Phosphorylation of the glucose transporter in vitro and in vivo by protein kinase C

Lead activates protein kinase C in immature rat brain microvessels

We investigated the effects of inorganic lead upon calcium-, phospholipid-dependent protein kinase (protein kinase C) in brain microvessels isolated from 6-day-old rat pups. We found that (a) in broken cell preparations, lead at micromolar concentrations activates this enzyme to an extent equivalent to that of micromolar calcium (10.3 +/- 1.3 and 9.2 +/- 1.6 pmol/mg/min, respectively) and (b) preincubation of intact microvessels with lead results in a translocation of protein kinase C from the soluble to the particulate fraction. The cytosolic kinase activity stimulated by lead has the same requirements for diacylglycerol and phospholipid as the calcium-stimulated enzyme, suggesting that lead activates the kinase by mimicking calcium. The hypothesis that lead affects protein kinase C activity through a mechanism similar to that of calcium is supported by the similar time courses of substrate phosphorylation and dephosphorylation mediated by lead and calcium. When intact microvessels are preincubated with micromolar concentrations of lead, the translocation of protein kinase C occurs in a dose- and time-dependent manner. The relocalization is virtually complete at 0.1 microM lead and by 30 min of exposure. We propose that the sensitivity of protein kinase C to lead, described here in immature brain microvessels, makes this regulatory enzyme a potential mediator of lead toxicity.

[Coordinate control of cell growth and transport functions in a kidney cell line]

Several protein kinases have been shown to be involved in the regulation of cell growth and differentiation. Molecules, regulating the activity of these protein kinases, also effect the activity of certain transport systems. Genetic experiments, suggesting a similar connection between the regulation of cell growth and transport functions in MDCK cells, are discussed.

Platelet-derived growth factor induces multisite phosphorylation of pp60c-src and increases its protein-tyrosine kinase activity

We have shown previously that pp60c-src is a substrate for protein kinase C in vivo and that the target of protein kinase C phosphorylation in mammalian pp60c-src is serine 12. We now demonstrate that in addition to tumor promoters, all activators of phosphatidylinositol turnover that we have tested in fibroblasts (platelet-derived growth factor, fibroblast growth factor, serum, vasopressin, sodium orthovanadate, and prostaglandin F2 alpha) lead to the phosphorylation of pp60c-src at serine 12. In addition to stimulating serine 12 phosphorylation in pp60c-src, platelet-derived growth factor treatment of quiescent fibroblasts induces phosphorylation of one or two additional serine residues and one tyrosine residue within the N-terminal 16 kilodaltons of the enzyme and activates its immune complex protein-tyrosine kinase activity.

Apparent involvement of protein kinase C in the central glucoregulatory action of insulin

We have studied the possible involvement of the calcium- and phospholipid/diacylglycerol-dependent enzyme, protein kinase C (PKC) in mediating insulin action in the central nervous system (CNS) by testing the effect of direct activation or blockade of the CNS PKC system on the plasma glucose responses to central insulin injection in mice. Insulin (0.1-1 microgram), injected into the CNS, produced rapid transient hypoglycemia. This effect appeared to involve interaction of insulin with specific receptors, since insulin analogs exhibiting diminished receptor binding affinity and peripheral bioactivity compared to the native hormone were much less active (i.e., insulin much greater than acetyl 3 insulin greater than proinsulin greater than IGF-I) or not active at all (i.e., insulin chain A and chain B). Central injection of the specific PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA) (0.01-0.5 microgram), but not the inactive TPA analog, 4-alpha-phorbol or the unstable synthetic diacylglycerol analog, 1-oleoyl-2-acetyl-sn-glycerol (OAG), significantly enhanced the hypoglycemic response to co-administered insulin (0.5 microgram) or the insulin derivative, acetyl 3 insulin (2.5 micrograms). Central TPA had no effect on basal glucose levels. Furthermore, central administration of the selective PKC blockers, polymyxin B (PMB, 1-25 micrograms) or 1-beta-galactosylsphingosine (psychosine, 0.5-10 micrograms) but not their respective inactive analogs, polymyxin E and sphingomyelin, strongly inhibited the hypoglycemic response to insulin (1 microgram) or acetyl 3 insulin (5 micrograms). PMB and psychosine, injected alone had no effect on basal glucose levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased protein kinase C activity in fibroblast membranes from psoriatic patients

