Perspectives in Biology and Medicine

MALIGNANT TRANSFORMATION OF CELLS BY VIRUSES HOWARD M. TEMIN* Most of the present approaches to an answer to the question How do viruses cause cancer? (as opposed to the question Do viruses cause human cancer?) come from studies involving cells in culture. In this article, I shall discuss from this perspective—and in a somewhat personal and speculative fashion—the general nature of cancer, some properties ofcancer cells in cellculture, andsomeresultsfromthe study oftumorvirusesincellculture. Cancer and Control ofCell Multiplication Multiplication ofcells in a multicellular animal is always under some type ofcontrol. This control is exerted inways ranging from alterations in general environmental parameters (like temperature) to alterations in the concentrations of specific molecules (like hormones). The range of controls is integrated so that every multicellular animal has a particular form and character. The controls can vary from control of development, to control of maintenance of an adult animal in a steady-state situation, to control ofrecovery ofthis steady-state situation when it has been altered by injury. The controls can be classified as to whether or not they are cell bound. Alternatively, they can be classified as to whether their effect is positive (stimulating cell multiplication) or negative (inhibiting cell multiplication ). Cell cultures are convenient for experimentation on the nature ofthe controls ofcell multiplication. Most ofthe experimentation has involved either fibroblasts attached to a solid surface, or colony-forming cells from spleen or bone marrow in soft agar. For both types ofcells, cell multiplication appears to be controlled by specific humoral factors [1-4]. However , from the point ofview oftumor viruses, only the fibroblast systems * McArdle Laboratory, University ofWisconsin, Madison, Wisconsin 537°6. II are useful at the present time because the other systems have not yet been used for transformation studies. Culture of fibroblasts requires an appropriate temperature, pH, ionic environment, and low and high molecular weight materials. Any ofthese factors can be made limiting for cell multiplication. However, under the usual conditions for cell culture (e.g., 37°C, pH 7.2, andamediumisotonic for cells and containing suitable amino acids, vitamins, and glucose), the amount ofserum appears to be the controlling factor for fibroblast multiplication . The more serum present in the culture medium, the more cell multiplication occurs. In fact, there is a linear relationship between the amount of serum in the medium and the amount ofcell multiplication, both for one cycle ofcell multiplication and for several cycles [3-5]. Previously, it was thought that the amount ofmultiplication offibroblasts incell culture was controlled by a phenomenon called "contact inhibition of cell multiplication." In 1954, Abercrombie and Heaysman [6] observed that the movement offibroblasts in culture appeared to be controlled by contact relations between cells. In particular, fibroblasts did not migrate over other fibroblasts. To describe this phenomenon, the phrase "contact inhibition ofcell movement" was introduced. Cultures ofnonmultiplying or stationary fibroblasts are usually monolayer . The monolayering appears to be the result ofcontact inhibition of movement. The cessation ofcell multiplication in these stationary cultures was, therefore, assumed to be caused by contact inhibition. However, the discoveries that the addition offresh serum to stationary cultures caused a resumption of cell multiplication, and that the medium of stationary cultures was depleted ofmultiplication-stimulating activity, led to the new interpretationthat cell multiplication had ceased as a result ofdepletion of multiplication-stimulating activity from the culture medium [3]. Multiplication-stimulating activity is defined by its ability to stimulate cell multiplication in stationary populations ofcells. It usually is added to cell culture medium as serum, but some cells produce it [4, 7]. In addition to multiplication-stimulating activity, a number ofother non-cell-bound controls seem to exist. These include antagonists of multiplication-stimulating activity and potentiators of multiplication-stimulating activity. The antagonists prevent cells from being stimulated by multiplicationstimulating activity. It is not known whether these antagonists act because ofa generaltoxiceffect oncells or because ofa specificeffect onmultiplica12 Howard M. Temin · Malignant Transformation by Viruses Perspectives in Biology and Medicine · Autumn 1970 tion-stimulating activity. The potentiators cause an increased efficiency of stimulation of cells by multiplication-stimulating activity [8, 9]. This potentiation is seen experimentally by a shift in the dose-response curve, when known quantities of multiplication-stimulating activity are mixed with certain media...