypic junctions could not be predicted from the behavior of the two connexons in the pair. When alpha 2 was paired with alpha 3, the pH sensitivity curve was similar to that obtained from alpha 2 homotypic pairs. Yet, pairing alpha 2 with alpha 1 shifted the curve similar to homotypic alpha 1 channels. Pairing alpha 2 with a less pH sensitive mutant of alpha 1 (M257) yielded the same curve as when alpha 1 was used. However, the pH sensitivity curve of alpha 3/alpha 1 channels was similar to alpha 3/alpha 3, while alpha 3/M257 was indistinguishable from alpha 3/alpha 1. Our results could not be consistently predicted by a probabilistic model of two independent gates in series. The data show that dissimilarities in the pH regulation of gap junctions are due to differences in the primary sequence of connexins. Moreover, we found that pH regulation is an intrinsic property of the hemichannels, but pH sensitivity is modified by the interactions between connexons. These interactions should provide a higher level of functional diversity to gap junctions that are formed by more than one connexin.

Growth control of 3T3 fibroblast cell lines established from connexin 43-deficient mice

Connexins are considered to be involved in cell growth control, on the basis of studies mainly with tumorigenic cells. To study the role of connexin genes in normal cell growth control, we established fibroblast cell lines from connexin 43 (Cx43)-deficient mice and characterized their growth. Embryonic fibroblasts from wild-type mice (Cx43+/+) and those with heterozygous (Cx43+/-) and homozygous (Cx43+/-) deficiencies of the Cx43 gene were cultured and passaged by a 3T3 protocol (every 3 d, 3 x 10(5) cells/60-mm dish). All cell lines showed a growth crisis during passages 6-15 and then started to grow well. All cell lines grew at similar rates under the 3T3 protocol, but Cx43-deficient (Cx43-/-) cell lines tended to grow faster when they were plated at 10(5) cells per dish. Cx43-/- cells did not express Cx43 and showed little gap-junctional intercellular communication (GJIC), confirming that Cx43 is the major connexin responsible for GJIC of these fibroblasts. While all Cx43+/+ and Cx43+/- cell lines expressed Cx43 protein, some of them showed very little GJIC. Those cell lines with high GJIC showed higher levels of the P2 form of Cx43 protein, and more Cx43 was localized in the plasma membrane than in cell lines with lower GJIC levels. We investigated effects of serum concentration on cell growth in these cell lines. Although different cell lines responded differentially to these agents, there was no clear relationship between Cx43 expression and cell growth stimulation by them. This suggests that Cx43 expression alone is not a strong regulator of mouse fibroblast growth.

Inhibitory effect of pentachlorophenol on gap junctional intercellular communication in rat liver epithelial cells in vitro

To understand the initiating/promoting actions of pentachlorophenol (PCP), a non-mutagenic hepatocarcinogen, and its metabolite, tetrachlorohydroquinone (TCHQ), we investigated the effects of each chemical on gap junctional intercellular communication (GJIC) in rat liver epithelial cells (WB cells) by the scrape-loading and dye transfer method. After treatment with PCP, the GJIC was initially inhibited at 4 h but was restored in 6-8 h, followed by a second phase of inhibition between 16 and 24 h. Both the first and second inhibitions were concentration-dependent and were restored by 2-4 h after removal of PCP. The phosphorylation state of connexin 43 (CX43) and its localization on the plasma membrane were unchanged up to 24 h after treatment; however, this was accompanied by a decrease in the CX43 protein level. No inhibitory effect was apparent on the GJIC of cells treated with TCHQ. These results suggest that PCP may play a critical role of promoting activity via non-mutagenic mechanisms.

Neoplastic phenotype of gap-junctional intercellular communication-deficient WB rat liver epithelial cells and its reversal by forced expression of connexin 32

Gap-junctional intercellular communication (GJIC) is involved in cellular growth control and is often reduced in neoplastic cells. In this study, four GJIC-deficient rat liver epithelial cell lines (WB-aB1, WB-bA2, WB-cD6, and WB-dA2) were examined for altered growth and tumorigenicity in comparison with their GJIC-competent parental cell line, WB-F344. WB-aB1 cells were also forced to express connexin 32 (Cx32) by transduction with a Cx32 cDNA retroviral expression vector to help determine whether the restoration of GJIC could reverse their neoplastic phenotype. WB-aB1 and WB-bA2 cells had faster population doubling times (PDTs) and higher saturation densities (SDs) than did WB-F344 cells. In contrast, the growth of WB-cD6 and WB-dA2 cells was not significantly different from that of WB-F344 cells. WB-aB1 and WB-bA2 cells formed tumors in male F344 rats, but WB-cD6 and WB-dA2 cells did not. After transduction of WB-aB1 cells with Cx32, four stable clones (WB-a/32-3, -8, -9, and -10) were isolated that had GJIC levels of 5.2%, 44.5%, 69.8%, and 90.5%, respectively. The growth of poorly coupled clones 3 and 8 was similar to that of parental WB-aB1 cells, but the growth of well-coupled clones 9 and 10 was similar to that of WB-F344 cells. The tumorigenicity of WB-a/32-9 and WB-a/32-10 cells was also significantly lower than that of WB-aB1 cells. Our results suggest that reduced GJIC contributes to neoplastic transformation of WB cells, that additional changes are necessary, and that restoration of GJIC by forced Cx32 protein expression can suppress the neoplastic phenotype of these cells.

