Regulation of Ca2+ influx in myocardial cells by beta adrenergic receptors,cyclic nucleotides,and phosphorylation

Calcium channels in the heart play a major role in cardiac function. These channels are modulated in a variety of ways, including protein phosphorylation. Cyclic AMP-mediated phosphorylation is the best understood phosphorylation mechanism which regulates calcium influx into cardiac cells. Binding of an agonist (e.g., a catecholamine) to the appropriate receptor stimulates production of cyclic AMP by adenylate cyclase. The cyclic AMP may subsequently bind to and activate a cyclic AMP-dependent protein kinase, which then can phosphorylate a number of substrates, including the calcium channel (or a closely-associated regulatory protein). This results in stimulation of the calcium channels, greater calcium influx, and increased contractility. The cyclic AMP system is not the only protein kinase system in the heart. Thus, the possibility exists that other protein kinases may also regulate the calcium channels and, hence, cardiac function. Recent evidence suggests that cyclic GMP-mediated phosphorylation may play a role opposite to cyclic AMP-mediated phosphorylation, i.e., inhibition of the calcium current rather than stimulation. Other recent evidence also suggests that a calcium/calmodulin-dependent protein kinase and calcium/phospholipid-dependent protein kinase (protein kinase C) may also regulate the myocardial calcium channels. Thus, protein phosphorylation may be a general mechanism whereby calcium channels and cardiac function are modulated under a variety of conditions.     

Properties of protein kinase and adenylate cyclase-deficient variants of a macrophage-like cell line.

Stable variants of the macrophage-like cell line J774.2, defective in adenylate cyclase and protein kinase activities, were selected by cloning cells resistant to the growth-inhibitory effect of cholera toxin and 8-bromo-adenosine 3':5' cyclic monophosphoric acid (8 Br-cAMP), respectively. These variants were analyzed for their ability to respond to cyclic AMP-mediated enhancement of phagocytosis and cyclic AMP-mediated inhibition of plasminogen activator secretion and growtn. The adenylate cyclase variants were unaffected by cholera toxin but were sensitive to 8 Br-cAMP-mediated inhibition of plasminogen activator secretion and growth. One of these variants exhibited a defect in phagocytosis that could be corrected by 8 Br-cAMP. The protein kinase variants exhibited normal basal phagocytosis that could not be stimulated by either 8 Br-cAMP or cholera toxin; they were also insensitive to cyclic AMP-mediated inhibition of plasminogen activator secretion and growth. The studies demonstrate that the three effects of cyclic AMP in J774.2--inhibition of growth and plasminogen activator secretion, and enhancement of basal Fc-mediated phagocytosis--are mediated by a cyclic AMP-dependent portein kinase. The results support the usefulness of variants in cyclic nucleotide metabolism in understanding the regulation of differentiated cell function by cyclic AMP.     

Effect of parathyroid hormone and cyclic AMP on protein phosphorylation in rabbit kidney cortex.

Suspensions of renal cortical tubules were incubated with 33Pi and exposed to parathyroid hormone (40 mlg/ml) or 1 mM dibutyryl cyclic AMP. In other experiments homogenates of renal cortex were assayed for protein kinase and phosphoprotein phosphatase activity using [gamma-32P]ATP with or without 5 mM cyclic AMP. Proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and phosphorylation of proteins measured by liquid scintillation counting of gel slices. The pattern of protein phosphorylation was similar in control tissue from both tubule suspensions and homogenates. In intact tubules, parathyroid hormone stimulated the phosphorylation of four proteins with molecular weights of approx. 150 000, 125 000, 100 000 and 50 000 by 28%, 24%, 13%, and 20%, respectively. Results with dibutyryl cyclic AMP were comparable but more variable. Stimulation of phosphorylation by cyclic AMP in homogenates was more generalized with the major effect on a 50 000 dalton protein (50% stimulation). No effect of cyclic AMP on dephosphorylation of proteins was observed. The results are interpreted as indicating that increased phosphorylation of cell proteins is part of the cyclic AMP-mediated response of the renal cortex to parathyroid hormone.     

