The human phosphatidylinositol phosphatase SAC1 interacts with the coatomer I complex

The Saccharomyces cerevisiae SAC1 gene encodes an integral membrane protein of the endoplasmic reticulum (ER) and the Golgi apparatus. Yeast SAC1 mutants display a wide array of phenotypes including inositol auxotrophy, cold sensitivity, secretory defects, disturbed ATP transport into the ER, or suppression of actin gene mutations. At present, it is not clear how these phenotypes relate to the finding that SAC1 displays polyphosphoinositide phosphatase activity. Moreover, it is still an open question whether SAC1 functions similarly in mammalian cells, since some phenotypes are yeast-specific. Potential protein interaction partners and, connected to that, possible regulatory circuits have not been described. Therefore, we have cloned human SAC1 (hSAC1), show that it behaves similar to ySac1p in terms of substrate specificity, demonstrate that the endogenous protein localizes to the ER and Golgi, and identify for the first time members of the coatomer I (COPI) complex as interaction partners of hSAC1. Mutation of a putative COPI interaction motif (KXKXX) at its C terminus abolishes interaction with COPI and causes accumulation of hSAC1 in the Golgi. In addition, we generated a catalytically inactive mutant, demonstrate that its lipid binding capacity is unaltered, and show that it accumulates in the Golgi, incapable of interacting with the COPI complex despite the presence of the KXKXX motif. These results open the possibility that the enzymatic function of hSAC1 provides a switch for accessibility of the COPI interaction motif.     

DACH1 inhibits transforming growth factor-beta signaling through binding Smad4

The vertebrate homologues of Drosophila dachsund, DACH1 and DACH2, have been implicated as important regulatory genes in development. DACH1 plays a role in retinal and pituitary precursor cell proliferation and DACH2 plays a specific role in myogenesis. DACH proteins contain a domain (DS domain) that is conserved with the proto-oncogenes Ski and Sno. Since the Ski/Sno proto-oncogenes repress AP-1 and SMAD signaling, we hypothesized that DACH1 might play a similar cellular function. Herein, DACH1 was found to be expressed in breast cancer cell lines and to inhibit transforming growth factor-beta (TGF-beta)-induced apoptosis. DACH1 repressed TGF-beta induction of AP-1 and Smad signaling in gene reporter assays and repressed endogenous TGF-beta-responsive genes by microarray analyses. DACH1 bound to endogenous NCoR and Smad4 in cultured cells and DACH1 co-localized with NCoR in nuclear dotlike structures. NCoR enhanced DACH1 repression, and the repression of TGF-beta-induced AP-1 or Smad signaling by DACH1 required the DACH1 DS domain. The DS domain of DACH was sufficient for NCoR binding at a Smad4-binding site. Smad4 was required for DACH1 repression of Smad signaling. In Smad4 null HTB-134 cells, DACH1 inhibited the activation of SBE-4 reporter activity induced by Smad2 or Smad3 only in the presence of Smad4. DACH1 participates in the negative regulation of TGF-beta signaling by interacting with NCoR and Smad4.     

Smac3, a novel Smac/DIABLO splicing variant, attenuates the stability and apoptosis-inhibiting activity of X-linked inhibitor of apoptosis protein

X-linked inhibitor of apoptosis protein (XIAP), the most potent member of the inhibitor of apoptosis protein (IAP) family, plays a crucial role in the regulation of apoptosis. XIAP is structurally characterized by three baculovirus IAP repeat (BIR) domains that mediate binding to and inhibition of caspases and a RING domain that confers ubiquitin ligase activity. The caspase inhibitory activity of XIAP can be eliminated by the second mitochondria-derived activator of caspases (Smac)/direct IAP-binding protein with low pI (DIABLO) during apoptosis. Here we report the identification and characterization of a novel isoform of Smac/DIABLO named Smac3, which is generated by alternative splicing of exon 4. Smac3 contains an NH2-terminal mitochondrial targeting sequence required for mitochondrial targeting of Smac3 and an IAP-binding motif essential for Smac3 binding to XIAP. Smac3 is released from mitochondria into the cytosol in response to apoptotic stimuli, where it interacts with the second and third BIR domains of XIAP. Smac3 disrupts processed caspase-9 binding to XIAP, promotes caspase-3 activation, and potentiates apoptosis. Strikingly, Smac3, but not Smac/DIABLO, accelerates XIAP auto-ubiquitination and destruction. Smac3-stimulated XIAP ubiquitination is contingent upon the physical association of XIAP with Smac3 and an intact RING domain of XIAP. Smac3-accelerated XIAP destabilization is, at least in part, attributed to its ability to enhance XIAP ubiquitination. Our study demonstrates that Smac3 is functionally additive to, but independent of, Smac/DIABLO.     

