Guggulsterone induces heme oxygenase-1 expression through activation of Nrf2 in human mammary epithelial cells: PTEN as a putative target

Abstract

Guggulsterone (GS) [4,17(20)-pregnadiene-3,16-dione] is a phytosterol found in the gum resin of the Commiphora mukul . GS exists naturally in two stereoisomers: E -GS ( cis -GS) and Z -GS ( trans -GS). In this study, the effects of both isomers on expression of the cytoprotective enzyme heme oxygenase-1 (HO-1) were evaluated in human mammary epithelial (MCF10A) cells. NF-E2-related factor 2 (Nrf2) is considered a master regulator in activating antioxidant response element (ARE)-driven expression of HO-1 and many other antioxidant/cytoprotective proteins. cis -GS upregulated the transcription and protein expression of HO-1 to a greater extent than did trans -GS. cis -GS treatment enhanced nuclear translocation and ARE-binding activity of Nrf2. MCF10A cells transfected with an ARE luciferase construct exhibited significantly elevated Nrf2 transcriptional activity upon cis -GS treatment compared with cells transfected with the control vector. In addition, silencing of the Nrf2 gene abrogated cis -GS-induced expression of HO-1. Incubation of MCF10A cells with cis -GS increased phosphorylation of Akt. The pharmacological inhibition of phosphoinositide 3-kinase (PI3K), an upstream kinase responsible for Akt phosphorylation, abrogated cis -GS-induced Nrf2 nuclear translocation. Pretreatment with the thiol-reducing agents attenuated Akt phosphorylation, Nrf2 activation and HO-1 expression, suggesting that cis -GS may cause thiol modification of an upstream signaling modulator. Phosphatase and Tensin Homologue Deleted on Chromosome 10 (PTEN) is a negative regulator of the PI3K–Akt axis. The mutation in cysteine 124 present in the catalytic domain of PTEN abolished cis -GS-induced HO-1 expression as well as Akt phosphorylation. Whether this cysteine is a ‘bona fide’ target of cis -GS in its activation of Nrf2 needs additional investigation.