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

Involvement of TRPC3 channels in calcium oscillations mediated by OX(1) orexin receptors

Oscillations of intracellular Ca2+ provide a novel mechanism for sustained activation of cellular processes. Receptor-activated oscillations are mainly thought to occur through rhythmic IP3-dependent store discharge. However, as shown here in HEK293 cells 1 nM orexin-A (Ox-A) acting at OX1 receptors (OX1R) triggered oscillatory Ca2+ responses, requiring external Ca2+. These responses were attenuated by interference with TRPC3 channel (but not TRPC1/4) function using dominant negative constructs, elevated Mg2+ (a blocker of many TRP channels) or inhibition of phospholipase A2. These treatments did not affect Ca2+ oscillations elicited by high concentrations of Ox-A (100 nM) in the absence of external Ca2+. OX1R are thus able to activate TRPC(3)-channel-dependent oscillatory responses independently of store discharge.

Cell density-dependent changes in intracellular Ca2+ mobilization via the P2Y2 receptor in rat bone marrow stromal cells

Bone marrow stromal cells (BMSCs) are an interesting subject of research because they have characteristics of mesenchymal stem cells. We investigated intracellular Ca(2+) signaling in rat BMSCs. Agonists for purinergic receptors increased intracellular Ca(2+) levels ([Ca(2+)](i)). The order of potency followed ATP = UTP > ADP = UDP. ATP-induced rise in [Ca(2+)](i) was suppressed by U73122 and suramin, but not by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suggesting the functional expression of G protein-coupled P2Y(2) receptors. RT-PCR and immunohistochemical studies also showed the expression of P2Y(2) receptors. [Ca(2+)](i) response to UTP changed with cell density. The UTP-induced rise in [Ca(2+)](i) was greatest at high density. V(max) (maximum Ca(2+) response) and EC(50) (agonist concentration that evokes 50% of V(max)) suggest that the amount and property of P2Y(2) receptors were changed by cell density. Note that UTP induced Ca(2+) oscillation at only medium cell density. Pharmacological studies indicated that UTP-induced Ca(2+) oscillation required Ca(2+) influx by store-operated Ca(2+) entry. Carbenoxolone, a gap junction blocker, enhanced Ca(2+) oscillation. Immunohistochemical and quantitative real-time PCR studies revealed that proliferating cell nuclear antigen (PCNA)-positive cells declined but the mRNA expression level of the P2Y(2) receptor increased as cell density increased. Co-application of fetal calf serum with UTP induced Ca(2+) oscillation at high cell density. These results suggest that the different patterns observed for [Ca(2+)](i) mobilization with respect to cell density may be associated with cell cycle progression.

Divergent responses of ras-transfected and non-ras-transfected human keratinocytes to extracellular calcium

Raising extracellular calcium (Cao) induces terminal differentiation in cultured epidermal keratinocytes. The introduction of the ras oncogene into keratinocytes results in resistance to Cao-mediated differentiation. To understand the signaling mechanism involved, we examined the Cao-induced formation of inositol triphosphate (IP3) and changes in intracellular Ca2+ (Cai) concentration in non-ras-transfected and ras-transfected HaCaT lines of human keratinocytes. When switched from 0.05- to 1.5-mM Cao medium, the non-ras HaCaT line showed a rapid twofold increase in IP3 formation, whereas the IP3 level in the ras-transfected I-7 line was slightly affected. G-protein-coupled activation of phospholipase was intact in both lines, as evidenced by the generation of similar amounts of IP3 in response to addition of bradykinin or guanosine 5'-[γ-thio]-triphosphate. Addition of 1.0 mM Cao evoked similar Cai responses in both non-ras- and ras-transfected cells: a transient elevation, followed by a sustained lower plateau. However, the two lines differed in their later responses: after being maintained in 1.0 mM Ca2+ for 24 h, the Cai level was significantly lower in ras-transfected cells than in non-ras-transfected HaCaT cells. The Cao-induced increase in Cai in both lines was inhibited by the Ca2+ entry blocker SK&F 96365 or depolarization in high K+ bathing solution, demonstrating its dependence of calcium influx. The results suggest fundamental differences in the early signal that are generated in response to an increase in Cao in ras-transfected keratinocytes, with the absence of a Cao-induced rise in IP3-a signaling pathway defect that may play a role in the differentiation block the cells exhibit. In addition, the inability of ras-transfected cells to sustain a prolonged Cai plateau may also contribute to their inability to differentiate in response to the Cao signal.Key words: cell differentiation, intracellular Ca2+, IP3, keratinocytes, ras transfection, signal transduction.

