PU.1 (Sfpi1), a pleiotropic regulator expressed from the first embryonic stages with a crucial function in germinal progenitors

Retinoic acid triggers c-kit gene expression in spermatogonial stem cells through an enhanceosome constituted between transcription factor binding sites for retinoic acid response element (RARE), spleen focus forming virus proviral integration oncogene (SPFI1) (PU.1) and E26 transformation-specific (ETS)

Restricted availability of retinoic acid (RA) in the testicular milieu regulates transcriptional activity of c-kit (KIT, CD117), which aids in the determination of spermatogonial stem-cell differentiation. The effect of RA on c-kit has been reported previously, but its mode of genomic action remains unresolved. We studied the molecular machinery guiding RA responsiveness to the c-kit gene using spermatogonial stem-cell line C18-4 and primary spermatogonial cells. A novel retinoic acid response element (RARE) positioned at -989 nucleotides upstream of the transcription start site (TSS) was identified, providing a binding site for a dimeric RA receptor (i.e. retinoic acid receptor gamma (RARγ) and retinoic X receptor). RA treatment influenced c-kit promoter activity, along with endogenous c-kit expression in C18-4 cells. A comprehensive promoter deletion assay using the pGL3B reporter system characterised the region spanning -271bp and -1011bp upstream of the TSS, which function as minimal promoter and maximal promoter, respectively. In silico analysis predicted that the region -1011 to +58bp comprised the distal enhancer RARE and activators such as spleen focus forming virus proviral integration oncogene (SPFI1) (PU.1), specificity protein 1 (SP1) and four E26 transformation-specific (ETS) tandem binding sites at the proximal region. Gel retardation and chromatin immunoprecipitation (ChIP) assays showed binding for RARγ, PU.1 and SP1 to the predicted consensus binding sequences, whereas GABPα occupied only two out of four ETS binding sites within the c-kit promoter region. We propose that for RA response, an enhanceosome is orchestrated through scaffolding of a CREB-binding protein (CBP)/p300 molecule between RARE and elements in the proximal promoter region, controlling germ-line expression of the c-kit gene. This study outlines the fundamental role played by RARγ, along with other non-RAR transcription factors (PU.1, SP1 and GABPα), in the regulation of c-kit expression in spermatogonial stem cells in response to RA.

Coordinated regulation of the immunoproteasome subunits by PML/RARα and PU.1 in acute promyelocytic leukemia

Recognition and elimination of malignant cells by cytotoxic T lymphocytes depends on antigenic peptides generated by proteasomes. It has been established that impairment of the immunoproteasome subunits, that is, PSMB8, PSMB9 and PSMB10 (PSMBs), is critical for malignant cells to escape immune recognition. We report here the regulatory mechanism of the repression of PU.1-dependent activation of PSMBs by PML/RARα in the pathogenesis of acute promyelocytic leukemia (APL) and the unidentified function of all-trans retinoic acid (ATRA) as an immunomodulator in the treatment of APL. Chromatin immunoprecipitation and luciferase reporter assays showed that PU.1 directly bound to and coordinately transactivated the promoters of PSMBs, indicating that PSMBs were transcriptional targets of PU.1 and PU.1 regulated their basal expression. Analysis of expression profiling data from a large population of acute myeloid leukemia (AML) patients revealed that the expression levels of PSMBs were significantly lower in APL patients than in non-APL AML patients. Further evidence demonstrated that the decrease in their expression was achieved through PML/RARα-mediated repression of both PU.1-dependent transactivation and PU.1 expression. Moreover, ATRA but not arsenic trioxide induced the expression of PSMBs in APL cells, indicating that ATRA treatment might activate the antigen-processing/presentation machinery. Finally, the above observations were confirmed in primary APL samples. Collectively, our data demonstrate that PML/RARα suppresses PU.1-dependent activation of the immunosubunits, which may facilitate the escape of APL cells from immune surveillance in leukemia development, and ATRA treatment is able to reactivate their expression, which would promote more efficient T-cell-mediated recognition in the treatment.

