Abstract
The effects of a wide range of selected chemical carcinogens on the frequency of Epstein‐Barr virus (EBV)‐induced transformation have been investigated. The carcinogens tested included direct‐acting chemicals and chemicals requiring either activation via reactions with nucleophiles, or cell‐mediated enzyme activation. Treatment with some but not all chemicals suspected of being carcinogens resulted in enhanced EBV‐induced transformation of neonatal or adult peripheral blood lymphocytes (PBLs). The temporal relationship between carcinogen exposure and EBV infection could dramatically influence the results of the chemical carcinogen‐cellular interaction as measured by the cells' ability to subsequently undergo morphologic transformation. This relationship was particularly evident when cells were treated with alkylating agents such as dimethylsulfonate (DMS) or N‐methyl‐N′‐nitro‐N‐nitro‐soguanidine (MNNG). Beginning at 24h, and at later times following EBV‐infection, cellular transformation became more resistant to the cytotoxic effects of DMS and, in contrast, more sensitive to the cytotoxic effects of MNNG. These diametrically opposed results clearly demonstrate the ability of EBV infection to alter the response of lymphocytes to chemical carcinogens as measured by transformation. The lymphoblastoid cell lines (LCLs) established from carcinogen‐treated PBLs had increased cloning efficiency. Furthermore, using radiolabelled, molecularly cloned subgenomic fragments of EBV DNA and DNA‐DNA hybridization, we have been able to detect an increased number of EBV genome equivalents in whole‐cell and high‐molecular‐weight cellular DNA extracted from LCLs established from MNNG as well as DMS‐treated PBLs. We propose that carcinogen enhancement of EBV‐induced transformation is an example of a two‐step mechanism of oncogenic transformation in primary human lymphoid cells. The possible significance of these findings in relation to potential development of lymphomas following EBV exposure will be discussed.