Survival of naive T cells is dependent upon IL-7, which is present in vivo in limiting amounts with the result that naive T cells must compete for IL-7-mediated survival signals. It would seem imperative during T cell homeostasis that limiting IL-7 be shared by the greatest possible number of T cells. We now describe a novel regulatory mechanism that specifically suppresses IL7Ralpha transcription in response to IL-7 and other prosurvival cytokines (IL-2, IL-4, IL-6, and IL-15). Consequently, IL7R expression is reduced on T cells that have received cytokine-mediated survival signals so they do not compete with unsignaled T cells for remaining IL-7. Interestingly, cytokine-mediated suppression of IL7Ralpha transcription involves different molecular mechanisms in CD4+ and CD8+ T cells, as CD8+ T cells utilize the transcriptional repressor GFI1 while CD4+ T cells do not. We suggest that this homeostatic regulatory mechanism promotes survival of the maximum possible number of T cells for the amount of IL-7 available.

No abstract

The growth factor independent 1 (Gfi1) transcriptional regulator oncoprotein plays a crucial role in hematopoietic, inner ear, and pulmonary neuroendocrine cell development and governs cell processes as diverse as selfrenewal of hematopoietic stem cells, proliferation, apoptosis, differentiation, cell fate specification, and oncogenesis. However, the molecular basis of its transcriptional functions has remained elusive. Here we show that Gfi1 recruits the histone lysine methyltransferase G9a and the histone deacetylase 1 (HDAC1) in order to modify the chromatin of genes targeted for repression by Gfi1. G9a and HDAC1 are both in a repressive complex assembled by Gfi1. Endogenous Gfi1 colocalizes with G9a, HDAC1, and K9-dimethylated histone H3. Gfi1 associates with G9a and HDAC1 on the promoter of the cell cycle regulator p21 Cip/WAF1, resulting in an increase in K9 dimethylation at histone H3. Silencing of Gfi1 expression in myeloid cells reverses G9a and HDAC1 recruitment to p21Cip/WAF1 and elevates its expression. These findings highlight the role of epigenetics in the regulation of development and oncogenesis by Gfi1.

Apoptosis or programmed cell death is a series of events in a cell that leads to its death. Human polymorphonuclear leukocytes (PMN) 3 take part in host defense mechanisms against infection and inflammatory diseases. Inappropriate termination of PMN activation or failure to remove apoptotic PMNs results in inflammation. This apoptotic process has been suggested to represent an in vivo mechanism limiting oxidant-induced tissue injury caused by PMNs at the sites of inflammation. Although PMNs are constitutively committed to apoptosis from the time they enter circulation, the rate of apoptosis is not fixed. We reported that interleukin-8, granulocytemacrophage colony-stimulating factor, LTB 4 , and bacterial lipopolysaccharide (LPS) delay constitutive PMN apoptosis through the activation of the serine/threonine kinase Akt (1, 2). We demonstrated that p38 mitogen-activated protein kinase (MAPK) activity is required for Akt phosphorylation and activation (3). Additionally, we showed that Akt exists in a signaling module with p38 MAPK, MAPK-activated protein kinase-2 (MK2), and heat shock protein 27 (Hsp27) (3).Heat shock proteins represent a group of chaperone proteins that protect the cells against a variety of stresses. Besides being involved in functioning as a chaperone, Hsp27 has also been shown to regulate stability of the cytoskeleton, cell motility (4 -7), and apoptosis (8 -13). When overexpressed in tumor cells, Hsp27 increases their tumorigenicity by overexpressing MMP-9 expression and down-regulating Src tyrosine kinase Yes expression (14 -16) and protects against apoptotic cell death triggered by various stimuli, including cytotoxic drugs and ligation of the Fas/Apo-1/CD95 death receptor (17)(18)(19). Mice overexpressing Hsp27 were protected from lethal ischemia/reperfusion injury compared with their negative littermates (20). Possible mechanisms of Hsp27 anti-apoptotic activity are proposed to result from its activity as a molecular chaperone. Hsp27 binds to and inactivates the pro-apoptotic molecules Smac, caspase 3, caspase 9, and cytochrome c (21-25). Hsp27-mediated suppression of Bid translocation to the mitochondria correlates with an inhibition of cytochrome c release (25). Hsp27 has also been shown to promote survival * This work was supported by American Heart Association-Scientist Development Grant 0335278N (to M. J. R.) and National Institutes of Health Grant 1R56AI059165-01A2 (to M. J. R.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Long interspersed nuclear elements (LINEs or L1 elements) are targeted for epigenetic silencing during early embryonic development and remain inactive in most cells and tissues. Here we show that E2F-Rb family complexes participate in L1 elements epigenetic regulation via nucleosomal histone modifications and recruitment of histone deacetylases (HDACs) HDAC1 and HDAC2. ChIP experiments demonstrated that (i) Rb and E2F interact with human and mouse L1 elements, (ii) L1 elements are deficient in both heterochromatin-associated histone marks H3 tri methyl K9 and H4 tri methyl K20 in Rb family triple knock out (Rb, p107, p130) fibroblasts (TKO), (iii) L1 promoter exhibits increased histone H3 acetylation in the absence of HDAC1 and HDAC2 recruitment, (iv) L1 expression in TKO fibroblasts is upregulated compared to wild type counterparts, (v) L1 expression increases in the presence of the HDAC inhibitor TSA. On the basis of these findings we propose a model in which L1 sequences throughout the genome serve as centers for heterochromatin formation in an Rb family-dependent manner. As such, Rb proteins and L1 elements may play key roles in heterochromatin formation beyond pericentromeric chromosomal regions. These findings describe a novel mechanism of L1 reactivation in mammalian cells mediated by failure of co-repressor protein recruitment by Rb, loss of histone epigenetic marks, heterochromatin formation, and increased histone H3 acetylation.