Relationship between exposure to TPA and appearance of transformed cells in MNNG-initiated transformation of BALB/c 3T3 cells

In the BALB/c-3T3-cell transformation system, the effect of 12-O-tetradecanoylphorbol-13-acetate (TPA) exposure on the appearance of transformed cells was examined in order to investigate the mechanisms of in vitro tumor promotion. Optimal duration of TPA exposure on N-methyl-N'-nitro-N-nitrosoguanidine(MNNG)-initiated cells was at least 11 days. To investigate the effect of transformation frequencies of altering inoculating cell density at the replating of MNNG-exposed cells and of altering the time of starting TPA exposure, MNNG-exposed cells were replated at various inoculum sizes. With lower inoculum sizes (1 x 10(3) to 3 x 10(4) cells/dish), maximum TPA-induced transformation occurred for TPA commencement at confluence, while with higher inoculum size (1 x 10(5) cells/dish), maximum transformation frequency was observed when TPA exposure was started on day 7 after replating, being some 2 days after confluence. This may suggest that there are different mechanisms involved, depending on inoculum size, and that these may involve cell-cell interactions (at lower inoculum) and mutation expression periods (at higher inoculum). By means of redispersion experiments, it was demonstrated that the appearance of transformed cells begins on about day 7 after replating at a cell density of 1 x 10(4) cells/dish. These results suggest the usefulness of the replating method for optimizing transformation in the BALB/c-3T3-cell transformation assay, and provide insight into the time frame of expression of MNNG-initiated transformants and TPA-induced expansion of these transformants.

Cell association increases RPE outgrowth from primary explant

PURPOSE

Most studies of cell growth of the RPE employ cultures which have been previously passaged, but here we investigated freshly-explanted RPE cells, which may have been growth-quiescent for years, within the eye to determine whether co-culture affects initial outgrowth in primary culture.

METHODS

Bovine or human RPE were co-cultured in primary culture with several cell types to test the effects of homologous or heterologous cell association. For bovine RPE, cell number was measured over 14 days in cultures of RPE alone, or RPE in co-culture with irradiated living cells, with fixed-killed cells, with cells separated from the RPE by a semipermeable membrane, or in medium conditioned by the cell types used for co-culture. For human RPE, isolates from all donors were randomized, over a 13-month period, to co-culture or to culture alone. The number of cultures attaining a cell number at one month that was sufficient for further propagation was compared.

RESULTS

Co-culture with irradiated living cells increased the growth of primary cultures of both bovine and human RPE. Living cells were required; fixed-killed cells were ineffective. The outgrowth-promoting activity was not tissue or species specific, and it appeared to require close cell association between the RPE and the co-culture cell population. Conditioned media were ineffective and rather were slightly growth inhibitory. Primary RPE cells showed an earlier expression of vimentin (a marker of Gzero-G1 transition), more rapid cell spreading, and a greater increase in cell number between 7 and 14 days after explant when grown in co-culture than when cultured alone.

CONCLUSIONS

The results indicate that cell association between non-mitotic RPE cells and previously cultured cells of many types increases the outgrowth of the RPE by accelerating the early stages of growth activation in vitro. Co-culture methods offer a practical means for increasing the likelihood of producing cultures from small RPE isolates. Further, should cells involved in proliferative pathologies in situ associate with non-mitotic RPE within the monolayer, the latter cells may also be activated, leading to an augmentation of the pathology.

Role of disrupted gap junctional intercellular communication in detection and characterization of carcinogens

Results from short-term tests for carcinogens and our advanced knowledge on cellular and molecular mechanisms of carcinogenesis strongly suggest that carcinogens do not induce genetic changes necessarily by directly interacting with DNA. Therefore, it is not surprising to see that many carcinogens are not detectable by available genetic toxicology tests. Thus, it has become necessary to study nongenotoxic mechanisms of carcinogenesis and to provide methods to predict those carcinogens which escape from conventional mutation tests. One possible nongenotoxic mechanism of carcinogenesis which is supported by abundant experimental evidence is inhibition of gap junctional intercellular communication. Many, but not all, tumor-promoting agents have been shown to inhibit the communication of cultured cells as well as in vivo. Molecular mechanisms of gap junctional intercellular communication control revealed that connexin (gap junction) genes form a family of tumor suppressor genes. Control mechanisms of expression as well as function of connexins are vulnerable to various carcinogenic insults, notably to nongenetoxic carcinogens. Thus, studies on the role of connexins in cell growth and carcinogenesis may prove to be useful for establishing a mechanism-based test to detect certain types of nongenotoxic carcinogens.

