Phosphorylation of a 16-kDa protein by diacylglycerol-activated protein kinase C in vitro and by vasopressin in intact hepatocytes

Cytoplasmic targeting signals mediate delivery of phospholemman to the plasma membrane

The FXYD protein family consists of several small, single-span membrane proteins that exhibit a high degree of homology. The best-known members of the family include the gamma-subunit of the Na(+)-K(+)-ATPase and phospholemman (PLM), a phosphoprotein of cardiac sarcolemma. Other members of the family include corticosteroid hormone-induced factor (CHIF), mammary tumor protein of 8 kDa (Mat-8), and related to ion channels (RIC). The exact physiological roles of the FXYD proteins remain unknown. To better characterize the function of the members of the FXYD protein family, we expressed several members of the family in Madin-Darby canine kidney (MDCK) cells. All of the FXYD proteins, with the exception of PLM, were primarily found in the basolateral plasma membrane. Surprisingly, PLM, a previously characterized plasma membrane protein, was found to colocalize with the endoplasmic reticulum marker protein disulfide isomerase. Treatment of MDCK cells expressing PLM with an agonist of PKC caused some of the PLM to be redistributed to the plasma membrane. Site-directed mutagenesis of residues within the cytoplasmic domain of PLM indicated that a negative charge at Ser69 is necessary to shift the localization of PLM to the plasma membrane. In addition, other regions of PLM necessary for either its endoplasmic reticulum or plasma membrane localization have been elucidated. In contrast to PLM, the plasma membrane localization of CHIF and RIC was not altered by mutation of potential cytoplasmic phosphorylation sites. Overall, these results suggest that phosphorylation of specific residues of PLM may direct PLM from an intracellular compartment to the plasma membrane.

Identification of a cytoskeleton-bound form of phospholemman with unique C-terminal immunoreactivity

Phospholemman (PLM) is a 72-amino acid transmembrane protein thought to function in Na,K-ATPase regulation or assembly, similar to other members of the FXYD family of proteins. Unique to PLM among these regulatory proteins are sites for C-terminal phosphorylation by PKA and PKC, although a role for phosphorylation in PLM function remains unclear. To study PLM phosphorylation, we used PLM phosphopeptides to generate antibodies to specifically detect phosphorylated PLM. Peptide affinity chromatography isolated two populations of antibodies: one reacting with standard PLM, a collection of closely-spaced 15-kDa protein bands by SDS-PAGE. About 20% of PLM antibodies reacted specifically with a single distinct form of PLM. Levels of this second immunological form (PLM-b) were increased with overexpression of PLM cDNA, and also reacted with a monoclonal antibody against the PLM N-terminus. In complete contrast to standard PLM, however, PLM-b was quantitatively insoluble in nonionic detergents and was released from tight binding by colchicine. Antibodies to PLM-b were present in two different antisera raised to the phosphorylated C-terminal peptide (residues 57-70), but not in antiserum raised to the non-phosphorylated C-terminal peptide. Despite an apparent relationship between PLM-b and phosphorylated PLM, PLM-b levels were not affected by treatment of heart cells with isoproterenol. PLM-b appears to represent a cytoskeleton-attached detergent-insoluble form of PLM with distinctive C-terminal immunoreactivity that might have implications for PLM structure and function.

Reduction of phospholemman expression decreases osmosensitive taurine efflux in astrocytes

