Effect of some carcinogenic and non-carcinogenic polycyclic aromatic hydrocarbons on gap junction intercellular communication in hepatoma cell cultures

One of the systems that regulate tissue homeostasis is gap junction intercellular communication (GJIC). It is accepted that the down-regulation of GJIC is linked to the tumor-promoting properties of carcinogens. In this study, the effect of some carcinogenic and non-carcinogenic polycyclic aromatic hydrocarbons (PAH) on GJIC was investigated. It was found that in hepatoma cell culture (Hep G2) carcinogenic PAH inhibited GJIC after 24 h exposure by 75-100% depending on the PAH structure. The inhibition effect on GJIC is reversible because removing the PAH by changing of culture medium restores the GJIC. The non-carcinogenic PAH do not significantly influence GJIC. alpha-Naphthoflavone, an inhibitor of PAH metabolism, has no effect on inhibition of GJIC by carcinogenic PAH. 2,3,7,8-Tetrachloro-p-dibenzodioxin, an aryl hydrocarbon (Ah) receptor ligand, inhibits GJIC by about 50% only after 48 h exposure. To clarify the role of formation of PAH metabolites and interaction with Ah receptor on inhibition of GJIC, we determined the effect of benzo/a/pyrene on hepatoma G27 cells in which neither mRNA of CYP1A1 nor Ah receptor was determined. As in Hep G2 cells, benzo/a/pyrene, unlike non-carcinogenic benzo/e/pyrene, inhibits GJIC. We conclude that in the studied hepatoma cells carcinogenic PAH inhibit GJIC directly (that is, not via their metabolites) and this effect is not associated with Ah receptor interaction.     

Inhibition of Gap Junction Intercellular Communications in Cell Culture by Polycyclic Aromatic Hydrocarbons (PAH) in the Absence of PAH Metabolism

We have studied the effect of polycyclic aromatic hydrocarbons (PAH) on gap junction intercellular communications (GJIC) in culture of hepatoma cells Hep G2 and G27. Carcinogenic PAH inhibited GJIC in both cultures in contrast to non-carcinogenic PAH. We showed that both constitutive and inducible expressions of mRNAs of Ah receptor and cytochrome P4501A1 (the main isoform involved in PAH metabolism) were absent in hepatoma G27 cells. We concluded that the initial, non-metabolized molecules of carcinogenic PAH are responsible for changes in GJIC through interaction with an unknown factor in the cellular membrane.     

A common pathway in differentiation and inflammation: p38 mediates expression of the acute phase SIP24 iron binding lipocalin in chondrocytes

SIP24 is an acute phase iron binding lipocalin physiologically expressed in vivo in developing cartilage by prehypertrophic/hypertrophic chondrocytes. Taking advantage of the chondrocytic cell line MC615 and using SIP24 as a marker we investigated the pathways active in cartilage differentiation and inflammation. MC615 cells were cultured as: (i) proliferating prechondrogenic cells expressing type I collagen (ii) differentiated hyperconfluent cells expressing Sox9 and type II collagen. In proliferating cells the pathway PKC/ERK1, ERK2 was activated and SIP24 was not expressed while in differentiated cells the pathway p38/NF-kappaB was activated and SIP24 was expressed. Proliferating cells treated with inflammatory agents expressed a large amount of SIP24 and showed activation of p38/NF-kappaB pathway and inhibition of PKC/ERK1, ERK2 pathway indicating that in inflammation and differentiation the same factors are activated (p38, NF-kappaB) or inactivated (PKC, ERKs). Treatment of proliferating cells with the p38 specific inhibitor SB203580 inhibited the inflammation induced activation of p38 and the synthesis of SIP24. PMA treatment induced activation of PKC, inactivation of p38 and suppression of SIP24 synthesis, suggesting that PKC activation inhibits p38 activation. In differentiated hyperconfluent cells the same factors (p38/NF-kappaB/SIP24) are constitutively activated: treatment with inflammatory agents does not increase synthesis of SIP24 while treatment with SB203580 and with PMA does not repress activation of p38 nor synthesis of SIP24. We propose that the SIP24 stress related protein is expressed via p38 activation/NF-kappaB recruitment both in chondrocyte differentiation and inflammation and that a signaling pathway active in the acute phase response is physiologically activated in differentiation.