Aberrant expression and function of gap junctions during carcinogenesis

Connexin Expression in Pituitary Adenomas and the Effects of Overexpression of Connexin 43 in Pituitary Tumor Cell Lines

Gap junction intercellular communication (GJIC) is considered a key mechanism in the regulation of tissue homeostasis. GJIC structures are organized in two transmembrane channels, with each channel formed by connexins (Cxs). GJIC and Cxs expression alterations are related to the process of tumorigenesis in different cell types. Pituitary neuroendocrine tumors (PitNETs) represent 15–20% of intracranial neoplasms, and usually display benign behavior. Nevertheless, some may have aggressive behavior, invading adjacent tissues, and featuring a high proliferation rate. We aimed to assess the expression and relevance of GJIC and Cxs proteins in PitNETs. We evaluated the mRNA expression levels of Cx26, 32, and 43, and the protein expression of Cx43 in a series of PitNETs. In addition, we overexpressed Cx43 in pituitary tumor cell lines. At the mRNA level, we observed variable expression of all the connexins in the tumor samples. Cx43 protein expression was absent in most of the pituitary tumor samples that were studied. Moreover, in vitro studies revealed that the overexpression of Cx43 decreases cell growth and induces apoptosis in pituitary tumor cell lines. Our results indicate that the downregulation of Cx43 protein might be involved in the tumorigenesis of most pituitary adenomas and have a potential therapeutic value for pituitary tumor therapy.

Connexin-46 Contained in Extracellular Vesicles Enhance Malignancy Features in Breast Cancer Cells

Under normal conditions, almost all cell types communicate with their neighboring cells through gap junction channels (GJC), facilitating cellular and tissue homeostasis. A GJC is formed by the interaction of two hemichannels; each one of these hemichannels in turn is formed by six subunits of transmembrane proteins called connexins (Cx). For many years, it was believed that the loss of GJC-mediated intercellular communication was a hallmark in cancer development. However, nowadays this paradigm is changing. The connexin 46 (Cx46), which is almost exclusively expressed in the eye lens, is upregulated in human breast cancer, and is correlated with tumor growth in a Xenograft mouse model. On the other hand, extracellular vesicles (EVs) have an important role in long-distance communication under physiological conditions. In the last decade, EVs also have been recognized as key players in cancer aggressiveness. The aim of this work was to explore the involvement of Cx46 in EV-mediated intercellular communication. Here, we demonstrated for the first time, that Cx46 is contained in EVs released from breast cancer cells overexpressing Cx46 (EVs-Cx46). This EV-Cx46 facilitates the interaction between EVs and the recipient cell resulting in an increase in their migration and invasion properties. Our results suggest that EV-Cx46 could be a marker of cancer malignancy and open the possibility to consider Cx46 as a new therapeutic target in cancer treatment.

Elasticity and tumorigenic characteristics of cells in a monolayer after nanosecond pulsed electric field exposure

Nanosecond pulsed electric fields (nsPEFs) applied to cells can induce different biological effects depending on pulse duration and field strength. One known process is the induction of apoptosis whereby nsPEFs are currently investigated as a novel cancer therapy. Another and probably related change is the breakdown of the cytoskeleton. We investigated the elasticity of rat liver epithelial cells WB-F344 in a monolayer using atomic force microscopy (AFM) with respect to the potential of cells to undergo malignant transformation or to develop a potential to metastasize. We found that the elastic modulus of the cells decreased significantly within the first 8 min after treatment with 20 pulses of 100 ns and with a field strength of 20 kV/cm but was still higher than the elasticity of their tumorigenic counterpart WB-ras. AFM measurements and immunofluorescent staining showed that the cellular actin cytoskeleton became reorganized within 5 min. However, both a colony formation assay and a cell migration assay revealed no significant changes after nsPEF treatment, implying that cells seem not to adopt malignant characteristics associated with metastasis formation despite the induced transient changes to elasticity and cytoskeleton that can be observed for up to 1 h.

Transient suppression of gap junctional intercellular communication after exposure to 100-nanosecond pulsed electric fields

Gap junctional intercellular communication (GJIC) is an important mechanism that is involved and affected in many diseases and injuries. So far, the effect of nanosecond pulsed electric fields (nsPEFs) on the communication between cells was not investigated. An in vitro approach is presented with rat liver epithelial WB-F344 cells grown and exposed in a monolayer. In order to observe sub-lethal effects, cells were exposed to pulsed electric fields with a duration of 100ns and amplitudes between 10 and 20kV/cm. GJIC strongly decreased within 15min after treatment but recovered within 24h. Gene expression of Cx43 was significantly decreased and associated with a reduced total amount of Cx43 protein. In addition, MAP kinases p38 and Erk1/2, involved in Cx43 phosphorylation, were activated and Cx43 became hyperphosphorylated. Immunofluorescent staining of Cx43 displayed the disassembly of gap junctions. Further, a reorganization of the actin cytoskeleton was observed whereas tight junction protein ZO-1 was not significantly affected. All effects were field- and time-dependent and most pronounced within 30 to 60min after treatment. A better understanding of a possible manipulation of GJIC by nsPEFs might eventually offer a possibility to develop and improve treatments.

SRC points the way to biomarkers and chemotherapeutic targets

The role of Src in tumorigenesis has been extensively studied since the work of Peyton Rous over a hundred years ago. Src is a non-receptor tyrosine kinase that plays key roles in signaling pathways controlling tumor cell growth and migration. Src regulates the activities of numerous molecules to induce cell transformation. However, transformed cells do not always migrate and realize their tumorigenic potential. They can be normalized by surrounding nontransformed cells by a process called contact normalization. Tumor cells need to override contact normalization to become malignant or metastatic. In this review, we discuss the role of Src in cell migration and contact normalization, with emphasis on Cas and Abl pathways. This paradigm illuminates several chemotherapeutic targets and may lead to the identification of new biomarkers and the development of effective anticancer treatments.