Cyclic AMP-receptor proteins in human salivary glands

In mammalian species, cyclic AMP receptor proteins (cARP) are the regulatory (R) subunits of cyclic AMP-dependent protein kinase (PKA), the cellular effector of cyclic AMP-mediated signal transduction. An isoform of the PKA type II R subunit (RII), cARP, is a polyfunctional protein, present in most tissues and cells. It is expressed in salivary and other glands of rodents, and secreted into the saliva of rats and Man. The aim of the present study was to determine the expression of cARP in human salivary glands using immunoelectron microscopy. Thin sections of normal salivary glands embedded in LR Gold resin were labeled with anti-cARP primary antibody, then with gold-conjugated secondary antibody. Labeling was present in the secretory granules and cytoplasm of parotid, submandibular (SMG) and sublingual gland serous cells. Quantitative analysis showed considerable variability in granule labeling from sample to sample, indicating shifts in expression and cellular location of cARP. Unlike rodent salivary glands, the granules of intercalated and striated duct cells also were labeled. The cytoplasm and granules of mucous cells of the SMG and sublingual glands were unlabeled, while the Golgi complex and filamentous bodies in these cells showed moderate reactivity. Mitochondria and nuclei of both serous and mucous cells were unlabeled. Labeling also was present in the connective tissue adjacent to the epithelial cells. The results indicate that serous cells of the parotid and SMG are the major source of salivary cARP. They also reveal significant species differences in the glandular distribution of RII. RII binds to cytoskeletal and nuclear proteins, and may function to regulate extracellular cyclic AMP levels. Thus, the tissue and cellular distribution of RII may serve as an index of regulation of gene expression and cell differentiation.     

Effect of parathyroid hormone and cyclic AMP on protein phosphorylation in rabbit kidney cortex

Suspensions of renal cortical tubules were incubated with ³³P_i and exposed to parathyroid hormone (40 μg/ml) or 1 mM dibutyryl cyclic AMP. In other experiments homogenates of renal cortex were assayed for protein kinase and phosphoprotein phosphatase activity using [γ-³²P]ATP with or without 5 mM cyclic AMP. Proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and phosphorylation of proteins measured by liquid scintillation counting of gel slices. The pattern of protein phosphorylation was similar in control tissue from both tubule suspensions and homogenates. In intact tubules, parathyroid hormone stimulated the phosphorylation of four proteins with molecular weights of approx. 1500 000, 125 000, 100 000 and 50 000 by 28%, 24%, 13%, and 20%, respectively. Results with dibutyryl cyclic AMP were comparable but more variable. Stimulation of phosphorylation by cyclic AMP in homogenates was more generalized with the major effect on a 50 000 dalton protein (50% stimulation). No effect of cyclic AMP on dephosphorylation of proteins was observed. The results are interpreted as indicating that increased phosphorylation of cell proteins is part of the cyclic AMP-mediated response of the renal cortex to parathyroid hormone.     

Characterization of two adenosine 3':5'-phosphate-dependent protein kinase species from Chinese hamster ovary cells.

