Evidence for cAMP-dependent protein kinase in mediating the parathyroid hormone-stimulated rise in cytosolic free calcium in rabbit connecting tubules

We investigated cellular mechanisms mediating the parathyroid hormone (PTH)-induced increase in cytosolic free Ca2+ concentration ([Ca2+]i) in isolated perfused rabbit connecting tubules. Prior and/or concomitant exposure to 0.5 mM of N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-8), a cyclic nucleotide-dependent protein kinase inhibitor, abolished the rise in [Ca2+]i produced by 0.1 nM PTH in five connecting tubules and suppressed it by approximately 50% in another five. In the latter, there was a delayed onset in the rise of [Ca2+]i. Such responses contrasted to the prompt increase in [Ca2+]i in PTH-stimulated control tubules. However, when H-8 was withdrawn, [Ca2+]i rose within minutes to reach a plateau value similar to the uninhibited response to PTH in controls, indicating rapidly reversible inhibition by H-8. In an otherwise identical protocol, 0.5 mM H-8 also reversibly suppressed the rise in [Ca2+]i induced by 0.175 mM 8-Br-cAMP. In contrast to the stimulatory effect of 8-Br-cAMP on [Ca2+]i, 1 mM 8-Br-cGMP caused no increase. At a concentration of 0.4 mM, the Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS), a well-characterized cAMP-dependent protein kinase inhibitor, totally abolished the rise in [Ca2+]i caused by 0.1 nM PTH. We conclude that a cAMP-dependent protein kinase plays an important role in the PTH-stimulated rise in [Ca2+]i in the rabbit connecting tubule. Since the increase in [Ca2+]i was shown previously to depend on extracellular Ca2+, we propose that cAMP-dependent protein phosphorylation is important in mediating PTH-stimulated Ca2+ fluxes across plasma membranes of connecting tubule cells.     

Remote estimation of carbon dioxide uptake by a Mediterranean forest

The estimation of the carbon balance in ecosystems, regions, and the biosphere is currently one of the main concerns in the study of the ecology of global change. Current remote sensing methodologies for estimating gross primary productivity are not satisfactory because they rely too heavily on (i) the availability of climatic data, (ii) the definition of land‐use cover, and (iii) the assumptions of the effects of these two factors on the radiation‐use efficiency of vegetation (RUE). A new methodology is urgently needed that will actually assess RUE and overcome the problems associated with the capture of fluctuations in carbon absorption in space and over time. Remote sensing techniques such as the widely used reflectance vegetation indices (e.g. NDVI, EVI) allow green plant biomass and therefore plant photosynthetic capacity to be assessed. However, there are vegetation types, such as the Mediterranean forests, with a very low seasonality of these vegetation indices and a high seasonality of carbon uptake. In these cases it is important to detect how much of this capacity is actually realized, which is a much more challenging goal. The photochemical reflectance index (PRI) derived from freely available satellite information (MODIS sensor) presented for a 5‐year analysis for a Mediterranean forest a positive relationship with the RUE. Thus, we show that it is possible to estimate RUE and GPP in real time and therefore actual carbon uptake of Mediterranean forests at ecosystem level using the PRI. This conceptual and technological advancement would avoid the need to rely on the sometimes unreliable maximum RUE.     

Cytoplasmic protein tyrosine phosphatases, regulation and function: the roles of PTP1B and TC-PTP

PTP1B and TC-PTP are closely related protein tyrosine phosphatases, sharing 74% homology in their catalytic domain. However, their cellular localization, function, and regulation are found to be different. Their substrate specificity has implicated these enzymes in various signaling pathways, regulating metabolism, proliferation and cytokine signaling. For instance, PTP1B has been shown to regulate the activation of cytokine receptors through the dephosphorylation of specific members of the JAK family, namely JAK2 and TYK2, whereas TC-PTP is involved in the modulation of cytokine signaling via JAK1 and JAK3 molecules. Gene-targeting approaches will help us to unravel the physiological functions of these enzymes.     