Advanced glycation end products down-regulate gap junctions in human hepatoma SKHep 1 cells via the activation of Src-dependent ERK1/2 and JNK/SAPK/AP1 signaling pathways

Hyperglycemia and advanced glycation end products (AGEs) are associated with an elevated risk of developing several cancers in diabetic patients. However, the detailed mechanisms remain to be elucidated. The mechanism of AGE-bovine serum albumin (BSA) on gap junction intercellular communication in human hepatoma cell line, SKHep 1, was investigated. Both Cx32 and Cx43 are major gap junction forming proteins in the liver, the loss of which has been shown to facilitate tumorigenesis. Although the MTT assay results showed that AGE-BSA significantly increased cell growth by 31%, AGE-BSA down-regulated Cx32 and Cx43 expression in a dose- and time-dependent manner. The present study also demonstrated that ERK1/2 and JNK/SAPK were significantly activated by AGE-BSA and that Src, ERK1/2, and JNK/SAPK inhibitors significantly reversed the reduction of Cx32 and Cx43 proteins by AGE-BSA. Taken together, these results strongly support the hypothesis that Src-dependent ERK1/2 and JNK/SAPK/AP1 signaling pathways play a key role in AGE-BSA-mediated down-regulation of Cx32 and Cx43 protein expression in SKHep 1 cells.

Analogues of Y27632 increase gap junction communication and suppress the formation of transformed NIH3T3 colonies

BACKGROUND

Constitutive activation of RhoA-dependent RhoA kinase (ROCK) signalling is known to promote cellular transformation and the ROCK inhibitor Y-27632 has the ability to suppress focus formation of RhoA transformed NIH3T3 cells.

METHODS

Sixty-four novel structural analogues of Y27632 were synthesised and tested for their ability to persistently inhibit the transformation of NIH3T3 cells by Rho guanidine exchange factor 16 (ARHGEF16) or Ras. In vitro kinase inhibitor profiling, co-culture of transformed cells with non-transformed cells and a novel Lucifer yellow/PKH67 dye transfer method were used to investigate their mode of action.

RESULTS

Four Y27632 analogues inhibited transformed focus formation that persisted when the compound was withdrawn. No toxicity was observed against either transformed or non-transformed cells and the effect was dependent on co-culture of these two cell types. In vitro kinase inhibitor profiling indicated that these compounds had reduced activity against ROCK compared with Y27632, targeting instead Aurora A (AURKA), p38 (MAPK14) and Hgk (MAP4K4). Dye transfer analysis showed they increased gap junction intercellular communication (GJIC) between transformed and non-transformed cells.

CONCLUSIONS

These data are the first to suggest that transient blockade of specific kinases can induce a persistent inhibition of non-contact inhibited transformed colony formation and can also remove pre-formed colonies. These effects could potentially be mediated by the observed increase in GJIC between transformed and non-transformed cells. Selection of kinase inhibitors with this property may thus provide a novel strategy for cancer chemoprevention.

Expression and distribution of connexin 32 in rat liver with experimentally induced fibrosis

The connexin 32 (Cx32) is a protein that forms the channels that promote the gap junction intercellular communication (GJIC) in the liver, allowing the diffusion of small molecules through cytosol from cell-to-cell. Hepatic fibrosis is characterized by a disruption of normal tissue architeture by cellular lesions, and may alter the GJIC. This work aimed to study the expression and distribution of Cx32 in liver fibrosis induced by the oral administration of dimethylnitrosamine in female Wistar rats. The necropsy of the rats was carried out after five weeks of drug administration. They presented a hepatic fibrosis state. Sections from livers with fibrosis and from control livers were submitted to immunohistochemical, Real Time-PCR and Western-Blot analysis to Cx32. In fibrotic livers the Cxs were diffusely scattered in the cytoplasm, contrasting with the control livers, where the Cx32 formed junction plaques at the cell membrane. Also it was found a decrease in the gene expression of Cx32 without reduction in the protein quantity when compared with controls. These results suggest that there the mechanism of intercellular communication between hepatocytes was reduced by the fibrotic process, which may predispose to the occurrence of a neoplastic process, taken in account that connexins are considered tumor suppressing genes.

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.

Cellular sublocalization of Cx43 and the establishment of functional coupling in IMR-32 neuroblastoma cells

Neuroblastoma (NB) is the most common solid pediatric tumor. IMR-32 cells are a highly malignant human NB cell line with uncontrolled proliferation but with the potential to be differentiated under specific conditions. Preliminary research indicated that connexin 43 (Cx43), the most widespread of the Cx family, is aberrantly located in IMR-32 cells, which renders these cells incapable of gap junction (GJ) intercellular communication. Functioning GJ intercellular communication has been strongly associated with growth control and a decrease in tumorigenicity. 8-br-cAMP, known to initiate the differentiation process in cancer cells, was used to examine changes in Cx43 localization and expression via immunocytochemistry, Western blot analysis, and flow cytometry. Exposure of IMR-32 cells to 8-br-cAMP decreased cell proliferation, restored the abnormally localized Cx43 from around the nucleus to the cell membrane, increased de novo Cx43 protein expression, and appeared to phosphorylate Cx43 on serine (Ser) 255 and Ser262. Forskolin, an activator of cAMP dependent protein kinase (PKA), produced identical results to 8-br-cAMP demonstrating the effect that was not unique to a cAMP analog. The use of a PKA inhibitor further confirmed the specificity of 8-br-cAMP and forskolin's effect on Cx43. The cellular relocation of Cx43, combined with the increased protein expression, established first ever GJ intercellular communication between IMR-32 cells as revealed by scrape loading. These results suggest that the GJ-mediated return of growth control, as a prerequisite for further differentiation, offers a new therapeutic avenue in the treatment of NB.

Connexin43 interacts with NOV: a possible mechanism for negative regulation of cell growth in choriocarcinoma cells

The gap junction protein connexin43 (Cx43) is thought to be involved in growth control in several tissues. Using the doxycycline inducible tet-on system, we generated human malignant trophoblast Jeg3 cells transfected with either Cx40, Cx43, or C-terminal truncated Cx43 (trCx43). Cx43, but not Cx40 or trCx43, displayed a reduced cell growth of Jeg3 cells in vitro and tumor growth in nude mice, suggesting a role of the C terminus of Cx43 in growth regulation. Using gene array analysis, the growth regulator NOV (CCN3), a member of the CCN gene family, was found to be up-regulated only in the Cx43-transfected cells. Validation by reverse transcriptase-PCR confirmed an up-regulation of the NOV transcript exclusively upon Cx43 induction. In contrast to Cx40 or trCx43, induction of Cx43 led to a switch in localization of NOV from the nucleus to the cell membrane, where it is colocalized with Cx43. Coimmunoprecipitation showed a binding of NOV to the C terminus of Cx43 in vitro as well as in transfected cells. Jeg3 cells transfected only with NOV revealed that NOV itself acts as a growth regulator. We suggest that Cx43 is able to regulate cell growth via an up-regulation of NOV transcription, a change in localization of the NOV protein and a binding of NOV to the C terminus of Cx43.

