Second messengers and phosphoproteins最新文献

The antihelminthic drug suramin inhibits the binding of monoradioiodinated VIP (125I-VIP) to two human cancerous cell lines, namely HT 29-D4 and IGR 39 derived from a colic adenocarcinoma and a superficial melanoma respectively, with an IC50 of 280 micrograms/ml. The drug is not able to remove 125I-VIP previously bound to either types of cells even with concentration as high as 1500 micrograms/ml. Neither 125I-VIP binding nor VIP binding sites molecular weight are affected by pretreatment of the cells by the drug. Suramin at 1000 micrograms/ml inhibits by 56% to 99% the cAMP accumulation induced by VIP, depending on the VIP concentrations and the cell lines used for the experiments. On the contrary the drug does not have any effects on the cAMP accumulation induced by the beta receptor agonist isoproterenol. Also suramin does not affect the basal accumulation of cAMP in both types of cells either in acute or chronic treatment with the drug. We speculate that these observations may account, at least in part, for the in vivo and in vitro effects of VIP and suramin on cell proliferation and survival.

Calcium-calmodulin dependent protein kinase II (CaM-KII) has been implicated in the inhibition of Na(+)-H+ exchange activity in the brush border of the renal proximal convoluted tubule. Conversely, the activity of the antiporter is stimulated in response to phosphorylation by calcium-phospholipid dependent protein kinase (PKC). In these experiments, we explored the potential for direct interaction between these two protein kinases by determining the effect of PKC activation by tumor promoting phorbol esters on the expression of mRNA for CaM-KII in the rabbit renal proximal tubule. The results indicate that activation of PKC reduced the steady-state levels of the mRNA for the alpha subunit of CaM-KII in a dose and time dependent manner. This suggests a novel mechanism by which PKC can antagonize the action of CaM-KII in selected tissues.

Alpha 1-adrenergic stimulation of phosphatidyl inositide (PI) hydrolysis was measured in liver slices and in cultured hepatocytes prelabeled with 3H-myoinositol. Incubation of cultured hepatocytes for 15 min with l-epinephrine stimulated the release of 3H-inositol phosphates with an EC50 of 0.28 +/- 0.19 microM and Emax representing 2.88 +/- 0.39% of incorporated radiolabel. Pretreatment of cultures with epinephrine resulted in reductions both in the maximum response and in the sensitivity to epinephrine. In liver slices, epinephrine elicited a similar maximum response (3.38 +/- 0.32%), but the sensitivity to epinephrine (EC50 = 2.27 +/- 1.27 microM) was considerably lower than for cultured cells. Prior treatment of liver slices with 200 microM l-epinephrine had no effect either on the maximum response to l-epinephrine or on the sensitivity to epinephrine. The affinity of alpha 1-adrenergic receptors for agonists was reduced 2.11 fold during incubation of liver slices for 60 min at 37 degrees C in the absence of added agonist (as is required for 3H-myoinositol labeling). Agonist affinity was not further reduced when 200 microM l-epinephrine was included in the incubation. It was concluded that alpha 1-adrenergic stimulation of PI hydrolysis in liver undergoes agonist-induced desensitization. It was also concluded that a limitation exists in the methodology for measuring PI hydrolysis in tissue slices prelabeled with 3H-myoinositol, in that this method assesses the activity of receptors in a desensitized state.