Chinese hamster ovary cells exhibit several characteristic morphological and physiological responses upon treatment with agents which increase the intracellular level of adenosine 3':5'-phosphate (cyclic AMP). To better understand the mechanism of these cyclic AMP-mediated responses, we separated two cyclic AMP-dependent protein kinases (ATP:protein phosphotransferase, EC 2.7.1.37) (protein kinase I and protein kinase II) from the cytosol of Chinese hamster ovary cells by DEAE-cellulose chromatography and studied their properties. Protein kinase I is eluted at a lower salt concentration than protein kinase II and is stimulable to 10 times its basal catalytic activity, while protein kinase II is stimulable only 2-fold. Both kinases are completely dissociated by cyclic AMP and inhibited by specific cyclic AMP-dependent protein kinase inhibitor. They have similar Km values for magnesium (approximately 1 mM), cyclic AMP (approximately 60 nM), and ATP (approximately 0.1 mM), and the dissociation constant (Kdis) for cyclic AMP (approximately 13 nM) is the same for both enzymes. However, they appear to have different substrate preferences and cyclic AMP-binding properties in that cyclic AMP bound to protein kinase II exchanges readily with free cyclic AMP, while that bound to protein kinase I is not exchangeable. The native enzymes have different sedimentation coefficients (6.4 S for protein kinase I and 4.8 S for protein kinase II), whereas those of the activated enzymes are the same (2.9--3.0 S). It appears that the two cyclic AMP-dependent protein kinases which differ from each other in their regulatory subunits may play different roles in the mediation of cyclic AMP action in Chinese hamster ovary cells.     

Binding proteins for adenosine 3′:5′-cyclic monophosphate in bovine adrenal cortex

Five peaks of cyclic AMP-binding activity could be resolved by DEAE-cellulose chromatography of bovine adrenal-cortex cytosol. Two of the binding peaks co-chromatographed with the catalytic activities of cyclic AMP-dependent protein kinases (ATP-protein phosphotransferase, EC 2.7.1.37) of type I or type II respectively. A third binding protein was eluted between the two kinases, and appeared to be the free regulatory moiety of protein kinase I. Two of the binding proteins for cyclic AMP, sedimenting at 9S in sucrose gradients, could also bind adenosine. They bound cyclic AMP with an apparent equilibrium dissociation constant (K(d)) of about 0.1mum, and showed an increased binding capacity for cyclic AMP after preincubation in the presence of K(+), Mg(2+) and ATP. The two binding proteins differed in their apparent affinities for adenosine. The isolated regulatory moiety of protein kinase I had a very high affinity for cyclic AMP (K(d)<0.1nm). At low ionic strength or in the presence of MgATP, the high-affinity binding of cyclic AMP to the regulatory subunit of protein kinase I was decreased by the catalytic subunit. At high ionic strength and in the absence of MgATP the high-affinity binding to the regulatory subunit was not affected by the presence of catalytic subunit. Under all experimental conditions tested, dissociation of protein kinase I was accompanied by an increased affinity for cyclic AMP. To gain some insight into the mechanism by which cyclic AMP activates protein kinase, the interaction between basic proteins, salt and the cyclic nucleotide in activating the kinase was studied.     

Tryptophan Hydroxylase Is Phosphorylated by Protein Kinase A

Abstract: The effect of protein kinase A on the catalytic activity and phosphorylation of brain tryptophan hydroxylase was examined. Stimulation of endogenous protein kinase A by cyclic AMP or its analogues, dibutyryl-cyclic AMP and 8-thiomethyl-cyclic AMP, failed to activate tryptophan hydroxylase. The activation of tryptophan hydroxylase by calcium/calmodulin-phosphorylating conditions was not modified by cyclic AMP. Endogenous protein kinase A phosphorylated a large number of proteins and tryptophan hydroxylase could be identified as one substrate by sucrose gradient centrifugation, immunoprecipitation, and immunoblotting. These results indicate that tryptophan hydroxylase is phosphorylated by protein kinase A in brain and question whether this protein kinase exerts direct regulatory influence over tryptophan hydroxylase activity via phosphorylation.     

Regulation of protooncogenes c-fos and c-myc expressions by protein tyrosine kinase, protein kinase C, and cyclic AMP mitogenic pathways in dog primary thyrocytes: a positive and negative control by cyclic AMP on c-myc expression

