Cellular and molecular methods to study the role of gap junctional intercellular communication in toxicology

Models and methods for in vitro testing of hepatic gap junctional communication

Inherent to their pivotal roles in controlling all aspects of the liver cell life cycle, hepatocellular gap junctions are frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity. Hepatic gap junctions, which are mainly built up by connexin32, are specifically targeted by tumor promoters and epigenetic carcinogens. This renders inhibition of gap junction functionality a suitable indicator for the in vitro detection of nongenotoxic hepatocarcinogenicity. The establishment of a reliable liver gap junction inhibition assay for routine in vitro testing purposes requires a cellular system in which gap junctions are expressed at an in vivo-like level as well as an appropriate technique to probe gap junction activity. Both these models and methods are discussed in the current paper, thereby focusing on connexin32-based gap junctions.

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.

Carotenoids in health and disease: recent scientific evaluations, research recommendations and the consumer

In addition to his many research achievements, James Allen Olson made important contributions to evaluations of the role of dietary carotenoids in prevention of chronic diseases such as cancer, heart disease and age-related macular degeneration in well-nourished populations. This paper reviews recent scientific evaluations of the role of carotenoids in disease, recommendations for future research, and consumer-related trends of relevance to carotenoids. Authoritative scientific evaluations by key leading thinkers have not been able to ascribe a disease prevention function to carotenoids because of the absence of definitive evidence. These leading thinkers recommend that future research on the role of carotenoids in disease focus on the complexities of diet, genetics and environment in the disease process. This research is important to make it possible for consumers to derive optimal health promoting benefits from fruits and vegetables in the context of changing dietary patterns.

The use of structure-activity relationships and markers of cell toxicity to detect non-genotoxic carcinogens

In contrast to the situation for genotoxic carcinogens, few in vitro tests exist that can detect early markers of the events thought to be associated with non-genotoxic carcinogenesis. Also, comparatively little is known about the quantitative structure-activity relationships (Q)SARs of these agents. This review discusses published SAR studies conducted on non-genotoxic carcinogens, in relation to the use of several markers of in vitro cell toxicity (inhibition of gap-junctional intercellular communication, inhibition of tubulin polymerization, modulation of apoptosis and induction of cell proliferation), which are used as endpoints for screening this class of carcinogen. Much of the work has involved the identification of new biophores (substructural features of molecules associated with toxicity), as well as other structural features, which are thought to predispose the chemicals to ligand binding with specific target molecules acting as possible receptors (e.g. protein kinase C, the oestrogen, peroxisome-proliferator and tubulin protein receptors), implicated in the mechanism of toxicity involved. It is concluded that (a) there is an urgent need for more information on (Q)SARs for non-genotoxic carcinogens; (b) this information should be acquired by using several different approaches in a variety of laboratories; and (c) such research should proceed together with more studies on the mechanisms of cell toxicity caused by these chemicals, including the identification and characterisation of further specific receptors involved in mediating the various types of cell toxicity associated with this type of carcinogenesis.

Intercellular communication in the immune system: differential expression of connexin40 and 43, and perturbation of gap junction channel functions in peripheral blood and tonsil human lymphocyte subpopulations

The distribution and function of connexins (integral membrane proteins assembled into gap junction intercellular communication channels) were studied in human lymphocyte subpopulations. The expression of mRNA encoding connexins in peripheral blood and tonsil-derived T, B and natural killer (NK) lymphocytes was examined. Connexin43 (Cx43) mRNA was expressed in peripheral blood and tonsil lymphocytes, but Cx40 mRNA expression was confined to tonsil-derived T and B lymphocytes; Cx26, Cx32, Cx37 and Cx45 were not detected by reverse transcription-polymerase chain reaction (RT-PCR). Western blot analysis also demonstrated the presence of Cx40 and Cx43 proteins in T and B lymphocytes in a manner coincidental to the mRNA detection. Stimulation in vitro of T and B lymphocytes with phytohaemagglutinin (PHA) and lipopolysaccharide (LPS), respectively, increased Cx40 and Cx43 protein expression. Flow cytometric analysis, using antibodies to extracellular loop amino acid sequences of connexins, confirmed the surface expression of connexins in all lymphocyte subpopulations. Assembly of connexins into gap junctions providing direct intercellular channels linking attached lymphocytes was demonstrated by using a dye transfer technique. The exchange of dye between lymphocytes was inhibited by a connexin extracellular loop mimetic peptide and alpha-glycyrrhetinic acid, two reagents that restrict intercellular communication across gap junctions. Dye coupling occurred between homologous and heterologous co-cultures of T and B lymphocytes, and was not influenced by their stimulation with PHA and LPS. The connexin mimetic peptide caused a significant decrease in the in vitro synthesis of immunoglobulin M (IgM) by T- and B-lymphocyte co-cultured populations in the presence or absence of stimulation by PHA. The results identify connexins as important cell surface components that modulate immune processes.

Biological activities of Apo-canthaxanthinoic acids related to gap junctional communication

Apo-carotenoids with different numbers of conjugated double bonds are formed upon eccentric cleavage of carotenoids. These compounds may exhibit biological activities similar to those of the parent carotenoids or their central cleavage products, the retinoids. 11-Apo-canthaxanthin-11-oic acid, 13-apo-canthaxanthin-13-oic acid, and 14'-apo-canthaxanthin-14'-oic acid, carrying 2, 3, or 5 conjugated double bonds in the polyene chain, respectively, were tested for their effects on gap junctional communication (GJC), on stabilization of connexin43 mRNA, and on the activation of the retinoic acid-beta2 receptor (RAR-beta2 receptor); the effects were compared to those of retinoic acid and 4-oxo-retinoic acid, known to stimulate GJC and to activate the RAR-beta2 receptor. The effects of 4-oxo-retinoic acid were comparable to those of retinoic acid. 4-Oxo-retinoic acid, like retinoic acid, influences the stability of connexin 43 mRNA via elements located in the 3'-UTR. No effects were observed with the short-chain apo-canthaxanthinoic acids. A small but statistically significant induction of GJC and transactivation activity towards the RARbeta2 was found with 14'-apo-canthaxanthin-14'-oic acid. This might be due to biological effects of the compound itself or to biologically active breakdown products. The data suggest that the major biological effects of canthaxanthin on retinoid signaling pathways are related to activities mediated by the products of the central cleavage.

Dietary carotenoids and lung cancer: a review of recent research

Several hundred carotenoid research studies have been published since 1996, when two major intervention trials showed a lack of protective effect of beta-carotene supplements against lung cancer. Recent epidemiologic studies continue to show an association between high dietary intake of beta-carotene and lower risk of lung cancer. New research is attempting to clarify the apparently contradictory results of intervention and epidemiologic studies. Promising areas of investigation include characterizing biologic activities of carotenoids and gaining further insight into whether they may serve primarily as markers for a healthy lifestyle or diet.