Vasopressin rapidly stimulates protein kinase C in digitonin-permeabilized Swiss 3T3 cells: involvement of a pertussis toxin-insensitive guanine nucleotide binding protein

Guanine nucleotides and pertussis toxin were used to test for the involvement of a guanine nucleotide binding protein in the vasopressin V1 receptor-mediated stimulation of protein kinase C activity in Swiss 3T3 cells. Addition of vasopressin in the presence of [gamma-32P]ATP and digitonin caused a marked and rapid increase (8 +/- 1-fold after 1 min) in the phosphorylation of an Mr = 80,000 cellular protein (80K), a specific marker for protein kinase C activation. This phosphorylation was selectively blocked by the V1 receptor antagonist Pmp1-0-Me-Tyr2 [Arg8] vasopressin, indicating that the effect was mediated through the vasopressin V1 receptor. Down regulation of protein kinase C by prior prolonged pretreatment of intact cells with phorbol 12,13-dibutyrate (PBt2) blocked the ability of vasopressin to stimulate the phosphorylation of 80K in digitonin-permeabilized cells. Addition of a submaximal concentration of vasopressin together with the GTP analogue GTP-gamma-S caused a synergistic stimulation of 80K phosphorylation. The GDP analogue GDP-beta-S caused a 50% inhibition of the phosphorylation of 80K induced by a saturating concentration of vasopressin and shifted the vasopressin dose-response curve to the right. GDP-beta-S had no effect on the dose-response for the stimulation of 80K phosphorylation induced by PBt2. Prior incubation of intact quiescent cultures of Swiss 3T3 cells with pertussis toxin did not impair either vasopressin-induced increase in cytosolic [Ca2+] or activation of protein kinase C. These findings provide functional evidence for the involvement of a pertussis toxin-insesitive G protein in the vasopressin V1 receptor-mediated stimulation of protein kinase C in Swiss 3T3 cells.     

Vasopressin rapidly stimulates NA entry and NA-K pump activity in quiescent cultures of mouse 3T3 cells

Addition of (Arg) vasopressin to quiescent cultures of Swiss 3T3 cells rapidly stimulates an ouabain-sensitive ⁸⁶Rb uptake. In contrast the hormone has no significant effect on the rate of efflux of this cation from preloaded cells. The stimulation of ⁸⁶Rb uptake is cycloheximide-insensitive, occurs within minutes of hormone addition and results from an increase in the Vmax of the uptake system. Vasopressin stimulates ion uptake in a concentration-dependent fashion (1-100 ng/ml); oxytocin also stimulated the Na-K pump but at significantly higher concentrations. The stimulation of the Na-K pump by vasopressin is apparently mediated by an increase in Na entry into the cells, since the hormone (1) strikingly shifts the concentration dependence on Na⁺ of the Na-K pump, (2) increases ²²Na uptake, and (3) increases intracellular Na contents when the efflux of this ion is blocked by ouabain. Since vasopressin is a potent mitogen for Swiss 3T3 cells, the results provide further evidence in support of a possible role of monovalent ion fluxes in signalling the initiation of growth stimulation.     

Rapid microassay for protein kinase C translocation in Swiss 3T3 cells

The Ca2+/phosphatidylserine-stimulated protein kinase C (PKC) appears to exist as interconvertible inactive, soluble and active, membrane-bound forms. Changes in the bimodal distribution of PKC induced by diacylglycerol or tumor-promoting phorbol esters have been proposed to regulate the activity of this kinase [Nishizuka, Y. (1984) Nature (London) 308, 693-698]. A rapid microassay for assessment of protein kinase C translocation between cytosol and membranes was developed. This procedure, which relied on the selective digitonin-mediated release of cytoplasmic proteins, eliminated potential homogenization and fractionation artifacts. PKC activity toward histone H1 was determined after limited trypsinolysis, which abolished the Ca2+/phospholipid requirement of the enzyme and prevented interference by inhibitory proteins. Complete translocation of PKC to the membrane fraction and subsequent down-regulation of the kinase in response to 12-O-tetradecanoylphorbol-13-acetate treatment of Swiss 3T3 cells could be demonstrated by this method. Platelet-derived growth factor, insulin-like growth factor 1, vasopressin, and prostaglandin F2 alpha facilitated partial conversions of PKC to the membrane-bound form in quiescent 3T3 cells.     