The role of phospholemman (PLM) in taurine and Cl(-) efflux elicited by 30% hyposmotic solution was studied in cultured cerebellar astrocytes with reduced PLM expression by antisense oligonucleotide (AO) treatment. PLM, a substrate for protein kinases (PK) C and A, is a protein that increases an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. Two antisense oligonucleotides (AO1 and AO2) effectively decreased the expression of the PLM protein by 40% and 30%, respectively, and markedly reduced [(3)H]taurine efflux by 67% and 62%. AO treatment also decreased the osmosensitive release of Cl(-), followed as (125)I. The inhibition of Cl(-) efflux (23% for AO1 and 13% for AO2) was notably lower than for [(3)H]taurine. The contribution of PKC and PKA in the function of PLM was also evaluated in astrocytes. Pharmacological activation or inhibition of PKC and PKA revealed that the osmosensitive taurine efflux is essentially PKC-independent while (125)I efflux is reduced by the PKC blockers H-7 (21%) and Gö6983 (41%). The PKA activator forskolin and dbcAMP increased taurine efflux by 66-70% and (125)I efflux by 21-45%. Norepinephrine increased the osmosensitive taurine efflux at about the same extent as dbcAMP and forskolin, and this was reduced by PKA blockers. These results suggest that PLM plays a role in RVD in astrocytes by predominantly influencing taurine fluxes, which are modulated by PKA but not PKC.

Characterization of the human and rat phospholemman (PLM) cDNAs and localization of the human PLM gene to chromosome 19q13.1

Previous reports have demonstrated that the phospholemman (PLM), a 72-residue plasma-membrane protein enriched in skeletal muscle and heart, is a major substrate phosphorylated in response to insulin and adrenergic stimulation. Here we describe the isolation and characterization of human and rat PLM cDNA from the heart. Both PLM proteins share significant nucleotide and amino acid sequence and structural similarities with the previously published canine PLM and, to a lesser degree, with Na+/K(+)-ATPase gamma subunit, Mat-8 protein, and CHIF protein. Despite the functional diversity, all these proteins are quite small and possess a single transmembrane domain. Human PLM appears to be a unique gene localized on chromosome 19q13.1. The PLM mRNA is widely distributed in human tissues, with the highest expression in skeletal muscle and heart, suggesting a functional role in muscle contraction. Like canine PLM, both human and rat PLM induce a hyperpolarization-activated chloride current when expressed in Xenopus oocytes. The high degree of sequence and functional conservation among the mammalian PLM proteins indicates that this gene is conserved throughout evolution.

Vasopressin stimulates phospholipase D activity against phosphatidylcholine in vascular smooth muscle cells

It is now clear that various hormones and agonists can stimulate the production of lipid mediators from non-phosphoinositide phospholipids. We have investigated the production of diacylglycerol from nonphosphoinositide sources, and we demonstrated that vasopressin and other vasoactive agents stimulate hydrolysis of phosphatidylcholine in a variety of cultured vascular smooth muscle cells of rat and human origin. We used vasopressin to characterize this response and found that vasopressin stimulates phospholipase D activity against phosphatidylcholine in A-10 vascular smooth muscle cells. The vasopressin-stimulated phosphatidylcholine hydrolysis is both time- and concentration-dependent. The half-maximal dose of vasopressin required for phosphatidylcholine hydrolysis (ED50 approximately 1 nM) correlates well with vasopressin binding to A-10 cells (Kd approximately 2 nM). The phosphatidylcholine in A-10 cells can be preferentially radiolabeled with [3H]myristic acid; subsequent treatment with vasopressin stimulates a rapid increase in 3H-labeled phosphatidate (approximately 4 X control values at 3 min), and after a short lag, 3H-labeled diacylglycerol rises and reaches maximal levels at 10 min (approximately 2 X control values). Similar temporal elevations of phosphatidate and diacylglycerol occur in A-10 cells labeled with [3H] glycerol. In A-10 cells radiolabeled with [3H] choline, the elevation of cellular phosphatidate and diacylglycerol is concomitant with the release of [3H] choline metabolites (predominantly choline) to the culture medium. The temporal production of phosphatidate and diacylglycerol as well as the release of choline to the culture medium are consistent with vasopressin activating phospholipase D. In addition, vasopressin stimulates a transphosphatidylation reaction that is characteristic of phospholipase D. The transphosphatidylation reaction is detected by the production of phosphatidylethanol that occurs when A-10 cells are incubated with ethanol and stimulated with vasopressin. The phospholipase D is active in the absence of extracellular Ca++ whereas the vasopressin-stimulated mobilization of arachidonic acid is dependent on extracellular Ca++. The data indicate that vasopressin stimulates phospholipase D which hydrolyzes phosphatidylcholine to phosphatidate. The phosphatidate is then metabolized, presumably by a phosphatidate phosphohydrolase, to produce sustained levels of cellular diacylglycerol. These sustained levels of diacylglycerol may activate protein kinase C and thereby function in the "sustained phase" of cellular responses.

