Breakdown products of neoglucobrassicin inhibit activation of Nrf2 target genes mediated by myrosinase-derived glucoraphanin hydrolysis products

Abstract

Glucosinolates (GLSs) present in Brassica vegetables serve as precursors for biologically active metabolites, which are released by myrosinase and induce phase 2 enzymes via the activation of Nrf2. Thus, GLSs are generally considered beneficial. The pattern of GLSs in plants is various, and contents of individual GLSs change with growth phase and culture conditions. Whereas some GLSs, for example, glucoraphanin (GRA), the precursor of sulforaphane (SFN), are intensively studied, functions of others such as the indole GLS neoglucobrassicin (nGBS) are rather unknown as are functions of combinations thereof. We therefore investigated myrosinase-treated GRA, nGBS and synthetic SFN for their ability to induce NAD(P)H:quinone oxidoreductase 1 (NQO1) as typical phase 2 enzyme, and glutathione peroxidase 2 (GPx2) as novel Nrf2 target in HepG2 cells. Breakdown products of nGBS potently inhibit both GRA-mediated stimulation of NQO1 enzyme and Gpx2 promoter activity. Inhibition of promoter activity depends on the presence of an intact xenobiotic responsive element (XRE) and is also observed with benzo[a]pyrene, a typical ligand of the aryl hydrocarbon receptor (AhR), suggesting that suppressive effects of nGBS are mediated via AhR/XRE pathway. Thus, the AhR/XRE pathway can negatively interfere with the Nrf2/ARE pathway which has consequences for dietary recommendations and, therefore, needs further investigation.

Myrosinase-treated glucoerucin is a potent inducer of the Nrf2 target gene heme oxygenase 1 — studies in cultured HT-29 cells and mice

“…2B) expression of the Nrf2 target gene HO-1 via GER+MYR, whereas treatment with GER alone did not cause any changes. These results are supported by the investigations performed by Haack et al, who showed that MYR-treated GRA, the predominant GL present in broccoli, produced an increase in Nrf2 translocation in HepG2 cells, whereas untreated GRA did not [8,39]. Because little information regarding the gene-regulatory activity of ERN is available in the literature, we determined whether the effects obtained in cell culture could be transferred to an in vivo model.…”

Identification of glucosinolate congeners able to form DNA adducts and to induce mutations upon activation by myrosinase

“…In contrast, neoglucobrassicin was a potent mutagen. It does not activate NRF2; it even counteracts some inductive effects of sulphoraphane [25].…”

A Glance at… Broccoli, glucoraphanin, and sulforaphane

“…Because rats fed broccoli containing heat-inactivated myrosinase do not exhibit such increases, it has been concluded that sulforaphane is responsible for broccoli-associated hepatic phase 2 enzyme induction [28]. Supporting this hypothesis are data indicating that sulforaphane and myrosinase-treated glucoraphanin induce the expression of glutathione-S-transferase (GST), a major phase 2 detoxification enzyme [29], NQO1, and hydroperoxide-reducing glutathione peroxidase-2 (GPx2) in cultured human hepatocytes, whereas intact glucoraphanin is without effect [30][31][32]. Table 1 provides several actions that are directly or indirectly attributable to sulforaphane.…”

“…Table 1 provides several actions that are directly or indirectly attributable to sulforaphane. Among these are attenuation by sulforaphane of the cytotoxicity of a number of oxidizers (including menadione, tert-butyl hydroperoxide, 4-hydroxynonenal, dexamethasone, peroxynitrile, gentamycin, and amyloid-b [25][26][27][28][29][30][31][32][33][34][35], while inducing increased expression of QR, NQO1, HO-1, Tr1, and glutathione in human retinal pigment epithelial cells, neuroblastoma cells, osteoblasts, keratinocytes, endothelial cells, and the renal cortex [33][34][35][36][37][38][39][40]. Oral sulforaphane has increased hepatocyte expression of superoxide dismutase, catalase, GST, GPx2, NQO1, HO-1, and glutathione in mice [41] and has increased nasal cell expression of GST, NQO1, and HO-1 [42] and jejunal enterocyte expression of GST [9] in humans.…”

“…Nrf2 activation and the indirect antioxidant effects of sulforaphane appear to be ubiquitous; they have been observed in the liver [30][31][32]41,59,60], kidney [60], lungs [60][61][62][63], lens [58], central nervous system [49,60], immune system [50], choroid plexus [51], small intestine [9], large intestine [60], prostate [60], retinal pigment epithelial cells [34,39,64], corneal epithelial cells [55], astroglial cells [54], fetal neural crest cells [52], neuroblastoma cells [36], insulin-secreting pancreatic cells [65], differentiated peripheral blood neutrophils [53], endothelial cells [40], osteoblasts [37], and keratinocytes [33][34][35].…”