title: SerpinB2 protection of retinoblastoma protein from calpain enhances tumor cell survival.
abstract: The tumor suppressor retinoblastoma protein (Rb) plays a pivotal role in the regulation of cell proliferation and sensitivity to apoptosis through binding to E2F transcription factors. Loss of Rb in response to genotoxic stress or inflammatory cytokines can enhance cell death, in part, by eliminating Rb-mediated repression of proapoptotic gene transcription. Here we show that calpain cleavage of Rb facilitates Rb loss by proteasome degradation and that this may occur during tumor necrosis factor alpha-induced apoptosis. The cytoprotective, Rb-binding protein SerpinB2 (plasminogen activator inhibitor type 2) protects Rb from calpain cleavage, increasing Rb levels and enhancing cell survival. Chromatin immunoprecipitation assays show that the increased Rb levels selectively enhance Rb repression of proapoptotic gene transcription. This cytoprotective role of SerpinB2 is illustrated by reduced susceptibility of SerpinB2-deficient mice to multistage skin carcinogenesis, where Rb-dependent cell proliferation competes with apoptosis during initiation of papilloma development. These data identify SerpinB2 as a cell survival factor that modulates Rb repression of proapoptotic signal transduction and define a new posttranslational mechanism for selective regulation of the intracellular levels of Rb.
authors.affiliation: Center for Vascular and Inflammatory Diseases, Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA.
authors.forename: Laura
authors.initials: L
authors.lastname: Tonnetti
authors.forename: Sarah
authors.initials: S
authors.lastname: Netzel-Arnett
authors.forename: Grant A
authors.initials: GA
authors.lastname: Darnell
authors.forename: Tamara
authors.initials: T
authors.lastname: Hayes
authors.forename: Marguerite S
authors.initials: MS
authors.lastname: Buzza
authors.forename: Ian E
authors.initials: IE
authors.lastname: Anglin
authors.forename: Andreas
authors.initials: A
authors.lastname: Suhrbier
authors.forename: Toni M
authors.initials: TM
authors.lastname: Antalis
mesh_terms: D000818:Animals; D017209:Apoptosis; D002154:Calpain / Q000378:metabolism*; D002470:Cell Survival; D005347:Fibroblasts / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic; D020022:Genetic Predisposition to Disease; D006367:HeLa Cells; D006801:Humans; D019169:Jurkat Cells; D051379:Mice; D017396:Plasminogen Activator Inhibitor 2 / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D016160:Retinoblastoma Protein / Q000378:metabolism*; D015398:Signal Transduction; D012878:Skin Neoplasms / Q000378:metabolism
chemical_list: D017396:Plasminogen Activator Inhibitor 2; D016160:Retinoblastoma Protein; D002154:Calpain