ISSN:1094-9313    

DOI:10.1159/000154633

出版商:S. Karger AG    

年代:1992

数据来源: Karger

ISSN:1094-9313    

DOI:10.1159/000154635

出版商:S. Karger AG    

年代:1992

数据来源: Karger

摘要:

The Na+/H+ exchanger NHE1 is an ubiquituously expressed transmembrane protein which is involved in intracellular pH and cell volume regulation. It is blocked by the compound amiloride and activated by all known mitogens and has therefore been hypothesized to be involved in cell proliferation control. In addition, several other isoforms of the Na+/H+ anti-porter (NHE2, NHE3, etc.) possessing a distinct pharmacological profile have been documented to perform specialized sodium transport across small intestine and kidney proximal tubule epithelia. In this review article we first describe the results obtained by a genetic approach which has led us to the molecular identification of the Na+/H+ antiporter NHE1 cDNA, and the characterization of the corresponding protein. The second part presents progress concerning the molecular mechanism of NHE1 activation by growth factors, the molecular identification of its amiloride-binding site and the sequence analysis of recently cloned cDNAs coding for various Na+/H+ antiporter isoforms pointing out the conserved domains which are presumed to be involved in ion translocation and internal pH sensing.

ISSN:1094-9313    

DOI:10.1159/000154636

出版商:S. Karger AG    

年代:1992

数据来源: Karger

摘要:

The vacuolar system of eukaryotic cells contains a large number of organelles that are primarily energized by an H+-ATPase that was named V-ATPase. Several genes encoding subunits of the enzyme were cloned and sequenced. The sequence information revealed the relations between V-ATPases and F-ATPases that evolved from common ancestral genes. Genes encoding subunits of V-ATPase in yeast cells were interrupted to yield mutants that are devoid of the enzyme and are sensitive to pH and calcium concentrations in the medium. The mutants were used for the study of structure, function, molecular biology and biogenesis of the V-ATPase. The function of V-ATPases in energizing the vacuolar system of eukaryotes is discussed.

ISSN:1094-9313    

DOI:10.1159/000154637

出版商:S. Karger AG    

年代:1992

数据来源: Karger

摘要:

Cytoplasmic pH of mammalian cells is regulated by Na+/H+ exchange and HCO-3 transport. In most proliferating cells, these systems collaborate to maintain pH in within a window of values that are permissive for growth. The enzyme systems whose ionization states and activities are affected by pH in this range are not known, but can include glycolysis, protein synthesis and degradation, and the DNA synthetic machinery. Thus, proper regulation of pH is a necessary, but not sufficient, criterion for proliferative control. On the other hand, there is mounting evidence that pH (and/or ΔΨ) ‘crises’ might be important to carcinogenesis. At least two different stages of in vitro carcinogenesis appear to have distinct (e.g. one acidic, one alkaline) pH optima. Once these stages have been passed, and the new phenotype is expressed, cells exhibit a decreased sensitivity to pH changes. The stage with the alkaline pH optimum can be induced by transfection with a H+-ATPase. This results in cells with homogeneous secondary phenotypic characteristics, such as: increased glycolysis, pH, Ca2+, constitutive fos expression and PDGF-independent growth. Furthermore, expression of H+-ATPases on the cell surface is observed in a subset of naturally occurring tumor

ISSN:1094-9313    

DOI:10.1159/000154638

出版商:S. Karger AG    

年代:1992

数据来源: Karger

摘要:

Alkaline stress leads to genomic transformation of Madin-Darby canine kidney (MDCK) cells. Transformed cells are spindle shaped and pleomorphic, lacking contact inhibition and exhibiting poor adhesion to the culture support, typical characteristics of dedifferentiated tumor cells. They generate spontaneous membrane potential oscillations at a frequency of about 1/min. These oscillations are calcium-dependent and due to the periodic activation/inactivation of plasma membrane K+ channels. Cells exhibit intracellular pH transients in phase with the membrane potential oscillations due to transmembrane HCO–3 shifts. Each voltage spike is followed by a transient period of intracellular alkalosis inactivating spiking activity. Aldosterone blunts oscillations through stimulation of plasma membrane Na+/H+ exchange and subsequent intracellular alkalinization. The pH changes may play an important role in the initiation and maintenance of abnormal epithelial cell growt

