Abstracts:
The Effects of DHEA, a Dietary Supplement, and Natural Anticarcinogenic Steroid, on Protein and Glutathione Homeostasis in vitro
John E. Biaglow, Jerry J. Donahue, Ann R. Kennedy and Stephen Tuttle
The Department of Radiation Oncology, University of Pennsylvania School of Medicine, 195 John Morgan Building, 3620 Hamilton Walk, Philadelphia, Pennsylvania, USA 19104-6072
In spite of many years of study very little is known about the mechanism of action of the anticarcinogenic steroid, DHEA. It is reported to be a specific inhibitor of glucose-6-phosphate dehydrogenase (G6PD). However, DHEA inhibits the growth of G6PD deficient cells. This unexpected effect led us to investigate alternative mechanisms for its action. We discovered that DHEA inhibited flavoprotein-linked reductions, in G6PD deficient cells, involving respiration and various flavoprotein enzymes. DHEA inhibits the selenium enzyme "thioredoxin reductase" (a flavoprotein) linked reduction of lipoate in G6PD- and all other cells tested in vitro. DHEA also inhibits G6PD- cell reduction of hydroxyethyldisulfide catalyzed by glutaredoxin and glutathione (GSH). In the later case this inhibits deglutathionylation of protein disulfides. DHEA induces the cell loss of GSH. GSH depletion can induce apoptotic responses in G6PD- cells. The inhibition of growth caused by DHEA in G6PD- cells may be due to inhibition of respiration. Notably, the G6PD- cells require a source of ribose-5-phosphate for DNA synthesis. This substrate is provided by the non-oxidative limb of the pentose cycle involving the enzymes transketolase and transaldolase. Apparently there is enough NADPH, provided by malate and isocitrate enzymes, for ribonucleotide reductase to reduce nucleotide diphosphates needed for DNA synthesis. We used hydroxythiamine (thiamine antimetabolite) to inhibit the thiamine containing transketolase enzyme. This antimetabolite blocked the growth of the G6PD- cells. In conclusion the mechanism of action of DHEA involves the inhibition of respiration, flavoprotein enzymes and a loss of GSH. All of these effects result in a disruption of cell redox. The G6PD deficient cells can maintain growth, i.e., DNA synthesis, by the non-oxidative limb of the pentose cycle and the enzyme transketolase. This research work is supported by NIH Grant CA44982.