Effect of DDT on hepatic gap junctional intercellular communication in rats

DDT and Other Chlorinated Insecticides

The use of organochlorine insecticides such as DDT, lindane and cyclodieneshas declined markedly worldwide over the last decades. Most are now banned or not used. At an acute toxicity level they have been relatively safe in use for humans. However, the greatest concerns are their persistence in people, wildlife and the environment due to their slow metabolism. Although their carcinogenicity for humans has not been supported by strong epidemiological evidence, their potential to be modulators of endocrine and immune function at levels remaining in the environment or associated with residual spraying of DDT continue to be of concern. At present, DDT is still allowed by the United Nations for combating malaria, with continual monitoring and assessment where possible. The toxicological consequences of exposure of animals and people to DDT is discussed as well as some analogues and other insecticides such as lindane, dieldrin and chlordecone that, although little used, continue to persist in surroundings and people. Because of circumstances of world health brought about by climate change or human activities that have yet to develop, there may come a time when the importance of some may re-emerge.

Transient disruption of liver gap junctional intercellular communication and induction of apoptosis after administration of 1,4-bis[2-(3,5 dichloropyridyloxy)]benzene in mice

Gap junctional intercellular communication (GJIC) and connexin expression (Cx26 and Cx32) in mouse liver were studied after administration of 4-bis[2-(3,5 dichloropyridyloxy)]benzene (TCPOBOP), a phenobarbital-like enzyme inducer. Female C57Bl/6 mice were administered TCPOBOP (5.8 mg/kg BW) and euthanized 0, 24, 48 and 72 hours later. Liver samples were snap frozen, or fixed in formalin, or submitted to GJIC analysis. The proliferating cell nuclear antigen (PCNA) immunohistochemistry and the Western blotting for Cx26 and Cx32 were performed. After 48 and 72 h of drug administration the liver-to-body weight ratio was increased 70% and 117% (p<0.0001), respectively. There were temporal-dependent alterations in liver histopathology and a significant increase in cell proliferation was noted after 48 h and sustained after 72 h, though to a lesser extent (p<0.0001). In addition, TCPOBOP administration induced apoptosis, which appeared to be time-dependent showing statistical significance only after 72 h (p<0.0001). Interestingly, a transient disruption by nearly 50% of GJIC capacity was detected after 48 h of drug ingestion, which recovered after 72 h (p=0.003). These GJIC changes were due to altered levels of Cx26 and Cx32 in the livers of TCPOBOP-treated mice. We concluded that a single administration of TCPOBOP transiently disrupted the levels of GJIC due to decreased expression of connexins and increased apoptotic cell death in mouse liver.

Gap junctional intercellular communication as a target for liver toxicity and carcinogenicity

Direct communication between hepatocytes, mediated by gap junctions, constitutes a major regulatory platform in the control of liver homeostasis, ranging from hepatocellular proliferation to hepatocyte cell death. Inherent to this pivotal task, gap junction functionality is frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity and carcinogenicity. In the present paper, the deleterious effects of a number of chemical and biological toxic compounds on hepatic gap junctions are discussed, including environmental pollutants, biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. Particular attention is paid to the molecular mechanisms that underlie the abrogation of gap junction functionality. Since hepatic gap junctions are specifically targeted by tumor promoters and epigenetic carcinogens, both in vivo and in vitro, inhibition of gap junction functionality is considered as a suitable indicator for the detection of nongenotoxic hepatocarcinogenicity.

Different mechanisms of modulation of gap junction communication by non-genotoxic carcinogens in rat liver in vivo

This is a comparative study of the mechanisms by which three different rodent non-genotoxic carcinogens modulate connexin-mediated gap junction intercellular communication in male rat liver in vivo. In the case of the peroxisome proliferating agent Wy-14,643, a non-hepatotoxic dose of 50mg/kg led to a marked loss of inter-hepatocyte dye transfer associated with a loss of both Cx32 and Cx26 protein expression. In contrast, p,p'-dichlorodiphenyltrichloroethane (DDT) at a non-hepatotoxic dose (25mg/kg) was not found to alter Cx32 or Cx26 expression or to produce a measurable Cx32 serine phosphorylation but did give a small, significant reduction of cell communication. Carbon tetrachloride (CCl(4)) did not affect cell communication (despite a small significant reduction of Cx32 content) at a non-hepatotoxic dose. Both loss of communication and Cx32 expression was observed only at a dose that caused hepatocyte toxicity as evidenced by increased serum alanine aminotransferase activity. Overall, the findings emphasise that loss of gap junctional communication in vivo can contribute to carcinogenesis by non-genotoxic carcinogens through different primary mechanism. In contrast to Wy-14,643 and DDT, the results with CCl(4) are consistent with a requirement for hepatotoxicity in its carcinogenic action.