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.

Retroviral delivery of connexin genes to human breast tumor cells inhibits in vivo tumor growth by a mechanism that is independent of significant gap junctional intercellular communication

The mechanism by which gap junction proteins, connexins, act as potent tumor suppressors remains poorly understood. In this study human breast tumor cells were found to exhibit diverse gap junction phenotypes including (a) undetectable Cx43 and no intercellular communication (HBL100); (b) low levels of Cx43 and sparse intercellular communication (MDA-MB-231); and (c) significant levels of Cx43 and moderate intercellular communication (Hs578T). Although retroviral delivery of Cx43 and Cx26 cDNAs to MDA-MB-231 cells did not achieve an expected substantial rescue of intercellular communication, overexpression of connexin genes did result in a dramatic suppression of tumor growth when connexin-expressing MDA-MB-231 cells were implanted into the mammary fat pad of nude mice. Subsequent immunolocalization studies on xenograph sections revealed only cytoplasmic stores of Cx43 and no detectable gap junctions. Moreover, DNA array and Western blot analysis demonstrated that overexpression of Cx43 or Cx26 in MDA-MB-231 cells down-regulated fibroblast growth factor receptor-3. Surprisingly, these results suggest that Cx43 and Cx26 induce their tumor-suppressing properties by a mechanism that is independent of significant gap junctional intercellular communication and possibly through the down-regulation of key genes involved in tumor growth. Moreover, our studies show that retroviruses are effective vehicles for delivering connexins to human breast tumor cells, facilitating potential gene therapy applications.

Gap junctions, homeostasis, and injury

Gap junctions (Gj) play an important role in the communication between cells of many tissues. They are composed of channels that permit the passage of ions and low molecular weight metabolites between adjacent cells, without exposure to the extracellular environment. These pathways are formed by the interaction between two hemichannels on the surface of opposing cells. These hemichannels are formed by the association of six identical subunits, named connexins (Cx), which are integral membrane proteins. Cell coupling via Gj is dependent on the specific pattern of Cx gene expression. This pattern of gene expression is altered during several pathological conditions resulting in changes of cell coupling. The regulation of Cx gene expression is affected at different levels from transcription to post translational processes during injury. In addition, Gj cellular communication is regulated by gating mechanisms. The alteration of Gj communication during injury could be rationalized by two opposite theories. One hypothesis proposes that the alteration of Gj communication attenuates the spread of toxic metabolites from the injured area to healthy organ regions. The alternative proposition is that a reduction of cellular communication reduces the loss of important cellular metabolisms, such as ATP and glucose.

A cellular automaton model of early tumor growth and invasion

A hybrid cellular automaton model is described and used to simulate early tumor growth and examine the roles of host tissue vascular density and tumor metabolism in the ability of a small number of monoclonal transformed cells to develop into an invasive tumor. The model incorporates normal cells, tumor cells, necrotic or empty space, and a random network of native microvessels as components of a cellular automaton state vector. Diffusion of glucose and H(+)ions (the latter largely resulting from the tumor's excessive reliance on anaerobic metabolism) to and from the microvessels, and their utilization or production by cells, is modeled through the solution of differential equations. In this way, the cells and microvessels affect the extracellular concentrations of glucose and H(+)which, in turn, affect the evolution of the automaton. Simulations of the model demonstrate that: (i) high tumor H(+)ion production is favorable for tumor growth and invasion; however for every H(+)ion production rate, there exists a range of optimal microvessel densities (leading to a local pH favorable to tumor but not to normal cells) for which growth and invasion is most effective, (ii) at vascular densities below this range, both tumor and normal cells die due to excessively low pH, (iii) for vascular densities above the optimal range the microvessel network is highly efficient at removing acid and therefore the tumor cells lose their advantage over normal cells gained by high local H(+)concentration. While significant spatial gradients of glucose formed, no regions of detrimentally poor glucose perfusion (for either cell type) were observed, regardless of microvessel density. Depending on metabolic phenotype, a variety of tumor morphologies similar to those clinically observed were realized in the simulations. Lastly, a sharp transition (analogous to that of the adenoma-carcinoma sequence) between states of initial tumor confinement and efficient invasiveness was observed when H(+)production reached a critical value.

Control of intracellular movement of connexins by E-cadherin in murine skin papilloma cells

The gap junctional intercellular communication-deficient mouse skin papilloma cell line P3/22 expresses Cx43 but not E-cadherin. The E-cadherin gene-transfected cells (P3E1) communicate in a calcium-dependent manner and they were used to study how E-cadherin restores the function of connexins. At low calcium, Cx43 molecules remain in the cytoplasm of P3E1 cells and appear at cell-cell contact areas only in high-calcium medium. While Cx43 is unphosphorylated in P3E1 cells in low-calcium medium, two phosphorylated bands appeared at high calcium. However, when Cx26, which has no C-terminal tail that can undergo phosphorylation, was expressed in P3E1 cells, this connexin also moved to the plasma membrane after the calcium shift and partly colocalized with Cx43, suggesting that C-terminal phosphorylation is not essential for E-cadherin-mediated intracellular transport of connexins. In low calcium, both Cx26 and Cx43 remained and colocalized in the endoplasmic reticulum. As early as 30 min after the shift to high-calcium medium, both Cx43 and Cx26 began to accumulate in the Golgi apparatus. Intracellular movement of connexins to the cytoplasmic membrane at high calcium was effectively blocked by cytochalasin D and brefeldin A. These results suggest that E-cadherin junction formation at high calcium leads to formation of actin cables, which directly or indirectly transport connexins from the cytoplasm to the cell-cell contact membranes via the Golgi apparatus.

A useful marker for evaluating tissue-engineered products: gap-junctional communication for assessment of the tumor-promoting action and disruption of cell differentiation in tissue-engineered products

An in vitro system for evaluating the safety of tissue-engineered products is a convenient because of its rapidity and low cost. On the basis of recent studies, intercellular channels called gap-junctions are considered to play an important role on the tumor-promotion stage during the tumorigenesis induced by polyurethanes. Further, we also demonstrate the significance of the intercellular communication during neuronal cell differentiation. From these results, we propose a survey of the function of the gap-junctional communication as a probable useful marker for evaluating the safety of tissue-engineered products.

