ABSTRACT
BACKGROUND: Recent studies indicate that angiogenesis is important in the pathogenesis of acute myeloid leukemias (AMLs). Among the various AMLs, the bone marrow angiogenetic response is particularly pronounced in acute promyelocytic leukemia (APL). However, the molecular mechanisms responsible for this angiogenetic response are largely unknown. In the present study, we have explored the role of HHEX, a homeodomain transcription factor, as a possible mediator of the pro-angiogenetic response observed in APL. This transcription factor seems to represent an ideal candidate for this biologic function because it is targeted by PML-RARα, is capable of interaction with PML and PML-RARα, and acts as a regulator of the angiogenetic response. METHODS: We used various cellular systems of APL, including primary APL cells and leukemic cells engineered to express PML-RARα, to explore the role of the PML-RARα fusion protein on HHEX expression. Molecular and biochemical techniques have been used to investigate the mechanisms through which PML-RARα downmodulates HHEX and the functional consequences of this downmodulation at the level of the expression of various angiogenetic genes, cell proliferation and differentiation. RESULTS: Our results show that HHEX expression is clearly downmodulated in APL and that this effect is directly mediated by a repressive targeting of the HHEX gene promoter by PML-RARα. Studies carried out in primary APL cells and in a cell line model of APL with inducible PML-RARα expression directly support the view that this fusion protein through HHEX downmodulation stimulates the expression of various genes involved in angiogenesis and inhibits cell differentiation. CONCLUSIONS: Our data suggest that HHEX downmodulation by PML-RARα is a key event during APL pathogenesis.
Subject(s)
Down-Regulation , Gene Expression Regulation, Leukemic , Homeodomain Proteins/genetics , Oncogene Proteins, Fusion/genetics , Transcription Factors/genetics , Blotting, Western , Cell Differentiation/genetics , Cell Line, Tumor , Cell Proliferation/genetics , Cells, Cultured , HEK293 Cells , Homeodomain Proteins/metabolism , Humans , Leukemia, Promyelocytic, Acute/genetics , Leukemia, Promyelocytic, Acute/metabolism , Neovascularization, Pathologic/genetics , Neovascularization, Pathologic/metabolism , Oncogene Proteins, Fusion/metabolism , Promoter Regions, Genetic/genetics , Reverse Transcriptase Polymerase Chain Reaction , Transcription Factors/metabolism , Tumor Cells, Cultured , U937 Cells , Vascular Endothelial Growth Factor A/genetics , Vascular Endothelial Growth Factor A/metabolism , Vascular Endothelial Growth Factor Receptor-2/genetics , Vascular Endothelial Growth Factor Receptor-2/metabolismABSTRACT
The tyrosine kinase Tie-2 and its ligands Angiopoietins (Angs) transduce critical signals for angiogenesis in endothelial cells. This receptor and Ang-1 are coexpressed in hematopoietic stem cells and in a subset of megakaryocytes, though a possible role of angiopoietins in megakaryocytic differentiation/proliferation remains to be demonstrated. To investigate a possible effect of Ang-1/Ang-2 on megakaryocytic proliferation/differentiation we have used both normal CD34(+) cells induced to megakaryocytic differentiation and the UT7 cells engineered to express the thrombopoietin receptor (TPOR, also known as c-mpl, UT7/mpl). Our results indicate that Ang-1/Ang-2 may have a role in megakaryopoiesis. Particularly, Ang-2 is predominantly produced and released by immature normal megakaryocytic cells and by undifferentiated UT7/mpl cells and slightly stimulated TPO-induced cell proliferation. Ang-1 production is markedly induced during megakaryocytic differentiation/maturation and potentiated TPO-driven megakaryocytic differentiation. Blocking endogenously released angiopoietins partially inhibited megakaryocytic differentiation, particularly for that concerns the process of polyploidization. According to these data it is suggested that an autocrine angiopoietin/Tie-2 loop controls megakaryocytic proliferation and differentiation.
