Mechanistic implications of alterations in HL-60 cell nascent DNA after exposure to 1-β- d -arabinofuranosylcytosine

To improve our understanding of the mechanism of 1-β-d-arabinofuranosylcytosine (ara-C) incorporation into DNA, we investigated the physical properties (size, position of nucleoside incorporation) of small fragments of nascent DNA (nDNA) obtained by pH-step alkaline elution of intact HL-60 cells following their exposure to ara-C. In the pH-step alkaline elution procedure, the smallest fragments of nDNA elute at pH 11. Anion-exchange high-performance liquid chromatography (HPLC) of nDNA obtained by 1 h elution at pH 11.0 of lysed HL-60 cells revealed a preponderance of nDNA fragments ranging from 0.5 to 40 kb in control ([3H]-dThd-labeled) cells. Exposure of cells to ara-C (0.8–1 μm) resulted in a loss of the preponderance of radiolabel in fragments of 0.5–40 kb along with redistribution of the radiolabel (from [3H]-dThd or [3H]-ara-C) into smaller nDNA fragments (predominantly <100 bases in length) as determined by HPLC. We used the ability of pH-step alkaline elution to provide these small nDNA fragments produced by ara-C to investigate the paradoxical behavior of ara-C as a chain terminator in cell-free DNA synthetic systems while being incorporated into an internucleotide position in intact cells. Following the digestion of purified nDNA with micrococcal nuclease and spleen phosphodiesterase II, the proportion of radiolabel in 3′-dNMP (indicating an internucleotide position) or free nucleoside (indicating a chain terminus position) was determined by reverse-phase HPLC. In digests of prelabeled genomic DNA, as expected, <90% of the radiolabel from [14C]-dThd or [3H]-ara-C was found to exist in an internucleotide position (as determined by co-chromatography with authentic 3′-dTMP or 3′-ara-CMP). In contrast, digests of nDNA that eluted at pH 11.0 revealed a significantly higher proportion of radiolabel in the chain terminus position (29%–35%) when the nDNA was obtained from cells exposed to 1 μm [3H]-ara-C as compared with cells exposed to [3H]-dThd or [3H]-dCyd alone (<10%). These data obtained from pH-step alkaline elution of intact cells suggest that by causing the inhibition of chain elongation while failing to inhibit the formation of new nDNA replication intermediates, ara-C exposure leads to the production of very small nDNA fragments. This relative chain-terminating effect of ara-C is most apparent in the small nDNA replication fragments that elute at pH 11.0. Since ara-C is accumulated predominantly in an internal position, even in the small nDNA fragments that elute at pH 11.0, ligation and gap-filling mechanisms in the intact cell must rapidly transform ara-C from a chain terminus to an internal position.