Tyrosine kinase nerve growth factor receptor switches from prosurvival to proapoptotic activity via Abeta-mediated phosphorylation

The present study shows that increased Abeta production in hippocampal neurons, due to a failure of NGF signal, induces an unexpected phosphorylation of tyrosine kinase receptor A (TrkA), followed by activation of the phospholipase C γ (PLCγ) pathway and neuronal death. Such phosphorylation seems causally connected with 2 kinases known be involved in amyloidogenesis, Src and CDK5, and associated with α and γ secretase–mediated p75 processing. Pharmacologic inhibition of TrkA phosphorylation and partial silencing of TrkA and/or p75 receptors prevent PLCγ activation and protect neurons from death. Concomitantly with these events, TrkA, p75, Abeta peptides, and PS1 protein coimmunoprecipitate, suggesting their direct interplay in the subsequent onset of apoptotic death. Together, these findings depict a cellular mechanism whereby the same cellular transducing system may invert its intracellular message from trophic and antiapoptotic to a death signaling, which could also have relevance in the onset of Alzheimer''s disease.     

Disulfide isomerization switches tissue factor from coagulation to cell signaling

Cell-surface tissue factor (TF) binds the serine protease factor VIIa to activate coagulation or, alternatively, to trigger signaling through the G protein-coupled, protease-activated receptor 2 (PAR2) relevant to inflammation and angiogenesis. Here we demonstrate that TF.VIIa-mediated coagulation and cell signaling involve distinct cellular pools of TF. The surface-accessible, extracellular Cys186-Cys209 disulfide bond of TF is critical for coagulation, and protein disulfide isomerase (PDI) disables coagulation by targeting this disulfide. A TF mutant (TF C209A) with an unpaired Cys186 retains TF.VIIa signaling activity, and it has reduced affinity for VIIa, a characteristic of signaling TF on cells with constitutive TF expression. We further show that PDI suppresses TF coagulant activity in a nitric oxide-dependent pathway, linking the regulation of TF thrombogenicity to oxidative stress in the vasculature. Furthermore, a unique monoclonal antibody recognizes only the noncoagulant, cryptic conformation of TF. This antibody inhibits formation of the TF.PAR2 complex and TF.VIIa signaling, but it does not prevent coagulation activation. These experiments delineate an upstream regulatory mechanism that controls TF function, and they provide initial evidence that TF.VIIa signaling can be specifically inhibited with minimal effects on coagulation.     

Tumour suppressor menin is essential for development of the pancreatic endocrine cells

Mutations of the multiple endocrine neoplasia type 1 (MEN1) gene predispose patients to MEN1 that affects mainly endocrine tissues, suggesting important physiological functions of the gene in adult endocrine cells. Homozygous disruption of Men1 in mice causes embryonic lethality, whereas the eventual involvement of the gene in embryonic development of the endocrine cells remains unknown. Here, we show that homozygous Men1 knockout mice demonstrate a reduced number of glucagon-positive cells in the E12.5 pancreatic bud associated with apoptosis, whereas the exocrine pancreas development in these mice is not affected. Our data suggest that menin is involved in the survival of the early pancreatic endocrine cells during the first developmental transition. Furthermore, chimerism assay revealed that menin has an autonomous and specific effect on the development of islet cells. In addition, using pancreatic bud culture mimicking the differentiation of alpha- and beta-cells during the second transition, we show that loss of menin leads to the failure of endocrine cell development, altered pancreatic structure and a markedly decreased number of cells expressing neurogenin 3, indicating that menin is also required at this stage of the endocrine pancreas development. Taken together, our results suggest that menin plays an indispensable role in the development of the pancreatic endocrine cells.     

Conversion of soluble immune response suppressor to macrophage-derived suppressor factor by peroxide.

After incubation with soluble immune response suppressor (SIRS), a product of concanavalin A-activated Ly2+ T cells, macrophages release a factor that suppresses in vitro antibody responses, DNA synthetic responses to T-cell and B-cell mitogens, and division of several tumor cell lines. This factor, macrophage-derived suppressor factor (M phi-SF), is a protein with an apparent Mr of 55,000 that is inactivated by sulfhydryl compounds, certain amines, and iodide but not by other halides. In experiments reported here, conventional SIRS and SIRS produced by a cloned T-cell hybridoma were used to analyze formation of M phi-SF by SIRS-treated macrophages. Formation of M phi-SF was insensitive to inhibitors of protein and prostaglandin synthesis but was sensitive to catalase and cyanide, indicating that M phi-SF was not a newly synthesized product and that peroxide was important to its formation. As M phi-SF and SIRS have similar molecular weights and other properties, it is possible that M phi-SF is SIRS modified by peroxide. To test this possibility, SIRS was treated with H2O2 and M phi-SF activity was determined. H2O2 at 0.1-1 pM was sufficient to convert SIRS to M phi-SF; the reaction required approximately 15-20 min and was sensitive to cyanide. Several conventional peroxidase substrates inactivated M phi-SF produced by the SIRS-H2O2 reaction or by SIRS-treated macrophages. In addition, catalase and several of the compounds that directly inactivate M phi-SF also partially interfere with SIRS-mediated suppression of antibody responses. Collectively, these data suggest that SIRS-treated macrophages produce H2O2, which converts SIRS to M phi-SF, which has properties of an oxidized peroxidase-like protein and acts by oxidizing cellular components essential for cell division.     