Expression of the gap junction connexins Cx43, Cx45 and Cx26 in human uterine leiomyomata

Uterine leiomyomata of 34 premenopausal women undergoing leiomyomectomy or hysterectomy, and in four cases the corresponding myometrium, were collected at laparotomy or laparoscopy to investigate the ability of these benign smooth muscle cell tumors to express different connexins. Immunohistochemical and Northern blot analyses were performed for the characterization of the expression of connexins Cx43, Cx45, Cx26 and Cx32. Immunofluorescence revealed the presence of Cx43 in most leiomyomata. Only seven leiomyomata lacked Cx43 expression. Cx45 was expressed in 13, a weak Cx26 immunostaining was found in seven cases, whereas Cx32 could not be detected. No correlations between the 17 beta-estradiol or progesterone serum levels and the expression patterns of the connexins Cx43, Cx45 and Cx26 could be observed. Gonadotropin-releasing hormone (GnRH)-agonist or progestin treatment did not influence the connexin expression pattern. Northern blot analyses confirmed these results; however, transcripts of Cx26 were not detectable. Connexin transcripts between myomata and the corresponding myometrium showed no obvious differences. Our data show that uterine leiomyomata are capable of expressing different connexins comparable to the corresponding myometrium, but do not respond to different hormonal conditions. The ability to express the appropriate connexins could explain why these tumors, though developing independently of hormonal levels, are still differentiated benign smooth muscle tumors.

Cadmium-induced alterations of connexin expression in the promotion stage of in vitro two-stage transformation

During the multistage carcinogenesis, functions of several key genes involved in the cell cycle control and cell-cell communication can be damaged. Gap junction intercellular communication (GJIC) is known to transfer small, water-soluble molecules through intercellular channels composed of proteins called connexins (Cxs). Therefore, aberrant expression of Cx may be one of the critical factors for the clonal expansion of initiated cells during the two-stage transformation. We already improved the classical in vitro two-stage transformation method using N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) as an initiator and cadmium as a promoter on Balb/3T3 A31 cells, and reconfirmed the promotional effect of cadmium with this method (Fang, M.Z., Cho, M.H., Lee, H.W., 2001. Improvement of in vitro two-stage transformation assay and detection of the promotional effect of cadmium, Toxicol. In Vitro (in press). In this study, precise roles of Cd on Cx expression in normal Balb/3T3 A31 and during the promotion stage of the in vitro two-stage transformation were elucidated. For this purpose, the Cx43, Cx32 and Cx26 protein levels, Cx43 and Cx26 mRNA levels and the cellular distribution location of Cx43 protein were determined. Normal Balb/3T3 cells expressed Cx43 and Cx32, but not Cx26. After a short-term treatment of cadmium on normal cells, phosphorylation of Cx43 protein increased and Cx32 protein level decreased. However, during the promotion stage of the in vitro two-stage transformation, transformed cells treated with cadmium for long periods expressed Cx43 and Cx32 highly, similar to the level of normal Balb/3T3 cells, compared to the nontransformed cells. Moreover, Cx43 of the transformed cells was distributed mostly in the perinuclear region rather than the intercellular membrane. These data suggest that cadmium may inhibit the GJIC by increasing the phosphorylation of Cx43 and decreasing the expression of Cx32 in the normal Balb/3T3 A31 cells. Our results also suggest that these changes are not associated with the cell transformation; transformed cells may reexpress Cx43 and Cx32 similar to the normal cells, though Cx43 protein is distributed aberrantly during the transformation process. Further studies are needed to clarify the relationship between transformation and posttranslational modification of the Cx proteins.

Reduced expression of connexin-43 and functional gap junction coupling in human gliomas

Gap junctions are an important means for intercellular communication during development, processes of tissue differentiation, and in maintenance of adult tissue homeostasis. We investigated the expression levels and distribution of connexin-43 (Cx-43), the most abundant astrocytic gap junction protein, in acutely isolated astrocytes and glioma cells from biopsy tissue obtained from patients diagnosed with glioblastoma multiforme (GBM), low-grade astrocytomas (LGAs), or mesial temporal lobe epilepsy. Western blot and immunohistochemical analyses indicated an inverse correlation between the amount of Cx-43 protein and tumor malignancy grade, as assessed by calculating tissue mitotic indexes (MI) obtained using anti-Ki-67 nuclear antigen staining. Samples from epilepsy patients had a low MI and were intensely positive for Cx-43 staining, while LGA tissue samples exhibited moderate staining for Cx-43 and average MI, and GBM biopsies showed significantly lower levels of Cx-43 and high MI. Functional coupling was assayed using fluorescence recovery after photobleach (FRAP). We found that cells from glioma cell lines and primary cultures of human astrocytes from GBM tissues displayed significantly lower degrees of gap junction intercellular communication (GJIC) as indicated by longer and less complete recovery from photobleaching. Mean recovery values were GBM 23.8% +/- 11.4%, LGA 49.4% +/- 47%, and nontumor astrocytes 67.2% +/- 8.4%. Western blot analysis of several human glioma cell lines and tissue biopsies showed variable expression levels of Cx-43, which correlated negatively with the extent of recovery in the same samples. Taken together, our findings suggest that high-grade brain tumors show reduced intercellular communication and a decrease in connexin-43 protein levels.

Epigenetic mechanisms of chemical carcinogenesis

Chemically induced cancer is a multi-step process involving damage to the genome initially followed by clonal expansion of the DNA damaged cell eventually leading to a neoplasm. Chemical carcinogens have been shown to impact at all of the stages of the tumorigenesis process. It has become apparent that chemical and physical agents that induce cancer may do so through several different cellular and molecular mechanisms. Epigenetic (nongenotoxic) chemical carcinogens are those agents that function to induce tumor formation by mechanisms exclusive of direct modification or damage to DNA. These agents appear to modulate cell growth and cell death and exhibit dose response relationships between exposure and tumor formation. The exact and/or exclusive mechanisms by which these agents function have not been established, however, changes in cell growth regulation and gene expression are important to tumor formation. This review focuses on several potential mechanisms and cellular processes that may be involved in nongenotoxic chemical carcinogenesis.

