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

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.