NIH-3T3 cells transformed with a ras oncogene exhibit a protein kinase C-mediated inhibition of agonist-stimulated Ca2+ inflow

Cellular calcium mobilization in response to phosphoinositide delivery

Intracellular calcium [Ca(2+)](i) is mobilized in many cell types in response to activation of phosphoinositide (PIP(n)) signaling pathways involving PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3). To further explore the relationship between increases in intracellular PIP(n) concentrations and mobilization of [Ca(2+)](i), each of the seven phosphorylated phosphoinositides (PIP(n)s) were delivered into cells and the metabolism and physiological effects of the exogenously administered PIP(n)s were determined. The efficient cellular delivery of fluorophore-tagged and native PIP(n)s was accomplished using histone protein, neomycin, and dendrimeric polyamines. PtdIns(4,5)P(2) fluorophore-tagged analogs with short- and long-acyl chains were substrates for cellular enzymes in vitro and for phospholipases in stimulated fibroblasts. PtdIns(4)P, PtdIns(3,4)P(2) and PtdIns(4,5)P(2), each induced calcium mobilization rapidly after exogenous addition to fibroblasts. PtdIns(3,4,5)P(3) induced a significant, but smaller increase in intracellular calcium. These observations suggest that PIP(n)s other than PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3) may have direct roles in signaling involving [Ca(2+)](i).

G-protein-stimulated phospholipase D activity is inhibited by lethal toxin from Clostridium sordellii in HL-60 cells

Lethal toxin (LT) from Clostridium sordellii has been shown in HeLa cells to glucosylate and inactivate Ras and Rac and, hence, to disorganize the actin cytoskeleton. In the present work, we demonstrate that LT treatment provokes the same effects in HL-60 cells. We show that guanosine 5'-O-(3-thiotriphosphate)-stimulated phospholipase D (PLD) activity is inhibited in a time- and dose-dependent manner after an overnight treatment with LT. A similar dose response to the toxin was found when PLD activity was stimulated by phorbol 12-myristate 13-acetate via the protein kinase C pathway. The toxin effect on actin organization seemed unlikely to account directly for PLD inhibition as cytochalasin D and iota toxin from Clostridium perfringens E disorganize the actin cytoskeleton without modifying PLD activity. However, the enzyme inhibition and actin cytoskeleton disorganization could both be related to a major decrease observed in phosphatidylinositol 4,5-bisphosphate (PtdIns(4, 5)P2). Likely in a relationship with this decrease, recombinant ADP-ribosylation factor, RhoA, Rac, and RalA were not able to reconstitute PLD activity in LT-treated cells permeabilized and depleted of cytosol. Studies of phosphoinositide kinase activities did not allow us to attribute the decrease in PtdIns(4,5)P2 to inactivation of PtdIns4P 5-kinase. LT was also found to provoke a major inhibition in phosphatidylinositol 3-kinase that could not account for the inhibition of PLD activity because wortmannin, at doses that fully inhibit phosphatidylinositol 3-kinase, had no effect on the phospholipase activity. Among the three small G-proteins, Ras, Rac, and RalA, inactivated by LT and involved in PLD regulation, inactivation of Ral proteins appeared to be responsible for PLD inhibition as LT toxin (strain 9048) unable to glucosylate Ral proteins did not modify PLD activity. In HL-60 cells, LT treatment appeared also to modify cytosol components in relationship with PLD inhibition as a cytosol prepared from LT-treated cells was less efficient than one from control HL-60 cells in stimulating PLD activity. Phosphatidylinositol transfer proteins involved in the regulation of polyphosphoinositides and ADP-ribosylation factor, a major cytosolic PLD activator in HL-60 cells, were unchanged, whereas the level of cytosolic protein kinase Calpha was decreased after LT treatment. We conclude that in HL-60 cells, lethal toxin from C. sordellii, in inactivating small G-proteins involved in PLD regulation, provokes major modifications at the membrane and the cytosol levels that participate in the inhibition of PLD activity. Although Ral appeared to play an essential role in PLD activity, we discuss the role of other small G-proteins inactivated by LT in the different modifications observed in HL-60 cells.

