Single-cell analysis of the mitogen-induced calcium responses of normal and protein kinase C-depleted Swiss 3T3 cells

Mechanics regulates ATP-stimulated collective calcium response in fibroblast cells

Cells constantly sense their chemical and mechanical environments. We study the effect of mechanics on the ATP-induced collective calcium response of fibroblast cells in experiments that mimic various tissue environments. We find that closely packed two-dimensional cell cultures on a soft polyacrylamide gel (Young's modulus E = 690 Pa) contain more cells exhibiting calcium oscillations than cultures on a rigid substrate (E = 36 000 Pa). Calcium responses of cells on soft substrates show a slower decay of calcium level relative to those on rigid substrates. Actin enhancement and disruption experiments for the cell cultures allow us to conclude that actin filaments determine the collective Ca(2+) oscillatory behaviour in the culture. Inhibition of gap junctions results in a decrease of the oscillation period and reduced correlation of calcium responses, which suggests additional complexity of signalling upon cell-cell contact. Moreover, the frequency of calcium oscillations is independent of the rigidity of the substrate but depends on ATP concentration. We compare our results with those from similar experiments on individual cells. Overall, our observations show that collective chemical signalling in cell cultures via calcium depends critically on the mechanical environment.

Regulation of myofibroblast activities: calcium pulls some strings behind the scene

Myofibroblast-induced remodeling of collagenous extracellular matrix is a key component of our body's strategy to rapidly and efficiently repair damaged tissues; thus myofibroblast activity is considered crucial in assuring the mechanical integrity of vital organs and tissues after injury. Typical examples of beneficial myofibroblast activities are scarring after myocardial infarct and repair of damaged connective tissues including dermis, tendon, bone, and cartilage. However, deregulation of myofibroblast contraction causes the tissue deformities that characterize hypertrophic scars as well as organ fibrosis that ultimately leads to heart, lung, liver and kidney failure. The phenotypic features of the myofibroblast, within a spectrum going from the fibroblast to the smooth muscle cell, raise the question as to whether it regulates contraction in a fibroblast- or muscle-like fashion. In this review, we attempt to elucidate this point with a particular focus on the role of calcium signaling. We suggest that calcium plays a central role in myofibroblast biological activity not only in regulating contraction but also in mediating intracellular and extracellular mechanical signals, structurally organizing the contractile actin-myosin cytoskeleton, and establishing lines of intercellular communication.

Myofibroblast communication is controlled by intercellular mechanical coupling

Neoformation of intercellular adherens junctions accompanies the differentiation of fibroblasts into contractile myofibroblasts, a key event during development of fibrosis and in wound healing. We have previously shown that intercellular mechanical coupling of stress fibres via adherens junctions improves contraction of collagen gels by myofibroblasts. By assessing spontaneous intracellular Ca2+ oscillations, we here test whether adherens junctions mechanically coordinate myofibroblast activities. Periodic Ca2+ oscillations are synchronised between physically contacting myofibroblasts and become desynchronised upon dissociation of adherens junctions with function-blocking peptides. Similar uncoupling is obtained by inhibiting myofibroblast contraction using myosin inhibitors and by blocking mechanosensitive ion channels using Gd3+ and GSMTx4. By contrast, gap junction uncouplers do not affect myofibroblast coordination. We propose the following model of mechanical coupling for myofibroblasts: individual cell contraction is transmitted via adherens junctions and leads to the opening of mechanosensitive ion channels in adjacent cells. The resulting Ca2+ influx induces a contraction that can feed back on the first cell and/or stimulate other contacting cells. This mechanism could improve the remodelling of cell-dense tissue by coordinating the activity of myofibroblasts.

