Calcitonin inhibits the growth of human gastric carcinoma cell line KATO III.

Calcitonin has a wide variety of actions on gastrointestinal function. In this study, we investigated the effects of calcitonin on the growth of human gastric carcinoma cell line KATO III in comparison with those of calcitonin gene-related peptide (CGRP). Calcitonin, but not CGRP, significantly and dose-dependently inhibited the growth of KATO III cells. This inhibition of cell growth was accompanied by an increase in cyclic AMP production. The proliferation of KATO III cells was also inhibited by forskolin and dibutyryl cyclic AMP, although agents which do not stimulate cyclic AMP production had no effect. Furthermore, in the presence of GTP, calcitonin stimulated adenylate cyclase activity in KATO III cell membranes, and this increase was reduced in the absence of GTP. On the other had, neither calcitonin nor CGRP enhanced the turnover of inositolphospholipid or the intracellular Ca2+ level. In addition, 125I-labeled human calcitonin was specifically bound to KATO III cell membranes, and this binding was dose-dependently displaced by unlabeled calcitonin but not CGRP. Furthermore, the specific binding of 125I-labeled human calcitonin to KATO III cell membranes was significantly reduced by addition of GTP but not ATP. These results suggest that calcitonin inhibits the growth of human gastric carcinoma cell line KATO III by stimulating cyclic AMP production via a GTP-dependent process coupled to specific calcitonin receptors.     

Covalent cross-linking of a photoactive derivative of calcitonin to human breast cancer cell receptors

A photoaffinity derivative of salmon calcitonin has been produced by transglutaminase-mediated incorporation of N-(beta-aminoethyl)-4-azido-2-nitroaniline into the hormone. The derivative, purified by high pressure liquid chromatography, retained the abilities to bind to cultured T47D breast cancer cells and to stimulate adenylate cyclase in these cells. In both these respects it was equipotent with synthetic salmon calcitonin. Photolysis of the 125I-labeled photoactive salmon calcitonin derivative bound to T47D cells was accompanied by specific labeling of only one component (Mr approximately 85,000) as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Labeling was observed only upon photolysis and was inhibited by unlabeled synthetic salmon calcitonin but not by the inactive calcitonin analogue, 8-glycine human calcitonin. Reduction did not alter the apparent molecular weight of the calcitonin receptor complex. No macromolecular forms of calcitonin were produced by photolysis in the absence of T47D cells.     

Effect of thioacetamide on cytochrome P-450 synthesis in rat liver.

Both calcitonin and prostaglandin E2 (PGE2) stimulate adenylate cyclase activity in the human breast cancer cell line (T 47D). The maximum cyclic AMP response to calcitonin exceeds that of PGE2. When maximal concentrations of the two hormones were added simultaneously to the cells, the amount of cyclic AMP generated was less than that seen with calcitonin alone. When cells were treated with the protein toxin of Bordetella pertussis (islet-activating protein; IAP) which inactivates the inhibitory regulatory component (Ni) of adenylate cyclase, there was no change in basal or calcitonin-responsive adenylate cyclase in intact cells. However, the PGE2 response was augmented at all dose levels, and this effect was dependent on the concentration of IAP. Moreover, in cells pretreated with IAP, simultaneous addition of PGE2 and calcitonin resulted in additivity rather than in inhibition of cyclic AMP production. The additivity of the response to calcitonin and PGE2 after IAP treatment implies activation of separate pools of adenylate cyclase catalytic subunit by the two hormones. These data are consistent with a model in which calcitonin acts on adenylate cyclase in T 47D cells through stimulatory regulatory components alone, whereas PGE2 acts on the same cells through both stimulatory and inhibitory components. The Ni input can limit the maximum effect of PGE2 and is capable of limiting calcitonin effects when the two agonists are used simultaneously.     

Receptor-mediated autocrine growth-stimulatory effect of 5-hydroxytryptamine on cultured human pancreatic carcinoid cells.

