Cell growth stimulation by the redox protein thioredoxin occurs by a novel helper mechanism

Thioredoxins are a class of low molecular weight redox proteins that undergo reversible reduction-oxidation of two active-site cysteine residues with reduction catalyzed by the NADPH-dependent flavoenzyme thioredoxin reductase. Human thioredoxin has been shown to be identical to a previously reported leukemic cell growth factor. We now report that recombinant human thioredoxin added to minimal culture medium in the absence of serum stimulates the proliferation of a number of human solid tumor cell lines measured over several days. The concentration of thioredoxin producing half-maximal stimulation of MCF-7 breast cancer cell proliferation was 350 nM, and the maximum stimulation occurred at 5 microM. The maximum increase in cell proliferation caused by thioredoxin was up to 90% of that seen with 10% bovine serum in the medium. There was a positive correlation between the ability of cell lines to proliferate in minimal medium, presumably, due to the autocrine production of growth factors by the cells, and the stimulation of proliferation by thioredoxin. Neither a redox inactive, mutant human thioredoxin, C32S/C35S, nor reduced Escherichia coli thioredoxin were able to stimulate MCF-7 cell proliferation. MCF-7 cell proliferation caused by human thioredoxin was completely abolished if the culture medium was changed each day. Antibody to thioredoxin blocked the cell proliferation caused by thioredoxin. Studies with 125I-labeled thioredoxin showed time-dependent binding to the surface of MCF-7 cells, but the binding was not saturable, indicating the absence of specific binding of thioredoxin to a cell surface receptor. Most of the thioredoxin associated with the cell could be released by trypsinization, and relatively little intact thioredoxin was taken up by the cell. The results of the study suggest that thioredoxin acts by a novel helper, redox mechanism to increase the cell proliferation response to growth factor(s) produced by the cell itself.