Coamplification of mu class glutathione S-transferase genes and an adenylate deaminase gene in coformycin-resistant Chinese hamster fibroblasts

GNAI3, GNAT2, AMPD2, GSTM are clustered in 120 kb of Chinese hamster chromosome 1q

We studied a polygenic region located on Chromosome (Chr) 1q in Chinese hamster cells that is coamplified along with the AMPD2 gene. Previous sequence analysis identified both members of the GSTM family and the GNAI3 gene within a cloned 120-kb region surrounding the AMPD2 locus. We show here that the GNAT2 gene, which is inactive in the fibroblastic cells, lies within the 20 kb separating the transcriptionally active GNAI3 and AMPD2 genes. We map most gene ends by sequence comparison with human homologs; one is inferred from the presence of an unmethylated CpG island. This Chinese hamster locus corresponds to a region of conserved linkage between human Chr 1 (locus 1p13) and mouse Chr 3 (position 52.5 cM), where Gnai-3 and Gnat-2 have been mapped. The AMPD2 gene is presently unlocalized in human genome; its proposed position on mouse Chr 3 is at 53.4 cM. Our results, obtained by physical mapping, strongly suggest that the order and possibly the tight linkage of these genes are conserved on all three genomes.

Reversion in Chinese hamster lines amplified at the AMPD2 locus: spontaneous and benzamide-stimulated gradual loss of amplified alleles of marker genes

The HC47 and HC474 cell lines of Chinese hamster fibroblasts resist coformycin through the intrachromosomal amplification of the AMP deaminase 2 (AMPD2) gene. Due to the coamplification of a mu glutathione S-transferase (GST) gene, these mutant lines are more sensitive than GMA32 wild-type parental cells to buthionine sulfoximine (BSO), an inhibitor of glutathione biosynthesis. This property was exploited to select revertants of amplification from HC474 cells. Reversion in that line is frequently a gradual process that does not involve extrachromosomal intermediates. The terminal products of this process are commonly cells with a complete deletion of the amplified allele of marker genes and are therefore haploid for these loci on the homologous chromosome. Exposing HC474 cells to benzamide (BA), an inhibitor of polyADP-ribosylation, increased the recovery of revertants to an extent allowing the detection of reverting cells without BSO selection. This effect of BA was used to isolate revertant cells from the HC47 line that is extremely stable and to demonstrate that the mechanism of gradual reversion also occurs in this line. The gradual deletion of amplified copies within the chromosomes suggests that breakage-fusion-bridge (BFB) cycles drive this process.

A glutathione depletion selectively imposed on mu glutathione S-transferase overproducing cells increases nitrogen mustard toxicity

Glutathione (GSH) contributes to the detoxification of anticancer drugs through the operation of specific glutathione S-transferases (GST) and innate, or acquired, overexpression of this enzyme family has been frequently observed in tumor cell lines. In the GMA32 line of Chinese hamster fibroblasts, we showed that GSH starvation produced by exposing cells to buthionine sulfoximine (BSO) increased the toxicity of chlorambucil and melphalan, but not that of N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU), cisplatine and doxorubicin. This indicates that efficient mechanisms of detoxification using GSH operate for chlorambucil and melphalan, but not for the other drugs in these cells. We then showed that GSH depletion could be selectively and transiently induced in the mu GST overexpressing cell line derived from GMA32, HC474, by exposing cells to substrates specific to the overexpressed isozyme. Exposing cells to such a substrate, trans-stilbene oxide, does not alter the sensibility of GMA32 cells to melphalan and chlorambucil, but increases that of HC474 cells to these drugs, to an extent comparable to that obtained with BSO. This observation highlights the possibility of exploiting GST overexpression, a frequent feature of tumor cells, to selectively sensitize these undesirable cells to anticancer drugs.