Intracellular calcium oscillations in cell populations of ras-transfected I-7 subline of human HaCaT keratinocytes

We have observed oscillations of intracellular Ca2+ (Ca[i]) concentration in populations of ras-transfected HaCaT keratinocytes of I-7 subline. In postconfluent monolayers of I-7 keratinocytes, an increase in extracellular Ca2+ (Ca[o]) concentration to 0.25-0.5 mM induced sinusoidal Ca(i) oscillations, which persisted longer than 1 h with amplitudes of 50-150 nM and periods of 5-10 min. Thapsigargin, which depletes internal Ca2+ stores, did not prevent Ca(o)-induced Ca(i) oscillations, and it also induced Ca(i) oscillations in the ras-transfected I-7 line. Removal of extracellular Ca2+ or addition of Ca2+-entry blocker La3+ or SK&F 96365 inhibited Ca(i) oscillations, suggesting that Ca(i) oscillations in ras-transfected HaCaT keratinocytes were dependent on Ca2+ influx across the plasma membrane. Because the Ca(o)-induced Ca(i) oscillations have been observed only in ras-transfected I-7 subline and not in its nontransfected parental HaCaT line, this may provide a partial explanation for the divergent responses of ras-transfected and nontransfected keratinocytes to Ca(o) signal for control of growth and differentiation.

Transient release of calcium from inositol 1,4,5-trisphosphate-specific stores regulates mouse preimplantation development

Inositol 1,4,5-trisphosphate can regulate growth and differentiation by modulating the release of intracellular Ca2+ in a variety of cellular systems, and it is involved in oocyte activation. Recent studies suggest that mammalian preimplantation development may also be regulated by the release of Ca2+ from intracellular stores. The rate of cavitation and cell division was accelerated after a transient elevation of intracellular Ca2+ levels was induced in morulae by exposure to ethanol or ionomycin. Embryos exposed to BAPTA-AM, a chelator of intracellular Ca2+, exhibited a brief dose-dependent reduction in basal Ca2+ levels, a temporal inhibition of ionophore-induced Ca2+ signalling and a subsequent delay in blastocoel formation. BAPTA-AM at 0.5 microM did not significantly alter the basal intracellular calcium level, but chelated Ca2+ that was released after ethanol exposure and thereby attenuated the ethanol-induced acceleration of cavitation. BAPTA-AM also inhibited cell division to the 16-cell stage in a dose-dependent manner, which correlated with the inhibition of cavitation. Thimerosal and inositol 1,4,5-trisphosphate significantly elevated the intracellular Ca2+ concentration in mouse morula-stage embryos, providing evidence for the existence of inositol 1,4,5-trisphosphate-sensitive Ca2+ stores. Although caffeine failed to release intracellular Ca2+, ryanodine induced a small biphasic release of Ca2+, suggesting that ryanodine-sensitive Ca2+ stores may also exist in mouse embryos. Morulae exposed to the calmodulin inhibitor W-7 exhibited a dose-dependent delay in blastocoel formation. A 4 hour exposure to 10 microM W-7 did not significantly alter cavitation, but attenuated the ionophore-induced stimulation of blastocoel formation. This finding suggests that the developmental effects produced through Ca2+ signalling are mediated by calmodulin. Our results demonstrate that Ca2+ release in mouse morulae occurs predominantly through the inositol 1,4,5-trisphosphate receptor, and that alteration of intracellular Ca2+ levels can accelerate or delay embryonic growth and differentiation, providing a mechanistic link between the regulation of oocyte and embryonic development.

Activation of Ca2+ influx by transforming Ha-ras

The effect of an induction of transforming Ha-ras on Ca2+ influx into NIH3T3 cells was studied employing Fura-2 quenching by Mn2+. The expression of transforming p21Ha-ras caused a significant increase in Mn2+ influx which was blocked by Cd2+, La3+, niguldipine and the Ca(2+)-channel blocker SK&F96365. This effect was specific for transforming Ha-ras and was not seen after overexpression of the Ha-ras proto-oncogene or v-mos. In addition to the enhanced Mn2+ influx, transforming p21Ha-ras elicited an increased efflux of the K(+)-congener 86Rb+ which was inhibitable by Ca(2+)-channel blockers and charybdotoxin, a selective inhibitor of high and intermediate conductance Ca(2+)-dependent K+ channels. Charybdotoxin did not reduce the increase in Mn2+ influx by ras, demonstrating that the activation of Ca(2+)-dependent K+ channels was not required for the sustained Mn2+/Ca2+ influx in the presence of transforming Ha-ras. In ras-expressing cells, the bradykinin-induced Mn2+ influx and charybdotoxin sensitive 86Rb+ efflux were markedly potentiated. The increase in the inositol- 1,4,5-trisphosphate and inositol-1,3,4,5-tetrakisphosphate levels by ras is not sufficient to explain the elevated Mn2+ influx. The mitogenic response to an expression of transforming Ha-ras was inhibited by the Ca(2+)-channel blockers not, however, by charybdotoxin. These data suggest the existence of an agonist-independent activation of a receptor- or second messenger-operated Ca2+ channel by transforming Ha-ras which is necessary for the mitogenic response to the activation of the oncogene.

