Ageing adipokinetic cells in Locusta migratoria: an ultrastructural morphometric study

Summary A morphometric study was made of the ultrastructure of adipokinetic cells in resting adults of Locusta migratoria at 3, 23, and 43 days after imaginal ecdysis. The nucleus, rough endoplasmic reticulum, and Golgi apparatus enlarge with age, which indicates that the synthesis and packaging of secretory substances increases during ageing. The size of the storage compartment, consisting of secretory and ergastoplasmic granules, does not increase earlier than 23–43 days after imaginal ecdysis. The lysosomal compartment markedly enlarges between 3 and 23 days; later on, the growth of this compartment, especially of autophagosomes, is less prominent. This suggests that lysosomal destruction initially compensates for the production of new secretory granules, assuming that exocytosis of secretory granules by adipokinetic cells is insignificant in resting locusts. Afterwards, lysosomal destruction may no longer be sufficient to prevent over-production of secretory granules, as is suggested by the increase in the number of these granules between 23 and 43 days. This coincides with the appearance of a considerable number of large ergastoplasmic granules, which represent a spatially more efficient form of storage of secretory material than the much smaller secretory granules. The increase with age in the amount of secretory products indicates that the biosynthetic activity of the adipokinetic cells is not (finely) tuned to their releasing activity.     

Proline-specific aminopeptidase P prevents replication-associated genome instability

Genotoxic stress during DNA replication constitutes a serious threat to genome integrity and causes human diseases. Defects at different steps of DNA metabolism are known to induce replication stress, but the contribution of other aspects of cellular metabolism is less understood. We show that aminopeptidase P (APP1), a metalloprotease involved in the catabolism of peptides containing proline residues near their N-terminus, prevents replication-associated genome instability. Functional analysis of C. elegans mutants lacking APP-1 demonstrates that germ cells display replication defects including reduced proliferation, cell cycle arrest, and accumulation of mitotic DSBs. Despite these defects, app-1 mutants are competent in repairing DSBs induced by gamma irradiation, as well as SPO-11-dependent DSBs that initiate meiotic recombination. Moreover, in the absence of SPO-11, spontaneous DSBs arising in app-1 mutants are repaired as inter-homologue crossover events during meiosis, confirming that APP-1 is not required for homologous recombination. Thus, APP-1 prevents replication stress without having an apparent role in DSB repair. Depletion of APP1 (XPNPEP1) also causes DSB accumulation in mitotically-proliferating human cells, suggesting that APP1’s role in genome stability is evolutionarily conserved. Our findings uncover an unexpected role for APP1 in genome stability, suggesting functional connections between aminopeptidase-mediated protein catabolism and DNA replication.     

Cysteinyl leukotrienes mediate histamine hypersensitivity ex vivo by increasing histamine receptor numbers

BACKGROUND: Hyperresponsiveness to histamine is a key feature of a variety of pathological conditions, including bronchial asthma, food allergy, colitis ulcerosa, and topical allergic disorders. Cells isolated from hyperresponsive individuals do not display exaggerated histamine responses ex vivo and thus the molecular mechanisms underlying histamine responsiveness remain obscure. Importantly, several in vivo observations implicate cysteinyl leukotrienes as possible mediators of increased histamine responses. We decided to investigate whether cysteinyl leukotrienes enhance the cellular reaction to histamine in cell types involved in pathological and immunological histamine hyperresponsiveness, as this might provide an in vitro system for studying histamine responsiveness and could shed light on the underlying molecular mechanisms. MATERIALS AND METHODS: Histamine responsiveness was determined by measuring histamine-induced prostaglandin E(2) production. Scatchard analysis was performed to determine the number of histamine H(1) receptors. Mouse macrophages, primary isolated human peripheral blood monocytes, and human umbilical smooth muscle cells were investigated before and after cysteinyl leukotriene stimulation. Results: In all three cell types tested, cysteinyl leukotrienes instantaneously enhanced histamine-induced prostaglandin E(2) production. This increase in prostaglandin E(2) production coincided with the immediate and transient appearance of additional H(1) receptors on the plasma membrane. CONCLUSIONS: Cysteinyl leukotrienes prime histamine responses by recruiting additional histamine receptors in immunologically relevant cell types in vitro.     

Activation of the canonical beta-catenin pathway by histamine

Histamine signaling is a principal regulator in a variety of pathophysiological processes including inflammation, gastric acid secretion, neurotransmission, and tumor growth. We report that histamine stimulation causes transactivation of a T cell factor/beta-catenin-responsive construct in HeLa cells and in the SW-480 colon cell line, whereas histamine did not effect transactivation of a construct containing the mutated response construct FOP. On the protein level, histamine treatment increases phosphorylation of glycogen synthase kinase 3-beta in HeLa cells, murine macrophages, and DLD-1, HT-29, and SW-480 colon cell lines. Furthermore, histamine also decreases the phosphorylated beta-catenin content in HeLa cells and murine macrophages. Finally, pharmacological inhibitors of the histamine H1 receptor counteracted histamine-induced T cell factor/beta-catenin-responsive construct transactivation and the dephosphorylation of beta-catenin in HeLa cells and in macrophages. We conclude that the canonical beta-catenin pathway acts downstream of the histamine receptor H1 in a variety of cell types. The observation that inflammatory molecules, like histamine, activate the beta-catenin pathway may provide a molecular explanation for a possible link between inflammation and cancer.     

Factor VIIa/tissue factor-induced signaling via activation of Src-like kinases, phosphatidylinositol 3-kinase, and Rac

Tissue factor (TF), apart from activating the extrinsic pathway of the blood coagulation, is a principal regulator of embryonic angiogenesis and oncogenic neoangiogenesis, but also influences inflammation, leukocyte diapedesis and tumor progression. The intracellular domain of TF lacks homology to other classes of receptors and hence the signaling mechanism is poorly understood. Here we demonstrate that factor VIIa (the natural ligand for TF) induces the activation of the Src family members c-Src, Lyn, and Yes, and subsequently phosphatidylinositol 3-kinase (PI3K), followed by stimulation of c-Akt/protein kinase B as well as the small GTPases Rac and Cdc42. In turn Rac mediates p38 mitogen-activated protein (MAP) kinase activation and cytoskeletal reorganization, whereas factor VIIa-induced p42/p44 MAP kinase stimulation required PI3K enzymatic activity but was not inhibited by dominant negative Rac proteins. We propose that this Src family member/PI3K/Rac-dependent signaling pathway is a major mediator of factor VIIa/TF effects in pathophysiology.     

The role of phosphatidylinositide-3-kinase in basal mitogen-activated protein kinase activity and cell survival

Phosphatidylinositide-3-OH-kinase (PI 3-kinase) is an upstream activator of p42/p44 mitogen-activated protein kinase (MAPK), but the role of PI 3-kinase-dependent MAPK remains obscure. Here we demonstrate that in a variety of different cell types, PI 3-kinase inhibition results in an inhibition of MAPK in unstimulated cells but does not interfere with growth factor-, or TPA-induced MAPK activity. Furthermore, inhibition of either PI 3-kinase or MEK/MAPK results in cell death in serum-starved cells. We concluded that basal, but not induced MAPK activity is mediated by PI 3-kinase and that this PI 3-kinase-mediated MEK/MAPK activity is essential for cell survival in quiescent cells.