Preservation of DNA integrity in human and mouse leukemic cells induced to terminally differentiate by chemical agents

Camptothecin derivatives induce regression of human ovarian carcinomas grown in nude mice and distinguish between non-tumorigenic and tumorigenic cells in vitro

We have recently shown that the plant alkaloid 20(S)-camptothecin and its derivatives 9-nitro-20(S)-camptothecin(9NC) and 9-amino-20(S)-camptothecin(9AC) inhibit the growth of a variety of human tumors xenografted in nude mice. In this report, we demonstrate that 9NC and 9AC effectively inhibit growth, and subsequently induce regression, of human ovarian tumors grown in nude mice. Tumor regression is accompanied by degenerative changes in the tumor cells as assessed by microscopic observations of histological sections prepared from the tumors. Parallel experiments in vitro show that 9NC inhibits in a similar manner the growth of human ovarian carcinoma cells, regardless of their ability to induce tumors when xenografted in nude mice, and induces similar morphological changes in both non-tumorigenic and tumorigenic cells, as assessed by microscopic observation. Flow cytometry studies show that 9NC-induced growth inhibition of the non-tumorigenic cells is associated with accumulation of these cells in G2. In contrast, 9NC-induced growth inhibition of the tumorigenic cells is associated with the generation of cells containing a reduced DNA content, that is, cells programmed to die. In conclusion, camptothecins appear to be cytostatic for non-tumorigenic, but cytotoxic for tumorigenic cells, an important finding from viewpoints of cell biology, pharmacology and cancer chemotherapy.

Radiolabeling of DNA can induce its fragmentation in HL-60 human promyelocytic leukemic cells

Incorporation of radiolabeled thymidine is commonly used to investigate DNA damage. Using a filter-binding assay, we observed that the addition of various doses of [methyl-3H]thymidine (0.2 and 2 microCi/ml) or [2-14C]thymidine (0.02 and 0.2 microCi/ml) in the culture medium for 2 days, a standard method for cell-labeling, induces DNA fragmentation in HL-60 human promyelocytic cells. This effect was dose- and time-dependent and the DNA fragments were not protein-linked since the levels of DNA fragmentation were identical in the presence and in the absence of proteinase K (0.5 mg/ml). Radiolabeled thymidine-induced DNA fragmentation was associated with an inhibition of cell growth, but cells remained able to exclude trypan blue, suggesting that plasma membrane integrity was conserved, except at very high doses of [methyl-3H]thymidine (2 microCi/ml). By agarose-gel electrophoresis, the DNA-fragmentation was demonstrated to be internucleosomal with a typical ladder pattern. Addition of unlabeled thymidine to the culture medium prevented DNA fragmentation in a dose-dependent manner, indicating that radiolabeled thymidine incorporation in DNA was directly responsible for DNA fragmentation. We conclude that radiolabeling of DNA using thymidine incorporation can induce DNA fragmentation in some cell lines such as HL-60. This observation must be taken into account in methods using radiolabeling to study DNA damage in these cells.

PolyADP ribosylation and Friend erythroleukemic-cell differentiation: action of poly(ADP-ribose) polymerase inhibitors

Previous studies have shown that benzamide and nicotinamide, two inhibitors of poly(ADP-ribose) polymerase, induce erythroid differentiation in Friend erythroleukemic cells. In contrast, we observed that two other commonly used inhibitors, i.e., 3-aminobenzamide and 3-methoxybenzamide, not merely failed to induce differentiation but actually inhibited it. Furthermore, we observed that benzamide at high concentrations induced differentiation, whereas at low concentrations, it inhibited differentiation. We propose that the induction occurring at high concentrations is due to the polar-planar properties of the molecule, while the inhibition at low concentrations might be due to the inhibition of poly(ADP-ribose) polymerase. Cells that were first exposed to an inducer (hexamethylene bisacetamide) and an inhibitor (3-aminobenzamide) and were subsequently incubated in medium lacking these substances did not differentiate. However, re-exposure to the inducer resulted in a very rapid increase in commitment to terminal differentiation. Therefore, 3-aminobenzamide appears to block differentiation prior to the commitment stage. We infer from these findings that polyADP-ribosylation is required for the terminal differentiation of Friend erythroleukemia cells.

