Antitumor agents XLVIII: Structure-activity relationships of quassinoids as in vitro protein synthesis inhibitors of P-388 lymphocytic leukemia tumor cell metabolism

Brusatol: A potential anti-tumor quassinoid from Brucea javanica

Brusatol, a triterpene lactone compound mainly from Brucea javanica, sensitizes a broad spectrum of cancer cells. It is known as a specific inhibitor of nuclear factor-erythroid 2-related factor 2 (Nrf2) pathway. In this review, we provide a comprehensive overview on the antitumor effect and molecular mechanisms of brusatol in vitro and in vivo. This review also covers pharmacokinetics studies, modification of dosages forms of brusatol. Increasing evidences have validated the value of brusatol as a chemotherapeutic agent in cancers, which may contribute to drug development and clinical application.

Safety assessment of standardized aqueous Brucea javanica extract in rats

ETHNOPHARMACOLOGICAL RELEVANCE

The seeds of Brucea javanica and its aqueous decoction is a traditional medicine consumed by diabetic patients in Malaysia. The daily consumption of B. javanica seeds and it's aqueous decoction causes much concern as the quassinoids and its glycosides from the seeds exhibited various pharmacological activity at low doses.

AIMS OF STUDY

The aim of the present study is to evaluate the repeated dose toxicity of the standardized aqueous extract administered daily for 30 days through oral administration at its effective hypoglycemia doses.

MATERIALS AND METHODS

The seeds were dried, ground and extracted in deionized water. A HPLC-photodiode array method was developed and validated for the standardization of both the hypoglycemia agents, namely bruceine D and E in aqueous extract. Both normoglycemia and streptozotocin (STZ)-induced diabetic rats were fed orally with 15, 30 and 60mg/kg body weight of standardized aqueous extract. The blood glucose was measured at 0-8h. In repeated dose toxicity, similar doses were administered orally to rats for 30 days. At the end of 30 days, the blood was withdrawn and subjected to biochemical and haematology analysis while organs were harvested for histology analysis.

RESULTS

Oral administration of standardized aqueous extract exhibited a dose-response relationship in both the normoglycemia and STZ-induced diabetic rats. Daily oral administration of 15, 30 and 60mg/kg standardized aqueous extract for 30 days to rats did not show signs to toxicity in its biochemical, haematology and histology analysis.

CONCLUSION

In conclusion, although the seeds were reported to contain compounds with various pharmacological activity, the daily oral administration to rats for 30 days do not showed signs of toxicity at its effective hypoglycemia doses.

UVA Irradiation Enhances Brusatol-Mediated Inhibition of Melanoma Growth by Downregulation of the Nrf2-Mediated Antioxidant Response

Brusatol (BR) is a potent inhibitor of Nrf2, a transcription factor that is highly expressed in cancer tissues and confers chemoresistance. UVA-generated reactive oxygen species (ROS) can damage both normal and cancer cells and may be of potential use in phototherapy. In order to provide an alternative method to treat the aggressive melanoma, we sought to investigate whether low-dose UVA with BR is more effective in eliminating melanoma cells than the respective single treatments. We found that BR combined with UVA led to inhibition of A375 melanoma cell proliferation by cell cycle arrest in the G1 phase and triggers cell apoptosis. Furthermore, inhibition of Nrf2 expression attenuated colony formation and tumor development from A375 cells in heterotopic mouse models. In addition, cotreatment of UVA and BR partially suppressed Nrf2 and its downstream target genes such as HO-1 along with the PI3K/AKT pathway. We propose that cotreatment increased ROS-induced cell cycle arrest and cellular apoptosis and inhibits melanoma growth by regulating the AKT-Nrf2 pathway in A375 cells which offers a possible therapeutic intervention strategy for the treatment of human melanoma.

Eurycomalactone Inhibits Expression of Endothelial Adhesion Molecules at a Post-Transcriptional Level

The C-19 quassinoid eurycomalactone (1) has recently been shown to be a potent (IC₅₀ = 0.5 μM) NF-κB inhibitor in a luciferase reporter model. In this study, we show that 1 with similar potency inhibited the expression of the NF-κB-dependent target genes ICAM-1, VCAM-1, and E-selectin in TNFα-activated human endothelial cells (HUVECtert) by flow cytometry experiments. Surprisingly, 1 (2 μM) did not inhibit TNFα-induced IKKα/β or IκBα phosphorylation significantly. Also, the TNFα-induced degradation of IκBα remained unchanged in response to 1 (2 μM). In addition, pretreatment of HUVECtert with 1 (2 μM) had no statistically significant effect on TNFα-mediated nuclear translocation of the NF-κB subunit p65 (RelA). Quantitative RT-PCR revealed that 1 (0.5-5 μM) exhibited diverse effects on the TNFα-induced transcription of ICAM-1, VCAM-1, and SELE genes since the mRNA level either remained unchanged (ICAM-1, E-selectin, and VCAM-1 at 0.5 μM 1), was reduced (VCAM-1 at 5 μM 1), or even increased (E-selectin at 5 μM 1). Finally, the time-dependent depletion of a short-lived protein (cyclin D1) as well as the measurement of de novo protein synthesis in the presence of 1 (2-5 μM) suggested that 1 might act as a protein synthesis inhibitor rather than an inhibitor of early NF-κB signaling.