Gap-junction disassembly and connexin 43 dephosphorylation induced by 18 beta-glycyrrhetinic acid

Gap-junction channels connect the interiors of adjacent cells and can be arranged into aggregates or plaques consisting of hundreds to thousands of channel particles. The mechanism of channel aggregation into plaques and whether plaques can disaggregate are not known. Many carcinogenic and tumor-promoting chemicals have been identified that inhibit cell-cell gap-junctional coupling. Here, we provide morphological evidence that 18 beta-glycyrrhetinic acid (18 beta-GA), a saponin isolated from licorice root that is an inhibitor of gap-junctional communication, caused the disassembly of gap-junction plaques in WB-F344 rat liver epithelial cells. This effect was dose (5-40 microM) and time dependent (1-4 h treatment). Gap-junction channels in WB-F344 cells are comprised of connexin 43 (Cx43), and the protein is phosphorylated to a species known as Cx43-P2 coincident with the assembly of channels into plaques. Consistent with this, the disassembly of plaques induced by 18 beta-GA was correlated with decreases in Cx43-P2 levels and increases in nonphosphorylated Cx43. Biochemical evidence indicated that these changes in the P2 and NP forms of Cx43 represented 18 beta-GA-induced dephosphorylation of Cx43-P2 and not its degradation or the inhibition of Cx43-NP phosphorylation. Okadaic acid and calyculin A, which are inhibitors of type 1 and type 2A protein phosphatases, prevented the dephosphorylation of Cx43, suggesting that one or both of these phosphatases were involved in Cx43 dephosphorylation. These data indicate that 18 beta-GA causes type 1 or type 2A protein phosphatase-mediated Cx43 dephosphorylation coincident with the disassembly of gap-junction plaques.

Intramolecular interactions mediate pH regulation of connexin43 channels

We have previously proposed that acidification-induced regulation of the cardiac gap junction protein connexin43 (Cx43) may be modeled as a particle-receptor interaction between two separate domains of Cx43: the carboxyl terminal (acting as a particle), and a region including histidine 95 (acting as a receptor). Accordingly, intracellular acidification would lead to particle-receptor binding, thus closing the channel. A premise of the model is that the particle can bind its receptor, even if the particle is not covalently bound to the rest of the protein. The latter hypothesis was tested in antisense-injected Xenopus oocyte pairs coexpressing mRNA for a pH-insensitive Cx43 mutant truncated at amino acid 257 (i.e., M257) and mRNA coding for the carboxyl terminal region (residues 259-382). Intracellular pH (pHo) was recorded using the dextran form of the proton-sensitive dye seminaphthorhodafluor (SNARF). Junctional conductance (Gj) was measured with the dual voltage clamp technique. Wild-type Cx43 channels showed their characteristic pH sensitivity. M257 channels were not pH sensitive (pHo tested: 7.2 to 6.4). However, pH sensitivity was restored when the pH-insensitive channel (M257) was coexpressed with mRNA coding for the carboxyl terminal. Furthermore, coexpression of the carboxyl terminal of Cx43 enhanced the pH sensitivity of an otherwise less pH-sensitive connexin (Cx32). These data are consistent with a model of intramolecular interactions in which the carboxyl terminal acts as an independent domain that, under the appropriate conditions, binds to a separate region of the protein and closes the channel. These interactions may be direct (as in the ball-and-chain mechanism of voltage-dependent gating of potassium channels) or mediated through an intermediary molecule. The data further suggest that the region of Cx43 that acts as a receptor for the particle is conserved among connexins. A similar molecular mechanism may mediate chemical regulation of other channel proteins.

Inhibition of gap junctional intercellular communication and enhancement of growth in BALB/c 3T3 cells treated with connexin43 antisense oligonucleotides

Many studies have correlated reductions in gap junctional intercellular communication (GJIC) with altered cellular growth, tumor promotion, and neoplastic transformation. To test directly whether reduced GJIC affects cellular growth, GJIC was inhibited in murine BALB/c 3T3 fibroblasts by treatment with a phosphorothioate-modified antisense oligonucleotide targeted against the connexin43 translation start codon, and in vitro cell growth was monitored. The cells were incubated with the oligonucleotide (0.1-0.5 microM) in liposomes in serumless culture medium for 16 h; washed and refed with serum-containing medium; and analyzed for dye-coupling, connexin43 protein and mRNA levels, and cell growth over the next 5 d. The antisense oligonucleotide inhibited dye-coupling and reduced connexin43 protein levels in a concentration-dependent manner but had no effect on connexin43 mRNA levels. Cell growth rate was not affected, but saturation density was increased approximately threefold by the oligonucleotide. These data support a role for GJIC in the establishment of contact inhibition of in vitro cell growth.