Obtusaquinone: A Cysteine-Modifying Compound That Targets Keap1 for Degradation

Abstract

We have previously identified the natural product obtusaquinone (OBT) as a potent antineoplastic agent with promising in vivo activity in glioblastoma and breast cancer through the activation of oxidative stress; however, the molecular properties of this compound remained elusive. We used a multidisciplinary approach comprising medicinal chemistry, quantitative mass spectrometry-based proteomics, functional studies in cancer cells, and pharmacokinetic analysis, as well as mouse xenograft models to develop and validate novel OBT analogs and characterize the molecular mechanism of action of OBT. We show here that OBT binds to cysteine residues with a particular affinity to cysteine-rich Keap1, a member of the CUL3 ubiquitin ligase complex. This binding promotes an overall stress response and results in ubiquitination and proteasomal degradation of Keap1 and downstream activation of the Nrf2 pathway. Using positron emission tomography (PET) imaging with the PET-tracer 2-[ 18 F]fluoro-2-deoxy-D-glucose (FDG), we confirm that OBT is able to penetrate the brain and functionally target brain tumors. Finally, we show that an OBT analog with improved pharmacological properties, including enhanced potency, stability, and solubility, retains the antineoplastic properties in a xenograft mouse model.

Effect of 1α,25(OH)2 Vitamin D3 in Mutant P53 Glioblastoma Cells: Involvement of Neutral Sphingomyelinase1

“…The standard initial approach is maximal safe surgical resection, followed by radio-and chemotherapy, and treatment options in the recurrent setting are less well defined, with no established standard of care [2]. Some molecules have recently been identified that can influence the development of glioblastoma and the course of the disease, such as obtusaquinone [3], coronarin D [4], betulinic acid [5], icaritin [6], and others. As early as 1995 Magrassi et al [7] had demonstrated that activated 1α,25(OH) 2 vitamin D 3 or calcitriol (1α,25(OH) 2 VD3) was capable of inducing a significant (>50%) reduction in growth of glioblastoma cells at dosages over 5 µM.…”

“…As early as 1995 Magrassi et al [7] had demonstrated that activated 1α,25(OH) 2 vitamin D 3 or calcitriol (1α,25(OH) 2 VD3) was capable of inducing a significant (>50%) reduction in growth of glioblastoma cells at dosages over 5 µM. Classically, the inactive form of vitamin D3 or cholecalciferol is first hydroxylated in carbon 25 in the liver to form 25(OH) vitamin D 3 or calcidiol and then it is hydroxylated in carbon 1 in the kidney to form 1α,25(OH) 2 VD3. Interestingly, glioblastoma was able to metabolize cholecalciferol to calcidiol [8].…”

Role of Nrf2 in Parkinson’s Disease: Toward New Perspectives

“…OBT could bind to cysteine residues of Keap1 with specific affinity and lead to Keap1 somatic degradation and activation of Nrf2 downstream expression factors. The ability of OBT was also confirmed to penetrate the protective barrier of the brain and work in the brain (Badr et al, 2020). The possible mechanism of action of OBT and its ability to penetrate the BBB greatly enhances its feasibility in studies related to PD prevention and treatment.…”

Exploring the role of Nrf2 signaling in glioblastoma multiforme

“…Multiple compounds have been tested for clinical efficacy, however, none of the proposed approaches has demonstrated success in clinical trials. A natural product obtusaquinone (OBT) has been shown to downregulate the Nrf2 pathway through binding to Keap1 cysteine residues and promoting its proteasomal degradation [ 74 ]. Malignant GBM phenotype is dependent on metabolic alterations which also show an association with the Nrf2 activity.…”