The involvement of nuclear factor-kappa B in cyclooxygenase-2 overexpression in murine colon cancer cells transduced with herpes simplex virus thymidine kinase gene

We have previously reported that transduction of murine colon cancer cells (MC38) with herpes simplex virus thymidine kinase (HSV-tk) gene results in a significant enhancement of tumor growth rate in vivo and overexpression of cyclooxygenase-2 (COX-2). Our current study aimed to investigate the involvement of nuclear factor-kappa B (NF-kappaB), a pivotal transcriptional regulator of COX-2, in the upregulation of COX-2 expression by HSV-tk. It was found that HSV-tk gene transduction of MC38 cells results in significantly enhanced NF-kappaB activity, increased phosphorylation and degradation of inhibitor-kappa Balpha (IkappaBalpha) and enhanced translocation of NF-kappaB to the nucleus. Treatment of HSV-tk-transduced MC38 cells with sulfasalazine, a potent NF-kappaB inhibitor, led to dose-dependent inhibition of NF-kappaB activity, IkappaB phosphorylation and nuclear translocation of NF-kappaB, accompanied by significantly decreased COX-2 expression and reduced release of prostaglandin E2. Transient transfection experiments with COX-2 promoter constructs fused to luciferase reporter gene revealed that mutation in NF-kappaB-responsive element of COX-2 promoter significantly reduced promoter activity in HSV-tk-transduced MC38 and COS-7 cells, whereas it had no effect on promoter activity in the respective wild-type cells. At last, it was found that HSV-tk gene transduction causes significant enhancement of NF-kappaB activity and COX-2 expression in two additional tumor cell lines, 9L and T24. These findings suggest that HSV-tk gene transduction results in NF-kappaB pathway activation, which is essential for COX-2 overexpression by HSV-tk.

Prostaglandin H synthase-2 inhibitors interfere with prostaglandin H synthase-1 inhibition by nonsteroidal anti-inflammatory drugs

Ram seminal vesicle microsomes, a rich source of prostaglandin H synthase-1, were incubated with 100 nM of the prostaglandin H synthase-2 inhibitors N-(2-cyclohexyloxy-4-nitrophenyl) methanesulfonamide (NS-398) and 5-bromo-2-(4-fluorophenyl)-3-(4-methylsulfonyl) thiophene (DuP-697) prior to exposure to the prostaglandin H synthase inhibitors aspirin, indomethacin, ibuprofen or naproxen. Activity of the enzyme was measured by following the conversion of arachidonic acid to prostaglandin E(2) and prostaglandin F2alpha. Although prostaglandin H synthase-1 activity was not altered by these concentrations of the prostaglandin H synthase-2 inhibitors, it was found that exposure to these agents prior to aspirin or indomethacin (irreversible prostaglandin H synthase inhibitors) significantly attenuated the inhibition obtained by the latter inhibitors. On the other hand, the same concentrations of the prostaglandin H synthase-2 inhibitors did not interfere with prostaglandin H synthase-1 inhibition that was induced by naproxen or ibuprofen (competitive prostaglandin H synthase inhibitors). Attenuation of the indomethacin inhibition of prostaglandin H synthase-1 by prostaglandin H synthase-2 inhibitors was observed only when the microsomes were pre-exposed to DuP-697 or NS-398 in the absence, but not in the presence, of arachidonic acid. The effect of DuP-697 was found to be irreversible, however, washing away the agent reversed the action of NS-398. Similar phenomena have been reported by us in bovine aortic endothelial cells and in human dermal fibroblasts. Attenuation of the inhibition by aspirin and indomethacin, without altering the enzyme's basal activity or the inhibition induced by ibuprofen or naproxen may suggest the possibility that the prostaglandin H synthase-2 specific inhibitors DuP-697 and NS-398 affect prostaglandin H synthase-1 by interaction with a site different from the enzyme's catalytic site.

PGHS-2 inhibitors, NS-398 and DuP-697, attenuate the inhibition of PGHS-1 by aspirin and indomethacin without altering its activity

Since the discovery of the inducible form of prostaglandin (PG) H synthase (PGHS), PGHS-2, considerable effort has been made to design selective inhibitors of this isozyme. N-(2-cyclohexyloxy-4-nitrophenyl) methanesulfonamide (NS-398) and 5-bromo-2-(4-fluorophenyl)-3-(4-methylsulfonyl) thiophene (DuP-697) have been shown to interact reversibly with PGHS-1, while irreversibly inhibiting PGHS-2 in a time-dependent manner. In the present study we have tested the effects of DuP-697 and NS-398 on the activity of PGHS-1 and further explored the interactions between these agents and the inhibition of PGHS-1 by aspirin, indomethacin and ibuprofen. Three independent experimental systems, namely bovine aortic endothelial cells (BAEC), human fibroblasts and ram seminal vesicle microsomes were used to investigate the effects of DuP-697 and NS-398 on PGHS-1. The results show that DuP-697 and NS-398, at concentrations ranges which do not inhibit PGHS-1 activity, significantly attenuated the inhibition of PGHS-1 that was caused by aspirin and indomethacin. The same concentrations of DuP-697 and NS-398 did not affect the inhibition of PGHS-1 that was induced by the competitive reversible inhibitors ibuprofen and naproxen. Similar effects of DuP-697 and NS-393 were obtained with ram seminal vesicle microsomes. These results suggest that PGHS-2 inhibitors DuP-697 and NS-398 possibly interact with PGHS-1 at a site different from the enzyme's catalytic site, thus causing attenuation of PGHS-1 inhibition by aspirin and indomethacin without altering PGHS-1 basal activity or the ibuprofen-induced inhibition.

