MECHANISMS OF THE REGULATION OF THIOREDOXIN REDUCTASE-ACTIVITY IN CANCER-CELLS BY THE CHEMOPREVENTIVE AGENT SELENIUM

Selenium is an essential trace element, the deficiency of which is ass ociated with an increased incidence of some human cancers, Dietary sup plementation with selenium has been reported to produce a decrease in the incidence of some cancers in humans, Thioredoxin reductase (TR) is a newly discovered homodimeric selenocysteine (SeCys)-containing prot ein that catalyzes the NADPH-dependent reduction of the redox protein thioredoxin (Trx), Trx is overexpressed by a number of human tumors, a nd experimental studies have shown that Trx contributes to the growth and to the transformed phenotype of some human cancer cells, Thus, TR, by reducing Trx, could play a role in regulating the growth of normal and cancer cells, We have investigated mechanisms by which selenium, in the form of sodium selenite, added to serum-free growth medium regu lates TR activity in cancer cell lines, Selenium caused a dose-depende nt increase in cellular TR activity, The increase in TR activity produ ced by 1 mu M Se compared to medium with no added selenium was: for MC F-7 breast cancer cells, 37-fold; for HT-29 colon cancer cells, 19-fol d; and for A549 lung cancer cells, g-fold, In contrast, Jurkat and HL- 60 leukemia cells showed no increase in TR activity, The half-life of the time course of induction of TR in HT-29 cells after adding seleniu m was 10 h. The increase in TR activity was accompanied by an increase in TR protein levels up to 3-fold and an increase in the specific act ivity of the enzyme of 5-32-fold, depending on the cell line. Studies using Se-75 showed that the amount of selenium incorporated into TR in creased with increasing selenium concentration up to a ratio of 1 sele nium per TR monomer, There was an increase in TR mRNA levels of 2-5-fo ld at 1 mu M selenium and an increase in the stability of TR mRNA with a half-life for degradation of 21 h compared to 10 h in the absence o f selenium. Trx mRNA and protein levels and Trx mRNA stability were no t affected by selenium. The results of the study show that the increas e in TR activity caused by selenium is specific and due to several eff ects, including an increase in the stability of TR mRNA leading to inc reased TR mRNA levels, an increase in TR protein, but predominantly to an increase in the specific activity of TR associated with increased incorporation of selenium into the enzyme.