The activity of phospholipid/Ca2+-dependent protein kinase (PKc) was measured in the membrane and cytosolic fractions of normal and psoriatic human fibroblasts. The psoriatic fibroblasts displayed higher membrane-associated PKc activity than normal cells. In contrast, no significant difference in PKc activities was observed in cytosolic fractions from normal and psoriatic fibroblasts. These data suggest that PKc is preferentially associated with the membrane in psoriatic fibroblasts and that such elevated PKc activity in membranes may play a role in the pathogenesis of this disease.

Activation and proliferation signals in murine macrophages: stimulation of glucose uptake by hemopoietic growth factors and other agents

Purified colony-stimulating factor (CSF-1) (or macrophage colony stimulating factor [M-CSF]) stimulated the glucose uptake of murine bone marrow-derived macrophages (BMM) and resident peritoneal macrophages (RPM) as measured by 3H-2-deoxyglucose (2-DOG) uptake. Similar concentrations of CSF-1 stimulated the 2-DOG uptake and DNA synthesis in BMM. Other purified hemopoietic growth factors, granulocyte-macrophage CSF (GM-CSF) and interleukin-3 (IL-3) (or multi-CSF), and the tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), even though differing in their mitogenic capabilities on BMM, were also stimulators of 2-DOG uptake in BMM and RPM. The nonmitogenic agents, lipopolysaccharide (LPS) and concanavalin A (Con A), were also active. The inhibition by cytochalasin B and by high concentrations of D-glucose suggest that the basal and stimulated 2-DOG uptake occurred via a carrier-facilitated D-glucose transport system. The responses of the two macrophage populations to the hemopoietic growth factors and to the other agents were quite similar, suggesting that events that are important for the induction of DNA synthesis are not tightly coupled to the earlier rise in glucose uptake. For the BMM, the ability of a particular agent to stimulate glucose uptake did not parallel its ability to promote cell survival. However, stimulation of glucose uptake could still be a necessary but insufficient early macrophage response for cell survival and subsequent DNA synthesis.

Dexamethasone and sphingolipids inhibit concanavalin A stimulated glucose uptake in 3T3-L1 fibroblasts

Studies were carried out in 3T3-L1 fibroblasts to determine the effect of sphingolipids and corticosteroids on concanavalin A stimulated glucose uptake. Sphinganine (40 microM) and sphingosine (40 AND 100 microM) significantly inhibited concanavalin A stimulated glucose uptake (P less than 0.025 & less than 0.005). Prior incubation with 10(-7) M dexamethasone also significantly inhibited concanavalin A stimulated glucose uptake (P less than 0.005). The known effects of corticosteroids on sphingolipid synthesis, degradation, and tissue levels, suggest inhibition by corticosteroids of glucose uptake may be through effects of the steroid on sphingolipid metabolism.

Degradation and biosynthesis of the glucose transporter protein in chicken embryo fibroblasts transformed by the src oncogene

The rate of glucose transport in cultured fibroblasts is regulated to a number of physiological variables, including malignant transformation by src, glucose starvation, and stimulation with mitogens. Much of this transport regulation can be accounted for by variations in the amount of transporter protein in the cells. To determine the mechanisms by which levels of the transporter are regulated, we measured the rates of synthesis and degradation of the transporter by pulse-chase experiments and immunoprecipitation of the transporter. We found that transformation by the src oncogene results in a large decrease in the rate at which the transporter protein is degraded but that it does not appreciably increase the rate of transporter biosynthesis. On the other hand, glucose starvation and mitogen stimulation increase the rate of transporter biosynthesis, although a role for control of degradation is possible in these circumstances also. Variations in the rate of glucose transport or the amount of the transporter are not associated with phosphorylation of the transporter protein.

