Regulation of the meiosis-inhibited protein kinase, a p38(MAPK) isoform, during meiosis and following fertilization of seastar oocytes

A p38(MAPK) homolog Mipk (meiosis-inhibited protein kinase) was cloned from seastar oocytes. This 40-kDa protein shares approximately 65% amino acid identity with mammalian p38-alpha isoforms. Mipk was one of the major tyrosine-phosphorylated proteins in immature oocytes arrested at the G(2)/M transition of meiosis I. The tyrosine phosphorylation of Mipk was increased in response to anisomycin, heat, and osmotic shock of oocytes. During 1-methyladenine-induced oocyte maturation, Mipk underwent tyrosine dephosphorylation and remained dephosphorylated in mature oocytes and during the early mitotic cell divisions until approximately 12 h after fertilization. At the time of differentiation and acquisition of G phases in the developing embryos, Mipk was rephosphorylated on tyrosine. In oocytes that were microinjected with Mipk antisense oligonucleotides and subsequently were allowed to mature and become fertilized, differentiation was blocked. Because MipK antisense oligonucleotides and a dominant-negative (K62R)Mipk when microinjected into immature oocytes failed to induce germinal vesicle breakdown, inhibition of Mipk function was not sufficient by itself to cause oocyte maturation. These findings point to a putative role for Mipk in cell cycle control as a G-phase-promoting factor.

A role for amplified protein kinase C activity in the pathogenesis of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a human neurodegenerative disorder of unknown origin that is characterized by progressive degeneration of corticospinal tracts and anterior horn cells in the brainstem and spinal cord. Previous studies have indicated that motoneuron degeneration associated with ALS may be triggered by mechanisms leading to increased intracellular Ca2+. In the present report, Ca(2+)-activated phospholipid-dependent protein kinase C (PKC) was evaluated in cervical spinal cords from ALS patients and control subjects. In patients who died with ALS, PKC histone H1 phosphotransferase activity was significantly increased by 330% in cytosolic- and 118% in particulate-derived extracts compared with controls. This increase in PKC phosphotransferase activity appeared to be partially due to an increase in the amount of PKC protein present in ALS spinal cord tissue. PKC histone H1 phosphotransferase activities of cytosolic- and particulate-derived extracts from motor and visual cortex of ALS patients and controls were not statistically different, nor were there differences in PKC histone H1 phosphotransferase activity in platelets and leukocytes. The specific nature of PKC alterations in affected regions of the CNS supports a role for PKC in the events leading to motoneuron death in sporadic ALS.

Identification of p42 mitogen-activated protein kinase as a tyrosine kinase substrate activated by maximal electroconvulsive shock in hippocampus

Recent studies have demonstrated that administration of an electroconvulsive shock produces a rapid and transient increase in tyrosyl phosphorylation of a approximately 40-kDa protein in rat brain. Initial characterization of this protein's chromatographic properties indicated that it might be a member of a recently identified family of kinases, referred to as mitogen-activated protein (MAP) kinases, that are activated by tyrosyl phosphorylation. In the present study, we have used MAP kinase antisera to assess the identity of this protein. We have found that the approximately 40-kDa phosphotyrosine-containing protein comigrates with p42 MAP kinase (p42mapk) and not with two other 44-kDa MAP kinase family members detected by these antisera. Western blots of proteins immunoprecipitated with MAP kinase antibodies confirm that p42mapk displays increased tyrosyl phosphorylation after an electroconvulsive stimulus. Chromatographic separation of hippocampal extracts indicates that MAP kinase activity elutes in parallel with p42mapk. Accordingly, these studies identify p42mapk as a tyrosyl kinase substrate that is activated by this stimulus and suggest that this form of MAP kinase may be selectively regulated by neuronal stimulation.

