title: Tumor suppressor p53 as a component of the tumor necrosis factor-induced, protein kinase PKR-mediated apoptotic pathway in human promonocytic U937 cells.
abstract: Despite what is known about the early signaling events in tumor necrosis factor (TNF) alpha-induced apoptosis, characterization of the downstream events remains largely undefined. It is now known that a cross-talk exists between the interferon and TNF-alpha pathways. This linkage allows recruitment of the cell proliferation suppressor PKR (dsRNA-dependent protein kinase) from the interferon pathway to play a pivotal role in TNF-alpha-induced apoptosis. In this study, we took advantage of the differential TNF-alpha susceptibilities of human promonocytic U937 subclones, deficient in or overexpressing PKR, to further characterize the role of PKR in apoptosis. By reverse transcription-polymerase chain reaction, we demonstrated that TNF-alpha transiently induces the tumor suppressor p53 in U937 cells. This p53 induction lags behind the TNF-alpha induction of PKR by 1 h. By cell viability determination, ultrastructural studies, apoptotic DNA laddering, and antisense techniques, it was shown that inhibition of p53 expression in PKR-overexpressing U937 cells abrogates the TNF-alpha-induced apoptosis in these cells. Conversely, overexpressing wild type p53 in PKR-deficient U937 cells confers the susceptibility of these cells to TNF-alpha-induced apoptosis. This latter result indicates that p53 induction is an event downstream of TNF-alpha-induced up-regulation of PKR, thereby further establishing the critical role of p53 in TNF-alpha-induced apoptosis in U937 cells. PKR-overexpressing U937 cells were found to possess a constitutively higher level of p53, which partly explains why these cells spontaneously undergo apoptosis even without TNF-alpha treatment. Finally, a model is presented on the interplay between PKR and p53 in effecting TNF-alpha-induced apoptosis in U937 cells.
authors.affiliation: The Moses Grossman Infectious Diseases Laboratory, Department of Pediatrics, San Francisco General Hospital and University of California, San Francisco, California 94110, USA.
authors.forename: M C
authors.initials: MC
authors.lastname: Yeung
authors.forename: A S
authors.initials: AS
authors.lastname: Lau
mesh_terms: D017209:Apoptosis; D001483:Base Sequence; D002460:Cell Line; D017931:DNA Primers; D006801:Humans; D007700:Kinetics; D008854:Microscopy, Electron; D009707:Nucleosomes / Q000378:metabolism; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism*; D016159:Tumor Suppressor Protein p53 / Q000096:biosynthesis / Q000378:metabolism*; D019892:eIF-2 Kinase / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D009707:Nucleosomes; D014409:Tumor Necrosis Factor-alpha; D016159:Tumor Suppressor Protein p53; D019892:eIF-2 Kinase