The difference in contact inhibition of cell replication between normal cells and cells transformed by different carcinogens

Fibroblasts as Turned Agents in Cancer Progression

Differentiated epithelial cells reside in the homeostatic microenvironment of the native organ stroma. The stroma supports their normal function, their G0 differentiated state, and their expansion/contraction through the various stages of the life cycle and physiologic functions of the host. When malignant transformation begins, the microenvironment tries to suppress and eliminate the transformed cells, while cancer cells, in turn, try to resist these suppressive efforts. The tumor microenvironment encompasses a large variety of cell types recruited by the tumor to perform different functions, among which fibroblasts are the most abundant. The dynamics of the mutual relationship change as the sides undertake an epic battle for control of the other. In the process, the cancer “wounds” the microenvironment through a variety of mechanisms and attracts distant mesenchymal stem cells to change their function from one attempting to suppress the cancer, to one that supports its growth, survival, and metastasis. Analogous reciprocal interactions occur as well between disseminated cancer cells and the metastatic microenvironment, where the microenvironment attempts to eliminate cancer cells or suppress their proliferation. However, the altered microenvironmental cells acquire novel characteristics that support malignant progression. Investigations have attempted to use these traits as targets of novel therapeutic approaches.

The Concept of "Cancer Stem Cells" in the Context of Classic Carcinogenesis Hypotheses and Experimental Findings

In this Commentary, the operational definition of cancer stem cells or cancer initiating cells includes the ability of certain cells, found in a heterogeneous mixture of cells within a tumor, which are able to sustain growth of that tumor. However, that concept of cancer stem cells does not resolve the age-old controversy of two opposing hypotheses of the origin of the cancer, namely the stem cell hypothesis versus the de-differentiation or re-programming hypothesis. Moreover, this cancer stem concept has to take into account classic experimental observations, techniques, and concepts, such as the multi-stage, multi-mechanism process of carcinogenesis; roles of mutagenic, cytotoxic and epigenetic mechanisms; the important differences between errors of DNA repair and errors of DNA replication in forming mutations; biomarkers of known characteristics of normal adult organ-specific stem cells and of cancer stem cells; and the characteristics of epigenetic mechanisms involved in the carcinogenic process. In addition, vague and misleading terms, such as carcinogens, immortal and normal cells have to be clarified in the context of current scientific facts. The ultimate integration of all of these historic factors to provide a current understanding of the origin and characteristics of a cancer stem cell, which is required for a rational strategy for prevention and therapy for cancer, does not follow a linear path. Lastly, it will be speculated that there exists evidence of two distinct types of cancer stem cells, one that has its origin in an organ-specific adult stem cell that is 'initiated' in the stem cell stage, expressing the Oct4A gene and not expressing any connexin gene or having functional gap junctional intercellular communication (GJIC). The other cancer stem cell is derived from a stem cell that is initiated early after the Oct4A gene is suppressed and the connexin gene is expressed, which starts early differentiation, but it is blocked from terminal differentiation.

Cancer Prevention and Therapy of Two Types of Gap Junctional Intercellular Communication⁻Deficient "Cancer Stem Cell"

Early observations showed a lack of growth control and terminal differentiation with a lack of gap junctional intercellular communication (GJIC). Subsequent observations showed that epigenetic tumor promoters and activated oncogenes, which block gap junction function, provide insights into the multi-stage, multi-mechanism carcinogenic process. With the isolation of embryonic induced pluri-potent stem cells and organ-specific adult stem cells, gap junctions were linked to early development. While tumors and tumor cell lines are a heterogeneous mixture of "cancer stem cells" and "cancer non-stem cells", the cancer stem cells seem to be of two types, namely, they express (a) no connexin genes or (b) connexin genes, but do not have functional GJIC. These observations suggest that these "cancer stem cells" originate from normal adult stem cells or from the de-differentiation or re-programming of somatic differentiated cells. This "Concept Paper" provides a hypothesis that "cancer stem cells" either originate from (a) organ-specific adult stem cells before the expression of the connexin genes or (b) organ-specific adult stem cells that just express gap junction genes but that the connexin proteins are rendered dysfunctional by activated oncogenes. Therefore, cancer prevention and therapeutic strategies must account for these two different types of "cancer stem cell".

Potential Role of Intercellular Communication in the Rate-Limiting Step in Carcinogenesis

In order to ascertain whether there might be a scientific basis for determining practical “thresholds” for “carcinogens,” the concepts of thresholds and carcinogens were examined in the context of some current ideas on cardnogenesis. The observation that cardnogenesis seems to involve the donal expansion of a pre-malignant cell through a series of pheno-typic changes was explained by the initiation/promotion model of cardnogenesis. Unrepaired DNA lesions, acting as substrates for mutations in dividing cells, were speculated to play a role in the initiation phase of cardnogenesis (and indirectly to the promotion phase if the lesions lead to significant cell killing, forcing “compensatory hyperplasia”). Inhibition of intercellular communication, either by cell removal, cell death, growth factors or chemical promoters, was speculated to allow the donal expansion of initiated cells to reach a “critical mass.” During that donal expansion of initiated cells, additional phenotypic changes were speculated to occur during cell replication by mutational and/or epigenetic events. Therefore, it was concluded, on the basis of this model, that conditions which prevented the inhibition of intercellular communication between normal cells and the initiated cell(s) contributed to the rate limiting step of cardnogenesis.

Assuming the initiation and promotion model of cardnogenesis, the classical concepts of “thresholds” and “carcinogens” were viewed as grossly inadequate because they did not symbolically represent the known determinants of the complex carcinogenic process. Unless genetic, developmental stage, tissue, nutritional, stress, life style, as well as concurrent antagonists and/or synergists, factors are known, extrapolation about the potential carcinogenicity of a given chemical from molecular, in vitro or even in vivo experiments or epidemiological data would be extremely risky. It was concluded that, at this stage of our understanding of the mech-anism(s) of carcinogenesis, attempts to determine “thresholds” for “carcinogens” naively assume “carcinogens” are the single determinants for carcinogenesis, and that all chemicals which might influence the appearance of tumors act the same way.