Phosphoinositide hydrolysis in Ki-ras-transformed fibroblasts stimulated by platelet-derived growth factor and bradykinin

Signal transduction in response to platelet-derived growth factor (PDGF)-BB and bradykinin (BK) have been examined by measuring inositol polyphosphate formation in NIH3T3 fibroblast and v-Ki-ras-transformed NIH3T3 fibroblast (DT). The PDGF-induced inositol polyphosphate formation in NIH3T3 was greater than that in DT cells, in which autophosphorylation of PDGF receptor and tyrosine phosphorylation of phospholipase C (PLC)-gamma 1 were suppressed when examined by immunoblotting with anti-phosphotyrosine antibody. On the other hand, BK-stimulation produced a much higher level of inositol polyphosphate in DT cells which have a greater number of BK receptors. These results indicate that in Ki-ras transformed cells the decrease (caused by PDGF) and the increase (caused by BK) in phosphoinositide hydrolysis are due to the defective autophosphorylation of PDGF receptors leading to a reduction in PLC-gamma 1 tyrosine phosphorylation and the overexpression of BK receptors, respectively.

Spatial and temporal signalling by calcium

Recent research has shown the importance of the spatial and temporal aspects of calcium signals, which depend upon regenerative properties of the inositol trisphosphate and ryanodine receptors that regulate the release of calcium from internal stores. Initiation sites have been found to spontaneously release calcium, recognized as 'hot spots' or 'sparks', and can trigger a wave that spreads through a process of calcium-induced calcium release.

Thy-1 inhibits mitogen-induced Ca2+ oscillation in ras-transformed mouse fibroblasts

Cell surface glycoprotein Thy-1 functions as a transformation suppressor in v-ras-transformed NIH/3T3 cells [Sugimoto et al., (1991) Cancer Res. 51, 99-104.]. In order to understand the mechanism of action of Thy-1, we examined the effect of Thy-1 expression on mitogen-induced Ca2+ oscillation which was correlated with v-ras-transformation [Fu et al., (1991) FEBS Lett. 281, 263-266.]. Forced expression of Thy-1 in v-ras-transformed cells inhibited mitogen-induced Ca2+ oscillation. Although v-Ras-free, Thy-1-positive NIH/3T3 cells (major population) did not show Ca2+ oscillation, whereas in Thy-1-negative NIH/3T3 cells (less than 1% of the population) Ca2+ oscillation was observed. Finally, replacement of the carboxyl-half of Thy-1 with that of CD4 abolished the inhibitory effect of Thy-1. These results suggest that Thy-1 directly or indirectly participates in the negative regulation of Ca2+ response by inhibiting Ca2+ oscillation.

Abolishment of bradykinin-induced calcium oscillations in ras-transformed fibroblasts by the expression of 80 kDa diacylglycerol kinase

Our previous study showed bradykinin-induced periodic Ca2+ changes (Ca2+ oscillations) in v-Ki-ras-transformed NIH/3T3 (DT) cells in which protein kinase C (PKC) activity is partially down-regulated by a sustained high level of 1,2-diacylglycerol (DAG) [FEBS Lett. (1991) 281, 263-266]. In the present study, DAG kinase with 80 kDa mass (80K DGK) has been successfully transfected in DT cells, which exhibited enhanced cellular DAG kinase activities, decreased cellular DAG contents, and increased PKC activities compared to the control vector-transfected cells. Furthermore, these DGK-transfectants showed strong inhibition in bradykinin-induced Ca2+ oscillations. The results suggest that the sustained DAG increase down-regulates the PKC activity, thereby leading to the induction of Ca2+ oscillations in DT cells.

Cytosolic calcium oscillations induced by hepatocyte growth factor (HGF) in single fura-2-loaded cultured hepatocytes: effects of extracellular calcium and protein kinase C

Hepatocyte growth factor (HGF) induced the periodic fluctuations of cytosolic calcium concentration ([Ca2+]i) in primary cultured rat hepatocytes, which were dependent on extracellular calcium. The HGF-induced [Ca2+]i oscillations were suppressed by the pretreatment with phorbol 12-myristate 13-acetate (PMA). Administration of PMA during oscillations also caused their blockade, but the subsequent addition of protein kinase C (PKC) inhibitor H-7 reversed the inhibitory effects of PMA, thereby resulting in the resumption of the oscillatory responses. Moreover, the prior exposure to H-7 caused apparent increases in [Ca2+]i spike peaks elicited by HGF. These results suggest a negative modulation via PKC in HGF-induced repetitive [Ca2+]i transients. The absence of HGF-induced oscillations after the thapsigargin treatment indicates that the agonist-sensitive intracellular Ca2+ pool plays a crucial role in the [Ca2+] oscillations.

