Effects of vanadate on protein kinases in rat hepatocytes.

In rat hepatocytes, vanadate modifies neither the intracellular concentration of cyclic AMP nor the --cyclic AMP/+cyclic AMP activity ratio for cyclic AMP-dependent protein kinase. Vanadate can, however, counteract the increase in cyclic AMP and the increase in the --cyclic AMP/+cyclic AMP activity ratio of cyclic AMP-dependent protein kinase induced by glucagon. On the other hand, vanadate treatment of hepatocytes can produce a time- and concentration-dependent increase in cyclic AMP- and Ca2+-independent casein kinase activity. Maximal activation at the optimal time with 5 mM-vanadate was about 70% over control. A clear relationship was observed between the activation of casein kinase and the inactivation of glycogen synthase after vanadate treatment. These results suggest that casein kinase activity may be involved in vanadate actions in rat hepatocytes.     

Two different mechanisms for activation of cyclic PIP synthase: by a G protein or by protein tyrosine phosphorylation

The biosynthesis of the functional, endogenous cyclic AMP antagonist, prostaglandylinositol cyclic phosphate (cyclic PIP) is performed by the plasma membrane-bound enzyme cyclic PIP synthase, which combines prostaglandin E (PGE) and activated inositol phosphate (n-IP) to cyclic PIP. The Km values of the enzyme for the substrates PGE and n-IP are in the micromolar range. The plasma membrane-bound synthase is activated by fluoride, by the stable GTP analog GMP-PNP, by protamine or biguanide, by noradrenaline, and by insulin. The activation by protamine or biguanide and fluoride (10 mM) is additive, which may indicate the presence of two different types of enzyme, comparable to phospholipase Cbeta and phospholipase Cgamma. Plasma membrane-bound cyclic PIP synthase is inhibited by the protein tyrosine kinase inhibitor tyrphostin B46 with an IC50 of 1.7 microM. However, the solubilized and gel-filtrated enzyme is no longer inhibited by tyrphostin, indicating that the activity of cyclic PIP synthase is connected with the activity of a membrane-bound protein tyrosine kinase. Cyclic PIP synthase activity of freshly prepared plasma membranes is unstable. Upon freezing and rethawing of liver plasma membranes, this instability is increased about 2-fold. Protein tyrosine phosphatase inhibitors [vanadate, fluoride (50-100 mM)] stabilize the enzyme activity, but protease inhibitors do not, indicating that inactivation of the enzyme is connected with protein tyrosine dephosphorylation. Cyclic PIP synthase is present in all tissues tested, like brain, heart, intestine, kidney, liver, lung, skeletal muscle, spleen, and testis. Apart from liver, cyclic PIP synthase activity in most tissues is rather low, but it can be increased up to 5-fold when protein tyrosine phosphatase inhibitors like vanadate are present in the homogenization buffer. Preincubation of cyclic PIP synthase of liver plasma membranes with the tyrosine kinase src kinase causes a 2-fold increase of cyclic PIP synthase activity, though this is certainly not the physiological role played by src kinase in intact cells. The data indicate that cyclic PIP synthase can be activated by two separate mechanisms: by a G protein or by protein tyrosine phosphorylation.     

Potentiation of Agonist-Stimulated Cyclic AMP Accumulation by Tyrosine Kinase Inhibitors in Rat Pinealocytes

Abstract: To study cross-talk mechanisms in rat pinealocytes, the role of tyrosine kinase or kinases in the regulation of adrenergic-stimulated cyclic AMP production was investigated. Both norepinephrine- and isoproterenol-stimulated cyclic AMP accumulation were increased by two distinct tyrosine kinase inhibitors, genistein or erbstatin, in a concentration-dependent manner. A similar increase was observed with two other inhibitors, tyrphostin B44 and herbimycin. In contrast, daidzein, an inactive analogue of genistein, was ineffective; whereas vanadate, a phosphotyrosine phosphatase inhibitor, reduced the adrenergic-stimulated cyclic AMP accumulation. The tyrosine kinase inhibitors were effective in potentiating the cholera toxin-or forskolin-stimulated cyclic AMP accumulation, indicating that their sites of action are at the postreceptor level. Neither an activator nor inhibitors of protein kinase C influenced the potentiation of the cyclic AMP responses by genistein, suggesting that the potentiation effect by tyrosine kinase inhibitors does not involve the phospholipase C/protein kinase C pathway. However, when the phosphodiesterase was inhibited by isobutylmethylxanthine, genistein failed to potentiate and vanadate did not inhibit the adrenergic-stimulated cyclic AMP accumulation, indicating that the phosphodiesterase is a probable site of action for these inhibitors. These results suggest that cyclic AMP metabolism in the pinealocytes is tonically inhibited by tyrosine kinase acting on the cyclic AMP phosphodiesterase.     

