Glutathione S-transferase isoenzyme patterns in different subtypes of enzyme-altered rat liver foci treated with the peroxisome proliferator nafenopin or with phenobarbital.

It is known that phenobarbital (PB) and the peroxisome proliferator (PP) nafenopin (NAF) promote tumor formation by stimulating selective growth of different subtypes of liver foci. While PB enhanced the gamma-glutamyltranspeptidase (GGT)-positive eosinophilic-clear cell foci (ECF), NAF amplified the GGT-negative weakly basophilic foci (WBF). These findings provide the possibility of using the occurrence of these foci subtypes as early indicators for the carcinogenic potential of PB- and PP-type promoters. In order to improve the methods for the discrimination between ECF and WBF we studied further differences in their phenotype, as determined by the expression pattern of glutathione S-transferase (GST) subunits. GST subunits of the alpha (Ya, Yc), mu (Yb1, Yb2) and pi family (Yp), which compose different GST isoenzymes, were demonstrated by immunohistochemical methods. ECF were the only foci subpopulation that expressed GST subunit Yp, while this subunit was always absent in WBF and in another focus subtype, the tigroid foci (TF). Neither PB nor NAF changed this pattern. Thus Yp expression was rather a function of the focus type than of the promoter used. Upon PB treatment expression of the GST subunits Yb1 and Yb2 was frequently elevated in ECF, while Ya and Yc remained more or less unchanged. In NAF-treated livers large WBF, however, showed diminished expression of all investigated GST subunits of the alpha and mu family. In conclusion, PB seems to promote mostly ECF with elevated levels of mu and pi class GSTs, while low levels or absence of all GSTs tested may be associated with growth selection of WBF through the PP NAF.