Differential involvement of ERK1-2and p38MAPKactivation on Swiss 3T3 cell proliferation induced by prostaglandin F2α

Prostaglandin F2alpha (PGF2alpha) induces cyclin D1 expression and DNA synthesis in Swiss 3T3 cells. In order to assess which signaling mechanisms are implicated in these processes, we have used both a pharmacological approach and interfering mutants. We demonstrate that PGF2alpha induces extracellular-signal-regulated kinase (ERK1-2) and p38MAPK activation, and inhibition of any of these signaling pathways completely blocks PGF2alpha-stimulated DNA synthesis. We also show that ERK1-2, but not p38MAPK activation is required to induce cyclin D1 expression, strongly suggesting that the concerted action of cyclin D1 gene expression and other events are required to induce complete phosphorylation of retinoblastoma protein and S-phase entry in response to PGF2alpha.

Prostaglandin F(2alpha) (PGF(2alpha)) induces cyclin D1 expression and DNA synthesis via early signaling mechanisms in Swiss mouse 3T3 cells

Prostaglandin F(2alpha) (PGF(2alpha)), a mitogen for Swiss 3T3 cells, triggers cyclin D1 mRNA/protein expression prior to cellular entry into the S phase, but fails to raise cdk4 or cyclin D3 levels, while 1-oleoyl-2-diacylglycerol (OAG), a protein kinase C (PKC) and tyrosine kinase (TK) activator, induces only cyclin D1 expression with no mitogenic response. In contrast, in PKC-depleted or -inhibited cells, PGF(2alpha), but not OAG, increases cyclin D1 expression with no mitogenic response. Finally, OAG, in the presence of orthovanadate (Na(3)VO(4)) or TGF(beta1), induces DNA synthesis. Thus, it appears that PGF(2alpha) triggers cyclin D1 expression via two independent signaling events that complement with TGF(beta1)-triggered events to induce DNA synthesis.

Mechanisms through which PDGF alters intracellular calcium levels in U-1242 MG human glioma cells

PDGF-BB induces a rapid, sustained increase in intracellular calcium levels in U-1242 MG cells. We used several calcium channel blockers to identify the types of channels involved. L channel blockers (verapamil, nimodipine, nicardipine, nitrendipine and taicatoxin) had no effect on PDGF-BB induced alterations in intracellular calcium. Blockers of P, Q and N channels (omega-agatoxin-IVA, omega-conotoxin MVIIC and omega-conotoxin GVIA) also had no effect. This indicates that these channels play an insignificant role in supplying the Ca2+ necessary for PDGF stimulated events in U-1242 MG cells. However, a T channel blocker (NDGA) and the non-specific (NS) calcium channel blockers (FFA and SK&F 9365) abolished PDGF-induced increases in intracellular calcium. This indicates that PDGF causes calcium influx through both non-specific cationic channels and T channels. To study the participation of intracellular calcium stores in this process, we used thapsigargin, caffeine and ryanodine, all of which cause depletion of intracellular calcium stores. The PDGF effect was abolished using both thapsigargin and caffeine but not ryanodine. Collectively, these data indicate that in these human glioma cells PDGF-BB induces release of intracellular calcium from caffeine- and thapsigargin-sensitive calcium stores which in turn lead to further calcium influx through both NS and T channels.

Ways of looking at calcium

What we understand about signalling pathways depends very much on the ways we can measure them. I review ways of measuring calcium and explore how changes in methods have led to new ways of thinking about calcium signals. I also suggest how the ways we have of looking at calcium will influence the analysis of other signalling pathways that, until now, have not been studied with the spatiotemporal precision available to those studying calcium signalling.

Involvement of protein kinase C in bradykinin-induced intracellular calcium increase in primary cultured human keratinocytes

