Although phospholipid scramblase 1 (PLSCR1) was originally identified based on its capacity to promote transbilayer movement of membrane phospholipids, subsequent studies also provided evidence for its role in cell proliferation, maturation, and apoptosis. In this report, we investigate the potential role of PLSCR1 in leukemic cell differentiation. We show that all-trans retinoic acid (ATRA) , IntroductionHuman phospholipid scramblase 1 (PLSCR1), 1,2 also known as MmTRA1b (Mm-1 cell-derived transplantability-associated gene 1b), 3 is a multiply palmitoylated, calcium-binding, lipid raftassociated endofacial plasma membrane protein. 4 PLSCR1 was originally identified based on its capacity to promote rapid transbilayer movement of phospholipids (PLs) in response to the elevation of Ca 2ϩ and has been proposed to play a role in the cell-surface exposure of phosphatidylserine (PtdSer) following cell activation, injury, or apoptosis. [1][2][3] However, its role in membrane PL scrambling is controversial. While overexpression of PLSCR1 was reported to increase cell-surface exposure of PtdSer or to induce apoptosis in a variety of cells, [5][6][7] an increase in PL scrambling activity with elevated expression of PLSCR1 was not observed in other studies. 8,9 Blood platelets from PLSCR1 Ϫ/Ϫ mice also showed normal capacity to expose PtdSer upon cell activation. 10 Although the precise biologic function(s) of PLSCR1 remains to be determined, recent studies provide strong evidence for its role in cell signaling and in cell maturation. PLSCR1 has been reported to be a substrate of several kinases that participate in cell proliferation, differentiation, or apoptotic responses, including c-Abl, c-Src, and protein kinase C␦ (PKC␦). [11][12][13][14] In response to growth factors such as epidermal growth factor (EGF), tyrosine phosphorylation of PLSCR1 by c-Src resulted in association of phosphorylated PLSCR1 with the adaptor protein Shc and the activated EGF receptor complex, and, in cells deficient in PLSCR1, signaling through the EGF receptor is markedly attenuated. 12,13 Furthermore, PLSCR1 itself is transcriptionally up-regulated by a number of selective growth factors and cytokines, including interferon. 9,15 Interestingly, whereas palmitoylation of PLSCR1 is required for its anchoring in the plasma membrane, PLSCR1 was found in the nucleus after transcriptional induction by cytokines and in circumstances in which palmitoylation is prevented. Such nuclear localization of PLSCR1 was shown to be actively mediated by import through a nuclear localization signal within the polypeptide, and once imported, the protein was shown to bind to genomic DNA, implying a possible effect on gene transcription. 16,17 Proliferation and terminal differentiation of myeloid precursor cells in response to selective growth factors are impaired in PLSCR1 Ϫ/Ϫ mice, and in both monocytic and granulocytic lineages, the expression of PLSCR1 markedly increases upon K.-W.Z, X.L., and Q.Z. contributed equally to this study. Guo-Qiang Chen, No. 280, Chon...

Angiogenesis and hematopoiesis are closely linked and interactive with each other, but few studies were given to identify possible links between angiogenesis-promoting proteins and hematopoiesis-related transcription factors. Here we investigated the potential relationship of oxygensensitive a-subunit of angiogenesis-related hypoxiainducible factor-1a (HIF-1a) with Runt-related protein 1 (Runx1, also known as acute myeloid leukemia-1, AML-1), an important hematopoietic transcription factor. The results demonstrated that Runx1 and HIF-1a proteins directly interacted with each other to a degree, in which Runt homology domain of Runx1 was mainly involved. Leukemia-related abnormal Runx1 fusion protein AML1-ETO, which fuses the N-terminal 177 amino acid residues of the Runx1 protein in frame to ETO (eight-twenty-one) protein, also interacted with HIF-1a protein with greater ability than Runx1 itself. More intriguingly, Runx1 overexpression inhibited DNA-binding and transcriptional activity of HIF-1 protein with reduced expression of HIF-1-targeted genes such as vascular endothelial growth factor, while silence of Runx1 expression by specific small interfering RNA significantly increased transcriptional activity of HIF-1 protein, suggesting that Runx1 inhibited transcription-dependent function of HIF-1. Vice versa, HIF-1a increased DNA-binding ability and transcriptional activity of Runx1 protein. All these data would shed new insight to understanding Runx1 and HIF-1a-related hematopoietic cell differentiation and angiogenesis.