Effect of okadaic acid on hepatocyte structure and function

The aim of this study with rat hepatocytes was to describe the effect of okadaic acid (OKA) (a potent and specific inhibitor of protein phosphatases) on the biosynthesis, processing and/or secretion of various lipid and protein molecules. Gel radioautograms indicated that low concentrations of okadaic acid (100 nM) induced hyperphosphorylation of a number of hepatocyte phosphoserine/threonine residues in the Mr range of 35-220 kDa. The effects of okadaic acid on the morphology of the hepatocytes was time and dose-dependent; early changes included cell rounding, loss of typical Golgi staining of beta COP, and fragmentation of the Golgi compartment at the EM level. General hepatocyte cell functions such as protein synthesis, lactate dehydrogenase activity, and ATP levels were unchanged with 100 nM okadaic acid as were all hepatocyte functions carried out in the endoplasmic reticulum of these cells. As such, incubation with okadaic acid did not alter the biosynthesis of phosphatidylcholine (from labeled choline), or very low density lipoproteins (VLDL) from labeled fatty acids or glycerol. Likewise, the biosynthesis of various endoplasmic reticulum synthesized proteins (transferrin, albumin, apolipoprotein E, and HMG CoA Reductase) continued normally in the presence of okadaic acid. However, incubation with okadaic acid led to major changes in all hepatocyte functions normally carried out in the Golgi compartment; i.e., the incorporation of labeled ceramide into sphingomyelin was profoundly reduced, as was the Golgi-required packaging and secretion of various proteins synthesized in the endoplasmic reticulum. These findings point to the Golgi compartment as an specific target for okadaic acid and suggest that one or more okadaic acid-sensitive phosphoproteins may be involved in maintaining its normal structure and function.