DNA polymerase activities in differentiating mouse neuroblastoma N-18 cells.

Abstract

The activities of two DNA polymerases (DNA nucleotidyltransferases) were characterized in mouse neuroblastoma clone N-18 on the basis of their apparent molecular weights (determined by sucrose density gradient centrifugation: polymerase-a, 7.5-8 S; polymerase-i, 3-4 S) and relative inhibition by sulfhydryl-blocking agents. N-Ethylmaleimide (10 mM) and iodoacetamide (1.5 mM) inhibited DNA polymerase-a activity completely, whereas only 3540% inhibition was observed for DNA polymerase-# under similar conditions. DNA polymerase-activity was reduced 50-70% in N-18 cells that had been induced to differentiate by 4MpM bromodeoxyuridine, and the low molecular weight DNA polymerase-$ activity remain unchanged. With activated calf thymus DNA as template, only DNA polymerase-a activity was stimulated in the presence of added ribonucleotides and purified Escherichia coli RNA polymerase.Clones and N-18 (neutral) isolated from mouse neuroblastoma C-1300 cells (1, 2) appear to be useful model systems (3) for studying chemically induced (4,5) neurite formation. The halogenated thymidine analogue 5-bromodeoxyuridine (BrdUrd) has been used in the study of differentiation of neuroblastoma cells (4), activation of latent viruses in certain cultured cells (6), and modulation of gene expression (7,8) in a variety of eukaryotic cells. Low concentrations (2-5 gM) of BrdUrd enhance cellular differentiation, whereas higher concentrations (40-700 gM) are cytotoxic (9-12). Further studies have revealed that BrdUrd (0.1-33 ,uM) inhibits the synthesis of a number of cell-specific proteins and enzymes (13)(14)(15), with concomitant incorporation of the thymidine analogue into DNA (16)(17)(18)(19)(20)(21)(22). Meuth and Green (23) reported that BrdUrd acted as an allosteric inhibitor of ribonucleotide reductase, which depletes deoxycytidine pools in mouse fibroblast lines 3T6 and 3T3 at concentrations of 20-50 ,M. Hill et al. (24) reported alterations in the incorporation of nucleotides during RNA transcription as a results of BrdUrd-induced errors in the genome during repair or replication. A BrdUrdadapted Syrian hamster melanoma clone (HAB) contained a greater proportion of heat-labile glucose 6-phosphate dehydrogenase molecules than the parent, unsubstituted cell line, and this correlated well with the level of BrdUrd substitution in the genome (25). While most reports suggest that many of the biochemical changes after BrdUrd (10 ,uM) treatment are directly related to its incorporation into DNA (26), none of the reports mentioned above has compared levels of DNA polymerase (DNA nucleotidyltransferase) activity after BrdUrd treatment. BrdUrd-induced differentiation of mouse neuroblastoma cells has not been correlated with simultaneous alteration of DNA polymerase activities (27). The possibility of specific inhibition of DNA biosynthesis by BrdUrd was sug-The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U...