The insulin-stimulated cyclic AMP phosphodiesterase binds to a single class of protein sites on the liver plasma membrane

Determination of the structure of the N-terminal splice region of the cyclic AMP-specific phosphodiesterase RD1 (RNPDE4A1) by 1H NMR and identification of the membrane association domain using chimeric constructs

A 25-residue peptide representing the membrane targeting N-terminal splice region of the cyclic AMP phosphodiesterase RD1 (RNPDE4A1) was synthesized, and its structure was determined by 1H NMR. Two independently folding helical regions were identified, separated by a highly mobile "hinge" region. The first helical region was formed by an N-terminal amphipathic alpha-helix, and the second consisted of multiple overlapping turns and contained a distinct compact, hydrophobic, tryptophan-rich domain (residues 14-20). Chimeric molecules, formed between the N-terminal region of RD1 and the soluble bacterial protein chloramphenicol acetyltransferase, were used in an in vitro system to determine the features within the splice region that were required for membrane association. The ability of RD1-chloramphenicol acetyltransferase chimera to become membrane-associated was not affected by deletion of any of the following regions: the apolar section (residues 2-7) of the first helical region, the polar part of this region together with the hinge region (residues 8-13), or the polar end of the C-terminal helical region (residues 21-25). In marked contrast, deletion of the compact, hydrophobic tryptophan-rich domain (residues 14-20) found in the second helical region obliterated membrane association. Replacement of this domain with a hydrophobic cassette of seven alanine residues also abolished membrane association, indicating that membrane-association occurred by virtue of specific hydrophobic interactions with residues within the compact, tryptophan-rich domain. The structure of this domain is well defined in the peptide, and although the region is helical, both the backbone and the distribution of side chains are somewhat distorted as compared with an ideal alpha-helix. Hydrophobic interactions, such as the "stacked" rings of residues Pro14 and Trp15, stabilize this domain with the side chain of residue Leu16 adopting a central position, interacting with the side chains of all three tryptophan residues 15, 19, and 20. These bulky side chains thus form a hydrophobic cluster. In contrast, the side chain of residue Val17 is relatively exposed, pointing out from the opposite "face" of the peptide. Although it appears that this compact, tryptophan-rich domain is responsible for membrane association, at present the target site and hence the specific interactions involved in membrane targeting by the RD1 splice region remain unidentified.

Engineered deletion of the unique N-terminal domain of the cyclic AMP-specific phosphodiesterase RD1 prevents plasma membrane association and the attainment of enhanced thermostability without altering its sensitivity to inhibition by rolipram

Full-length cDNA for the rat brain rolipram-sensitive cyclic AMP phosphodiesterase (PDE), RD1 was introduced into the expression vector pSVL. COS cells transfected with the recombinant vector pSVL-RD1 exhibited a 30-55% increase in homogenate PDE activity, which was abolished by rolipram (10 microM). Removal of the first 67 nucleotides of the RD1 cDNA yielded a truncated enzyme called Met26-RD1 which lacked the N-terminal first 25 amino acids. Whereas approx. 75% of RD1 activity was membrane-associated, Met26-RD1 activity was found exclusively in the cytosol fraction. Expression of RD1 nearly doubled membrane-associated PDE activity, while expression of Met26-RD1 increased cytosolic activity by approx. 30%. Membrane RD1 activity was found to be primarily associated with the plasma membrane, was not released by either high concentrations of NaCl or by a 'hypotonic shock' treatment, but was solubilized with low concentrations of Triton X-100. Phase separation of membrane components with Triton X-114 showed partition of RD1 into both the aqueous and detergent-rich phases, whereas Met26-RD1 partitioned exclusively into the aqueous phase. Both RD1 and Met26-RD1 specifically hydrolysed cyclic AMP; were unaffected by either Ca2+/calmodulin or by low cyclic GMP concentrations; exhibited linear Lineweaver-Burke plots with similar Km values for cyclic AMP (4 microM); both were potently and similarly inhibited by rolipram (Ki approx. 0.5 microM) and were similarly inhibited by cilostamide and 3-isobutyl-1-methylxanthine. Thermal inactivation, at 50 degrees C, showed that while the cytosolic-located fraction of RD1 (t0.5 approx. 3 min) and Met26-RD1 (t0.5 approx 3 min) were similarly thermolabile, membrane-bound RD1 was considerably more thermostable (t0.5 approx. 11 min). Treatment of both cytosolic RD1 and Met26-RD1 with Triton X-100 did not affect their thermostability, but solubilization of membrane RD1 activity with Triton X-100 markedly decreased its thermostability (t0.5 approx. 5 min). The N-terminal domain of RD1 appears not to influence either the substrate specificity or inhibitor sensitivity of this enzyme, but it does contain information which can allow RD1 to become plasma membrane-associated and thereby adopt a conformation which has enhanced thermostability.