Differential regulation of cyclooxygenase isoenzymes by cAMP-elevating agents

Bovine aortic endothelial cells produce prostacyclin as their major arachidonic acid metabolite. cAMP, in turn, is the second messenger for prostacyclin. In the present study, we investigated the effects of cAMP-elevating agents on prostacyclin production by bovine aortic endothelial cells. Treatment of resting bovine aortic endothelial cells with cAMP-elevating agents inhibited prostacyclin production and cyclooxygenase activity, without affecting arachidonic acid release. No change was detected in cyclooxygenase-1 protein expression. The specific inhibitor of protein kinase A, Rp-cAMPS (adenosine 3',5'-cyclic monophosphorothioate, Rp-isomer, triethylammonium salt), and the phosphatase inhibitor, okadaic acid, both suppressed cAMP-induced inhibition, suggesting that this inhibition is mediated by a phosphorylation-dephosphorylation cascade, which is possibly protein kinase A-dependent. In lipopolysaccharide-treated cyclooxygenase-2 expressing bovine aortic endothelial cells, where cyclooxygenase-1 activity was selectively inhibited, dibutyryl cAMP failed to inhibit cyclooxygenase-2 activity. Cyclooxygenase-2 protein was induced upon treatment with dibutyryl cAMP and further induction of cyclooxygenase-2 protein was effected by IBMX (3-isobutyl-1-methyl-xanthine) and dibutyryl cAMP in bacterial lipopolysaccharide-stimulated cells. These results suggest that increased cellular cAMP selectively inhibits cyclooxygenase-1 activity without altering cyclooxygenase-1 protein expression, and at the same time, up-regulates cyclooxygenase-2 protein. This complex regulation of cyclooxygenase activity and protein expression by cAMP may represent a prostacyclin-induced autoregulatory mechanism in bovine aortic endothelial cells.

Differential effect of TPA on PGE2 and cicaprost-induced cAMP synthesis in UMR-106 cells

PGE2 and prostacyclin each enhance cAMP synthesis in the osteoblast-like cell line UMR-106. The amount of cAMP induced by PGE2 was 5-7-fold greater than the amount induced by cicaprost or iloprost, stable prostacyclin analogues. Both PGE2 and the two prostacyclin analogues enhanced cAMP synthesis with similar time dependence. The EC50 values of PGE2 and cicaprost were 3 X 10(-6) and 5 x 10(-8) M, respectively. Short-term incubation of the cells with 12-o-tetradecanoylphorbol 13-acetate (TPA) markedly reduced the PGE2-induced cAMP synthesis. In contrast, cells that were incubated with the same concentrations of TPA in the presence of cicaprost or iloprost showed a 1.6-fold increase in cAMP formation. The marked disparity between the cAMP response to cicaprost and PGE2 in the presence of TPA suggests that the two prostanoids induce cAMP synthesis in the UMR-106 cells by interaction with different receptors. These observations support the idea that the osteoblastic UMR-106 cells may express specific prostacyclin receptors and suggest that prostacyclin may have a unique role in osteoblasts.

Corticotropin releasing factor modulates interleukin-1-induced prostaglandin synthesis in fibroblasts: receptor binding and effects of antagonists

Corticotropin releasing factor (CRF) is a predominant regulator of the neuroendocrine, autonomic and behavioral responses to stress. In addition, numerous studies support autocrine/paracrine roles for this peptide at peripheral sites. CRF and CRF binding sites have been identified in different regions of the central nervous system as well as in the heart, spleen, adrenal and testis, and high levels of CRF were detected in inflamed fibroblasts. However, the precise physiological or pathophysiological role of peripheral CRF cannot yet be discerned. Here we show that CRF, through interaction with specific membrane receptors, blocks the interleukin-1alpha (IL-1alpha)-stimulated prostaglandin (PG) synthesis in fibroblasts. Binding of [125I]-labeled CRF in fibroblasts was saturable and fitted a two sites model. K(D) for the higher-affinity class of receptors was 20+/-2.2 pM, and Bmax 1.95+/-0.22 fmol/mg protein. For the lower-affinity class of receptors K(D) was 160+/-17 nM, and Bmax 2.38+/-0.27 fmol/mg protein. CRF blocked the effect of IL-1alpha on PGE2 synthesis, and this was antagonised by D-PheCRF12-41. In addition, the CRF receptor antagonists alpha helical CRF9-41 and D-PheCRF12-41 at high concentrations inhibited the IL-1alpha-induced PG synthesis similarly to CRF, suggesting partial agonistic action. Taken together, these results suggest a modulatory role of CRF in inflammation.

