Upon DNA damage, p53-binding protein 1 (53BP1) relocalizes to sites of DNA double-strand breaks and forms discrete nuclear foci, suggesting its role in DNA damage responses. We show that 53BP1 changed its localization from the detergent soluble to insoluble fraction after treatment of cells with x-ray, but not with ultraviolet or hydroxyurea. Either DNase or phosphatase treatment of the insoluble fraction released 53BP1 into the soluble fraction, showing that 53BP1 binds to chromatin in a phosphorylation-dependent manner after X-irradiation of cells. 53BP1 was retained at discrete nuclear foci in X-irradiated cells even after detergent extraction of cells, showing that the chromatin binding of 53BP1 occurs at sites of DNA double-strand breaks. The minimal domain for focus formation was identified by immunofluorescence staining of cells ectopically expressed with 53BP1 deletion mutants. This domain consisted of conserved Tudor and Myb motifs. The Tudor plus Myb domain possessed chromatin binding activity in vivo and bound directly to both double-stranded and singlestranded DNA in vitro. This domain also stimulated endjoining by DNA ligase IV/Xrcc4, but not by T4 DNA ligase in vitro. We conclude that 53BP1 has the potential to participate directly in the repair of DNA double-strand breaks.

OBJECTIVE-Adipose tissue serves as an integrator of various physiological pathways, energy balance, and glucose homeostasis. Forkhead box-containing protein O subfamily (FoxO) 1 mediates insulin action at the transcriptional level. However, physiological roles of FoxO1 in adipose tissue remain unclear.RESEARCH DESIGN AND METHODS-In the present study, we generated adipose tissue-specific FoxO1 transgenic mice (adipocyte protein 2 [aP 2 ]-FLAG-⌬256) using an aP 2 promoter/ enhancer and a mutant FoxO1 (FLAG⌬256) in which the carboxyl terminal transactivation domain was deleted. Using these mice, we analyzed the effects of the overexpression of FLAG⌬256 on glucose metabolism and energy homeostasis. RESULTS-The aP 2 -FLAG-⌬256 mice showed improved glucose tolerance and insulin sensitivity accompanied with smallersized adipocytes and increased adiponectin (adipoq) and Glut 4 (Slc2a4) and decreased tumor necrosis factor ␣ (Tnf) and chemokine (C-C motif) receptor 2 (Ccr2) gene expression levels in white adipose tissue (WAT) under a high-fat diet. Furthermore, the aP 2 -FLAG-⌬256 mice had increased oxygen consumption accompanied with increased expression of peroxisome proliferator-activated receptor ␥ coactivator (PGC)-1␣ protein and uncoupling protein (UCP)-1 (Ucp1), UCP-2 (Ucp2), and ␤3-AR (Adrb3) in brown adipose tissue (BAT). Overexpression of FLAG⌬256 in T37i cells, which are derived from the hibernoma of SV40 large T antigen transgenic mice, increased expression of PGC-1␣ protein and Ucp1. Furthermore, knockdown of endogenous FoxO1 in T37i cells increased Pgc1␣ (Ppargc1a), Pgc1␤ (Ppargc1b), Ucp1, and Adrb3 gene expression.CONCLUSIONS-These data suggest that FoxO1 modulates energy homeostasis in WAT and BAT through regulation of adipocyte size and adipose tissue-specific gene expression in response to excessive calorie intake. Diabetes 57:563-576, 2008

Forkhead transcription factors FoxOs are conserved beyond species and regulated by insulin signaling pathway. FoxOs have diverse functions on differentiation, proliferation and cell survival. In calorie restriction (CR) or starvation, FoxOs are in nucleus, active transcriptionally, and increase hepatic glucose production, decrease insulin secretion, increase food intake and cause degradation of skeletal muscle for supplying substrates for glucose production. However, even in insulin resistance due to excessive calorie intake, FoxOs are active and causes type 2 diabetes and hyperlipidemia. The understanding of molecular mechanism how FoxOs affect glucose or lipid metabolism will shed light on the novel therapy of type 2 diabetes and the metabolic syndrome.

The forkhead transcription factor FoxO1 has been identified as a negative regulator of insulin/IGF-1 signaling. Its function is inhibited by phosphorylation and nuclear exclusion through a PI3K-dependent pathway. However, the structure/function relationship of FoxO1 has not been elucidated completely. In this study, we carried out mutation analysis of the FoxO1 coactivator-interacting LXXLL motif (amino acids 459-463). Expression of a 3A/LXXAA mutant, in which 3 Akt phosphorylation sites (T24, S253, and S316) and 2 leucine residues in the LXXLL motif (L462 and L463) were replaced by alanine, decreased both Igfbp-1 and G6Pase promoter activity and endogenous Igfbp-1 and G6Pase gene expression in simian virus 40-transformed (SV40-transformed) hepatocytes. Importantly, mutagenesis of the LXXLL motif eliminated FoxO1 interaction with the nicotinamide adenine dinucleotide-dependent (NAD-dependent) deacetylase sirtuin 1 (Sirt1), sustained the acetylated state of FoxO1, and made FoxO1 nicotinamide and resveratrol insensitive, supporting a role for this motif in Sirt1 binding. Furthermore, intravenous administration of adenovirus encoding 3A/LXXAA FoxO1 into Lepr db/db mice decreased fasting blood glucose levels and improved glucose tolerance and was accompanied by reduced G6Pase and Igfbp-1 gene expression and increased hepatic glycogen content. In conclusion, the LXXLL motif of FoxO1 may have an important role for its transcriptional activity and Sirt1 binding and should be a target site for regulation of gene expression of FoxO1 target genes and glucose metabolism in vivo.

Both insulin and leptin signaling converge on phosphatidylinositol 3-OH kinase [PI( 3 )K]/3-phosphoinositide-dependent protein kinase-1 (PDK-1)/protein kinase B (PKB, also known as Akt) in proopiomelanocortin (POMC) neurons. Forkhead box-containing protein-O1 (FoxO1) is inactivated in a PI( 3 )K-dependent manner. However, the interrelationship between PI( 3 )K/PDK-1/Akt and FoxO1, and the chronic effects of the overexpression of FoxO1 in POMC neurons on energy homeostasis has not been elucidated. To determine the extent to which PDK-1 and FoxO1 signaling in POMC neurons was responsible for energy homeostasis, we generated POMC neuron-specific Pdk1 knockout mice ( POMCPdk1−/−) and mice selectively expressing a constitutively nuclear (CN)FoxO1 or transactivation-defective (Δ256)FoxO1 in POMC neurons ( CNFoxO1 POMC or Δ256FoxO1 POMC). POMCPdk1−/− mice showed increased food intake and body weight accompanied by decreased expression of Pomc gene. The CNFoxO1 POMC mice exhibited mild obesity and hyperphagia compared with POMCPdk1−/− mice. Although expression of the CNFoxO1 made POMCPdk1−/− mice more obese due to excessive suppression of Pomc gene, overexpression of Δ256FoxO1 in POMC neurons had no effects on metabolic phenotypes and Pomc expression levels of POMCPdk1−/− mice. These data suggest a requirement for PDK-1 and FoxO1 in transcriptional regulation of Pomc and food intake.