The proliferation of dog thyrocytes in primary culture is stimulated by three distinct intracellular signaling pathways: (1) the thyrotropin or forskolin-cyclic AMP-mediated cascade which is compatible with the differentiated state of the cell; (2) the protein kinase C pathway activated by diacylglycerol and phorbol esters; and (3) a protein tyrosine kinase system activated by epidermal growth factor. The two latter pathways also induce dedifferentiation. The activation of the three cascades induced the expression of the protooncogenes c-fos and c-myc with dose-response curves similar to those for DNA synthesis. After TPA and EGF, the time courses of stimulation of c-fos and c-myc were the same as those for mitogenically stimulated fibroblasts. However, after the cyclic AMP stimulation, c-myc expression was biphasic with an enhancement at 1 h followed by a down-regulation. A similar inhibition by cyclic AMP was also observed on the increased c-myc expression induced by EGF. This down-regulation is suppressed by cycloheximide, which suggests the involvement of a neosynthesized or a labile protein intermediate. The action of cyclic AMP on c-myc mRNA levels could be related to the opposite requirements of the stimulation of both proliferation and differentiation expression by the cyclic AMP pathway in the differentiated thyrocytes.     

Nuclear events during early chondrogenesis: phosphorylation of the precartilage 35.5-kDa domain-specific chromatin protein and its regulation by cyclic AMP

During chondrogenesis in vivo and in vitro, a family of nonhistone proteins (Mr 35,500), designated PCP 35.5, is lost from the nuclei of precartilage mesenchyme cells. A basic subcomponent of this family, designated PCP 35.5b, is phosphorylated during the first few hours of chondrogenesis in vitro by a phosphorylating system whose activity is enhanced 12- to 15-fold by exposure of differentiating precartilage cells to dibutyryl cyclic AMP. This phosphorylating system is present in isolated precartilage cell nuclei, where it retains its dependence on cyclic AMP and its specificity for PCP 35.5b. Assays for nuclear cyclic AMP inhibitable protein phosphatase activity capable of dephosphorylating PCP 35.5b were negative, indicating that the system responsible for phosphorylating this protein is a cyclic AMP-dependent protein kinase. Chromatin fractionation studies indicate that PCP 35.5b is localized at sites previously shown to be closely associated with DNase I-sensitive domains of precartilage cell chromatin. These studies define PCP 35.5b as a strategically located component of precartilage cell chromatin which is the major or sole chromatin target of cyclic AMP-dependent phosphorylation during chondrogenesis. This chromatin modification occurs prior to overt cartilage differentiation and may therefore play a regulatory role in the acquisition of the cartilage cell phenotype.     

Adenosine 3'':5''-cyclic monophosphate-binding proteins in bovine and rat tissues.

1. At least two classes of high-affinity cyclic AMP-binding proteins have been identified: those derived from cyclic AMP-dependent protein kinases (regulatory subunits) and those that bind a wide range of adenine analogues (adenine analogue-binding proteins). 2. In fresh-tissue extracts, regulatory subunits could be further subdivided into 'type I or 'type II' depending on whether they were derived from 'type I' or 'type II' protein kinase [see Corbin et al. (1975) J. Biol. Chem. 250, 218-225]. 3. The adenine analogue-binding protein was detected in crude tissue supernatant fractions of bovine and rat liver. It differed from the regulatory subunit of cyclic AMP-dependent protein kinase in many of its properties. Under the conditions of assay used, the protein accounted for about 45% of the binding of cyclic AMP to bovine liver supernatants. 4. The adenine analogue-binding protein from bovine liver was partially purified by DEAE-cellulose and Sepharose 6B chromatography. It had mol.wt. 185000 and was trypsin-sensitive. As shown by competition and direct binding experiments, it bound adenosine and AMP in addition to cyclic AMP. At intracellular concentrations of adenine nucleotides, binding of cyclic AMP was essentially completely inhibited in vitro. Adenosine binding was inhibited by only 30% under similar conditions. 5. Rat tissues were examined for the presence of the adenine analogue-binding protein, and, of those examined (adipose tissue, heart, brain, testis, kidney and liver), significant amounts were only found in the liver. The possible physiological role of the adenine analogue-binding protein is discussed. 6. Because the adenine analogue-binding protein or other cyclic AMP-binding proteins in tissues may be products of partial proteolysis of the regulatory subunit of cyclic AMP-dependent protein kinase, the effects of trypsin and aging on partially purified protein kinase and its regulatory subunit from bovine liver were investigated. In all studies, the effects of trypsin and aging were similar. 7. In fresh preparations, the cyclic AMP-dependent protein kinase had mol.wt. 150000. Trypsin treatment converted it into a form of mol.wt 79500. 8. The regulatory subunit of the protein kinase had mol.wt. 87000. It would reassociate with and inhibit the catalytic subunit of the enzyme. Trypsin treatment of the regulatory subunit produced a species of mol.wt. 35500 which bound cyclic AMP but did not reassociate with the catalytic subunit. Trypsin treatment of the protein kinase and dissociation of the product by cyclic AMP produced a regulatory subunit of mol.wt. 46500 which reassociated with the catalytic subunit. 9. These results may be explained by at least two trypsin-sensitive sites on the regulatory subunit. A model for the effects of trypsin is described.     

