Membrane and cytosolic protein phosphorylation patterns in the early stages of DEN-induced hepatocarcinogenesis in rats fed a high or low protein diet

The membrane and cytosolic protein phosphorylation patterns in the early stages of diethylnitrosamine-induced rat liver carcinogenesis, promoted by 2-acetylaminofluorene in the diet plus partial hepatectomy (DEN-AAF-PH), were analyzed by two-dimensional gel electrophoresis in animals fed a low protein (5% casein) diet, or the original high protein (24% casein) diet, in order to modulate the development of GST-P-positive preneoplastic lesions. Compared with untreated controls, membrane and cytosolic protein phosphorylation patterns changed only slightly in low protein-fed rats 7 days post-hepatectomy, with no appearance of enzyme-altered hyperplastic foci in the liver sections. By contrast, high protein-fed animals demonstrated GST-P-positive preneoplastic lesions 7 days post-hepatectomy and several acidic and more basic high Mr phosphorylated membrane (between 97 and 116 kDa) as well as cytosolic (between 97 and 200 kDa) proteins could be detected. In the presence of enzyme-altered hepatocytes in the liver sections, low protein-fed rats demonstrated at 60 days post-hepatectomy cytosolic protein phosphorylation patterns remarkably similar to those shown by 24% casein-fed animals at 7 days posthepatectomy, suggesting close correlation between protein phosphorylation patterns and development of preneoplastic lesions during the early stages of DEN-AAF-PH liver carcinogenesis. This may arise by a constitutive activation of one or more signal transduction pathways, possibly involving protein kinase C, during liver tumour promotion.