Reduced gap junctional intercellular communication and altered biological effects in mouse osteoblast and rat liver oval cell lines transfected with dominant-negative connexin 43

Gap junctional intercellular communication (GJIC) maintains normal growth and differentiation of cells in a tissue. The intercellular molecules traversing gap junctions are largely unknown, but the molecular weight (MW) cutoff is normally 1200 Da. No differences in dye transfer were observed in normal or vector controls of WB-F344 rat liver epithelial or mouse osteoblastic MC3T3-E1 cells with either Lucifer Yellow (LY) with a MW of 457 Da (LY-457) or LY with a MW of 649 Da (LY-649). Transfection of a dominant negative-connexin 43 (Cx43) gene decreased GJIC (>50%) when LY-649 was used, however, normal GJIC was observed in both cell lines when LY-457 was used. Therefore, the MW cut off in these clones was considerably less than the wild type. The dominant negative clones of the MC3T3-E1 cells exhibited over 90% less alkaline phosphatase (ALPase) activity and calcium deposition after the induction of differentiation. Similarly, dominant negative Cx43 inhibited gene expression of ALPase and bone sialoprotein but not osteocalcin in MC3T3-E1. WB-F344 cells normally exhibit a biphasic response to 12-O-tetradecanoylphorbol-13-acetate (TPA) where inhibition of GJIC recovers after 2 h, but the dominant negative clones showed no recovery from inhibition of GJIC by TPA. Dominant negative Cx43 also inhibited the formation of network-like structures by WB-F344 cells on Matrigel. These results demonstrate that the dominant negative gene transfected into cell types containing the wild-type connexins result in diminished channel sizes, thus allowing the determination of whether distinct biological endpoints, i.e., differentiation, are dependent upon either small or high MW intercellular signals.     

Inhibition of gap-junctional intercellular communication by environmentally occurring polycyclic aromatic hydrocarbons.

Polycyclic aromatic hydrocarbons (PAHs) are a broad class of ubiquitous environmental pollutants with known or suspected carcinogenic properties. Tumor promotion is a cell-proliferative step of cancer that requires the removal of cells from growth suppression via the inhibition of gap-junctional intercellular communication (GJIC). Inhibition of GJIC measured with an in vitro WB-F344 rat liver epithelial cell system was used to assess the relative potencies of 13 PAHs suggested by the U.S. Environmental Protection Agency (EPA) as the principal contaminants and 22 other PAHs, most of them identified in environmental samples. Maximal inhibition of GJIC was detected after 30 min of exposure, followed by a recovery in intercellular communication after an additional 30 min of exposure, suggesting a transient character of inhibition. Although microM concentrations of PAHs were required to reach the inhibition level equal to the model tumor promoter phorbol 12-myristate 13-acetate (IC50 = 8 nM), 12 of the PAHs under study were found to be strong inhibitors of GJIC (strongest effects were observed with fluoranthene, picene, 5-methylchrysene and nine additional PAHs). The other nine PAHs, including benzo[a]pyrene, inhibited GJIC only up to 50-75% of the control level. Interestingly, several high molecular weight PAHs with known strong carcinogenic properties possessed only weak (dibenzopyrenes) or no inhibition potency (dibenzofluoranthenes, naphtho[2,3-a]pyrene and benzo[a]perylene). Based on the IC50 values related to the reference PAH benzo[a]pyrene, we suggested arbitrary values of inhibition equivalency factors (GJIC-IEFs) ranging from 0 (noninhibiting PAHs) to 10.0 (strongest inhibitors), suitable for the purposes of environmental risk assessment.     

Two formulations of the agricultural pesticide adjuvant, Toximul, reduce the glycogen content of HepG2 cells

Young mice exposed dermally to the Toximul (Tox) class of agricultural pesticide adjuvants have reduced levels of hepatic glycogen, a marker of subclinical toxicity. The present study determined whether these effects on glycogen also occurred in cultured HepG2 cells. Exposure (3 hr) to Tox resulted in significant, concentration-dependent glycogen reductions (up to 70%) relative to control values (76 +/- 3 microg glycogen/mg protein). These reductions did not appear to be due to loss of cell viability, and were reversible with Tox removal. Two different formulations of Tox (3409F and MP-A) differed significantly in the magnitudes of glycogen reduction in the HepG2 cells.     

Cigarette smoke components inhibited intercellular communication and differentiation in human pancreatic ductal epithelial cells

Smoking is a well-documented risk factor for the development of pancreatic adenocarcinoma. Although the most abundant polycyclic aromatic hydrocarbons (PAHs) in cigarette smoke are methylated anthracenes and phenanthrenes, the epigenetic toxicity of these compounds has not been extensively studied. We previously showed that methylanthracenes, which possess a bay-like structure, affect epigenetic events such as an induced release of arachidonic acid, inhibition of gap junctional intercellular communication (GJIC) and induction of mitogen-activated protein kinases in a pluripotent rat liver epithelial stem cell line. Anthracenes with no bay-like structures were inactive. These biological effects are all molecular events associated with the promotional phase of cancer. A human immortalized, nontumorigenic pancreatic ductal epithelial cell line, H6c7, was examined to study the epigenetic toxicity of PAHs related to pancreatic cancer by using scrape-loading dye transfer, immunostaining, RT-PCR and telomerase assay methods. H6c7 cells were GJIC-incompetent and exhibited high telomerase activity when grown in growth factor and hormone-supplemented medium. In the presence of the cAMP elevating drugs (forskolin and IBMX) the cells became GJIC competent and expressed connexins. Telomerase activity was also decreased by cAMP elevating drug treatment. After induction of cAMP, 1-methylanthracene with bay-like structures inhibited GJIC, whereas the 2-methylanthracene lacking a bay-like structure had no effect on GJIC. Telomerase activity remained high in 1-methylanthracene treatment but not with 2-methylanthracene. These results indicate that a prominent component of cigarette smoke, namely methylanthracenes with distinct structural configurations, could be a potential etiological agent contributing to the epigenetic events of pancreatic cancer.     

Inhibition of gap junctional intercellular communication by perfluorinated compounds in rat liver and dolphin kidney epithelial cell lines in vitro and Sprague-Dawley rats in vivo.

Gap junctional intercellular communication (GJIC) is the major pathway of intercellular signal transduction, and is thus important for normal cell growth and function. Recent studies have revealed a global distribution of some perfluorinated organic compounds, especially perfluorooctane sulfonic acid (PFOS) in the environment. Because other perfluoroalkanes had been shown to inhibit GJIC, the effects of PFOS and related sulfonated fluorochemicals on GJIC were studied using a rat liver epithelial cell line (WB-F344) and a dolphin kidney epithelial cell line (CDK). In vivo effects on GJIC were studied in Sprague-Dawley rats orally exposed to PFOS for 3 days or 3 weeks. Effects on GJIC were measured using the scrape loading dye technique. PFOS, perfluorooctane sulfonamide (PFOSA), and perfluorohexane sulfonic acid (PFHA) were found to inhibit GJIC in a dose-dependent fashion, and this inhibition occurred rapidly and was reversible. Perfluorobutane sulfonic acid (PFBS) showed no significant effects on GJIC within the concentration range tested. A structure activity relationship was established among all 4 tested compounds, indicating that the inhibitory effect was determined by the length of fluorinated tail and not by the nature of the functional group. The results of the studies of the 2 cell lines and the in vivo exposure were comparable, suggesting that the inhibitory effects of the selected perfluorinated compounds on GJIC were neither species- nor tissue-specific and can occur both in vitro and in vivo.