Inhibition of c-myc expression in human promyelocytic leukemia and colon adenocarcinoma cells by 6-thioguanine

Antimetabolite-induced increases in the invasive capacity of murine leukaemia L1210 cells

Pretreatment of murine leukaemia L1210 cells with non-lethal concentrations of various antimetabolites increased the in vitro invasive capacity of these cells into monolayers of rat embryo fibroblasts. The increase in invasive capacity was partly correlated with the induced cell cycle arrest. The concomitant increase in cell surface fucosylation and inhibition of invasion with sulphate indicate a role for glycoproteins in this process. Our results suggest that treatment with antimetabolites may lead to a more aggressive phenotype by altering cell surface properties.

Influence of amsacrine (m-AMSA) on bulk and gene-specific DNA damage and c-myc expression in MCF-7 breast tumor cells

In the MCF-7 human breast tumor cell line, the aminoacridine, m-AMSA, induces protein-associated DNA strand breaks consistent with inhibition of topoisomerase II. However, neither single-strand nor double-strand breaks in DNA, determined using conventional assays, show a consistent relationship with m-AMSA-induced inhibition of growth. In contrast, when DNA strand breaks are determined by alkaline unwinding under the high salt conditions of the alkaline unwinding/Southern blotting (AU/SB) assay, developed by our laboratories, damage to DNA corresponds closely with growth inhibition. The AU/SB assay, which is capable of assessing breaks within large-scale domains (upwards of 1 megabase) surrounding genes of interest, was further utilized to explore the capacity of m-AMSA to induce damage within specific genomic regions that may regulate cell growth. Regions encompassing the transcriptionally active oncogenes, c-myc and c-fos, were found to be more susceptible to m-AMSA-induced strand breaks than the region encompassing the non-transcribed alpha-satellite DNA or the genome as a whole (bulk DNA). These findings demonstrate that m-AMSA may produce more pronounced damage within specific genomic regions than in bulk DNA, m-AMSA also preferentially altered expression of the c-myc oncogene; at an m-AMSA concentration where growth was inhibited by between 70 and 80%, steady-state c-myc mRNA levels declined to approximately 10-15% of control levels within 2-3 hr; furthermore, concentration-dependent reductions in c-myc expression appeared to coincide with growth inhibition. In addition, inhibition of [3H]thymidine incorporation after 2 hr directly paralleled inhibition of growth, suggesting an early effect at the level of DNA biosynthesis, possibly related to the down-regulation of c-myc expression. It is proposed that specific lesions, e.g., in regions surrounding the c-myc gene, as well as generalized lesions in DNA may lead to growth inhibition mediated by down-regulation of the expression of select growth regulatory genes, such as c-myc.

Effect of 6-thioguanine on Chlamydia trachomatis growth in wild-type and hypoxanthine-guanine phosphoribosyltransferase-deficient cells

Chlamydiae have evolved a biphasic life cycle to facilitate their survival in two discontinuous habitats. The unique growth cycle is represented by two alternating forms of the organism, the elementary body and the reticulate body. Chlamydiae have an absolute nutritional dependency on the host cell to provide ribonucleoside triphosphates and other essential intermediates of metabolism. This report describes the pleiotropic effects of the purine antimetabolite 6-thioguanine on chlamydial replication. In order to display cytotoxicity, 6-thioguanine must first be converted to the nucleotide level by the host cell enzyme hypoxanthine-guanine phosphoribosyltransferase. Our results show that 6-thioguanine is an effective inhibitor of chlamydial growth with either wild-type or hypoxanthine-guanine phosphoribosyltransferase-deficient cell lines as the host. Interestingly, the mechanism of 6-thioguanine-induced inhibition of chlamydial growth is different depending on which cell line is used. With wild-type cells as the host, the cytotoxic effects of 6-thioguanine on chlamydial growth are relatively fast and irreversible. Under these circumstances, cytotoxicity likely results from the combined effect of starving chlamydiae for purine ribonucleotides and incorporation of host-derived 6-thioguanine-containing nucleotides into chlamydial nucleic acids. With hypoxanthine-guanine phosphoribosyltransferase-deficient cells as the host, 6-thioguanine must be present at the start of the chlamydial infection cycle to be effective and the growth inhibition is reversible upon removal of the antimetabolite. These findings suggest that in hypoxanthine-guanine phosphoribosyltransferase-deficient cells, the free base 6-thioguanine may inhibit the differentiation of elementary bodies to reticulate bodies. With hypoxanthine-guanine phosphoribosyltransferase-deficient cells as the host, 6-thioguanine was used as a selective agent in culture to isolate a Chlamydia trachomatis isolate resistant to the effects of the drug. This drug resistant C. trachomatis isolate was completely resistant to 6-thioguanine in hypoxanthine-guanine phosphoribosyltransferase-deficient cells; however, it displayed wildtype sensitivity to 6-thioguanine when cultured in wild-type host cells.