DNA strand scission and ADP-ribosyltransferase activity during murine erythroleukemia cell differentiation

The accumulation of DNA strand breaks and activation of ADP-ribosyltransferase (ADPRT) have recently been associated with cellular differentiation. Murine erythroleukemia (MEL) cells undergo erythropoietic differentiation when exposed to dimethyl sulfoxide (Me2SO) and several studies have suggested that DNA strand scission induced by this agent is a prerequisite for expression of the differentiated phenotype. Me2SO induction of MEL cells has also been associated with increases in ADPRT activity in one study, but not in another. We have monitored the effects of Me2SO on DNA strand breaks in preformed and replicating MEL cell DNA. The results clearly demonstrate that DNA fragmentation is not detectable during Me2SO induction of MEL differentiation, even in the presence of 3-aminobenzamide, an inhibitor of ADPRT. Further, these results are consistent with an absence of detectable changes in both endogenous and total potential ADPRT activity during Me2SO-induced MEL differentiation. These findings would argue against Me2SO induction of DNA strand scission and ADPRT in MEL cells undergoing differentiation.

Chemical inducers of differentiation cause conformational changes in the chromatin and deoxyribonucleic acid of murine erythroleukemia cells

The chemical inducers of murine erythroleukemia cell differentiation, dimethyl sulfoxide, sodium butyrate, and ethionine, elicited conformational changes in the DNA and chromatin of treated cells. The chromatin from dimethyl sulfoxide treated and butyrate-treated cells exhibited circular dichroic spectra different from that of the noninduced control. The molar ellipticity [theta]282.5 in isotonic saline decreased from 4900 deg X cm2 X dmol-1 for control chromatin to 3800 and 3600 deg X cm2 X dmol-1 for dimethyl sulfoxide treated and butyrate-treated chromatin, respectively, while that from ethionine-treated chromatin remained virtually unchanged (5400 deg X cm2 X dmol-1). Increasing the ionic strength to 2.5 or 5 M with NaCl resulted in a substantial, uniform, decrease in molar ellipticity. Thermal denaturation profiles of high molecular weight DNA prepared from each of the inducer-treated cells showed a pronounced hyperchromic shift but no change in Tm when compared to control DNA. Circular dichroic spectra of the DNA indicated a decrease in ellipticity [theta]277 from 9600 deg X cm2 X dmol-1 to 8900, 8300, and 8800 deg X cm2 X dmol-1 for ethionine, dimethyl sulfoxide, and butyrate treated cells, respectively. Treatment of the DNA with 3 M NaCl canceled the UV and CD differences. These measurements indicate an increased stacking of bases or an increased compactness of the DNA from induced cells. Concomitant with specific modifications such as hypomethylation of DNA, the data can be interpreted in terms of conformational changes in chromatin resulting from core histone acetylation.

Maintenance of DNA integrity during murine erythroleukemia cell differentiation induced by ethionine and other hypomethylation agents

The extent of single-strand nicks in DNA from murine erythroleukemia cells induced to differentiate to hemoglobin synthesis in the presence of the hypomethylating agent ethionine was estimated and compared to those levels in uninduced cells and from cells induced to differentiate upon exposure to dimethylsulfoxide or butyrate ion. Although ethionine has been shown to cause more extensive hypomethylation in the DNA of induced cells than that caused by dimethylsulfoxide or butyrate ion, the frequency of detected single-strand breaks in the DNA of uninduced, control cells was not significantly different from that of cells exposed to any of these inducing chemicals. This data indicates that no correlation exists between DNA hypomethylation and DNA single-strand breaks and that unmethylated CpG loci likely do not operate as specific endonuclease recognition sites or as potential origins of transcription in these mammalian cells.