5-hydroxytryptamine (5-HT) is a mitogen for fibroblasts, vascular smooth muscle cells, renal mesangial cells, and jejunal crypt cells. The human carcinoid cell line (termed BON) that we established in our laboratory from a pancreatic carcinoid tumor produces and secretes 5-HT. In this study, therefore, we examined the effect of 5-HT on growth of BON cells. Furthermore, by use of selective 5-HT receptor antagonists, we examined receptor and post-receptor mechanisms by which 5-HT-induced responses were produced. 5-HT stimulated growth of BON cells. 5-HT stimulated phosphatidylinositol (PI) hydrolysis in a dose-dependent fashion and inhibited cyclic AMP production in a dose-dependent fashion. The 5-HT1A/1B receptor antagonist, SDZ 21-009, prevented the reduction of cyclic AMP production evoked by 5-HT and inhibited the mitogenic action of 5-HT. The 5-HT1C/2 receptor antagonist, mesulergine, competitively inhibited PI hydrolysis, but did not affect the mitogenic action of 5-HT. The mitogenic action of 5-HT and the reduction of cyclic AMP production evoked by 5-HT were also inhibited by pertussis toxin. These results suggest that 5-HT is an autocrine growth factor for BON cells and that mitogenic mechanism of 5-HT involves receptor-mediated inhibition of the production of cyclic AMP which may be linked to pertussis toxin-sensitive GTP binding protein. 8-bromo-cyclic AMP inhibited growth of BON cells whereas 8-bromo-cyclic GMP had no effect on cell growth. Involvement of protein kinase A in BON cell growth regulation was confirmed by the observation that a cAMP-dependent protein kinase antagonist (Rp-cAMPS) could stimulate BON cell growth.     

Somatostatin analogs: correlation of receptor affinity with inhibition of cyclic AMP formation in pancreatic acinar cells

Somatostatin inhibited secretin-stimulated cyclic AMP formation in pancreatic acinar cells. The inhibition was only partial. Maximal inhibition reached about 50%. Somatostatin analogs tested inhibited secretin-stimulated cyclic AMP formation with a lower potency than somatostatin. Cys-Aza Ala-Phe-Phe-DTrp-Lys-Thr-Phe-Phe-Cys was found to be an antagonist of somatostatin in inhibiting secretin-stimulated cyclic AMP. Analogs inhibited the binding of 125I-[Tyr11] somatostatin to pancreatic acini. There was a good correlation (r = 0.97) between concentration for inhibiting 50% secretin-stimulated cyclic AMP and receptor binding affinities.     

Effect of tunicamycin on functions of PGE1 receptors from mouse mastocytoma P-815 cells

Prostaglandin E1 (PGE1) receptors from mouse mastocytoma P-815 cells were found to bind to a wheat germ agglutinin (WGA)-Agarose column, suggesting that the receptors are glycoproteins. To further elucidate the role of carbohydrate moieties in the PGE1 receptors for their binding activity to ligand, the P-815 cells were treated with tunicamycin, swainsonine or monensin. Tunicamycin, an inhibitor of N-glycosylation, dose- and time-dependently inhibited the binding of PGE1 to mastocytoma P-815 cells. Neither swainsonine, an inhibitor of Golgi mannosidase II, nor monensin, an inhibitor of processing beyond the high mannose stage, altered PGE1 binding properties of the cells. The inhibition of PGE1 binding by tunicamycin was observed when incorporation of [3H]glucosamine into macromolecules was inhibited. The inhibitory effect was not on their affinity but on their number of binding sites. Subcellular distributions of [3H]PGE1-binding activity showed that decreases in the binding activity by tunicamycin were highest in plasma membrane fractions. Treatment of membranes with various endo- and exoglycosidases did not affect PGE1 binding. PGE1-stimulated cyclic AMP accumulation in the cells was also inhibited by tunicamycin. These results suggest that PGE1 receptors of mastocytoma P-815 cells are glycoproteins and that inhibition of N-glycosylation of PGE1 receptors by tunicamycin results in the arrest of the translocation of newly synthesized receptors to the surface of mastocytoma P-815 cells.     