Human adult stem cells as the target cells for the initiation of carcinogenesis and for the generation of "cancer stem cells"

The inference to stem cells has been found in ancient myths and the concept of stem cells has existed in the fields of plant biology, developmental biology and embryology for decades. In the field of cancer research, the stem cell theory was one of the earliest hypotheses on the origin of a cancer from a single cell. However, an opposing hypothesis had it that an adult differentiated somatic cell could "de-differentiate" to become a cancer cell. Only within the last decade, via the "cloning" of Dolly, the sheep, did the field of stem cell biology really trigger an exciting revolution in biological research. The isolation of human embryonic stem cells has created a true revolution in the life sciences that has led to the hope that these human stem cells could lead to (a) basic science understanding of gene regulation during differentiation and development; (b) stem cell therapy; (c) gene therapy via stem cells; (d) the use of stem cells for drug discovery; (e) screening for toxic effects of chemicals; and (f) understand the aging and diseases of aging processes.

Evolution of energy metabolism, stem cells and cancer stem cells: how the warburg and barker hypotheses might be linked

The evolutionary transition from single cells to the metazoan forced the appearance of adult stem cells and a hypoxic niche, when oxygenation of the environment forced the appearance of oxidative phosphorylation from that of glycolysis. The prevailing paradigm in the cancer field is that cancers start from the "immortalization" or "re-programming" of a normal, differentiated cell with many mitochondria, that metabolize via oxidative phosphorylation. This paradigm has been challenged with one that assumes that the target cell for carcinogenesis is the normal, immortal adult stem cell, with few mitochondria. This adult organ-specific stem cell is blocked from "mortalizing" or from "programming" to be terminally differentiated. Two hypotheses have been offered to explain cancers, namely, the "stem cell theory" and the "de-differentiation" or "re-programming" theory. This Commentary postulates that the paleochemistry of the oceans, which, initially, provided conditions for life' s energy to arise via glycolysis, changed to oxidative phosphorylation for life' s processes. In doing so, stem cells evolved, within hypoxic niches, to protect the species germinal and somatic genomes. This Commentary provides support for the "stem cell theory", in that cancer cells, which, unlike differentiated cells, have few mitochondria and metabolize via glycolysis. The major argument against the "de-differentiation theory" is that, if re-programming of a differentiated cell to an "induced pluri-potent stem cell" happened in an adult, teratomas, rather than carcinomas, should be the result.

Induction of iPS cells and of cancer stem cells: the stem cell or reprogramming hypothesis of cancer?

This article as designed to examine whether the "stoichiometric" or "elite models" of the origin of the "induced pluripotent stem" (iPS) cells fits some experiment facts from the developmental biology of adult stem cells and from the field of cancer research. In brief, since the evidence presented to support the stoichiometric model failed to recognize the factual existence of adult organ specific stem cells, the model has not been rigorously tested. In addition, the demonstration of a subset of cells (MUSE cells) in normal primary in vitro cultures of human fibroblasts (the usual source of iPS cells) seems to be the origin of the iPS cells. Moreover, from the field of carcinogenesis, the "stem cell" versus "de-differentiation" or "reprogramming" hypotheses were examined. Again, using the role of glycolysis, known to be associated with the Warburg effect in cancer cells, a list of experiments showing that (a) normal stem cells, which have few mitochondria, metabolize via glycolysis; (b) the stem cells are targets for "initiation" or "immortalization" or the blockage of differentiation and apoptosis of the stem cells by "immortalizing viruses"; (c) Lactate dehydrogenase A (LDHA), when expressed, is associated with glycolysis and therefore, must be expressed in normal adult stem cells, as well as in cancer cells; and (d) p53, depleted or rendered dysfunctional by SV40 Large T antigen, is associated with the reduction of mitochondrial function and mass and is associated with the Warburg effect. Together, these observations from the iPS and "cancer stem cell" fields support the idea that both iPS cells and cancer stem cell are derived from adult organ-specific stem cells that do not restore or switch their metabolism of glucose from oxidative metabolism to glycolysis but, rather, in both cases, the adult stem cell, which metabolizes by glycolysis, is prevented from differentiation or from metabolizing by oxidative phosphorylation.

The modulatory effect of cell–cell contact on the tumourigenic potential of pre-malignant epithelial cells: a computational exploration

Malignant development cannot be attributed alone to genetic changes in a single cell, but occurs as a result of the complex interplay between the failure of cellular regulation mechanisms and the presence of a permissive microenvironment. Although E-cadherin is classified as a 'metastasis suppressor' owing to its role in intercellular adhesion, the observation that it may be downregulated at a premalignant stage is indicative of additional roles in neoplastic development. We have used an agent-based computational model to explore the emergent behaviour resulting from the interaction of single and subpopulations of E-cadherin-compromised cells with unaffected normal epithelial cells within a monolayer environment. We have extended this to investigate the importance of local tissue perturbations in the form of scratch-wounding, or ablation of randomly-dispersed normal cells, on the growth of a single cell exhibiting E-cadherin loss. Our results suggest that the microenvironment with respect to localized cell density and normal/E-cadherin-compromised neighbours is crucial in determining whether an abnormal individual cell proliferates or remains dormant within the monolayer. These predictions raise important questions relating to the propensity for individual mutations to give rise to disease, and future experimental exploration of these will enhance our understanding of a complex, multifactorial pathological process.

