## 3 Keep1-Nrf2-ARE Signaling System
### Brusatol (BT)  
Brusatol (3), a quassinoid present in _Brucea sumatrana_ and _B. javanica_, exhibits a potent tumor suppressing effect [42]. It suppresses Nrf2 expression by upregulating protein ubiquitination, disrupting intracellular redox balance. In many cancer types, it has been observed that high constitutive expression of Nrf2 builds up a conducive environment for the survival and proliferation of cancer cells [101]. Therefore, specific Nrf2 inhibitors are a must in such circumstances in order to tackle these cancer cells. BT sensitized a broad spectrum of cancer cells, including A549 xenografts to cisplatin, where it selectively decreased the protein levels of Nrf2 via increased ubiquitination and degradation of Nrf2 [101]. Consequently, the expressions of Nrf2-downstream genes were reduced, and the Nrf2-dependent cellular response was suppressed. This co-treatment of BT and cisplatin reduced cell proliferation, inhibited tumor growth, and induced apoptosis more substantially than cisplatin treatment alone. Combinatorial therapy of BT with chemotherapeutic drugs has demonstrated better outcomes than using BT alone. It synergistically enhanced the antitumor activity of trastuzumab against human epidermal  
Figure 5: Mechanism of Nrf2 activation by ZB. The diagram was adapted and modified from [PERSON] et al., 2018 [86]. growth factor receptor 2 (HER2)-positive SKOV3 and BT-474 cells, which was modulated by the inhibition of the Nrf2/HO-1 and HER2-AKT/ERK1/2 signaling pathways [102]. Combination of BT and metformin overcame progestein resistance by downregulating Nrf2/aldo-keto reductase family 1 member C1 (AKR1C1) in endometrial cancer [103]. BT also improved sensitivity to taxol as a result of Nrf2 suppression and ROS level upregulation in MCF-7 and MDA-MB-231 breast cancer cells [104]. BT and sorafenib in combination ameliorated hepatitis C virus-associated hepatocellular carcinoma and increased the efficacy of the anticancer drug [105]. This combinatorial therapy inhibited Nrf2 expression and enhanced NF-\(\kappa\)B, tumor necrosis factor (TNF), and MAPK pathways in order to exert its effect. BT treatment in combination with UVA inhibited A375 melanoma cell growth via G1 phase cell cycle arrest and apoptosis induction which involved partial suppression of Nrf2 and its target gene, HO-1, through the PI3K/ AKT signaling pathway [106]. A quite similar mechanism was found for the amelioration of nasopharyngeal carcinoma [107]. Here, BT mediated the mitochondrial apoptosis and cell cycle arrest that followed the suppression of the AKT/mammalian target of rapamycin (mTOR) signaling. BT and cytarabine in combination exerted an anti-tumor effect in acute myeloid leukemia by inhibiting Nrf2, which in turn downregulated the expression of glycolysis-related proteins and reduced glucose uptake and lactate production [108]. In Hepa-1 clc7 hepatoma cells, BT imparted improved chemoresistance by inhibiting Keap1-independent Nrf2 signaling [109]. Again based on Nrf2 inhibition, BT remarkably suppressed the viability of A549 cells and promoted apoptotic processes [110]. The Nrf2 inhibitory action of BT is correlated with its capacity to suppress tumorigenicity and tumor cell migration and invasion [111]. For instance, in PATU-8988 and PANC-1 pancreatic cancer cells, it mediated apoptosis by inactivating NF-kB/signal transducer and activator of transcription 3 (STAT3) and activating JNK/p38 MAPK signaling [112]. As a potential inhibitor of Nrf2 and STAT3, BT actively suppressed tumor formation and progression in head and neck squamous cell carcinoma, as well [113]. Disruption of PI3K/AKT/NF-\(\kappa\)B signaling by BT caused inhibition of epithelial mesenchymal transition (EMT) in order to exert its potent anti-gastric cancer effect [114]. Figure 6: Mechanism of Nrf2-ARE activation by KLA. The diagram was adapted and modified from [PERSON] et al., 2018 [40].