Epigenetic Theories of Cancer Initiation

I argue that carcinogenic insults injure many cells rather than mutate a few. This results from evidence that such insults convert too many cells to a precancerous state and that too many of the converted cells then revert to plausibly involve mutation and its repair; from evidence that the delays between such insults and chemically demonstrable mutations are long enough to easily allow nonmutational mechanisms to work; from evidence that even ionizing radiation first acts on the cytoplasm and mainly affects cells unhit by it; from the fact that such insults induce proto-oncogene expression far too quickly to do so by mutation; and from the fact that fusions of various cells and cell parts show that the tumorous or nontumorous nature of the product depends on its cytoplasmic rather than its nuclear component. I further argue that reduced DNA methylation, modifications of the histone code, and tissue disorganization are the three main mechanisms of epigenetic cancer initiation. Hypomethylation would result from DNA excision repair. Moreover, a methyl-deficient diet is carcinogenic and demethylation is also known to be carcinogenic via the histone code. Finally, I strongly argue for tissue disorganization as a mechanism of cancer initiation. This results from evidence that skin carcinogens disrupt the dermal/epidermal connection and from the fact that tumorigens swiftly disrupt gap junctions, as well as from evidence that such disruption is tumorigenic.

Non-genotoxic carcinogens: early effects on gap junctions, cell proliferation and apoptosis in the rat

Non-genotoxic carcinogens are thought to induce tumour formation by disturbing the balance between cell growth and cell death. Gap junctions (GJ) contribute to the maintenance of tissue homeostasis by allowing the intercellular exchange of growth regulatory signals and potential inhibition of GJ intercellular communication through loss of connexin (Cx) plaques has been shown to be involved in the cancer process. We have investigated the time- and dose-dependent effects of the non-genotoxic hepatocarcinogens Wy-14,643, 2,3,7,8-tetrachlorodibenzo-p-dioxin, methapyrilene and hexachlorobenzene and the male rat kidney carcinogens chloroform, p-dichlorobenzene and d-limonene on gap junction plaque expression in relation to proliferation and apoptosis. With the exception of limonene, all non-genotoxic carcinogens significantly reduced the expression of GJ plaques containing Cx32 in their respective target tissue. No dose-dependent, significant effects were seen in non-target organs. Although alteration of Cx32 expression did not appear to correlate with induction of cell proliferation, out data suggest that the interaction of both processes-interference of GJ coupled with a proliferative stimulus (at the carcinogenic dose)-may be important in non-genotoxic carcinogenesis and provide a potential alert for non-genotoxic carcinogens in short-term toxicity tests.

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.

Detailed low-dose study of 1,1-bis(p-chlorophenyl)-2,2,2- trichloroethane carcinogenesis suggests the possibility of a hormetic effect

