Inhibition of intercellular communication of rat hepatocytes by nafenopin: involvement of protein kinase C.

Peroxisome proliferators (PPs) have been shown to cause tumours in rodent liver. The mechanism of action of these chemicals is only poorly understood. Current evidence, however, suggests that they may cause tumours through a tumour promoting activity. In the present study we therefore evaluated the effect of three peroxisome proliferators on gap junctional intercellular communication (IC) of cultured hepatocytes. Interference with IC is thought to be one of the mechanisms involved in tumour promotion. IC was detected by dye coupling of hepatocytes using microinjection of Lucifer Yellow CH. Five hours after plating, coupling of the cells amounted to approximately 90%. Incubation of hepatocytes with the PPs mono(2-ethylhexyl)phthalate (MEHP), nafenopin and [4-chloro-6-(2,3- xylidino)-2-pyrimidylthio]acetic acid (Wy-14,643) decreased dye coupling of the hepatocytes. Half maximal effects were obtained at approximately 50 microM nafenopin, 150 microM Wy-14,643 and 200 microM MEHP. Addition of the specific inhibitor of Ca(2+)-dependent protein kinase C isoenzymes, Gö 6976 (2 microM), prevented inhibition of IC by nafenopin, but not by the two other peroxisome proliferators. Further studies suggest significant differences in the mechanisms underlying inhibition of dye coupling between hepatocytes by nafenopin and by phenobarbital, a known tumour promoter in the liver. The results show that the PPs nafenopin, MEHP and Wy-14,643 decrease IC between cultured hepatocytes. Inhibition of IC by nafenopin, but not by MEHP and Wy-14,643, is most likely mediated by Ca(2+)-dependent protein kinase C isoenzymes.