Insulin increases the synthesis of phospholipid and diacylglycerol and protein kinase C activity in rat hepatocytes

The effects of insulin on phospholipid metabolism and generation of diacylglycerol (DAG) and on activation of protein kinase C in rat hepatocytes were compared to those of vasopressin and angiotension II. Insulin provoked increases in [3H]glycerol labeling of phosphatidic acid (PA), diacylglycerol (DAG), and other glycerolipids within 30 s of stimulation. Similar increases were also noted for vasopressin and angiotensin II. Corresponding rapid increases in DAG mass also occurred with all three hormones. As increases in [3H]DAG (and DAG mass) occurred within 30-60 s of the simultaneous addition of [3H]glycerol and hormone, it appeared that DAG was increased, at least partly, through the de novo synthesis of PA. That de novo synthesis of PA was increased is supported by the fact that [3H]glycerol labeling of total glycerolipids was increased by all three agents. Increases in [3H]glycerol labeling of lipids by insulin were not due to increased labeling of glycerol 3-phosphate, and were therefore probably due to activation of glycerol-3-phosphate acyltransferase. Unlike vasopressin, insulin did not increase the hydrolysis of inositol phospholipids. Insulin- and vasopressin-induced increases in DAG were accompanied by increases in cytosolic and membrane-associated protein kinase C activity. These findings suggest that insulin-induced increases in DAG may lead to increases in protein kinase C activity, and may explain some of the insulin-like effects of phorbol esters and vasopressin on hepatocyte metabolism.

Phosphorylation of low molecular mass cytosolic proteins by protein kinase C and protein kinase A in the rabbit exocrine pancreas

Subcellular fractionation of rabbit pancreatic acini was performed to study the distribution of endogenous substrates for protein kinase C. Substrates for protein kinase C were found to be predominantly low molecular mass proteins of cytosolic origin. At least three of these soluble substrates, with molecular masses of 17-19 kDa, were relatively heavily phosphorylated by endogenous as well as purified pancreatic protein kinase C. In the same molecular mass range, 16-18 kDa, soluble proteins were also phosphorylated by protein kinase A. Moreover, addition of cyclic AMP under conditions that activated protein kinase C gave a more than additive labelling of these low molecular mass proteins. The latter observation may be of interest in view of the potentiating effect cyclic-AMP-activated protein kinase A has on amylase secretion stimulated by secretagogues which increase free cytosolic Ca2+ and activate protein kinase C.

Alterations in binding of inositol 1,4,5-trisphosphate to subcellular structures of rat liver during chronic endotoxemia

Inositol 1,4,5-trisphosphate (IP3) binding to, and Ca2+ uptake and release by plasma membrane- and endoplasmic reticulum-enriched fractions of rat liver were measured after continuous Escherichia coli endotoxin (ET) administration in vivo. IP3 binding to both fractions was significantly reduced by ET treatment. This was associated with decreased Ca2+ uptake and impaired IP3-dependent Ca2+ release. A decrease of 5'-nucleotidase specific activity of plasma membrane-enriched fraction was also observed in ET treated rats. The results suggest that previously observed impairments in the ability of hepatocytes to mobilize Ca2+, to activate glycogen phosphorylase and to respond--when saponin permeabilized--by Ca2+ release upon IP3 addition during chronic endotoxemia are due to alterations in both IP3 binding to the subcellular fractions that are imputed to be targets of IP3, and a decrease in the size of IP3-sensitive pool of releasable Ca2+.