Reversible in activation of purified (Na+ + K+)-ATPase from human renal tissue by cyclic AMP-dependent protein kinase.

Human renal (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) preparations which exhibited a non-linear reaction rate, contained high levels of membrane-bound cyclic AMP-dependent protein kinase, while this latter activity was much less or absent in purified preparations. A non-linear reaction rate was observed in a purified preparation of (Na+ + K+)-ATPase by reconstituting the enzyme into lipid vesicles with cyclic AMP-dependent protein kinase. The addition of cyclic AMP to the ATPase assay of these lipid vesicles inactivated the (Na+ + K+)-ATPase. The cytoplasmic fraction of the cell contained a nondialyzable factor, which prevented (or reversed) the cyclic AMP-mediated inactivation of the enzyme.     

The cardiac sarcoplasmic reticulum - glycogenolytic complex, an internal beta adrenergic receptor.

Agents which stimulate the formation of cyclic AMP are well known to cause positive inotropic effects on the heart. This review suggests that the myocardial cell is multicompartmented and has several effector sites for cyclic AMP-mediated events and therefore several possible receptor sites for cyclic AMP-mediated agents. It is possible that some of the complex relationships between cyclic AMP-mediated agents and cardiac physiologic functions result from differences among agonist receptors and enzymes in the intracellular compartments. Examination of the characteristics of intracellular drug receptors may yield a new frontier for cardiac pharmacology.     

Periodic stimuli are more successful than randomly spaced ones for inducing development inDictyostelium discoideum

Aggregation in the cellular slime moldDictyostelium discoideum is due to chemotaxis. The chemoattractant, cyclic AMP, is synthesised and released periodically by the cells. Externally applied periodic pulses of cyclic AMP can also induce differentiation in this organism. The present work examines the role of periodicityper se in cyclic AMP-mediated stimulation of cell differentiation. For this purpose we use Agip53, aDictyostelium mutant which does not develop beyond the vegetative state but can be made to aggregate and differentiate by reiterated applications of cyclic AMP. Importantly, Agip53 cells do not make or release any cyclic AMP themselves even in response to an increase in extracellular cyclic AMP. A comparison of the relative efficiencies of periodic and aperiodic stimulation shows that whereas the two patterns of stimulation are equally effective in inducing the formation of EDTA-stable cell contacts, periodic stimuli are significantly superior for inducing terminal differentiation. This suggests that there must be molecular pathways which can only function when stimulation occurs at regular intervals.     

