Levels of bovine papillomavirus RNA and protein expression correlate with variations in the tumorigenic phenotype of hamster cells.

Three independent cell lines were established from primary cultures of LSH hamster embryo cells infected with bovine papillomavirus type 1 (BPV-1). Although these cell lines differed in their in vitro saturation densities, none was capable of colony formation in soft agar. Interestingly, two cell lines (BPV-HE1 and BPV-HE3) were tumorigenic in nude mice, syngeneic hamsters, and allogeneic hamsters, whereas BPV-HE2 was not. All three cell lines contained similar numbers of the BPV-1 genome (approximately 50 to 200 copies per cell). However, the nontumorigenic BPV-HE2 cell line contained very low levels of BPV-specific RNA and only small amounts of the BPV-1 E5 transforming protein. The efficiency and rate of tumor formation by BPV-HE1 and BPV-HE3 correlated directly with the apparent amount of viral E5 protein. This analysis suggests that there is a threshold level of BPV protein synthesis required for tumorigenicity, there is a continuum of tumorigenic phenotypes which may depend upon the level of BPV protein expression, and BPV-transformed hamster cells can withstand allogeneic transplantation.