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Dependence of the in vitro antiproliferative activity of recombinant human gamma-interferon on the concentration of tryptophan in culture media.

In addition to its antiviral and antibacterial activities, recombinant human gamma-interferon (rHuIFN-gamma) can exert an antiproliferative effect on human cell lines. The mechanisms involved in this antiproliferative activity are poorly understood, but it is known that IFN-gamma can induce indoleamine 2,3-dioxygenase, which enhances tryptophan metabolism and thus depletes the cellular pool of this amino acid. In the present study we have examined the effect of different tryptophan concentrations on the antiproliferative activity of rHuIFN-gamma on four human tumor cell lines, HeLa 229, HEp-2, A549, and T24. Cells were grown in the presence of rHuIFN-gamma (0.01 to 100 ng/ml) and/or tryptophan (10 to 400 micrograms/ml) for 7 days at which time they were counted. rHuIFN-gamma (4 ng/ml) inhibited the growth of A549 and T24 cells by 50%. Hep-2 and HeLa 229 cells were more sensitive to the rHuIFN-gamma induced antiproliferative effects, requiring only 0.4 ng/ml for a 50% inhibition. Addition of tryptophan to the media at concentrations from 10 to 100 micrograms/ml resulted in a significant blockage of the antiproliferative activity of rHuIFN-gamma. For example, when 50 micrograms/ml of tryptophan were added to the media, 10 times more rHuIFN-gamma (4 ng/ml) was needed to inhibit HeLa 229 cells by 50% of the control. The A549 was the most sensitive cell line to the modulatory activity of the tryptophan. Addition of 10 micrograms/ml of tryptophan changed the amount of rHuIFN-gamma needed to produce a 50% inhibition from 4 ng/ml to 100 ng/ml. In summary, in the four human tumor cell lines tested, the antiproliferative activity of rHuIFN-gamma could be modulated by the concentration of tryptophan in the media.