The tumor suppressor p53 is a transcription factor that responds to cellular stresses by initiating cell cycle arrest or apoptosis. One transcriptional target of p53 is Mdm2, an E3 ubiquitin ligase that interacts with p53 to promote its proteasomal degradation in a negative feedback regulatory loop. Here we show that the wild-type p53-induced phosphatase 1 (Wip1), or PPM1D, downregulates p53 protein levels by stabilizing Mdm2 and facilitating its access to p53. Wip1 interacts with and dephosphorylates Mdm2 at serine 395, a site phosphorylated by the ATM kinase. Dephosphorylated Mdm2 has increased stability and affinity for p53, facilitating p53 ubiquitination and degradation. Thus, Wip1 acts as a gatekeeper in the Mdm2-p53 regulatory loop by stabilizing Mdm2 and promoting Mdm2-mediated proteolysis of p53.

Notch signaling plays an important role in developmental processes and adult tissue homeostasis. Altered Notch signaling has been associated with various diseases including cancer. While the importance of altered Notch signaling in cancers of hematopoietic and epithelial origins has been established, its role in tumors of mesenchymal origin is less clear. Here, we report that human osteosarcoma cell lines and primary human osteosarcoma tumor samples show significant up-regulation of Notch, its target genes and Osterix. Notch inhibition by gamma-secretase inhibitors or by using lentiviral mediated expression of dominant negative Mastermind-like protein (DN-MAML) decreases osteosarcoma cell proliferation in vitro. In vivo, established human tumor xenografts in nude mice show decreased tumor growth after chemical or genetic inhibition of Notch signaling. Finally, transcriptional profiling of osteosarcomas from p53 mutant mice confirmed up-regulation of Notch1 target genes Hes1, Hey1 and its ligand Dll4. Our data suggest that activation of Notch signaling contributes to the pathogenesis of human osteosarcomas and its inhibition may be a therapeutic approach for the treatment of this mesenchymal tumor.

Deletion of Runx1 in adult mice produces a myeloproliferative phenotype. We now find that Runx1 gene deletion increases marrow monocyte while reducing granulocyte progenitors and that exogenous RUNX1 rescues granulopoiesis. Deletion of Runx1 reduces Cebpa mRNA in lineage-negative marrow cells and in granulocyte-monocyte progenitors or common myeloid progenitors. Pu.1 mRNA is also decreased, but to a lesser extent. We also transduced marrow with dominant-inhibitory RUNX1a. As with Runx1 gene deletion, RUNX1a expands lineage ؊ Sca-1 ؉ c-kit ؉ and myeloid cells, increased monocyte CFUs relative to granulocyte CFUs, and reduced Cebpa mRNA. Runx1 binds a conserved site in the Cebpa promoter and binds 4 sites in a conserved 450-bp region located at ؉37 kb; mutation of the enhancer sites reduces activity 6-fold in 32Dcl3 myeloid cells. Endogenous Runx1 binds the promoter and putative ؉37 kb enhancer as assessed by ChIP, and RUNX1-ER rapidly induces Cebpa mRNA in these cells, even in cycloheximide, consistent with direct gene regulation. The ؉37 kb region contains strong H3K4me1 histone modification and p300-binding, as often seen with enhancers. Finally, exogenous C/EBP␣ increases granulocyte relative to monocyte progenitors in Runx1-deleted marrow cells. Diminished CEBPA transcription and consequent impairment of myeloid differentiation may contribute to leukemic transformation in acute myeloid leukemia cases associated with decreased RUNX1 activity. (Blood. 2012; 119(19):4408-4418) IntroductionThe Runx1 transcription factor contributes to formation of pluripotent adult HSCs from hemogenic endothelium during embryogenesis. [1][2][3] Deletion of floxed Runx1 alleles in adult mice preserves pluripotent HSCs and erythroid cells but leads to thrombocytopenia and a marked reduction in B-and T-lymphoid cells and their precursors. [4][5][6] In addition, these mice develop an expansion of myeloid progenitors, with increased numbers of lineage Ϫ Sca-1 ϩ c-kit ϩ (LSK) cells, common myeloid progenitors (CMPs), granulocyte-monocyte progenitors (GMPs), myeloid CFUs, and mature myeloid cells. 4,5 This myeloproliferative phenotype is transplantable and so intrinsic to the hematopoietic system. 5 The myeloid expansion associated with absence of Runx1 may be relevant to myeloid leukemogenesis. Reduced RUNX1 activity occurs commonly in acute myeloid leukemia (AML), often because of expression of the RUNX1-ETO or core binding factor ␤ (CBF␤)-SMMHC oncoproteins or as a result of RUNX1 genomic mutation. [7][8][9] It has been suggested that these and other "type II" alterations, including CEBPA mutation, contribute to myeloid transformation by inhibiting myeloid differentiation, with coincident "type I" alterations, for example, activation of N-Ras or receptor tyrosine kinases, inhibiting apoptosis while stimulating proliferation. 10 In this study, we further assess the effect of Runx1 gene deletion on myeloid differentiation in adult mice. Comparison of Runx1-deleted myeloid CFUs with those of control mice uncovered a marked increase in m...

