Roquin proteins preclude spontaneous T cell activation and aberrant differentiation of T follicular helper (Tfh) or T helper 17 (Th17) cells. Here we showed that deletion of Roquin-encoding alleles specifically in regulatory T (Treg) cells also caused the activation of conventional T cells. Roquin-deficient Treg cells downregulated CD25, acquired a follicular Treg (Tfr) cell phenotype, and suppressed germinal center reactions but could not protect from colitis. Roquin inhibited the PI3K-mTOR signaling pathway by upregulation of Pten through interfering with miR-17∼92 binding to an overlapping cis-element in the Pten 3' UTR, and downregulated the Foxo1-specific E3 ubiquitin ligase Itch. Loss of Roquin enhanced Akt-mTOR signaling and protein synthesis, whereas inhibition of PI3K or mTOR in Roquin-deficient T cells corrected enhanced Tfh and Th17 or reduced iTreg cell differentiation. Thereby, Roquin-mediated control of PI3K-mTOR signaling prevents autoimmunity by restraining activation and differentiation of conventional T cells and specialization of Treg cells.

This review focuses on the regulation of transcription factors, many of which are DNA-binding proteins that recognize cis-regulatory elements of target genes and are the most direct regulators of gene transcription. Transcription factors serve as integration centres of the different signal-transduction pathways affecting a given gene. It is obvious that the regulation of these regulators themselves is of crucial importance for differential gene expression during development and in terminally differentiated cells. Transcription factors can be regulated at two, principally different, levels, namely concentration and activity, each of which can be modulated in a variety of ways. The concentrations of transcription factors, as of intracellular proteins in general, may be regulated at any of the steps leading from DNA to protein, including transcription, RNA processing, mRNA degradation and translation. The activity of a transcription factor is often regulated by (de) phosphorylation, which may affect different functions, e.g. nuclear localization DNA binding and trans-activation. Ligand binding is another mode of transcription-factor activation. It is typical for the large super-family of nuclear hormone receptors. Heterodimerization between transcription factors adds another dimension to the regulatory diversity and signal integration. Finally, non-DNA-binding (accessory) factors may mediate a diverse range of functions, e.g. serving as a bridge between the transcription factor and the basal transcription machinery, stabilizing the DNA-binding complex or changing the specificity of the target sequence recognition. The present review presents an overview of different modes of transcription-factor regulation, each illustrated by typical examples.

Damaged replication forks activate poly(ADP-ribose) polymerase 1 (PARP1), which catalyses poly(ADP-ribose) (PAR) formation; however, how PARP1 or poly(ADP-ribosyl)ation is involved in the S-phase checkpoint is unknown. Here we show that PAR, supplied by PARP1, interacts with Chk1 via a novel PAR-binding regulatory (PbR) motif in Chk1, independent of ATR and its activity. iPOND studies reveal that Chk1 associates readily with the unperturbed replication fork and that PAR is required for efficient retention of Chk1 and phosphorylated Chk1 at the fork. A PbR mutation, which disrupts PAR binding, but not the interaction with its partners Claspin or BRCA1, impairs Chk1 and the S-phase checkpoint activation, and mirrors Chk1 knockdown-induced hypersensitivity to fork poisoning. We find that long chains, but not short chains, of PAR stimulate Chk1 kinase activity. Collectively, we disclose a previously unrecognized mechanism of the S-phase checkpoint by PAR metabolism that modulates Chk1 activity at the replication fork.

The immunosuppressive macrolide rapamycin and its derivative everolimus (SDZ RAD, RAD) inhibit the mammalian target of rapamycin (mTOR) signaling pathway. In this study, we provide evidence that RAD has profound antiproliferative activity in vitro and in NOD/SCID mice in vivo against Hodgkin lymphoma (HL) and anaplastic large cell lymphoma (ALCL) cells. Moreover, we identified 2 molecular mechanisms that showed how RAD exerts antiproliferative effects in HL and ALCL cells. RAD down-regulated the truncated isoform of the transcription factor CCAAT enhancer binding protein ␤ (C/ EBP␤), which is known to disrupt terminal differentiation and induce a transformed phenotype. Furthermore, RAD inhibited constitutive nuclear factor B (NF-B) activity, which is a critical sur- IntroductionHodgkin lymphoma (HL) and anaplastic large cell lymphoma (ALCL) share morphologic and immunophenotypic markers in a subgroup of cases although they are biologically distinct entities. 1 Therefore, pathologic diagnosis is sometimes difficult to achieve and these cases are classified as "gray-zone lymphomas." 2 Moreover, in both entities novel therapeutic options are needed, as curative therapy of HL is compromised by a high risk of long-term complications, and anaplastic lymphoma kinase (ALK)-negative ALCL still has a very unfavorable prognosis with current treatment strategies. [3][4][5] The macrocyclic lactone everolimus (SDZ, RAD) RAD is a rapamycin derivative with potent immunosuppressive and antiproliferative properties. [6][7][8][9][10] It is further known to inhibit growth factor-driven cell proliferation of hematopoietic and nonhematopoietic cells. 6,10 In addition, RAD is a potent inhibitor of human Epstein-Barr virus (EBV)-transformed B lymphocytes in vitro and in vivo, arresting cell-cycle progression and increasing the apoptotic rate of EBV ϩ B cells. 10 Therefore, it has been suggested that RAD might be effective in the prevention and treatment of human posttransplant lymphoproliferative disorders. 10 Here, we investigated whether RAD inhibits tumor cell proliferation of HL and ALCL. We show that RAD significantly inhibits proliferation of HL and ALCL cells in vitro and arrests cell-cycle progression in G 0 /G 1 . Furthermore, we demonstrate that in vivo, RAD markedly suppresses tumor cell proliferation of HL and ALCL cells, xenotransplanted into NOD/SCID mice. Our data suggest that RAD might be used in combination chemotherapy for the treatment of HL and ALCL. Moreover, we studied the mechanisms of proliferation arrest mediated by the mammalian target of rapamycin (mTOR) inhibitor RAD to identify the molecular targets in HL and ALCL. The mTOR pathway controls the translation initiation machinery in response to nutrients and growth factors thereby coordinating cell growth with cell division. 11 A transcription factor that is a critical target of mTOR is the CCAAT enhancer binding protein (C/EBP)␤. 11-13 C/EBP␤ has previously been identified as an essential downstream target in tumors expressing activated cyclin D1. 14 Our dat...