Purification and characterization of echinoderm casein kinase II. Regulation by protein kinase C

Casein kinase II (CKII) is one of several protein kinases that become activated before germinal-vesicle breakdown in maturing sea-star oocytes. Echinoderm CKII was purified over 11,000-fold with a recovery of approximately 10% by sequential fractionation of the oocyte cytosol on tyrosine-agarose, heparin-agarose, casein-agarose and MonoQ. The purified enzyme contained 45, 38 and 28 kDa polypeptides, which corresponded to its alpha, alpha' and beta subunits respectively. The beta-subunit was autophosphorylated on one major tryptic peptide on serine residues, whereas the alpha'-subunit incorporated phosphate into at least two tryptic peptides primarily on threonine residues. Western-blotting analysis of sea-star oocyte extracts with two different anti-peptide antibodies that recognized conserved regions of the alpha-subunit indicated that the protein levels of the alpha- and alpha'-subunits of CKII were unchanged during oocyte maturation. The purified CKII was partly inactivated (by 25%) by preincubation with protein-serine/threonine phosphatase 2A, but protein-tyrosine phosphatases had no effect. The beta-subunit of CKII was phosphorylated on a serine residue(s) up to 0.54 mol of P/mol of beta-subunit by purified protein kinase C, and this correlated with a 1.5-fold enhancement of its phosphotransferase activity with phosvitin as a substrate. CKII was not a substrate for the maturation-activated myelin basic protein kinase p44mpk from sea-star oocytes, nor for cyclic-AMP-dependent protein kinase. These studies point to possible regulation of CKII by protein phosphorylation.

Role of protein phosphorylation in the maturation-induced activation of a myelin basic protein kinase from sea star oocytes

We have previously described the purification of a myelin basis protein (MBP) kinase from maturing sea star oocytes (Sanghera, J. S., Paddon, H. B., Bader, S. A., and Pelech, S. L. (1990) J. Biol. Chem. 265, 52-57). The ability of the purified 44-kDa protein to bind azido-ATP and undergo autophosphorylation on the serine residue implied that it is a protein kinase. Furthermore, partial amino acid sequence data has revealed that it is a novel protein kinase, which we have provisionally designated p44mpk. Autophosphorylation of p44mpk to 0.7 mol of phosphate/mol of enzyme was correlated with a modest (approximately 17%) increase in the MBP-phosphorylating activity of the kinase. Rabbit polyclonal antibody raised against purified p44mpk recognized on immunoblots the protein in highly purified preparations as well as crude oocyte extracts. The affinity-purified anti-p44mpk antibody could immunoprecipitate active kinase, but a subpopulation of the antibody also appeared to be inhibitory. Using this antibody, we have demonstrated that the up to 12-fold stimulation of the cytosolic MBP-phosphorylating activity of this kinase that occurs during sea star oocyte maturation is not due to an increase in the amount of enzyme protein, either from a redistribution within the oocyte or protein synthesis. A slight retardation of the migration of the activated p44mpk on sodium dodecyl sulfate-polyacrylamide gels and its tighter interaction with a MonoQ column is consistent with phosphorylation of the kinase during maturation. p44mpk underwent enhanced phosphorylation when oocytes prelabeled with [32P]orthophosphate were induced to mature with 1-methyladenine. The stimulated MBP-phosphorylating activity of p44mpk in cytosols from maturing oocytes was partly stabilized by the presence of the phosphatase inhibitor beta-glycerol phosphate. Furthermore, treatment of purified p44mpk with protein phosphatase 2A and alkaline phosphatase resulted in 56 and 86% decreases, respectively, in the activity of the kinase. Together, these findings strongly implicate a role for phosphorylation of p44mpk in its activation during sea star oocyte maturation.