Modulation of endothelial prostaglandin synthesis by corticotropin releasing factor and antagonists

Corticotropin releasing factor (CRF) is a hypothalamic hormone that also displays autocrine/paracrine roles at peripheral sites. High concentrations of CRF have been identified in endothelial cells and other inflammatory tissues. We investigated the effects of CRF and antagonists in the regulation of prostaglandin synthesis in bovine aortic endothelial cells, and also characterized the binding of CRF in these cells. Interleukin-1alpha increased prostacyclin (prostaglandin I2) synthesis in endothelial cells and this response to interleukin-1alpha was abolished by simultaneous exposure to CRF. The effect of CRF on interleukin-1alpha-induced prostaglandin synthesis was antagonised by the CRF receptor antagonist alpha-helical CRF-(9-41). In addition, this as well as another CRF receptor antagonist, namely [D-Phe12]CRF-(12-41), when applied alone at low concentrations inhibited the interleukin-1alpha-induced prostaglandin synthesis similarly to CRF, suggesting partial agonistic action. Binding of [125I]-labeled CRF in endothelial cells was saturable and fitted a two sites model. Kd for the higher-affinity class of receptors was 0.2 +/- 0.02 nM, and Bmax 0.79 +/- 0.095 fmol/mg protein. The lower-affinity class of receptors had a Kd of 1.77 +/- 0.14 microM and Bmax 0.97 +/- 0.12 fmol/mg protein. These findings suggest a direct role for CRF in the local regulation of inflammation.

Regulation of a common, low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

Bovine aortic endothelial cells contain a prostaglandin site which binds with similar low-affinity PGE2, PGF2alpha and the thromboxane agonist U-46619. Treatment of the cells with agents that increase the level of cellular cAMP such as forskolin, a direct activator of adenylate cyclase or IBMX, a phosphodiesterase inhibitor, decreased the binding of PGE2 to the cells. Addition of dibutyryl cAMP to intact cells caused a quick reduction in PGE2 binding with a half time of less than 2 min. The reduction in PGE2 binding was completely reversible after removing the dibutyryl cAMP. The reduction in PGE2 binding after addition of dibutyryl cAMP to the intact cells was also observed after a mechanical disruption of the cells or after permeabilization with digitonin. Incubation of the cells with myristoylated PKI(14-22) amide, a specific protein kinase A inhibitor, resulted in partial suppression of the reduction of PGE2 binding by dibutyryl cAMP. Pretreatment of intact cells for 24 h with 10(-6) M PGE2 or a PKC activator did not reduce the specific binding of [3H]-PGE2. These results suggest that PKA, but not PKC, is involved in a fast reversible regulation of the common prostanoid receptor on bovine endothelial cells.

Prostaglandin H synthase: protein synthesis-independent regulation in bovine aortic endothelial cells

The objective of the present study was to examine whether prostaglandin H synthase (PGHS) can be regulated by pathways independent of de novo synthesis of PGHS. Incubation of bovine aortic endothelial cells (BAEC) for as short as 5 min with NaF (40 mM) resulted in a 60% increase in PGHS activity. PGHS activity induced by NaF was unaffected by either 10 microM cycloheximide or 1 microM actinomycin D. Aspirin (25 microM) completely inhibited resting PGHS activity, and NaF did not induce further stimulation. NS-398 (500 nM), a specific PGHS-2 inhibitor, was ineffective. Basic fibroblast growth factor (bFGF) induced a significant increase in PGHS activity within 30 min and was insensitive to cycloheximide. The levels of PGHS-1 and PGHS-2 proteins, as measured by Western blots, were not affected by NaF or bFGF. The tyrosine kinase inhibitor genistein attenuated PGHS activity that was induced by NaF and bFGF, whereas the tyrosine phosphatase inhibitor, sodium orthovanadate, augmented these responses. The G protein activators 5'-guanylyl imidodiphosphate and guanosine 5'-O-(3-thiotriphosphate) inhibited both resting and NaF-induced PGHS activities. These results suggest-that, in BAEC, PGHS-1 activity can be regulated by tyrosine kinase and/or G proteins, independently of de novo protein synthesis.

Increased surface phosphatidylserine is an early marker of neuronal apoptosis

Neuronal apoptosis is the subject of intense investigation and is beginning to be understood in some molecular detail. In the present study, we show that PC12 cells, like certain other cell types, redistribute phosphatidylserine (PS) from the inner leaflet to the outer leaflet of the plasma membrane early in the process of apoptosis. The externalised PS can be readily visualised by incubating intact cells with a fluorescent derivative of the protein annexin V. When apoptosis is blocked with an inhibitor of interleukin-1beta-converting-enzyme-like proteases, the increased annexin binding is also blocked. Fluorescent annexin V binding provides a rapid and convenient way to identify apoptotic neurones.