Stem cells in toxicology: fundamental biology and practical considerations

This "Commentary" has examined the use of human stem cells for detection of toxicities of physical, chemical, and biological toxins/toxicants in response to the challenge posed by the NRC Report, "Toxicity Testing in the 21st Century: A vision and Strategy." Before widespread application of the use of human embryonic, pluripotent, "iPS," or adult stem cells be considered, the basic characterization of stem cell biology should be undertaken. Because no in vitro system can mimic all factors that influence cells in vivo (individual genetic, gender, developmental, immunological and diurnal states; niche conditions; complex intercellular interactions between stem, progenitor, terminal differentiated cells, and the signaling from extracellular matrices, oxygen tensions, etc.), attempts should be made to use both embryonic and adult stem cells, grown in three dimension under "niche-like" conditions. Because many toxins and toxicants work by "epigenetic" mechanisms and that epigenetic mechanisms play important roles in regulating gene expression and in the pathogenesis of many human diseases, epigenetic toxicity must be incorporated in toxicity testing. Because modulation of gap junctional intercellular communication by epigenetic agents plays a major role in homeostatic regulation of both stem and progenitor cells in normal tissues, the modulation of this biological process by both endogenous and endogenous chemicals should be incorporated as an end point to monitor for potential toxicities or chemo-preventive attributes. In addition, modulation of quantity, as well as the quality, of stem cells should be considered as potential source of a chemical's toxic potential in affecting any stem cell-based pathology, such as cancer.

The gap junction as a "Biological Rosetta Stone": implications of evolution, stem cells to homeostatic regulation of health and disease in the Barker hypothesis

The discovery of the gap junction structure, its functions and the family of the "connexin" genes, has been basically ignored by the major biological disciplines. These connexin genes code for proteins that organize to form membrane-associated hemi-channels, "connexons", co-join with the connexons of neighboring cells to form gap junctions. Gap junctions appeared in the early evolution of the metazoan. Their fundamental functions, (e.g., to synchronize electrotonic and metabolic functions of societies of cells, and to regulate cell proliferation, cell differentiation, and apoptosis), were accomplished via integrating the extra-cellular triggering of intra-cellular signaling, and therefore, regulating gene expression. These functions have been documented by genetic mutations of the connexin genes and by chemical modulation of gap junctions. Via genetic alteration of connexins in knock-out and transgenic mice, as well as inherited connexin mutations in various human syndromes, the gap junction has been shown to be directly linked to many normal cell functions and multiple diseases, such as birth defects, reproductive, neurological disorders, immune dysfunction and cancer. Specifically, the modulation of gap junctional intercellular communication (GJIC), either by increasing or decreasing its functions by non-mutagenic chemicals or by oncogenes or tumor suppressor genes in normal or "initiated" stem cells and their progenitor cells, can have a major impact on tumor promotion or cancer chemoprevention and chemotherapy. The overview of the roles of the gap junction in the evolution of the metazoan and its potential in understanding a "systems" view of human health and aging and the diseases of aging will be attempted.

Structure-activity-dependent regulation of cell communication by perfluorinated fatty acids using in vivo and in vitro model systems

BACKGROUND

Perfluoroalkanoates, [e.g., perfluorooctanoate (PFOA)], are known peroxisome proliferators that induce hepatomegaly and hepatocarcinogenesis in rodents, and are classic non-genotoxic carcinogens that inhibit in vitro gap-junctional intercellular communication (GJIC). This inhibition of GJIC is known to be a function of perfluorinated carbon lengths ranging from 7 to 10.

OBJECTIVES

The aim of this study was to determine if the inhibition of GJIC by PFOA but not perfluoropentanoate (PFPeA) observed in F344 rat liver cells in vitro also occurs in F344 rats in vivo and to determine mechanisms of PFOA dysregulation of GJIC using in vitro assay systems.

METHODS

We used an incision load/dye transfer technique to assess GJIC in livers of rats exposed to PFOA and PFPeA. We used in vitro assays with inhibitors of cell signaling enzymes and antioxidants known to regulate GJIC to identify which enzymes regulated PFOA-induced inhibition of GJIC.

RESULTS

PFOA inhibited GJIC and induced hepatomegaly in rat livers, whereas PFPeA had no effect on either end point. Serum biochemistry of liver enzymes indicated no cytotoxic response to these compounds. In vitro analysis of mitogen-activated protein kinase (MAPK) indicated that PFOA, but not PFPeA, can activate the extracellular receptor kinase (ERK). Inhibition of GJIC, in vitro, by PFOA depended on the activation of both ERK and phosphatidylcholine-specific phospholipase C (PC-PLC) in the dysregulation of GJIC in an oxidative-dependent mechanism.

CONCLUSIONS

The in vitro analysis of GJIC, an epigenetic marker of tumor promoters, can also predict the in vivo activity of PFOA, which dysregulated GJIC via ERK and PC-PLC.

Review Paper: Cancer Stem Cells and Cancer Nonstem Cells: From Adult Stem Cells or from Reprogramming of Differentiated Somatic Cells

Two opposing hypotheses of the origin of cancer have existed for many decades. One hypothesis postulates that the adult stem cell is needed to initiate the carcinogenic process, whereas the other claims a somatic differentiated cell can dedifferentiate or be reprogrammed to regain properties associated with cancer cells. Recent reemergence of the cancer stem cell hypothesis and the isolation of presumptive cancer stem cells from many types of cancer have forced a reexamination of these 2 hypotheses of the origin of cancer. In addition, normal embryonic and adult stem cells have now been isolated and partially characterized. Furthermore, the demonstration of embryonic-like stem cells, being isolated from adult-differentiated skin fibroblast cells of mice, monkey, and human beings, provides a newer opportunity to determine which of these 2 hypotheses might explain the cell type for initiating the carcinogenic process. The goal of this review is to integrate these recent findings, concerning the isolation of normal and cancer stem cells, with several of the classical concepts of carcinogenesis (initiation/promotion/progression; mutation/epigenetic; stem cell theory/dedifferentiation hypotheses; oncogenetumor suppressor theory). Although the weight of the evidence in this review seems to support the stem cell hypothesis, only future studies, probably using comparative animal and human oncologic studies, will determine if targeting the cancer stem cell, with individualized medical approaches, will improve cancer prevention and therapy.

Toward a genetics of cancer resistance

Two of three humans never get cancer. Even the majority of heavy smokers remain cancer free. Is this a matter of chance, or are there cancer-resistant genotypes? Based on the evidence discussed, it would appear that evolution has favored a limited number of relatively common resistance genes that may nip incipient cancerous foci in the bud, i.e., to stop them at their inception. It is further suggested that resistance genes may act at the level of tissue organization in a dominant fashion.

