E1A Oncogene Induction of Cellular Susceptibility to Killing by Cytolytic Lymphocytes Through Target Cell Sensitization to Apoptotic Injury James L. Cook,* ,1 Barbara A. Routes,† Thomas A. Walker,† Kelley L. Colvin,† and John M. Routes† , ‡ *Department of Medicine, College of Medicine, University of Illinois at Chicago, Chicago, Illinois 60612; †Department of Medicine, National Jewish Medical and Research Center, Denver, Colorado 80206; and ‡Department of Medicine and Department of Immunology, University of Colorado Health Sciences Center, Denver, Colorado 80262 E1A oncogene expression increases mammalian cell susceptibility to lysis by cytolytic lymphocytes (CLs) at a stage in this intercellular interaction that is inde- pendent of cell surface recognition events. Since CLs can induce either apoptotic or necrotic cell death, we asked whether E1A sensitization to injury-induced apoptosis is sufficient to explain E1A-induced cyto- lytic susceptibility. Mouse, rat, hamster, and human cells that were rendered cytolytic susceptible by E1A were also sensitized to CL-induced and chemically in- duced apoptosis. In contrast, E1A-positive cells were no more susceptible to injury-induced necrosis than E1A-negative cells. Similar to induction of cytolytic susceptibility and in contrast to other E1A activities, cellular sensitization to chemically induced apoptosis depended on high-level E1A oncoprotein expression. Loss of both cytolytic susceptibility and sensitization to chemically induced apoptosis was coselected dur- ing in vivo selection of E1A-positive sarcoma cells for increased tumorigenicity. Furthermore, E1A mutant proteins that cannot bind the cellular transcriptional coactivator, p300, and that fail to induce cytolytic sus- ceptibility also failed to sensitize cells to injury-in- duced apoptosis. These data indicate that E1A induces susceptibility to killer cell-induced lysis through sen- sitization of cells to injury-induced apoptosis. © 1999 Academic Press Key Words: adenovirus; E1A; oncogene; cytolytic lymphocyte; beauvericin; apoptosis; NIH-3T3. INTRODUCTION In addition to its roles in controlling viral gene ex- pression and the cell cycle, the E1A oncogene of human adenovirus (Ad) types 2 and 5 also actively induces cells to become highly susceptible to lysis by several components of the antitumor immune response, includ- ing different types of cytotoxic lymphocytes (CLs), ac- tivated macrophages, and tumor necrosis factor  (TNF) [15–17, 19, 45, 55]. This E1A-induced cellular phenotype has been termed “cytolytic susceptibility.” We and others proposed that E1A-induced cytolytic susceptibility explains the lack of tumorigenicity of cells transformed by these nononcogenic Ad serotypes in immunocompetent animals [8, 10, 19, 44, 45, 49, 50, 55]. We reported that the mechanism by which E1A in- duces the cytolytic susceptible phenotype involves a stage in the interaction between CLs and their E1A- expressing target cells that follows, and is independent of, interactions between killer cell or cytokine ligands and target cell receptors—a “postrecognition” stage in cellular injury [18]. These observations suggest that E1A induces cytolytic susceptibility by causing a qual- itative change in the cellular response to CL-induced injury. Most reports indicate that CLs kill their target cells by inducing apoptosis through two mechanisms: one caused by the joint actions of CL granule-associated perforin and granzymes and the other resulting from CL surface Fas-ligand crosslinking of Fas antigen on target cells (reviewed in [2, 5]). However, there is also evidence suggesting that CL-induced apoptosis may not explain all killing activity and that CLs can also cause a necrotic cell death response in at least some types of target cells [20, 51, 54]. Therefore, it was not possible, using only assays of CL-induced injury, to determine whether E1A induction of cytolytic suscep- tibility is caused by E1A-induced cellular sensitivity to apoptosis or necrosis or to both cell death responses. In the studies presented in this report, fibroblastic cells stably transfected with the E1A oncogene and expressing the E1A oncoprotein were used to test the relationship between E1A induction of cytolytic sus- ceptibility and E1A sensitization to apoptotic injury. 1 To whom correspondence and reprint requests should be sent at Infectious Diseases Section (MC735), Room 885, CME Building, De- partment of Medicine, College of Medicine, University of Illinois at Chicago, 808 South Wood Street, Chicago, IL 60612. Fax: (312) 413–1657. 414 0014-4827/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved. Experimental Cell Research 251, 414 – 423 (1999) Article ID excr.1999.4597, available online at http://www.idealibrary.com on