To obtain information on the effects of nongenotoxic carcinogens at low doses for human cancer risk assessment, the carcinogenic potential of the organochlorine insecticide, 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), in the liver was assessed in F344 rats. In experiment 1, 240 male animals, 21 days old, were administered 0, 0.5, 1.0, 2.0, 5.0, 20, 100 and 500 ppm DDT in the diet for 16 weeks. Experiment 2 was conducted to elucidate the carcinogenic potential of DDT at lower levels using 180 rats given doses of 0, 0.005, 0.01, 0.1, 0.2 and 0.5 ppm. The livers of all animals were immunohistochemically examined for expression of glutathione S-transferase placental form (GST-P), putative preneoplastic lesions. Quantitative values for GST-P-positive foci in the liver were increased dose-dependently in rats given 20 ppm DDT and above with statistical significance as compared with the concurrent control value. In contrast, doses of 0.005 and 0.01 ppm were associated with a tendency for decrease below the control value, although not significantly. Western blotting analysis show that cytochrome P-450 3A2 (CYP3A2) protein expression tended to decrease at 0.005 and 0.01 ppm, a good correlation being observed with the change in the number of GST-P-positive foci. These findings suggest that a DDT hepatocarcinogenicity may show nonlinear response, that is, hormetic response at low doses. Furthermore, since CYP3A2 protein expression appears to be important for the effects of phenobarbital and the alpha-isomer of benzene hexachloride, mRNAs for IL-1 receptor type 1 (IL-1R1) and TNF-alpha receptor type 1 (TNFR1) whose ligands have roles not only in downregulating CYP3A2 expression but also in inducing antiproliferative effect or apoptosis in hepatocyte were examined. Increase was observed at low doses of DDT. Oxidative stress in liver DNA, assessed in terms of 8-hydroxydeoxyguanosine as a marker, was also decreased. These findings suggest that the possible hormetic effect that was observed in our detailed low-dose study of DDT carcinogenesis, although not statistically significant, may be linked to levels of oxidative stress and proinflammatory cytokines.

Intercellular communication and cell proliferation in precision-cut rat liver slices: effect of medium composition and DDT

Gap junctional intercellular communication (GJIC) and cell proliferation were studied in control and 1,1'-bis(p-chlorophenyl)-2, 2,2,-trichloroethane (DDT) treated precision-cut liver slices of rat by evaluating connexin 32 (Cx32) expression and 5-bromo-2'-deoxyuridine (BrdU) incorporation. In addition, the effect of different culture media (RPMI and WME) on control and DDT influenced Cx32 expression and cell proliferation was determined. Cx32 expression in control precision-cut liver slices was maintained during 8 h of culturing, but decreased after prolonged culturing. Control cell proliferation was significantly higher when WME was used as culture medium than when RPMI was used. In slices treated with DDT Cx32 expression was decreased. In slices cultured in RPMI medium, this decrease preceded a dose-dependent increase in cell proliferation. These results show the usefulness of precision-cut liver slices in studying cellular proliferation and intercellular communication.

Development of gap junctional channels and intercellular communication in rat liver during ontogenesis

BACKGROUND/AIMS

We investigated the expression of connexin (Cx) 32 and 26 subunit proteins of the gap junction (GJ) in the rat liver during ontogenesis to clarify their roles in control of growth and differentiation, and observed their channels in association with development of gap junctional intercellular communication (GJIC).

METHODS

The expression of Cx32 and 26 in prenatal and postnatal livers was examined by Western blot and immunofluorescence. GJ channels were investigated not only by double immunofluorescence study but also by immunogold electron microscopy. The spread of lucifer yellow 5 min after its microinjection was examined in the cultured liver tissues.

RESULTS

1) Western blot showed the expression of both Cx from the late stage of gestation and their peak a week after birth. 2) Cx32- or 26-positive plaques were scattered on hepatocytes of the fetal liver and some of them were colocalized; both were increased just after birth. On day 7 after birth, Cx32-positive plaques were present on all hepatocytes within a lobule, and Cx26-positive plaques were distributed in the periportal area. 3) Double-immunogold electron microscopy just after birth showed that most GJ channels were homotypic type of Cx32 or 26, and that few were heterotypic. On day 7 after birth, most channels had the homotypic type of type of Cx32 in the middle and pericentral areas, and there was a heterotypic type of Cx32 and 26 in the periportal area. 4) The dye transfer of lucifer yellow showed a wider spread in the liver tissues on day 7 after birth than on day 1.

CONCLUSION

Increased GJ formation and compatibility or incompatibility of GJ channels are closely associated with development of GJIC, and GJIC may develop at cytodifferentiation during ontogenesis.