Epithelial to mesenchymal transition of a primary prostate cell line with switches of cell adhesion modules but without malignant transformation

BACKGROUND

Epithelial to mesenchymal transition (EMT) has been connected with cancer progression in vivo and the generation of more aggressive cancer cell lines in vitro. EMT has been induced in prostate cancer cell lines, but has previously not been shown in primary prostate cells. The role of EMT in malignant transformation has not been clarified.

METHODOLOGY/PRINCIPAL FINDINGS

In a transformation experiment when selecting for cells with loss of contact inhibition, the immortalized prostate primary epithelial cell line, EP156T, was observed to undergo EMT accompanied by loss of contact inhibition after about 12 weeks in continuous culture. The changed new cells were named EPT1. EMT of EPT1 was characterized by striking morphological changes and increased invasion and migration compared with the original EP156T cells. Gene expression profiling showed extensively decreased epithelial markers and increased mesenchymal markers in EPT1 cells, as well as pronounced switches of gene expression modules involved in cell adhesion and attachment. Transformation assays showed that EPT1 cells were sensitive to serum or growth factor withdrawal. Most importantly, EPT1 cells were not able to grow in an anchorage-independent way in soft agar, which is considered a critical feature of malignant transformation.

CONCLUSIONS/SIGNIFICANCE

This work for the first time established an EMT model from primary prostate cells. The results show that EMT can be activated as a coordinated gene expression program in association with early steps of transformation. The model allows a clearer identification of the molecular mechanisms of EMT and its potential role in malignant transformation.

Tumor promoting properties of a cigarette smoke prevalent polycyclic aromatic hydrocarbon as indicated by the inhibition of gap junctional intercellular communication via phosphatidylcholine-specific phospholipase C

Inhibition of gap junctional intercellular communication (GJIC) and the activation of intracellular mitogenic pathways are common hallmarks of epithelial derived cancer cells. We previously determined that the 1-methyl and not the 2-methyl isomer of anthracene, which are prominent cigarette smoke components, activated extracellular receptor kinase, and inhibited GJIC in WB-F344 rat liver epithelial cells. Using these same cells, we show that an immediate upstream response to 1-methylanthracene was a rapid (<1 min) release of arachidonic acid. Inhibition of phosphatidylcholine-specific phospholipase C prevented the inhibition of GJIC by 1-methylanthracene. In contrast, inhibition of phosphatidylinositol specific phospholipase C, phospholipase A(2), diacylglycerol lipase, phospholipase D, protein kinase C, and tyrosine protein kinases had no effect on 1-methylanthracene-induced inhibition of GJIC. Inhibition of protein kinase A also prevented inhibition of GJIC by 1-methylanthracene. Direct measurement of phosphatidylcholine-specific phospholipase C and sphingomyelinase indicated that only phosphatidylcholine-specific phospholipase C was activated in response to 1-methylanthracene, while 2-methylanthracene had no effect. 1-methylanthracene also activated p38-mitogen activated protein kinase; however, like extracellular kinase, its activation was not involved in 1-methylanthracene-induced regulation of GJIC, and this activation was independent of phosphatidylcholine-specific phospholipase C. Although mitogen activated protein kinases were activated, Western blot analyzes indicated no change in connexin43 phosphorylation status. Our results indicate that phosphatidylcholine-specific phospholipase C is an important enzyme in the induction of a tumorigenic phenotype, namely the inhibition of GJIC; whereas mitogen activated protein kinases triggered in response to 1-methylanthracene, were not involved in the deregulation of GJIC.

From adult stem cells to cancer stem cells: Oct-4 Gene, cell-cell communication, and hormones during tumor promotion

Carcinogenesis is characterized by "initiation," "promotion," and "progression" phases. The "stem cell theory" and "de-differentiation" theories are used to explain the origin of cancer. Growth control for stem cells, which lack functional gap junctional intercellular communication (GJIC), involves negative soluble or niche factors, while for progenitor cells, it involves GJIC. Tumor promoters, hormones, and growth factors inhibit GJIC reversibly. Oncogenes stably inhibit GJIC. Cancer cells, which lack growth control and the ability to terminally differentiate and to apoptose, lack GJIC. The Oct3/4 gene, a POU (Pit-Oct-Unc) family of transcription factors was thought to be expressed only in embryonic stem cells and in tumor cells. With the availability of normal adult human stem cells, tests for the expression of Oct3/4 gene and the stem cell theory in human carcinogenesis became possible. Human breast, liver, pancreas, kidney, mesenchyme, and gastric stem cells, HeLa and MCF-7 cells, and canine tumors were tested with antibodies and polymerase chain reaction (PCR) primers for Oct3/4. Adult human breast stem cells, immortalized nontumorigenic and tumor cell lines, but not the normal differentiated cells, expressed Oct3/4. Adult human differentiated cells lose their Oct-4 expression. Oct3/4 is expressed in a few cells found in the basal layer of human skin epidermis. The data demonstrate that normal adult stem cells and cancer stem cells maintain expression of Oct3/4, consistent with the stem cell hypothesis of carcinogenesis. These Oct-4 positive cells might represent the "cancer stem cells." A strategy to target "cancer stem cells" is to suppress the Oct-4 gene in order to cause the cells to differentiate.

