Effect of malondialdehyde–acetaldehyde–protein adducts on the protein kinase C-dependent secretion of urokinase-type plasminogen activator in hepatic stellate cells

Abstract

Previous studies from our laboratory have shown that malondialdehyde–acetaldehyde–protein adducts (MAA adducts) are formed in hepatocytes of ethanol-fed rats and directly influence the hepatic stellate cells (HSCs) to induce their secretion of chemokines and to up-regulate their expression of adhesion molecules. Since protein kinase C (PKC) is known to play a major role in many diverse intracellular signal transduction processes, we investigated whether MAA adducts influence the function of HSCs via a PKC-dependent pathway. HSCs in culture were exposed to MAA adducts, and PKC activity was determined. We observed a time- and concentration-dependent activation of PKC when cultures were exposed to BSA–MAA as compared with unmodified BSA. Using PKC isoform-specific inhibitors, we also showed that BSA–MAA induces the activation of a specific isoform of PKC, PKC-α, in HSCs. No activation of PKC was observed when HSCs were exposed to other aldehyde adducts such as BSA–acetaldehyde or BSA–malondialdehyde, indicating that the effects of MAA adducts on HSCs were somewhat specific. We further examined whether the observed increase in PKC activation induced by MAA adducts in HSCs, in turn, causes a functional effect. We observed that BSA–MAA induces the increased secretion of urokinase-type plasminogen activator, a key component of the plasmin-generating system, and that PKC activation is necessary for this enhanced urokinase-type plasminogen activator secretion. These results indicate that MAA adducts via a PKC-mediated pathway may regulate plasmin-mediated matrix degradation in the liver, thereby contributing to the progression of hepatic fibrosis.