Carcinogenesis and the plasma membrane

Presented is a two-stage hypothesis of carcinogenesis based on: (1) plasma membrane defects that produce abnormal electron and proton efflux; and (2) electrical uncoupling of cells through loss of intercellular communication. These changes can be induced by a wide variety of stimuli including chemical carcinogens, oncoviruses, inherited and/or acquired genetic defects, and epigenetic abnormalities. The resulting loss of electron/proton homeostasis leads to decreased transmembrane potential, electrical microenvironment alterations, decreased extracellular pH, and increased intracellular pH. This produces a positive feedback loop to enhance and sustain the proton/electron efflux and loss of intercellular communication. Low transmembrane potential is functionally related to rapid cell cycling, changes in membrane structure, and malignancy. Intracellular alkalinization affects a variety of pH-sensitive systems including glycolysis, DNA synthesis, DNA transcription and DNA repair, and promotes genetic instability, accounting for the accumulation of genetic defects seen in malignancy. The abnormal microenvironment results in the selective survival and proliferation of malignant cells at the expense of contiguous normal cell populations.

Altered gap and tight junctions in human thyroid oncocytic tumors: a study of 8 cases by freeze-fracture

Human oncocytic tumors of the thyroid gland may be either adenomas or carcinomas. The morphology and the ultrastructure of these oncocytes are well-known. Numerous studies have demonstrated the role of gap and tight junctions in experimental and human carcinogenesis; however, the junctional complexes of the oncocytic tumors have never been studied. The aim of this study is to analyze gap and tight junctions in the oncocytic tumors of the thyroid. Because they are morphologically similar, whether benign or malignant, they offer an attractive model for studying the junctional complexes in both benign and malignant lesions. Eight oncocytic human thyroid tumors were collected and studied by freeze-fracture. Four of these cases were benign and four were malignant. Four cases of normal gland were also studied to represent the control group. Normal tight and gap junctions were described for the control group. No gap junctions could be found for the oncocytic tumors. Furthermore, alterations of the tight junctions were described; especially focal tights in the oncocytic adenomas and well organized and labyrinthic tight junctions in the oncocytic carcinomas. The lack of gap junction in the benign as well as in the malignant oncocytomas may suggest that the absence of gap junction is not sufficient for malignancy. The alterations of the tight junctions found in the oncocytic tumors of the thyroid are similar to those observed in poorly differentiated tissues or tumors, and may suggest a cellular regression rather than a tumorogenic factor.

SV40 Tag transformation of the normal invasive trophoblast results in a premalignant phenotype. II. Changes in gap junctional intercellular communication

Poor gap junctional intercellular communication (GJIC) has been associated with uncontrolled cell growth and neoplasia. We have successfully propagated normal first trimester invasive extravillous trophoblast (EVT) cells, and have produced premalignant EVT lines after SV40 Tag transformation: RSVT-2 is an uncloned line that is long-lived; RSVT2/C is a clonal line that is immortal. Both are hyperproliferative, hyperinvasive and variably refractory to the anti-proliferative and anti-invasive effects of transforming growth factor beta (TGFbeta). Possible changes in gap junctions during the transition of normal invasive EVT cells to the premalignant stage were examined by comparing expression of connexin proteins (by immunolabeling for Cx26, Cx32, Cx40, Cx43), and mRNA (by Northern blot with cDNA probes for Cx26, Cx32, Cx43), and functional GJIC (by dye transfer using the preloading method) in normal parental EVT cells and their SV40 Tag transformants. Results from immunofluorescence and Northern blot analysis revealed that, of the panel of connexins examined, only Cx43 was variably expressed in these cell lines in vitro. Expression of Cx43 protein and mRNA was abundant in normal EVT cell line HTR8, reduced in long-lived RSVT-2 cells and undetectable in immortalized RSVT2/C cells. GJIC, as measured by dye transfer between donor and recipient cells, was also similarly reduced in recipient RSVT-2 cells, and drastically reduced in RSVT2/C cells, irrespective of whether the dye donor was of the same cell type (homocellular coupling) or HTR8 cells (heterocellular coupling). Treatment with TGFbeta reduced Cx43 mRNA expression as well as GJIC in normal EVT cells, but not in the SV40 Tag transformants. Our findings suggest that downregulation of connexins with the resultant impairment in GJIC is an early event in tumor progression, as observed in the premalignant SV40 Tag transformants.

Expression of connexin32 and connexin43 gap junction proteins and E-cadherin in human lung cancer

We used immunohistochemical staining to examine the expression of the gap junction proteins connexin32 and connexin43 and of the intercellular adhesion molecule, E-cadherin, that is thought to be a prerequisite for gap junctional intercellular communication (GJIC), in 24 specimens of human lung cancer. Connexin32 was not found in cancer tissue and there were significantly fewer spots of connexin43 in the poorly differentiated versus the well differentiated (P = 0.0005) and moderately differentiated (P = 0.0002) adenocarcinomas and in the poorly differentiated versus the well differentiated (P = 0.0182) and moderately differentiated (P = 0.004) squamous cell carcinomas of the lung. E-Cadherin was expressed in all but three cases of poorly differentiated non-small cell lung cancer that showed a heterogeneously decreased expression of E-cadherin. These findings suggest that GJIC is decreased in poorly differentiated non-small cell lung cancer.

Differential decrease in connexin 32 expression in ischemic and nonischemic regions of rat liver during ischemia/reperfusion

The effect of a localized hepatic injury, regional ischemia/reperfusion, on the expression of connexin 32 (Cx32) was studied. Cx32 is the component of the major hepatic gap junction. Two regions of the injured liver were analyzed: the area directly affected by the ischemic insult (ischemic liver), and the remainder of the organ (nonischemic liver). In the ischemic liver, there were simultaneous reductions in Cx32 mRNA steady-state levels and the encoding polypeptide from the plasma membrane within 1 h of reperfusion. In contrast, Cx32 mRNA steady-state levels were only reduced after 4 h of reperfusion in the nonischemic liver. This reduction of Cx32 mRNA levels was followed by the disappearance of Cx32 on the plasma membrane within 24 h of the insult. Administration of actinomycin D prior to the ischemic insult prevented the reduction in Cx32 mRNA in both ischemic and nonischemic liver regions. Protein synthesis was blocked during the first hour of reperfusion in the ischemic liver but not in the nonischemic liver. To mimic this effect, animals were treated with cycloheximide in absence of the ischemic insult. A reduction in Cx32 mRNA and polypeptide in the liver was observed in cycloheximide treated animals. This finding suggests that the decrease in Cx32 expression in the ischemic, but not in the nonischemic, liver may be due to the inhibition of protein synthesis during ischemia/reperfusion. These observations suggest that an ischemic insult produces a selective deteriorating effect on Cx32 expression in both ischemic and nonischemic liver regions probably through different mechanisms.

