TPA inhibition of gap junctional intercellular communication in Syrian hamster embryo cells through two different pathways

Pharmacological evidence for system-dependent involvement of protein kinase C isoenzymes in phorbol ester-suppressed gap junctional communication

Several phorbol esters are potent activators of protein kinase C. They down-regulate gap junctional intercellular communication and induce phosphorylation of connexin43, but the sensitivity and extent of responses vary much between systems. We asked whether the total protein kinase C enzyme activity or the protein kinase C isoenzyme constitution was of importance for such variations. Some fibroblastic culture systems were compared. It was concluded that the total protein kinase C enzyme activity did not determine the sensitivity to phorbol esters. Furthermore, the use of isotype-specific inhibitors of protein kinase C indicated that protein kinase C alpha, delta, and epsilon may be involved to different extents in different fibroblastic systems in the response to phorbol esters.

Morphological transformation and effect on gap junction intercellular communication in Syrian hamster embryo cells as screening tests for carcinogens devoid of mutagenic activity

A large fraction of chemicals observed to cause cancer in experimental animals is devoid of mutagenic activity. It is therefore of importance to develop methods that can be used to detect and study environmental carcinogenic agents that do not interact directly with DNA. Previous studies have indicated that induction of in vitro cell transformation and inhibition of gap junction intercellular communication are endpoints that could be useful for the detection of non-genotoxic carcinogens. In the present work, 13 compounds [chlordane, Arochlor 1260, di(2-ethylhexyl)phthalate, 1,1,1-trichloro-2, 2-bis(4-chlorophenyl)ethane, limonene, sodium fluoride, ethionine, o-anisidine, benzoyl peroxide, o-vanadate, phenobarbital, 12-O-tetradecanoylphorbol 13-acetate and clofibrate] have been tested for their ability to induce morphological transformation and affect intercellular communication in Syrian hamster embryo cells. The substances were selected on the basis of being proven or suspected non-genotoxic carcinogens, and thus difficult to detect in short-term tests. The data show that nine of the 13 compounds induced morphological transformation, and seven of the 13 inhibited intercellular communication in hamster embryo cells. Taken together, 12 of the 13 substances either induced transformation or caused inhibition of communication. The data suggest that the combined use of morphological transformation and gap junction intercellular communication in Syrian hamster embryo cells may be beneficial when screening for non-genotoxic carcinogens.

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.

Effect of 1,1'-(2,2,2-trichloroethylidene)-bis(4-chlorobenzene) (DDT) on gap junctional intercellular communication and morphological transformation of Syrian hamster embryo cells

The organochlorine insecticide 1,1'-(2,2,2-trichloroethylidene)bis(4-chlorobenzene) (DDT) did not induce or promote induction of morphological transformation in Syrian hamster embryo (SHE) cells, but it was a potent inhibitor of gap junctional intercellular communication (GJIC). The kinase inhibitor staurosporine did not affect DDT induced inhibition of GJIC, although it has been shown to decrease the inhibitory effect of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on GJIC. In addition, pretreatment with TPA made the cells refractory to further TPA induced inhibition of GJIC, while they remained sensitive to DDT. Thus, DDT and TPA inhibit GJIC through different mechanisms. Elevation of cellular cyclic adenosine monophosphate (cAMP) level by exposure to forskolin counteracted the inhibitory effect of DDT similar to that observed for TPA. Continuous exposure to DDT at concentrations near the effective concentration (50%) value (EC50 value) resulted in a slight recovery of GJIC following the initial inhibition. This recovery was not accompanied by the cells becoming refractory to further DDT induced inhibition of GJIC. The recovery of GJIC after removal of the DDT containing medium seemed to be related to a reduction in the amount of cell-associated DDT.