A comparison of topoisomerase I activity in normal and transformed cells

Increment of DNA topoisomerases in chemically and virally transformed cells

The activities of topoisomerases I and II were assayed in subcellular extracts obtained from nontumorigenic BALB/c 3T3 A31 and normal rat kidney (NRK) cell lines and from the same cells transformed by benzo[a]pyrene (BP-A31), Moloney (M-MSV-A31) and Kirsten (K-A31) sarcoma viruses, and simian virus 40 (SV-NRK). The enzymatic activity of topoisomerase I was monitored by the relaxation of negatively supercoiled pBR322 DNA and by the formation of covalent complexes between 32P-labeled DNA and topoisomerase I. Topoisomerase II activity was determined by decatenation of kinetoplast DNA (k-DNA). It was found that nuclear and cytoplasmic type I topoisomerase specific activities were higher in every transformed cell line than in the normal counterparts. These differences cannot be attributed to an inhibitory factor present in A31 cells. When chromatin was treated at increasing ionic strengths, the 0.4 M NaCl extract showed the highest topoisomerase I specific activity. Moreover, in this fraction the transformed cells exhibited the most significant increment in the enzymatic activity as compared with nontransformed cultures. Spontaneously transformed A31 cells showed topoisomerase I activity similar to that of extracts of cells transformed by benzo[a]pyrene. Topoisomerase II specific activity was also increased in SV-NRK cells, as judged by the assay for decatenation of k-DNA to yield minicircle DNA.

Abelson-transformed fibroblasts contain nuclear phosphotyrosyl-proteins which preferentially bind to murine DNA

Protein-tyrosine kinases, either in the form of growth-factor receptors or as the polypeptide products of oncogenes, appear to be important in the regulation of cell growth and transformation. A major question, however, is how their substrates mediate changes in gene expression (reviewed in refs 6-8). Because binding of proteins to specific DNA sequences represents the most direct mechanism for regulating transcription, we have investigated the possibility that some DNA-binding proteins may be substrates of protein-tyrosine kinases. Here, we present evidence for nuclear phosphotyrosyl-proteins in murine fibroblasts transformed by the v-abl protein-tyrosine kinase. Furthermore, we have found that these proteins are not significantly phosphorylated in normal NIH 3T3 cells. Finally, using affinity competition chromatography with bacterial and mouse DNA, we have demonstrated that some of these proteins preferentially bind to mouse DNA. The identification of phosphotyrosyl-proteins with selective DNA-binding properties suggests a possible mechanism through which protein-tyrosine kinases may effect changes in gene transcription.