A common low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

[3H]PGE2 and [3H]PGF2 alpha were shown to bind with similar binding capacity and dissociation constants to bovine aorta endothelial cells. The similarity in the binding parameters suggests that both agonists may bind to the same binding site. Displacement of [3H]PGE2 performed with PGE2, PGF2 alpha or U-46619, a thromboxane agonist, shows that all three prostanoids displaced the bound [3H]PGE2 with comparable potency (IC50 = 10(-7) M). These results indicated that the three different prostanoids, which serve as specific agonists to different prostanoid receptors, also compete for the same binding site in bovine endothelial cells with similar affinity. Comparison of the displacement of [3H]PGE2 or [3H]PGF2 alpha by a number of prostaglandin agonists and antagonists further supports the notion that the natural prostanoids bind with similar affinities to the same binding site. Thus, sulprostone, an EP1/EP3 agonist, displaced bound [3H]PGE2 and [3H]PGF2 alpha with IC50 of about 10(-7) M. On the other hand, thromboxane antagonists (BAY u-3405 and GR-32191B), EP1 specific antagonist (SC-19220) EP1/DP antagonist (AH-6809) and iloprost, a stable prostacyclin agonist, failed to displace bound [3H]PGE2 or [3H]PGF2 alpha at a concentration range of 10(-9)-10(-6) M. Gradual increase of sodium fluoride (NaF), a general activator of G binding proteins, or incubation of permeabilized cells with GTP gamma S resulted in a decrease in [3H]PGE2 binding, suggesting that the binding site represents a low-affinity common prostanoid receptor which, similar to other prostanoid receptors, is probably coupled with G binding proteins.

Dual regulation of PLA2 and PGI2 production by G proteins in bovine aortic endothelial cells

NaF, a nonselective activator of heterotrimeric guanine nucleotide-binding proteins (G proteins), increased the release of arachidonic acid (AA) and prostacyclin (PGI2) production in bovine aortic endothelial cells (BAEC) at low concentrations (40-60 mM). On the other hand, higher concentrations (100 mM) inhibited phospholipase A2 (PLA2) compared with the basal activity. Intracellular Ca2+ levels did not rise after treatment with stimulatory concentrations of NaF, and, moreover, neither neomycin nor Ca(2+)-free medium affected the biphasic pattern of PGI2 synthesis in response to NaF. CGP-43187, an inhibitor of the 14-kDa secretory PLA2, did not affect NaF-induced AA release. However, AACOCF3, a specific inhibitor of the cytosolic 85-kDa PLA2 (cPLA2), abrogated AA release and PGI2 production in response to 60 mM NaF. A biphasic pattern of PGI2 production was also obtained with the guanosine 5'-triphosphate analogues guanosine 5'-O-(3-thiotriphosphate) and guanylylimidodiphosphate in permeabilized BAEC. Pretreatment of the cells with guanosine 5'-O-(2-thiodiphosphate) suppressed the inhibition and the stimulation of AA release induced by guanylylimidodiphosphate. In addition, phenylisopropyl adenosine inhibited the release of AA and PGI2, whereas ATP and bradykinin increased PGI2. Pertussis toxin not only inhibited ATP- and bradykinin-stimulated PGI2 release, it also reversed the inhibitory effect of phenylisopropyl adenosine, resulting in a significant stimulation. These findings strongly suggest that, in BAEC, cPLA2 is coupled with more than one G protein that are involved in inhibition and stimulation of cPLA2 activity.

Expression in vivo of CD45RA, CD45RB and CD44 on T cell receptor-transgenic CD8+ T cells following immunization

We used mice transgenic for a major histocompatibility complex class I-restricted T cell receptor to study the changes of phenotype in vivo which follow priming by antigen of CD8 T cells. We show that following priming with peptide, CD44 on CD8 T cells is up-regulated. The change of phenotype was relatively stable, as primed CD8 cells isolated from thymectomized mice 6 weeks after priming still expressed increased levels of CD44. CD8 T cells in these mice are still responsive to peptide and could represent long-lived primed cells. No down-regulation in vivo of the CD45RA or CD45RB isoforms was found, indicating that there is a differential regulation of the expression of CD44 and CD45RB by activated CD8 transgenic T cells. These results contradict earlier studies in vitro which showed that CD8 T cells which have been primed earlier belong to the CD45RA- or CD45RB- subset.

The molecular mechanisms of neuronal apoptosis

During the past year, apoptosis has been recognized as a process that is perpetually poised to be initiated--often from the cytoplasm rather than from the nucleus--unless it is suppressed by survival factors. Suspected mediators of apoptosis that have recently been investigated include the cysteine protease interleukin-1 beta-converting enzyme, free radicals and cell cycle kinases. Known inhibitors of programmed cell death, such as Bcl-2 and its homologues, have been further studied, and the results suggest that cell death may be regulated by multiple pathways. With the recent identification of the Drosophila gene reaper, which appears to play a role in the initiation of apoptosis, another genetic system for studying cell death has become available.

Cyclooxygenase inhibitors suppress inhibitory effect of PGE2 on Na-K-ATPase in MDCK cells

In a previous work, we showed that picomolar concentrations of prostaglandin E2 (PGE2) inhibit Na-K-ATPase activity and ouabain binding in a clone of Madin-Darby canine kidney (MDCK) cells. In the present study, we demonstrate that the inhibitory effects of PGE2 on Na-K-ATPase activity, ouabain-sensitive Rb+ uptake, and ouabain binding in MDCK cells were diminished by treatment of the cells with nonsteroidal anti-inflammatory drugs. These results suggested that products of arachidonic acid synthesized through the cyclooxygenase pathway are involved in the inhibitory mechanism of PGE2. Treatment of the cells with arachidonic acid resulted in inhibition of ouabain binding, and the inhibition was eliminated by cyclooxygenase inhibitors. These observations further support the involvement of cyclooxygenase products in the PGE2-induced inhibitory process. Finally, we demonstrated that dopamine inhibits Rb+ influx and ouabain binding in MDCK cells similarly to PGE2. Cyclooxygenase inhibitors suppressed the inhibition of ouabain binding by dopamine, thus also suggesting the involvement of cyclooxygenase products in the inhibitory effect of dopamine.

