Role of enzymes involved in the 4-hydroxy-2,3-nonenal metabolism

Oxidative stress arises from an imbalanced redox status between the production of reactive oxygen species (ROS) and the biological systems able to remove them. A common biochemical consequence of oxidative stress is the induction of lipid peroxidation phenomena and the most intensively studied end product of lipid peroxidation is 4-hydroxy-2,3-nonenal (HNE). It is known that in carcinogenesis, increased ROS level can inhibit tumor cell growth and high concentrations of HNE can induce apoptosis in cancer cells. Indeed, several anticancer drugs and radiation therapy, that increase oxidative stress, can overcome the antioxidant defenses of cancer cells and drive them to apoptosis. However recent advances shown an important and physiological role of HNE in cell signalling (Parola et al. 1999). In order to evaluate the contribution of HNE and its metabolites to redox signaling and proliferative/apoptotic events, it is essential to identify the metabolism of these molecule not only under physiological condition but also undergoing stress conditions. The research started on the study of the pattern of HNE-metabolizing enzymes in different cell lines and then it was focused on the modulation of their activity in a specific cell line, the human astrocytoma ADF cells, exposed to different oxidative stress conditions. In these cells the major route of HNE metabolism seems to be the conjugation with glutathione by glutathione S-transferase enzymes to form the GS-HNE which can be metabolize essentially by NADPH/NADP+-dependent pathways. ADF cells seem to be resistant to oxidative stress and able to metabolize toxic aldehydes even after oxidative insult. The main consequences of hydrogen peroxide treatment consist in a reversible inactivation of the NAD+-dependent dehydrogenases which are not involved in the aldehyde oxidation and an increased level of glutathione. It was also relevant the identification in cellular crude extract of a novel NADP+-dependent enzymatic activity able to oxidize GS-HNE which could in part explain ADF resistance to the oxidative insult. The enzyme was isolated and purified from ADF cells and preliminary kinetic characterization was reported.