A major aldehydic end product of the peroxidation of arachidonic acid, 4-hydroxy-2,3-nonenal (HNE), has recently been considered for its potential involvement in a variety of cell functions. Here we report on the differential regulation of rat hepatocyte protein kinase C (PKC) isoforms by concentrations of HNE actually detectable in specific biological fluids or tissues. PKC I and, to a much greater extent, PKC II activities were markedly increased by 0.1 mol/L HNE (final concentration in cell medium) whereas they were unaffected or even inhibited by 1 to 10 mol/L HNE. On the contrary, the calcium independent PKC ␦ activity was inhibited by 0.1 mol/L and increased by 1 and 10 mol/L. The carbonyl compound 4-hydroxy-2,3-nonenal (HNE) is one of the major products of the oxidative breakdown of -6 fatty acids, the featuring process of membrane lipid peroxidation. 1 This aldehyde, being strongly electrophylic, readily reacts at neutral or slightly alkaline pH with sulphydryl and amino groups. HNE production was quantified several years ago in rat liver microsomes and isolated hepatocytes undergoing various experimental conditions of enhanced lipid peroxidation. 2,3 In more recent in vivo experimental reproduction of liver 4 and arterial 5,6 fibrosclerotic lesions, the detectable steady-state HNE concentration reached the low-micromolar range, up to 5 to 10 µmol/L.Until rather recently, in vitro biochemical studies were oriented towards seeking evidence of the many biochemical effects of HNE, generally using concentrations above 10 µmol/L; these were normally performed by adding the aldehyde to cell cultures or suspensions or cellular subfractions. 1 Such an approach was adopted to counteract the high aldehyde metabolism shown by the majority of the cell types tested 7 ; however, it directed the investigation to quantities of HNE of rather unlikely biological interest. Further, the overall negative effects exerted by relatively high HNE dosage contributed to the short-sighted opinion that this and related lipid peroxidation products were simply detrimental and acutely toxic to cells and tissues and consequently were of limited interest in pathophysiology.This state of affairs was rectified in particular by extensive research on the role of oxidatively modified low density lipoproteins in the pathogenesis of atherosclerosis 8 and the detection in the human plasma of autoantibodies against HNE-modified low-density lipoproteins 9 or of HNE-modified proteins, 10 and a role of HNE in pathophysiology became more favorably considered.At present, because of the use of specific monoclonal antibodies raised against tissue HNE-protein adducts, the number of human diseases with detectable accumulation of HNE is rapidly growing: increased HNE levels have been recognized in the liver of patients with alcohol-induced Abbreviations: HNE, 4-hydroxy-2,3-nonenal; PKC, protein kinase C; CD, cathepsin D; DAG, diacylglycerol.From the