Identification and selective inhibition of four distinct soluble forms of cyclic nucleotide phosphodiesterase activity from kidney

Homogenization of rat kidney under isotonic conditions and in the presence of protease inhibitors showed that some 92% of the cyclic AMP phosphodiesterase activity and some 83% of the cyclic GMP phosphodiesterase activity was released into the soluble fraction. Analysis of soluble phosphodiesterase activity by FPLC on a Mono-Q column resolved four distinct fractions expressing cyclic nucleotide phosphodiesterase activity. Lineweaver-Burk plots for the hydrolysis of both cyclic GMP and cyclic AMP yielded linear results. The first two peaks (KPDE-MQ-II, KPDE-MQ-III) showed higher activities towards cyclic GMP than cyclic AMP with the ratio of their Vmax values for the hydrolysis of cyclic AMP/cyclic GMP being 0.66 and 0.16, respectively. For the second two peaks (KPDE-MQ-IV, KPDE-MQ-V) the Vmax ratios for the hydrolysis of cyclic AMP/cyclic GMP were 6.4 and 16.7, respectively. All enzymes exhibited similar low Km values for both cyclic AMP and cyclic GMP but had very different Vmax values. KPDE-MQ-II was activated by Ca2+/calmodulin. The cyclic AMP phosphodiesterase activity of KPDE-MQ-III was augmented by the presence of low concentrations of cyclic GMP. Thermal denaturation studies showed that the phosphodiesterase activity of each fraction decayed as a single exponential indicating that each phosphodiesterase fraction contained but a single phosphodiesterase activity. The inhibitors IBMX, zaprinast, milrinone, amrinone, buquineran, carbazeran, ICI 118233, ICI 63197 exerted selective effects on the activities of these enzymes. We compared the action of these compounds on cyclic GMP phosphodiesterases from bovine retina. Over the concentration ranges used, the bovine retinal enzyme was only inhibited by IBMX, zaprinast and carbazeran. The cytosolic isoenzymes of cyclic AMP phosphodiesterases play a much more important role in metabolizing cyclic AMP in kidney compared with liver, where the activity of membrane-bound isoenzymes predominate.

Insulin alters the target size of the peripheral cyclic AMP phosphodiesterase but not the integral cyclic GMP-stimulated cyclic AMP phosphodiesterase in liver plasma membranes

Radiation inactivation of the two high affinity cyclic AMP phosphodiesterases (PDE) found in liver plasma membranes afforded an estimation of their molecular target sizes in situ. The activity of the peripheral plasma membrane PDE decayed as a single exponential with a target size corresponding to a monomer of circa 54 kDa. The integral, cyclic GMP-stimulated PDE decayed as a dimer of circa 125 kDa. Preincubation of plasma membranes with insulin (10nM), prior to irradiation, caused the target size of only the peripheral plasma membrane PDE to increase. We suggest that insulin addition causes the peripheral plasma membrane PDE to alter its coupling to an integral plasma membrane protein with a target size of circa 90 kDa.

Insulin stimulates the tyrosyl phosphorylation and activation of the 52 kDa peripheral plasma-membrane cyclic AMP phosphodiesterase in intact hepatocytes

The 52 kDa subunit of the peripheral-plasma-membrane insulin-stimulated high-affinity cyclic AMP phosphodiesterase can be specifically detected by the antibody PM1 by Western-blotting procedures and also can be immunoprecipitated from a hepatocyte extract. PM1-mediated immunoprecipitation from hepatocyte extracts showed that insulin treatment of intact 32P-labelled hepatocytes caused the rapid phosphorylation of the peripheral-plasma-membrane cyclic AMP phosphodiesterase. Phosphoamino acid analysis and the use of a phosphotyrosine-specific antibody indicated that phosphorylation occurred on tyrosyl residue(s) of this phosphodiesterase. Prior treatment of hepatocytes with glucagon (10 nM) completely blocked the insulin-mediated tyrosyl phosphorylation of this 52 kDa protein, as detected with both the PM1 and the anti-phosphotyrosine antibodies. Treatment of hepatocytes with glucagon alone did not increase the phosphorylation state of the peripheral-plasma-membrane cyclic AMP phosphodiesterase. The specific anti-phosphotyrosine antibody also detected the insulin-stimulated phosphorylation of proteins of 180 kDa, 95 kDa and 39 kDa. Prior treatment of hepatocytes with glucagon decreased the ability of insulin to phosphorylate the 180 kDa and 39 kDa species, but not the 95 kDa species.

