Pyruvate diminishes the cytotoxic activity of ascorbic acid in several tumor cell lines in vitro

Trihydroxybenzaldoximes are Redox Cycling Inhibitors of ThDP-Dependent DXP Synthase

Pathogenic bacteria must swiftly adapt to dynamic infection environments in order to survive and colonize in the host. 1-Deoxy-d-xylulose-5-phosphate synthase (DXPS) is thought to play a critical role in bacterial adaptation during infection and is a promising drug target. DXPS utilizes a thiamine diphosphate (ThDP) cofactor to catalyze the decarboxylative condensation of pyruvate and d-glyceraldehyde-3-phosphate (d-GAP) to form DXP, a precursor to isoprenoids and B vitamins. DXPS follows a ligand-gated mechanism in which pyruvate reacts with ThDP to form a long-lived lactyl-ThDP (LThDP) adduct which is coordinated by an active-site network of residues. d-GAP binding ostensibly disrupts this network to activate LThDP for decarboxylation. Our lab previously reported trihydroxybenzaldoxime inhibitors which are competitive with respect to d-GAP, and uncompetitive with respect to pyruvate, suggesting they bind after E-LThDP complex formation. Here, we conducted mechanistic studies to determine if these compounds inhibit DXPS by preventing LThDP activation or if they act as inducers of LThDP activation. We discovered that the catechol moiety of the trihydroxybenzaldoxime scaffold undergoes oxidation under alkaline aerobic conditions, and inhibitory potency is reduced under oxygen restriction. Leveraging long-range 1H-15N HSQC NMR and electrochemical measurements, we demonstrated that the oxidized form of the trihydroxybenzaldoxime induces LThDP decarboxylation and accepts electrons from the resulting carbanion, resulting in reduction to the catechol and formation of acetyl-ThDP which hydrolyzes to form acetate. Under aerobic conditions the catechol is reoxidized. Thus, these compounds act as redox cycling, substrate-wasting inhibitors of DXP formation. These findings uncover a novel activity and mechanism of DXPS inhibition which may have implications for DXPS-mediated redox activity in bacteria. Further exploration of redox active DXPS probes may provide new insights for inhibition strategies and selective probe development.

Trihydroxybenzaldoximes are redox cycling inhibitors of ThDP-dependent DXP synthase

Pathogenic bacteria must swiftly adapt to dynamic infection environments in order to survive and colonize in the host. 1-Deoxy-d-xylulose-5-phosphate synthase (DXPS) is thought to play a critical role in bacterial adaptation during infection and is a promising drug target. DXPS utilizes a thiamine diphosphate (ThDP) cofactor to catalyze the decarboxylative condensation of pyruvate and D-glyceraldehyde-3-phosphate (d-GAP) to form DXP, a precursor to isoprenoids and B vitamins. DXPS follows a ligand-gated mechanism in which pyruvate reacts with ThDP to form a long-lived lactyl-ThDP (LThDP) adduct which is coordinated by an active-site network of residues. d-GAP binding ostensibly disrupts this network to activate LThDP for decarboxylation. Our lab previously reported trihydroxybenzaldoximes inhibitors which are competitive with respect to D-GAP, and uncompetitive with respect to pyruvate, suggesting they bind after E-LThDP complex formation. Here, we conducted mechanistic studies to determine if these compounds inhibit DXPS by preventing LThDP activation or if they act as inducers of LThDP activation. We discovered that the catechol moiety of the trihydroxybenzaldoxime scaffold undergoes oxidation under alkaline aerobic conditions, and inhibitory potency is reduced under oxygen restriction. Leveraging long range 1H-15N HSQC NMR and electrochemical measurements, we demonstrated that the oxidized form of the trihydroxybenzaldoxime induces LThDP decarboxylation. The oxime moiety accepts electrons from the resulting carbanion, resulting in formation of acetyl-ThDP which hydrolyzes to form acetate. SAR studies revealed that the catechol attenuates the redox activity of the oxime moiety, and under aerobic conditions these compounds are oxidized and thus act as redox cycling inhibitors of DXPS. Further exploration of redox active DXPS probes may provide new insights for inhibition strategies and selective probe development.