Absence of endothelin receptors and receptor mRNA in mammalian fibroblasts transformed with SV40 or ras oncogene

Endothelin-1 (ET-1), a peptide isolated from the culture medium of endothelial cells, mediates a variety of physiological and pathological responses including mitogenesis. We have compared the expression of ET receptors in untransformed versus ras-transformed NIH-3T3 murine fibroblasts and in untransformed versus SV40-transformed W138 (VA13) human fibroblasts by ligand binding and Northern analysis. NIH-3T3 and W138 cells displayed high affinity (200 and 220 pM) and high density (23,000 sites/cell and 14,000 sites/cell for NIH-3T3 and W138 cells, respectively) ET receptors. Competition binding experiments using subtype-selective ligands identified these receptors as the ETA subtype. Addition of ET-1 to the cells produced a concentration-dependent increase in intracellular calcium release. Both ras-transformed NIH-3T3 cells and SV40-transformed W138 cells (VA13) completely lacked [125I]ET-1 binding and failed to release calcium when exposed to ET-1. Northern analysis of the polyadenylated RNA (polyA RNA) isolated from untransformed and transformed cells revealed that the steady-state level of ETA receptor RNA was 90-95% less in transformed cells compared to untransformed cells. Thus, the loss of ET receptors as well as the receptor-mediated responses in transformed cells can be explained by down-regulation of ET receptor mRNA.

Purification of histidine-tagged ras and its use in the detection of ras binding proteins

Recombinant histidine-tagged v-Ha-ras (his-ras) was purified to homogeneity from extracts of E. coli M15 using a one-step procedure which involved immobilised metal ion chromatography on Ni(2+)-nitriloacetic acid agarose (Ni-NTA). The optimal pH for elution by imidazole was 6.6 and the yield of his-ras protein (greater than 95% pure) was about 4 mg/litre E. coli culture. Chromatography of a mixture of purified his-ras and rat brain cytosol on Ni-NTA together with SDS-PAGE and silver staining of proteins were employed to search for ras-binding proteins present in rat brain cytosol. Chromatography of rat brain cytosol alone on Ni-NTA revealed several protein species which were not readily eluted with imidazole. These are likely to be low-abundance brain metal ion binding proteins. Pre-treatment of rat brain cytosol with Ni-NTA before a second round of chromatography on Ni-NTA removed most of these proteins. Chromatography of a mixture of pre-treated rat brain cytosol and purified his-ras protein revealed four new protein bands with molecular weights of 250, 90, 80 and 70 kDa. These were considered to be candidate ras-binding proteins. It is concluded that the use of his-ras and immobilised metal ion chromatography does provide an approach which can be used to identify ras binding proteins present in cellular extracts.

The effects of N-ras oncogene expression on PDGF-BB stimulated responses in cultured mouse myoblasts

The role of the ras oncogene in the signalling pathway triggered by platelet-derived growth factor BB (PDGF-BB) has been investigated in a cell line which normally differentiates into myotubes. Following the activation of the N-ras oncogene, however, the cells proliferate and form foci. PDGF-BB stimulated the phosphorylation of tyrosine in several cellular proteins of molecular weight 185, 160, 94, 54, 44, 42 kDa and furthermore Ca2+ was released from internal stores. Activation of the N-ras gene by treatment of cells with dexamethasone (DEX) inhibited these responses to PDGF-BB. On the other hand, both ras-induced and -non induced cells responded to bradykinin (BK), foetal calf serum (FCS) and ionomycin (ION) by releasing Ca2+ from intracellular stores. The inhibition of the response to PDGF-BB in ras-activated cells has been further investigated. The binding of [125I]-PDGF-BB to its receptors was low and western blotting showed a low level of PDGF-BB receptor protein. This was in marked contrast to the receptor number seen in cells grown in growth medium or fusion promoting medium. These results indicate that cells transformed with the N-ras oncogene fail to respond to platelet-derived growth factor and exhibit a very low level of PDGF receptors. This suggests a role for the ras oncogene in the earliest steps of the signalling pathway.

