ExperimentalGerontology, Vol.27, pp. 383-389, 1992 0531-5565/92 $5.00 + .00 Printed in the USA.All rightsreserved. Copyright© 1992 PergamonPressLtd. THE TWO-STAGE MECHANISM CONTROLLING CELLULAR SENESCENCE AND IMMORTALIZATION WOODRING E. WRIGHT and JERRY W. SHAY University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75235-9039 Key Words: aging, senescence, retinoblastoma gene product, p53, telomeres, life span INTRODUCTION IN ORDER for cancer cells to become immortal, they must first escape the controls that limit the proliferative capacity of all normal diploid cells. We began these experiments with the hope that understanding the molecular details of immortalization would identify the key factors involved in establishing the senescent state. The knowledge of these factors would in turn provide the starting point for studying the programming events that regu- lated the timing of cellular senescence. Most of the work on cellular immortalization has come from the identification of a vari- ety ofoncogenes in rodent cells. Although claims have been made for the identification of immortalizing nuclear versus "transforming" cytoplasmic oncogenes, virtually all of these studies have employed some form of clonal or focus-forming assay. The functions that have been examined can thus be reinterpreted as those that increase the clonability of cells or their ability to overcome contact inhibition of cell growth, rather than those that directly cause immortalization. The very high spontaneous rate of immortalization in mouse cells could then account for an immortalization event occurring in the expanded population of cells produced as a result of the oncogene, with the eventual production of a truly immortal population. Although carcinogens, mutagens, and oncogenes increase the rate of immortalization in rodent cells, none of these have been found to be effective in human cells. The only agents that are known rarely but consistently to immortalize human diploid fibroblasts are DNA tumor viruses such as simian virus 40 (SV40), adenovirus, and the human papilloma virus. Even these tumor viruses fail to directly immortalize human cells. Rather, they first confer an increased growth rate and abnormal morphology, aneuploidy rapidly develops, and the cells eventually enter a state called crisis. During crisis, the rate of cell division and cell death are initially balanced so that the population size remains constant. Later, divi- sion decreases and death increases, with the result that the culture is lost. In rare cases, a Correspondence to: W.E. Wright, Department of Cell Biology and Neuroscience, U.T. Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75235-9039. 383