The release of superoxide anion from granulocytes: effect of inhibitors of anion permeability.

In vivo administration of ecdysterone produced a decrease in cyclic AMP levels and cyclic AMP-binding protein activity in mouse liver 40 min after injection. These changes were accompanied by a concomitant decrease in cyclic AMP-dependent protein kinase. The effect on phosphoprotein phosphatase was the opposite pattern of that on protein kinase. These results support the idea that the cyclic AMP-protein kinase system may be involved in the heterophylic action of ecdysterone.     

Peroxide(s) of vanadium: a novel and potent insulin-mimetic agent which activates the insulin receptor kinase

The actions of insulin, vanadate (V) and hydrogen peroxide (H2O2) on IGF-II binding and insulin receptor tyrosine kinase activity were studied in rat adipocytes. Incubating adipocytes with a combination of V plus H2O2 resulted in a potent synergistic effect on both the increase in IGF-II binding and the activation of the insulin receptor kinase. Catalase, which removes H2O2, abolished this synergism if added at the time of mixing of V plus H2O2 but not if added 10 min. later, suggesting that the formation of peroxide(s) of vanadate generated a potent insulin mimicker. The data support a critical role for the insulin receptor kinase in insulin action. The novel insulin-mimetic compound, a presumed peroxide of vanadate, could prove useful for investigating insulin action and may be valuable for treating insulin resistance.     

Decrease of liver protein kinase C in streptozotocin-induced diabetic rats and restoration by vanadate treatment

Effects of streptozotocin-induced diabetes and administration of the insulinmimetic agent, vanadate in rats on the liver protein kinase C-induced phosphorylation of exogenous C (Histone III-S) and endogenous substrates were investigated. Diabetes caused a significant fall (40-60%) in liver cytosolic protein C activity measured using both types of substrates. Vanadate treatment for a period of 5 weeks restored them to normal levels. Phosphorylation of cytosolic target proteins for protein kinase C followed a similar pattern in response to diabetes and vanadate. These treatments had no effect on particulate protein kinase C activity. Vanadate also had no effect in normal livers with respect to the protein kinase C system.     

The mixed-lineage kinase DLK undergoes Src-dependent tyrosine phosphorylation and activation in cells exposed to vanadate or platelet-derived growth factor (PDGF)

Some data in the literature suggest that serine/threonine phosphorylation is required for activation of the mixed-lineage kinases (MLKs), a subgroup of mitogen-activated protein kinase kinase kinases (MAPKKKs). In this report, we demonstrate that the MLK family member DLK is activated and concurrently tyrosine-phosphorylated in cells exposed to the protein tyrosine phosphatase inhibitor vanadate. Tyrosine phosphorylation appears crucial for activation as incubation of vanadate-activated DLK molecules with a tyrosine phosphatase substantially reduced DLK enzymatic activity. Interestingly, the effects of vanadate on DLK are completely blocked by treatment with a Src family kinase inhibitor, PP2, or the expression of short hairpin RNA (shRNA) directed against Src. DLK also fails to undergo vanadate-stimulated tyrosine phosphorylation and activation in fibroblasts which lack expression of Src, Yes and Fyn, but reintroduction of wild-type Src or Fyn followed by vanadate treatment restores this response. In addition to vanadate, stimulation of cells with platelet-derived growth factor (PDGF) also induces tyrosine phosphorylation and activation of DLK by a Src-dependent mechanism. DLK seems important for PDGF signaling because its depletion by RNA interference substantially reduces PDGF-stimulated ERK and Akt kinase activation. Thus, our findings suggest that Src-dependent tyrosine phosphorylation of DLK may be important for regulation of its activity, and they support a role for DLK in PDGF signaling.     

Activity and subcellular distribution of protein kinase dependent on adenosine 3':5'-monophosphate in liver from normal and adrenalectomized rats.

