Dietary sphingolipids suppress a subset of preneoplastic rat liver lesions exhibiting high PTEN, low phospho-Akt and high levels of ceramide species

Abstract

Rat liver glutathione- S-transferase Pi-(GST-P)-positive enzyme-altered foci (EAF) are preneoplastic lesions that develop in response to carcinogenic stress. They are often used as endpoints in e.g. chemopreventive studies. In this study we characterize a pAkt-negative/ceramide-positive (pAkt−/cer+) EAF phenotype, as defined by immunohistochemistry for pAkt and ceramide species, in diethylnitrosamine(DEN)-, phenobarbital- or aflatoxinB1-treated rats. There was a close to 100% overlap for the pAkt and the ceramide marker. Furthermore, serial sections stained for PTEN indicated a close correlation between PTEN-positive and pAkt-negative lesions in DEN-treated rats. Experiments with DEN-treated rats given sphingomyelin in the diet suggested that sphingomyelin selectively targeted these lesions. In in vitro experiments sphingosine rapidly decreased pAkt levels in hepatocytes, and in experiments with hepatocytes from DEN-treated rats sphingosine selectively killed EAF cells. Furthermore, pretreatment with antisense Akt oligonucleotides in vitro sensitized non-EAF hepatocytes, so that EAF and non-EAF cells became equally sensitive to sphingosine. It is concluded that rat liver, in response to carcinogenic stress, develops a distinct EAF phenotype exhibiting low pAkt levels and concomitant alterations in sphingolipid metabolism. Our data also suggest that pAkt−/cer+ EAF are selectively targeted by sphingolipids in the diet and that lesions with this phenotype should be of particular interest for future studies on chemopreventive effects that may affect sphingolipid metabolism.