Cytotoxic activity of high dose ascorbic acid is enhanced by 2-deoxy-d-glucose in glycolytic melanoma cells

Although, numerous in vitro studies showed that cancer cells are killed after exposure to pharmacological doses of ascorbic acid (AA), significant clinical data proving the efficacy of AA is still absent. A hallmark of most tumor cells is an altered glucose metabolism characterized by an upregulation of glycolysis despite normoxic conditions (Warburg effect). Since pyruvate is capable of detoxifying hydrogen peroxide (H₂O₂), the alleged mediator of AA-induced toxicity, it seems likely that enhanced glycolysis and subsequent higher pyruvate formation might be an explanation for the attenuated effect of pharmacological AA in vivo. Therefore, inhibition of glycolysis might be a promising approach to enhance the anticancer effect of AA by diminishing the generation of pyruvate. Considering the altered metabolism of cancer cells, we examined the cytotoxic potential of 2-DG and/or AA using SRB assay in two different cell lines: a glycolytic human melanoma (451Lu) and a non-glycolytic breast cancer (MCF-7) cell line. Inhibition of glycolysis increased AA cytotoxicity in 451Lu cells, whereas same treatment had a marginal effect on MCF-7 cells. We also investigated the influence of glycolysis inhibition on H₂O₂ generation. H₂O₂ concentrations were higher in presence of 451Lu cells when pretreated with 2-DG, but not in MCF-7 cells. Treatment with 10 mM 2-DG decreased pyruvate and lactate concentrations in both cell lines in a concentration-dependent manner. In summary, 2-DG enhances the cytotoxic effect of AA in glycolytic 451Lu cells by increasing AA-induced H₂O₂ concentration. This result indicates that lower pyruvate levels, as a result of glycolysis inhibition, may be responsible for the enhanced effect of 2-DG on AA toxicity. Further experiments are needed to clarify the underlying mechanism and the potential use in cancer therapy.

Standardized Arrabidaea chica Extract Shows Cytoprotective Effects in Zoledronic Acid-Treated Fibroblasts and Osteoblasts

INTRODUCTION

Osteonecrosis of the jaw is a condition associated with intraoral ulceration and bone necrosis induced by antiresorptive medications, such as zoledronic acid, a bisphosphonate. Previous data on Arrabidaea chica (H&B.) Verlot wound healing activity prompted the study reported herein on A. chica standardized hydro alcoholic extract in vitro cytoprotective activity data on epithelial and osteoblastic cells exposed to zoledronic acid (ZA).

METHODS

Primary human gingival fibroblasts and murine pre-osteoblasts were treated with ZA 10 µM together with 5 or 10 µg.mL-1 A. chica extract for 24h and 48 h. At both times, cells were submitted to viability assay and caspase 3/7 activation evaluation. Statistical analysis used one-way ANOVA and p=0.05.

RESULTS

In cell viability assay, a drastic damage effect of ZA appeared after 48 h in both epithelial (55.8%) and pre-osteoblastic cells (39.7%). When treated with ZA in combination with A. chica extract, cells showed higher viability values: 74.1%-82.3% for fibroblasts and 66% for pre-osteoblasts. Furthermore, the combined treatment presented lower caspase 3/7 activation in fibroblasts and pre-osteoblasts.

CONCLUSION

At low concentrations, A. chica extract showed promising cytoprotective effects against ZA-induced damage actions; however, further in vitro and in vivo studies are required to establish the mechanism of action.

Dietary antioxidants as a source of hydrogen peroxide

Studies of 54 antioxidants revealed that 27 of them, mainly polyphenols, generated hydrogen peroxide (H₂O₂) when added to Dulbecco's modified Eagle's medium (DMEM), other media used for culture of mammalian and yeast cells and phosphate-buffered saline. The most active antioxidants were: propyl gallate (PG), (-)-epigallocatechin gallate (EGCG) and quercetin (Q). Chelex treatment and iron chelators decreased H₂O₂ generation suggesting that transition metal ions catalyze antioxidant autoxidation and H₂O₂ production. Green tea also generated H₂O₂; tea prepared on tap water generated significantly more H₂O₂ than tea prepared on deionized water. Ascorbic acid decreased H₂O₂ production although it generated H₂O₂ itself, in the absence of other additives. Lemon added to the tea significantly reduced generation of H₂O₂. Hydrogen peroxide generated in the medium contributed to the cytotoxicity of PG, EGCG and Q to human prostate carcinoma DU-145 cells, since catalase increased the survival of the cells subjected to these compounds in vitro.