Growth inhibition of human breast cancer cells induced by calcitonin

The human breast cancer cell line (T47D) has specific, high affinity calcitonin receptors and calcitonin-responsive adenylate cyclase. Human, salmon and [Asu1,7]eel calcitonin inhibited cell growth in a dose-related manner with almost equipotency. Analogues of human calcitonin demonstrated slight cell growth inhibition. We found extreme growth inhibition with daily treatment with dibutyryl cyclic AMP (10(-4) M). In contrast to calcitonin 1,25-(OH)2D3 had a biphasic effect on cell growth. Physiological doses (5 X 10(-10) M) of 1,25-(OH)2D3 stimulated growth of T47D, whereas treatment by supraphysiological amounts (2.5 X 10(-7) M) caused significant inhibition of growth. Calcitonin and 1,25-(OH)2D3 appeared to have additive effects.     

The relationship between glycosylation and glycoprotein metabolism of mouse neuroblastoma N18 cells.

Two inhibitors of glycosylation, glucosamine and tunicamycin, were utilized to examine the effect of glycosylation inhibition in mouse neuroblastoma N18 cells on the degradation of membrane glycoproteins synthesized before addition of the inhibitor. Treatment with 10 mM-glucosamine resulted in inhibition of glycosylation after 2h, as measured by [3H]fucose incorporation into acid-insoluble macromolecules, and in a decreased rate of glycoprotein degradation. However, these results were difficult to interpret since glucosamine also significantly inhibited protein synthesis, which in itself could cause the alteration in glycoprotein degradation [Hudson & Johnson (1977) Biochim. Biophys. Acta 497, 567-577]. N18 cells treated with 5 microgram of tunicamycin/ml, a more specific inhibitor of glycosylation, showed a small decrease in protein synthesis relative to its effect on glycosylation, which was inhibited by 85%. Tunicamycin-treated cells also showed a marked decrease in glycoprotein degradation in experiments with intact cells. The inhibition of glycoprotein degradation by tunicamycin was shown to be independent of alterations in cyclic AMP concentration. Polyacrylamide-gel electrophoresis of isolated membranes from N18 cells, double-labelled with [14C]fucose and [3H]fucose, revealed heterogeneous turnover rates for specific plasma-membrane glycoproteins. Comparisons of polyacrylamide gels of isolated plasma membranes from [3H]fucose-labelled control cells and [14C]fucose-labelled tunicamycin-treated cells revealed that both rapidly and slowly metabolized, although not all, membrane glycoproteins became resistant to degradation after glycosylation inhibition.     

Dopamine Stimulates [3H]Phorbol 12,13-Dibutyrate Binding in Cultured Striatal Cells

The effect of dopamine (DA) on the binding of [3H]phorbol 12,13-dibutyrate ([3H]PdBu) in cultured rat striatal cells was examined. DA maximally increased specific [3H]PdBu binding by 70 +/- 10%, an increase comparable to that observed with norepinephrine (NE). This finding suggests that DA activates protein kinase C in cultured striatal cells, because increases in [3H]PdBu binding reflect translocation of protein kinase C. Half-maximal stimulation was observed with 10(-6) M DA. The peak response was observed at 2-3 min after addition of 10(-4) M DA, but [3H]PdBu binding was still increased above basal at 30 min. DA was not acting via an adrenergic receptor. Prazosin (10(-6) M) blocked the response to NE, suggesting mediation by an alpha 1-adrenergic receptor, but had little effect on the response to DA. Conversely, the D1 receptor antagonist SCH-23390 (10(-6) M) blocked the response to DA, but only partially inhibited the response to NE. Morphine (10(-6) M) inhibited the response to DA by 46 +/- 14%, but did not affect significantly the response to NE. The DA effect on [3H]PdBu binding is apparently independent of the increase in cyclic AMP seen on D1 receptor activation. Forskolin, apomorphine, and the D1 agonist SKF-38393 all increased cyclic AMP in striatal cells, but were less effective than DA in stimulating [3H]PdBu binding. The D2 agonist quinpirole was ineffective in stimulating either cyclic AMP or [3H]PdBu binding.     