Identification of a major maturation-activated acetyl-CoA carboxylase kinase in sea star oocytes as p44mpk

Maturation-activated protein-serine/threonine kinases were investigated in the high-speed supernatant fractions from sea-star oocytes harvested at the time of germinal vesicle breakdown. One of the major stimulated protein kinases able to phosphorylate acetyl-CoA carboxylase in these extracts was found to co-purify with a 44 kDa myelin basic protein kinase (p44mpk) that is activated with a similar time course during oocyte maturation. Purified sea-star oocyte p44mpk phosphorylated acetyl-CoA carboxylase (purified from rat liver) predominantly on serine and to a small extent on threonine. Furthermore, the phosphorylation of acetyl-CoA carboxylase occurred principally on a tryptic phosphopeptide which displayed electrophoretic and chromatographic properties very similar to those of the peptide that has previously been shown to undergo increased phosphorylation in response to insulin in rat adipocytes [Brownsey & Denton (1982) Biochem. J. 202, 77-86]. The acetyl-CoA carboxylase was phosphorylated at a similar rate and to a similar extent by casein kinase II, which was also purified from maturing sea-star oocytes. Although casein kinase II was also activated approximately 3-fold near the time of nuclear envelope breakdown, it was responsible for only a minor component of the total enhanced acetyl-CoA carboxylase kinase activity measured in the soluble extracts from maturing oocytes. Acetyl-CoA carboxylase was a relatively poor substrate for the major S6 peptide kinase activity that was also stimulated during resumption of meiosis in the oocytes. The properties of the p44mpk are reminiscent of those of a microtubule-associated protein 2 (MAP-2) kinase that is activated in response to insulin and other mitogens in mammalian cells [Ray & Sturgill (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 3753-3757; Hoshi, Nishida & Sakai (1988) J. Biol. Chem. 263, 5396-5401]. It is intriguing that several of the mammalian protein kinases that are acutely activated after mitogenic prompting of quiescent mouse fibroblasts (i.e. G0 to G1 transition), such as MAP-2 kinase, casein kinase II and S6 kinase II, have counterparts that are activated during M-phase in maturing sea star oocytes.

Catecholamine responses to histamine infusion in man

To evaluate the effects of histamine-induced hypotension on plasma catecholamine levels, eight normal men, aged 20 to 40 years, were infused with incremental doses of histamine starting at 0.2 microgram/kg/min at a 30 degree tilt position with monitoring of blood pressure (BP) and heart rate. Histamine dosage was increased every 5 minutes by 0.1 to 0.2 microgram/kg/min until mean BP fell greater than 15 mm Hg or a dosage of 1.6 micrograms/kg/min was reached. Plasma catecholamine samples were taken between the fourth and fifth minute of each histamine dosage. Identical measurements were made during nitroglycerin-induced hypotension in these subjects. Histamine produced threefold greater increases in heart rate and plasma norepinephrine (NE) levels than did nitroglycerin for comparable decreases in BP. Although NE levels increased twofold to fivefold from baseline with histamine infusion, epinephrine levels increased minimally at the highest doses or not at all. Our data demonstrate that histamine selectively releases NE from adrenergic nerve terminals without significant adrenal catecholamine release. We suggest that neural NE release plays an important role in the cardiac effects of histamine.

IL-3-induced activation of protein kinases in the mast cell/megakaryocyte R6-XE.4 line

A role for second messenger-regulated protein kinases in the early post-IL-3 receptor signal transduction pathway was investigated in the mast cell/megakaryocyte line R6-XE.4. The activity of the calcium- and phospholipid-dependent protein kinase C (PKC) was assessed by the ability of the enzyme to phosphorylate histone H1 in the presence of calcium, diacylglycerol, and phosphatidylserine or after proteolytic activation of PKC with trypsin. In high serum-supplemented cells, but not in cells that were preincubated in serum-deficient media for 6 h, subsequent treatment for 15 min with synthetic IL-3 (10 micrograms/ml) caused up to a sixfold increase in the calcium- and lipid-stimulated histone H1 phosphorylating activity of particulate-associated PKC after fractionation on MonoQ. However, there was no corresponding reduction of cytosolic PKC activity. Therefore, IL-3 appeared to modify the activity of preexisting membrane-associated PKC rather than eliciting its recruitment from the cytoplasm in R6-XE.4 cells. This was in contrast to the situation with FDC-P1 cells, where IL-3 induced PKC translocation. IL-3 also stimulated a cytosolic protein kinase that phosphorylated a synthetic peptide patterned after a phosphorylation site in ribosomal protein S6, but this IL did not alter the activity of cAMP-dependent protein kinase.