Kinetics of diacylglycerol accumulation in response to vasopressin stimulation in hepatocytes of continuously endotoxaemic rats

The content and composition of 1,2-diacyl-sn-glycerol (1,2-DAG) was determined in hepatocytes from saline (0.9% NaCl)- and Escherichia coli endotoxin (ET)-infused rats upon continuous vasopressin (VP) (10(-8) M) stimulation. In both experimental groups the accumulation of 1,2-DAG was detected after a lag period (2-5 min), was sustained up to the last time analysed (10 min), and C18:0- and C20:4-fatty-acid-containing-DAG accumulation preceded that of DAG containing other acyl groups. In hepatocytes from ET-infused rats the VP-induced accumulation of DAG was delayed and was decreased by 50%, showing a C18:0/C20:4 molar ratio of 1.6 as compared with 1.1 for cells from saline-infused rats. A similar lower cellular response to VP stimulation was observed in cells prelabelled with [14C]C20:4 fatty acid. The accumulation of [14C]C20:4-DAG (lower in ET than in saline-infused rats) was paralleled by a decrease in phosphatidylinositol (PI) labelling, whereas phosphatidic acid showed a transient increase by 5 min in saline- but not in ET-infused rats. The present results demonstrate that the previously reported impairment in the early degradation of poly-PI and later in the 'PI cycle' during VP stimulation [Rodriguez de Turco & Spitzer (1987) Metab. Clin. Exp. 36, 753-760] is also reflected at the level of their phosphodiesteratic product, DAG. Moreover, the kinetics of the accumulation of DAG acyl groups is consistent with the idea that the initial release of C18:0- and C20:4-DAG (possibly derived from inositol lipids) could regulate the subsequent enlargement of this pool by stimulating a phospholipase C-mediated degradation of other phospholipids (e.g. phosphatidylcholine).

Vasopressin is the only component of serum-free medium that stimulates phosphatidylcholine hydrolysis and accumulation of diacylglycerol in cultured REF52 cells

Vasopressin stimulates phosphatidylcholine hydrolysis in REF52 cells, and this phosphatidylcholine hydrolysis results in increases in choline containing metabolites in the culture medium (2.3 x control levels) and accumulation of cellular diacylglycerol (6.5 x control levels). Vasopressin is the only component of a 6-component mixture of the serum-free medium for REF52 cells that induces the phosphatidylcholine hydrolysis response. The effect of vasopressin is both time- and concentration-dependent. Maximal levels of both phosphatidyl-choline hydrolysis and accumulation of diacylglycerol are observed between 10 and 20 min after treatment with vasopressin. Effects are maximal at vasopressin concentrations of 100 ng/ml; the ED50 for vasopressin-stimulated phosphatidyl-choline hydrolysis is approximately 0.7 ng/ml. The evolution of diacylglycerol occurs in a time frame that is consistent with the diacylglycerol activating protein kinase C in a "second phase" agonist response.

Vasopressin, phorbol diesters and serum elicit choline glycerophospholipid hydrolysis and diacylglycerol formation in nontransformed cells: transformed derivatives do not respond