Fibroblast growth factor stimulates protein kinase C in quiescent 3T3 cells without Ca2+ mobilization or inositol phosphate accumulation

To elucidate the transmembrane signalling processes initiated by fibroblast growth factor (FGF), we have studied the effect of recombinant basic FGF (bFGF) on various early events associated with mitogenesis in Swiss 3T3 fibroblasts. bFGF, at mitogenic concentrations, neither induced Ca2+ mobilization from intracellular stores nor increased the accumulation of inositol phosphates. In contrast, bFGF stimulated the phosphorylation of the Mr 80,000 (80K) cellular protein which is a major substrate of protein kinase C. This effect was potentiated by the diacylglycerol kinase inhibitor R59022. Two-dimensional polyacrylamide gel electrophoresis and phosphopeptide mapping showed that the 80K phosphoproteins generated in response to bFGF, bombesin, and phorbol 12,13-dibutyrate were indistinguishable. Down-regulation of protein kinase C prevented bFGF stimulation of 80K phosphorylation. Other protein kinase C-dependent early events such as transmodulation of the epidermal growth factor receptor, cytoplasmic alkalinization, inhibition of vasopressin induced increase in cytosolic [Ca2+], and enhancement of cAMP accumulation in response to forskolin were also induced by bFGF. Similar results were obtained when bFGF was added to quiescent cultures of tertiary mouse embryo fibroblasts. We conclude that bFGF stimulates protein kinase C through a signal transduction pathway distinct from inositol phospholipid turnover and Ca2+ mobilization.     

Cyclic AMP increasing agents rapidly stimulate vimentin phosphorylation in quiescent cultures of Swiss 3T3 cells

The results presented here demonstrate that an elevation in the cellular levels of cyclic AMP (cAMP) increases the phosphorylation of an Mr = 58,000 cellular protein in quiescent cultures of Swiss 3T3 cells. The enhancement of 32Pi incorporation into the Mr 58,000 cellular protein was detected as early as 1 min and reached a maximum after 20 min of treatment. The role of cAMP in the phosphorylation of Mr = 58,000 protein is substantiated by the following lines of evidence: a) a variety of agents that cause cAMP accumulation in 3T3 cells, including cholera toxin, 5'-N-ethylcarboxamideadenosine (NECA), PGE1, and 3-isobutyl-1-methyl-xanthine (IBMX) increased the phosphorylation of the same Mr 58,000 cellular protein as demonstrated by peptide mapping; b) inhibitors of cyclic nucleotide phosphodiesterase potentiated the ability of low concentrations of the adenylate cyclase activators NECA, PGE1, and forskolin to increase Mr 58,000 phosphorylation; and c) permeable derivatives of cAMP such as 8BrcAMP were also effective and specific in promoting Mr 58,000 phosphorylation. Detergent extraction, immunoblotting, and immunoprecipitation identified the Mr = 58,000 phosphoprotein as vimentin, the main protein subunit of the intermediate filaments of mesenchymal cells including Swiss 3T3 cells. Studies with intact 3T3 cells revealed that an increase in the intracellular level of cAMP induced a marked redistribution and collapse of the intermediate filaments. These results raise the possibility that an intact intermediate filament network may restrict the reinitiation of DNA synthesis.     

Calcium free inositol (1,4,5)-trisphosphate stimulates protein kinase C dependent protein phosphorylation in nuclei isolated from mitogen-treated Swiss 3T3 cells

As a step towards the elucidation of the role played by nuclear polyphosphoinositides, we have investigated the effect of exogenous calcium free inositol (1,4,5)-trisphosphate on the in vitro phosphorylation of proteins in nuclei prepared from Swiss 3T3 cells treated with bombesin and insulin-like growth factor I. When present in combination with phosphatidylserine, inositol (1,4,5)-trisphosphate enhanced the phosphorylation of two nuclear proteins, Mr 21,000 and 31,000, as well as of exogenous histone H1, to the same extent as a combination of phosphatidylserine and diacylglycerol. Inositol (1,4,5)-trisphosphate alone had no effect. This stimulation could be abolished by the protein kinase C inhibitor sphingosine and by EGTA, while could be restored by a combination of phosphatidylserine and exogenous Ca+(+) ions. These results raise the possibility that inositol (1,4,5)-trisphosphate is capable of liberating Ca+(+) ions from a nuclear store thus stimulating protein kinase C activity.     