Site-selective regulation of platelet-derived growth factor beta receptor tyrosine phosphorylation by T-cell protein tyrosine phosphatase

The platelet-derived growth factor (PDGF) beta receptor mediates mitogenic and chemotactic signals. Like other tyrosine kinase receptors, the PDGF beta receptor is negatively regulated by protein tyrosine phosphatases (PTPs). To explore whether T-cell PTP (TC-PTP) negatively regulates the PDGF beta receptor, we compared PDGF beta receptor tyrosine phosphorylation in wild-type and TC-PTP knockout (ko) mouse embryos. PDGF beta receptors were hyperphosphorylated in TC-PTP ko embryos. Fivefold-higher ligand-induced receptor phosphorylation was observed in TC-PTP ko mouse embryo fibroblasts (MEFs) as well. Reexpression of TC-PTP partly abolished this difference. As determined with site-specific phosphotyrosine antibodies, the extent of hyperphosphorylation varied among different autophosphorylation sites. The phospholipase Cgamma1 binding site Y1021, previously implicated in chemotaxis, displayed the largest increase in phosphorylation. The increase in Y1021 phosphorylation was accompanied by increased phospholipase Cgamma1 activity and migratory hyperresponsiveness to PDGF. PDGF beta receptor tyrosine phosphorylation in PTP-1B ko MEFs but not in PTPepsilon ko MEFs was also higher than that in control cells. This increase occurred with a site distribution different from that seen after TC-PTP depletion. PDGF-induced migration was not increased in PTP-1B ko cells. In summary, our findings identify TC-PTP as a previously unrecognized negative regulator of PDGF beta receptor signaling and support the general notion that PTPs display site selectivity in their action on tyrosine kinase receptors.     

Cleavage of mitochondria-like transfer RNAs expressed in Escherichia coli

Mitochondrial (mt) transfer RNAs (tRNAs) often harbor unusual structural features causing their secondary structure to differ from the conventional cloverleaf. tRNAs designed with such irregularities, termed mt-like tRNAs, are active in Escherichia coli as suppressors of reporter genes, although they display low steady-state levels. Characterization of fragments produced during mt-like tRNA processing in vitro and in vivo suggests that these RNAs are not fully processed at their 5' ends and are cleaved internally. These abnormal processing events may account for the low levels of mature mt-like RNAs in vivo and are most likely related to defective processing by RNase P.     

Circumventing senescence is associated with stem cell properties and metformin sensitivity

Most cancers arise in old individuals, which also accumulate senescent cells. Cellular senescence can be experimentally induced by expression of oncogenes or telomere shortening during serial passage in culture. In vivo, precursor lesions of several cancer types accumulate senescent cells, which are thought to represent a barrier to malignant progression and a response to the aberrant activation of growth signaling pathways by oncogenes (oncogene toxicity). Here, we sought to define gene expression changes associated with cells that bypass senescence induced by oncogenic RAS. In the context of pancreatic ductal adenocarcinoma (PDAC), oncogenic KRAS induces benign pancreatic intraepithelial neoplasias (PanINs), which exhibit features of oncogene‐induced senescence. We found that the bypass of senescence in PanINs leads to malignant PDAC cells characterized by gene signatures of epithelial‐mesenchymal transition, stem cells, and mitochondria. Stem cell properties were similarly acquired in PanIN cells treated with LPS, and in primary fibroblasts and mammary epithelial cells that bypassed Ras‐induced senescence after reduction of ERK signaling. Intriguingly, maintenance of cells that circumvented senescence and acquired stem cell properties was blocked by metformin, an inhibitor of complex I of the electron transport chain or depletion of STAT3, a protein required for mitochondrial functions and stemness. Thus, our studies link bypass of senescence in premalignant lesions to loss of differentiation, acquisition of stemness features, and increased reliance on mitochondrial functions.     

A spectacular new species of Nepenthes L. (Nepenthaceae) pitcher plant from central Palawan,Philippines

A new species of Nepenthes L., N. attenboroughii (Nepenthaceae), from Palawan Island in the Philippines, is described and illustrated. It is restricted to rocky, ultramafic soils that comprise the summit region of Mount Victoria, Municipality of Narra, where it occurs in isolation from other members of the genus. On the basis of the morphological features, this new taxon appears to be related to both N. mira Jebb & Cheek of Palawan and N. rajah Hook.f. of Borneo. Its substantial size places it among the largest of known pitcher plants. The diagnostic morphological characters are discussed and an updated key is provided for a revised complex of Nepenthes species from the Palawan and North Borneo phytogeographical region. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 195–202.