REF52, a rat embryo cell line, and several transformed derivatives were used to examine the lipid-related events associated with agonist treatment (phorbol diesters, vasopressin, fetal bovine serum). Exposure of cells, prelabeled with [3H]glycerol, to TPA (12-O-tetradecanoylphorbol 13-acetate) resulted in 3-4-fold increase in the amount of intracellular diacyl[3H]glycerols as early as 10 min after treatment. Continued incubation (up to 60 min) revealed that the diacyl[3H]glycerol formed was under dynamic metabolic regulation as shown by the production of triacyl[3H]glycerols and free [3H]glycerol. Serum and vasopressin likewise induced the generation of intracellular diacyl[3H]glycerol, thereby illustrating that physiological agents provoke a similar reaction. In the three SV-40-transformed variants examined, the diacylglycerol generative-response to TPA, serum and vasopressin, was greatly diminished or totally absent. Experiments employing REF52 cells prelabeled with [3H]choline demonstrated that both TPA and vasopressin induce the hydrolysis of cellular choline-containing glycerophospholipids; this was measured by both a decrease in cell-associated phosphatidylcholine radioactivity and an increase in the production of water-soluble [3H]choline-containing metabolites in the culture medium. 92-97% of the tritium released to the medium was identified as [3H]choline. Vasopressin treatment of REF52 cells prelabeled with [3H]arachidonic acid elicited an increase of more than 11-fold in the amount of cellular diacyl[3H]glycerol and a concomitant release of arachidonic acid to the culture medium that was 12-fold higher than controls. These data demonstrate that tumor-promoting phorbol esters (agonists of protein kinase C), serum and vasopressin, increase the levels of cellular diacylglycerol by stimulating the hydrolysis of choline-containing glycerophospholipids. This agonist-directed mechanism is inoperable in transformed cells. Further, collateral with vasopressin-induced phosphatidylcholine hydrolysis, the cellular release of arachidonic acid occurs. The participation of these lipid-related responses in the signaling of agonist-directed events and their relation to cellular homeostasis is currently being explored.

Phosphorylation of multiple sites in a 15,000 dalton proteolipid from rat skeletal muscle sarcolemma, catalyzed by adenosine 3',5'-monophosphate-dependent and calcium/phospholipid-dependent protein kinases

This study reports a partial characterization of a 15,000 dalton (15 kDa) proteolipid present in rat skeletal muscle sarcolemma. The proteolipid is phosphorylated by both cyclic AMP-dependent and calcium/phospholipid-dependent protein kinases, displays an isoelectric point (pI) of 5.9, and can be extracted from sarcolemma by acidified chloroform/methanol (2:1) or non-ionic detergents. Phosphoamino acid analysis and tryptic fingerprinting of the phosphorylated proteolipid indicate that both cyclic AMP- and calcium/phospholipid-dependent protein kinases predominantly phosphorylate serine residue(s) on a single tryptic peptide. Additivity experiments and thermolytic fingerprinting demonstrate a minimum of two distinct phosphorylation sites on the proteolipid, the phosphorylation of which is independently catalyzed by cyclic AMP-dependent and calcium/phospholipid-dependent protein kinases in vitro. This sarcolemma proteolipid, which appears to be identified to a sarcolemma protein previously reported to be phosphorylated upon addition of insulin in a GTP-dependent manner (Walaas, O., Walaas, E., Rye-Alertsen, A. and Horn, R.S. (1979) Mol. Cell. Endocrinol. 16, 45-55), therefore represents a possible membrane target for those neuronal and hormonal stimuli which can regulate cyclic AMP-dependent or calcium/phospholipid-dependent protein kinase activities in skeletal muscle.

Impairments in hepatocyte phosphoinositide metabolism in endotoxemia

The status of phospholipid metabolism and inositol lipids-mediated transmembrane signaling in rat hepatocytes was analyzed during chronic, nonlethal endotoxemia. Rats were infused intravenously (IV) with Escherichia coli endotoxin (ET) via subcutaneously implanted osmotic pumps at a rate of 0.1 mg/100 g bw/day. The experiments were performed after 30 hours of ET or sterile saline (NaCl) infusion, in hepatocytes prelabelled "in vitro" with 32P (15 microCi/mL) and further stimulated with vasopressin (VP, 0.23 mumol/L). Similar experiments were done with food-restricted animals, whose food intake was matched with the voluntary intake of ET-infused rats. Uptake of 32P label into phosphatidic acid (PA), phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP2) occurs rapidly in cells from pair-fed, saline and ET-infused animals, and reaches a plateau between 60 and 80 minutes of incubation. Labeling of phosphatidylinositol (PI), phosphatidylethanolamine (PE), and phosphatidylcholine (PC) proceeds linearly after a ten-minute lag period for PI and 20 minutes for the two other lipids. The nutritional state greatly affects the distribution of 32P uptake into lipids, resulting in very low labeling of PA and PI and a high labeling of poly-PI as compared with control (taken from untreated rats) cells. In ET-v saline-infused rats, the labeling of PI and PE was depressed concomitantly with a proportional increase in the labeling of PIP and PC. The ability of VP to induce polyphosphoinositide (poly-PI) degradation in hepatocytes from saline-infused animals was similar to that observed in control cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The stimulation by sodium fluoride of plasma-membrane Ca2+ inflow in isolated hepatocytes. Evidence that a GTP-binding regulatory protein is involved in the hormonal stimulation of Ca2+ inflow