Ion selectivity of Ba2+ inward current oscillations in ras-transformed fibroblasts that elicit cytoplasmic Ca2+ oscillations by bradykinin

Ion selectivity of divalent cations on Ba2+ inward current oscillations was examined by voltage-clamp recording in v-Ki-ras-transformed NIH/3T3 (DT) fibroblasts where repetitive transient increases in cytoplasmic Ca2+ concentration were evoked by bradykinin. Application of bradykinin onto DT cells in 50 mM Ba2+ solution initiated Ba2+ inward current oscillations. The inward currents were inhibited in equimolar Sr2+ or Ca2+ solutions. Ba2+ current oscillations were dependent upon extracellular Ba2+ concentration. The results suggest that inward current oscillations are highly selective to Ba2+.

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.

Differential pathways (phospholipase C and phospholipase D) of bradykinin-induced biphasic 1,2-diacylglycerol formation in non-transformed and K-ras-transformed NIH-3T3 fibroblasts. Involvement of intracellular Ca2+ oscillations in phosphatidylcholine breakdown

Bradykinin (BK) induced a biphasic increase in 1,2-diacylglycerol (DAG) in both K-ras-transformed fibroblasts (DT) and the parent NIH-3T3 cells. The first phase was coincident with the increase in Ins(1,4,5)P3 resulting from PtdIns(4,5)P2 hydrolysis, and the second, sustained, phase was derived from phosphatidylcholine (PtdCho) hydrolysis. In NIH-3T3 cells, stimulation by BK induced greater production of choline than phosphocholine in [3H]choline-labelled cells and appreciable phosphatidylethanol (PtdEtOH) formation in [3H]myristic acid-labelled cells, suggesting that PtdCho was hydrolysed mainly by a phospholipase D (PLD) activity. Pretreatment with propranolol, an inhibitor of phosphatidate phosphohydrolase, markedly diminished the second DAG accumulation, supporting the above notion. In DT cells, BK induced predominantly phosphocholine generation and little PtdEtOH formation, indicating that the PtdCho hydrolysis was due to a phospholipase C (PLC) activity. The BK-induced oscillations in intracellular Ca2+ concentration ([Ca2+]i) observed in single DT cells [Fu, Sugimoto, Oki, Murakami, Okano & Nozawa (1991) FEBS Lett. 281, 263-266] were detected as a sustained [Ca2+]i elevation when assayed in a cell suspension. A receptor-operated Ca2+ channel blocker, SK&F 96365, suppressed both the BK-induced phosphocholine generation and the sustained [Ca2+]i elevation in a similar dose-dependent manner. These results thus suggested that oscillations in [Ca2+]i are involved in the activation of PtdCho-specific PLC in DT cells.

Ba2+ current oscillations modulated by cyclic AMP and phorbol esters in ras-transformed fibroblasts

An oscillatory influx of divalent cations was measured as Ba2+ inward currents (Ba2+ current oscillations) by voltage-clamp recording in v-Ki-ras-transformed NIH/3T3 (DT) fibroblasts after activation with bradykinin or serum. Application of forskolin or dibutyryl cyclic AMP onto DT cells initiated Ba2+ current oscillations. Increasing intracellular cyclic AMP reduced the amplitude but increased the frequency of the Ba2+ current oscillations. Activation of protein kinase C by phorbol esters terminated Ba2+ current oscillations. No inhibition of Ba2+ current oscillations by phorbol esters was observed in down-regulated cells that had been pretreated with phorbol esters for 24 hrs. The results suggest that Ba2+ current oscillations are regulated by intracellular second messengers.

Ba2+ current oscillation evoked by bradykinin in ras-transformed fibroblasts

By voltage-clamp recording, we show a novel inward current which oscillates after activation with bradykinin or serum in v-Ki-ras-transformed NIH/3T3 cells. The current oscillation was infrequently observed in control NIH/3T3 fibroblasts. The same stimulation evokes Ca2+ oscillations in the ras-transformed cells but not in parental cells (Fu et al., FEBS Lett. 281, 263-266, 1991). The results suggest that the oscillatory currents are generated by influxes of divalent cations to maintain Ca2+ oscillations in ras-transformed NIH/3T3 cells.