Bradykinin (BK) is one of the key mediators of inflammation and a weak mitogen. We have previously demonstrated that BK induced the generation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) which caused Ca2+ mobilization in human keratinocytes. In this study, BK-induced Ca2+ responses were examined in primary cultured human keratinocytes by video imaging fluorescence microscopy using fura-2. Intracellular calcium concentration ([Ca2+]i) level increased to a peak within 30 s after BK addition and decreased gradually to the basal level. The existence of the broad shoulder in the [Ca2+]i profile was suggested to be due to the Ca2+ influx from the external medium, because this disappeared in the presence of 0.5 mM EGTA. Pretreatment with phorbol-12-myristate-13-acetate (PMA), a protein kinase C (PKC) activator, significantly resulted in reduction of the descending shoulder of BK-induced increase in [Ca2+]i. A 20-min pretreatment with PKC inhibitors, H-7 or staurosporine, reversed the decrease by PMA in the shoulder of BK-induced Ca2+ response. Furthermore, the BK-induced [45Ca] uptake was inhibited by EGTA and PMA. Ins(1,4,5)P3 generation induced by BK peaked at 20 s and returned to the basal level at 60 s. There were no significant differences in Ins(1,4,5)P3 levels at 20 and 60 s among the cells exposed to BK alone, BK with PMA pretreatment (20 min) and BK with PMA+H-7 pretreatment. These results suggest that the BK-induced Ca2+ influx, which was shown as shoulder, may be negatively modulated by PKC in primary cultured human keratinocytes.

Transforming growth factor beta 1, insulin and prostaglandin E1 enhance prostaglandin F2 alpha mitogenic action in Swiss 3T3 cells via separate events

Transforming growth factor beta 1 (TGF beta 1) had no mitogenic effect in Swiss 3T3 cells, but could increase prostaglandin F2 alpha (PGF2 alpha)-induced DNA synthesis. Insulin, but not prostaglandin E1 (PGE1), further enhanced PGF2 alpha action at low TGF beta 1 concentrations. TGF beta 1 also acted concertedly with the protein kinase C (PKC) activator 1-oleoyl-2-acetylglycerol to induce mitogenesis. Thus, it appears that TGF beta 1 and insulin act via separate signals, while TGF beta 1 and PGE1 might share a common pathway not involving TGF beta 1-mediated prostaglandin synthesis. These results suggest that TGF beta 1 might elicit various signalling mechanisms to enhance PGF2 alpha-triggered events.

Activation of AT1 angiotensin receptors induces DNA synthesis in a rat intestinal epithelial (RIE-1) cell line

Proliferation of the rat intestinal epithelial cell-line, RIE-1, has previously been shown to be stimulated by certain polypeptide growth factors acting via receptors that possess intrinsic tyrosine kinase activity. In this study, we show that the octapeptide hormone angiotensin II (AII), apparently acting through the AT1 G-protein-coupled receptor, is also a mitogen for RIE-1 cells. Maximal stimulation of DNA synthesis and cellular proliferation occurred at an AII concentration of 10-100 nM, with half-maximal stimulation at 1 nM. The mitogenic response to AII was completely inhibited by the AT1 angiotensin-receptor antagonist, DuP753, but not by the AT2-receptor antagonist, PD123319. The early signalling responses activated by AII in RIE-1 cells include increased production of inositol phosphates, a transient increase in the intracellular concentration of free calcium, an activation of protein kinase C, and a rapid change in the pattern of cellular protein-tyrosine phosphorylation. These results implicate an activation of the inositol lipid signalling pathway via the AT1 receptor subtype in the AII-stimulated mitogenic response of this normal epithelial cell line.

Homologous desensitization of bombesin-induced increases in intracellular Ca2+ in quiescent Swiss 3T3 cells involves a protein kinase C-independent mechanism

Addition of bombesin to Swiss 3T3 cells causes a rapid and transient increase in the intracellular concentration of Ca2+ ([Ca2+]i), which is followed by desensitization to a subsequent addition of the peptide. The concentrations of bombesin used to study this acute cellular desensitization (0.1-0.5 nM) did not deplete the intracellular pool of Ca2+ released by inositol(1,4,5)trisphosphate, as shown by addition of vasopressin after consecutive additions of bombesin. Two lines of evidence support the conclusion that activation of protein kinase C (PKC) does not mediate the acute homologous desensitization of Ca2+ responses induced by bombesin. First, long-term treatment (48 h) of Swiss 3T3 cells with phorbol 12,13-dibutyrate (PDB) to deplete PKC did not prevent homologous desensitization. The responses to second additions of bombesin at 0.1, 0.25, and 0.5 nM were 42%, 26% and 11% of the initial responses, respectively. Second, the PKC inhibitor GF 109203X did not alter homologous desensitization at concentrations that completely prevented the inhibition of Ca2+ mobilization induced by PDB and blocked PDB-mediated phosphorylation of the prominent PKC substrate 80K/MARCKS. We conclude that acute homologous desensitization of Ca2+ responses induced by bombesin occurs through a PKC-independent mechanism.