Two different signal transductions of neuropeptide Y1 receptor in SK-N-MC cells.

The signal transduction systems of the neuropeptide Y (NPY) Y1 receptor were studied in SK-N-MC human neuroblastoma cells. NPY induced an increase in intracellular calcium ion concentration ([Ca2+]i) and inhibition of forskolin-stimulated cyclic AMP accumulation, which were mediated through Y1 receptors. One-min preincubation of cells with phorbol 12-myristate 13-acetate (PMA) inhibited both signal transductions dose-dependently, but its effect on [Ca2+]i was about 100-fold more potent than that on cyclic AMP. PMA had no effect on [125I]BH-NPY binding in SK-N-MC cells and hardly inhibited the endothelin-1-induced increase in [Ca2+]i. Pertussis toxin also inhibited the NPY-induced [Ca2+]i increase 30-fold more effectively than the NPY-mediated inhibition of cyclic AMP accumulation. These results indicate that Y1 receptors in SK-N-MC cells couple to two signal transduction systems that have different sensitivities to phorbol ester and pertussis toxin treatments.     

The binding of cyclic AMP to renal brush border membranes.

The binding of cyclic AMP to the proximal tubule luminal (brush border) membrane isolated from the rabbit renal cortex was studied. The rate of binding was dependent on temperature; at 37 degrees equilibrium was attained in 45 min, whereas at 0 degrees 120 min was required. The final levels of binding were identical. The binding of 3H-cyclic AMP was reversed by dilution or addition of unlabeled cyclic nucleotide. Debinding was markedly temperature sensitive. Binding was only partially saturable with respect to cyclic AMP concentration, apparently with more than one binding site. The cyclic AMP bound to the membrane was recovered unchanged. When bound to the membrane cyclic AMP was resistant to hydrolysis by endogenous membrane or exogenously added phosphodiesterase. The binding to the membranes was relatively specific for cyclic AMP, although other cyclic purine nucleotides inhibited, cyclic IMP greater than dibutyryl cyclic AMP greater than cyclic GMP. The renal membranes did bind cyclic GMP, but this binding was relatively non-specific. Hormones and drugs, that mediate cyclic AMP generation or renal function, as well as other compounds common to the proximal tubule were without significant effect on cyclic AMP binding. Binding was inhibited by sulfhydryl reacting agents and this inhibition could be blocked and partially reversed by mercaptoethanol.     

Cyclic AMP-dependent and -independent effects on tissue-type plasminogen activator activity in osteogenic sarcoma cells; evidence from phosphodiesterase inhibition and parathyroid hormone antagonists

The plasminogen activator (PA) in clonal osteogenic sarcoma cells of rat origin (UMR 106-01 and UMR 106-06) and in osteoblast-rich rat calvarial cells has been characterized using specific antibodies to be tissue-type PA (tPA). An Mr value of 75,000 by SDS-polyacrylamide gel electrophoresis and fibrin autoradiography supports this characterization. There was also evidence for an Mr 105,000 component, which could be due to a proteinase-inhibitor complex. The mechanism of regulation of this tPA activity has been studied in the clonal osteogenic sarcoma cells. Parathyroid hormone (PTH) and prostaglandin E2, which increase cyclic AMP production in the sarcoma cells, also increased tPA activity. The sensitivity and magnitude of the tPA response to PTH and prostaglandin E2 were increased by simultaneous treatment with isobutylmethylxanthine (IBMX) at drug concentrations which had little effect themselves on tPA activity. In UMR 106-06 cells, which unlike UMR 106-01 cells show a cyclic AMP response to calcitonin, tPA activity was also increased in response to calcitonin, and the effect was enhanced by IBMX. 1,25-Dihydroxyvitamin D-3 also increased tPA activity in the cells, but this response was not modified by IBMX. Synthetic peptide antagonists of PTH-responsive adenylate cyclase, [34Tyr]-hPTH (3-34) amide and [34Tyr]-hPTH (5-34) amide, inhibited the PTH-induced increase in tPA activity over the same concentration range at which they inhibited cyclic AMP production, but the antagonist peptides had no effect on the tPA responses to prostaglandin E2, calcitonin or 1,25-dihydroxyvitamin D-3. These data indicate that cyclic AMP mediates the actions of PTH, prostaglandin E2 and calcitonin in increasing tPA activity in the clonal osteogenic sarcoma cells. 1,25-Dihydroxyvitamin D-3, on the other hand, increases tPA activity through a mechanism independent of cyclic AMP.     

