6-thioguanine resistance in a human colon carcinoma cell line with unaltered levels of hypoxanthine guanine phosphoribosyltransferase activity

Cell populations resistant to high doses (30 mu M) of 6-thio-guanine (6-TG, 6-TG(r) cells) were selected from a human colon carcinoma cell line, LoVo. This cell line, which lacks hMSH2, a component of the human mismatch bindi ng heterodimer hMutS alpha, is resistant to low doses of 6-TG. The level of activity of hypoxanthine-guanine phosphoribosyltransferase, the enzyme res ponsible for the phosphoribosylation of the thiopurine, was comparable to t hat expressed in the parental cells, Mo significant difference was found in the levels of enzyme activities involved in the conversion of 6-TG or its derivatives into non-toxic compounds, In contrast, a significant difference was found in the uptake kinetics of 6-TG in the 2 cell types. Met uptake o f 6-TG ceased after 100-sec incubation in the 6-TG(r) cells, while it appea red to continue throughout the 10 min incubation in the wild-type cells, As a consequence, after IG-min incubation, the total amount of 6-TG taken up by the parental LoVo cells was approximately 3 times higher than that prese nt in the 6-TG(r) cells. (C) 1999 Wiley-Liss, Inc.