Enhancement of drug sensitivity and a bystander effect in PC-9 cells transfected with a platelet-derived endothelial cell growth factor thymidine phosphorylase cDNA

5'-Deoxy-5-fluorouridine (5'-DFUR) and 1-(tetrahydro-2-furyl)-5-fluorouracil (tegafur), prodrugs of 5-fluorouracil (5-FU), are anticancer agents activated by thymidine phosphorylase (dThdPase). As it is well known that the levels of dThdPase are higher in tumours than in normal tissue, it should be advantageous to use such pyrimidine antimetabolites for the selective inhibition of tumour growth. However, tumours are not necessarily sensitive to 5'-DFUR and tegafur because their levels of dThdPase vary. In this study, we examined whether transfection of tumour cells with a human platelet-derived endothelial cell growth factor (PD-ECGF) complementary DNA (cDNA) expressing dThdPase would sensitize the cells to the cytotoxic effects of pyrimidine antimetabolites in vitro. A cDNA encoding PD-ECGF was transfected into PC-9 cells (human lung adenocarcinoma). The transfected cells, PC9-DPE2, had a more than 50 times higher activity of dThdPase than the parental PC-9 cells or control PC-9 cells transfected with the pcDNA3 vector alone (PC9-D1). They were more sensitive than parental PC-9 or PC9-D1 cells not only to 5'-DFUR and tegafur but also to 5-FU. In addition, we demonstrated that PC9-DPE2 cells are able to potentiate the cytotoxic effects of 5'-DFUR towards co-cultured parental PC-9 cells. This "bystander effect' did not require cell-cell contact. These results suggest that transfection of PD-ECGF (dThdPase) genes may be useful as a gene therapy strategy for cancer treatment.

Pulse treatment with the tumor promoter TPA delays the onset of desensitization response and prolongs the inhibitory effect on gap junctional intercellular communication of a rat liver epithelial cell line WB F-344

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) is an inhibitor of gap junctional intercellular communication (GJIC) of the rat liver epithelial cell line, WB F-344. We have previously reported that prolonged treatment of the WB cells with TPA (10 ng/ml) caused a reversal of the inhibition of GJIC that was initially induced (Oh, S.Y., et al. (1988) Carcinogenesis, 9, 135-139). Under this condition, addition of fresh TPA did not inhibit GJIC of these cells. In the present investigation we examined whether pulse exposure to TPA delays the onset of this desensitization response. Cultures were treated for 5 or 15 min with TPA and shifted to normal medium. Intercellular communication was measured at 15 min, 1 h and 6 h after the 5 or 15 min pulse treatments. Under these pulse treatment conditions, GJIC of the cells was markedly inhibited for up to 4 h and gradually reverted to near control levels by 6-8 h. At every sixth hour of pulse treatment the cells were given an additional pulse treatment (5 or 15 min) and the inhibitory effect of TPA on the GJIC of the cells was assayed 15 min after each such treatment. The results clearly showed that, when the cells were treated with 10 ng/ml TPA for 5 or 15 min every 6 h they maintained their sensitivity to the inhibitory effect of TPA on GJIC. This response to TPA was sustained for a considerably longer time when the duration of the pulse treatment was 5 min. Our data suggested that pulse exposure to TPA delays the desensitization response normally observed in prolonged treatment regimens and that this delay is possibly due to maintenance of the TPA activatable pool of protein kinase C under these conditions.

Properties of connexin40 gap junction channels endogenously expressed and exogenously overexpressed in human choriocarcinoma cell lines

We have investigated the properties of gap junction channels of three human malignant trophoblast (choriocarcinoma) cell lines: BeWo, Jeg-3 and JAr, as well as in Jeg-3 cells stably transfected with rat connexin40 (Cx40). Reverse-transcriptase polymerase chain reaction (RT-PCR), Northern blot analysis and immunostaining demonstrated expression of Cx40 in BeWo and JAr cell lines. JAr cells also expressed minor amounts of Cx43. Very low levels of Cx40 transcripts were revealed by RT-PCR in parental Jeg-3 cells, but Cx40 protein was not detected. To compare properties of endogenously and exogenously expressed Cx40 channels we have transfected Jeg-3 cells with rat Cx40. Recordings with dual whole-cell methods were used to determine the junctional conductance (gj) in the various cell lines and transfectants. Cx40 channels exogenously expressed in Jeg-3 cells demonstrated steep voltage sensitivity in the transjunctional voltage range of +/-30 to +/-40 mV and a unitary mainstate conductance of 175 pS, values which are similar to the data obtained from endogenously expressed Cx40 in BeWo cell pairs. In addition, greater driving forces resulted in a lower unitary conductance of about 30 pS, exclusively in BeWo cells. Between JAr cell pairs we determined a gj of 10 nS and unitary conductances were predominantly 100 and 152 pS. Voltage dependence was less sensitive in JAr cells compared to Cx40 transfectants and BeWo cells. Thus, coexpression of Cx43 and Cx40 leads to a macroscopic conductance with a mixture of properties expected for each connexin, whereas single-channel properties of each connexin type are maintained.

Ras is involved in gap junction closure in proliferating fibroblasts or preadipocytes but not in differentiated adipocytes

A decrease in gap junctional, intercellular communication (GJIC) has been associated with cells neoplastically transformed by a variety of factors. To investigate the role of the Ras oncogene product in gap junction function, a panel of murine C3H10T1/2 (10T1/2) fibroblasts was constructed in which the levels of ras gene expression could be effectively up- or down-regulated. Intercellular communication was measured using a novel technique of in situ electroporation of adherent cells on a partly conductive slide. The introduction of increasing amounts of activated Ras(leu61) in mouse 10T1/2 fibroblasts proportionally reduced GJIC, while the downregulation of endogenous c-ras gene expression increased junctional permeability. These results indicate that Ras plays an important role in the junction closure pathway leading to the proliferation of normal cells. However, differentiation of c-Ras-deficient preadipocytes entirely abolished their initially extensive GJIC, indicating that junction closure in response to adipocytic differentiation is independent of Ras.

Cellular Ras partly mediates gap junction closure by the polyoma virus middle tumor antigen

Endogenous, cellular Ras proteins (c-Ras) mediate the transforming action of the polyoma virus middle Tumor antigen (mT), which is accompanied by elimination of gap junctional, intercellular communication (GJIC). In this report we show that reducing the c-Ras content of murine C3H10T1/2 fibroblasts (10T1/2) through the expression of an anti-sense ras gene, increased GJIC by 60-80% mT totally eliminated GJIC in normal 10T1/2 cells but it reduced GJIC no more than 50% in the c-Ras deficient lines. These results indicate that endogenous c-Ras is at least partly responsible for the mT-induced gap junction closure.

