The ETS family transcription factor PU.1 is a key regulator of haematopoietic differentiation. Its expression is dynamically controlled throughout haematopoiesis in order to direct appropriate lineage specification. Elucidating the biological role of PU.1 has proved challenging. This paper will discuss how a range of experiments in cell lines and mutant and transgenic mouse models have enhanced our knowledge of the mechanisms by which PU.1 drives lineage-specific differentiation during haematopoiesis. 1. Introduction Haematopoiesis is a lifelong process that generates the range of blood cell types that exhibit distinct and specialised functions. Transcription factors play a critical role in this complex and highly orchestrated process, directing multipotent haematopoietic stem cells (HSCs) towards lineage commitment by regulating lineage-specific gene expression, proliferation, and differentiation. The ETS family member PU.1 is one such transcription factor The PU.1 gene was first identified as a proviral integration site for the spleen focus forming virus (SFFV) in erythroleukaemias [1]. SFFV integration in the PU.1 locus leads to increased PU.1 transcription and subsequent erythroleukaemic transformation. It has since emerged that PU.1 is one of the major haematopoietic regulators, with a particular role in directing differentiation within the myeloid and lymphoid pathways [2]. Several PU.1 null and mutant mouse lines have been generated and exhibit varied phenotypes depending on the nature of PU.1 defect [3]. PU.1 knockout mice succumb to neonatal death and show a marked lack of myeloid cells, T and B cells [4, 5]. Erythropoiesis is also altered in the foetal livers of PU.1 null mice with erythroid progenitors displaying reduced self-renewal capacity and a propensity to differentiate prematurely [6]. PU.1 is thus crucial in directing many facets of haematopoiesis and concordant with this, its expression fluctuates dynamically in the various haematopoietic differentiation pathways (Figure 1). Importantly, the regulation of differentiation by PU.1 is not merely via a “presence or absence of expression” mechanism but by a dose-dependent effect. For instance, the expression of PU.1 is low in long-term reconstituting (LT)-HSCs but rises as these progenitors become more lineage restricted and form precursor cells known as common myeloid progenitors (CMPs) and common lymphoid progenitors (CLPs). Upon further lineage differentiation and maturation, PU.1 is expressed at varied levels in mature blood cells, with higher levels found in macrophages than B cells
References
[1]
F. Moreau-Gachelin, A. Tavitian, and P. Tambourin, “Spi-1 is a putative oncogene in virally induced murine erythroleukaemias,” Nature, vol. 331, no. 6153, pp. 277–280, 1988.
[2]
R. P. DeKoter and H. Singh, “Regulation of B lymphocyte and macrophage development by graded expression of PU.1,” Science, vol. 288, no. 5470, pp. 1439–1441, 2000.
[3]
P. Kastner and S. Chan, “PU.1: a crucial and versatile player in hematopoiesis and leukemia,” International Journal of Biochemistry and Cell Biology, vol. 40, no. 1, pp. 22–27, 2008.
[4]
E. W. Scott, M. C. Simon, J. Anastasi, and H. Singh, “Requirement of transcription factor PU.1 in the development of multiple hematopoietic lineages,” Science, vol. 265, no. 5178, pp. 1573–1577, 1994.
[5]
S. R. McKercher, B. E. Torbett, K. L. Anderson et al., “Targeted disruption of the PU.1 gene results in multiple hematopoietic abnormalities,” EMBO Journal, vol. 15, no. 20, pp. 5647–5658, 1996.
[6]
J. Back, A. Dierich, C. Bronn, P. Kastner, and S. Chan, “PU.1 determines the self-renewal capacity of erythroid progenitor cells,” Blood, vol. 103, no. 10, pp. 3615–3623, 2004.
[7]
J. Back, D. Allman, S. Chan, and P. Kastner, “Visualizing PU.1 activity during hematopoiesis,” Experimental Hematology, vol. 33, no. 4, pp. 395–402, 2005.
[8]
S. L. Nutt, D. Metcalf, A. D'Amico, M. Polli, and L. Wu, “Dynamic regulation of PU.1 expression in multipotent hematopoietic progenitors,” Journal of Experimental Medicine, vol. 201, no. 2, pp. 221–231, 2005.
[9]
M. J. Klemsz, S. R. McKercher, A. Celada, C. van Beveren, and R. A. Maki, “The macrophage and B cell-specific transcription factor PU.1 is related to the ets oncogene,” Cell, vol. 61, no. 1, pp. 113–124, 1990.
[10]
F. Rosenbauer, B. M. Owens, L. Yu et al., “Lymphoid cell growth and transformation are suppressed by a key regulatory element of the gene encoding PU.1,” Nature Genetics, vol. 38, no. 1, pp. 27–37, 2006.
[11]
F. Moreau-Gachelin, F. Wendling, T. Molina et al., “Spi-1/PU.1 transgenic mice develop multistep erythroleukemias,” Molecular and Cellular Biology, vol. 16, no. 5, pp. 2453–2463, 1996.