The KEAP1-NRF2 System in Cancer

Cancer cells first adapt to the microenvironment and then propagate. Mutations in tumor suppressor genes or oncogenes are frequently found in cancer cells. Comprehensive genomic analyses have identified somatic mutations and other alterations in the KEAP1 or NRF2 genes and in well-known tumor suppressor genes or oncogenes, such as TP53, CDKN2A, PTEN, and PIK3CA, in various types of cancer. Aberrant NRF2 activation in cancer cells occurs through somatic mutations in the KEAP1 or NRF2 gene as well as through other mechanisms that disrupt the binding of KEAP1 to NRF2. Unregulated NRF2 confers on cancer cells high-level resistance to anticancer drugs and reactive oxygen species (ROS) and directs cancer cells toward metabolic reprogramming. Therefore, NRF2 has been studied as a therapeutic target molecule in cancer. Two strategies have been used to target NRF2 via therapeutic drugs: inhibition of NRF2 and induction of NRF2. NRF2 inhibitors may be effective against NRF2-addicted cancer cells in which NRF2 is aberrantly activated. These inhibitors have not yet been established as NRF2-targeted anticancer drugs for the treatment of human cancers. Diagnosis of NRF2 activation could facilitate the use of NRF2 inhibitors for the treatment of patients with NRF2-addicted cancers. Conversely, NRF2 inducers have been used or are being developed for non-cancer diseases. In addition, NRF2 inducers may be useful for cancer chemotherapy in combination with conventional anticancer agents or even NRF2 inhibitors.

Application of Mass Spectrometry Profiling to Establish Brusatol as an Inhibitor of Global Protein Synthesis

The KEAP1/Nrf2 pathway senses and responds to changes in intracellular oxidative stress. Mutations that result in constitutive activation of Nrf2 are present in several human tumors, especially non-small cell lung cancer. Therefore, compounds that inhibit Nrf2 activity might be beneficial in treating patients whose tumors show activation of this pathway. Recent reports suggest that the natural product brusatol can potently and selectively inhibit Nrf2 activity, resulting in cell cytotoxicity, and can be effectively combined with chemotherapeutic agents. Here, we analyzed the effects of brusatol on the cellular proteome in the KEAP1 mutant non-small cell lung cancer cell line A549. Brusatol was found to rapidly and potently decrease the expression of the majority of detected proteins, including Nrf2. The most dramatically decreased proteins are those that display a short half-life, like Nrf2. This effect was confirmed by restricting the analysis to newly synthesized proteins using a labeled methionine analogue. Moreover, brusatol increased the expression of multiple components of the ribosome, suggesting that it regulates the function of this macromolecular complex. Finally, we show that brusatol induces its potent cellular cytotoxicity effects on multiple cancer cell lines in a manner independent of KEAP1/Nrf2 activity and with a profile similar to the protein translation inhibitor silvestrol. In conclusion, our data show that the activity of brusatol is not restricted to Nrf2 but, rather, functions as a global protein synthesis inhibitor.

The quassinoid isobrucein B reduces inflammatory hyperalgesia and cytokine production by post-transcriptional modulation

Isobrucein B (1) is a quassinoid isolated from the Amazonian medicinal plant Picrolemma sprucei. Herein we investigate the anti-inflammatory and antihyperalgesic effects of this quassinoid. Isobrucein B (1) (0.5-5 mg/kg) inhibited carrageenan-induced inflammatory hyperalgesia in mice in a dose-dependent manner. Reduced hyperalgesia was associated with reduction in both neutrophil migration and pronociceptive cytokine production. Pretreatment with 1 inhibited in vitro production/release of cytokines TNF, IL-1β, and KC/CXCL1 by lipopolysaccharide-stimulated macrophages. To investigate its molecular mechanism, RAW 264.7 macrophages with a luciferase reporter gene controlled by the NF-κB promoter were used (RAW 264.7-Luc). Quassinoid 1 reduced the luminescence emission by RAW 264.7-Luc stimulated by different compounds. Unexpectedly, NF-κB translocation to macrophage nuclei was not inhibited by 1 when evaluated by Western blotting and immunofluorescence. Furthermore, quassinoid 1 did not change the levels of TNF mRNA transcription in stimulated macrophages, suggesting post-transcriptional modulation. In addition, constitutive expression of luciferase in RAW 264.7 cells transiently transfected with a plasmid containing a universal promoter was inhibited by 1. Thus, isobrucein B (1) displays anti-inflammatory and antihyperalgesic activities by nonselective post-transcriptional modulation, resulting in decreased production/release of pro-inflammatory cytokines and neutrophil migration.