IL-3-induced activation of protein kinases in the mast cell/megakaryocyte R6-XE.4 line

A role for second messenger-regulated protein kinases in the early post-IL-3 receptor signal transduction pathway was investigated in the mast cell/megakaryocyte line R6-XE.4. The activity of the calcium- and phospholipid-dependent protein kinase C (PKC) was assessed by the ability of the enzyme to phosphorylate histone H1 in the presence of calcium, diacylglycerol, and phosphatidylserine or after proteolytic activation of PKC with trypsin. In high serum-supplemented cells, but not in cells that were preincubated in serum-deficient media for 6 h, subsequent treatment for 15 min with synthetic IL-3 (10 micrograms/ml) caused up to a sixfold increase in the calcium- and lipid-stimulated histone H1 phosphorylating activity of particulate-associated PKC after fractionation on MonoQ. However, there was no corresponding reduction of cytosolic PKC activity. Therefore, IL-3 appeared to modify the activity of preexisting membrane-associated PKC rather than eliciting its recruitment from the cytoplasm in R6-XE.4 cells. This was in contrast to the situation with FDC-P1 cells, where IL-3 induced PKC translocation. IL-3 also stimulated a cytosolic protein kinase that phosphorylated a synthetic peptide patterned after a phosphorylation site in ribosomal protein S6, but this IL did not alter the activity of cAMP-dependent protein kinase.

Protein kinase C activation by platelet-activating factor is independent of enzyme translocation

The subcellular distribution and activation state of protein kinase C (PKC) was studied after short-term exposure of rabbit platelets to platelet-activating factor (PAF). Cytosolic and nonidet P-40-solubilized particulate extracts prepared from treated platelets were subjected to analytical column chromatography on MonoQ, hydroxylapatite and Superose 6/12. PKC activity was assayed by the ability of the enzyme to phosphorylate the following substrates: (i) histone H1 in the presence of the activators calcium, diacylglycerol and phosphatidylserine; (ii) histone H1 following proteolytic activation of PKC with 0.5 micrograms trypsin/ml; and (iii) protamine in the absence of calcium and lipid. PAF treatment for 1-20 min elicited a rapid 2-4-fold activation of both cytosolic and particulate-derived PKC as assessed by all three methods. On the other hand, there were no significant PAF-induced changes in the level of [3H]phorbol-12,13-dibutyrate binding by soluble and particulate-associated PKC. Hydroxyapatite column chromatography revealed that in non-treated rabbit platelets the type II (beta) form of PKC predominated, but PAF appeared to induce a shift in the elution profile from this resin. The stability of the PAF activation of PKC to column chromatography and the altered binding affinity to hydroxylapatite indicated that the stimulation might be a consequence of covalent modification, albeit minor, since PKC still eluted as an 80 kDa protein from Superose 6/12. As the PAF-induced increases in the kinase activity of PKC were preserved even after proteolytic activation with trypsin, but were without effect on the phorbol ester binding activity, such a putative modification may have occurred within or near the catalytic domain of PKC. These findings imply that PAF may directly modulate the activity of preexisting membrane-associated PKC by a novel mechanism, rather than by eliciting its recruitment from the cytoplasm.

Purification and characterization of a maturation-activated myelin basic protein kinase from sea star oocytes