Altered calcium regulation in SV40-transformed Swiss 3T3 fibroblasts

Calcium homeostasis has long been thought to be altered in transformed cells but mechanisms have not been established. In this study, the photoprotein, aequorin, was used to examine calcium regulation in 3T3 and SV40-transformed 3T3 cells. It was found that calcium transients induced by bradykinin or serum in serum-starved cells are lower and delayed in the transformed cells and decay kinetics are altered. These changes are not related to differences in cell cycle distribution. Though the serum transient is insensitive to nifedipine, verapamil, or lanthanum, removal of extracellular calcium accelerates transient decay in both cell types. Treatment of unstimulated cells with the ER Ca(2+)-ATPase inhibitor, thapsigargin, causes a 4-5-fold greater increase in [Ca2+]i in the transformed than in the nontransformed cells. Following serum stimulation, transformed cells still exhibit a large thapsigargin-induced increase in [Ca2+]i whereas the response in nontransformed cells is nearly abolished. When the 3T3 or SV3T3 cells are exposed to serum or thapsigargin in the absence of extracellular calcium and subsequently exposed to 11.8 mM Ca2+, a much greater influx of calcium again occurs in the SV3T3 cells. The observed changes in SV3T3 cells are most likely due to an alteration in a capacitative mechanism which regulates influx of calcium through the plasma membrane.

Calcium mobilization and cell proliferation activated by extracellular ATP in human ovarian tumour cells

The effects of ATP on cell proliferation and intracellular calcium concentration ([Ca2+]i) were examined in a human ovarian cancer cell line (OVCAR-3). Micromolar concentrations of ATP promoted a biphasic rise in [Ca2+]i representing a phase with a rapid peak followed by a phase in which the rise was slower and sustained. When the influx of extracellular calcium was blocked by calcium chelation to EGTA, the ATP stimulated rise in [Ca2+]i was rapid and monophasic. Voltage-sensitive calcium channel blockers like nifedipine and verapamil had no effect on the action of ATP while prenylamine totally blocked calcium influx. ATP inclusion in the medium significantly stimulated growth of OVCAR-3 cells. Fetal calf serum (FCS) increased [Ca2+]i with similar biphasic kinetics representing both the entry of extracellular calcium and release of calcium from intracellular stores. FCS also caused a substantial increase in cell growth. From these experiments it was concluded that an increase in [Ca2+]i is obligatory for stimulation of cell growth in OVCAR-3 cells and that this increase probably requires a contribution from the entry of extracellular calcium. The involvement of both pertussis toxin sensitive G-protein and protein kinase C in ATP induced responses was indicated by the data showing interference of the response by pertussis toxin and phorbol-myristate acetate.

Early cell cycle diacylglycerol (DAG) content and protein kinase C (PKC) activity enhancement potentiates prostaglandin F2 alpha (PGF2 alpha) induced mitogenesis in Swiss 3T3 cells

R59022, a diacylglycerol kinase inhibitor, enhances the prostaglandin F2 alpha (PGF2 alpha)-induced diacylglycerol (DAG) synthesis in Swiss 3T3 cells. It also potentiates the PGF2 alpha-mediated protein kinase C (PKC)-dependent 80 kDa protein (80K) phosphorylation and initiation of DNA replication. R59022 enhances the PGF2 alpha mitogenic response by increasing the rate of entry into the S phase. Insulin does not cause 80K phosphorylation, and does not enhance its induction but it potentiates the PGF2 alpha mitogenic response. These results suggest that mitogenically triggered fluctuations in DAG content and PKC activity play a pivotal role in controlling the PGF2 alpha-induced DNA synthesis while insulin acts via a different mechanism.

