IMPAIRMENT OF MOLONEY MURINE LEUKEMIA-VIRUS INTEGRATION IN A CELL-LINE UNDEREXPRESSING DNA TOPOISOMERASE-II

The possible intervention of nuclear proteins as cofactors of integras e-catalyzed integration of retroviral DNA into the host cell genome is not fully understood, Among various nuclear proteins, DNA topoisomera se II appears to be a plausible candidate, This hypothesis is supporte d by a series of evidence, including the fact that integration is mark edly affected by the topology of the target DNA and mainly occurs in t ranscribed regions in which topoisomerase II is preferentially located . In an attempt to confirm the validity of this hypothesis, we have co mparatively investigated the early stages of a recombinant Moloney mur ine leukemia virus (psi neo) in two related Chinese hamster cell lines (DC3F and R/DC3F) expressing different levels of both isoforms of top oisomerase II, R/DC3F is derived from the parental cell line DC3F and displays a resistant phenotype towards the usual anticancer topoisomer ase II inhibitors (actinomycin D, doxorubicin, and taxol), Results sho w that the early stages of the retroviral cycle are markedly impaired in cells underexpressing topoisomerase II (R/DC3F), This alteration mi mics Fv-1 restriction and is characterized by about a 6-fold decrease in viral DNA synthesis and total inhibition of viral genome integratio n. The specific impairment of integration in R/DC3F cells compared to DC3F cells is assessed by the absence of G418-resistant colonies upon viral infection and a lack of the viral genome in cellular nuclear DNA as detected by the PCR procedure, These features are observed in rele vant infecting conditions leading, in both cell lines, to the same amo unt of linear viral DNA and to the occurrence of two long terminal rep eats containing circular DNA in the nuclear fractions.