Mechanism of Cell Transformation by Polyoma Virus

MECHANISM OF CELL TRANSFORMATION BY POLYOMA VIRUS R. DULBECCO* An important lecture like this one is a good occasion for halting a moment from the experimental work and looking back at the original research object, at the path followed, at the goals reached and those to be reached. One would like to be able to come with a perfect, complete story which leaves no questions unanswered, nor doubts possible. Always, however, the story is different; the results as they are achieved answer some questions, lead to others, and raise new doubts while eliminating others. At first one is disappointed with this state ofaffairs; it is soon realized , however, that it is an attribute ofliving science to be in a state of flux and that only dead science can provide sets ofdefinite answers. Ifyou agree with this view, you will bear with me and follow me as I discuss the results obtained and the questions raised by the recent work ofmy group on the growth regulation ofanimal cells and on the breakdown ofthis regulation caused by polyoma virus. Polyoma virus is a small DNA virus which causes tumors in a variety of rodents. Mien inoculated into newborn mice or hamsters in large doses, it causes a severe disease in which many cells of the body are destroyed. Later on, tumors develop in the animals. The virus, therefore, causes two types of effects: a cell-killing and a cell-transforming effect leading to cancer. In our work we have used animal cells cultivated in vitro rather than animals, since the cultures enable us to make a precise, analytical study of these phenomena. Both the killing effect and the transforming effect can * The SaIk Institute for Biological Studies, San Diego, California. This lecture was delivered at the University of Chicago May 12, 1965, on the occasion of the author's receiving the Ricketts Award. Part of the experimental results reported in the article were obtained from research conducted under a Research Grant from the National Institutes of Health, No. CA 07592. Publication costs were kindly contributed to perspectives inbiology and medicine by The National Foundation. 298 R. Dulbecco · Transformation by Polyoma Vims Perspectives in Biology and Medicine · Winter 1966 be observed and measured in vitro by using different cells [i]. In cultures ofmouse embryo or mouse embryo kidney cells, the virus causes a predominantly cell-killing effect; in cultures of hamster embryo cells, it causes mainly a cell-transforming effect. It is very important for our studies that the DNA ofpolyoma virus is very small (it has a molecular weight ofabout 3 X io6 daltons) [2]. This DNA contains about 5,000 nucleotide pairs and can code for proteins containing about 1,600 amino acids in all; since more than one-third of these amino acids are used for building the protein molecules ofthe viral coat, about 1,000 are available for other virus-specified proteins. The number of genetic functions performed by the virus in the infected cells is therefore small, probably between two and four; and there is a good chance, therefore, of identifying the functions that affect the growth regulatory mechanism ofthe cell. Before discussing the effect of the virus on the cells, we must define regulation of cellular growth in the experimental system we use. The growth regulation oftissue culture cells on a solid substrate, such as the bottom ofa petri dish, is easily demonstrated. The normal cells, i.e., those recently derived from an animal, grow actively at certain optimal cell concentrations; as the population density increases, however, growth declines and may stop altogether. This is the consequence oftwo phenomena: reciprocal cell-to-cell contact, and presence in the medium of inhibitory substances produced by the cell. In contrast to normal cells, cells transformed by polyoma virus and other cancer viruses continue to grow actively under crowded conditions. Among the biochemical consequences of crowding of normal cells are the cessation of the synthesis of DNA and ofa group ofenzymes related to DNA synthesis. We shall now turn to the cell transformation caused by polyoma virus. The transformed cells differ from normal cells in two ways: they grow to very crowded, multilayered cultures, and they have less adhesion to...