We have examined whether glucocorticoids control the activity and (or) the subcellular distribution of protein kinase dependent on cyclic AMP (adenosine 3':5'-monophosphate), since they influence cyclic-AMP-dependent responses to other hormones. Protein kinase activity was determined in rat liver homogenates and subcellular fractions, nuclear, large granular, microsomal and supernatant obtained by differential sedimentation in 0.25 M sucrose. 63% of the tissue protein kinase activity detected in absence of cyclic AMP reside in the particulate fractions. Upon addition of exogenous cyclic AMP, protein kinase activity is stimulated 1.8, 1.2, 1.2 and 4.5-fold in nuclear, large granular, microsomal and supernatant fractions, respectively. Under these conditions, 66% of tissue activity are found in the supernatant fraction. The activity sensitive to exogenous cyclic AMP resolves into a major (84%) cytosoluble and a minor (16%) nucleomicrosomal component. The latter activity resists elution with isotonic saline and is increased in the presence of Triton X-100. Three groups of rats were studied: control and adrenalectomized with or without cortisol treatment. In whole liver homogenates, both protein kinase activity detected in absence of exogenous cyclic AMP and sensitivity of the enzyme to cyclic AMP were comparable in all groups. Moreover, the distribution patterns of proteins kinase activity amoung the fractions were essentially the same in all groups of animals, whether or not particles had been treated with Triton X-100. Finally, in cell-free experiments, glucocorticoids alone or in combination with their intracellular receptor did not modify protein kinase activity of rat liver. Thus the results reported do not support the possibility that glucocorticoids influence cyclic AMP-dependent protein kinase in rat liver. Yet, this study provides data, not available before, on subcellular distribution of this enzyme in rat liver.     

Effects of vanadate, insulin and fenugreek (Trigonella foenum graecum) on creatine kinase levels in tissues of diabetic rat

The in vivo effects of insulin, and other insulino mimetic agents like vanadate and fenugreek (T. foenum graecum) were followed on the changes in the activities of creatine kinase in heart, skeletal muscle and liver of experimental diabetic rats. As compared to control rats, creatine kinase activities were found to decrease significantly in the tissues during experimental diabetes. All the antidiabetic compounds used namely, insulin, vanadate and Fenugreek seed powder normalised the decreased activities to almost control values. The effects of insulin and vanadate were comparable in restoring normoglycemia and the creatine kinase activities.     

In vivo effects of vanadate on hepatic glycogen metabolizing and lipogenic enzymes in insulin-dependent and insulin-resistant diabetic animals

The insulin-mimetic action of vanadate is well established but the exact mechanism by which it exerts this effect is still not clearly understood. The role of insulin in the regulation of hepatic glycogen metabolizing and lipogenic enzymes is well known. In our study, we have, therefore, examined the effects of vanadate on these hepatic enzymes using four different models of diabetic and insulin-resistant animals. Vanadate normalized the blood glucose levels in all animal models. In streptozotocin-induced diabetic rats, the amount of liver glycogen and the activities of the active-form of glycogen synthase, both active and inactive-forms of phosphorylase, and lipogenic enzymes like glucose 6-phosphate dehydrogenase and malic enzyme were decreased and vanadate treatment normalized all of these to near normal levels. The other three animal models (db/db mouse, sucrose-fed rats and fa/fa obese Zucker rats) were characterized by hyperinsulinemia, hypertriglyceridemia, increases in activities of lipogenic enzymes, and marginal changes in glycogen metabolizing enzymes. Vanadate treatment brought all of these values towards normal levels. It should be noted that vanadate shows differential effects in the modulation of lipogenic enzymes activities in type I and type II diabetic animals. It increases the activities of lipogenic enzymes in streptozotocin-induced diabetic animals and prevents the elevation of activities of these enzymes in hyperinsulinemic animals. The insulin-stimulated phosphorylation of insulin receptor subunit and its tyrosine kinase activity was increased in streptozotocin-induced diabetic rats after treatment with vanadate. Our results support the view that insulin receptor is one of the sites involved in the insulin-mimetic actions of vanadate.     

Independent modulation of hepatic protein kinase activities.