ISSN:1094-9313    

DOI:10.1159/000154639

出版商:S. Karger AG    

年代:1992

数据来源: Karger

摘要:

Mitogenic activation of quiescent cells is generally accompanied by rapid changes in ionic permeability and transport resulting in alterations in electrical membrane potential. This brief review summarizes current knowledge on growth factor-induced membrane potential changes and their underlying mechanisms. Although the membrane potential changes per se are unlikely to serve an essential role in long-term mitogenic signalling, they represent a convenient and very sensitive assay for elucidating receptor-linked signal transduction cascades.

ISSN:1094-9313    

DOI:10.1159/000154640

出版商:S. Karger AG    

年代:1992

数据来源: Karger

摘要:

Phosphoinositide hydrolysis and stimulated ion fluxes are part of the host of rapid intracellular events triggered by the activation of growth factor receptors. In the case of EGF and other factors (PDGF, FGF) addressed to receptors endowed with endogenous tyrosine kinase activity, the phospholipase C responsible for the signalling reaction is the γ1 isoform. The enzyme can bind directly to a recently identified domain of the EGF receptor when the latter undergoes autophosphorylation as a consequence of ligand binding. The bound enzyme is then phosphorylated at multiple sites, an event that markedly increases its activity. Of the second messengers generated by the hydrolysis, one, diacylglycerol, activates protein kinase C, an enzyme that mediates (among other effects) the feedback desensitization of the EGF receptor; the other messenger, inositol-1,4,5-trisphosphate, activates the release of Ca2+ from intracellular stores. Concomitantly, Ca2+ influx across the plasma membrane is stimulated via cationic, nonselective channels, and the membrane potential is increased by the activation of Ca2+-dependent K+ channels. Recent results with selective blockers of those two responses (imidazole drugs and charibdotoxin, respectively) have revealed that hyperpolarization has only a marginal role in the EGF-induced cell proliferation whereas Ca2+ influx is important since its block results in a substantial decrease of cell growth. The final part of the review concerns the role of phosphoinositide hydrolysis in the process of T lymphocyte activation. In these cells, the T cell receptor has been shown to interact with tyrosine kinases that are not directly inserted across the plasma membrane, and to activate them. The mechanism of transmembrane signalling resembles therefore (from this point of view) that of the EGF receptor. The mechanism by which changes in [Ca2+]i and of other cytoplasmatic parameters are transferred to the nucleus and stimulate proliferation are still poorly understood. The possible involvement of G proteins, such as ras, and of protein phosphatases is discussed

ISSN:1094-9313    

DOI:10.1159/000154641

出版商:S. Karger AG    

年代:1992

数据来源: Karger

摘要:

The expression of ras oncogene in NIH 3T3 fibroblasts is followed by profound alterations of ion transport across the cell membrane. On the one hand, ras-oncogene-expressing cells -unlike normal cells – respond to bradykinin, bombesin and serum with sustained oscillations of cell membrane potential, due to oscillations of intracellular calcium activity and subsequent activation of calcium-sensitive K+ channels. These oscillations depend on the hormone-stimulated entry of calcium from the extracellular space but at least partially involve calcium release from intracellular stores. On the other hand, the expression of the ras oncogene leads to a marked shift of the set point for volume-regulatory ion transport with subsequent activation of the Na+/H+ exchanger and the Na+, K+,2C1- cotransport, both carriers serving the regulatory cell volume increase. As a result, ras-oncogene-expressing cells are larger and more alkaline than cells that do not express the ras oncogene. The growth-factor-independent proliferation of ras-oncogene-expressing cells is abolished following inhibition of the Na+/K+,2Cl- cotransporter (by furosemide), inhibition of the Na+/H+ exchanger (by dimethylamiloride) and inhibition of bradykinin-induced calcium entry (by nifedipine). Thus, activation of the volume-regulatory ion transporters and the agonist-induced oscillations of intracellular calcium activity are apparently prerequisites for the growth-factor-independent proliferation of ras-oncogene-expressing cell

ISSN:1094-9313    

DOI:10.1159/000154642

出版商:S. Karger AG    

年代:1992

数据来源: Karger