The baculovirus p35 protein is a potent inhibitor of programmed cell death induced by a variety of stimuli in insects, nematodes, and mammalian cell lines. The broad ability of p35 in preventing apoptosis has led us to investigate its effect on mouse embryo fibroblasts in vitro and in vivo. For this purpose, we have used R- cells (3T3-like fibroblasts derived from mouse embryos with a targeted disruption of the insulin-like growth factor I receptor (IGF-IR) genes) and R508 cells (derived from R- and with 15 x 10(3) IGF-IRs per cell). Both cell lines grow normally in monolayer, but they do not form colonies in soft agar, and they are non-tumorigenic in nude mice. We show here that, in addition to its anti-apoptotic effect, p35 causes transformation of R508 cells, as evidenced by the following: 1) decreased growth factor requirements, 2) ability to form foci in monolayer and colonies in soft agar, and 3) ability to form tumors in nude mice. Since R- cells stably transfected with p35 do not transform, our observations suggest that in addition to its effect as an inhibitor of apoptosis, the baculovirus p35 protein has transforming potential that requires the presence of the IGF-IR. The possibility that these two properties could be separated was confirmed by demonstrating that R508 cells expressing another anti-apoptotic protein, Bcl-2, could not form tumors in nude mice
