FUNCTIONAL ANALYSIS OF BIPARTITE NRF2 ACTIVATORS THAT OVERCOME FEEDBACK REGULATION FOR AGE-RELATED CHRONIC DISEASES

Abstract

Activating Nrf2 with small molecules is a promising strategy for countering aging, oxidative stress, inflammation, and various disorders, including neurodegeneration. The primary regulator of Nrf2 protein stability is Keap1, a redox sensor protein and an adapter in the Cullin III ubiquitin ligase complex, which labels Nrf2 for proteasomal degradation. The known Nrf2 activators either chemically modify sensor thiols in Keap1 or competitively displace Nrf2 from the ubiquitin ligase complex. The latter approach is considered the most suitable for continuous administration, as non-specific chemical modifiers of Keap1 thiols also modify active thiols on other proteins, thus causing side effects. However, when transitioning from homogeneous to cell-based assays, genuine displacement activators show a significant loss in potency by several orders of magnitude. As we demonstrate here, this offset is due to the presence of high micromolar concentrations of Keap1 in both the cell lines and brain tissue. A potential solution could involve targeted delivery of an alkylating agent to Keap1 to achieve the desired specificity. Transcriptomic analysis of a cell-permeable Nrf2 peptide bearing an alkylating fumarate moiety indicates selective activation of the Nrf2 genetic program, confirming the high specificity of this approach. The Nrf2-triggered genetic program has a feedback regulation mechanism through the activation of Bach1, an Nrf2 transcriptional repressor, which is elevated in age-related neurodegeneration. Thus, a benign bipartite Nrf2 activator with Bach1 inhibition properties is needed for maximal benefits. The recently developed heterocyclic carboxamide, HPPE, shows overlap with the Nrf2 pathway activated by the fumarate-linked Nrf2 peptide and with zinc and tin protoporphyrins, which are recognized inhibitors of Bach1. Therefore, HPPE presents a promising and unique combination of the two desired activities that could be further optimized to treat age-related neurodegeneration. Highlights: The decrease in potency for reversible displacement activators of Nrf2 in biological assays is attributed to high micromolar concentrations of Keap1 and competition with endogenous Keap1 client proteins. Nrf2 activators specific for Keap1 should combine a displacement scaffold with a substitution that undergoes intracellular conversion into active pro-oxidant or alkylating species. Cell-permeable fumarate-linked Nrf2 peptide solely activates the Nrf2 antioxidant genetic program, as demonstrated by transcriptomic analysis. HPPE, a small bipartite molecule, exhibits properties of both Nrf2 activation and Bach1 inhibition, to bypass feedback regulation by targeting both Keap1 and Bach1.