The glucose transporter in human fibroblasts is phosphorylated in response to phorbol ester but not in response to growth factors

The possibility that the stimulation of hexose transport in human fibroblasts by phorbol myristate acetate (PMA), insulin, platelet-derived growth factor (PDGF) or epidermal growth factor (EGF) is associated with phosphorylation of the glucose transporter has been investigated. The time and concentration dependencies of the stimulation of transport by these agents under conditions identical to those used for phosphorylation were determined. Each agent, when used at the concentration that resulted in the maximal increase in transport rate, elicited this effect within 30 min of exposure. The extent of stimulation ranged from 15 to 70%. For determination of phosphorylation of the glucose transporter, fibroblasts were incubated for 16 h with [32P]Pi and exposed to the agonist for 30 min; the transporter was then isolated from a detergent lysate of the cells by immunoprecipitation with a monoclonal antibody. Under these conditions, there was no phosphorylation of transporter in basal cells and only PMA caused detectable incorporation of phosphate into the transporter. Thus, it is unlikely that the stimulation of glucose transport by insulin, PDGF and EGF involve transporter phosphorylation.

Parallel changes in amino acid transport and protein kinase C localization in LLC-PK1 cells treated with TPA or diradylglycerols

Protein kinase C is considered to be a major target for tumor promoting phorbol esters such as 12-0-tetradecanoylphorbol-13-acetate (TPA). We have analyzed the correlation between A-system amino acid transport and the distribution of protein kinase C (PKC) between a membrane-rich fraction (100,000 g pellet) and cytosol (supernatant) from homogenized LLC-PK1 cells, a pig kidney epithelial cell line grown in culture. During log growth 1 day after seeding the cells onto culture plates, PKC activity is high in the membrane fraction and low in the cytosol. As the cells become confluent the PKC distribution shifts to a cytosolic pool. Concomitantly, A-system amino acid transport, as measured by methylaminoisobutyric acid [14C]MeAIB uptake, decreases. TPA (0.01-1.0 microM) induces a shift of PKC activity from the cytosol back to the membrane-rich fraction in post-confluent cells with a concomitant 2-3 fold stimulation of MeAIB uptake. The same responses can be achieved by treating cells with certain diradylglycerols, either diacylglycerols such as 1-oleyl-2-acetyl-sn-glycerol (OAG) or alkylacylglycerols such as 1-hexadecenyl-2-oleyl-sn-glycerol. Both responses to TPA are blocked by cytochalasin B, but cycloheximide inhibits the transport response without affecting PKC redistribution. It is suggested that the redistribution may be a necessary but not sufficient concomitant to the transport activation.

Contraction-associated translocation of protein kinase C in rat skeletal muscle

Electrical stimulation of the sciatic nerve of the anaesthetized rat in vivo led to a time-dependent translocation of protein kinase C from the muscle cytosol to the particulate fraction. Maximum activity of protein kinase C in the particulate fraction occurred after 2 min of intermittent short tetanic contractions of the gastrocnemius-plantaris-soleus muscle group and coincided with the loss of activity from the cytosol. Translocation of protein kinase C may imply a role for this kinase in contraction-initiated changes in muscle metabolism.

Degradation and biosynthesis of the glucose transporter protein in chicken embryo fibroblasts transformed by the src oncogene

The rate of glucose transport in cultured fibroblasts is regulated to a number of physiological variables, including malignant transformation by src, glucose starvation, and stimulation with mitogens. Much of this transport regulation can be accounted for by variations in the amount of transporter protein in the cells. To determine the mechanisms by which levels of the transporter are regulated, we measured the rates of synthesis and degradation of the transporter by pulse-chase experiments and immunoprecipitation of the transporter. We found that transformation by the src oncogene results in a large decrease in the rate at which the transporter protein is degraded but that it does not appreciably increase the rate of transporter biosynthesis. On the other hand, glucose starvation and mitogen stimulation increase the rate of transporter biosynthesis, although a role for control of degradation is possible in these circumstances also. Variations in the rate of glucose transport or the amount of the transporter are not associated with phosphorylation of the transporter protein.