Thy-1 antibody-triggered neurite outgrowth requires an influx of calcium into neurons via N- and L-type calcium channels

We present evidence that the neurite out-growth stimulated by the binding of Thy-1 antibodies to PC12 cells is mediated by calcium influx through both N- and L-type calcium channels. PC12 cells cultured on a noncellular substratum in the presence of NGF, or on a cellular substratum in the absence of NGF, responded to soluble Thy-1 antibody by extending longer neurites. The response required bivalent antibody and could be blocked by removing Thy-1 from the surface of PC12 cells with phosphatidylinositol specific phospholipase C. The response could also be blocked by reducing extracellular calcium to 0.25 mM, or by antagonists of L- and N-type calcium channels. Additionally, the response could be fully inhibited by preloading PC12 cells with BAPTA/AM which buffers changes in intracellular calcium. A heterotrimeric G-protein is also implicated in the pathway as the response could be fully inhibited by pertussis toxin. These data suggest that antibody-induced clustering of Thy-1 stimulates neurite outgrowth by activating a second messenger pathway that has previously been shown to underlie cell adhesion molecule (NCAM, N-cadherin, and L1), but not integrin or NGF-dependent neurite outgrowth.

PGE2 inhibits Na-K-ATPase activity and ouabain binding in MDCK cells

In the present study we report on a direct effect of prostaglandin E2 (PGE2) on ouabain binding and Na-K-adenosinetriphosphatase (Na-K-ATPase) activity in a clone of Madin-Darby canine kidney cells, a renal cell line with collecting duct properties. Incubation of the cells with low concentrations (pM) of PGE2 produced a concomitant reduction of approximately 50% in the activity of Na-K-ATPase in the cell homogenate and in ouabain binding to the intact cells (half-maximal inhibition of approximately 0.1 pM). The inhibition was apparent within 10 min of preincubation of the cells with PGE2. Scatchard analysis of the binding demonstrated that the treatment with PGE2 reduced the number of ouabain binding sites without a change in the dissociation constant. PGE1 and PGF2 alpha (10 nM) did not affect ouabain binding or Na-K-ATPase activity. The fast, potent, and specific effect of PGE2 suggests that the diuretic/natriuretic effect of prostaglandins of the E series in the collecting tubule, in addition to the interference with the activity of arginine vasopressin, may result from a direct reduction in the number of the Na-K-ATPase active units, via a prostaglandin receptor.

Alternative splicing of the cytoplasmic domain of neural cell adhesion molecule alters its ability to act as a substrate for neurite outgrowth

A full-length cDNA encoding 180-kDa neural cell adhesion molecule (NCAM 180) has been transfected into mouse NIH-3T3 fibroblasts, and stable clones expressing the transgene have been isolated and characterised. Transfection was associated with the expression of a major protein band of 180 kDa and a minor related band of 140 kDa. Antibodies reactive exclusively with human NCAM immunoprecipitated both proteins but failed to coprecipitate any other proteins. The ability of transfected NCAM to stimulate neurite outgrowth was determined by culturing rat cerebellar neurons on top of confluent monolayers of parental 3T3 cells or clones of transfected 3T3 cells expressing either NCAM 140 or NCAM 180. The results show that NCAM 180 is less able to act as a substrate for neurite outgrowth than NCAM 140.

Prostaglandin E2 can bimodally inhibit and stimulate the epididymal adipocyte adenylyl cyclase activity

Measurements of prostaglandin E2 (PGE2)-induced adenylyl cyclase activity in membranes isolated from epididymal rat adipocytes revealed inhibition of cAMP production at low concentrations of PGE2 (less than 10 mM) and stimulation at higher concentrations. This biphasic effect of PGE2 was obtained when adenylyl cyclase was stimulated with GTP or NaF. In the presence of forskolin only the inhibitory phase by PGE2 was observed. Sulprostone, a PGE2 analogue, did not affect cAMP synthesis in the presence of either GTP or NaF; however, in the presence of forskolin, it inhibited cAMP production similarly to PGE2. Treatment of the membranes with cholera or pertussis toxin did not alter the biphasic effect of PGE2 on cAMP production. These findings raise the possibility that PGE2 acts through several receptor subtypes which are coupled to GTP binding proteins different from the classical Gi or Gs proteins.

Evidence that the CD45 phosphatase regulates the activity of the phospholipase C in mouse T lymphocytes

The importance of the tyrosine phosphatase CD45 in the regulation of lymphocyte activation was first demonstrated using antibodies against the extracellular domain of CD45 in functional assays. More recently it was reported that CD45-negative mutants were nonresponsive to stimulation through the T cell receptor-CD3 complex. We have studied the effect of CD45 cross-linking on the early signals induced by CD3 in mouse T cells. We show that CD45 cross-linking inhibits the increase in inositol phosphates and cytoplasmic Ca2+ induced by cross-linking of CD3. This indicates that CD45 is involved in the regulation of phospholipase C.

