The heme oxygenase/biliverdin reductase system: a potential drug target in Alzheimer s disease

Alzheimer s disease (AD) is a neurodegenerative disorder characterized by the progressive loss of cognitive function, the inability to perform the activities of daily living and psychiatric symptoms. The formation of toxic aggregates of amyloid-beta-peptide (Abeta), through the activities of beta - and gamma- secretases, is considered as the earlier event in the pathogenesis of the disease. The deposition of both Abeta and the following hyperphosphorylation of tau protein, trigger an exaggerate immune-inflammatory response culminating with the production of excess reactive oxygen and nitrogen species responsible for damage on cellular nucleic acids, proteins and lipids. One of the mechanisms used by neural cells to counteract oxidative/nitrosative damage in AD is the enhancement of the cell stress response. Among the main components of the cell stress response is the heme oxygenase/biliverdin reductase (HO/BVR) axis, which catalyzes the degradation of heme which is toxic if produced in excess or under redox unbalanced conditions. However, the HO/BVR system and its by-products, carbon monoxide and bilirubin, have also been shown to be neuroprotective by activating pro-survival pathways and scavenging free radicals. Nevertheless, recent research demonstrated as both the inducible isoform of HO, known as HO-1, and BVR undergo oxidative/nitrosative/phosphorylative post-translational modifications in AD brain which alter the ability of HO-1 and BVR to activate the cell stress response. In this light, naturally occurring substances or drugs (e.g. statins) that prevent the post-translational modifications leading to a controlled up-regulation of the HO/BVR system have been proposed as potential new tools for the treatment of AD.