1. In isolated hepatocytes NaF increased the rate of 45Ca2+ exchange, the cytoplasmic free Ca2+ concentration ([Ca2+]i) (monitored by using quin2), and the activity of glycogen phosphorylase a in a Ca2+-dependent manner. 2. In cells previously incubated in the absence of extracellular Ca2+(Ca2+o), NaF caused a pronounced enhancement in the increases in the activity of glycogen phosphorylase and in [Ca2+]i observed when Ca2+ was subsequently added. The effect of NaF on glycogen phosphorylase activity was inhibited by verapamil and deferoxamine, and was potentiated by AlCl3. 3. The actions of NaF were associated with (a) increases in [3H]inositol polyphosphates, which were slower in onset and about half the magnitude of those induced by vasopressin, in hepatocytes labelled with [3H]inositol, and (b) enhanced rates of O2 utilization and decreased concentrations of ATP. The latter effects were not potentiated by AlCl3. 4. Preincubation of hepatocytes with vasopressin in the absence of added Ca2+o for times up to 30 min did not diminish the ability of a subsequent addition of extracellular Ca2+ to activate glycogen phosphorylase. 5. 12-O-Tetradecanoylphorbol 13-acetate had little effect on 45Ca2+ exchange and did not enhance the activation by Ca2+o of phosphorylase in hepatocytes incubated in the absence of Ca2+o. 6. On the basis of the observation that AlF4- activates GTP-binding regulatory proteins [Sternweiss & Gilman (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 4888-4891], it is concluded that the present results provide evidence for the function of a GTP-binding regulatory protein in the mechanism by which hormones stimulate plasma-membrane Ca2+ inflow in the liver cell, and indicate that an increase in [Ca2+]i and the activation of protein kinase C are not part of this mechanism.

A 16 kDa protein substrate for protein kinase C and its phosphorylation upon stimulation of vasopressin receptors in rat aortic myocytes

Phosphorylation induced by protein kinase C was examined in a plasma membrane fraction from rat aortic myocytes. Labelled phosphate incorporation produced by addition of kinase C to the membrane preparation allowed to identify a 16 kDa protein as the major substrate of the enzyme. This protein electrophoretically migrated with a protein phosphorylated by cAMP dependent protein kinase, but the two kinases produced phosphorylation of different sites since their effects were additive. Pretreatment of the myocytes with a kinase C activating phorbol ester or with vasopressin decreased further phosphate incorporation into the 16 kDa protein under the influence of exogenous kinase C. The results provide evidence that vasopressin produced in situ phosphorylation of the 16 kDa protein in rat aortic myocytes, with a time course and at concentrations consistent with a role of kinase C activation in the response of aortic myocytes to stimulation of V1 receptors.

Endogenous substrates of protein kinase C in experimentally induced and regressed rat thyroid goitres

The presence of endogenous substrates of the protein kinase C (PKc) in rat thyroid glands has been demonstrated in in vitro phosphorylated cytosolic proteins by polyacrylamide gel electrophoresis (PAGE). Rat thyroid PKc specifically catalyzes the phosphorylation of the 35 kDa and 18 kDa proteins. These proteins were not labelled in the presence of Ca2+ alone, but they were phosphorylated when phospholipids alone were added. In hyperplastic glands the total phosphorylation of endogenous proteins is stimulated, due to the increased labelling of the 35 kDa and 18 kDa proteins. No extra phosphorylated bands were revealed by PAGE analysis. After suppression of growth activity the labelling of the two PKc-specific substrates was strongly inhibited.