PDGF-BB triggered cytoplasmic calcium responses in cells with endogenous or stably transfected PDGF beta-receptors

Platelet-derived growth factor-BB (PDGF-BB) triggered signal transduction was investigated in human foreskin fibroblasts with endogenous PDGF beta-receptors, and porcine aortic endothelial (PAE) cells with stably transfected PDGF beta-receptors. Immunoprecipitation and immunoblotting showed that PDGF induced dose-dependent autophosphorylation of PDGF beta-receptor, and the PLC-gamma associates with autophosphorylated PDGF beta-receptors and becomes phosphorylated. Activation of PLC-gamma is known to induce fluctuations of the concentration of cytoplasmic calcium ([Ca2+]i). Microfluorometry and digital imaging were employed for measurements of the concentration of [Ca2+]i. In both cell types the growth factor induced four types of [Ca2+]i responses; no rise, a small and sluggish monophasic rise, a biphasic rise with an initial transient peak followed by a sustain elevation, and finally regular oscillations. The frequencies and amplitudes of the oscillatory responses were independent of agonist concentration after stimulation with PDGF-BB. Latency, the period from application of stimulus to the first [Ca2+]i peak, was reduced at higher concentrations of agonist. Also, the proportion of responding cells increased with higher concentrations of ligand. Oscillations of [Ca2+]i were elicited at submaximal concentrations of agonist. In PAE cells PDGF-BB triggered a single [Ca2+]i peak in absence of external Ca2+. Ligand-induced oscillations and sustained increases of [Ca2+]i were counteracted by the inorganic Ca2+ channel blocker Ce3+. These results show that similar types of [Ca2+]i responses occur in different cell types independently of whether the PDGF beta-receptors are expressed endogeneously or after transfection. Potentially, the different [Ca2+]i responses have distinct physiological consequences.

Effect of glucose on pHin and [Ca2+]in in NIH-3T3 cells transfected with the yeast P-type H(+)-ATPase

NIH-3T3 cells transfected with yeast H(+)-ATPases (RN1a cells) are tumorigenic (Perona and Serrano, 1988, Nature, 334:438). We have previously shown that RN1a cells maintain a chronically high intracellular pH (pHin) under physiological conditions. We have also shown that RN1a cells are serum-independent for growth, maintain a higher intracellular Ca2+ ([Ca2+]in), and glycolyze more rapidly than their non-transformed counterparts (Gillies et al., Proc. Natl. Acad. Sci., 1990, 87:7414; Gillies et al., Cell. Physiol. Biochem., 1992, 2:159). The present study was aimed to understand the interrelationships between glycolysis, pHin, and [Ca2+]in in RN1a cells and their non-transformed counterparts, NIH-3T3 cells. Our data show that the higher rate of glycolysis observed in RN1a cells is due to the presence of low affinity glucose transporters. Consequently, the higher rate of glycolysis is exacerbated at high glucose concentration in RN1a cells. Moreover, the maximal velocity (Vmax) for glucose utilization is up to sixfold higher in RN1a cells than in the NIH-3T3 cells, suggesting that the number of glucose transporters is higher in RN1a than NIH-3T3 cells. Glucose addition to NIH-3T3 cells results in modest decreases in both pHin and [Ca2+]in. In contrast, RN1a cells respond to glucose with a large decrease in pHin, followed by a large decrease in [Ca2+]in. The decrease in [Ca2+]in observed upon glucose addition is likely due to activation of Ca(2+)-ATPase by glycolysis, since the Ca2+ decrease is abolished by the Ca2+ ATPase inhibitors thapsigargin and cyclopiazonic acid. Glucose addition to ATP-depleted cells results in a decrease in [Ca2+]in, suggesting that ATP furnished by glycolysis is utilized by this pump.

Competence induction by PDGF requires sustained calcium influx by a mechanism distinct from storage-dependent calcium influx