A meiosis-activated myelin basic protein (MBP) kinase was purified approximately 8700-fold from soluble post-germinal vesicle breakdown extracts from maturing oocytes of the sea star Pisaster ochraceus. Purification to apparent homogeneity was achieved by sequential chromatography on DEAE-cellulose, hydroxylapatite, phosphocellulose, phenyl-Sepharose, heparin-Sepharose, polylysine-Sepharose, and Mono-Q. The final product exhibited an apparent molecular mass of approximately 42 kDa by both native gradient and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and this precisely correlated with the chromatographic behavior of the recovered MBP kinase activity on a Superose 6/12 column. The kinase utilized the MBP as the major substrate with little or no phosphorylation of histones (H1, H2A, or H2B), casein, phosvitin, protamine, or 40 S ribosomal proteins. The purified enzyme was relatively insensitive to high concentrations of beta-glycerol phosphate, calmodulin, EGTA, NaCl, sodium fluoride, dithiothreitol, spermine, and heparin but was quite sensitive to inhibition by metal ions such as Mn2+, Zn2+, and Ca2+. The true Km values for ATP and myelin basic protein were determined to be 58 and 25 microM, respectively, using double-reciprocal plots. The purified enzyme was unable to utilize GTP in place of ATP. The enzyme was shown to rapidly undergo autophosphorylation. The autophosphorylation was sensitive to alkali treatment implying that phosphate was incorporated on serine/threonine residues. The properties of this MBP kinase are reminiscent of a protein kinase that is also activated in a cyclic fashion at M-phase during the early cell divisions of sea star and sea urchin embryos (Pelech, S. L., Tombe, R., Meijer, L., and Krebs, E. G. (1988) Dev. Biol. 130, 26-36).

Histamine releases PGI2 from human pulmonary artery

Histamine caused a triphasic response of human pulmonary artery strips in vitro, consisting of a small initial contraction followed by pronounced relaxation preceding a second contractile response. These characteristics were not seen with other contractile stimuli including 5-hydroxytryptamine, leukotriene D4, and KCl. The relaxant component of this response was ablated by removal of endothelium from the vascular strips or by pretreatment of the tissue with 1 microM indomethacin. Measurement of the PGI2 degradation product 6-keto-PGF1 alpha in supernatants from histamine-challenged tissues confirmed the synthesis of PGI2. Supernatants from unstimulated or leukotriene-challenged tissues contained no detectable amounts of 6-keto-PGF1 alpha. The histamine H1 antagonist diphenydramine inhibited both the contractile and relaxant responses to histamine whereas the H2 antagonist cimetidine affected neither component. The released PGI2 significantly altered the dose-response curve to histamine without inhibiting the maximal contractile responses. We conclude that histamine induces PGI2 formation from pulmonary arterial endothelium via an H1 receptor.

Phorbol esters stimulate phosphatidylcholine biosynthesis by translocation of CTP:phosphocholine cytidylyltransferase from cytosol to microsomes

Previous studies have demonstrated that 12-O-tetradecanoyl phorbol 13-acetate (TPA) stimulates phosphatidylcholine biosynthesis in HeLa cells. The stimulation was apparently caused by an acceleration of the reaction catalyzed by CTP:phosphocholine cytidylyltransferase (CTP:cholinephosphate cytidylyltransferase, EC 2.7.7.15) (Paddon, H.B. and Vance, D.E. (1980) Biochim. Biophys. Acta 620, 636-640). We now provide evidence that the enzyme activation is due to a translocation of the cytidylyltransferase from the cytosol to the microsomes. The rate of phospho[Me-3H]choline conversion into phosphatidylcholine was approx. 3-fold faster in HeLa cells treated with 100 nM TPA. This stimulation correlated with a 2.3-fold activation (P less than 0.05) of cytidylyltransferase in homogenates from treated cells. There was a 1.7-fold increase in the enzyme associated with microsomes (P less than 0.05) and a corresponding decrease in enzyme recovered from cytosol (P less than 0.01). The total amount of enzyme recovered from the homogenates was unchanged. Further evidence for TPA causing an increased association of cytidylyltransferase with cellular membranes was obtained when cells were treated with digitonin. The release of cytidylyltransferase into the medium was inhibited by 4-fold from cells previously treated with TPA. Similar results on phospho[Me-3H]choline incorporation into phosphatidylcholine were found with cells incubated with phorbol-12,13-dibutyrate, a water-soluble tumor-promoting agent.