Protein phosphorylation in rat hippocampal synaptic plasma membranes in response to neurohypophyseal peptides

The effect of arginine vasopressin (AVP) on protein phosphorylation in rat hippocampal synaptic plasma membranes (SPM) was examined. With a crude SPM preparation, AVP (10(-8)-10(-5) M) stimulated phosphorylation of a number of proteins which included a brain-specific protein of 48 kDa called B50 or protein F1, which is thought to be related to synaptic plasticity. Equimolar levels of oxytocin also stimulated B50/F1 phosphorylation. AVP and oxytocin at higher concentrations (10(-4)-10(-3) M) reduced SPM protein phosphorylation. When SPM was treated with both AVP and oxytocin, the effects were not additive; on the other hand, the effects of the phorbol ester (TPA) and AVP were additive. With SPM, partially purified by sucrose density centrifugation, only the inhibitory effect of AVP on B50/F1 phosphorylation was seen. These results suggest that AVP and oxytocin stimulation of B50/F1 phosphorylation requires cellular factors which are removed from SPM during membrane purification. In contrast, the inhibitory mechanism triggered by AVP and oxytocin appears to be associated with, or an integral part of, the synaptic membrane itself. Because the effects on membrane protein phosphorylation with maximal amounts of AVP and oxytocin were not additive, they must bind to the same sites on the membrane. This conclusion is supported by the additivity of the effects of AVP and phorbol ester, since the phorbol ester can act directly on the kinase and does not require a membrane recognition site.

Protein phosphorylation in isolated mitochondria and the effects of protein kinase C

When isolated intact rat liver mitochondria are incubated with [gamma-32P]ATP the major phosphorylated proteins are those of 47 and 36 kDa. Phosphorylation of the 47 kDa protein, but not of the 36 kDa protein, is inhibited by carboxyatractyloside, an inhibitor of mitochondrial ATP uptake, while phosphorylation of the 36 kDa protein is inhibited by various uncouplers and an inhibitor of mitochondrial respiration. Addition of purified protein kinase C to the isolated mitochondria leads to the phosphorylation of 69, 37 and 17 kDa proteins. As with other substrates for protein kinase C, phosphorylation of these proteins is dependent on Ca2+ and markedly stimulated by various tumor promoters.

Cyclic AMP-dependent phosphorylation of a 16 kDa protein in a plasma membrane-enriched fraction of rat aortic myocytes

Phosphorylation induced by cAMP-dependent protein kinase was examined in a plasma membrane-enriched fraction from control and beta-adrenergic-stimulated rat aortic myocytes. Phosphorylation of a 16 kDa protein which copurified with the plasma membrane marker (Na+ + K+)-ATPase was most prominent. It was decreased by pretreatment of the myocytes with isoproterenol and the effect of isoproterenol was inhibited by propranolol. Both phosphorylation induced by cAMP-dependent protein kinase and its inhibition by isoproterenol pretreatment declined in preparations exposed to endogenous phosphatase. These results provide strong evidence that beta-adrenergic stimulation of aortic myocytes induces in situ phosphorylation of a 16 kDa plasma membrane protein.

Action of a phorbol ester on B-cells: potentiation of stimulant-induced electrical activity

