Epac inhibits apoptosis of human leukocytes.

cAMP is known to participate in the regulation of apoptosis in leukocytes. Depending on the cell type, pro- and antiapoptotic effects of cAMP have been described. Thus far, most of the cAMP-dependent effects have been attributed to the activation of PKA. However, Epac proteins (direct cAMP targets and guanine nucleotide exchange factors for Ras-like GTPases) have been shown recently to contribute to cAMP-dependent regulation of apoptosis. Therefore, we investigated the effects of the selective Epac activators 8-pCPT and Sp on apoptosis in human leukocytic cells (U937, HL-60, primary human mononuclear cells). We report here that Epac activation inhibits leukocyte apoptosis significantly.

Cloning of human thymic stromal lymphopoietin (TSLP) and signaling mechanisms leading to proliferation.

Thymic stromal lymphopoietin (TSLP) is a novel cytokine that was found to promote the development of murine B cells in vitro. Here we describe the cloning and characterization of the human homologue of murine TSLP. This protein, which is expressed in a number of tissues including heart, liver and prostate, prevented apoptosis and stimulated growth of the human acute myeloid leukemia (AML)-derived cell line MUTZ-3. Anti-interleukin (IL)-7 receptor antibodies (Abs) neutralized this effect indicating that TSLP binds to at least part of the IL-7 receptor complex. TSLP induced phosphorylation of signal transducer and activator of transcription (STAT)-5. In contrast to IL-7, TSLP-triggered STAT-5 phosphorylation was not preceded by activation of janus kinase (JAK) 3. These findings would be in accordance with the notion, raised previously for the mouse system, that TSLP leads to STAT-5 phosphorylation by activating other kinases than the JAKs. Some other signaling pathways stimulated by many cytokines are not involved in TSLP activity; thus, TSLP did not stimulate activation of ERK1,2 and p70S6K. Furthermore, neutralizing Abs raised against cytokines known to stimulate the growth of MUTZ-3 cells did not inhibit the proliferative effects of TSLP, suggesting that TSLP-induced growth was a direct effect. In summary, we describe the cloning of human TSLP and its proliferative effects on a myeloid cell line. TSLP-induced proliferation is preceded by phosphorylation of STAT-5, but not of JAK 3.

Tumor necrosis factor-alpha-induced proliferation requires synthesis of granulocyte-macrophage colony-stimulating factor.

OBJECTIVE: Tumor necrosis factor- alpha (TNF-alpha) induces a variety of cellular responses, some of them being at least seemingly contradictory. Thus, we set out to find differences in the modes of proliferative and apoptotic responses to TNF- alpha. MATERIALS AND METHODS: We screened a panel of acute myeloid leukemia-derived cell lines for TNF- alpha-responsiveness. In two lines (OCI-AML-1, OCI-AML-11), TNF- alpha acted as an apoptotic agent; in others (HU-3, M-07e, TF-1), it had the opposite effect, preventing apoptosis and inducing proliferation. Direct and indirect signaling mechanisms, including NF-kappaB activation and cytokine synthesis, were analyzed. RESULTS: All cell lines tested expressed TNF- alpha receptors I and II and responded to TNF- alpha by upregulation of intercellular adhesion molecule-1. In contrast to granulocyte-macrophage colony-stimulating factor (GM-CSF), TNF- alpha did not activate the MAP kinase and p70S6 kinase pathways. Nevertheless, inhibitors of these pathways clearly reduced the TNF-alpha-induced cell growth, indicating that TNF- alpha-proliferative cells produced a growth factor that induced proliferation upon stimulation of the above pathways. Anti-GM-CSF antibodies inhibited the TNF-alpha-induced growth, suggesting the presence of an autocrine loop for cell proliferation mediated by GM-CSF. Supporting this notion, TNF-alpha-induced upregulation of GM-CSF mRNA levels and protein secretion in the TNF-alpha-proliferative, but not in the TNF-alpha-apoptotic cell lines. CONCLUSION: These data identify GM-CSF synthesis as an early and essential step in TNF- alpha-induced proliferation. We show for the first time that TNF-alpha-treated cell lines producing no or only minimal amounts of GM-CSF demonstrate an apoptotic phenotype, while cell lines with high GM-CSF expression rates can escape from growth arrest or even apoptosis. In this context, we discuss arguments pointing at NF-kappaB as regulator of GM-CSF synthesis and thus indirectly as regulator for the escape of TNF-alpha-induced apoptosis.

Yel013p (Vac8p), an armadillo repeat protein related to plakoglobin and importin alpha is associated with the yeast vacuole membrane.

Proteins of the armadillo family are involved in diverse cellular processes in higher eukaryotes. Some of them, like armadillo, beta-catenin and plakoglobins have dual functions in intercellular junctions and signalling cascades. Others, belonging to the importin-alpha-subfamily are involved in NLS recognition and nuclear transport, while some members of the armadillo family have as yet unknown functions. Here, we introduce the Saccharomyces cerevisiae protein Yel013p as a novel armadillo (arm) repeat protein. The ORF Yel013w was identified in the genome project on chromosome V (EMBL: U18530) and codes for an acidic protein of 578 residues with 8 central arm-repeats, which are closely related to the central repeat-domain of Xenopus laevis plakoglobin. We show that Yel013p (Vac8p) is constitutively expressed in diploid and haploid yeasts and that it is not essential for viability and growth. However, the vacuoles of mutant cells are multilobular or even fragmented into small vesicles and the processing of aminopeptidase I, representing the cytoplasm-to-vacuole transport pathway, is strongly impaired. Consistent with these observations, subcellular fractionation experiments, immunolocalization and expression of green fluorescent protein (GFP) fusion proteins revealed that Yel013p (Vac8p) is associated with the vacuolar membrane. Our data provide evidence for the involvement of an arm-family member in vacuolar morphology and protein targeting to the vacuole.