Membrane-bound CTP:phosphocholine cytidylyltransferase regulates the rate of phosphatidylcholine synthesis in HeLa cells treated with unsaturated fatty acids

The influence of fatty acids on phosphatidylcholine biosynthesis in HeLa cell cultures was investigated. Oleate and other unsaturated fatty acids stimulated the incorporation of phospho[Me-3H]choline into phosphatidylcholine from 5 to 20-fold, while saturated fatty acids were without effect. Stimulation of the reaction catalyzed by CTP:phosphocholine cytidylyltransferase (CTP:cholinephosphate cytidylyltransferase, EC 2.7.7.15) by 1 mM oleate was evident within 15 min and could be reversed within 40 min after removal of the oleate-supplemented cell medium. Cytidylyltransferase activity was 11-fold higher in homogenates from cells exposed to oleate. Treatment of the HeLa cells with oleate produced almost complete translocation of the cytidylyltransferase from the cytosol to the microsomal fraction. Additional support for conversion of the cytidylyltransferase to a membrane-bound form in oleate-treated cells was obtained in studies with digitonin. Exposure of control cells to digitonin for 2 min caused the release of 60% of the total cytidylyltransferase into the medium, while oleate-treated cells leaked only 5% of the enzyme in the presence of digitonin. Finally, oleate (50 microM) was shown to promote complete aggregation of cytosolic cytidylyltransferase with microsomes from HeLa cells and 22-fold stimulation of the enzyme activity. It appears that the rate of phosphatidylcholine biosynthesis is governed by the amount of cytidylyltransferase bound to endoplasmic reticulum in HeLa cells.

Stimulation of hepatic phosphatidylcholine biosynthesis in rats fed a high cholesterol and cholate diet correlates with translocation of CTP: phosphocholine cytidylyltransferase from cytosol to microsomes

A new model system for the study of phosphatidylcholine biosynthesis is presented. Young rats were fed a diet that contained 5% cholesterol and 2% cholate. After 6 days there was a 2-fold increase in the concentration of plasma phospholipid (243 mg/dl compared to 132 mg/dl for control animals) and a 3-fold increase in the concentration of plasma phosphatidylcholine. The rate of phosphatidylcholine biosynthesis was measured after injection of [Me-3H]choline into the portal veins. The incorporation of tritium into choline, phosphocholine and betaine by liver was similar for experimental and control animals, whereas there was a 3-fold increased incorporation into phosphatidylcholine of the cholesterol/cholate-fed rats. The activities of the enzymes of phosphatidylcholine biosynthesis in cytosol and microsomes were assayed. The only change detected was in the cytosolic and microsomal activities of CTP: phosphocholine cytidylyltransferase which were increased more than 2-fold in specific activity. When total cytidylyltransferase activity per liver was determined, a dramatic translocation of the enzyme to microsomes was observed. The control livers had 24% of the cytidylyltransferase activity associated with microsomes, whereas this value was 61% in the livers from cholesterol/cholate-fed rats. When the cytosolic cytidylyltransferase was assayed in the presence of phospholipid, the enzyme was stimulated several-fold and the difference in specific activity between control and cholesterol/cholate-fed rats was abolished. The increased activity in cytosol appears to be the result of a 2-fold increase in the amount of phospholipid in the cytosol from cholesterol/cholate-fed rats. The data strongly support the hypothesis that the special diet stimulates phosphatidylcholine biosynthesis by causing a translocation of the cytidylyltransferase from cytosol to microsomes where it is activated.

Diethylstilbestrol treatment increases the amount of choline kinase in rooster liver

Studies have been performed on the mechanism by which diethylstilbestrol stimulates the activity of choline kinase in livers from cockerels. The enzyme was purified 700-900-fold by affinity chromatography. The increased enzyme activity could not be accounted for by diethylstilbestrol alteration of the kinetic constants of the enzyme. Rabbit antibody was raised to the purified enzyme. Titration studies with antiserum demonstrated a 2-fold increase in the amount of choline kinase in diethylstilbestrol-treated cytosol, which correlated with a 2-fold elevation of the activity of the enzyme. We conclude that diethylstilbestrol stimulates the activities of choline kinase in cockerel liver by corresponding increase in the amount of enzyme.