Vascular endothelial growth factor causes translocation of p47phox to membrane ruffles through WAVE1

Growth factors initiate cytoskeletal rearrangements tightly coordinated with nuclear signaling events. We hypothesized that the angiogenic growth factor, vascular endothelial growth factor (VEGF), may utilize oxidants that are site-directed to a complex critical to both cytoskeletal and mitogenic signaling. We identified the WASP-family verprolin homologous protein-1 (WAVE1) as a binding partner for the NADPH oxidase adapter p47phox within membrane ruffles of VEGF-stimulated cells. Within 15 min of VEGF stimulation, p47phox coprecipitated with WAVE1, with the ruffle and oxidase agonist Rac1, and with the Rac1 effector PAK1. VEGF also increased p47phox phosphorylation, oxidant production, and ruffle formation, all of which were dependent upon PAK1 kinase activity. The antioxidant Mn (III) tetrakis(4-benzoic acid) porphyrin and ectopic expression of either the p47-binding WAVE1 domain or the WAVE1-binding p47phox domain decreased VEGF-induced ruffling, whereas the active mutant p4-(S303D, S304D,S328D) stimulated oxidant production and formation of circular dorsal ruffles. Both kinase-dead PAK1-(K298A) and Mn (III) tetrakis(4-benzoic acid) porphyrin decreased c-Jun N-terminal kinase (JNK) activation by VEGF, whereas dominant-negative JNK did not block ruffle formation, suggesting a bifurcation of mitogenic and cytoskeletal signaling events at or distal to the oxidase but proximal to JNK. Thus, WAVE1 may act as a scaffold to recruit the NADPH oxidase to a complex involved with both cytoskeletal regulation and downstream JNK activation.     

APP processing is regulated by cytoplasmic phosphorylation

Amyloid-beta peptide (Abeta) aggregate in senile plaque is a key characteristic of Alzheimer's disease (AD). Here, we show that phosphorylation of amyloid precursor protein (APP) on threonine 668 (P-APP) may play a role in APP metabolism. In AD brains, P-APP accumulates in large vesicular structures in afflicted hippocampal pyramidal neurons that costain with antibodies against endosome markers and the beta-secretase, BACE1. Western blot analysis reveals increased levels of T668-phosphorylated APP COOH-terminal fragments in hippocampal lysates from many AD but not control subjects. Importantly, P-APP cofractionates with endosome markers and BACE1 in an iodixanol gradient and displays extensive colocalization with BACE1 in rat primary cortical neurons. Furthermore, APP COOH-terminal fragments generated by BACE1 are preferentially phosphorylated on T668 verses those produced by alpha-secretase. The production of Abeta is significantly reduced when phosphorylation of T668 is either abolished by mutation or inhibited by T668 kinase inhibitors. Together, these results suggest that T668 phosphorylation may facilitate the BACE1 cleavage of APP to increase Abeta generation.     

Deregulation of Cdc2 kinase induces caspase-3 activation and apoptosis

Progression of the cell cycle and control of apoptosis are tightly linked processes. It has been reported that manifestation of apoptosis requires cdc2 kinase activity yet the mechanism(s) of which is largely unclear. In an attempt to study the role of human MDM2 (HDM2) in interphase and mitosis, we employed the Xenopus cell-free system to study HDM2 protein stability. Interestingly, HDM2 is specifically cleaved in Xenopus mitotic extracts but not in the interphase extracts. We demonstrate that HDM2 cleavage is dependent on caspase-3 and that activation of cdc2 kinase results in caspase-3 activation in the Xenopus cell-free system. Furthermore, expression of cdc2 kinase in mammalian cells leads to activation of caspase-3 and apoptosis. Taken together, these data indicate that deregulation of cdc2 kinase activity can trigger apoptotic machinery that leads to caspase-3 activation and apoptosis.     

Prolonged culture in low glucose induces apoptosis of rat pancreatic beta-cells through induction of c-myc

Prolonged culture in low-glucose concentrations (相似文献   

The gamma-frailty Poisson model for the nonparametric estimation of panel count data

In this article, we study nonparametric estimation of the mean function of a counting process with panel observations. We introduce the gamma frailty variable to account for the intracorrelation between the panel counts of the counting process and construct a maximum pseudo-likelihood estimate with the frailty variable. Three simulated examples are given to show that this estimation procedure, while preserving the robustness and simplicity of the computation, improves the efficiency of the nonparametric maximum pseudo-likelihood estimate studied in Wellner and Zhang (2000, Annals of Statistics 28, 779-814). A real example from a bladder tumor study is used to illustrate the method.