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 u ndergo reversible reduction-oxidation of two active-site cysteine resi dues with reduction catalyzed by the NADPH-dependent flavoenzyme thior edoxin 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 a bsence of serum stimulates the proliferation of a number of human soli d tumor cell lines measured over several days. The concentration of th ioredoxin producing half-maximal stimulation of MCF-7 breast cancer ce ll proliferation was 350 nM, and maximum stimulation occurred at 5 mu m. The maximum increase in cell proliferation caused by thioredoxin wa s up to 90% of that seen with 10% bovine serum in the medium. There wa s a positive correlation between the ability of cell lines to prolifer ate in minimal medium, presumably due to the autocrine production of g rowth factors by the cells, and the stimulation of proliferation by th ioredoxin. Neither a redox inactive, mutant human thioredoxin, (CS)-S- 32/(CS)-S-35, nor reduced Escherichia coli thioredoxin were able to st imulate MCF-7 cell proliferation. MCF-7 cell proliferation caused by h uman thioredoxin was completely abolished if the culture medium was ch anged each day. Antibody to thioredoxin blocked the cell proliferation caused by thioredoxin. Studies with I-125- 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 thio redoxin to a cell surface receptor. Most of the thioredoxin associated with the cell could be released by trypsinization, and relatively lit tle intact thioredoxin was taken up by the cell. The results of the st udy suggest that thioredoxin acts by a novel helper, redox mechanism t o increase the cell proliferation response to growth factor(s) produce d by the cell itself.