Diethylstilbestrol treatment modulates the enzymatic activities of phosphatidylcholine biosynthesis in rooster liver

The effect of diethylstilbestrol injection on the activities of phosphatidylcholine biosynthetic enzymes in rooster liver has been determined. Choline kinase activity was stimulated within 4 h after the first hormone injection. By the third day enzyme activity reached 5.47 nmol . min-1 . mg-1 protein compared to control values (1.83 nmol . min-1 . mg-1 protein) which were unchanged during the the experiment. CTP : phosphocholine cytidylyltransferase activity was unaffected until Day 3 when its activity was 50% that of control values. When assayed in the presence of exogenous phospholipid, no significant change was noted in cytidylyltransferase activity. The activity of CDPcholine : 1,2-diacylglycerol phosphocholinetransferase was not altered by the hormone injections. The activity of phosphatidylethanolamine-N-methyltransferase gradually increased so that by Day 3, the enzyme activity was elevated 2-fold (0.12 to 0.24 nmol methyl group transferred per mg microsomal protein). These results are consistent with earlier in vivo studies (Vigo, C. and Vance, D.E. (1981) Eur. J. Biochem., in the press) that indicated a stimulation of phosphatidylcholine biosynthesis via CDPcholine during the first 2 days of diethylstilbestrol injection and inhibition on the third day.

Tetradecanoyl-phorbol acetate stimulates phosphatidylcholine biosynthesis in HeLa cells by an increase in the rate of the reaction catalyzed by CTP:phosphocholine cytidylyltransferase

Studies have been performed on the mechanism by which 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulates phosphatidylcholine biosynthesis in HeLa cells. The phorbol acetate did not alter the transport of choline into the cells, nor did it stimulate choline phosphorylation. When [methyl-3H]choline was added to HeLa cells for a 1 h pulse, virtually all of the radioactivity in the aqueous phase of the cell extracts became associated with phosphocholine. The addition of the phorbol ester caused an accelerated disappearance of radioactivity from phosphocholine and a concomitant increase in the incorporation into phosphatidylcholine. The radioactivity associated with CDPcholine remained low and constant throughout the experiment. These results provide strong evidence that TPA accelerates phosphatidylcholine biosynthesis in HeLa cells by stimulation of the reaction catalyzed by CTP:phosphocholine cytidylyltransferase.

An increase in cytoplasmic CTP accelerates the reaction catalyzed by CTP:phosphocholine cytidylyltransferase in poliovirus-infected HeLa cells

Poliovirus increases phosphatidylcholine biosynthesis in HeLa cells by stimulation of the reaction catalyzed by CTP:phosphocholine cytidylyltransferase (Vance, D.E., Trip, E.M., and Paddon, H.B. (1980) J. Biol. Chem. 255, 1064-1069). The mechanism for the virus effect has been investigated. An assay for the cytidylyltransferase which mimics the physiological conditions within the cell was developed. The enzyme activity was not changed at 3 h but was stimulated more than 2-fold at 4 and 5 h after infection with poliovirus. Enzyme activity was stimulated by addition of CTP to the assay. At 0.10 mM CTP the difference in activities from poliovirus- and mock-infected cells was abolished. Mg2+ inhibited the cytidylyltransferase activities and eliminated the differences between the two activities at a concentration of 0.05 mM. However, the endogenous amount of Mg2+ in the postmitochondrial supernatants was the same for infected and mock-infected cells. The addition of CDP-choline or PPi inhibited the cytidylyltransferase activity but had no effect on the relative differences in activities from infected and mock-infected cells. Measurement of CTP in the postmitochondrial fraction showed no differences at 3 h but was elevated 2- to 3-fold in poliovirus-infected cells at 4 and 5 h. It appears that the cytidylyltransferase reaction is faster in poliovirus-infected HeLa cells because of an increase of CTP in the cytoplasmic compartment. Moreover, it appears that the concentration of CTP in the cytoplasm can determine the rate of phosphatidylcholine biosynthesis in HeLa cells.