Heterogeneity of cyclic nucleotide phosphodiesterases in liver endoplasmic reticulum

Resolution of soluble cyclic nucleotide phosphodiesterase isoenzymes, from liver and hepatocytes, identifies a novel IBMX-insensitive form

DEAE chromatography of a high speed supernatant fraction from a homogenate of rat liver, prepared under isotonic conditions in the presence of protease inhibitors, yielded three peaks of cyclic nucleotide phosphodiesterase activity (PDE activity). The first peak could be resolved on Affi-gel Blue chromatography to yield a Ca2+/calmodulin stimulated cyclic GMP specific PDE and a cyclic AMP and cyclic GMP hydrolysing PDE whose activity was insensitive to Ca2+/calmodulin. These two activities could also be clearly resolved by Mono-Q chromatography of soluble extracts from both liver and hepatocytes. These had different molecular weights, kinetics of substrate utilization, thermostabilities, dependence on Mg2+ and inhibitor sensitivities. The cyclic AMP and cyclic GMP utilizing PDE resolved in these procedures appears to be a novel enzyme form (PDE-MQ-I) which is insensitive to inhibition by the so-called non-selective PDE inhibitor IBMX and displays catalytic activity in the absence of Mg2+. None of the inhibitors tested were capable of inhibiting this form showing that the catalytic activity of this species could be distinguished from all the other soluble activities. This novel enzyme hydrolysed both cyclic AMP and cyclic GMP with Km values of 25 microM and 237 microM, respectively. The Vmax ratio of hydrolysis of cyclic GMP/cyclic AMP was above unity (1.4). It accounted for 30% of the soluble cyclic AMP PDE activity and 10% of the cyclic GMP PDE activity assessed at 1 microM substrate. Gel filtration of PDE-MQ-I indicated a size of 33,150 Da, in contrast to the size of 237,500 Da observed for the Ca2+/calmodulin PDE-MQ-II. Thermal inactivation of PDE-MQ-I and PDE-MQ-II yielded single exponential decays with t1/2 values of 6.33 min and 0.7 min at 60 degrees respectively. In the presence of saturating Ca2+, PDE-MQ-II was activated by calmodulin with an EC50 of ca. 30 ng/ml. In the presence of calmodulin, PDE-MQ-II was activated by Ca2+ with an EC50 of ca. 20 microM. Chromatography of homogenates on Mono-Q also identified a cyclic GMP-activated cyclic nucleotide PDE (PDE-MQ-III) and two cyclic AMP specific activities (PDE-MQ-IV and PDE-MQ-V). These exhibited very different inhibitor sensitivities and could be readily distinguished using the compound Ro-20-1724 which yielded IC50 values for inhibition of greater than 500 microM, 13 microM and 1.5 microM, respectively, for the hepatocyte enzymes.(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.

Identification and characterization of both the cytosolic and particulate forms of cyclic GMP-stimulated cyclic AMP phosphodiesterase from rat liver

Two enzymes displaying cyclic GMP-stimulated cyclic AMP phosphodiesterase activity were purified from rat liver to apparent homogeneity: a 'particulate enzyme' found as an integral membrane protein associated with the plasma membrane, and a 'soluble' enzyme found in the cytosol. The physical properties of these enzymes were very similar, being dimers of Mr 134,000, composed in each instance of two subunits of Mr = 66,000-67,000. Both enzymes showed similar kinetics for cyclic AMP hydrolysis. They are both high-affinity enzymes, with kinetic constants for the particulate enzyme of Km = 34 microM and Vmax. = 4.0 units/mg of protein and for the cytosolic enzyme Km = 40 microM and Vmax. = 4.8 units/mg of protein. In both instances hydrolysis of cyclic AMP appeared to show apparent positive co-operativity, with Hill coefficients (happ.) of 1.5 and 1.6 for the particulate and cytosolic enzymes respectively. However, in the presence of 2 microM-cyclic GMP, the hydrolysis of cyclic AMP obeyed Michaelis kinetics (happ. = 1) for both enzymes. The addition of micromolar concentrations of cyclic GMP had little effect on the Vmax. for cyclic AMP hydrolysis, but lowered the Km for cyclic AMP hydrolysis to around 20 microM in both cases. However, at low cyclic AMP substrate concentrations, cyclic GMP was a more potent activator of the particulate enzyme than was the soluble enzyme. The activity of these enzymes could be selectively inhibited by cis-16-palmitoleic acid and by arachidonic acid. In each instance, however, the hydrolysis of cyclic AMP became markedly more sensitive to such inhibition when low concentrations of cyclic GMP were present. Tryptic peptide maps of iodinated preparations of these two purified enzyme species showed that there was considerable homology between these two enzyme forms.