Effect of 1,25-dihydroxyvitamin D3 on cyclic AMP responses to hormones in clonal osteogenic sarcoma cells.

The effect of 1,25-dihydroxyvitamin D3 on adenylate cyclase responsiveness was studied in the clonal osteogenic sarcoma cell line, UMR 106-06, which responds to several bone active hormones. 1,25-dihydroxyvitamin D3 treatment had no consistent effect on basal formation of cyclic AMP in intact cells, but the responses to parathyroid hormone, isoproterenol, prostaglandin E2, salmon calcitonin and the plant diterpene, forskolin, were all attenuated, by up to 90%. The effect of 1,25-dihydroxyvitamin D3 was dose-dependent, with half-maximal effectiveness at 0.1 nM, and required 48 h treatment of cells before it became apparent. The relative potencies of other vitamin D3 compounds correlated closely with their relative affinities for the 1,25-dihydroxyvitamin D3 receptor and their biological activities in other systems. 1,25-dihydroxyvitamin D3 treatment had no effect on the kinetics of labelled calcitonin binding to UMR 106-06 cells. Furthermore, the fact that such a range of hormones was affected made a receptor mediated mechanism unlikely. Nucleotide stimulatory (Ns) unit activity was assayed after 1,25-dihydroxyvitamin D3 treatment and found to be unchanged. Islet activating protein, an inhibitor of nucleotide inhibitory unit (Ni) activity, failed to modify the 1,25-dihydroxyvitamin D3 effect. Thus the effect of 1,25-dihydroxyvitamin D3 appeared to be exerted beyond hormone receptor and nucleotide regulatory components of the adenylate cyclase complex. It is concluded that 1,25-dihydroxyvitamin D3 attenuates adenylate cyclase response to hormones by a direct or indirect action on the catalytic component of adenylate cyclase.     

Regulation of uracil uptake in Escherichia coli by adenosine 3',5'-monophosphate.

Culture of a wild-type strain of Escherichia coli in the presence of cyclic AMP leads to an impairment of uracil uptake. Half maximum inhibition of uracil uptake was observed at 1.5 mM cyclic AMP. The effect seems to be specific since no inhibition was found in cultures supplemented with ATP, ADP or 5'-AMP. Similarly the inhibition was not observed in cultures of a mutant deficient in the cyclic AMP receptor protein. The inhibition in uracil uptake, found in bacteria cultured in the presence of cyclic AMP, is not a consequence of a reduction in the growth rate. On the other hand, this inhibition was observed only in cultures containing glucose or pyruvate as carbon source.     

Altered interaction between binding and catalytic subunits of a cyclic AMP-stimulated protein kinase from hepatoma cells.