Membrane protein kinase C activity rapidly increases in quiescent tsRSV-infected NRK cells upon reactivation of the mitogenic v-src protein kinase

The viral src protein kinase, pp60^v-src, is a powerful intracellular mitogen which can initiate and maintain the proliferation of quiescent cells in the absence of any exogenous growth factors. In an attempt to understand how pp60^v-src induces proliferation, we examined the early events in the G₀ to G₁ transition caused by the activation of a thermolabile v-src protein in quiescent, serum-starved tsRSV-transformed NRK cells. The reactivation of pp60^v-src, in the presence of exogenous growth factors, triggered a rapid biphasic surge of membrane-associated protein kinase C (PKC) activity. Unlike TPA-stimulated PKC activity, the pp60^v-src-induced increase in PKC was readily extracted from membranes by EGTA. The down-regulation of PKC activity in these quiescent cells by prolonged exposure to TPA strongly inhibited the ability of the reactivated v-src protein to stimulate DNA replication in serum-deficient medium, suggesting that PKC plays a role in the initial signal by which the viral enzyme induces the G₀ to G₁ transition in NRK cells.     

Pasteurella multocida toxin, a potent mitogen, stimulates protein kinase C-dependent and -independent protein phosphorylation in Swiss 3T3 cells

Pasteurella multocida toxin, either native or recombinant (rPMT), is an extremely effective mitogen for Swiss 3T3 cells and acts at picomolar concentrations (Rozengurt, E., Higgins, T. E., Chanter, N., Lax, A. J., and Staddon, J. M. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 123-127). Here, we show that similar concentrations of rPMT markedly stimulated the phosphorylation of an acidic 80-kDa protein in [32P]Pi-labeled Swiss 3T3 cells. Co-migration on one- and two-dimensional gels and phosphopeptide analysis indicated that this phosphoprotein was indistinguishable from 80K, a known protein kinase C substrate. In parallel cultures, the stimulation of 80K phosphorylation by rPMT (5-10-fold) was comparable to that induced by bombesin or phorbol dibutyrate (PBt2). However, the increase in phosphorylation by rPMT occurred after a pronounced lag period (1-3 h, depending upon the concentration of rPMT) in contrast to the relatively immediate stimulation by PBt2 or bombesin. Early, but not late, addition of either PMT antiserum or the lysosomotrophic agent methylamine selectively inhibited 80K phosphorylation in response to rPMT. 80K phosphorylation persisted after removal of free toxin and was not inhibited by cycloheximide. It appears that rPMT enters the cells via an endocytotic pathway to initiate and perpetuate events leading to 80K phosphorylation. rPMT, like PBt2, also stimulated the phosphorylation of 87-kDa and 33-kDa proteins in Swiss 3T3 cells. Phosphorylation of the 80K and 87-kDa proteins by rPMT or PBt2 were greatly attenuated in cells depleted of protein kinase C. In contrast, phosphorylation of the 33-kDa protein by rPMT, but not by PBt2, persisted in the absence of protein kinase C. rPMT, like bombesin, caused a translocation of protein kinase C to the cellular particulate fraction. The toxin enhanced the cellular content of diacylglycerol. rPMT also caused a time- and dose-dependent decrease in the binding of 125I-epidermal growth factor to its receptor which was blocked by methylamine and dependent only in part upon the presence of protein kinase C. We conclude that rPMT stimulates protein kinase C-dependent and -independent protein phosphorylation in Swiss 3T3 cells.     

Vasopressin rapidly stimulates phosphatidic acid-phosphatidylinositol turnover in rat anterior pituitary cells

In cultured rat anterior pituitary cells, the agonist [Asu1,6, Arg8]vasopressin (AVP-A) increased by 1.5-fold 32Pi incorporation into phosphatidic acid (PA), as early as 15 s after its addition. Increased phosphatidylinositol (PI) labeling became significant 4 min after AVP-A addition. Dose-response measurements with AVP-A showed ED50 values of 76 and 62 nM for PA and PI labeling, respectively. Peptide corticotropin-releasing factor (CRF) (0.1 microM) did not affect the stimulatory effect of AVP-A on PA and PI labeling. These data suggest that stimulation of PI metabolism in corticotrophs may be one of the early events involved in the stimulation of ACTH release induced by vasopressin.     