Mechanisms of hormonal regulation of hepatic glucose metabolism

Acute hormonal regulation of liver carbohydrate metabolism mainly involves changes in the cytosolic levels of cAMP and Ca2+. Epinephrine, acting through beta 2-adrenergic receptors, and glucagon activate adenylate cyclase in the liver plasma membrane through a mechanism involving a guanine nucleotide-binding protein that is stimulatory to the enzyme. The resulting accumulation of cAMP leads to activation of cAMP-dependent protein kinase, which, in turn, phosphorylates many intracellular enzymes involved in the regulation of glycogen metabolism, gluconeogenesis, and glycolysis. These are (1) phosphorylase b kinase, which is activated and, in turn, phosphorylates and activates phosphorylase, the rate-limiting enzyme for glycogen breakdown; (2) glycogen synthase, which is inactivated and is rate-controlling for glycogen synthesis; (3) pyruvate kinase, which is inactivated and is an important regulatory enzyme for glycolysis; and (4) the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase bifunctional enzyme, phosphorylation of which leads to decreased formation of fructose 2,6-P2, which is an activator of 6-phosphofructo-1-kinase and an inhibitor of fructose 1,6-bisphosphatase, both of which are important regulatory enzymes for glycolysis and gluconeogenesis. In addition to rapid effects of glucagon and beta-adrenergic agonists to increase hepatic glucose output by stimulating glycogenolysis and gluconeogenesis and inhibiting glycogen synthesis and glycolysis, these agents produce longer-term stimulatory effects on gluconeogenesis through altered synthesis of certain enzymes of gluconeogenesis/glycolysis and amino acid metabolism. For example, P-enolpyruvate carboxykinase is induced through an effect at the level of transcription mediated by cAMP-dependent protein kinase. Tyrosine amino-transferase, serine dehydratase, tryptophan oxygenase, and glucokinase are also regulated by cAMP, in part at the level of specific messenger RNA synthesis. The sympathetic nervous system and its neurohumoral agonists epinephrine and norepinephrine also rapidly alter hepatic glycogen metabolism and gluconeogenesis acting through alpha 1-adrenergic receptors. The primary response to these agonists is the phosphodiesterase-mediated breakdown of the plasma membrane polyphosphoinositide phosphatidylinositol 4,5-P2 to inositol 1,4,5-P3 and 1,2-diacylglycerol. This involves a guanine nucleotide-binding protein that is different from those involved in the regulation of adenylate cyclase. Inositol 1,4,5-P3 acts as an intracellular messenger for Ca2+ mobilization by releasing Ca2+ from the endoplasmic reticulum.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute in vivo stimulation of low-Km cyclic AMP phosphodiesterase activity by insulin in rat-liver Golgi fractions

A low-Km phosphodiesterase activity, which is acutely stimulated by insulin in vivo, has been identified in plasma membranes and Golgi fractions prepared from rat liver homogenates in isotonic sucrose. Within seconds after insulin injection (25 micrograms/100 g body weight) cAMP phosphodiesterase activity increases by 30-60% in Golgi fractions and by 25% in plasma membranes; activity in crude particulate and microsomal fractions is unaffected. The increase in activity is short-lived in the light and intermediate Golgi fractions, but persists for at least 10 min in the heavy Golgi fraction. It precedes the translocation of insulin and insulin receptors to these fractions, which is maximal at 5 min. The doses of insulin required for half-maximal and maximal activation are, respectively, 7.5 micrograms/100 g and 25 micrograms/100 g body weight. Golgi-associated cAMP phosphodiesterase activity shows non-linear kinetics; a high-affinity component (Vmax, 13 pmol min-1 mg protein-1; Km, 0.35 microM) is detectable. Insulin treatment increases the Vmax 60-70%, but does not affect the Km. Unlike the low-Km cAMP phosphodiesterase associated with crude particulate fractions, the Golgi-associated activity is not easily extractable by solutions of low or high ionic strength. On analytical sucrose density gradients, low-Km cAMP phosphodiesterase associated with the total particulate fraction equilibrates at lower densities than endoplasmic reticulum and lysosomal markers, but at a higher densities than plasma membrane, Golgi markers and insulin receptors. Insulin treatment increases the specific activity of the enzyme by 20-60% at densities below 1.12 g cm-3, and by 20-40% in the density interval 1.23-1.25 g cm-3. Such treatment also causes a slight, but significant shift in the distribution of phosphodiesterase towards lower densities. It is suggested that Golgi elements or physically similar subcellular structures are a major site of localization of insulin-sensitive cAMP phosphodiesterase in rat liver. However, internalization of the insulin-receptor complex is probably not required for enzyme activation.

