No abstract

Unchecked accumulation of reactive oxygen species (ROS) compromises maintenance of hematopoietic stem cells. Regulation of ROS by the tumor suppressor protein ataxia telangiectasia mutated (ATM) is critical for preserving the hematopoietic stem cell pool. In this study we demonstrate that the Foxo3 member of the Forkhead Box O (FoxO) family of transcription factors is essential for normal ATM expression. In addition, we show that loss of Foxo3 leads to defects in hematopoietic stem cells, and these defects result from an overaccumulation of ROS. Foxo3 suppression of ROS in hematopoietic stem cells is mediated partly by regulation of ATM expression. We identify ROSindependent modulations of ATM and p16INK4a and ROSmediated activation of p53/p21 CIP1/WAF1/Sdi1 tumor suppressor pathways as major contributors to Foxo3-null hematopoietic stem cells defects. Our studies demonstrate that Foxo3 represses ROS in part via regulation of ATM and that this repression is required for maintenance of the hematopoietic stem cell pool.

TH1 and TH17 cells mediate neuroinflammation in experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. Pathogenic TH cells in EAE must produce the pro-inflammatory cytokine granulocyte-macrophage colony stimulating factor (GM-CSF). TH cell pathogenicity in EAE is also regulated by cell-intrinsic production of the immunosuppressive cytokine interleukin 10 (IL-10). Here, we demonstrate that mice deficient for the basic helix-loop-helix (bHLH) transcription factor Bhlhe40 (Bhlhe40−/−) are resistant to the induction of EAE. Bhlhe40 is required in vivo in a T cell-intrinsic manner, where it positively regulates the production of GM-CSF and negatively regulates the production of IL-10. In vitro, GM-CSF secretion is selectively abrogated in polarized Bhlhe40−/− TH1 and TH17 cells, and these cells show increased production of IL-10. Blockade of IL-10 receptor in Bhlhe40−/− mice renders them susceptible to EAE. These findings identify Bhlhe40 as a critical regulator of autoreactive T cell pathogenicity.

Stra13 is a transcriptional repressor related within its basic helixloop-helix domain with the Drosophila Hairy, Enhancer of Split, and the mouse Hes1 proteins that interact with the corepressor Groucho. Because Stra13 lacks the conserved WRPW motif for interaction with Groucho, we examined the function and mechanism of transcriptional repression mediated by Stra13 that exhibits several distinctive features. Here, we report that Stra13 expression is closely associated with cell growth arrest induced by several triggers such as retinoic acid and trichostatin A (TSA; a specific histone deacetylase inhibitor) as well as by serum starvation. Stra13 expression is transcriptionally repressed and maintained at a low level in cells through a negative autoregulatory mechanism that is brought about by its interaction with the corepressor histone deacetylase (HDAC1). This interaction requires the Stra13 C-terminal domain containing three ␣-helices, which are also functionally critical to its repressive activity. Thus, inhibition of HDAC activity by TSA abrogates Stra13-mediated repression of its promoter, resulting in induction of Stra13 expression that is coincident with TSA-induced growth arrest. Further, once induced, Stra13 strongly represses the expression of the cell proliferationassociated gene c-Myc through an HDAC1-independent pathway that involves its interaction with the basal transcription factor TFIIB. Our studies indicate that Stra13 may play a key role in signaling pathways that lead to growth arrest and terminal differentiation by repression of target genes via HDAC-dependent and HDAC-independent mechanisms. T ranscription factors of the basic helix-loop-helix (bHLH) family are important regulators of cellular growth, differentiation, and apoptosis (1). Stra13 is a novel bHLH gene (2) that exhibits the highest sequence identity in the bHLH domain with the Drosophila Hairy (H), Enhancer of Split [E(Spl)], and mouse Hes1 proteins (3). Members of this subfamily bind to an N-box sequence element and act as transcriptional repressors by recruiting the corepressor Groucho through a highly conserved ''WRPW'' motif (4). Outside the bHLH domain, Stra13 shares no significant identity with known proteins and is characterized by three putative ␣-helices in its C terminus. Although Stra13 does not bind the N-box element, it does exhibit a strong transcriptional repression activity that is mediated through the ␣-helices (2). Moreover, unlike E(Spl), Hairy, and Hes, Stra13 lacks a WRPW motif, suggesting that it may mediate transcriptional repression by interaction with corepressors other than Groucho.Recent studies have provided molecular evidence that modification of chromatin structure by histone deacetylation is an important mechanism in controlling gene transcription. Several transcriptional repressors such as YY-1, RB, and CBF-1 interact directly with histone deacetylases (HDAC), whereas nuclear hormone receptors, Mad, and PLZF are linked indirectly to HDAC through additional components [silencing mediator of retinoi...