The possible role of protein kinase c in regulating the electrical events in the B-cell plasma membrane was examined by using the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), a known activator of this enzyme. TPA has been found to enhance glucose- and sulfonylurea-induced insulin secretion with little or no effect on the fluxes of 86Rb+ or 45Ca2+ across the plasma membrane. TPA, 0.2 microM, did not influence the membrane potential from 0 to 5.6 mM glucose but increased by two- to threefold the fraction of the plateau phase of the oscillatory electrical activity induced by 7.0-11.1 mM glucose. This effect of TPA was completely blocked by 0.5 mM spermidine, an inhibitor of protein kinase c. However, spermidine had no influence on the electrical activity elicited by glucose alone. Glyburide, 10 nM, initiated slow depolarization and constant spike activity after about 18 and 25 min, respectively. TPA or 2.8 mM glucose reduced the lag period for glyburide to elicit an electrical response by about 75%. The duration of the spikes was increased two- to threefold by the presence of glucose or TPA with glyburide. There were also characteristic differences in the shape of the spikes under each experimental condition. Spermidine inhibited the influence of glucose, but not TPA, on the glyburide-induced electrical response. These results indicate that TPA may influence stimulant-induced electrical events via protein kinase c or by directly altering the ionic permeability of the plasma membrane.

Inositol trisphosphate and diacylglycerol as intracellular second messengers in liver

Receptor occupation by a variety of Ca2+-mobilizing hormones, such as alpha 1-adrenergic agents, vasopressin and angiotensin II, causes a rapid phosphodiesterase-mediated hydrolysis of phosphatidylinositol-4,5-bisphosphate in the plasma membrane with the production of the water soluble compound myo-inositol-1,4,5-trisphosphate (IP3) and the lipophilic molecule 1,2-diacylglycerol (DG). This review summarizes the recent evidence obtained in the liver that defines the roles of these products as intracellular messengers of hormone action. Intracellular Ca2+ mobilization is mediated by IP3, which releases Ca2+ from a subpopulation of the endoplasmic reticulum, resulting in a rapid increase of the cytosolic free Ca2+ concentration ( [Ca2+]i). Further effects of receptor occupancy are inhibition of the plasma membrane Ca2+-ATPase, despite net Ca2+ efflux, and an increased permeability of the plasma membrane to extracellular Ca2+. The activation of the phospholipid-dependent protein kinase C by DG does not alter Ca2+ fluxes across the plasma membrane. In contrast to some secretory cells, a synergism between protein kinase C activation and increased [Ca2+]i is not observed in liver. Activation of protein kinase C profoundly inhibits the response to alpha 1-adrenergic agonists, with only minimal effects on the vasopressin response. It is concluded that in liver the two inositol-lipid messenger systems, IP3 and DG, exert their effects by essentially separate pathways.

A transient increase in diacylglycerols is associated with the action of vasopressin on hepatocytes

Vasopressin induced a transient increase of 50% in the total concentration of diacylglycerols (determined by g.l.c.) in isolated hepatocytes. The increase was maximal at 0.25 min, and the concentration of diacylglycerols in cells treated with vasopressin had returned to the basal value by 4 min. No change in the concentration of diacylglycerols was observed after the treatment of cells with glucagon. The dependency of this effect on the concentration of vasopressin was similar to that of the effect of the hormone on 45Ca2+ efflux measured at 0.1 mM extracellular Ca2+. Vasopressin increased the proportion of arachidonic acid and stearic acid and decreased the proportion of oleic acid present in the diacylglycerols. In hepatocytes prelabelled with [14C]arachidonic acid, vasopressin increased the amount of [14C]diacylglycerol. The effects of vasopressin on the total concentration of diacylglycerols and [14C]diacylglycerol were mimicked by an exogenous phospholipid phosphodiesterase (phospholipase C) from Clostridium perfringens. The results are consistent with the conclusion that the transient increase in diacylglycerols induced by vasopressin is caused by the rapid hydrolysis of both the phosphoinositides and one or more other phospholipids.

12-O-Tetradecanoylphorbol 13-acetate stimulates inositol lipid phosphorylation in intact human platelets

The phorbol esters are among the most potent tumor promoters. On addition of 12-O-tetradecanoylphorbol 13-acetate (TPA) to isolated human platelets prelabelled with [32P]orthophosphate we found a rapid increase in 32P incorporation into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. In view of similar findings with cells infected with the oncogene Rous sarcoma virus, it is suggested that inositol lipid phosphorylation might be a key event in the molecular action of phorbol esters.