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

Growth control in colon epithelial cells: gadolinium enhances calcium-mediated growth regulation

Gadolinium, a member of the lanthanoid family of transition metals, interacts with calcium-binding sites on proteins and other biological molecules. The overall goal of the present investigation was to determine if gadolinium could enhance calcium-induced epithelial cell growth inhibition in the colon. Gadolinium at concentrations as low as 1-5 μM combined with calcium inhibits proliferation of human colonic epithelial cells more effectively than calcium alone. Gadolinium had no detectable effect on calcium-induced differentiation in the same cells based on change in cell morphology, induction of E-cadherin synthesis, and translocation of E-cadherin from the cytosol to the cell surface. When the colon epithelial cells were treated with gadolinium and then exposed to increased calcium concentrations, movement of extracellular calcium into the cell was suppressed. In contrast, gadolinium treatment had no effect on ionomycin-induced release of stored intracellular calcium into the cytoplasm. Whether these in vitro observations can be translated into an approach for reducing abnormal proliferation in the colonic mucosa (including polyp formation) is not known. These results do, however, provide an explanation for our recent findings that a multi-mineral supplement containing all of the naturally occurring lanthanoid metals including gadolinium are more effective than calcium alone in preventing colon polyp formation in mice on a high-fat diet.

A multimineral natural product from red marine algae reduces colon polyp formation in C57BL/6 mice

The goal of this study was to determine if a multimineral natural product derived from red marine algae could reduce colon polyp formation in mice on a high-fat diet. C57BL/6 mice were maintained for up to 18 mo either on a high-fat "Western-style" diet or on a low-fat diet (AIN 76A), with or without the multimineral-supplement. To summarize, colon polyps were detected in 22 of 70 mice (31%) on the high-fat diet but in only 2 of 70 mice (3%) receiving the mineral-supplemented high-fat diet (P < 0.0001). Colon polyps were detected in 16 of 70 mice (23%) in the low-fat group; not significantly different from high-fat group but significantly higher than the high-fat-supplemented group (P = 0.0006). This was in spite of the fact that the calcium level in the low-fat diet was comparable to the level of calcium in the high-fat diet containing the multimineral-product. Supplementation of the low-fat diet reduced the incidence to 8 of 70 mice (11% incidence). Taken together, these findings demonstrate that a multimineral natural product can protect mice on a high-fat diet against adenomatous polyp formation in the colon. These data suggest that increased calcium alone is insufficient to explain the lower incidence of colon polyps.

Glucose increases extracellular [Ca2+] in rat insulinoma (INS-1E) pseudoislets as measured with Ca2+-sensitive microelectrodes

Secretory granules of pancreatic β-cells contain high concentrations of Ca2+ ions that are co-released with insulin in the extracellular milieu upon activation of exocytosis. As a consequence, an increase in the extracellular Ca2+ concentration ([Ca2+]ext) in the microenvironment immediately surrounding β-cells should be expected following the exocytotic event. Using Ca2+-selective microelectrodes we show here that both high glucose and non-nutrient insulinotropic agents elicit a reversible increase of [Ca2+]ext within rat insulinoma (INS-1E) β-cells pseudoislets. The glucose-induced increases in [Ca2+]ext are blocked by pretreatment with different Ca2+ channel blockers. Physiological agonists acting as positive or negative modulators of the insulin secretion and drugs known to intersect the secretory machinery at different levels also induce [Ca2+]ext changes as predicted on the basis of their described action on insulin secretion. Finally, the glucose-induced [Ca2+]ext increase is strongly inhibited after disruption of the actin web, indicating that the dynamic [Ca2+]ext changes recorded in INS-1E pseudoislets by Ca2+-selective microelectrodes occur mainly as a consequence of exocytosis of Ca2+-rich granules. In conclusion, our data directly demonstrate that the extracellular spaces surrounding β-cells constitute a restricted domain where Ca2+ is co-released during insulin exocytosis, creating the basis for an autocrine/paracrine cell-to-cell communication system via extracellular Ca2+ sensors.