New findings on Simalikalactone D, an antimalarial compound from Quassia amara L. (Simaroubaceae)

Quassia amara L. (Simaroubaceae) is a species widely used as tonic and is claimed to be an efficient antimalarial all over the Northern part of the Amazon basin. Quassinoid compound Simalikalactone D (SkD) has been shown to be one of the molecules responsible for the antiplasmodial activity of a watery preparation made out of juvenile fresh leaves of this plant. Because of its strong antimalarial activity, we decided to have a further insight of SkD pharmacological properties, alone or in association with classical antimalarials. At concentrations of up to 200μM, we showed herein that SkD did not exert any apoptotic or necrotic activities in vitro on lymphoblastic cells. However, an antiproliferative effect was evident at concentrations higher than 45nM. SkD was inefficient at inhibiting heme biomineralization and the new permeability pathways induced by the parasite in the host erythrocyte membrane. With respect to Plasmodium falciparum erythrocytic stages, SkD was almost inactive on earlier and later parasite stages, but potently active at the 30th h of parasite cycle when DNA replicates in mature trophozoites. In vitro combination studies with conventional antimalarial drugs showed that SkD synergizes with atovaquone (ATO). The activity of ATO on the Plasmodium mitochondrial membrane potential was enhanced by SkD, which on its own had a poor effect on this cellular parameter.

Brusatol enhances the efficacy of chemotherapy by inhibiting the Nrf2-mediated defense mechanism

The major obstacle in cancer treatment is the resistance of cancer cells to therapies. Nrf2 is a transcription factor that regulates a cellular defense response and is ubiquitously expressed at low basal levels in normal tissues due to Keap1-dependent ubiquitination and proteasomal degradation. Recently, Nrf2 has emerged as an important contributor to chemoresistance. High constitutive expression of Nrf2 was found in many types of cancers, creating an environment conducive for cancer cell survival. Here, we report the identification of brusatol as a unique inhibitor of the Nrf2 pathway that sensitizes a broad spectrum of cancer cells and A549 xenografts to cisplatin and other chemotherapeutic drugs. Mechanistically, brusatol selectively reduces the protein level of Nrf2 through enhanced ubiquitination and degradation of Nrf2. Consequently, expression of Nrf2-downstream genes is reduced and the Nrf2-dependent protective response is suppressed. In A549 xenografts, brusatol and cisplatin cotreatment induced apoptosis, reduced cell proliferation, and inhibited tumor growth more substantially when compared with cisplatin treatment alone. Additionally, A549-K xenografts, in which Nrf2 is expressed at very low levels due to ectopic expression of Keap1, do not respond to brusatol treatment, demonstrating that brusatol-mediated sensitization to cisplatin is Nrf2 dependent. Moreover, a decrease in drug detoxification and impairment in drug removal may be the primary mechanisms by which brusatol enhances the efficacy of chemotherapeutic drugs. Taken together, these results clearly demonstrate the effectiveness of using brusatol to combat chemoresistance and suggest that brusatol can be developed into an adjuvant chemotherapeutic drug.

Structure–activity relationships of quassinoids for eukaryotic protein synthesis

The effect of 63 quassinoids on eukaryotic protein synthesis has been investigated. Seventeen of the tested compounds showed potent in vitro activity, with IC50s below 2 microM for inhibition in Krebs ascites translation extracts. Sixteen of these quassinoids were also potent inhibitors of in vivo protein synthesis when exposed to Hela cells for 1 h. Our results led to the following structure-activity relationships for quassinoids regarding translation inhibition. Activity is influenced by (i) the nature of the C-15 side chain, (ii) the nature of A ring modifications, (iii) the presence or absence of a sugar moiety, and (iv) the presence of an epoxymethano bridge.

Rapid action of Qinghaosu and related drugs on incorporation of [3H]isoleucine by Plasmodium falciparum in vitro

Using the incorporation of [3H]isoleucine into acid-insoluble products as an index of protein-synthetic activity, it was shown that Qinghaosu and two related drugs had a rapid effect on this process in human erythrocytes infected with Plasmodium falciparum in vitro. Inhibition could be seen 1 hr or less after addition of the drugs at concentrations from 5 mumole/1. to 50 nmole/1. It is recommended that the effects of these drugs be studied in cell-free protein-synthetic systems.