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.

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.

Differential potentiation of mitogen-stimulated phosphoinositide hydrolysis in protein kinase C-depleted Swiss 3T3 cells

In Swiss 3T3 cells, depletion of protein kinase C (PKC) by prolonged incubation with phorbol esters potentiates the formation of total inositol phosphates in response to bombesin or vasopressin [Blakeley, Corps & Brown (1989) Biochem. J. 258, 177-185]. The characteristics of the accumulation of inositol phosphates in control and PKC-depleted cells stimulated by bombesin, vasopressin or prostaglandin F2 alpha (PGF2 alpha) have now been compared. The potentiation of the PGF2 alpha response was greater than that of the vasopressin response which was, in turn, greater than that of the bombesin response. The time courses of the responses to all three agonists were biphasic, and both phases of the response were amplified in the PKC-depleted cells. These results provide further evidence for the involvement of a PKC-mediated negative-feedback loop regulating phosphoinositide hydrolysis in response to several 3T3 cell mitogens. The differential potentiation of the response to these agonists suggests that PKC might act at multiple sites within the signal transduction pathway.

Endothelin-1 stimulates DNA synthesis and anchorage-independent growth of Rat-1 fibroblasts through a protein kinase C-dependent mechanism

Stimulation of quiescent cultured fibroblasts with a variety of growth-promoting factors induces release of diacylglycerol (DG) and subsequent activation of protein kinase C (pkC), but the role of pkC in the induction of DNA synthesis and cell proliferation remains unclear. We have investigated the involvement of pkC in the response of Rat-1 fibroblasts to the newly described peptide endothelin-1 (Et-1), an agonist that is secreted by the vascular endothelium and that may play a role in the proliferative response of cells in the vessel wall. Addition of Et-1 to serum-deprived Rat-1 cells promoted DNA synthesis in the absence of additional factors and stimulated anchorage-independent growth in the presence of epidermal growth factor (EGF), indicating that Et-1 has many of the characteristics of a mitogen. The ability of Et-1 to stimulate both DNA synthesis and anchorage-independent growth was markedly reduced by the depletion of cellular pkC activity induced by prolonged exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA). In contrast, the ability of Et-1 to induce both second messenger production and transcription of c-fos and c-jun was largely independent of cellular pkC activity. Production of DG in response to Et-1 persisted for greater than 12 h and may account for the ability of Et-1 to augment the G1-S phase transition. Although these observations indicate that functional pkC is not an essential component of the proximal pathway leading to rapid changes in gene transcription and second messenger production in response to Et-1 treatment, the data suggest that activation of pkC is an essential component of the downstream events responsible for the stimulation of cell proliferation and anchorage-independent growth in Rat-1 cells exposed to Et-1.

Neuropeptide stimulation of calcium flux in human lung cancer cells: delineation of alternative pathways

Calcium ion flux following the administration of a series of neuropeptides, N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate, and serum was monitored by flow cytometry in selected lung and breast cancer cell lines and Chinese hamster ovary cell line CHO-K1. Calcium ion flux was monitored in individual cells by flow cytometry using the indicator indo-1 AM. Five groups of neuropeptides produced calcium flux changes in lung cancer cell lines and CHO-K1 cells but not in breast cancer cells. The peak increase in free calcium was reached within 10 sec of peptide administration and declined to resting levels in 70-120 sec. When two or more members of the same group were administered simultaneously, calcium flux changes were identical to that produced by each single peptide. When two or more members of different groups were administered simultaneously, an increased calcium release occurred. When identical peptides or peptides from the same group were administered sequentially after the return of calcium concentrations to resting values, no calcium flux resulted from the second peptide. When peptides from different active groups were administered sequentially, a new calcium flux occurred after each peptide. These data are interpreted to mean that members of each active group of peptides trigger a different calcium flux pathway. Thus, many such pathways and different metabolic states exist within the cell. Elucidation of calcium flux pathways in normal and cancer cells may lead to greater understanding of the nature of the malignant defect.