Dietary modulation of the multistage, multimechanisms of human carcinogenesis: effects on initiated stem cells and cell-cell communication

Diet can influence the risk to cancer in both negative and positive ways. Worldwide, more than 10 million persons develop cancer annually. Diet could prevent many cancers. Carcinogenesis is a multistage, multimechanism process, consisting of "initiation," "promotion," and "progression" phases. Although diet could affect each phase, an efficacious strategy for dietary chemoprevention would be intervention during the promotion phase. The tumor-promotion process requires sustained exposure to agents that stimulate the growth and inhibition of apoptosis of initiated cells in the absence of antipromoters. Chronic inflammation has been associated with the promotion process. The mechanism affecting the promotion process appears to be the inhibition of cell-cell communication between normal and initiated cells. Most, if not all, tumor-promoting agents and conditions, reversibly, inhibit cell-cell communication, whereas antipromoters, antioxidants, and anti-inflammatory agents have been shown to ameliorate the effects of tumor promoters on cell-cell communication. Additionally, adult stem cells are hypothesized to be the target cells for initiating the carcinogenic process. A new paradigm has been presented that postulates the first function of the carcinogenic process is to block the "mortalization" of a normal, "immortal" adult stem cell rather than the induction of "immortalization" of a normal mortal cell.

Modulated gap junctional intercellular communication as a biomarker of PAH epigenetic toxicity: structure-function relationship

Cancer is a multistage multimechanism process involving gene and/or chromosomal mutations (genotoxic events), altered gene expression at the transcriptional, translational, and post-translational levels (epigenetic events), and altered cell survival (proliferation and apoptosis or necrosis), resulting in an imbalance of the organism's homeostasis. Maintenance of the organism's homeostasis depends on the intricate coordination of genetic and metabolic events between cells via extracellular and intercellular communication mechanisms. The release of a quiescent cell, whether normal or premalignant, from the suppressing effects of communicating neighbors requires the downregulation of intercellular communication via gap junctions, thereby allowing factors that control intracellular events to exceed a critical mass necessary for the cell to either proliferate or undergo apoptosis. Therefore, determining the role an environmental pollutant must play in the multistage carcinogenic process includes mechanisms of epigenetic toxicity such as the effects of a compound on gap junctional intercellular communication (GJIC). A classic example of a class of compounds in which determination of carcinogenicity focused on genotoxic events and ignored epigenetic events is polycyclic aromatic hydrocarbons (PAHs). The study of structure-activity relationships of PAHs has focused exclusively on the genotoxic and tumor-initiating properties of the compound. We report on the structure-activity relationships of two- to four-ringed PAHs on GJIC in a rat liver epithelial cell line. PAHs containing a bay or baylike region were more potent inhibitors of GJIC than the linear PAHs that do not contain these regions. These are some of the first studies of determine the epigenetic toxicity of PAHs at the epigenetic level.

Hierarchical and cybernetic nature of biologic systems and their relevance to homeostatic adaptation to low-level exposures to oxidative stress-inducing agents

During evolution in an aerobic environment, multicellular organisms survived by adaptive responses to both the endogenous oxidative metabolism in the cells of the organism and the chemicals and low-level radiation to which they had been exposed. The defense repertoire exists at all levels of the biological hierarchy--from the molecular and biochemical level to the cellular and tissue level to the organ and organ system level. Cells contain preventive antioxidants to suppress oxidative damage to membranes. Cells also contain proteins and DNA; built-in redundancies for damaged molecules and organelles; tightly coupled redox systems; pools of reductants; antioxidants; DNA repair mechanisms and sensitive sensor molecules such as nuclear factor kappa beta; and signal transduction mechanisms affecting both transcription and post-translational modification of proteins needed to cope with oxidative stress. The biologic consequences of the low-level radiation that exceeds the background level of oxidative damage could be necrosis or apoptosis, cell proliferation, or cell differentiation. These effects are triggered by oxidative stress-induced signal transduction mechanisms--an epigenetic, not genotoxic, process. If the end points of cell proliferation, differentiation, or cell death are not seen at frequencies above background levels in an organism, it is unlikely that low-level radiation would play a role in the multistep processes of chronic diseases such as cancer. The mechanism linked to homeostatic regulation of proliferation and adaptive functions in a multicellular organism could provide protection of any one cell receiving deposited energy by the radiation tract through the sharing of reductants and by triggering apoptosis of target stem cells. Examples of the role of gap junctional intercellular communication in the adaptive response of cells and the bystander effect illustrate how the interaction of cells can modulate the effect of radiation on the single cell.

Inhibition of endothelial cell movement by pericytes and smooth muscle cells: activation of a latent transforming growth factor-beta 1-like molecule by plasmin during co-culture

When a confluent monolayer of bovine aortic endothelial (BAE) cells is wounded with a razor blade, endothelial cells (ECs) spontaneously move into the denuded area. If bovine pericytes or smooth muscle cells (SMCs) are plated into the denuded area at low density, they block the movement of the ECs. This effect is dependent upon the number of cells plated into the wound area and contact between ECs and the plated cells. Antibodies to transforming growth factor-beta 1 (TGF-beta 1) abrogate the inhibition of BAE cell movement by pericytes or SMCs. TGF-beta 1, if added to wounded BAE cell monolayers, also inhibits cell movement. When cultured separately, BAE cells, pericytes, and SMCs each produce an inactive TGF-beta 1-like molecule which is activated in BAE cell-pericyte or BAE cell-SMC co-cultures. The activation appears to be mediated by plasmin as the inhibitory effect on cell movement in co-cultures of BAE cells and pericytes is blocked by the inclusion of inhibitors of plasmin in the culture medium.

Radiation and chemically induced transformation: free radicals, antioxidants and cancer

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Distinctive transforming genes in x-ray-transformed mammalian cells

DNAs from hamster embryo cells and mouse C3H/10T1/2 cells transformed in vitro by x-irradiation into malignant cells transmit the radiation transformation phenotype by producing transformed colonies (transfectants) in two mouse recipient lines, the NIH 3T3 and C3H/101/2 cells, and in a rat cell line, the Rat-2 cells. DNAs from unirradiated cells or irradiated and visibly untransformed cells do not produce transformed colonies. The transfectants grow in agar and form tumors in nude mice. Treatment of the DNAs with restriction endonucleases prior to transfection indicates that the same transforming gene (oncogene) is present in each of the transformed mouse cells and is the same in each of the transformed hamster cells. Southern blot analysis of 3T3 or Rat-2 transfectants carrying oncogenes from radiation-transformed C3H/10T1/2 or hamster cells indicates that the oncogenes responsible for the transformation of 3T3 cells are not the Ki-ras, Ha-ras, or N-ras genes, nor are they neu, trk, raf, abl, or fms, although quick blot analysis using 11 oncogene probes detected increased transcripts of c-abl and c-fms in the 3T3 transformants containing oncogenic sequences from the x-ray-transformed C3H/10T1/2 cells. The work demonstrates that DNAs from mammalian cells transformed into malignancy by direct exposure in vitro to radiation contain genetic sequences with detectable transforming activity in three recipient cell lines. The results provide evidence that DNA is the target of radiation carcinogenesis induced at a cellular level in vitro. The experiments indicate that malignant radiogenic transformation in vitro of hamster embryo and mouse C3H/10T1/2 cells involves the activation of unique non-ras transforming genes, which heretofore have not been described.