Fatty acid binding protein (FABP) modulates prostaglandin E binding to rat epididymal adipocyte membrane similarly to albumin

Albumin enhances prostaglandin E2 (PGE2) binding to isolated epididymal adipocyte membrane and also binds PGE2 with low affinity. On the other hand, S-100, ovalbumin and albumin-stearate failed to bind PGE2, as shown by ultrafiltration, and also failed to enhance PGE2 binding to the isolated adipocyte membranes. These results suggested that albumin enhances PGE2 binding possibly by serving as a carrier for the prostaglandin molecules. 3 mM warfarin or 1 mM phenylbutazone inhibited PGE2 binding to albumin by 70% and 95%, respectively, but both drugs failed to affect the enhancement of PGE2 binding to the isolated adipocyte membrane in the presence of albumin. These results exclude the possibility that PGE2 bound to albumin is more accessible to the prostaglandin receptor than free PGE2 in solution. Finally it is shown that fatty acid binding protein (FABP), a cytosolic protein which binds specifically PGE1 but not PGE2, enhances PGE1 and PGE2 binding to isolated adipocyte membranes similarly to albumin. The physiological implications of these findings are discussed.

Biphasic effect of sodium fluoride and guanyl nucleotides on binding to prostaglandin E2 receptors in rat epididymal adipocyte membranes

Both NaCl and NaF promoted PGE2 binding to epididymal adipocyte membranes by apparent increase in the binding affinity. In order to distinguish between the effect of fluoride and the 'salt effect' of sodium on PGE2 binding, the effects of Mg2+ and guanyl nucleotides on PGE2 binding in the presence of NaCl or NaF were compared. Mg2+ decreased PGE2 binding; high NaF concentration abolished this inhibition, while increased NaCl concentrations did not affect the Mg2+ inhibition. In the presence of Mg2+ the effects of NaCl and NaF were additive. The enhancement of PGE2 binding by fluoride, unlike sodium, was dependent on the presence of Mg2+. Incubation of the membranes with GDP beta S, Gpp(NH)p, GTP or GTP gamma S increased PGE2 binding. Gradual increase in NaF concentrations in the presence of guanyl nucleotides resulted in stimulation of PGE2 binding at low NaF concentrations and inhibition of PGE2 binding at high NaF concentrations. No changes in the stimulatory action of NaCl on PGE2 binding were observed in the simultaneous presence of NaCl and guanyl nucleotides. A biphasic effect on PGE2 binding was observed with a wide concentration range of guanyl nucleotides. Treatment of the isolated membranes with cholera or pertussis toxins stimulated the adenylyl cyclase activity of the membranes, but failed to influence PGE2 binding. The implications of these findings are discussed.

Exposure of latent prostaglandin binding sites in the rat epididymal adipocyte membrane and the effects of albumin, heating and alkylating agents

Brief incubation (1 min) of the adipocyte isolated membranes at 60 degrees C caused an increase in prostaglandin E2 binding, similar to that obtained with albumin. The increase in the membranal binding capacity after a short heating of the membranes was concomitant with a substantial decline in the ability of albumin to induce a further increase in the binding capacity of the treated membranes. Pretreatment of the isolated adipocyte membranes with 10 mM N-ethylmaleimide inhibited the enhancement of prostaglandin E2 binding in the presence of albumin, but did not affect the prostaglandin E2 binding in the absence of albumin. Identical treatment of the isolated membranes with glutathione-maleimide, an impermeable SH reagent with comparable alkylation reactivity, enhanced the binding of prostaglandin E2 in the absence of albumin and failed to inhibit the enhancement of prostaglandin E2 binding in its presence. In contrast to the effect of albumin on prostaglandin E2 binding to the isolated membranes, albumin failed to alter prostaglandin E2 specific binding to intact adipocytes.

Exposure of latent prostaglandin-binding sites in the rat epididymal adipocyte membrane

The addition of albumin to a suspension of plasma membranes isolated from rat epididymal fat cells increases both the initial binding rate and the maximal specific binding of prostaglandin E2 to these membranes. The presence of albumin affects neither the dissociation rate of bound prostaglandin E2 nor its dissociation constant. The results indicate that the interaction of albumin with the isolated membranes exposes latent prostaglandin-binding sites. Albumin fails to alter the specific binding of prostaglandin E2 to membranes isolated from frog erythrocytes or from rat peritoneal fat cells. These results indicate that the effect of albumin on the specific binding of prostaglandin E2 characterizes a specific molecular property of the prostaglandin receptors in rat epididymal adipocytes.

Incidence of neuronal dye-coupling in neocortical slices depends on the plane of section

The fluorescent dye Lucifer Yellow CH was intracellularly injected into neurons in slices of guinea-pig visual neocortex which had been prepared by sectioning either in a plane normal to the pial surface (radial slices) or in a plane parallel to the pial surface (tangential slices). In radial slices 44.3% of the injections resulted in dye-coupling and the number of cells coupled to the impaled neuron per injection followed a Poisson distribution. In contrast dye-coupling was not observed in tangential slices. Incidence of dye-coupling in slices that had been sectioned in both the radial and tangential planes was the same as in intact radial slices, indicating that slicing in the radial plane induced the formation of pathways for dye movement between neurons. The results suggest that formation and/or strengthening of direct intercellular junctions between neocortical neurons may occur as a specific neuronal response to partial dendrotomy.