RNA-mediated epigenetic heredity requires the cytosine methyltransferase Dnmt2

RNA–mediated transmission of phenotypes is an important way to explain non-Mendelian heredity. We have previously shown that small non-coding RNAs can induce hereditary epigenetic variations in mice and act as the transgenerational signalling molecules. Two prominent examples for these paramutations include the epigenetic modulation of the Kit gene, resulting in altered fur coloration, and the modulation of the Sox9 gene, resulting in an overgrowth phenotype. We now report that expression of the Dnmt2 RNA methyltransferase is required for the establishment and hereditary maintenance of both paramutations. Our data show that the Kit paramutant phenotype was not transmitted to the progeny of Dnmt2^−/− mice and that the Sox9 paramutation was also not established in Dnmt2^−/− embryos. Similarly, RNA from Dnmt2-negative Kit heterozygotes did not induce the paramutant phenotype when microinjected into Dnmt2-deficient fertilized eggs and microinjection of the miR-124 microRNA failed to induce the characteristic giant phenotype. In agreement with an RNA–mediated mechanism of inheritance, no change was observed in the DNA methylation profiles of the Kit locus between the wild-type and paramutant mice. RNA bisulfite sequencing confirmed Dnmt2-dependent tRNA methylation in mouse sperm and also indicated Dnmt2-dependent cytosine methylation in Kit RNA in paramutant embryos. Together, these findings uncover a novel function of Dnmt2 in RNA–mediated epigenetic heredity.

The possibility of a mode of inheritance distinct from the Mendelian model has been considered since the early days of genetics. Only recently, however, suitable experimental models were created. We now see the development of new experimental systems detecting non-Mendelian inheritance in a variety of organisms, from worms to mice. We have previously shown that RNA molecules act as transgenerational inducers of epigenetic variations in mice. We are currently using Mendelian genetics to dissect the factors involved in RNA–mediated transgenerational signalling. By showing an absolute requirement for Dnmt2 in this process, our study extends our knowledge of this still somewhat enigmatic protein. We confirmed that RNA rather than DNA methylation by the protein is involved in epigenetic heredity, and our genetic results indicate a requirement during an early step in the reproductive process, between parental gametogenesis and the preimplantation stage.

Regulation of the hematopoietic cell kinase (HCK) by PML/RARα and PU.1 in acute promyelocytic leukemia

This study investigates the dynamic regulation of human hematopoietic cell kinase (HCK) in acute promyelocytic leukemia (APL) and the underlying molecular mechanisms. First, the level of HCK in APL blasts was found lower than that in normal granulocytes and monocytes. Second, the HCK promoter was repressed by PML/RARα and this repression required PU.1. PU.1 was capable of transactivating the HCK promoter through a region encompassing three PU.1 motifs. Chromatin immunoprecipitation assays provided evidence that PU.1 and PML/RARα bound to the HCK promoter in vivo. Finally, we found an unequivocal increase of HCK expression upon treatment with all-trans retinoic acid.

Two distinct auto-regulatory loops operate at the PU.1 locus in B cells and myeloid cells

The transcription factor PU.1 occupies a central role in controlling myeloid and early B-cell development, and its correct lineage-specific expression is critical for the differentiation choice of hematopoietic progenitors. However, little is known of how this tissue-specific pattern is established. We previously identified an upstream regulatory cis element whose targeted deletion in mice decreases PU.1 expression and causes leukemia. We show here that the upstream regulatory cis element alone is insufficient to confer physiologic PU.1 expression in mice but requires the cooperation with other, previously unidentified elements. Using a combination of transgenic studies, global chromatin assays, and detailed molecular analyses we present evidence that PU.1 is regulated by a novel mechanism involving cross talk between different cis elements together with lineage-restricted autoregulation. In this model, PU.1 regulates its expression in B cells and macrophages by differentially associating with cell type-specific transcription factors at one of its cis-regulatory elements to establish differential activity patterns at other elements.

Transcription factor PU.1 is expressed in white adipose and inhibits adipocyte differentiation

PU.1 transcription factor is a critical regulator of hematopoiesis and leukemogenesis. Because PU.1 interacts with transcription factors GATA-2 and C/EBPalpha, and both are involved in the regulation of adipogenesis, we investigated whether PU.1 plays a role in the regulation of adipocyte differentiation. Our data indicate that PU.1 is expressed in white adipose tissue. PU.1 protein can also be detected in cultured 3T3-L1 adipocytes. Forced expression of PU.1 in 3T3-L1 cells inhibits adipocyte differentiation, whereas deletion of the transactivation domain of PU.1 abolishes this effect. The inhibition of adipocyte differentiation by PU.1 is achieved, at least in part, through repression of the transcriptional activity of C/EBPalpha and C/EBPbeta. Furthermore, GATA-2 and PU.1 have an additive inhibitory effect on C/EBP transactivation and adipogenesis. Finally, the expression of PU.1 is increased in white adipose of obese mice.