Mitogen-stimulated phosphorylation of nuclear proteins in Swiss 3T3 cells: evidence for a protein kinase C requirement

When Swiss 3T3 fibroblasts are treated with a combination of IGF-I2 and bombesin at mitogenic concentrations, in vivo phosphorylation of some nuclear proteins occurs within 45-90 min. Among these proteins, histone H1 and a 0.75 M PCA soluble polypeptide with an apparent Mr of 21,000, as revealed by electrophoretic analysis, are phosphorylated in vitro by protein kinase C in isolated nuclei purified from 3T3 cells treated for 90 min with IGF-I and bombesin. Since these phosphorylative events follow the earlier changes, recently demonstrated, in nuclear polyphosphoinositide metabolism induced by the same mitogen combination, it seems possible that these two phenomena are related to each other and trigger the synthetic machinery responsible for replicating DNA.     

Initiation of DNA synthesis in quiescent Swiss 3T3 and 3T6 cells by lanthanum

La³⁺ stimulated quiescent Swiss 3T3 and 3T6 ceils to enter the DNA-synthesizing S phase of the cell cycle. La³⁺ and insulin interacted synergistically to increase DNA synthesis. A brief exposure of the cells to soluble LaCl₃ optimally stimulated entry into S. La³⁺ was similar to Al³⁺ in its mitogenic properties (J. Cell. Physiol.118 , 298, 1984), but La³⁺ was 10 times more potent than Al³⁺.     

Transforming growth factor-beta 1 stimulates glucose uptake and the expression of glucose transporter mRNA in quiescent Swiss mouse 3T3 cells

Transforming growth factor-beta 1 (TGF-beta 1) is a multifunctional polypeptide that regulates the proliferation and differentiation of various types of animal cells. TGF-beta 1 stimulated glucose uptake and the expression of a brain-type glucose transporter (GLUT1) mRNA in quiescent mouse 3T3 cells. TGF-beta 1 also synergistically stimulated these activities when given together with calf serum, phorbol ester, fibroblast growth factor, or epidermal growth factor. The increases in glucose uptake and the GLUT1 mRNA level were induced by picomolar concentrations of TGF-beta 1 within 3 h of stimulation, reached a peak between 6 and 9 h, and then decreased gradually to basal levels before an increase in DNA synthesis. The stimulation of GLUT1 mRNA expression was completely abolished by actinomycin D, but was not affected by cycloheximide, suggesting that new protein synthesis was not required for the expression of GLUT1 mRNA. TGF-beta 1 had little mitogenic activity and did not affect serum-induced DNA synthesis in quiescent 3T3 cells. However, it stimulated DNA synthesis synergistically when given with fibroblast growth factor, epidermal growth factor, phorbol ester, or insulin. These results suggest that TGF-beta 1 mediates the stimulation of glucose uptake, GLUT1 mRNA expression, and DNA synthesis via a pathway(s) and cellular components distinct from those for other growth factors. The possible role of the TGF-beta 1-induced stimulation of glucose transport activity in the control of mouse fibroblast proliferation is also discussed.     

The diacylglycerol kinase inhibitor R 59022 potentiates bombesin stimulation of protein kinase C activity and DNA synthesis in Swiss 3T3 cells

Addition of the inhibitor of diacylglycerol kinase R 59022 to quiescent Swiss 3T3 cells potentiated protein kinase C activation by the neuropeptide bombesin, a potent mitogen for these cells. This was detected as a marked shift in the dose-response relationship for bombesin-stimulated phosphorylation of a Mr 80,000 protein, which is a major, specific substrate of protein kinase C. R 59022 also promoted the inhibition of [125I]epidermal growth factor binding induced by bombesin, an effect mediated by protein kinase C. A salient feature of our results is that R 59022, at concentrations that enhanced the activation of protein kinase C in intact cells (4-6 microM), potentiated long-term mitogenesis elicited by bombesin. Thus, R 59022 may be a valuable tool for elucidating the contribution of the protein kinase C pathway in mitogenesis.     

