Hormetic dietary phytochemicals from Western Canadian plants: Identification, characterization and mechanistic insights

Abstract

Activation of mammalian stress responsive pathways by plant secondary metabolites may contribute to the protection against certain chronic diseases afforded by fruit and vegetable consumption. This work focuses on the identification of plant compounds that activate the stress-responsive enzyme quinone reductase (QR) by stabilizing the transcription factor NF-E2 related factor-2 (Nrf2). Screening methanolic extracts of plants from Western Canada for QR induction in a mouse hepatoma cell line (Hepa-1c1c7) led to the identification of twenty-one extracts capable of doubling the activity of QR. Bioassay-guided fractionation of six extracts led to the identification of novel classes of compounds with QR-inducing activity including fatty-acid derived polyacetylenes, phthalides, and cannabinoids. Studies using low molecular weight thiols and the recombinantly expressed protein Keap1, the principal negative regulator of Nrf2, supported a mechanism of QR activation involving covalent modification of Keap1 cysteines for the polyacetylenes and phthalides. Analysis of transcriptional changes in response to treatment with a panel of QR-inducing compounds provided strong support for Nrf2 activation by the polyacetylene (3S,8S)-falcarindiol and the isothiocyanate (R)-sulforaphane and weaker support for the compounds (3R,8S)-falcarindiol, 6-isovaleryl-umbelliferone (6-IVU) and (Z)-ligustilide. Additionally, transcript level analyses supported a role for the aryl-hydrocarbon receptor in QR-activation by (3R,8S)-falcarindiol, (Z)-ligustilide, (R)-sulforaphane, 6-IVU and cannabidiol and suggested that treatment with polyacetylenes with a (3R)-configuration, (Z)-ligustilide and 6-IVU causes substantial changes in the expression of genes associated with lipid homeostasis and energy metabolism. As a whole, this work provides evidence that compounds that activate QR (and Nrf2) are widely distributed in the Canadian flora. However, of these QR activators, few are active at concentrations that are expected to be achieved through dietary consumption. Nevertheless, the most exceptional compounds isolated in this work, the compounds (3S,8S)-falcarindiol and epoxyfalcarindiol are highly potent and appear to be or are expected to be specific for activating Nrf2 and thus warrant attention with respect to dietary implications and as drug candidate leads.