Inhibitory effects of organochlorine pesticides on intercellular transfer of Lucifer Yellow in cultured bovine oviductal cells

The present study investigated the effects of dichlorodiphenyltrichloroethane (DDT), methoxychlor (MXC), and gamma-hexachlorocyclohexane (gammaHCH, lindane) on gap junctional intercellular communication (GJIC) in cultured bovine oviductal cells. GJIC was evaluated by microinjecting fluorescent dye Lucifer Yellow and observing the inhibition of the spreading of dye into adjacent cells. After incubation for 1 h at 37 degrees C, a dose-dependent inhibition of GJIC was observed over a concentration range of 16 to 128 microM DDT, MXC, or gammaHCH compared with nonexposed controls. A significant inhibition began at 32 microM DDT, MXC, or gammaHCH. After incubation for 5 h, a dose-dependent inhibition of GJIC was obtained in the concentration range from 8 to 64 microM of the pesticides. The first significant inhibitory effect on GJIC was caused by 8 microM DDT, 16 microM MXC, and 32 microM gammaHCH. The 128 microM concentration of the pesticides was toxic. At pesticide concentration of 64 microM, the decrease in dye-coupling observed was not due to lethal cell injury, as is indicated by the use of trypan blue dye exclusion. After removal of 64 microM DDT from the culture medium, intercellular communication was reestablished within 3 h. Measurement of cytosolic free Ca2+ concentration [Ca2+]i in fura-2/AM-loaded oviductal cells showed that the inhibition of GJIC by addition of DDT, MXC, or gammaHCH was not associated with a detectable increase in [Ca2+]i. Coincubation of the DDT with dibutyryl-cAMP prevented the 64 microM DDT-induced inhibition of intercellular communication in adherent oviduct cells. It is suggested that organochlorine pesticides can influence cells responsible for reproduction.

Effect of phenobarbital on hepatic gap junctional intercellular communication in rats

The effects of in vivo exposure to phenobarbital (PB) on hepatic gap junctional intercellular communication (GJIC) and connexin protein expression in Sprague-Dawley rats were examined by in vivolin vitro dye-transfer assay, immunohistochemical staining, and by Western blot analysis. PB (50 mg/kg) was administered orally once a day for up to 6 wk. The average size of the dye spread after injection of Lucifer Yellow decreased at week 1 and remained at the same level until week 6. The area and number of connexin 32 (Cx32) spots per hepatocyte in the central zone of liver lobules decreased from week 1 to week 6, but no change of Cx32 spots in the peripheral zone was observed. The average area and number of connexin 26 (Cx26) spots per hepatocytes showed no clear change through the experimental periods. The decreased level of Cx32 protein in plasma membranes was observed in the PB group. These results suggest that PB, a liver tumor-promoting agent, inhibits hepatic GJIC in vivo in rats and that aberrant Cx32 protein expression and/or localization may be responsible for this effect.

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.

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

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

Effect of diverse tumor promoters on the expression of gap-junctional proteins connexin (Cx)26, Cx31.1, and Cx43 in SENCAR mouse epidermis

The inhibition of gap-junctional intercellular communication (GJIC) between initiated and surrounding normal cells by tumor promoters is believed to be important in the promotion stage of carcinogenesis. Therefore, we examined the effect of skin-tumor promoters on the expression of the gap-junctional proteins connexin (Cx) 26, Cx43, and Cx31.1 in SENCAR mouse skin. Animals were treated with 12-0-tetradecanoylphorbol-13-acetate (TPA) (8.3 nmol), okadaic acid (OA) (2.5 nmol), chrysarobin (220 nmol), or benzoyl peroxide (BzPo) (83 micromol). Northern blot and immunofluorescence analyses revealed that keratinocytes in adult mouse skin expressed Cx31.1 and Cx43 but not Cx26. All four of the skin-tumor promoters switched on the Cx26 gene, transiently increased expression of Cx43, and significantly inhibited the expression of Cx31.1. The time courses for changes in Cx26, Cx3l. 1, and Cx43 mRNA levels coincided in most cases and in general corresponded well to the time-response curves for hyperplastic changes in mouse skin. The peaks of Cx26 and Cx43 expression and Cx31.1 inhibition appeared 12 h after TPA application and 24 h after OA and chrysarobin application. BzPo elevated the levels of Cx26 and Cx43 transcripts later (peak at 2-4 d). In tumor promoter-treated skin, Cx26 and Cx43 plaques were on the plasma membrane of most keratinocytes. Cx31.1 staining was much weaker than in untreated epidermis. Thus, tumor promoters induce a large change in the expression of several Cxs, which in turn may affect both the level of GJIC and the sensitivity of GJlC to regulatory factors.