Lateral mobility of phospholipids in the external and internal leaflets of normal and hereditary spherocytic human erythrocytes

The lateral diffusion coefficients (D) and the mobile fractions of the fluorescent phospholipid N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phosphatidylethanolamine (NBD-PE) and of membrane proteins labelled with fluorescein isothiocyanate, were measured by fluorescence photobleaching recovery on erythrocytes from healthy persons and from a hereditary spherocytosis patient. Measurements of lipid probe mobility were performed on ghosts labelled by NBD-PE exclusively at the external monolayer, or at both sides of the membrane. Our results indicate the following: (1) The mean values and the temperature dependence of D are different at the external and internal membrane leaflets. (2) In both normal and HS ghosts the mobile fraction of NBD-PE in the external monolayer does not depend significantly on temperature. On the other hand, the mobile fraction in the internal monolayer is reduced as the temperature is decreased. (3) At low temperatures, the mobile fraction of NBD-PE in the internal monolayer of spherocytic ghosts is significantly lower than the mobile fraction in the internal monolayer of normal ghosts. (4) No differences were observed between the mobilities of membrane proteins in normal and in spherocytic ghosts. However, differences were observed between the two cell populations in the temperature-dependence of the intrinsic fluorescence of unlabelled membrane proteins. The implications of these results for membrane phospholipid asymmetry and for cytoskeletal interactions with the internal lipid monolayer are discussed.

Functional molecular weight of the lac carrier protein from Escherichia coli as studied by radiation inactivation analysis

Cytoplasmic membrane vesicles prepared from Escherichia coli containing multiple copies of the lac y gene were frozen in liquid nitrogen before or after generation of a proton electrochemical gradient (interior negative and alkaline) and irradiated with a high-energy electron beam at -135 degrees C. Subsequently, the lac carrier protein was extracted into octyl beta-D-glucopyranoside, reconstituted into proteoliposomes, and assayed for transport activity. Under all conditions tested, activity decreased as a single exponential function of radiation dosage, allowing straightforward application of target theory for determination of functional molecular mass. When lac carrier activity solubilized from nonenergized vesicles was assayed, the results obtained were consistent with a functional molecular size of 45-50 kDa, a value similar to the size of the protein as determined by other means. Similar values were obtained when the octyl beta-D-glucopyranoside extract was irradiated, and the target size observed for D-lactate dehydrogenase was in good agreement with the molecular size of this enzyme. Strikingly, when the same procedures were carried out with vesicles that were energized with appropriate electron donors prior to freezing and irradiation, a functional molecular size of 85-100 kDa was obtained for the lac carrier with no change in the target size of D-lactate dehydrogenase. In contrast, when the vesicles were energized under conditions in which the proton electrochemical gradient was collapsed, the target mass of the lac carrier returned to 45-50 kDa. The results indicate that the functional mass of the lac carrier protein is no greater than a dimer and suggest that the proton electrochemical gradient may cause an alteration in subunit interactions.

Topology of the lac carrier protein in the membrane of Escherichia coli

Proteolysis of topologically sealed right-side-out and inside-out membrane vesicles from Escherichia coli with chymotrypsin, trypsin, or papain inactivates lac carrier function in a symmetrical manner. Concomitantly, the electrophoretic mobility of lac carrier protein photoaffinity labeled in situ with p-nitro[2-3H]phenyl-alpha-D-galactopyranoside is altered from a relative Mr of 33,000 to 20,000, and the time course of proteolysis is almost identical in vesicles of opposite polarities. In contrast, solubilization of the vesicles in NaDodSO4 followed by proteolysis causes fragmentation of the Mr 33,000 band into material that electrophoreses at the solvent front. Notably, proteolysis has no effect whatsoever on the ability of the lac carrier protein to bind substrate, as judged by photoaffinity-labeling experiments. Furthermore, the electrophoretic patterns of samples proteolyzed prior to photoaffinity labeling are the same as those observed when the procedures are reversed. These results show that the lac carrier protein spans the membrane and indicate that the binding site resides within a segment that is embedded in the bilayer.

Lateral mobility of phospholipids in turkey erythrocytes. Implications for adenylate cyclase activation

Hormone activation of turkey erythrocyte adenylate cyclase is affected by temperature and by cis-vaccenic acid incorporation. This was correlated with changes in membrane "fluidity" which were previously assumed to affect lateral diffusion of the beta-adrenergic receptor (Rimon, G., Hanski, E., and Levitzki, A. (1980) Biochemistry 19, 4451-4460; Briggs, M. M., and Lefkowitz, R. J. (1980) Biochemistry 19, 4461-4466). To test this hypothesis, we measured the lateral mobility of the fluorescent phospholipid N-4-nitrobenzo-2-oxa-1,3-diazolyl-phosphatidylethanolamine (NBD-PE in turkey erythrocyte ghosts, using fluorescence photobleaching recovery. Ghosts labeled only in the external leaflet exhibited a linear Arrhenius plot of the lateral diffusion coefficient (D) with little change in the mobile fraction of NBD-PE between 45 and 5 degrees C. Labeling from both sides, however, yielded a break point (approximately 30 degrees C) in the Arrhenius plot and a 30% decrease in the mobile fraction of NBD-PE from 24 to 10 degrees C, indicating differences between the external and inner monolayers. The observed discontinuities are not due to gel-phase formation, since fluorescence depolarization of cis- and transparinaric acid did not reveal discontinuities at this temperature range. cis-Vaccenic acid incorporation did not markedly affect either D or the mobile fraction of NBD-PE at 25 degrees C. These results are discussed in view of the effects of temperature and cis-vaccenic acid incorporation on adenylate cyclase activation.