Since inositol 1,4,5-trisphosphate (1,4,5-IP3) and inositol 1,3,4,5-tetrakisphosphate (1,3,4,5-IP4) have been described to modulate Ca(2+)-channels, we investigated the possible participation of 1,4,5-IP3 and/or 1,3,4,5-IP4 in the bradykinin-induced Ca(2+)-influx into cultured porcine aortic endothelial cells. In our experiments bradykinin induced a quick release of Ca2+ from intracellular stores and a longlasting Ca(2+)-influx, which remained constant for at least 15 minutes. In contrast to its effect on [Ca2+]i, bradykinin only transiently elevated 1,4,5-IP3 and 1,3,4,5-IP4 levels. Ten minutes after addition of bradykinin, both 1,4,5-IP3 and 1,3,4,5-IP4 levels returned to basal values, whereas Ca(2+)-influx was still unaltered. Furthermore, preincubation of endothelial cell with phorbol-12-myristate-13-acetate (PMA) abolished the stimulatory effect of bradykinin on the formation of 1,4,5-IP3 and 1,3,4,5-IP4, but did not affect the longlasting Ca(2+)-influx. These data provide evidence that in endothelial cells inositolphosphates are not involved in the regulation of bradykinin-induced longlasting Ca(2+)-influx.

In streptolysin O permeabilized acini that were normally responsive to carbamylcholine and cholecystokinin octapeptide, amylase secretion was stimulated: a) by the stable guanyl nucleotides with a potency decreasing as follows: guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) greater than guanylyl-imidodiphosphate (GMP-PNP) = guanylyl-(beta, gamma-methylene)-diphosphonate (GMP-PCP), in the presence of 0.5 mM calcium and b) by calcium alone (EC50 3 microM). The maximal secretory effect of calcium alone (a 2-fold increase) was less effective than that of GTP gamma S and calcium offered in combination (an 8-fold increase). In the virtual absence of Ca2+, GTP gamma S still stimulated amylase release (a 3-fold increase) while 12-O-tetradecanoylphorbol 13-acetate (TPA) did not. The relative potencies of guanyl nucleotides were GTP gamma S greater than GMP-PNP = GMP-PCP = GTP on phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown, GTP gamma S greater than GMP-PNP greater than GMP-PCP = GTP on 45Ca2+ efflux, and GTP GMP-PNP = GMP-PCP = GTP gamma S on [1-14C]arachidonate efflux. Based on these data, the contribution of G proteins to stimulus-secretion coupling beyond the transduction of receptor signal is considered.

Cytidylate cyclase was demonstrated to be distributed in various tissues of rat, with the highest activity in brain, and it was shown to be a membrane-bound type enzyme. The enzyme was effectively dispersed from the membrane fraction of rat brain with 0.3% (w/v) Triton X-100. The dispersed cytidylate cyclase had an optimal pH of 9.4 and the activity at a physiological pH of 7.5 was less than 20% of the maximum value. This enzyme completely lost its activity in the absence of divalent cation such as Mn2+ and Mg2+. The Km value for CTP was calculated as 0.0156 mM, by Lineweaver-Burk analysis. It was also found that activity of dispersed enzyme was inhibited by ATP, but not GTP. Both forskolin and lanthanum chloride, which affect adenylate cyclase, showed no effect on cytidylate cyclase. These results indicate that cytidylate cyclase is a unique membrane-bound enzyme distinct from purine nucleotide cyclases, adenylate cyclase and guanylate cyclase.

In freshly collagenase-isolated rat pancreatic islets and in islets cultured for 72 hours, the effects of thiol reagents on glucagon (5 micrograms/ml) and/or glucose (16.7 mM)-mediated increases in cAMP formation as well as on clonidine (10 microM)-induced inhibition of these actions were studied. In freshly isolated islets and to a more pronounced degree in islets cultured for 72 hours glucagon (5 micrograms/ml) increased the cAMP content above the basal value. Clonidine (0.1-100 microM) had no significant effect on the basal cAMP formation, but inhibited the glucagon-mediated effect. The thiol reagents diamide (10-100 microM) and NEM affected neither the basal nor the glucagon-mediated effect, but abolished the inhibitory action of clonidine on cAMP formation. In freshly isolated islets, high glucose concentrations (8.3-16.7 mM) increased the cAMP formation. Diamide (100 microM) and NEM (100 microM) attenuated the stimulatory effect of 16.7 mM glucose. It is suggested that these selective effects of the thiol reagents on glucagon-mediated increase in cAMP formation in the presence of substimulatory concentration of glucose may be due to the differences in the sensitivity of the sulfhydryl groups of the G-proteins to thiol reagents i.e. Gi or proteins closely related to Gi being more sensitive than Gs. The data further suggest that glucose acts on the cAMP cascade at a step distinct from Rs. Since both glucose and glucagon effects were influenced by the addition of clonidine, it is possible to interpret the data as indicating that the effects of both stimulators eventually converge at some common step in the adenylate cyclase cascade.

