Downstream targets of altered sphingolipid metabolism in response to inhibition of ENOX2 by phenoxodiol

Abstract

Phenoxodiol, an ENOX2 inhibitor, alters cytosolic NADH levels to initiate a regulatory cascade linking sphingolipid metabolism and the PI3K/Akt pathway to programmed cell death. Specifically, the pyridine nucleotide products of plasma membrane redox, NAD+ and NADH, directly modulate in a recriprocal manner two key plasma membrane enzymes. NADH stimulation of sphingomyelinase and NADH inhibition of sphingosine kinase potentially lead to G1 arrest (increase in ceramide) and apoptosis (loss of sphingosine‐1‐phosphate). The findings link plasma membrane electron transport and the anticancer action of several clinically‐relevant anticancer agents targeted to ENOX2 such as phenoxodiol. Growth inhibition by phenoxodiol is unaffected by inhibitors of protein or mRNA synthesis. Findings with okadiaic acid, an inhibitor of serine/threonine phosphatases, suggest that hyperphosphorylation of intracellular substrates does not affect the action of phenoxodiol on ENOX2. Our findings and those of others are consistent with operation of the FAS signaling pathway of apoptosis and its suppression by sphingosine‐1‐phosphate. The prevailing hypothesis is that products of Akt activation, c‐FLIP and XIAP, which exhibit anticaspase activities to block FAS signaling when sphingosine‐1‐phospate is elevated, are down regulated to permit apoptosis when sphingosine‐1‐phosphate is decreased by inhibition of sphingosine kinase under conditions of elevated cytosolic NADH associated with anticancer drug inhibition of ENOX2.

Phase I/II Trial of the Combination of 177Lutetium Prostate specific Membrane Antigen 617 and Idronoxil (NOX66) in Men with End-stage Metastatic Castration-resistant Prostate Cancer (LuPIN)

“…We hypothesised that combining a tumour-specific radiation sensitiser with LuPSMA-617 would reduce radiation resistance, prolonging and deepening treatment responses compared with LuPSMA-617 alone. We proposed that idronoxil, via its effects on apoptosis, cell cycle arrest, and topoisomerase II, may impair DNA repair mechanisms and improve sensitivity to LuPSMA-617 [9,10,26]. Furthermore, we postulated 9) 0 (0) 11 (34) Pneumonitis a 0 (0) 1 ( 3) 0 (0) 1 (3) Neutropenia 3 ( 9) 0 (0) 0 (0) 3 ( 9) AE = adverse event.…”

“…Idronoxil is a synthetic flavonoid derivative of genistein that inhibits external NADH oxidase 2 (ENOX2) to induce apoptosis and cell cycle arrest, and inhibit DNA topoisomerase 2 [5][6][7][8][9][10]. ENOX2 inhibition activates the sphingomyelinase pathway and deactivates the antiapoptotic protein kinase B (Akt) pathway [10], a key driver of radiotherapy resistance [11]. The targeted effect on ENOX2 has the potential to limit toxicity to noncancer cells, which preferentially express ENOX1 [12].…”

Reciprocal Relationship Between Cytosolic NADH and ENOX2 Inhibition Triggers Sphingolipid‐Induced Apoptosis in HeLa Cells

“…Drug-induced ceramide accumulation inhibits proliferation, induces apoptosis and promotes differentiation [Geilen et al, 1997]. The results with okadaic acid [De Luca et al, 2009] support the concept that ceramide might block activation of Akt by inhibiting its translocation to the cell membrane [Stratford et al, 2001] rather than by promoting its dephosphorylation by phosphatases. Furthermore, an increase in longchain C16-ceramide, the most abundant species produced by SMase activation at the plasma membrane may contribute to the degradation of XIAP and FLIP by activating the proteosome [Kroesen et al, 2003].…”

Trans-Plasma Membrane Electron Transport in Mammals: Functional Significance in Health and Disease

“…Mouse embryonic fibroblasts derived from transgenic mice overexpressing a truncated form of ENOX2 showed accelerated growth rates with respect to wild-type fibroblasts and enhanced invasiveness (315). In contrast, decreased cell growth and migration, as well as increased cell death, are observed when the function of ENOX2 is impaired, either by inhibiting the catalytic activity with (3)-epigallocatechin-3-gallate, capsaicin, phenoxodiol, or anti-ENOX2 antibodies (47,72,306,315) or by down-modulating its expression by RNA interference (173). Morré and colleagues also viewed ENOX2 as both (i) a noninvasive, circulating tumor marker (based on their description of several cancer-type specific ENOX2 isoforms) (119,307) and (ii) a potential antitumoral drug target, since this enzyme is blocked by quinone site inhibitors, which also exhibit anticancer activity (54,119,208).…”