The reversal of the Gpp(NH)p-activated state of adenylate cyclase by GTP and hormone is by the "collision coupling" mechanism

The guanylyl imidodiphosphate-activated state of turkey erythrocyte cyclase can be reversed to the basal state by the simultaneous action of beta-agonists and GTP. The rate of reversal diminishes progressively with decreasing concentration of beta-adrenergic receptors on the membrane surface, whereas the extent of reversal is always maximal. The rate of reversal is found to be linearly dependent on the concentration of beta-adrenergic receptors within the membranes. This result supports the notion that the interaction of the enzyme unit and the beta-adrenergic receptor is catalytic and therefore of the "collision coupling" type. The dependence of the rate of reversal reaction on epinephrine concentration is noncooperative with an apparent dissociation constant of KD = 3.0 X 10(-6) M. The fraction of guanylyl imidodiphosphate-activated cyclase system which can be reversed by GTP and a beta-agonist strongly depends on temperature and reveals a sharp transition at 24 degrees C which is the freezing temperature of the inner monolayer. It is suggested that the GTP regulatory unit is inactive when the inner monolayer is frozen.

Temperature dependence of beta receptor, adenosine receptor, and sodium fluoride stimulated adenylate cyclase from turkey erythrocytes

The individual temperature dependencies of the process which control the activity of turkey erythrocyte adenylate cyclase have been determined. The temperature dependence of the fraction of activable cyclase units experiences a thermal transition at 24 degrees C for all three modes of enzyme activation: l-epinephrine, adenosine, and NaF. This thermal transition probably reflects the phase transition in the inner monolayer of the membrane which influences the behavior of the GTP regulatory unit which is involved in all three modes of enzyme activation. The "rate constant" of enzyme activation by adenosine reflects two thermal transitions, at 24 and at 35 degrees C; the apparent rate constant of cyclase activation by NaF activation experiences a transition only at 24 degrees C whereas the rate constant of the beta-receptor-bound agonist decreases monotonously with no "breaks" on the Arrhenium plot. Following the temperature dependence of the fluorescence intensity of dansylphosphatidylethanolamine embedded in both sides of the membrane and exclusively in the outer monolayer, one can assign the thermal transition of 24 degrees C to the inner monolayer and the other two transitions to the outer monolayer (10 and 35 degrees C). We interpret these results as follows. (a) The monomolecular rate constant characterizing the activation of cyclase by the precoupled adenosine receptor experiences both the transition at 24 and 35 degrees C, indicating that the latter may span the bilayer. (b) The bata receptor activates the cyclase units only in fluid areas since it can diffuse exclusively in the fluid areas of the membrane and is unable to interact with cyclase units in "frozen" areas. the linear dependence of the logarithm of the rate constant on 1/T for the bata receptor reflects the change of membrane fluidity as a function of temperature.

Glucose 6-phosphate transport in membrane vesicles isolated from Escherichia coli: effect of imposed electrical potential and pH gradient

Imposition of a membrane potential (delta psi, interior negative) or a pH gradient (delta pH, interior alkaline) across the membrane of Escherichia coli DF2000 leads to a marked, transient increase in glucose 6-phosphate transport that varies systematically with pH. Outwardly directed potassium diffusion gradients in the presence of valinomycin (i.e., generation of delta psi, interior negative) drive glucose 6-phosphate transport at pH 7.5 but much less effectively at pH 5.5, although the magnitudes of the transient delta psi generated are comparable at both pH values. Similarly, imposition of delta psi (interior negative) retards the rate of passive, carrier-mediated glucose 6-phosphate efflux down a concentration gradient at pH 7.5 but not at pH 5.5. In contrast, imposition of delta pH (interior alkaline) by means of outwardly directed acetate diffusion gradients drives glucose 6-phosphate accumulation at pH 5.5 but is relatively ineffective at pH 7.5. The results are independent of the pK of glucose 6-phosphate and provide strong support for the argument that the glucose 6-phosphate porter catalyzes an electrically neutral reaction at acid pH and an electrogenic reaction at alkaline pH. In addition, they are entirely consistent with the hypothesis that the proton/glucose 6-phosphate stoichiometry increases at alkaline pH [Rottenberg, H. (1976) FEBS Lett. 66, 159; Ramos, S., & Kaback, H.R. (1977) Biochemistry 16, 854, 4271].

Substrate inhibition of the (Na+, K+)-ATPase in the presence of excess Mg2+

1. High concentrations of ATP inhibit completely the activity of (Na+, K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) prepared from sheep brain. 2. The inhibition depends on the concentration of total ATP, i.e. complexed ATP+ free ATP. 3. The inhibition by high ATP concentrations persists in the absence of K+, and is then independent of the Na+ concentration between 2 and 140 mM Na+. 4. Raising the K+ concentration at 20 mM Na+ increases the ATP concentration required for the maximal hydrolysis rate. 5. The Hill number for the inhibition process is about three. 6. The inhibition by ATP is temperature-dependent, in that as the temperature is increased, higher ATP concentrations are required for inhibition.