Flavonoids protect neurons from oxidized low-density-lipoprotein-induced apoptosis involving c-Jun N-terminal kinase (JNK), c-Jun and caspase-3

Oxidative stress has been associated with neuronal loss in neurodegenerativ e diseases and during age-associated cognitive decline. Flavonoids have bee n proposed to play a useful role in protecting the central nervous system a gainst oxidative and excitotoxic stress, although the mechanism of action i s unknown. Using oxidized low-density lipoprotein (oxLDL) as the oxidative insult we investigated the mechanism of neurotoxicity and attempted to iden tify possible sites of action of two of the most potent protective flavonoi ds, epicatechin and kaempferol, in cultured primary neurons. Using cultured striatal neurons and selective phosphospecific antibodies we addressed the potential role of extracellular signal-regulated kinases 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK). OxLDL stimulated a Ca2+-dependent activation of both ERK1/2 and JNK that was strongly inhibited by pre-treatment with l ow micromolar concentrations of epicatechin. Neurotoxicity induced by oxLDL , however, was neither reduced nor enhanced by inhibiting ERK1/2 activation with mitogen-activated protein kinase kinase (MEK) inhibitors, suggesting that this cascade is unlikely to be involved in either oxLDL toxicity or th e protective effects of flavonoids. oxLDL caused a sustained activation of JNK that resulted in the phosphorylation of the transcription factor c-Jun, which was abolished in neurons pre-treated with flavonoids. Furthermore, o xLDL induced the cleavage of procaspase-3 and increased caspase-3-like prot ease activity in neurons, an effect which was strongly inhibited by pre-exp osure to either epicatechin or kaempferol. In addition, a caspase-3 inhibit or reduced oxLDL-induced neuronal death, implicating an apoptotic mechanism . A major in vivo metabolite of epicatechin, 3'-O-methyl-epicatechin was as effective as epicatechin in protecting neurons. Thus dietary flavonoids mi ght have potential as protective agents against neuronal apoptosis through selective actions within stress-activated cellular responses, including pro tein kinase signalling cascades.