Ultraviolet irradiation transforms C3H10T1/2 cells to a unique, suppressible phenotype

Transformation of C3H10T1/2 cells by exposure to ultraviolet (UV) irradiation followed by tetradecanoyl phorbol acetate (TPA) has been used as a model of two-stage carcinogenesis. However, cells cloned from UV-TPA-induced foci (UV-TDTx cells) had a unique phenotype. Cloned UV-TDTx cells appeared transformed in pure culture but were unable to form foci when cocultured with C3H10T1/2 cells. However, in the presence of TPA, UV-TDTx cells form foci in mixed culture with C3H10T1/2 cells. This phenotype was the only one observed for UV-TPA transformants. These data suggest that communal suppression of cell division is a discrete phenomenon that must be overcome as one step in the multistage process of transformation, and this protocol permits the routine isolation of transformed cells responsive to density-dependent growth suppression.

Selenium and vitamin E inhibit radiogenic and chemically induced transformation in vitro via different mechanisms

Results from in vivo and in vitro studies showing that antioxidants may act as anticarcinogens support the role of active oxygen in carcinogenesis and provide impetus for exploring the functions of dietary antioxidants in cancer prevention by using in vitro models. We examined the single and combined effects of selenium, a component of glutathione peroxidase, and vitamin E, a known antioxidant, on cell transformation induced in C3H/10T-1/2 cells by x-rays, benzo[a]pyrene, or tryptophan pyrolysate and on the levels of cellular scavenging systems and peroxide destruction. Incubation of C3H/10T-1/2 cells with 2.5 microM Na2SeO3 (selenium) or with 7 microM alpha-tocopherol succinate (vitamin E) 24 hr prior to exposure to x-rays or the chemical carcinogens resulted in an inhibition of transformation by each of the antioxidants with an additive-inhibitory action when the two nutrients were combined. Cellular pretreatment with selenium resulted in increased levels of cellular glutathione peroxidase, catalase, and nonprotein thiols (glutathione) and in an enhanced destruction of peroxide. Cells pretreated with vitamin E did not show these biochemical effects, and the combined pretreatment with vitamin E and selenium did not augment the effect of selenium on these parameters. The results support our earlier studies showing that free radical-mediated events play a role in radiation and chemically induced transformation. They indicate that selenium and vitamin E act alone and in additive fashion as radioprotecting and chemopreventing agents. The results further suggest that selenium confers protection in part by inducing or activating cellular free-radical scavenging systems and by enhancing peroxide breakdown while vitamin E appears to confer its protection by an alternate complementary mechanism.

Potential role of the src gene product in inhibition of gap-junctional communication in NIH/3T3 cells

The effects of the src gene on the activity of protein kinase C and intercellular communication have been studied in transformed NIH/3T3 clones isolated from soft agar following transfection with the plasmid carrying the v-src gene (psrc-11). Six transformed clones that were studied contained newly incorporated v-src genes in the genome, had an increased amount of pp60src, and showed enhanced activities of protein kinase C. Intercellular communication, studied by observing with autoradiography the transfer of [3H]uridine nucleotide from prelabeled donor cells to recipient cells in contact, was found to be reduced in transformed clones as compared to parental NIH/3T3 cells. Treatment with phorbol 12-myristate 13-acetate was also found to increase protein kinase C activity and to reduce intercellular communication in normal NIH/3T3 cells. These results suggest that the v-src gene product, in a manner similar to some of the powerful tumor promoters, may directly or indirectly affect cell-cell communication.

Inhibited intercellular communication as a mechanistic link between teratogenesis and carcinogenesis

Teratogenesis and carcinogenesis share many characteristics, leading to the speculation that they may also share pathogenic mechanisms. Direct intercellular communication mediated by membrane junctions is known to occur between a variety of cells and may play an important role in the control of cell growth and differentiation. Inhibition of junctional communication may be a mechanism common to both teratogenesis and carcinogenesis whereby cells and tissues are diverted from their normal differentiation paths. The multistage model of carcinogenesis predicts that the irreversibly initiated cell is at least partially regulated by the surrounding cells of a tissue, and that the initiated cell remains inactive until stimulated to proliferate by a tumor promotor. Tumor promoters may release the initiated cell from control of the surrounding tissue by interrupting intercellular communication, since many tumor promoters have now been shown to interfere with junctional communication in cultured mammalian cells. Furthermore, many tumorigenic cells have compromised junctional communication abilities. Similarly, it has been reasoned that the cells of an embryo must be able to communicate with each other to define tissue specificity and pattern formation, and to coordinate morphogenetic events. Many studies have chronicled alterations in junctional communication that occur coincident with major developmental events and some studies suggest that junctional communication may be modified at boundaries of morphogenetic fields. A recent in vivo study has provided evidence that inhibition of junctional communication may interfere with embryonic development, and several teratogens are known to interrupt junctional communication in mammalian cells in culture. These observations suggest that inhibition of junctional intercellular communication may be a shared mechanism of carcinogenesis and teratogenesis.

