A quantitative approach to assessing intercellular communication: studies on cigarette smoke condensates

Characterization of a gap-junctional intercellular communication (GJIC) assay using cigarette smoke

Inhibition of gap-junctional intercellular communication (GJIC) via exposure to various toxic substances has been implicated in tumor promotion. In the present study, cigarette smoke total particulate matter (TPM), a known inhibitor of GJIC, were used to characterize a new GJIC screening assay in three independent experiments. The main features of this assay were automated fluorescence microscopy combined with non-invasive parachute technique. Rat liver epithelial cells (WB-F344) were stained with the fluorescent dye Calcein AM (acetoxymethyl) and exposed to TPM from the Kentucky Reference Cigarette 2R4F (a blend of Bright and Burley tobaccos) and from two single-tobacco cigarettes (Bright and Burley) for 3h. Phorbol-12-myristate-13-acetate (TPA) was used as positive control and 0.5% dimethyl sulfoxide (DMSO) as solvent control. The transfer of dye to adjacent cells (percentage of stained cells) was used as a measure of cellular communication. A clear and reproducible dose-response of GJIC inhibition following TPM exposure was seen. Reproducibility and repeatability measurements for the 2R4F cigarette were 3.7% and 6.9%, respectively. The half-maximal effective concentration values were 0.34ng/ml for TPA, 0.050mg/ml for the 2R4F, 0.044mg/ml for the Bright cigarette, and 0.060mg/ml for the Burley cigarette. The assay was able to discriminate between the two single-tobacco cigarettes (P<0.0001), and between the single-tobacco cigarettes and the 2R4F (P=0.0008, 2R4F vs. Burley and P<0.0001, 2R4F vs. Bright). Thus, this assay can be used to determine the activity of complex mixtures such as cigarette smoke with high throughput and high precision.

The use of in vitro systems to assess cancer mechanisms and the carcinogenic potential of chemicals

Carcinogenesis is a highly complex, multi-stage process that can occur over a relatively long period before its clinical manifestation. While the sequence in which a cancer cell acquires the necessary traits for tumour formation can vary, there are a number of mechanisms that are common to most, if not all, cancers across the spectrum of possible causes. Many aspects of carcinogenesis can be modelled in vitro. This has led to the development of a number of mechanistically driven, cell-based assays to assess the pro-carcinogenic and anti-carcinogenic potential of chemicals. A review is presented of the current in vitro models that can be used to study carcinogenesis, with examples of cigarette smoke testing in some of these models, in order to illustrate their potential applications. We present an overview of the assays used in regulatory genotoxicity testing, as well as those designed to model other aspects that are considered to be hallmarks of cancer. The latter assays are described with a view to demonstrating the recent advances in these areas, to a point where they should now be considered for inclusion in an overall testing strategy for chemical carcinogens.

Evaluation of in vitro assays for assessing the toxicity of cigarette smoke and smokeless tobacco

BACKGROUND

In vitro toxicology studies of tobacco and tobacco smoke have been used to understand why tobacco use causes cancer and to assess the toxicologic impact of tobacco product design changes. The need for toxicology studies has been heightened given the Food and Drug Administration's newly granted authority over tobacco products for mandating tobacco product performance standards and evaluate manufacturers' health claims about modified tobacco products. The goal of this review is to critically evaluate in vitro toxicology methods related to cancer for assessing tobacco products and to identify related research gaps.

METHODS

PubMed database searches were used to identify tobacco-related in vitro toxicology studies published since 1980. Articles published before 1980 with high relevance also were identified. The data were compiled to examine (a) the goals of the study, (b) the methods for collecting test substances, (c) experimental designs, (d) toxicologic end points, and (e) relevance to cancer risk.

RESULTS

A variety of in vitro assays are available to assess tobacco smoke that address different modes of action, mostly using non-human cell models. However, smokeless tobacco products perform poorly in these assays. Although reliable as a screening tool for qualitative assessments, the available in vitro assays have been poorly validated for quantitative comparisons of different tobacco products. Assay batteries have not been developed, although they exist for nontobacco assessments. Extrapolating data from in vitro studies to human risks remains hypothetical.

CONCLUSIONS

In vitro toxicology methods are useful for screening toxicity, but better methods are needed for today's context of regulation and evaluation of health claims.

