Sulforaphane is a Nrf2-independent inhibitor of mitochondrial fission

Abstract

The KEAP1-Nrf2-ARE antioxidant system is a principal means by which cells respond to oxidative and xenobiotic stresses. Sulforaphane (SFN), an electrophilic isothiocyanate derived from cruciferous vegetables, activates the KEAP1-Nrf2-ARE pathway and has become a molecule-of-interest in the treatment of diseases in which chronic oxidative stress plays a major etiological role. We demonstrate here that the mitochondria of cultured, human retinal pigment epithelial (RPE-1) cells treated with SFN undergo hyperfusion that is independent of both Nrf2 and its cytoplasmic inhibitor KEAP1. Mitochondrial fusion has been reported to be cytoprotective by inhibiting pore formation in mitochondria during apoptosis, and consistent with this, we show Nrf2-independent, cytoprotection of SFN-treated cells exposed to the apoptosis-inducer, staurosporine. Mechanistically, SFN mitigates the recruitment and/or retention of the soluble fission factor Drp1 to mitochondria and to peroxisomes but does not affect overall Drp1 abundance. These data demonstrate that the beneficial properties of SFN extend beyond activation of the KEAP1-Nrf2-ARE system and warrant further interrogation given the current use of this agent in multiple clinical trials.

Keywords: Apoptosis; Drp1; Fission; Fusion; Mitochondria; Nrf2; Sulforaphane.

Fig. 1

SFN induces Nrf2/KEAP1-independent mitochondrial fusion. (A) RPE-1 cells were transfected with the indicated siRNAs and 3 days later treated with DMSO or the Nrf2 activators SFN (50 μM), MG132 (10 μM), or tBHQ (100 μM) for 4 h. Mitochondria (red) are labeled with an anti-Tom20 antibody, and nuclei (blue) are counterstained with DAPI. (B) Graph showing quantification of mitochondrial morphology scoring from (A). >50 cells per condition were evaluated in a blinded fashion. (C) Representative western blots from (A). (D) RPE-1 cells were transfected with 10 nM siRNA and 3 days later treated with SFN for 4 h prior to being fixed and stained as in (A). (E) Graph showing quantification of mitochondrial phenotype scoring from (D). >100 cells per condition were evaluated in a blinded fashion. (F) Representative western blots from (D). (G) Cells were transfected and treated as in (D) with siCON or siKEAP1. (H) Cells from (G) were scored as in (B) and (E) on the basis of mitochondrial morphology. (I) Representative western blots from (G). Data in (B), (E), and (H) were compiled from 3 independent experiments each and statistical significance was determined by two-tailed Student's t-test. Error bars reflect +/- S.D. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 2

SFN induces peroxisomal lengthening. (A) RPE-1 cells were transfected with 10 nM of the indicated siRNA and 3 days later treated with DMSO or 50 μM SFN for 4 h. Peroxisomes (green) were labeled with an anti-PMP70 antibody, mitochondria with MitoTracker (red), and DNA counterstained with DAPI. Enlarged insets of peroxisomes are shown on the right (panels d, h, and l) to facilitate visualization of the changes in morphology induced by SFN and Drp1 depletion. Arrowheads highlight constriction points. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 3

SFN causes a loss of Drp1 from the mitochondria. (A) Subcellular fractionation of RPE-1 cells following 4 h of DMSO or SFN. Whole-cell lysates (WCL), nuclear (Nuc), cytosolic (Cyto), and crude mitochondrial (Mito) fractions were resolved by SDS-PAGE and processed for western blotting with the indicated antibodies. The migration of molecular weight markers is indicated on the left. (B) Graphs showing densitometric quantification of Drp1 in the indicated fractions from (A). (C) RPE-1 cells were transfected with 10 nM siCON or siDrp1 and 3 days later treated with DMSO or SFN for 4 h. Drp1 (green) was visualized with an anti-Drp1 antibody, mitochondria with MitoTracker (red), and nuclei with DAPI (blue). (D) Automated co-localization analysis of Drp1 and MitoTracker signal from (C). Data in (B) and (D) were compiled from 3 and 5 independent experiments, respectively, and statistical significance was determined by two-tailed Student's t-test. Error bars reflect +/- S.D and asterisks denote statistical significance. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 4

The cytoprotective effects of SFN are independent of Nrf2 expression (A) RPE-1 cells were pre-treated with DMSO or 50 μM SFN for 2 h prior to treatment with DMSO, 1 μM staurosporine (STS), or 50 μM etoposide for 6 h and were processed for anti-PARP western blotting. (B) RPE-1 cells were transfected with 2.5 nM siCON, 1 nM siBcl-XL, or 2.5 nM siBcl-XL and 3 days later were treated with DMSO or 1 μM STS for 2, 4, or 6 h. Representative western blots are shown and the migration of molecular weight markers is indicated on the left. (C) CRISPR/Cas9-generated wild-type (Nrf2WT) and Nrf2 knockout (Nrf2KO) RPE-1 cells were transfected with 1 nM siBcl-XL and 3 days later were pre-treated with DMSO or 50 μM SFN for 2 h. Subsequently, the cells were treated with 1 μM STS for 2, 4, or 6 h. Representative western blots with the indicated antibodies are shown. (D) Quantification of cleaved PARP as a percentage of total PARP (cleaved+uncleaved) from 3 independent experiments. Importantly, the levels of cleaved PARP were comparable whether cells expressed Nrf2 or not, indicating that SFN protection from STS is independent of the transcription factor. (E) 20X phase-contrast images taken immediately prior to harvest of lysates from (C). Scale bar=65 µm. (F) Representative western blots demonstrating that depletion of Drp1 confers near-comparable protection from STS as SFN treatment. RPE-1 cells were transfected with 1 nM siBcl-XL and additionally transfected with either 10 nM siCON or 10 nM siDrp1. 3 days later, siCON cells were pre-treated with SFN as in (A) and (C) and then exposed to STS for 4 h prior to being harvested and processed for western blotting with the indicated antibodies. (G) Same as (D) for the data presented in (F) compiled from 3 independent experiments. Error bars reflect +/- S.E.M.