Role of tumor promotion in affecting the multi-hit nature of carcinogenesis

The assessment of risk from low-level exposure to radiation and chemicals is hindered by the basic lack of scientific understanding of the complex nature of the multiple levels of protective or synergistic interactions. Radiation and chemicals have the potential of inducing mutations, cell death and altered gene expression. The biological consequences of each of these effects is again complex, since many factors can enhance or mask the effect of a single mutation, cytotoxic or gene modulatory change. Carcinogenesis, representing but one chronic disease state, is a multi-step process, involving the clonal expansion of a single altered cell (that is, the initiation event). Radiation and chemicals appear to contribute to the initiation process by their ability to induce viable mutations. Insufficient theoretical and empirical knowledge precludes a determination of whether mutagenesis and, hence, initiation exhibit a threshold phenomenon. Because of a variety of redundancy mechanisms on the genetic and cellular levels, the physiological impact of a single dysfunctional cell is felt only after it is amplified to a large number (the promotion phase of carcinogenesis). Radiation and chemical-induced cytotoxicity, as well as noncytotoxic chemical induction of mitogenesis, can induce surviving single dysfunctional stem cells to multiply. If during this multiplication of dysfunctional cells, the initiated cells are further exposed chronically to low levels of mutagens, there is an enhanced probability of additional genetic changes. The accumulation of foci of dysfunctional cells can occur in any stem cell population of any tissue. As with mutagens, the existence of threshold levels for promoting conditions and chemicals is still not yet scientifically validated. However, specific examples of the actions of a few promoters does seem to be consistent with that idea. The concepts of initiation and promotion imply the existence of anti-initiation and antipromotion conditions. Together with genetic factors, the complex and unpredictable interactions of radiation and chemicals as initiators, anti-initiators, promoters and antipromoters, defy prospects of an easy means to predict the consequences of exposure to chronic low levels of radiation and chemicals.

Ultracytochemistry of the cell surface and microfilaments in dimethylhydrazine-induced colonic tumor cells

Studies were made of the ultracytochemical changes in the cell membrane and microfilaments of colonic epithelial cells during tumorigenesis induced by 1,2-dimethylhydrazine (DMH) in mice fed a high fat diet. The tumor cells showed reduced membrane ATPase activity and loss of contact with neighboring cells. Microfilaments in tumor cells showed an irregular intensity of fluorescent staining. Their actin filaments bound with heavy meromyosin (HMM) had an arrowhead pattern as in normal cells, but these complexes were shortened and detached from the cell membrane. The arrowheads were directed toward the interior in the terminal web of tumor cells. Microfilaments with long rootlets extended to the apical surface of some tumor cells. These results indicate that during development of colonic tumors, the structures of the cell membrane and microfilaments of the cells changes.

Loss of reciprocal modulations of growth and liver function of hepatoma cells in culture by contact with cells or cell membranes

In mature rat hepatocytes in primary culture, many metabolic functions and cell growth are controlled reciprocally by cell density, and this reciprocal regulation is mediated by a cell-surface modulator through cell-cell contact. Cultured RY-121B cells from Reuber hepatoma and MH1C1 cells from Morris hepatoma, which retain some liver-specific functions, did not show any cell-density dependency of either cell growth or hepatocyte-specific functions, such as induction of tyrosine aminotransferase by dexamethasone. However, when RY-121B cells were cocultured with a low density of rat hepatocytes as monolayers in direct contact, they exerted contact-dependent control of DNA synthesis and of differentiated function of the hepatocytes. Furthermore, plasma membranes from various tumor cells including these hepatoma cells had strong modulator activity on primary cultures of normal rat hepatocytes, and their activity mimicked the reciprocal effects of cell density on DNA synthesis and induction of tyrosine aminotransferase. On the contrary, addition of plasma membranes from normal adult rat liver to sparse cultures of RY-121B or MH1C1 cells did not cause any inhibition of active DNA synthesis or enhancement of induction of tyrosine aminotransferase in these cells. These results suggest that hepatoma cells have lost cell density-dependent regulations of many cellular activities and cell growth because they have lost the ability to respond to the cell surface modulator, although they retain modulator activity on their plasma membranes.

Inhibition of metabolic cooperation by cigarette smoke condensate and its fractions in V-79 Chinese hamster lung fibroblasts

This study was performed to determine the usefulness of the intercellular metabolic cooperation assay for analysis of a complex mixture and to compare the results obtained with previously conducted in vivo tumor promoter assays. One hundred 2R1 cigarettes were smoked according to Federal Trade Commission guidelines and the resulting condensate was separated into a water/methanol-soluble fraction (which was further partitioned into acidic and basic components) and an organic solvent-soluble fraction (which was then chromatographed on silicic acid with petroleum ether, benzene/petroleum ether, benzene, ether, and methanol). The following fractions were positive in the metabolic cooperation assay (in decreasing order of activity): organic solvent-soluble, acidic, whole condensate, and water/methanol-soluble fractions as well as the ether, benzene, and benzene/petroleum ether eluates. The basic fraction and the petroleum ether and methanol eluates were negative. In general, the metabolic cooperation assay results were comparable to previously published results obtained on mouse skin.

The role of inhibited cell-cell communication in teratogenesis

A mechanistic link between teratogenesis and carcinogenesis has been suggested by a wide variety of scientific observations. This report attempts to provide a theoretical explanation for one of the several possible mechanisms which might be shared during carcinogenesis and teratogenesis. The initiation and promotion concept of carcinogenesis was briefly reviewed and the role of intercellular communication during the complex tumor promotion phase was discussed. Inhibition of intercellular communication by a wide variety of physical, chemical and biological factors was speculated to disrupt the regulation of proliferation and differentiation in stem cells. Chemicals, which interfered with intercellular communication during early organogenesis, have the potential of being teratogens, while if they are present in the developed, initiated organisms have the potential of being tumor promoters. Evidence was presented showing that known tumor promoters which inhibited intercellular communication also had been shown to be teratogens. It was concluded that in vitro assays, designed to measure intercellular communication, although having known limitations, might be used as an in vitro means to screen for potential teratogens.