A review of in vitro methods to assess the biological activity of tobacco smoke with the aim of reducing the toxicity of smoke

In the last few years tobacco companies have been developing several research strategies in order to reduce the risks associated with smoking. These strategies include, for example, the refining of alternative cigarette designs that reduce the amount of hazardous chemicals in the mainstream smoke by introducing modified filters, and/or reducing the amount of biologically significant ingredients in tobacco-burning cigarettes. In the last few decades numerous studies have been published to assess the biological activity of tobacco smoke using in vivo and in vitro test systems. In this scenario a general scientific consensus on how to measure and characterize the risk associated with cigarette smoke is still lacking. Short-term in vitro assays, which are widely accepted by regulatory agencies around the world, are useful tools to evaluate both the biological activity and the progress towards a reduction of tobacco smoke toxicity. These assays could be mainly applied to evaluate cytotoxicity and genotoxicity properties on whole cigarette smoke as well as condensates or fractions of whole smoke. Cytotoxicity induction can be measured as cellular viability and growth rates using different end-points. Otherwise, the target of genotoxicity studies is the DNA molecule. For genotoxicity evaluation, the end-points and cell systems should be chosen from those that are relevant and appropriate as clinical surrogate markers. In this respect, the occurrence of early biological effect markers, such as mutational or clastogenic events (point mutations, frameshifts, micronuclei, SCE, DNA adducts) in bacterial and mammalian cells should be studied in a tiered approach following the guidelines of regulatory agencies. The choice of criteria shall be matter of discussion.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

Gap junction intercellular communication and cytotoxicity in normal human cells after exposure to smoke condensates from cigarettes that burn or primarily heat tobacco

Heating tobacco, rather than burning it, reduces tobacco combustion and pyrolysis products. This study tested the hypothesis that the simplified smoke chemistry of a cigarette which primarily heats tobacco (TOB-HT) significantly reduces the potential to alter the structure or function of cellular plasma membranes relative to low "tar" 1R4F and ultra low "tar" lR5F Kentucky reference cigarettes which burn tobacco. Gap junction intercellular communication (GJIC) and lactate dehydrogenase release (LDH) were used to quantify functional and structural changes to the plasma membrane, respectively. Cigarette smoke condensate (CSC) from the mainstream smoke of TOB-HT, lR4F and 1R5F cigarettes were compared in the GJIC and LDH release assays following a 1-hr exposure in vitro. Human bronchial/tracheal epithelial cells, coronary artery endothelial cells, coronary artery smooth muscle cells, foreskin keratinocytes and the WB-344 rat liver epithelial cell line were studied. TOB-HT did not inhibit GJIC in any of the human cell types tested (P0.05) at concentrations where 1R4F and lR5F did inhibit GJIC (P<0.05). TOB-HT did not elevate LDH release (P0.05) when tested at concentrations where lR4F and lR5F did elevate LDH release (P<0.05). Our results suggest that CSC from TOB-HT cigarettes is less damaging to the structure or function of the cellular plasma membranes of a variety of human cell lines than CSC from 1R4F and 1R5F tobacco burning reference cigarettes.

Inhibition of intercellular communication by condensates of high and low tar cigarettes

Inhibition of gap junctional intercellular communication (GJIC) is a predictive short term test for tumor promoting activity. A new metabolic cooperation assay has been developed, which takes the cytochrome P-450 metabolism into account. In this assay the inhibitory activity of tobacco smoke condensates (CSC) and CSC fractions from high and low tar cigarettes was tested. CSC of both high and low tar cigarettes and fractions thereof contained tumor promoting activity. The tar yield of the cigarettes did not closely reflect the effects in the GJIC assay and the major constituent nicotine had no effect. The effect was only marginally greater in cells expressing different cytochrome P-450 enzymes, indicating that the active substances are not metabolized by these enzymes. The activities of CSC fractions were considerably lower than the activities in the unfractionated CSC. This may indicate that compounds in the CSC act strongly synergistically. Furthermore, CSC and CSC fractions synergistically inhibit GJIC with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate, indicating different mechanisms of action.

Limitations of the scrape-loading/dye transfer technique to quantify inhibition of gap junctional intercellular communication

Gap junctional intercellular communication (GJIC) is recognized as playing an important role in normal cell proliferation and development. Chemically induced alteration of GJIC has been proposed to be associated with abnormal cellular growth and/or tumor promotion. Several in vitro assays are currently used to determine the effects of chemicals on GJIC between cultured mammalian cells. One of these assays, the scrape-loading dye transfer (SL/DT) technique, is based on monitoring the transfer of the fluorescent dye Lucifer yellow from one cell into adjacent cells via functional gap junctions. The objective of our study was to evaluate and compare various approaches for quantifying results obtained with the SL/DT technique. Confluent cultures of either WB rat liver epithelial cells or LC-540 rat leydig cells were exposed to the animal tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), solvent (0.1% ethanol), or culture medium for one hour at 37 degrees C prior to analysis of GJIC. Inhibition of dye transfer was clearly evident following TPA exposure. Quantification of this dye transfer was assessed via four approaches: manually counting the number of labeled cells; measuring the distance of dye travel from the scrape line; quantifying the amount of cellular dye uptake; and determining the distribution of dye away from the scrape line. Our results suggest that while the SL/DT technique can be effectively used as a tool to determine the qualitative presence or absence of GJIC, its use in quantifying changes in GJIC following chemical exposure is limited. Since concentration-dependent responses are critical in chemical testing, application of the SL/DT method should be restricted to a screening assay for qualitatively assessing the presence or absence of GJIC.