[12]
H. Iwasaki, C. Somoza, H. Shigematsu et al., “Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their differentiation,” Blood, vol. 106, no. 5, pp. 1590–1600, 2005.
[13]
F. Rosenbauer, K. Wagner, J. L. Kutok et al., “Acute myeloid leukemia induced by graded reduction of a lineage-specific transcription factor, PU.1,” Nature Genetics, vol. 36, no. 6, pp. 624–630, 2004.
[14]
A. Dakic, D. Metcalf, L. Di Rago, S. Mifsud, L. Wu, and S. L. Nutt, “PU.1 regulates the commitment of adult hematopoietic progenitors and restricts granulopoiesis,” Journal of Experimental Medicine, vol. 201, no. 9, pp. 1487–1502, 2005.
[15]
R. C. Fisher, J. D. Lovelock, and E. W. Scott, “A critical role for PU.1 in homing and long-term engraftment by hematopoietic stem cells in the bone marrow,” Blood, vol. 94, no. 4, pp. 1283–1290, 1999.
[16]
K. Akashi, D. Traver, and L. I. Zon, “The complex cartography of stem cell commitment,” Cell, vol. 121, no. 2, pp. 160–162, 2005.
[17]
J. Adolfsson, R. M？nsson, N. Buza-Vidas et al., “Identification of Flt3+ lympho-myeloid stem cells lacking erythro-megakaryocytic potential a revised road map for adult blood lineage commitment.,” Cell, vol. 121, no. 2, pp. 295–306, 2005.
[18]
A. D. Friedman, “Transcriptional control of granulocyte and monocyte development,” Oncogene, vol. 26, no. 47, pp. 6816–6828, 2007.
[19]
J. Seita and I. L. Weissman, “Hematopoietic stem cell: self-renewal versus differentiation,” Wiley Interdisciplinary Reviews: Systems Biology and Medicine, vol. 2, no. 6, pp. 640–653, 2010.
[20]
Y. Arinobu, S. Mizuno, Y. Chong et al., “Reciprocal activation of GATA-1 and PU.1 marks initial specification of hematopoietic stem cells into myeloerythroid and myelolymphoid lineages,” Cell Stem Cell, vol. 1, no. 4, pp. 416–427, 2007.
[21]
C. J. Spooner, J. X. Cheng, E. Pujadas, P. Laslo, and H. Singh, “A recurrent network involving the transcription factors PU.1 and Gfi1 orchestrates innate and adaptive immune cell fates,” Immunity, vol. 31, no. 4, pp. 576–586, 2009.
[22]
M. B. Kamath, I. B. Houston, A. J. Janovski et al., “Dose-dependent repression of T-cell and natural killer cell genes by PU.1 enforces myeloid and B-cell identity,” Leukemia, vol. 22, no. 6, pp. 1214–1225, 2008.
[23]
R. Pop, J. R. Shearstone, Q. Shen et al., “A key commitment step in erythropoiesis is synchronized with the cell cycle clock through mutual inhibition between PU.1 and S-phase progression,” PLoS Biology, vol. 8, no. 9, Article ID e1000484, 2010.
[24]
J. Starck, A. Doubeikovski, S. Sarrazin et al., “Spi-1/PU.1 is a positive regulator of the Fli-1 gene involved in inhibition of erythroid differentiation in friend erythroleukemic cell lines,” Molecular and Cellular Biology, vol. 19, no. 1, pp. 121–135, 1999.
[25]
K. S. Choe, O. Ujhelly, S. N. Wontakal, and A. I. Skoultchi, “PU.1 directly regulates cdk6 gene expression, linking the cell proliferation and differentiation programs in erythroid cells,” Journal of Biological Chemistry, vol. 285, no. 5, pp. 3044–3052, 2010.
[26]
G. Rao, N. Rekhtman, G. Cheng, T. Krasikov, and A. I. Skoultchi, “Deregulated expression of the PU.1 transcription factor blocks murine erythroleukemia cell terminal differentiation,” Oncogene, vol. 14, no. 1, pp. 123–131, 1997.
[27]
M. Papetti and A. I. Skoultchi, “Reprogramming leukemia cells to terminal differentiation and growth arrest by RNA interference of PU.1,” Molecular Cancer Research, vol. 5, no. 10, pp. 1053–1062, 2007.
[28]
O. Atar and B. Z. Levi, “PU.1 silencing leads to terminal differentiation of erythroleukemia cells,” Biochemical and Biophysical Research Communications, vol. 329, no. 4, pp. 1288–1292, 2005.
[29]
T. Yamada, N. Kondoh, M. Matsumoto, M. Yoshida, A. Maekawa, and T. Oikawa, “Overexpression of PU.1 induces growth and differentiation inhibition and apoptotic cell death in murine erythroleukemia cells,” Blood, vol. 89, no. 4, pp. 1383–1393, 1997.
[30]
I. Matushansky, F. Radparvar, and A. I. Skoultchi, “CDK6 blocks differentiation: coupling cell proliferation to the block to differentiation in leukemic cells,” Oncogene, vol. 22, no. 27, pp. 4143–4149, 2003.