Binding activity obtained from an established line of hepatoma tissue culture (HTC) cells has a lower apparent affinity for cyclic AMP at physiological pH than has the analogous binding activity from rat liver. However, the apparent binding affinity of HTC preparations can be reversibly increased by adding NaCl or guanidine · HCl. In the presence of such activating substances, a macromolecular inhibitory activity has been chromatographically separated from the cyclic AMP-binding activity. Removal of this inhibitory component causes the apparent affinity of the cyclic AMP-binding activity from HTC cells to increase and resemble that observed with liver preparations. Before treatment with salt, the inhibitory activity seems to be physically associated with the binding activity. Adding the isolated inhibitory component back to a suitably activated binding preparation from HTC cells results in a decrease in the apparent affinity for cyclic AMP. The isolated inhibitory component is devoid of cyclic AMP-binding and cyclic AMP phosphodiesterase activities and has an apparent minimal molecular weight of about 30,000 by gel filtration. It possesses protein kinase activity and seems to be identical to the catalytic subunit of a cyclic AMP-stimulated protein kinase on the basis of chromatographic properties and sensitivities to heat and low pH. This catalytic subunit represents only a minor portion of total cellular protein kinase activity and is also present in liver extracts. However, the binding activity from liver is not inhibited significantly under conditions where the binding from HTC cells is affected by the catalytic subunit. The difference in this inhibitory response between liver and HTC preparations appears to reflect differences in the cyclic AMP-binding proteins themselves.     

Calcitonin-sensitive adenylate cyclase in rat renal tubular membranes.

1. Renal tubular membranes from rat kidneys were prepared, and adenylate cyclase activity was measured under basal conditions, after stimulation by NaF or salmon calcitonin. Apparent Km value of the enzyme for hormone-linked receptor was close to 1 x 10(-8) M. 2. The system was sensitive to temperature and pH. pH was found to act both on affinity for salmon calcitonin-linked receptor and maximum stimulation, suggesting an effect of pH on hormone-receptor binding and on a subsequent step. 3. KCl was without effect areas whereas CoCl and CaCl2 above 100 muM and MnCl2 above 1 muM inhibited F- -and salmon calcitonin-sensitive adenylate cyclase activities. The Ca2+ inhibition of the response reflected a fall in maximum stimulation and not a loss of affinity of salmon calcitonin-linked receptor for the enzyme. 4. The measurement of salmon calcitonin-sensitive adenylate cyclase activity as a function of ATP concentration showed that the hormone increases the maximum velocity of the adenylate cyclase. GTP, ITP and XTP at 200 muM did not modify basal, salmon calcitonin- and parathyroid hormone-sensitive adenylate cyclase activities. 5. Basal, salmon calcitonin- and F- -sensitive adenylate cyclase activities decreased at Mg2+ concentrations below 10 mM. High concentrations of Mg2+ (100 mM) led to an inhibition of the F- -stimulated enzyme. 6. Salmon calcitonin-linked receptor had a greater affinity for adenylate cyclase than human or porcine calcitonin-linked receptors. There was no additive effect of these three calcitonin peptides whereas parathyroid hormone added to salmon calcitonin increased adenylate cyclase activity, thus showing that both hormones bound to different membrane receptors. Human calcitonin fragments had no effect on adenylate cyclase activity. 7. Salmon calcitonin-stimulated adenylate cyclase activity decreased with the preincubation time. This was due to progressive degradation of the hormone and not to the rate of binding to membrane receptors.     

Role of carbohydrate of human chorionic gonadotropin in the mechanism of hormone action.

The role of the carbohydrate part of human chorionic gonadotropin (hCG) was investigated by measuring the ability of hCG derivatives lacking various sugar residues to bind to rat Leydig cells and stimulate them to synthesize testosterone and cyclic adenosine 3':5'-monophosphate (cyclic AMP). Whereas sequential removal of the sialic acid, galactose, N-acetylglucosamine, and mannose residues led to a progressive increase in the effective dose of the hormone required to stimulate steroidogenesis, it resulted in a marked loss in the ability of the hormone to stimulate cyclic AMP accumulation. Low doses of the glycosidase-treated hormone derivatives were additive with hCG when their ability to stimulate testosterone synthesis was analyzed. Nevertheless, the glycosidase-treated derivatives were potent inhibitors of hCG-induced cyclic AMP accumulation, suggesting that removal of the sugars did not influence binding of the hormone to the cell as much as it reduced the ability of the bound hormone to activate adenyl cyclase. This hypothesis was further supported by our finding that the hCG derivatives were highly effective inhibitors of 125I-hGC binding to the intact cells. Removal of sialic acid and galactose enhanced the inhibition, whereas removal of all the sugar residues only decreased the inhibition slightly. The degree of these effects was comparatively small. The possibility that steroidogenesis and cyclic AMP accumulation are altered independently by hCG stimulation is discussed.     

