Activities of purine-metabolizing enzymes in human colon carcinoma cell lines and xenograft tumors

Abstract

The activities of eleven enzymes of purine metabolism have been determined in extracts of four human colon carcinoma cell lines (clones A and D of the heterogeneous DLD-1 line, HCT-15 and DLD-2) in culture and as xenografts in nude mice. Activities of the enzyme adenine phosphoribosyltransferase (APRT), hypoxanthine phosphoribosyltransferase (HPRT), adenosine kinase, 5'-deoxy-5'-methylthioadenosine (MTA) phosphorylase, adenylate (AMP) kinase, guanylate (GMP) kinase, and nucleoside diphosphokinase (NDP kinase) changed little from line to line in vitro. Adenosine deaminase (ADA) activity varied more than 5-fold between HCT-15 and clone A cells. Purine nucleoside phosphorylase (PNP) activity was about 3-fold higher in DLD-2 cells than in the other three lines. Guanine deaminase (guanase) activity was 4- to 6-fold higher in clone A and D cells than in HCT-15 and DLD-2 cells. Xanthine oxidase was not detected in any of the lines grown in vitro or in vivo. The activity of APRT was highest in clone D xenografts. HPRT was lowest in DLD-2 tumors. ADA activity in DLD-2 tumors was more than 2-fold elevated over that for clone A and HCT-15 xenografts. PNP activity in clone A and HCT-15 xenografts was lower than that for clone D or DLD-2 tumors. Guanase was lower in HCT-15 and DLD-2 tumors than in those of clones A and D. Clone D tumor GMP kinase activity was elevated more than 4-fold above that for the other three tumors. The activity of NDP kinase was highest in clone D tumors. MTA phosphorylase activity was similar in all four xenografts. In general, the activity of a given enzyme was similar in cells growing in monolayer culture and as xenografts in nude mice (37/44 comparisons). Enzyme activities were also measured in normal human colon and compared to those in the xenografts. Where significant differences were seen, the values for normal tissue were almost always lower than those of the tumors. The striking similarities between the in vitro and in vivo enzymic profiles for each of these four human colon carcinoma lines indicate that, at least on this basis, responses of these cells to purines and purine analogs in the two-dimensional monolayer culture system should be predictive of in vivo responses.