Growth inhibition of human tumor cells in soft-agar cultures by treatment with low levels of adenosine 5'-triphosphate.

Abstract

Treatment of a variety of human tumor cells in monolayer cultures with low levels (40 to 80 microM) of adenosine 5'-diphosphate (ADP) or adenosine 5'-triphosphate (ATP) was recently shown to produce arrest of cellular growth in the S phase of the cell cycle (E. Rapaport, J. Cell. Physiol., 114: 279-283, 1983). We now demonstrate that exposure of two well-characterized colonic adenocarcinoma (HT-29 and SW-620) and two pancreatic adenocarcinoma (CAPAN-1 and PANC-1) cell lines in soft-agar cultures to exogenously supplied 5 to 20 microM ATP results in substantial inhibition of cellular growth. Exposure of the cells to 5 to 20 microM ADP produces slightly smaller growth-inhibitory effects, while 20 microM adenosine 5'-monophosphate or adenosine have marginal effects on cellular proliferation in these systems. Successful demonstration of these effects requires the use of heat-inactivated fetal bovine serum, since normal fetal bovine serum possesses enzymatic activities which catalyze the rapid degradation of adenine nucleotides. Tumor cell growth was assayed by the well-established colony formation assay as well as by [3H]thymidine incorporation into acid-insoluble material. [3H]Thymidine incorporation is performed 4 to 14 days after plating and correlates well with results obtained by colony formation assays. Due to the ectoenzymatic activities of the cells which include adenosinetriphosphatase and adenosinediphosphatase catalyzing the dephosphorylation of ATP and ADP, the effective levels of ATP that inhibit the growth of human tumor cells in this system, which is widely claimed to predict the in vivo response of a tumor, are lower than the 5 to 20 microM which are exogenously supplied. The two previously characterized, well-differentiated pancreatic and colonic tumor cell lines (CAPAN-1 and HT-29) were shown to exhibit higher chemosensitivity towards treatment with ATP and ADP than did the lesser-differentiated pancreatic and colonic tumor cell lines (PANC-1 and SW-620).