The significance and mechanism of extracellular calcium influx in the stimulation by PDGF of cell replication was investigated in density-arrested C3H 10T1/2 mouse fibroblasts. PDGF consistently stimulated a biphasic increase in the [Ca2+]i composed of a rapid transient release of calcium from intracellular storage sites followed by a sustained elevation, significantly greater than prestimulated levels, which was dependent upon the [Ca2+]e and persisted for at least 1 h. The percentage of cells incorporating [3H]-TdR into DNA after stimulation with PDGF+insulin was closely correlated with the magnitude of the sustained [Ca2+]i increase and to the [Ca2+]e. Selective inhibition of the sustained [Ca2+]i increase, by blocking calcium influx with La3+, completely inhibited progression to S phase without affecting the release of calcium from intracellular storage sites. Progression to S phase was inhibited by La3+ or the omission of added extracellular calcium only during PDGF exposure and not during treatment with insulin. PDGF-induced calcium influx was completely inhibited by La3+ whereas storage-dependent calcium influx (SDCI) induced by thapsigargin was unaffected. Pretreatment with TPA, forskolin, dibutyryl-cAMP, dibutyryl-cGMP, nifedipine, and TMB-8 had no effect on PDGF-induced calcium influx. These data suggest that the induction of replicative competence by PDGF is dependent upon the maintenance of a sustained increase in the intracellular calcium concentration due to the influx of extracellular calcium through a calcium influx pathway distinct from SDCI.

Modulation by retinoic acid of spontaneous and benzo(a)pyrene-induced c-Ha-ras expression

The effects of retinoic acid on the expression of the Ha-ras gene were studied in transformed rat hepatoma cells (H4IIE) and in rat aortic smooth muscle cells (ASMC) treated with benzo(a)pyrene (30 microM) in vitro. In H4IIE cells, a dose-dependent increase in steady state Ha-ras mRNA levels was observed upon exposure to retinoic acid for 24 hr. Exposure of ASMC to 10 microM retinoic acid under similar experimental conditions was also associated with increased Ha-ras expression. In contrast, retinoic acid (1 and 10 microM) inhibited benzo(a)pyrene-induced expression of Ha-ras in ASMC. These results suggest that retinoic acid modulates spontaneous and carcinogen-induced expression of Ha-ras in a differential manner.

P2Y purinoceptors in normal NIH 3T3 and in NIH 3T3 overexpressing c-ras

The ability of purinergic agonists to induce Ca2+ responses has been tested in two lines of murine fibroblasts: normal NIH 3T3 fibroblasts and NIH 115.14, a clone expressing high levels [1] of the c-ras protooncogene. Both kinds of cells are responsive to ATP in the range 1 microM-1 mM; ADP and ATP gamma S are almost as potent as ATP, while AMP is unable to elicit a response. Ca2+ measurements performed in single cells by image analysis show great variability among cells but in each individual responding cell the Ca2+ rise occurs in an all-or-none fashion. The transient Ca2+ response does not depend on influx from the extracellular medium. Electrophysiological experiments reveal the activation of an outward current (at -50 mV) by ATP, probably due to Ca(2+)-activated K+ channels, confirming the absence of a substantial Ca2+ influx. Finally, stimulation by ATP produces a small but significant increase in the production of inositol phosphates. These results indicate that these cell lines possess purinergic receptors which are not integral membrane channels and which are coupled to InsP3 formation and may be therefore classified as P2Y.

Different protein kinase C isozymes could modulate bradykinin-induced extracellular calcium-dependent and -independent increases in osteoblast-like MC3T3-E1 cells

Effects of protein kinase C (PKC) on bradykinin (BK)-induced intracellular calcium mobilization, consisting of rapid Ca2+ release from internal stores and a subsequent sustained Ca2+ inflow, were examined in Fura-2-loaded osteoblast-like MC3T3-E1 cells. The sustained Ca2+ inflow as inferred with Mn2+ quench method was blocked by Ni2+ and a receptor-operated Ca2+ channel blocker SK&F 96365, but not by nifedipine. The short-term pretreatment with phorbol 12-myristate 13-acetate (PMA), inhibited BK-stimulated Ca2+ inflow, and the prior treatment with PKC inhibitors, H-7 or staurosporine, enhanced the initial internal release and reversed the PMA effect. Moreover, 6 h pretreatment with PMA caused similar effect on the BK-induced inflow to that obtained with PKC inhibitors, whereas 24 h pretreatment was necessary to affect the internal release. On the other hand, the translocation and down-regulation of PKC isozymes were examined after PMA treatment of MC3T3-E1 cells by immunoblot analyses of PKCs with the isozyme-specific antibodies. 6 h treatment with PMA induced down-regulation of PKC beta, whereas longer treatment was needed for down-regulation of PKC alpha. Taken together, it was suggested that the BK-induced initial Ca2+ peak and the sustained Ca2+ inflow through the activation of a receptor-operated Ca2+ channel, are differentially regulated by PKC isozymes alpha and beta, respectively, in osteoblast-like MC3T3-E1 cells.