Regulation of protein phosphorylation in pancreatic acini by cyclic AMP-mediated secretagogues: interaction with carbamylcholine

The effects on protein phosphorylation in mouse pancreatic acini of cyclic AMP-mediated secretagogues and the Ca2+-mediated agonist carbamylcholine were compared. Under the conditions adopted for the study of protein phosphorylation, carbamylcholine (3 microM) stimulated amylase release from pancreatic acini 6-fold, whereas vasoactive intestinal polypeptide (VIP) (100 nM) and the cyclic AMP analogue 8-bromo-cyclic AMP (1 mM) caused little or no increase in secretion. However, VIP and 8-bromo-cyclic AMP, when added in combination with carbamylcholine, potentiated the stimulation of amylase release to 170-180% of that caused by carbamylcholine alone. As assessed by two-dimensional gel electrophoresis, VIP reproduced four of the ten changes in protein phosphorylation elicited by carbamylcholine, these changes being the increased phosphorylation of one soluble protein and the decreased phosphorylation of three soluble proteins. VIP enhanced the carbamylcholine-induced changes in phosphorylation for three proteins. In addition, VIP increased the phosphorylation of a unique protein of Mr 52,000 and pI 5.66 which was not affected by carbamylcholine. All of the effects on protein phosphorylation exerted by VIP in the presence or absence of carbamylcholine were mimicked by 8-bromo-cyclic AMP. Secretin also reproduced most of the changes in protein phosphorylation caused by VIP, although concentrations of secretin of at least 100-fold higher were required to elicit a maximal response. It is concluded that cyclic AMP-mediated secretagogues alter the phosphorylation of a unique protein as well as of several pancreatic proteins affected by carbamylcholine. Moreover, these effects appear to be mediated primarily by VIP-preferring receptors and may be involved in the synergistic action of VIP to promote carbamylcholine-induced amylase release.     

The effects of triethyltin bromide on red cell and brain cyclic AMP-dependent protein kinases

Triethyltin bromide activates the cyclic AMP-dependent protein kinases of human red cell membranes and of bovine brain. Additions of 25-500 microM triethyltin to red cell ghosts resulted in enhanced phosphorylation of ghost proteins. When added to partially purified cyclic AMP-dependent protein kinases from red cell ghosts or bovine brain, stimulation of the phosphorylation of calf thymus histone was observed. The enhancement of kinase activity was due to release of catalytic subunits from the intact protein kinase. Brief exposure of the partially purified enzymes to triethyltin, followed by DE52 chromatography, resulted in elution profiles for regulatory and catalytic subunits that were similar to the profile resulting after cyclic AMP activation. Triethyltin interacts with both regulatory and catalytic subunits. When it was added to the partially purified cyclic AMP-dependent protein kinases from human red cell ghosts or bovine brain, noncompetitive inhibition of cyclic AMP binding to the regulatory subunit of the enzyme was observed. It interacted with the catalytic subunit to produce slow inhibition of catalytic activity. The inhibition was non-competitive with respect to both histone and ATP. When intact red cells were subjected to brief exposure with triethyltin, enhanced phosphorylation of certain membrane proteins occurred, suggesting that the activation of the cyclic AMP protein kinases by triethyltin may be physiologically significant.     

PGE1-independent MDCK cells have elevated intracellular cyclic AMP but retain the growth stimulatory effects of glucagon and epidermal growth factor in serum-free medium

