NRF2 ACTIVATORS FOR AGE-RELATED CHRONIC DISEASES: THEORETICAL LIMITATIONS AND PRACTICAL APPROACHES (HPPE and protoporphyrins)

One of the major metabolic responses to counteract aging, inflammation, and various chronic diseases including neurodegeneration, is the so call antioxidant pathway controlled by Nrf2 transcription factor. Nrf2 protein stability is regulated via interaction with the redox sensor Keap1, an adapter protein in Cullin III ubiquitin ligase complex. The search for Nrf2 activators is mainly focused on compounds that either chemically modify sensor thiols in Keap1 or competitively displace Nrf2 from the ubiquitin ligase complex. The latter approach is considered as the most appropriate one for continuous administration without side effects, which are unavoidable in case of non-specific chemical modifiers of Keap1. However, true displacement activators show a significant offset, by many orders of magnitude, when switched from homogeneous assay to cell-based assays. This phenomenon originates, as we show here, from high micromolar intracellular concentrations of Keap1. A guarantee for high specificity of an alkylating Nrf2 activator is its targeted delivery. A transcriptomic analysis for a cell-permeable Nrf2-peptide bearing a fumarate moiety shows the activation of the antioxidant program only, confirming the high specificity of such approach. However, aging-linked neurodegeneration is also known to build-up Bach1, an Nrf2 target gene and competitive transcriptional repressor for Nrf2. Therefore, an ideal combination could be a benign Nrf2 activator and Bach1 inhibitor in one molecule, as represented by the recently developed HPPE. HPPE demonstrates an intersection of the activated antioxidant pathway with the Nrf2 peptide, and, in addition to that, shows a big pool of genes overlapping with the known inhibitors of Bach1 - zinc and tin protoporphyrins. Hence, at present moment, HPPE presents a unique combination of the two desired activities beneficial for age-related neurodegeneration.