Interactions of insulin, glucagon and dexamethasone in controlling the activity of glycerol phosphate acyltransferase and the activity and subcellular distribution of phosphatidate phosphohydrolase in cultured rat hepatocytes

Rat hepatocytes were incubated in monolayer culture for 8 h. Glucagon (10nM) increased the total phosphatidate phosphohydrolase activity by 1.7-fold. This effect was abolished by adding cycloheximide, actinomycin D or 500 pM-insulin to the incubations. The glucagon-induced increase was synergistic with that produced by an optimum concentration of 100 nM-dexamethasone. Theophylline (1mM) potentiated the effect of glucagon, but it did not affect the dexamethasone-induced increase in the phosphohydrolase activity. The relative proportion of the phosphohydrolase activity associated with membranes was decreased by glucagon when 0.15 mM-oleate was added 15 min before the end of the incubations to translocate the phosphohydrolase from the cytosol. This glucagon effect was not seen at 0.5 mM-oleate. Since glucagon also increased the total phosphohydrolase activity, the membrane-associated activity was maintained at 0.15 mM-oleate and was increased at 0.5 mM-oleate. This activity at both oleate concentrations was also increased in incubations that contained dexamethasone, particularly in the presence of glucagon. Insulin increased the relative proportion of phosphatidate phosphohydrolase that was associated with membranes at 0.15 mM-oleate, but not at 0.5 mM-oleate. It also decreased the absolute phosphohydrolase activity on the membranes at both oleate concentrations in incubations that also contained glucagon and dexamethasone. None of the hormonal combinations significantly altered the total glycerol phosphate acyltransferase activity. However, glucagon significantly increased the microsomal activities, and insulin had the opposite effect. Glucagon also decreased the mitochondrial acyltransferase activity. There was a highly significant correlation between the total phosphatidate phosphohydrolase activity and the synthesis of neutral lipids from glycerol phosphate and 0.5 mM-oleate in homogenates of cells from all of the hormonal combinations. Phosphatidate phosphohydrolase activity is increased in the long term by glucocorticoids and also by glucagon through cyclic AMP. In the short term, glucagon increases the concentration of fatty acid required to translocate the cytosolic reservoir of activity to the membranes on which phosphatidate is synthesized. Insulin opposes the combined actions of glucagon and glucocorticoids. The long-term events explain the large increases in the phosphohydrolase activity that occur in vivo in a variety of stress conditions. The expression of this activity depends on increases in the net availability of fatty acids and their CoA esters in the liver.

Insulin-stimulated high affinity cyclic AMP phosphodiesterase in rat mammary acini

High affinity cyclic AMP phosphodiesterase activity in preparations of acini isolated from mammary tissue of lactating rats is shown to be stimulated by the addition of physiological concentrations of insulin to incubations of acini in vitro. This effect is expressed specifically on membrane-associated phosphodiesterase and occurs in the absence of concurrent protein synthesis. The possible functional role of this aspect of insulin's action on mammary tissue is discussed and compared with the well-known reversal by this hormone of the effects of lipolytic agents in adipose tissue and liver.

[Cyclic nucleotide phosphodiesterase in vertebrates]

The cellular concentration of cyclic nucleotides is largely dependent upon the activity of the enzymatic system responsible for their degradation: cyclic nucleotide phosphodiesterase. This enzymatic system thus plays a crucial role in the regulation of the multiple functions which are modulated by cyclic nucleotides in the organism. Many methodological problems, as well as the complexity of the phosphodiesterase system have long maintained a confusion in this field. Recent progresses (purification to homogeneity of some enzymatic forms, discovery of regulatory mechanisms, particularly) have brought a considerable evolution in the knowledge of the system. It is now well established that cyclic nucleotide phosphodiesterase exists under several isoenzymatic forms, the properties and distribution of which largely differ from a tissue to another. Some of these forms are relatively well characterized, while the representativity of others is still discussed. The significance of this multiplicity of isoenzymes, and their interrelationships are presently under study. A very interesting aspect in the study of this enzymatic system is that it is submitted to several physiological regulatory processes. Recent studies on this point suggest that phosphodiesterase might play a major role in the response of the organism to several hormones. These fundamental studies of phosphodiesterase system find a most interesting application in the pharmacological field. Indeed, numerous synthetic compounds which inhibit the enzyme present a strong pharmacological interest.

