Brusatol provokes a rapid and transient inhibition of Nrf2 signaling and sensitizes mammalian cells to chemical toxicity—implications for therapeutic targeting of Nrf2

Abstract

The transcription factor Nrf2 regulates the basal and inducible expression of a battery of cytoprotective genes. Whereas numerous Nrf2-inducing small molecules have been reported, very few chemical inhibitors of Nrf2 have been identified to date. The quassinoid brusatol has recently been shown to inhibit Nrf2 and ameliorate chemoresistance in vitro and in vivo. Here, we show that brusatol provokes a rapid and transient depletion of Nrf2 protein, through a posttranscriptional mechanism, in mouse Hepa-1c1c7 hepatoma cells. Importantly, brusatol also inhibits Nrf2 in freshly isolated primary human hepatocytes. In keeping with its ability to inhibit Nrf2 signaling, brusatol sensitizes Hepa-1c1c7 cells to chemical stress provoked by 2,4-dinitrochlorobenzene, iodoacetamide, and N-acetyl-p-benzoquinone imine, the hepatotoxic metabolite of acetaminophen. The inhibitory effect of brusatol toward Nrf2 is shown to be independent of its repressor Keap1, the proteasomal and autophagic protein degradation systems, and protein kinase signaling pathways that are known to modulate Nrf2 activity, implying the involvement of a novel means of Nrf2 regulation. These findings substantiate brusatol as a useful experimental tool for the inhibition of Nrf2 signaling and highlight the potential for therapeutic inhibition of Nrf2 to alter the risk of adverse events by reducing the capacity of nontarget cells to buffer against chemical and oxidative insults. These data will inform a rational assessment of the risk:benefit ratio of inhibiting Nrf2 in relevant therapeutic contexts, which is essential if compounds such as brusatol are to be developed into efficacious and safe drugs.

Transcription Factor NRF2 as a Therapeutic Target for Chronic Diseases: A Systems Medicine Approach

“…Natural products such as brusatol (Ren et al, 2011;Olayanju et al, 2015), ochratoxin A (Tarumoto et al, 2004;Limonciel and Jennings, 2014), and trigonelline (Arlt et al, 2013) have also been found to inhibit NRF2. However, their mechanism of action is not fully understood.…”

Keap1–Nrf2 signalling in pancreatic cancer

“…lung, colon, head and neck, breast, and endometrial) as well as some non-neoplastic diseases (Copple, 2012). To date, there is a small but growing number of Nrf2 inhibitor compounds, all of which are naturally occurring, namely; retinoic acid , brusatol Olayanju et al, 2015), luteolin (Tang et al, 2011), and trigonelline (Boettler et al, 2011). The latter two have been studied in pancreatic cancer.…”

“…In non-PDAC cancer studies, brusatol has been shown to be a highly specific Nrf2 inhibitor which produces rapid and transient effects, and can increase chemotherapy sensitisation (Olayanju et al, 2015;Ren et al, 2011). Thus brusatol would be a useful experimental tool for further study of Nrf2 in PDAC, although the precise mechanism of Nrf2 inhibition is still under investigation.…”

“…Although complicated by dosing considerations, Nrf2 inhibitors provide useful tools in exploring the mechanisms and effects of Nrf2 inhibition in the preclinical setting, from which assessments can be made as to the therapeutic context that these might be safely be applied to PDAC and other diseases with Keap1-Nrf2 dysregulation (Chapple et al, 2012;Copple, 2012;Olayanju et al, 2015).…”

Activated Nrf2 Interacts with Kaposi's Sarcoma-Associated Herpesvirus Latency Protein LANA-1 and Host Protein KAP1 To Mediate Global Lytic Gene Repression

“…While the stress-inducing, antiproliferative effects of Nrf2 knockdown have been previously reported, these effects do not become apparent until Nrf2 inhibition is coadministered with cytotoxic chemotherapeutic drugs. Similarly, brusatol's anticancerous effects on A549, a lung adenocarcinoma cell line with elevated Nrf2, were not significant until coadministration with cisplatin, whereas its toxic effects on PEL cells at the same concentration were readily observable (data not shown) (42,(63)(64)(65). These data suggest that PEL cell lines are highly dependent on Nrf2 for proliferation and survival, explaining the high Nrf2 levels in these cell lines.…”