Transformed rat pancreatic islet-cell lines established by BK virus infection in vitro.
Single-cell suspensions of primary rat pancreatic islet cells were infected with BK virus (BKV) prototype (Gardner) and the naturally occurring BKV strain (TU), isolated from human urine. These viral strains have different sequences in their non-coding control regions which contain promoter-enhancer elements and origin of DNA replication. Uninfected cell cultures disintegrated within 3 weeks, while a varying number of virus-infected cultures were immortalized and a majority exhibited focal areas of 2-dimensional growth. A significantly higher proportion of the BKV (TU)- than of the BKV (Gardner)-infected cultures were immortalized. Large tumor (T) antigen expression was evident in virus-infected cultures from day 4 post infection (p.i.), while insulin secretion decreased steadily during the first weeks of culture. Two cell lines were established from BKV (TU)-infected cultures. Line 5A4 was contact-inhibited, growing as a dense monolayer, while 6A3 demonstrated foci of 2-dimensional growth. Both cell lines have retained their morphology and T-antigen expression for approximately 130 passages. At high passages a low level of intracellular insulin, but no secretion into media, was detected. Based on standard biological tests (reduced serum requirements, growth at low cell density, anchorage-independent growth and tumor induction in nude mice) both cell lines have a fully transformed phenotype, but 6A3 appears to be more malignantly transformed. Since both cell lines were established simultaneously from the same primary cells with the same virus batch, they provide an opportunity to study the transforming mechanisms of BKV in a relative context.