Proteolytic activation and solubilization of endoplasmic-reticulum cyclic AMP phosphodiesterase activity

Dithiothreitol led to the activation and solubilization of the cyclic nucleotide phosphodiesterase activities associated with the smooth and various rough subfractions of rat liver endoplasmic reticulum. The activity in each of the subfractions exhibited somewhat different time courses, and sensitivities to dithiothreitol concentration, in respect of their solubilization and activation. Both activation and solubilization by dithiothreitol could be blocked by either thiol proteinase inhibitors or excess bovine serum albumin. Freeze-thaw solubilization was not blocked by the thiol proteinase inhibitor antipain and did not lead to the activation of the enzyme. After dithiothreitol-induced solubilization, all of the enzymes exhibited non-linear Lineweaver-Burk plots indicative of apparent negative co-operativity. In contrast, after freeze-thaw solubilization the enzyme in the smooth-endoplasmic-reticulum-plus-Golgi fraction still obeys Michaelis kinetics, as does the membrane-bound enzyme. It is possible to mimic the action of dithiothreitol in solubilizing and activating the enzyme by limited proteolysis with trypsin. Triton X-100 is highly efficient at solubilizing these enzymes, yet has little effect on their activities. Charged detergents exhibit highly selective effects on the enzymes as regards their solubilization and activity expressed.

Guanine nucleotides can activate the insulin-stimulated phosphodiesterase in liver plasma membranes

The insulin-stimulated cyclic AMP phosphodiesterase from liver plasma membranes is shown to be activated upon incubation with guanine nucleotides in the presence of ATP. The non-hydrolysable analogue of ATP, adenylyl imidodiphosphate failed to substitute for ATP in achieving activation. GTP, its non-hydrolysable analogues p[NH]ppG and GTP-gamma-S, as well as GDP, all elicited activation. It is suggested that guanine nucleotides, and probably insulin, exert their effect on this enzyme through a distinct species of guanine nucleotide regulatory protein.

The effects of glucagon, phenylephrine and insulin on the phosphorylation of cytoplasmic, mitochondrial and membrane-bound proteins of intact liver cells from starved rats

1. The effects of glucagon, insulin and phenylephrine on the phosphorylation of cytoplasmic, mitochondrial and membrane proteins were studied in intact hepatocytes from 24 h-starved rats incubated with [32P]Pi. A rapid cell-fractionation technique was used, followed by radioautography of the proteins separated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. 2. Glucagon consistently caused a significant increase in the phosphorylation of four readily separable cytoplasmic phosphoproteins, of Mr 93000, 50000, 46000 and 20000, and a decrease in phosphorylation of a phosphoprotein of Mr 22000. Phosphorylation of the protein of Mr 46000 was also enhanced by both phenylephrine and insulin, and that of Mr 93000 by phenylephrine. 3. The phosphoprotein of Mr 22000 was not precipitated by boiling for 5 min, and had a mobility identical with that of similar protein whose phosphorylation is enhanced in the adipocyte by insulin [Belsham & Denton (1980) Biochem. Soc. Trans. 8, 382-383]. 4. Glucagon, but not phenylephrine or insulin, enhanced the phosphorylation of a mitochondrial protein of Mr 35000 and of four plasma- or microsomal-membrane proteins of Mr 50000, 30000, 23000 and 19000. 5. Mitochondria from glucagon-treated animals or hepatocytes phosphorylated a protein of Mr 30000 when incubated in vitro with [32P]Pi and ADP. Phosphorylation of this protein did not occur with mitochondria from control, phenylephrine- or insulin-treated cells. 6. The significance of these hormonally induced changes in protein phosphorylation is discussed.

Submitochondrial localization and asymmetric disposition of two peripheral cyclic nucleotide phosphodiesterases

There are two distinct cyclic AMP phosphodiesterases associated with the liver mitochondrion: one with the outer membrane and one with the inner membrane. No activity is associated with the lysosomal fraction. Both of the enzymes are peripheral proteins and can be released from the membranes by high-ionic-strength treatment. Treatment of intact mitochondria with trypsin and insoluble trypsin localizes these enzymes to the cytosol-facing surface of their respective membranes. The enzymes differ in regard to sedimentation coefficient, thermostability and susceptibility to inactivation by trypsin. Both enzymes degrade cyclic AMP and cyclic GMP. Whereas the outer-membrane enzyme displays Michaelis kinetics and appears to be a low-affinity enzyme, the inner-membrane enzyme displays kinetics indicative of apparent negative co-operativity.