Clofibrate-induced neoplastic development in the rat liver is associated with decreased connexin 32 expression but not with a co-ordinated shift in expression of marker enzymes

Altered enzyme phenotype and expression of connexin 32 (Cx32), a gap junction protein were studied during the development of rat liver tumors induced by the non-genotoxic carcinogen, clofibrate. (1) In contrast to previous findings for nitrosamine-induced lesions, preneoplastic enzyme-altered foci (EAF) and neoplastic nodules (NN) lacked any clear association with degree of altered enzyme expression because of an almost complete negativity for GST-P and GGT. (2) Immunohistochemically demonstrated Cx32 spots on the hepatocyte membranes showed a clear decrease in clofibrate-induced lesions. (3) Naturally occurring EAF demonstrating GST-P and/or GGT positivity did not show a significant decrease of Cx32 counts suggesting a reversible nature. Therefore, the Cx32 decrease appears closely linked to progression of hepatocarcinogenesis irrespective of the enzyme phenotype of neoplastic focal lesions and the carcinogens used for their induction.

A connexin 43 antisense vector reduces the ability of normal cells to inhibit the foci formation of transformed cells

Antisense gene constructs have been very useful in the functional analysis of genes and their products. In this report we used a connexin 43 (Cx43) antisense gene construct to study the role that heterologous gap-junctional intracellular communication (GJIC) plays in the ability of untransformed fibroblasts to suppress the foci-forming ability of src oncogene-transformed cells. Untransformed Rat-1 fibroblasts transfected with the Cx43 antisense DNA construct showed marked decreases in Cx43 RNA and protein, which were accompanied by a corresponding decrease in GJIC. These Cx43 antisense-transfected cells maintained normal cell morphology, growth rates, and saturation densities and did not grow in soft-agar suspension. However, in coculture experiments, the Cx43 antisense cells were less effective than vector-alone-transfected, sense-transfected, and untransfected cells at inhibiting foci formation of pp60v-src-transformed cells. These effects of junctionally competent, normal cells were associated with the existence of heterologous GJIC with the transformed cells and did not appear to result from the elaboration of a stable, diffusible inhibitory factor. Thus, gap-junction-mediated transfer of putative regulatory molecules may play a role in the ability of untransformed cells to suppress the expression of certain properties of transformed cells.

A novel technique for the study of intercellular, junctional communication: electroporation of adherent cells on a partly conductive slide

One of the effects of neoplastic transformation by a variety of factors is a decrease in gap junctional, intercellular communication (GJIC). The investigation of junctional permeability is usually conducted through the microinjection of the fluorescent dye, Lucifer yellow, followed by observation of its migration into neighboring cells. This is a time-consuming approach, requiring expensive equipment. To overcome these problems, a novel technique was devised which takes advantage of the ability of short electric pulses to create transient "pores" on the cell membrane through which Lucifer yellow can enter, simultaneously and into large numbers of cells, with minimal disturbance to cellular metabolism. Cells were grown on a glass slide, half of which was coated with electrically conductive, optically transparent, indium-tin oxide. An electric pulse was applied in the presence of Lucifer yellow, causing its penetration into the cells growing on the conductive half of the slide, and the migration of the dye to the nonelectroporated cells growing on the nonconductive area was microscopically observed under fluorescence illumination. Using this technique, we investigated the relationship between expression of the middle tumor antigen of polyoma virus (mT) and GJIC in two representative cell systems with different responses to mT. The results show that low mT expression levels, although unable to transform rat F111 cells fully, are able to interrupt GJIC. Although parts of this mechanism might be mediated through protein kinase C (PKC), mT appears to have additional functions. PKC, however, had the opposite effect upon junctional permeability in a clone of mouse NIH-3T3 fibroblasts; intercellular communication in these cells appears to require PKC activity.

Multiple mechanisms are responsible for altered expression of gap junction genes during oncogenesis in rat liver

Although several abnormalities in gap junction (GJ) structure and/or function have been described in neoplasms, the molecular mechanisms responsible for many of the alterations remain unknown. The identification of a family of GJ proteins, termed connexins, prompted this study of connexin32 (Cx32), connexin26 (Cx26) and connexin43 (Cx43) expression during rat hepatocarcinogenesis. Using antibody, cDNA and cRNA probes, we investigated connexin mRNA and protein expression in preneoplastic and neoplastic rat livers. In normal liver, Cx32 is expressed in hepatocytes throughout the hepatic acinus, Cx26 is restricted to periportal hepatocytes, and Cx43 is expressed by mesothelial cells forming Glisson's capsule. Most preneoplastic altered hepatic foci generated by diethylnitrosamine (DEN) initiation and either phenobarbital (PB) or 2,3,7,8-dichlorodibenzo-p-dioxin (TCDD) promotion exhibited decreased Cx32 or increased Cx26 staining. Foci from either protocol failed to display Cx43 immunoreactivity. In the majority of PB-promoted foci, Cx32 immunoreactivity decreased independently of changes in mRNA abundance. Continuous thymidine labeling, following cessation of PB promotion, showed that downregulation of Cx32 staining is reversible in foci that are promoter-dependent for growth, but irreversible in lesions that are promoter-independent for growth. Hepatic neoplasms from rats initiated with DEN and promoted with PB or TCDD also displayed modified connexin expression. While all 24 neoplasms studied were deficient in normal punctate Cx32 and Cx26 staining, altered cellular localization of these proteins was apparent in some tumors. Immunoblotting of crude tissue extracts revealed that neoplasms with disordered Cx32 staining showed immunoreactive bands with altered electrophoretic mobility. These observations show that hepatomas may downregulate Cx32 expression through changes in the primary structure of Cx32 or by post-translational modifications. Northern blotting of total tumor mRNAs failed to demonstrate consistent changes in the abundance of Cx32, Cx26 or Cx43 transcripts. Some tumors expressed steady-state transcripts without observable immunoreactivity, indicating that some hepatomas downregulate connexin immunoreactivity independently of mRNA abundance. Increased levels of Cx43 mRNA and protein were found in several neoplasms, but immunostaining was always localized to nonparenchymal cells. Areas of bile duct proliferation and cholangiomas displayed Cx43 staining, whereas, cholangiocarcinomas were deficient in immunoreactivity. These findings show that alterations in the expression of connexins, by either downregulation or differential induction, represent common modifications during hepatocarcinogenesis. Although our results imply that connexins represent useful markers for the boundary between tumor promotion and progression, preneoplastic and neoplastic rat hepatocytes fail to use a common mechanism to modify connexin expression.