The effects of hormonal status on protein kinase activity was examined in homogenates of rat liver. Protein kinase activity was evaluated from incorporation of 32P from [gamma-32P]ATP into protamine or histone as receptor substrates. Protamine phosphorylation in the presence or absence of cyclic AMP exceeded histone phosphorylation by at least a factor or two. Hypophysectomy markedly increased protamine phosphorylation in the presence or absence of saturating amounts of cyclic AMP. In contrast, hypophysectomy only slightly increased cyclic AMP independent phosphorylation of histone. These results could not be amounted for by differences in ATPase or protein phosphase activities. Cortisone (2 mg/day x 3) decreased total protein kinase activity in livers of hypophysectomized rats when protamine was substrate, but had no effect on the total activity toward histone. Growth hormone (100 mug/day x 3) significantly increased histone, but not protamine phosphorylation in livers of hypophysectomized rats. Administration of 5 mug of triiodothyonine/day to hypophysectomized rats also markedly increased the phosphorylation of histone, but not protamine when saturating amounts of cyclic AMP were present. These results support the hypothesis that liver may contain more than one type of protein kinase activity and that the different protein kinase activities can be separately affected by hormones. Such control distal to cyclic AMP might allow selective modulation of cyclic AMP-dependent processes in cells which carry out more than one such process.     

Stimulation of tyrosine-specific protein phosphorylation and phosphatidylinositol phosphorylation by orthovanadate in rat liver plasma membrane

Orthovanadate stimulated the incorporation of 32P from [gamma-32P]ATP by Triton X-100-solubilized rat liver plasma membrane into endogenous, trichloroacetic acid-precipitable materials as well as added (Glu4:Tyr1) copolymers. Extraction of incubation mixture with chloroform-methanol-HCl revealed that the increase in 32P incorporation by vanadate was predominantly into endogenous phospholipids. [32P]Phosphatidylinositol 4-phosphate (PtdIns-4-P) was identified by thin-layer chromatography as the major phosphorylated product of vanadate stimulation, which also resulted in elevated 32P, predominantly in P-Tyr in endogenous membrane proteins. Vanadate effects on protein tyrosine and phosphatidylinositol phosphorylation were concomitant and exhibited similar sensitivity. These effects of vanadate were enhanced by the presence of either dithiothreitol or NAD(P)H. Phosphatidylinositol phosphorylation could also be stimulated by a substrate of and inhibited by a synthetic inhibitory copolymer of tyrosine kinase. These results suggest that vanadate, an oxygen radical producer, stimulates a tyrosine kinase-PtdIns kinase coupled system much like those described for a number of growth factors and oncogene encoded products.     

Activation of cyclic AMP-dependent protein kinase(s) by growth hormone in the liver and adrenal gland of the rat.

A single dose of growth hormone (10 mg/kg, i.p.) was injected into male weanling rats (50--60 g), and the temporal changes in cyclic AMP concentration, protein kinase activation, and ornithine decarboxylase activation were measured in the liver and adrenal gland. The level of cyclic AMP did not change significantly from control values in either liver or adrenal following growth hormone administration. Cyclic AMP-dependent protein kinase(s); however, was markedly activated in liver and adrenal within 30 min. Protein kinase remained activated for more than 4 hr in the liver, while activation of protein kinase in the adrenal returned to control value within 2 hr. Ornithine decarboxylase activity was elevated 20-fold in liver within 4 hr of injection and was increased 7- to 8-fold in be adrenal within l hr. These observations are discussed with regard to the generality of the role of cyclic AMP as the second messenger for target-specifici trophic hormone action and the significance of protein kinase activiation as an index of the cyclic nucleotide involvement in the growth response.     

Regulation of pp90rsk phosphorylation and S6 phosphotransferase activity in Swiss 3T3 cells by growth factor-, phorbol ester-, and cyclic AMP-mediated signal transduction.