Stimulation of leucine transport by a phorbol ester through activation of protein kinase C and Na+/H+ exchanger

A synthetic diacylglycerol, 1-oleoyl-2-acetyl-sn-glycerol (OAG), as well as 12-O-tetradecanoylphorbol-13-acetate (TPA) has been found to elevate the cytoplasmic pH and increase leucine uptake dose-dependently, when added to quiescent cultures of Chang liver cell. Addition of either a protein kinase C inhibitor, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), or an Na+/H+ antiporter inhibitor, ethylisopropylamiloride (EIPA), abolished completely or incompletely the TPA-stimulated leucine uptake and TPA-induced cytoplasmic alkalinization. Therefore the stimulation of leucine uptake by OAG and TPA is proposed to be elicited at least partly through activation of Na+/H+ antiporter. We suggest that activation of protein kinase C by the phorbol ester is responsible for the stimulation of Na+/H+ exchange system and also leucine uptake in the cell.

Induction of meiotic maturation in Xenopus oocytes by 12-O-tetradecanoylphorbol 13-acetate

Fully grown Xenopus oocytes are physiologically arrested at the G2/prophase border of the first meiotic division. Addition in vitro of progesterone or insulin causes release of the G2/prophase block and stimulates meiotic cell division of the oocyte, leading to maturation of the oocyte into an unfertilized egg. The possibility that the products of polyphosphoinositide breakdown, diacylglycerol and inositol-1,4,5-trisphosphate (IP3-, are involved in oocyte maturation was investigated. Microinjection of IP3 into oocytes just prior to addition of progesterone or insulin accelerated the rate of germinal vesicle breakdown (GVBD) by up to 25%. Half-maximal acceleration occurred at an intracellular IP3 concentration of 1 microM. Treatment of oocytes with the diacylglycerol analog and tumor promoter, 12-O-tetradecanoylphorbol 13-acetate (TPA) induced GVBD in the absence of hormone. Half-maximal induction of GVBD occurred with 150 nM TPA and was blocked by pretreatment of oocytes with 10 nM cholera toxin. Microinjection of highly purified protein kinase C from rat brain into oocytes did not induce maturation but markedly accelerated the rate of insulin-induced oocyte maturation. However, injection of the enzyme had no effect on progesterone action. In oocytes with a basal intracellular pH below 7.6, TPA increased intracellular pH, but GVBD occurred with TPA in Na-substituted medium. Neomycin, a putative inhibitor of polyphosphoinositide breakdown, reversibly inhibited insulin- but not progesterone-induced maturation. Half-maximal inhibition occurred at 1.6 mM neomycin. These results indicate that protein kinase C is capable of regulating oocyte maturation in Xenopus.

Elevated levels of glucose transport and transporter messenger RNA are induced by ras or src oncogenes

An accelerated rate of glucose transport is among the most characteristic biochemical markers of cellular transformation. To study the molecular mechanism by which transporter activity is altered, cultured rodent fibroblasts transfected with activated myc, ras, or src oncogenes were used. In myc-transfected cells, the rate of 2-deoxy-D-glucose uptake was unchanged. However, in cells transfected with activated ras and src oncogenes, the rate of glucose uptake was markedly increased. The increased transport rate in ras- and src-transfected cells was paralleled by a marked increase in the amount of glucose transporter protein, as assessed by immunoblots, as well as by a markedly increased abundance of glucose transporter messenger RNA. Exposure of control cells to the tumor-promoting phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) for 18 hours had a similar effect of increasing the rate of glucose transport and the abundance of transporter messenger RNA. For ras, src, and TPA, the predominant mechanism responsible for activation of the transport system is increased expression of the structural gene encoding the glucose transport protein.

A role for glucocorticoids in the polyphosphoinositide second messenger system

Glucocorticoids have been shown to be involved in numerous secretory and activation processes which are known to be mediated by the polyphosphoinositide second messenger system. A connection between glucocorticoids and the polyphosphoinositide system has not been made because of the marked temporal differences in their effects and the fact that most of the known effects of glucocorticoids involve transcription and/or protein synthesis. An attempt is made to to rationalize these apparent incongruities. The recently reported stimulation of glucose transport by kinase C suggests an experimental system to investigate glucocorticoid effects on the polyphosphoinositide system.

