Differential effects of NAD, nicotinamide and related compounds upon growth and nucleoside incorporation in human cells

Abstract

Two human melanoma cell lines, MM96 and MM127, were found to be highly sensitive to the toxicity of adenosine ( d 50 100–150 μg/ml) compared with other melanoma lines. HeLa cells and a lymphoblastoid line ( d 50 > 500 μg/ml). The MM127 line was also sensitive to NAD ( d 50 41 μg/ml) compared with the other lines ( d 50 > 400 μg/ml), and accumulated three-fold more NAD-derived isotopic label. Nicotinamide exhibited little toxicity in any cell type ( d 50 > 400 μg/ml); 25–100 μg/ml nicotinamide greatly increased the plating efficiency of melanoma cells and fibroblasts when low levels of foetal calf serum were used. The toxicity of DNA-damaging agents (alkylating agents and u.v.) in melanoma cells was not reduced in the presence of NAD, adenosine or nicotinamide. Studies of the effects of the latter compounds upon the incorporation of deoxynucleosides showed that: (a) melanoma cells have lower purine pools than fibroblasts; (b) [ 3H]deoxyguanosine incorporation was inhibited more than [ 3H]deoxyadenosine incorporation; (c) incorporation of [ 3H]deoxyadenosine and [ 3H]deoxyadenosine and [ 3H]deoxyguanosine into RNA was inhibited by adenosine, thus providing a method for determination of guanine-specific DNA repair; and (d) NAD enhanced thymidine incorporation in intact melanoma cells but not in fibroblasts, in a pattern similar to the release from template restriction previously reported for permeabilised tumour cells.