Prostaglandin E1 (PGE1), a component in the hormone-supplemented, serum-free medium for the Madin Darby canine kidney (MDCK) cell line, has been proposed to increase MDCK cell growth by increasing intracellular cyclic AMP levels. The association between increased intracellular cyclic AMP and the growth stimulatory effect of PGE1 has been examined in normal MDCK cells and in PGE1-independent variants of MDCK. These variant cells have lost the PGE1 requirement for long term growth in defined medium. Normal MDCK cells had almost twofold higher intracellular cyclic AMP levels during growth in Medium K-1 (9.0 pmol/mg protein) than in Medium K-1 minus PGE1. Furthermore, PGE1-independent clone 1 had higher intracellular cyclic AMP levels in Medium K-1 minus PGE1 than normal MDCK cells in Medium K-1. This latter observation suggests that the PGE1 requirement for MDCK cell growth is associated with the low intracellular cyclic AMP levels of this cell line. An involvement of cyclic AMP in the growth response to PGE1 is supported by these observations, as well as by the growth stimulatory effects of other agents that affect cyclic AMP metabolism in MDCK cells. These agents include glucagon, isobutyl methylxanthine (IBMX), and dibutyryl cyclic AMP. The growth of PGE1-independent clone 1 was inhibited rather than stimulated by PGE1. Similarly, PGE1-independent cell growth was inhibited by IBMX and dibutyryl cyclic AMP. However, the growth response to one agent which increases cyclic AMP (glucagon) was retained in PGE1-independent clone 1. This result suggests that the effect of glucagon is not associated with increases in intracellular cyclic AMP. The growth stimulatory effect of epidermal growth factor (EGF) on normal MDCK cells was also studied. Although EGF does not act via a cyclic AMP-mediated mechanism, EGF increased normal MDCK cell growth and substituted for PGE1 in Medium K-1. Thus, EGF and PGE1 could possibly affect similar growth-related functions in MDCK cells, although by different pathways. This possibility was examined further, using PGE1-independent clone 1. EGF, like glucagon, was still growth stimulatory to the PGE1-independent cells. Consequently, the biochemical pathways by which EGF and PGE1 increase MDCK cell growth probably do not converge.     

Species Specificity of Brain Cyclic AMP Receptor Proteins

The cyclic AMP binding proteins present in mouse, rat, bovine, and sheep brains were compared. Extracts were isotopically labeled with 8-azido-cyclic [32P]AMP, a photoaffinity analog specific for cyclic AMP binding sites, and then subjected to two-dimensional gel electrophoresis. The resulting autoradiographic patterns were generally similar, but showed consistent species variations. Proteins identified by their size and phosphorylatability as regulatory subunits of Type II protein kinase isozymes were present in all species, but with slight variations in pI. A series of charge variants identified as regulatory subunits of Type I kinase isozymes on the basis of their size was also ubiquitously present, as were several smaller proteins postulated to be proteolytic fragments derived from the regulatory subunits. The major species difference was a series of labeled proteins found only in rodent brains and not in the brains of any ruminant, or in other rodent tissues. These proteins had a molecular weight of 54,000 and a pI range of 5.89--6.26, and could not be endogenously phosphorylated. The identities of these proteins and their relationship to the protein kinase regulatory subunits are unknown.     

Cyclic AMP agonists induce the phosphorylation of phospholipase C-tau and of a 76 kDa protein co-precipitated by anti-(phospholipase C-tau) monoclonal antibodies in BALB/c-3T3 cells. Relationship to inositol phosphate formation.

Previous studies have demonstrated enhanced phosphorylation of phospholipase C-tau (PLC-tau), a key regulatory enzyme in phosphoinositide metabolism, in cells treated with platelet-derived growth factor (PDGF) and epidermal growth factor, both of which act via specific receptor tyrosine kinases. Our studies on BALB/c-3T3 cells show that agents that promote cellular cyclic AMP accumulation also increase the phosphorylation, specifically the serine phosphorylation, of this enzyme. Increased phosphorylation of PLC-t (2-3-fold) was evident within 5-10 min of addition of isobutylmethylxanthine (IBMX) and either cholera toxin or forskolin to cells, and persisted for at least 3 h. Treatment of cells with cyclic AMP agonists also enhanced, with similar kinetics, the phosphorylation of a 76 kDa protein co-precipitated by anti-PLC-tau monoclonal antibodies. Brief exposure of cells to cholera toxin/IBMX or forskolin/IBMX decreased inositol phosphate formation induced by the GTP-binding protein (G-protein) activator aluminium fluoride by approx. 50%, but was without effect on PDGF-stimulated inositol phosphate formation. These findings suggest that PLC-tau, and perhaps the 76 kDa co-precipitated protein, are substrates of cyclic AMP-dependent protein kinase in BALB/c-3T3 cells: however, the lack of effect of cyclic AMP elevation on PDGF-stimulated inositol phosphate formation indicates that the intrinsic activity of PLC-tau is unaltered by cyclic AMP-mediated phosphorylation.