Acute insulin action requires insulin receptor kinase activity: introduction of an inhibitory monoclonal antibody into mammalian cells blocks the rapid effects of insulin

The role of the insulin receptor tyrosine kinase (protein-tyrosine kinase, EC 2.7.1.112) in various rapid insulin effects was studied by injecting four different cell types (by osmotic lysis of pinocytotic vesicles) with a monoclonal antibody that specifically inhibits the kinase activity of the insulin receptor and the closely related receptor for insulin-like growth factor (IGF)-I. Injection of this inhibitory antibody resulted in a decreased ability of insulin to stimulate the uptake of 2-deoxyglucose in Chinese hamster ovary cells and freshly isolated rat adipocytes, ribosomal protein S6 phosphorylation in CHO cells, and glycogen synthesis in the human hepatoma cell line HepG2. The ability of insulin, IGF-I, and IGF-II to stimulate glucose uptake in TA1 mouse adipocytes was also inhibited. Studies with CHO cells demonstrated that these effects of the inhibitory antibody were specific, since there was no change in phorbol ester-stimulated glucose uptake and injection of a noninhibiting antibody to the kinase had no effect on insulin action. These studies indicate that the tyrosine kinase activity of the insulin receptor is important in mediating several rapid insulin effects in a variety of different cell types.

Evidence that protein kinase C is involved in regulating glucose transport in the adipocyte

The role of protein kinase C in the mechanism of stimulation of glucose transport in rat adipocytes was investigated. Glucose transport was stimulated by dioleoylglycerol (DOG), tetradecanoyl phorbol acetate (TPA) and phospholipase C (PLC). Agents that inhibit protein kinase C (polymyxin B, gossypol and quercitin) also inhibited glucose transport that had been stimulated by DOG, TPA, PLC and insulin.

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.

Possible involvement of protein kinase C in the stimulation of amino acid transport by phorbol ester, platelet-derived growth factor and A23187 in Swiss 3T3 cells

Stimulation of amino acid transport induced by phorbol-12,13-dibutyrate, platelet-derived growth factor or A23187 was not observed in cells lacking protein kinase C. On the other hand, stimulation of transport by epidermal growth factor or insulin was not affected. These results suggested that the stimulation of amino acid transport is mediated by at least two separate pathways.

Reconstitution of glucose transport activity from erythrocyte membranes without detergent and its use in studying effects of ATP depletion

The direct reconstitution of unsolubilized membrane proteins by the freeze-thaw procedure avoids possible changes in properties produced by detergent solubilization and fractionation. Glucose transport activity was reconstituted using human erythrocyte membranes, with about 2/3 of the glucose uptake being stereo-specific. The highest specific activity occurred at low ratios of protein to lipid in the reconstitution, where most transport was due to liposomes containing single transporter molecules. Transporters were reconstituted with a scrambling of orientations, indicated by a 50% inactivation by added trypsin. Separation of unreconstituted protein doubled the specific activity. Similar results were obtained using the purified transporter (Wheeler, T.J. and Hinkle, P.C. (1981) J. Biol. Chem. 256, 8907-8914). The same ratio of net uptake to equilibrium exchange was observed for the two preparations. Their relative reconstituted transport activities and cytochalasin B binding activities were equal, indicating that the two were reconstituted with similar efficiencies. The decrease in glucose transport in erythrocytes produced by ATP depletion and the stimulation produced by resealing with ATP (Jacquez, J.A. (1983) Biochim. Biophys. Acta 727, 367-378) were confirmed. However, no difference was observed in reconstituted transport activity using ghosts resealed with or without ATP, indicating that ATP produces indirect effects rather than modifications of the transporter.

Studies and perspectives of protein kinase C

Protein kinase C, an enzyme that is activated by the receptor-mediated hydrolysis of inositol phospholipids, relays information in the form of a variety of extracellular signals across the membrane to regulate many Ca2+-dependent processes. At an early phase of cellular responses, the enzyme appears to have a dual effect, providing positive forward as well as negative feedback controls over various steps of its own and other signaling pathways, such as the receptors that are coupled to inositol phospholipid hydrolysis and those of some growth factors. In biological systems, a positive signal is frequently followed by immediate negative feedback regulation. Such a novel role of this protein kinase system seems to give a logical basis for clarifying the biochemical mechanism of signal transduction, and to add a new dimension essential to our understanding of cell-to-cell communication.