Cellular transformation by radiation: induction, promotion, and inhibition

Mammalian cell cultures offer powerful tools for evaluating qualitatively and quantitatively the oncogenic potential of radiation over a wide range of doses with particular importance at the low dose range that is relevant to human exposure and risk. Our studies have shown that early events in the process of radiation induced transformation in both rodent and human cells requires initial replication for fixation of transformation as a hereditary property of cells and further clonal expansion for full expression. Early events (fixation) are inhibited by cell-cell contact and high cell density but can be modified at low temperature where repair processes are slowed. Cell-cell contact and communication in tissue organization may be in part responsible for our findings that radiation oncogenesis induced in utero in hamsters is expressed at a lower frequency than that induced in vitro. Quantitative studies carried out on hamster embryo cells indicate that neutrons are more effective in their carcinogenic potential than x-rays but also more toxic, that splitting the dose of x-rays at low doses leads to enhanced transformation, but that at high doses protracted radiation has a sparing effect. At all dose ranges survival was increased by protracting the radiation dose, thus suggesting that different repair processes must be involved for survival and transformation. Similar observations were seen when the protease inhibitor Antipain was found to enhance transformation in rodent and human cells when present at the time of radiation, but was protective when added after radiation. Survival was not modified under any of those conditions, and Antipain did not affect DNA replication and repair. In our qualitative studies, once cells are transformed by radiation, they exhibit a wide range of structural and functional phenotypic changes, some of which are membrane-associated and are expressed within days after induction. Our current studies on nutritional and hormonal influences on radiation transformation indicate the following: Pyrolysate products from broiled protein foods act in synergism with radiation to produce transformation, whereas vitamin A analogs are powerful, preventive agents. Retinoids inhibit both x-ray-induced transformation and its promotion by TPA; these modifications (enhancement by TPA, inhibition by retinoids) are not reflected in sister chromatid exchanges, but are reflected in the level of membrane associated enzymes Na/K ATPase. Whereas retinoids modify late events (expression, promotion), we find that thyroid hormone plays a crucial role in the early phases of radiation and chemically induced transformation. Under hypothyroid conditions no transformation is observed. The addition of triiodothyronine at physiological levels results in a transformation rate that is dose-related. Our recent success in transforming human skin fibroblasts will enable quantitative and qualitative studies of radiation carcinogenesis in a system relevant to man.

Elimination of metabolic cooperation in Chinese hamster cells by a tumor promoter

Wild-type Chinese hamster V79 cells (6-thioguanine-sensitive) reduce the recovery of 6-thioguanine-resistant cells when they are cultured together at high densities, through a form of intercellular communication (metabolic cooperation). Cooperation is inhibited by 12-O-tetradecanoyl phorbol-13-acetate, rescuing the 6-thioguanine-resistant cells. These results may be useful in the study of an aspect of the mechanism of tumor promotion and in assaying for promoters.

Locomotory behavior, contact inhibition and pattern formation of 3T3 and polyoma virus-transformed 3T3 cells in culture

The social behavior of 3T3 cells and their polynoma virus-transformed derivative (Py3T3 cells) was examined by time-lapse cinemicrography in order to determine what factors are responsible for the marked differences in the patterns formed by the two cell lines in culture. Contrary to expectations, both cell types have been found to exhibit contact inhibition of cell locomotion. Therefore, the tendency of 3T3 cells to form monolayers and of Py3T3 cells to form crisscrossed multilayers cannot be explained on the basis of the presence versus the absence of contact inhibition. Morevover, with the exception of cell division control, the social behavior of the two cell types is qualitively similar. Both exhibit cell underlapping and, after contact between lamelliopodia, both show inhibition of locomotory activity and adhesion formation. Neither cell type was observed to migrate over the surface of another cell. The two cell types do show quantitative differences in the frequency of underlapping, the frequency with which contact results in inhibition of locomotion, and the proportion of the cell margin that adheres to the substratum. The increased frequency pf Py3T3 underlapping is correlated with the reduced frequency of substratum adhesions, which in turn favors underlapping. On the basis of these observations, it is concluded that the differences in culture patterns are the result of differences in the shapes of the individual cells, such that underlapping, and hence crisscrossing, is favored in Py3T3 cell interactions and discouraged in 3T3 cells.

Inhibition of DNA synthesis in cultures of 3T3 cells by isolated surface membranes

When added to a sparse culture of 3T3 cells, a surface membrane-enriched fraction from 3T3 cells inhibited the rate of DNA synthesis in a time- and concentration-dependent manner. The membrane preparation had no effect on the rate of DNA synthesis of simian virus 40-transformed 3T3 cells. A similar membrane preparation from transformed cells had a lesser inhibitory effect on 3T3 cells and no effect on transformed cells. The inhibition by membranes was reversible. The data suggest that, when added to growing 3T3 cells, 3T3 surface membranes can mimic the effect of increasing cell density on DNA synthesis.

Inhibition of intracellular multiplication of adenovirus by interaction between infected and uninfected cells

The possibility that virus multiplication may be inhibited by interaction of infected cells with uninfected cells was tested by experiments, using human adenovirus type 12 (Ad12). Permissive human cells (human embryonic kidney-HEK,KB or HeLa) were infected and seeded on uninfected or infected "nonpermissive" cell (human embryonic lung=HEL) monolayers, and virus yields or proportions of viral antigen-synthesizing cells were compared with each other. Both the virus yields and the proportions of viral antigen-positive cells were not reduced significantly when seeded on infected HEL cells, while they seeded on uninfected HEL cells both of them were reduced remarkably, compared with the yield and the proportion of controls seeded on glass. Similar results were obtained regardless of the type of permissive cells, HEK, KB, or HeLa. Similar reduction of the yield was observed when seeded on HEL cells infected with Ad12 inactivated by heat or by antiserum, and partial reduction was observed when seeded on HEL cells infected with UV-inactivated Ad12, depending on the extent of UV dosis. These experiments showed that intracellular virus multiplication may be inhibited by interaction of infected cells with uninfected cells, and this may be due to the difference in the cell surface structure.