Glutamate-Stimulated, Guanine Nucleotide-Mediated Phosphoinositide Turnover in Astrocytes Is Inhibited by Cyclic AMP

The potential for cross-talk between the adenyl cyclase and phosphoinositide (PPI) lipid second messenger system was investigated in astrocytes cultured from neonatal rat brain. Glutamate-stimulated PPI turnover, measured by the formation of total inositol phosphates from myo-[3H]inositol-labeled lipids, was inhibited in a concentration-dependent manner by the elevation of intracellular cyclic AMP levels produced either by stimulation of the isoproterenol receptor linked to adenyl cyclase or by its direct activation by forskolin. N6,2'-O-Dibutyryl cyclic AMP, an analogue that can also activate cyclic AMP-dependent kinase, inhibited glutamate-stimulated PPI turnover in a concentration-dependent manner as well, a result suggesting that cyclic AMP-dependent kinase is involved in mediating the inhibition. Inclusion of an inhibitor of cyclic AMP-dependent kinase, 1-(5-isoquinolinesulfonyl)-2 methylpiperazine dihydrochloride or N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride, blocked the cyclic AMP-mediated inhibition in a concentration-dependent manner, a finding further supporting this hypothesis. The site of inhibition of the phosphoinositol lipid pathway by cyclic AMP was probed using a digitonin-permeabilized cell system. Guanosine 5'-O-(3-thiotriphosphate), a nonhydrolyzable analogue of GTP, stimulated PPI turnover and potentiated glutamate-stimulated PPI turnover, and guanosine 5'-O-(3-thiodiphosphate) inhibited glutamate-stimulated PPI turnover in these cells, results providing evidence that glutamate receptors are coupled to phospholipase C by a guanine nucleotide binding protein in astrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Presence of histamine H2-receptors on human gastric carcinoma cell line MKN-45 and their increase by retinoic acid treatment.

Histamine dose-dependently stimulated cyclic AMP production in human gastric carcinoma cell line MKN-45, and this effect was inhibited by cimetidine but not by pyrilamine. Moreover, not only histamine but also cimetidine displaced the specific binding of [3H]tiotidine to these cells, whereas pyrilamine had no effect. On the other hand, pretreatment of MKN-45 cells with retinoic acid (RA) significantly enhanced histamine-induced increase of cyclic AMP production, although the cyclic AMP response to either forskolin or NaF was not affected. Finally, RA treatment increased the number of histamine receptor without altering its affinity. Thus, it appears that histamine H2-receptors are present on MKN-45 cells, and that RA treatment enhances the action of histamine on these cells by increasing the number of H2-receptors.     

衣霉素对人肝癌细胞表面胰岛素受体的影响

衣霉素(Tunicamycin)是一种糖蛋白N-连接型糖链合成的抑制剂。它可抑制人肝癌细胞抹SMMC-7721的生长,其抑制率和剂量及处理时间有相关性。衣霉素处理细胞18h尚可显著抑制~3H-甘露糖和~3H-氨基葡萄糖参入细胞,但仅轻度抑制~3H-亮氨酸的参入。这些标记化合物的参入抑制都有量效关系。0.1gg/mL衣霉素处理18h后,细胞表面胰岛素受体和胰岛素的结合容量下降,而对照细胞和处理细胞的胰岛素竞争结合曲线基本平行。这主要由于衣霉素抑制新合成的胰岛素受体的糖基化所致。我们对糖基化障碍引起细胞膜胰岛素受体结合容量降低的机理作了讨论。