Sphingolipids inhibit insulin and phorbol ester stimulated uptake of 2-deoxyglucose

Studies are presented demonstrating inhibition of both insulin and phorbol myristate acetate stimulated uptake of 2-deoxyglucose uptake by 3T3-L1 fibroblasts. Greatest inhibition of uptake was seen with sphinganine while sphingosine was also potent in this regard. Ceramide inhibited phorbol myristate acetate but not insulin stimulation of uptake. It is suggested that sphingolipid inhibition of glucose transport relates to the previously demonstrated effect of corticosteroids to increase membrane sphingomyelin and inhibit glucose transport.

Protein kinase C of human erythrocytes phosphorylates bands 4.1 and 4.9

Addition of 10 nM 12-O-tetradecanoylphorbol 13-acetate (TPA) to intact human erythrocytes results in rapid phosphorylation of two cytoskeletal components, bands 4.1 and 4.9. The synthetic diacylglycerol, 1-oleoyl-2-acetylglycerol, shows a similar effect, while the biologically inactive phorbol ester, 4 alpha-phorbol didecanoate, fails to enhance phosphorylation. That TPA and 1-oleoyl-2-acetylglycerol stimulate this phosphorylation suggests that protein kinase C is being activated. In the presence of TPA, bands 4.1 and 4.9 incorporate 1.5 mol Pi/mol protein and 1.2 mol Pi/mol protein, respectively. The pattern and extent of phosphorylation shows that it is not due to cAMP-dependent protein kinases, which also phosphorylate bands 4.1 and 4.9. Ca2+-phospholipid-dependent protein kinase activity is demonstrable in the soluble fraction of erythrocytes, and has been partially purified (2200-fold) from the hemolysate by affinity chromatography (Uchida and Filburn, 1984. J. Biol. Chem. 259, 12311-12314). The affinity purified erythrocyte kinase has a 42 A Stokes' radius and phosphorylates purified bands 4.1 and 4.9 in vitro in a Ca2+- and phospholipid-dependent manner. These results show that human erythrocytes contain protein kinase C, and that band 4.1 and 4.9 are the major endogenous substrates for this kinase.

Protein kinase C and an endogenous substrate associated with adenohypophyseal secretory granules

Secretory granules isolated from anterior pituitary glands were examined for Ca2+/phospholipid-dependent protein kinase (protein kinase C) activity as well as the occurrence of granule-associated substrate proteins. Sheep adenohypophyses were fractionated by differential and sucrose-density-gradient centrifugation to yield a granule fraction enriched for luteinizing-hormone (lutropin)-containing secretory granules. Marker-enzyme analysis showed no detectable cytosolic contamination, although there were small amounts of plasma membranes (2-4%) and lysosomes (4-6%) associated with the preparation. As determined by histone-H1 phosphorylation after DEAE-cellulose DE-52 chromatography, protein kinase C activity with a marked dependence on Ca2+ and lipid (4-fold increase in their presence) was evident in the secretory-granule fraction. Phosphorylation in vitro of the secretory-granule fraction by endogenous and exogenous protein kinase C revealed a protein of Mr 36,000, which by two-dimensional SDS/polyacrylamide-gel electrophoresis showed multiple sites of phosphorylation. The Mr-36,000 protein was not found in cytosolic or plasma-membrane fractions and was not phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase. Several secretory-granule proteins served as substrates for the catalytic subunit, the most prominent of which were of Mr 63,000, 23,000 and 21,000. From these data, we suggest that phosphorylation of secretory-granule-associated proteins by protein kinase C and by cyclic AMP-dependent protein kinase may be important in secretion regulation in the anterior pituitary gland.

Effect of protein kinase C activation and Ca2+ mobilization on hexose transport in Swiss 3T3 cells

Down-modulation of Ca2+-activated, phospholipid-dependent protein kinase (protein binase C), which was accomplished by pretreatment with phorbol-12,13-dibutyrate for 24 h, resulted in the loss of a phorbol ester-induced stimulation of hexose transport activity in Swiss 3T3 cells. In these cells, however, platelet-derived growth factor as well as Ca2+ ionophore A23187 were still able to induce stimulation of hexose transport activity accompanied by the elevation of intracellular free Ca2+ concentration. Since chelation of extracellular Ca2+ inhibited this stimulation, inflow of extracellular Ca2+ into cytoplasm seemed to be essential for the stimulatory effect of platelet-derived growth factor and A23187 on hexose transport. Epidermal growth factor and insulin also stimulated hexose transport activity regardless of the absence of protein kinase C. However, in the case of epidermal growth factor, intracellular Ca2+, but not extracellular Ca2+, was found to be necessary for the stimulation. On the other hand, insulin stimulated the hexose transport independent of both intra- and extracellular Ca2+.