Mechanisms and function of intercellular calcium signaling

Intercellular Ca2+ waves initiated by mechanical or chemical stimuli propagate between cells via gap junctions. The ability of a wide diversity of cells to display intercellular Ca2+ waves suggests that these Ca2+ waves may represent a general mechanism by which cells communicate. Although Ca2+ may permeate gap junctions, the intercellular movement of Ca2+ is not essential for the propagation of Ca2+ waves. The messenger that moves from one cell to the next through gap junctions appears to be IP3 and a regenerative mechanism for IP3 may be required to effect multicellular communication. Extracellularly mediated Ca2+ signaling also exists and this could be employed to supplement or replace gap junctional communication. The function of intercellular Ca2+ waves may be the coordination of cooperative cellular responses to local stimuli.

Gap junctional intercellular communication and cell proliferation during rat liver carcinogenesis

During multistage liver carcinogenesis, there is a sequential decrease in gap junctional intercellular communication (GJIC), associated with reduced expression of a major liver gap-junction protein (connexin 32). There are also several lines of evidence indicating that the induction of cell proliferation plays an important role during liver carcinogenesis. The relationship between GJIC and cell proliferation and their roles in liver carcinogenesis are not yet known. Results from various experiments suggest that there is a close relationship between the inhibition of GJIC and stimulation of liver cell proliferation. However, our results also suggest that different stimuli may affect cell proliferation and GJIC differentially by different mechanisms.

Effect of glucocorticoids on TPA-induced inhibition of gap-junctional communication and morphological transformation in Syrian hamster embryo cells

The effect of glucocorticoids on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inhibition of gap-junctional intercellular communication (GJIC) and morphological transformation in Syrian hamster embryo (SHE) cells was examined. Fluocinolone acetonide (FA) and dexamethasone (DEX) almost completely suppressed the effect of TPA on induction of transformed morphology. On the other hand, up to 1000 times higher FA and DEX concentrations did not influence the inhibitory effect of TPA on GJIC. Neither treatment with these glucocorticoids for 4, 24 or 48 h before TPA exposure nor 24 h co-exposure with TPA altered the effect of TPA on GJIC. Thus the potent effect of glucocorticoids as inhibitors of the promotional effect of TPA on morphological transformation in SHE cells does not result in alterations of TPA-induced inhibition of GJIC.

An improved long-term culture of rat hepatocytes to detect liver tumour-promoting agents: results with phenobarbital

Among various cocultures of hepatocytes with other cell types, we found that mouse embryonal cells (BALB/c 3T3) are more effective in maintaining rat hepatocytes in vitro. Because most human cancers are epithelial in origin, we thought that such a hepatocyte culture system could be used for the detection of tumour-promoting agents, most of which are inhibitors of gap-junctional intercellular communication. We, therefore, have examined the effect of the strong rat liver tumour promoter, phenobarbital, on the gap-junctional intercellular communication capacity of hepatocytes in long-term cultures. A single application of phenobarbital drastically inhibited the gap-junctional intercellular communication between hepatocytes in a coculture for only several hours, but treatment for 3 weeks provoked a constant decrease of gap-junctional intercellular communication (50%) throughout the treatment period. This type of long-term culture of rat hepatocytes may be usable in a rapid in vitro assay to detect tumour-promoting agents.

Biological effects of electromagnetic fields

Life on earth has evolved in a sea of natural electromagnetic (EM) fields. Over the past century, this natural environment has sharply changed with introduction of a vast and growing spectrum of man-made EM fields. From models based on equilibrium thermodynamics and thermal effects, these fields were initially considered too weak to interact with biomolecular systems, and thus incapable of influencing physiological functions. Laboratory studies have tested a spectrum of EM fields for bioeffects at cell and molecular levels, focusing on exposures at athermal levels. A clear emergent conclusion is that many observed interactions are not based on tissue heating. Modulation of cell surface chemical events by weak EM fields indicates a major amplification of initial weak triggers associated with binding of hormones, antibodies, and neurotransmitters to their specific binding sites. Calcium ions play a key role in this amplification. These studies support new concepts of communication between cells across the barriers of cell membranes; and point with increasing certainty to an essential physical organization in living matter, at a far finer level than the structural and functional image defined in the chemistry of molecules. New collaborations between physical and biological scientists define common goals, seeking solutions to the physical nature of matter through a strong focus on biological matter. The evidence indicates mediation by highly nonlinear, nonequilibrium processes at critical steps in signal coupling across cell membranes. There is increasing evidence that these events relate to quantum states and resonant responses in biomolecular systems, and not to equilibrium thermodynamics associated with thermal energy exchanges and tissue heating.

Reversal of ras-induced inhibition of gap-junctional intercellular communication, transformation, and tumorigenesis by lovastatin

The plasma-membrane association and transforming activity of the ras oncoprotein p21 are dependent upon posttranslational farnesylation. Farnesyl synthesis and p21 ras farnesylation are inhibited by hydroxymethylglutaryl-CoA reductase inhibitors such as lovastatin. In this study, we examined whether lovastatin could reverse the transformed phenotype of a v-Ha-ras-transformed rat liver epithelial cell line (WB-ras cells) and if changes were associated with the enhancement of gap-junctional intercellular communication (GJIC). WB-ras cells grow in soft agar, have reduced GJIC, and are highly tumorigenic. Membrane association of p21 ras in these cells was inhibited after in vitro treatment with lovastatin (0.1-0.5 microM) for 48 h. Concomitantly, the cells displayed a more normal morphology, decreased growth in soft agar, and enhanced GJIC. These changes were prevented by cotreatment with mevalonic acid. The morphology and GJIC of rat liver epithelial cells transformed with other oncogenes (src, neu, and raf/myc) were not affected by lovastatin. Intrahepatic WB-ras tumors were induced in male rats by intraportal-vein injection of WB-ras cells. The size and DNA labeling index of these tumors were decreased approximately 75% by administration of lovastatin (5 mg/kg orally twice daily for 2 wk). These results suggest that lovastatin reversed the transformed phenotype of WB-ras cells by inhibiting p21 ras plasma membrane association. Furthermore, the concomitant enhancement of GJIC in lovastatin-treated cells suggests a role for reduced GJIC in the expression of the transformed phenotype.