Somatic cell homologs to the Xenopus laevis S6 protein kinases (referred to collectively as pp90rsk) have recently been identified and partially characterized. Here we examine alterations in pp90rsk phosphorylation and S6 phosphotransferase activity in response to regulators of multiple signal transduction systems: purified growth factors, phorbol ester, changes in cyclic AMP (cAMP) levels, and sodium vanadate. All reagents tested increased pp90rsk serine and threonine phosphorylation, but only those agents that regulate cell proliferation and sodium vanadate activated its S6 kinase activity. In addition to the cAMP-stimulated phosphorylation of pp90rsk, a simple correlation between the extent of growth-regulated pp90rsk phosphorylation and S6 phosphotransferase activity was not observed. Quantitative phosphorylation of pp90rsk continued to increase after its S6 kinase activity began its return towards basal levels. However, a close correlation between the appearance and disappearance of a slow-mobility form of phosphorylated pp90rsk (by electrophoresis) and pp90rsk activity was observed. In addition, pp90rsk was regulated by both protein kinase C-independent and -dependent signaling mechanisms. The extent of protein kinase C participation, however, varied depending on which growth factor receptor was activated. Furthermore, growth factor-specific differences in the temporal regulation of pp90rsk S6 phosphotransferase activity were also observed. These results support the notion that the complex regulation of the rsk gene product constitutes one of the primary responses of animal cells to mitogenic signals.     

Ontogeny of cyclic AMP-dependent protein phosphokinase during hepatic development of the rat.

The ontogeny of protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) and cyclic AMP-binding activity in subcellular fractions of liver was examined during prenatal and postnatal development of the male rat. 1. Protein kinase activity and cyclic AMP-binding activity were found in the nuclear, microsomal, lysosomal-mitochondrial, and soluble liver fractions. 2. The protein kinase activity of the soluble (105 000 X g supernatant) fraction measured with histone F1 as substrate was stimulated by cyclic AMP. Cyclic AMP did not stimulate the protein kinase activity of the particulate fractions. 3. The protein kinase activity of all subcellular fractions increased rapidly from the activity observed in prenatal liver (3-4 days before birth) to reach maximal activity in 2-day-old rats. Thereafter, the protein kinase activity declined more slowly and regained the prenatal levels at 10 days after birth. 4. Considerable latent protein kinase activity was associated with liver microsomal fractions which could be activated by treatment of microsomes with Triton X-100. The latent microsomal protein kinase activity was highest in prenatal liver, at the time of birth, and 2 days after birth. During the subsequent postnatal development the latent microsomal protein kinase activity gradually declined to insignificantly low levels. 5. During the developmental period examined (4 days before birth to age 60-90 days) marked alterations of the cyclic AMP-binding activity were determined in all subcellular fractions of rat liver. In general, cytosol, microsomal, and lysosomal-mitochondrial cyclic AMP-binding activity was highest in 10-11 day-old rats. Nuclear cyclic AMP-binding activity was highest 3-4 days before birth and declined at birth and during the postnatal period. There was no correlation between the developmental alteration of cyclic AMP-binding activity and cyclic AMP dependency of the protein kinase activity in any of the subcellular fractions. This suggests that the measured cyclic AMP-binding activity does not reflect developmental alterations of the cyclic AMP-binding regulatory subunit of cyclic AMP-dependent protein kinase.     

Vanadate, tungstate and molybdate activate rod outer segment phosphodiesterase in the dark

Anionic activation of rod outer segment phosphodiesterase by vanadate, molybdate and tungstate is demonstrated. Comparisons are made to adenylate cyclase, which is known to be activated by vanadate and molybdate but not by tungstate. In view of the differences in anionic activation between these two important enzymatic regulators of intracellular cyclic nucleotide metabolism, it is possible that tungstate can be used as a selective probe for the effects of phosphodiesterase activity in photoreceptors and other cells. The known electrophysiological stimulation of Limulus photoreceptors by these anions is also interpreted in light of our results. If anionic production of quantum bumps in Limulus photoreceptors is mediated by changes in cyclic nucleotides, then the electrophysiological response of Limulus photoreceptors to tungstate may indicate a role for phosphodiesterase rather than adenylate cyclase in mediating light-induced cyclic nucleotide alterations in this cell.     

Induction of c-jun independent of PKC, pertussis toxin-sensitive G protein, and polyamines in quiescent SV40-transformed 3T3 T cells.