Osteosarcoma is the primary malignant cancer of bone and particularly affects adolescents and young adults, causing debilitation, and sometimes death. As a model for human osteosarcoma we have been studying p53+/− mice, which develop osteosarcoma at high frequency. To discover genes that cooperate with p53 deficiency in osteosarcoma formation we have integrated array comparative genomic hybridization, microarray expression analyses in mouse and human osteosarcomas, and functional assays. In this study we found seven frequent regions of copy number gain and loss in the mouse p53+/− osteosarcomas, but have focused on a recurrent amplification event on mouse chromosome 9A1. This amplicon is syntenic with a similar chromosome 11q22 amplicon identified in a number of human tumor types. Three genes on this amplicon, the matrix metalloproteinase gene MMP13, and the anti-apoptotic genes Birc2 (cIAP1), and Birc3 (cIAP2) show elevated expression in mouse and human osteosarcomas. We developed a functional assay using clonal osteosarcoma cell lines transduced with lentiviral shRNA vectors to show that downregulation of MMP13, Birc2, or Birc3 resulted in reduced tumor growth when transplanted into immunodeficient recipient mice. These experiments revealed that high MMP13 expression enhances osteosarcoma cell survival and that Birc2 and Birc3 also enhance cell survival, but only in osteosarcoma cells with the chromosome 9A1 amplicon. We conclude that the anti-apoptotic genes Birc2 and Birc3 are potential oncogenic drivers in the chromosome 9A1 amplicon.Requests for reprints: Lawrence A.

C/EBPα is required for the formation of granulocyte-monocyte progenitors; however, its role in subsequent myeloid lineage specification remains uncertain. Transduction of murine marrow with either of two Cebpa shRNAs markedly increases monocyte and reduces granulocyte colonies in methylcellulose or the monocyte to neutrophil ratio in liquid culture. Similar findings were found after marrow shRNA transduction and transplantation and with CEBPA knockdown in human marrow CD34+ cells. These results apparently reflect altered myeloid lineage specification, as similar knockdown allowed nearly complete 32Dcl3 granulocytic maturation. Cebpa knockdown also generated lineage-negative blasts with increased colony replating capacity but unchanged cell cycle parameters, likely reflecting complete differentiation block. The shRNA having the greatest effect on lineage skewing reduced Cebpa 3-fold in differentiating cells but 6-fold in accumulating blasts. Indicating that Cebpa is the relevant shRNA target, shRNA-resistant C/EBPα-ER rescued marrow myelopoiesis. Cebpa knockdown in murine marrow cells also increased in vitro erythropoiesis, perhaps reflecting 1.6-fold reduction in PU.1 leading to GATA-1 derepression. Global gene expression analysis of lineage-negative blasts that accumulate after Cebpa knockdown demonstrated reduction in Cebpe and Gfi1, known transcriptional regulators of granulopoiesis, and also reduced Ets1 and Klf5. Populations enriched for immature granulocyte or monocyte progenitor/precursors were isolated by sorting Lin−Sca-1−c-Kit+ cells into GCSFR+MCSFR− or GCSFR−MCSFR+ subsets. Cebpa, Cebpe, Gfi1, Ets1, and Klf5 RNAs were increased in the c-Kit+GCSFR+ and Klf4 and Irf8 in the c-Kit+MCSFR+ populations, with PU.1 levels similar in both. In summary, higher levels of C/EBPα are required for granulocyte and lower levels for monocyte lineage specification, and this myeloid bifurcation may be facilitated by increased Cebpa gene expression in granulocyte compared with monocyte progenitors.