The stable GTP analog, guanosine 5'-(3-O-thiotriphosphate), GTP gamma S, stimulated both inositol trisphosphate (InsP3) and choline generation by NIH 3T3 cell membranes. Choline generation was stimulated by GTP gamma S over the dose range for activation of GTP-binding proteins. Membranes from control and c-Ha-ras- or c-Ha-ras(61 leu)-transformed cells did not differ in the extent to which GTP gamma S stimulated InsP3 or choline formation despite 5-10 fold over expression of Ras in the transformed cells. Unlike GTP gamma S, GTP did not stimulate phospholipid hydrolysis, even in membranes from cells expressing Ras61leu, a mutant protein having reduced GTPase activity. Thus there is G protein regulation of both phosphatidylcholine-specific phospholipase D and polyphosphoinositide-specific phospholipase C in NIH 3T3 cell membranes. However, the lack of difference in GTP gamma S-stimulated phospholipid metabolism between control and ras-transformed cell membranes suggests that Ras does not function as the G protein(s) that directly regulate either phospholipase.

The impact of changes in the activation state of the low Km cAMP phosphodiesterase (PDE) on cAMP output in adipocyte membranes was assessed by measuring the product of cAMP synthesis and degradation in the membrane preparation simultaneously. Crude membranes were prepared from adipocytes treated with the cAMP analog, 8-pCl phi S-cAMP and from adipocytes treated with 2 nM insulin. Using membranes from control and treated cells, adenylate cyclase was activated with various concentrations of forskolin and cAMP production (synthesis minus degradation) was measured with and without complete PDE inhibition using the specific inhibitor CI-914. Half maximal inhibition of the low Km cAMP PDEs in control membranes was produced by 1.16 +/- 0.07 microM CI-914 and greater than 98% of the activity was inhibited by 100 microM CI-914. The I50 and the concentration of CI-914 producing complete PDE inhibition in membranes from 8-pCl phi S-cAMP or insulin-treated cells were identical to those seen in membranes from control cells. Treatment of adipocytes with 8-pCl phi S-cAMP or with insulin did not modify basal rates of cAMP synthesis or alter the ability of adenylate cyclase to be activated by forskolin. The impact of PDE activity on cAMP accumulation was relatively small in membranes from control cells, but treatment of adipocytes with 8-pCl phi S-cAMP or with insulin activated the low Km cAMP PDE and caused a marked decrease in cAMP accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

A new kind of bisbenzylisoquinoline compounds, daurisoline alkaloid derivatives, has been found to be very potent calmodulin antagonists. The fluorescence spectra of interaction between these derivatives and calmodulin have been studied. The experimental results showed that these derivatives could interact with calmodulin resulting in forming a complex and diminishing fluorescence intensity. The process was Ca2(+)-dependent. These derivatives can bind to calmodulin and result in conformational change of calmodulin. But the binding site of these derivatives on calmodulin may be different from that of trifluoperazine. These derivatives can not displace all Ca2+ on calmodulin like trifluoperazine can do. Their abilities of antagonizing calmodulin to stimulate calmodulin-dependent cyclic nucleotide phosphodiesterase and the affinities of binding to calmodulin were related to hydrophobicity of substituting groups in side chain of these derivatives. Increase in hydrophobicity of these substituting groups increased binding of the derivatives and generally increased the inhibition of calmodulin stimulation of calmodulin-dependent cyclic nucleotide phosphodiesterase.