[31]
P. Burda, N. Curik, J. Kokavec et al., “PU.1 activation relieves GATA-1-mediated repression of Cebpa and Cbfb during leukemia differentiation,” Molecular Cancer Research, vol. 7, no. 10, pp. 1693–1703, 2009.
[32]
N. Rekhtman, F. Radparvar, T. Evans, and A. I. Skoultchi, “Direct interaction of hematopoietic transcription factors PU.1 and GATA- 1: functional antagonism in erythroid cells,” Genes and Development, vol. 13, no. 11, pp. 1398–1411, 1999.
[33]
P. Zhang, X. Zhang, A. Iwama et al., “PU.1 inhibits GATA-1 function and erythroid differentiation by blocking GATA-1 DNA binding,” Blood, vol. 96, no. 8, pp. 2641–2648, 2000.
[34]
P. Zhang, G. Behre, J. Pan et al., “Negative cross-talk between hematopoietic regulators: GATA proteins repress PU.1,” Proceedings of the National Academy of Sciences of the United States of America, vol. 96, no. 15, pp. 8705–8710, 1999.
[35]
I. B. Houston, M. B. Kamath, B. L. Schweitzer, T. M. Chlon, and R. P. DeKoter, “Reduction in PU.1 activity results in a block to B-cell development, abnormal myeloid proliferation, and neonatal lethality,” Experimental Hematology, vol. 35, no. 7, pp. 1056–1068, 2007.
[36]
S. Hohaus, M. S. Petrovick, M. T. Voso, Z. Sun, D. E. Zhang, and D. G. Tenen, “PU.1 (Spi-1) and C/EBPα regulate expression of the granulocyte-macrophage colony-stimulating factor receptor α gene,” Molecular and Cellular Biology, vol. 15, no. 10, pp. 5830–5845, 1995.
[37]
D. E. Zhang, C. J. Hetherington, H. M. Chen, and D. G. Tenen, “The macrophage transcription factor PU.1 directs tissue-specific expression of the macrophage colony-stimulating factor receptor,” Molecular and Cellular Biology, vol. 14, no. 1, pp. 373–381, 1994.
[38]
R. Dahl, J. C. Walsh, D. Lancki et al., “Regulation of macrophage and neutrophil cell fates by the PU.1: C/EBPα ratio and granulocyte colony-stimulating factor,” Nature Immunology, vol. 4, no. 10, pp. 1029–1036, 2003.
[39]
P. Laslo, C. J. Spooner, A. Warmflash et al., “Multilineage transcriptional priming and determination of alternate hematopoietic cell fates,” Cell, vol. 126, no. 4, pp. 755–766, 2006.
[40]
C. J. Hu, S. Rao, D. L. Ramirez-Bergeron et al., “PU.1/Spi-B regulation of c-rel is essential for mature B cell survival,” Immunity, vol. 15, no. 4, pp. 545–555, 2001.
[41]
G. H. Su, H. S. Ip, B. S. Cobb, M. M. Lu, H. M. Chen, and M. C. Simon, “The Ets protein Spi-B is expressed exclusively in B cells and T cells during development,” Journal of Experimental Medicine, vol. 184, no. 1, pp. 203–214, 1996.
[42]
M. K. Anderson, A. H. Weiss, G. Hernandez-Hoyos, C. J. Dionne, and E. V. Rothenberg, “Constitutive expression of PU.1 in fetal hematopoietic progenitors blocks T cell development at the pro-T cell stage,” Immunity, vol. 16, no. 2, pp. 285–296, 2002.
[43]
B. U. Mueller, T. Pabst, M. Osato et al., “Heterozygous PU.1 mutations are associated with acute myeloid leukemia,” Blood, vol. 100, no. 3, pp. 998–1007, 2002.
[44]
P. Burda, P. Laslo, and T. Stopka, “The role of PU.1 and GATA-1 transcription factors during normal and leukemogenic hematopoiesis,” Leukemia, vol. 24, no. 7, pp. 1249–1257, 2010.
[45]
N. Suraweera, E. Meijne, J. Moody et al., “Mutations of the PU.1 Ets domain are specifically associated with murine radiation-induced, but not human therapy-related, acute myeloid leukaemia,” Oncogene, vol. 24, no. 22, pp. 3678–3683, 2005.
[46]
V. Vegesna, S. Takeuchi, W. K. Hofmann et al., “C/EBP-β, C/EBP-δ, PU.1, AML1 genes: mutational analysis in 381 samples of hematopoietic and solid malignancies,” Leukemia Research, vol. 26, no. 5, pp. 451–457, 2002.
[47]
T. J. Ley, P. J. Minx, M. J. Walter et al., “A pilot study of high-throughput, sequence-based mutational profiling of primary human acute myeloid leukemia cell genomes,” Proceedings of the National Academy of Sciences of the United States of America, vol. 100, no. 24, pp. 14275–14280, 2003.