Regulation of hexose transporters in chicken embryo fibroblasts: stimulation by the phorbol ester TPA leads to increased numbers of functioning transporters

As has been observed with many types of cultured cells, chicken embryo fibroblasts (CEF) when exposed to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) develop a 3- to 4-fold increase in hexose transport activity in 4 h. This increase in transport activity occurred despite a modest decline of 20% in [3H]leucine incorporation into acid insoluble fractions. Cycloheximide largely, but not completely, blocked the increase in transport activity during TPA exposure. The effects of TPA were somewhat similar to those of glucose starvation induced enhancement of hexose transport activity. Furthermore, with TPA there was no additive effect to that produced by glucose starvation. Plasma membrane enriched fractions were prepared from CEF treated with or without TPA. Membranes prepared from TPA exposed cells had a two-fold enhancement of stereospecific D-glucose transport activity as well as D-glucose inhibitable [3H]cytochalasin B binding as compared to the membranes from control CEF. There was no effect on transport when membranes were exposed to TPA in vitro. These results provide strong evidence that TPA exposure leads to an increase in the number of functioning transporters, an effect largely requiring protein synthesis.

Purified pyruvate kinases type M2 from unfertilized hen's egg are substrates of protein kinase C

To characterize pyruvate kinase isoenzymes from cells with the capability to proliferate, this enzyme was purified from yolk and vitelline membrane of unfertilized hen's egg. Pyruvate kinase type M2 from vitelline membrane was obtained in a homogeneous form after a 1150-fold purification to a specific enzymatic activity of 450 mumol X min-1 X mg-1. It was saturated half-maximally with phosphoenolpyruvate at KPPrv0.5 = 0.36 mM phosphoenolpyruvate and was activity by fructose 1,6-bisphosphate and L-serine at suboptimal substrate concentrations. After 11 000-fold purification to a specific enzymatic activity of 60 mumol X min-1 X mg-1, the pyruvate kinase isoenzymes type M2 (KPPrv0.5 = 0.32 mM) and M1 (KPPrv0.5 = 0.04 mM) were obtained from the yolk substance. Kinetic differences were noted between the pyruvate kinase type-M2 isoenzymes from vitelline membrane and yolk. A comparison of the amino acid composition of the purified pyruvate kinase isoenzymes from hen's egg revealed that all isoenzymes were related to pyruvate kinase type M1 from chicken breast muscle. The M2-type isoenzyme from vitelline membrane was related to the M2-type isoenzyme from chicken tumors, but was not related to the M2-type pyruvate kinase from chicken lung or liver. Protein kinase C from chicken oviduct phosphorylated in vitro both pyruvate kinase M2 isoenzymes from the unfertilized hen's egg preferably at serine and less at threonine residues. Pyruvate kinase type M1 from egg yolk was a weak substrate of protein kinase C. An activation of pyruvate kinase type M2 from vitelline membrane was observed at suboptimal concentrations of phosphoenolpyruvate under the conditions of phosphorylation, in the presence of phosphatidylserine.

Proliferating and quiescent cells exhibit different subcellular distribution of protein kinase C activity

The activity of calcium, phospholipid-dependent protein kinase (PKc), which is thought to play an important role in cell proliferation, has been measured in the particulate and soluble fractions of cultured cells, under different proliferative conditions. Our results indicate that proliferating cells display higher PKc activity than quiescent cells. Furthermore, in both normal and transformed cells, PKc is preferentially associated with the particulate fraction when the cells are proliferating, while in mitotically quiescent cells the majority of the enzyme activity is found in the soluble fraction. These data suggest tha PKc activity and subcellular distribution undergo spontaneous changes according to the proliferative state of the cells.