CSV3 clones of simian virus 40 large T antigen-transformed murine 3T3 T cells can be made quiescent as part of a differentiation process. In these quiescent cells, insulin- and vanadate-induced mitogenesis are both associated with the induction of the c-jun proto-oncogene (Wang and Scott 1991 J. Cell. Physiol. 147, 102-110; Wang et al. 1991 Cell Growth Differ. 2, 645-652). The current studies were therefore designed to compare the early signal transduction pathways employed by insulin and vanadate to regulate c-jun expression. In quiescent CSV3-1 cells, down-regulation of protein kinase C by prolonged exposure to 12-O-tetra-decanoylphorbol-13-acetate or inhibition of protein kinase C activity by treatment with the protein kinase C antagonist staurosporine is shown not to affect c-jun induction by insulin or vanadate. This suggests that both insulin and vanadate act in a protein kinase C-independent manner. Insulin's effect on c-jun induction does, however, involve a G protein because insulin's effect can be inhibited by pertussis toxin. In contrast, vanadate induction of c-jun is not affected by pertussis toxin. Genistein, a general tyrosine kinase inhibitor, can inhibit the ability of vanadate to induce c-jun but it does not inhibit insulin's effect. Finally, the depletion of polyamines, particularly spermidine, by DL-alpha-difluoromethylornithine treatment also prevents c-jun induction by insulin but DL-alpha-difluoromethylornithine treatment has no effect on c-jun induction by vanadate. These observations indicate that the c-jun induction by insulin and vanadate in CSV3-1 cells is mediated by different signal transduction mechanisms. Together with our previously published data, these results suggest that c-jun can be induced independent of protein kinase C activation, without involvement of pertussis toxin-sensitive G protein, independent of induction of c-fos, and without expression of high levels of intracellular polyamines.     

Amino acid sequence of the phosphorylation site of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase from rat liver was phosphorylated by cyclic AMP-dependent protein kinase and [gamma-32P]ATP. Treatment of the 32P-labeled enzyme with thermolysin removed all of the radioactivity from the enzyme core and produced a single labeled peptide. The phosphopeptide was purified by ion exchange chromatography, gel filtration, and reverse phase high pressure liquid chromatography. The sequence of the 12-amino acid peptide was found to be Val-Leu-Gln-Arg-Arg-Arg-Gly-Ser(P)-Ser-Ile-Pro-Gln. Correlation of the extent of phosphorylation with activity showed that a 50% decrease in the ratio of kinase activity to bisphosphate activity occurred when only 0.25 mol of phosphate was incorporated per mol of enzyme subunit, and maximal changes occurred with 0.7 mol incorporated. The kinetics of cyclic AMP-dependent protein kinase-catalyzed phosphorylation of the native bifunctional enzyme was compared with that of other rat liver protein substrates. The Km for 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase (10 microM) was less than that for rat liver pyruvate kinase (39 microM), fructose-1,6-bisphosphatase (222 microM), and 6- phosphofructose -1-kinase (230 microM). Comparison of the initial rate of phosphorylation of a number of protein substrates of the cyclic AMP-dependent protein kinase revealed that only skeletal muscle phosphorylase kinase was phosphorylated more rapidly than the bifunctional enzyme. Skeletal muscle glycogen synthase, heart regulatory subunit of cyclic AMP-dependent protein kinase, and liver pyruvate kinase were phosphorylated at rates nearly equal to that of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase, while phosphorylation of fructose-1,6-bisphosphatase and 6-phosphofructo-1-kinase was barely detectable. Phosphorylation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase was not catalyzed by any other protein kinase tested. These results are consistent with a primary role of the cyclic AMP-dependent protein kinase in regulation of the enzyme in intact liver.     

The effect of vanadate on Pichia pastoris growth,protein kinase A activity and ribosomal protein phosphorylation

It was found that wild type yeast Pichia pastoris can tolerate vanadate concentration as high as 25 mM in the growth medium. Moreover, four vanadate-resistant P. pastoris strains designated JC100/1, JC100/3, JC100/9 and JC100/15 exhibiting tolerance up to 150 mM vanadate were selected. Growth of P. pastoris was correlated with vanadate to vanadyl reduction and its accumulation in the growth medium. In two selected strains, JC100/9 and JC100/15, protein kinase A activity was much higher in comparison to the wild type strain even without vanadate addition to the growth medium. Moreover, in the presence of vanadate, protein kinase A activity was significantly increased in the wild type and the vanadate-resistant JC100/1 and JC100/3 strains. It was also found that phosphorylation of a 40 kDa protein associated with ribosomes occured in all vanadate-resistant strains from the logarithmic, while in the wild type strain from the stationary growth phase. From the presented results it can be concluded that a protein kinase A signalling pathway(s) might be involved in the mechanism of P. pastoris vanadate resistance. The results also indicate a possible role of the 40 kDa protein in protection of P. pastoris against vanadate toxicity.