Mechanism of tumor promoter inhibition of cellular binding of epidermal growth factor

In previous studies we demonstrated that the tumor-promoting agent 12-O-tetradecanoyl phorbol 13-acetate (TPA) and related macrocyclic diterpenes are potent inhibitors of the binding of epidermal growth factor (EGF) to its cell surface receptors in HeLa cells. The present study explores the specificity and mechanism of this effect. We have found that the same effect is observed in various cell types derived from mice, rats, or humans. In HeLa cells TPA inhibits the initial binding of EGF and also accelerates the loss of previously bound EGF from cells. The released EGF is recovered largely intact in the medium, indicating that TPA does not induce increased proteolysis or increased cellular internalization and degradation of EGF. The TPA effect on EGF receptors is mediated by a highly temperature-dependent process because TPA inhibition of EGF binding, and TPA-induced release of prebound EGF, are much greater at 37 degrees C or 22 degrees C than at 4 degrees C. A curious feature is that when cells are grown in TPA for one or more days they escape or become refractory to TPA inhibition of EGF binding. Taken together, these results suggest that TPA inhibits EGF binding not by binding directly to the "active site" of the EGF receptor but by indirectly altering the conformation or inducing the clustering of EGF receptors. These and other membrane effects of this tumor promoter suggest that it is a valuable probe for elucidating complex aspects of membrane structure and function.     

Mutational analysis of Smad3, a candidate tumor suppressor implicated in TGF-beta and menin pathways,in parathyroid adenomas and enteropancreatic endocrine tumors

Based upon molecular allelotyping and comparative genomic hybridization studies, chromosome 15q is the likely location of a tumor suppressor gene important in the pathogeneses of sporadic enteropancreatic endocrine tumors and parathyroid adenomas. Interest has focused on Smad3 as a candidate endocrine tumor suppressor gene because 1) it is localized to 15q and 2) it encodes a TGF beta signaling molecule that has been identified as a binding partner of the multiple endocrine neoplasm type 1 gene product menin, itself involved in enteropancreatic and parathyroid neoplasia. To determine whether Smad3 plays a primary role in development of these tumors, 20 enteropancreatic tumors and 67 parathyroid adenomas were investigated for loss of heterozygosity at DNA markers surrounding Smad3. Twenty percent of enteropancreatic tumors and 24% of parathyroid adenomas showed loss. All 9 coding exons and intron-exon boundaries of the Smad3 gene were then sequenced in genomic DNA from all 20 enteropancreatic and 25 parathyroid tumors, including every case with loss of heterozygosity. No acquired clonal mutations, insertions, or microdeletions in Smad3 were detected in any tumors. Because inactivating somatic mutation is the hallmark of an authentic tumor suppressor, Smad3 is unlikely to function as a classical tumor suppressor gene in the pathogenesis of sporadic parathyroid or enteropancreatic endocrine tumors.     

Physical activity, insulin-like growth factor 1, insulin-like growth factor binding protein 3, and survival from colorectal cancer

BACKGROUND: Recent reports have shown that physical activity improves the outcome of patients with colorectal cancer as well as breast and prostate cancer. However, the mechanisms whereby physical activity reduces cancer mortality are not well established. METHODS: Incident cases of colorectal cancer were identified among participants of the Melbourne Collaborative Cohort Study, a prospective cohort study of 41,528 Australians recruited from 1990 to 1994. Information on tumour site and stage, treatments given, recurrences, and deaths were obtained from systematic review of the medical records. Baseline assessments of physical activity and body size were made, and cases with available plasma had pre-diagnosis insulin-like growth factor 1 (IGF-1) and insulin-like growth factor binding protein 3 (IGFBP-3) levels measured. We assessed associations between these hormones and colorectal cancer specific deaths with respect to physical activity. RESULTS: A total of 526 cases of colorectal cancer were identified, of which 443 had IGF-1/IGFBP-3 levels measured. Median follow up among survivors was 5.6 years. For the physically active, increasing IGFBP-3 by 26.2 nmol/l was associated with a 48% reduction in colorectal cancer specific deaths (adjusted hazard ratio (HR) 0.52 (0.33-0.83); p = 0.006). No association was seen for IGF-1 (adjusted HR 0.90 (0.55-1.45); p = 0.65). For the physically inactive, neither IGF-1 nor IGFBP-3 was associated with disease specific survival. CONCLUSIONS: This study supports the hypothesis that the beneficial effects of physical activity in reducing colorectal cancer mortality may occur through interactions with the insulin-like growth factor axis and in particular IGFBP-3.