Insulin activates the plasma-membrane and dense-vesicle cyclic AMP phosphodiesterase in hepatocytes by distinct routes

Insulin elicits the activation of two distinct membrane-bound cyclic AMP phosphodiesterases when incubated at 37 degrees C for 5 min with intact hepatocytes: the 'dense-vesicle' enzyme and the peripheral-plasma-membrane enzyme. In hepatocytes the lysosomotropic agents chloroquine, methylamine and NH4Cl, as well as intracellular ATP depletion elicited by fructose or incubation with insulin at 22 degrees C, blocks selectively the activation of the 'dense-vesicle' enzyme. Incubation of hepatocytes with bacitracin, leupeptin and a variety of proteinase inhibitors failed to affect insulin's activation of these two cyclic AMP phosphodiesterases by distinct routes. It is suggested that activation of the 'dense-vesicle' enzyme occurs through a pathway triggered by the endocytosis, processing and recycling of the insulin receptor. This might involve the delivery, with subsequent activation, of a latent phosphodiesterase into this fraction.

Insulin and glucagon regulate the activation of two distinct membrane-bound cyclic AMP phosphodiesterases in hepatocytes

Glucagon (10 nM) caused a transient elevation of intracellular cyclic AMP concentrations, which reached a peak in around 5 min, and slowly returned to basal values in around 30 min. When 1 mM-3-isobutyl-1-methylxanthine (IBMX) was present, this process yielded a Ka of 1 nM for glucagon. The addition of insulin (10 nM) after 5 min exposure to glucagon (10 nM) caused intracellular cyclic AMP concentrations to fall dramatically, attaining basal values within 10 min. The regulation of this process was dose-dependent, exhibiting a Ka of 0.4 nM for insulin. If insulin and glucagon were added together to hepatocytes, then insulin decreased the magnitude of the cyclic AMP response to glucagon. IBMX (1 mM) prevented insulin antagonizing the action of glucagon in both of these instances. A gentle homogenization procedure followed by a rapid subcellular fractionation of hepatocytes on a Percoll gradient was developed. This was used to resolve subcellular membrane fractions and to identify cyclic AMP phosphodiesterase activity in both membrane and cytosol fractions. Glucagon and insulin only affected the activity of two distinct membrane-bound species, a plasma-membrane enzyme and a 'dense vesicle' enzyme. Glucagon (10 nM), insulin (10 nM), IBMX (1 mM), dibutyryl cyclic AMP (10 microM) and cholera toxin (1 microgram/ml) all elicited the activation of the 'dense vesicle' enzyme. The plasma-membrane enzyme was not activated by glucagon, IBMX or dibutyryl cyclic AMP, although insulin and cholera toxin both led to its activation. The degree of activation of the plasma-membrane enzyme produced by insulin was increased in the presence of IBMX or dibutyryl cyclic AMP. Glucagon pretreatment (5 min) of hepatocytes blocked the ability of insulin to activate the plasma-membrane enzyme. The activity state of these phosphodiesterases is discussed in relation to the observed changes in intracellular cyclic AMP concentrations. It is suggested that insulin exerts its action on the plasma-membrane phosphodiesterase through a mechanism involving a guanine nucleotide-regulatory protein.

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.