Mutations in the gene encoding the transcriptional repressor methyl-CpG binding protein 2 (MeCP2) cause the neurodevelopmental disorder Rett syndrome. Loss of function as well as increased dosage of MECP2 gene cause a host of neuropsychiatric disorders. To explore the molecular mechanism(s) underlying these disorders, we examined gene expression patterns in the hypothalamus of mice that either lack or overexpress MeCP2. In both models, MeCP2 dysfunction induced changes in the expression levels of thousands of genes, but surprisingly the majority of genes (~85%) appeared to be activated by MeCP2. We selected six genes and confirmed that MeCP2 binds to their promoters. Furthermore, we showed that MeCP2 associates with the transcriptional activator CREB1 at the promoter of an activated target but not a repressed target. These studies suggest that MeCP2 regulates the expression of a wide range of genes in the hypothalamus and that it can function as both an activator and repressor of transcription.Rett syndrome (RTT, MIM 312750) is a progressive neurodevelopmental disorder caused by mutations in the X-linked gene encoding methyl-CpG binding protein 2 (MeCP2) (1). RTT patients appear to develop normally up to the first year of age after which they lose any acquired speech and replace purposeful hand use with stereotypies. MECP2 mutations also result in a host of neurobehavioral abnormalities, ranging from mild learning disabilities to autism, Xlinked mental retardation, and infantile encephalopathy. Interestingly, an increase in MECP2 dosage is equally detrimental to the nervous system: duplications spanning the MECP2 locus cause features overlapping with those of RTT and related neurological disorders (2).
Mammalian target of rapamycin (mTOR) controls cell growth and proliferation via the raptor-mTOR (TORC1) and rictor-mTOR (TORC2) protein complexes. Recent biochemical studies suggested that TORC2 is the elusive PDK2 for Akt/PKB Ser473 phosphorylation in the hydrophobic motif. Phosphorylation at Ser473, along with Thr308 of its activation loop, is deemed necessary for Akt function, although the regulatory mechanisms and physiological importance of each phosphorylation site remain to be fully understood. Here, we report that SIN1/MIP1 is an essential TORC2/PDK2 subunit. Genetic ablation of sin1 abolished Akt-Ser473 phosphorylation and disrupted rictor-mTOR interaction but maintained Thr308 phosphorylation. Surprisingly, defective Ser473 phosphorylation affected only a subset of Akt targets in vivo, including FoxO1/3a, while other Akt targets, TSC2 and GSK3, and the TORC1 effectors, S6K and 4E-BP1, were unaffected. Our findings reveal that the SIN1-rictor-mTOR function in Akt-Ser473 phosphorylation is required for TORC2 function in cell survival but is dispensable for TORC1 function.
SIRT1 is an NAD-dependent deacetylase critically involved in stress responses, cellular metabolism and, possibly, ageing [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] . The tumour suppressor p53 represents the first non-histone substrate functionally regulated by acetylation and deacetylation 16,17 ; we and others previously found that SIRT1 promotes cell survival by deacetylating p53 (refs 4-6 ). These results were further supported by the fact that p53 hyperacetylation and increased radiation-induced apoptosis were observed in Sirt1-deficient mice 10 . Nevertheless, SIRT1-mediated deacetylase function is also implicated in p53-independent pathways under different cellular contexts, and its effects on transcriptional factors such as members of the FOXO family and PGC-1α directly modulate metabolic responses [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] . These studies validate the importance of the deacetylase activity of SIRT1, but how SIRT1 activity is regulated in vivo is not well understood. Here we show that DBC1 (deleted in breast cancer 1) acts as a native inhibitor of SIRT1 in human cells. DBC1-mediated repression of SIRT1 leads to increasing levels of p53 acetylation and upregulation of p53-mediated function. In contrast, depletion of endogenous DBC1 by RNA interference (RNAi) stimulates SIRT1-mediated deacetylation of p53 and inhibits p53-dependent apoptosis. Notably, these effects can be reversed in cells by concomitant knockdown of endogenous SIRT1. Our study demonstrates that DBC1 promotes p53-mediated apoptosis through specific inhibition of SIRT1.To understand the regulation of SIRT1-mediated deacetylation in vivo, biochemical purification was used to identify cellular factors that stably interact with SIRT1. We isolated physiologically formed protein complexes containing SIRT1 from cell extracts of native HeLa cells by conducting affinity chromatography with affinity-purified antisera raised against the carboxy (C) terminus (amino acids 480-737) of SIRT1 ( Supplementary Fig. 1a). As expected, we identified SIRT1 as the major component of the complexes, but several protein bands were also co-purified with SIRT1. Mass spectrometry of a prominent protein band of approximately 130 kilodaltons (kDa) from the SIRT1 complexes revealed peptide sequences corresponding to the DBC1 protein ( Supplementary Fig. 1b, Gi: 24432106). The DBC1 gene was initially identified as it is localized to a region of chromosome 8p21 that was homozygously deleted in human breast cancer; however, the molecular function of DBC1 is poorly understood 18,19 .To examine the interaction between endogenous DBC1 and SIRT1, cell extracts from human osteosarcoma U2OS cells were immunoprecipitated with the anti-SIRT1 antibody or with the
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