Bradykinin transiently activates protein kinase C in Swiss 3T3 cells. Distinction from activation by bombesin and vasopressin

The results presented here demonstrate that bradykinin, acting through a B2 subtype receptor, induces a unique pattern of early signals in quiescent Swiss 3T3 cells. Bradykinin caused a rapid mobilization of calcium from internal stores, as judged by measurements of intracellular Ca2+ concentration in fura-2-loaded cells and by 45Ca2+ efflux from radiolabeled cells. Analysis of phosphoproteins from 32P-labeled Swiss 3T3 cells by one- and two-dimensional gel electrophoresis revealed that bradykinin stimulated transient phosphorylation of an acidic cellular protein migrating with an apparent Mr = 80,000 (termed 80K), identified as a major and specific substrate of protein kinase C. Down-regulation of protein kinase C by pretreatment with phorbol 12,13-dibutyrate (PDBu) completely abolished the increase in 80K phosphorylation. In contrast to the sustained effect induced by bombesin, vasopressin, or PDBu, the stimulation of 80K phosphorylation by bradykinin reached a maximum after 1 min of incubation, and then it rapidly decreased to almost basal levels. Furthermore, bradykinin did not induce protein kinase C-mediated events such as inhibition of 125I-epidermal growth factor binding or enhancement of cAMP accumulation. Bombesin and vasopressin elicited both responses in parallel cultures. Bradykinin induced rapid accumulation of total inositol phosphates in cells labeled with myo-[3H]inositol. In contrast to bombesin and vasopressin which stimulated a linear increase in inositol phosphate accumulation over a 10-min period, the effect of bradykinin reached a plateau after 2.5 min of incubation with no further increase up to 10 min. The results demonstrate that the early signaling events triggered by bradykinin can be distinguished from those elicited by bombesin and vasopressin in Swiss 3T3 cells.     

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 esential 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⁺.     

Aluminum ions stimulate DNA synthesis in quiescent cultures of Swiss 3T3 and 3T6 cells

Micromolar concentrations of AI3+ are shown to be strongly mitogenic for quiescent cultures of Swiss 3T3 and 3T6 cells. AI3+ caused a striking shift in the dose-response curve for the effect of fetal bovine serum on 3H-thymidine incorporation. In the absence of serum the mitogenic effect of aluminum was greatly potentiated by insulin or cholera toxin, but not epidermal growth factor or 12-0-tetradecanoyl-phorbol-13-acetate. The stimulation of DNA synthesis was maximal by 15-20 microM AI3+ X AI3+ at 100 microM had no inhibitory effect on DNA synthesis. AI3+ had no significant effect on cellular cyclic adenosine monophosphate in the presence or absence of insulin or an inhibitor of cyclic nucleotide phosphodiesterases.     

Increases in phosphatidic acid levels accompany sphingosine-stimulated proliferation of quiescent Swiss 3T3 cells

Sphingosine, a breakdown product of cellular sphingolipids, has recently been shown to stimulate DNA synthesis and act synergistically with known growth factors to induce proliferation of quiescent Swiss 3T3 fibroblasts (Hong, Z., Buckley, N. E., Gibson, K., and Spiegel, S. (1990) J. Biol. Chem. 265, 76-81). The present study demonstrates that mitogenic concentrations of sphingosine induce early increases in cytosolic phosphatidic acid, which is a potent mitogen for Swiss 3T3 cells. Structurally related analogs of sphingosine, such as N-stearoylsphingosine and other long chain aliphatic amines, did not mimic the mitogenic effect of sphingosine and did not elevate phosphatidic acid levels. Sphingosine not only stimulated [3H]thymidine incorporation with similar efficiency and kinetics as phosphatidic acid, it also induced similar morphological alterations. Both sphingosine and phosphatidic acid acted synergistically with a variety of growth factors, such as, insulin, epidermal growth factor, fibroblast growth factor, and 12-O-tetradecanoylphorbol 13-acetate. In sharp contrast, sphingosine and phosphatidic acid did not have additive or synergistic effects in either the presence or absence of other growth factors. Both sphingosine and phosphatidic acid stimulated DNA synthesis in cells made protein kinase C-deficient by prolonged treatment with phorbol ester and sphingosine still stimulated similar increases in phosphtidic acid in these cells. Furthermore, similar to the actions of phosphatidic acid on signal transduction in Swiss 3T3 cells, mitogenic concentrations of sphingosine also inhibit cAMP accumulation and trigger the hydrolysis of polyphosphoinositides. Our findings indicate that sphingosine and phosphatidic acid control cellular responses in Swiss 3T3 cells through a common pathway. In view of the prominent role of phosphatidic acid in signal transduction and cellular proliferation, our observations that sphingosine, at mitogenic concentrations, increases the level of phosphatidic acid and also mimics the effects of phosphatidic acid on signal transduction, have important implications for the mechanism of action of sphingosine.