Abstract 1141: p19Arf represses Pdgfrβ-driven pericyte proliferation in vivo

Abstract p19Arf, encoded by the mouse Arf gene, is a critical tumor suppressor with roles regulating cell cycle arrest in embryo development and suppressing oncogenesis. Previous studies established that p19Arf controls pericyte-like cells in the hyaloid vascular system (HVS) in the primary vitreous of the developing eye. Arf-deficient mice have hyperplasic expansion of pericytes in the vitreous, a developmental defect driven by excessive Pdgfrβ signaling. We have used complementary studies of cultured cells and mouse embryos to further explore the functional interactions between p19Arf and Pdgfrβ. To address whether Pdgfrβ acts as a mitogenic or a survival factor in the Arf-deficient mouse eye, we quantified cell proliferation and apoptosis at 13.5 days post coitum (dpc) in ArfGFP/GFP embryos that had or lacked Pdgfrβ. Cell proliferation, measured by BrdU incorporation, was increased by two-fold in the vitreous in the presence of Pdgfrβ. The presence or absence of Pdgfrβ did not change the small number of apoptotic cells in the vitreous of the eye. To explore mechanisms by which Arf expression impeded Pdgfrβ signaling, we used the 10T1/2 pericyte-like cell culture model to show that retroviral expression of p19Arf significantly repressed Pdgfrβ protein. Conversely, shRNA-mediated knockdown of p19Arf enhanced Pdgfrβ in serial-passaged mouse embryo fibroblasts (MEFs). In vivo measurements of Arf mRNA extracted from vitreous samples isolated by laser capture microdissection (LCM) from mouse embryos at 13.5 dpc demonstrated increased Pdgfrβ mRNA in Arf-deficient embryos as compared to wild-type. To confirm that p19Arf repressed Pdgfrβ transcription, we demonstrated that both mature and primary Pdgfrβ RNA transcripts were repressed in 10T1/2 cells. Co-transfection of p19Arf with a luciferase reporter plasmid driven by the Pdgfrβ promoter demonstrated that a 1.6kb segment of DNA conferred Arf-dependent regulation. Chromatin immunoprecipitation (ChIP) showed that p19Arf reduced RNA Pol II binding to the proximal Pdgfrβ promoter. Overall our data show that, in the absence of Arf, Pdgfrβ drives excess proliferation in cells that would normally express Arf in the developing eye. p19Arf acts to repress Pdgfrβ transcription, in part by blocking RNA Pol II binding to the proximal Pdgfrβ promoter. We are currently using additional ChIP assays to identify how the presence or absence of Arf influences the binding of several transcription factors known or suspected of regulating Pdgfrβ expression, and we are exploring the importance of the Arf-Pdgfrβ pathway deregulation in cancer models. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1141. doi:10.1158/1538-7445.AM2011-1141