Calcium signals in growth factor signal transduction

There is a substantial amount of information which has been obtained concerning the effects of growth factors on [Ca2+]i in proliferating cells. A number of different mitogens are known to induce elevations in [Ca2+]i and some characterization of the Ca2+ response to different classes of mitogens has been obtained. In addition, much is known about whether the Ca2+ response to a particular growth factor occurs as the result of an influx of external Ca2+ or a mobilization of internal Ca2+ stores. In addition, a considerable amount of information is available on the mechanism by which the Ins(1,4,5)P3-sensitive internal Ca2+ store takes up and releases Ca2+. However, there is still a large deficiency in our information concerning other Ca2+ stores in proliferating cells as well as in our knowledge of the mechanisms for regulating Ca2+ entry pathways. Much more data addressing these issues exists for other types of agonist-stimulated cells, and we have discussed much of it in this review article. While the wealth of data in nonproliferating cells provides some indications of what mechanisms might be involved in the growth factor-induced changes in [Ca2+]i, it is clear that much work must be done in proliferating cells to fully understand how external factors such as growth factors control [Ca2+]i. In addition, much work remains to be done in identifying the mechanisms for the internal control of [Ca2+]i as cells move through the cell cycle and in identifying the role that these changes in [Ca2+]i may play throughout the cell cycle.

Inhibition of Ca2+ inflow causes an abrupt cessation of growth-factor-induced repetitive free Ca2+ transients in single NIH-3T3 cells

In single NIH-3T3 fibroblasts loaded with fura-2, bombesin induced one of three patterns of increase in the concentration of intracellular free Ca2+ [( Ca2+]i): a single transient increase, a sustained increase, or repetitive transient increases in [Ca2+]i. Foetal-calf serum and ATP also gave these three patterns of response, although a lower proportion of cells gave repetitive Ca2+ transients in response to ATP. An increase in the concentration of bombesin from 1 to 25 nM increased the proportion of cells which exhibited repetitive Ca2+ transients. At 25 nM-bombesin, the proportion of cells which exhibited repetitive Ca2+ transients increased as the extracellular Ca2+ (Ca2+o) concentration was increased from 1 to 5 mM. Removal of Ca2+o by addition of EGTA, or inhibition of Ca2+ inflow by treatment of cells incubated in the presence of Ca2+o with verapamil or an activator of protein kinase C, abruptly terminated repetitive Ca2+ transients, with only one transient observed after the cessation of Ca2+ inflow. Repetitive Ca2+ transients were not observed in cells incubated in the absence of Ca2+o and in the presence of EGTA. Addition of Ca2+o to cells previously incubated in the presence of EGTA caused a resumption of repetitive Ca2+ transients. Addition of thapsigargin alone induced a large transient increase in [Ca2+]i, whereas much smaller transient increases in [Ca2+]i were induced in about 30% of cells tested by caffeine or carbonyl cyanide m-chlorophenylhydrazone (CCCP) plus oligomycin. Thapsigargin or the combination of CCCP plus oligomycin completely inhibited bombesin-induced repetitive Ca2+ transients, whereas caffeine had no effect. It is concluded from the studies of the role of Ca2+o that NIH-3T3 cells differ from other cell types in the anatomical or chemical links between extracellular Ca2+ and the intracellular stores involved in the generation of Ca2+ transients, whereas the results of the experiments with inhibitors indicate that the generation of repetitive Ca2+ transients in NIH-3T3 cells is unlikely to involve Ca(2+)-induced Ca2+ release from caffeine-sensitive stores.

Calcium oscillation associated with reduced protein kinase C activities in ras-transformed NIH3T3 cells

We show here novel intracellular Ca2+ oscillation in v-K-ras-transformed NIH3T3 cells induced by mitogenic peptide hormones, bradykinin and bombesin, as well as fetal calf serum. Induction of the Ca2+ oscillation is strongly correlated with the malignant properties and inversely with PKC activities in vitro and in vivo. These results suggest that the mitogen-induced Ca2+ oscillation is negatively regulated by PKC, which modulates Ca2+ influx in v-K-ras-transformed NIH3T3 cells.