Stimulation of fibroblast proliferation by insoluble gadolinium salts

The purpose of this study was to assess insoluble salts containing gadolinium (Gd(3+)) for effects on human dermal fibroblasts. Responses to insoluble Gd(3+) salts were compared to responses seen with Gd(3+) solubilized with organic chelators, as in the Gd(3+)-based contrast agents (GBCAs) used for magnetic resonance imaging. Insoluble particles of either Gd(3+) phosphate or Gd(3+) carbonate rapidly attached to the fibroblast cell surface and stimulated proliferation. Growth was observed at Gd(3+) concentrations between 12.5 and 125 μM, with toxicity at higher concentrations. Such a narrow window did not characterize GBCA stimulation. Proliferation induced by insoluble Gd(3+) salts was inhibited in the presence of antagonists of mitogen-activated protein kinase and phosphatidylinositol 3-kinase signaling pathways (similar to chelated Gd(3+)) but was not blocked by an antibody to the platelet-derived growth factor receptor (different from chelated Gd(3+)). Finally, high concentrations of the insoluble Gd(3+) salts failed to prevent fibroblast lysis under low-Ca(2+) conditions, while similar concentrations of chelated Gd(3+) were effective. In conclusion, while insoluble Gd(3+) salts are capable of stimulating fibroblast proliferation, one should be cautious in assuming that GBCA dechelation must occur in vivo to produce the profibrotic changes seen in association with GBCA exposure in the subset of renal failure patients that develop nephrogenic systemic fibrosis.

Fibroblast response to lanthanoid metal ion stimulation: potential contribution to fibrotic tissue injury

The purpose of this study was to compare each of the 14 naturally occurring lanthanoid metal ions for ability to stimulate pro-fibrotic responses in human dermal fibroblasts. When fibroblasts were exposed to individual lanthanoids over the concentration range of 1-100 μM, increased proliferation was observed with each of the agents as compared with control cells that were already proliferating rapidly in a growth factor-enriched culture medium. Dose-response differences were observed among the individual metal ions. Matrix metalloproteinase-1 (MMP-1) and tissue inhibitor of metalloproteinase-1 levels were also increased in response to lanthanoid exposure but type I procollagen production was not. A dose-response relationship between induction of proliferation and increased MMP-1 was observed. Non-lanthanoid transition metal ions (aluminum, copper, cobalt, iron, magnesium, manganese, nickel, and zinc) were examined in the same assays; there was little stimulation with any of these metals. When epidermal keratinocytes were examined in place of dermal fibroblasts, there was no growth stimulation with any of the lanthanoids. Several of the lanthanoid metals inhibited keratinocyte proliferation at higher concentrations (50-100 μM).

Casein phosphopeptides modulate proliferation and apoptosis in HT-29 cell line through their interaction with voltage-operated L-type calcium channels

At the intestinal level, proliferation and apoptosis are modulated by the extracellular calcium concentration; thus, dietary calcium may exert a chemoprotective role on normal differentiated intestinal cells, while it may behave as a carcinogenesis promoter in transformed cells. Calcium in milk is associated with casein and casein phosphopeptides (CPPs), hence is preserved from precipitation. CPPs were demonstrated to induce uptake of extracellular calcium ions by in vitro intestinal tumor HT-29 cells but only upon differentiation. Here, the hypothesis that CPPs could differently affect proliferation and apoptosis in undifferentiated and differentiated HT-29 cells through their binding with calcium ions was investigated. Results showed that CPPs protect differentiated intestinal cells from calcium overload toxicity and prevent their apoptosis favoring proliferation while inducing apoptosis in undifferentiated tumor cells. The CPP effect on undifferentiated HT-29 cells, similar to that exerted by ethyleneglycol-O, O'-bis(2-aminoethyl)-N, N, N', N'-tetraacetic acid (EGTA), is presumably due to the ability in binding the extracellular calcium. The effect on differentiated HT-29 cells is coupled to the interaction of CPPs with the voltage-operated L-type calcium channels, known to activate calcium entry into the cells under depolarization and to exert a mitogenic effect: the use of an agonist potentiates the cell response to CPPs, while the antagonists abolish the response to CPPs (36% of examined cells) or reduce both the percentage of responsive cells and the increase of intracellular calcium concentration. Taken together, these results confirm the potentialities of CPPs as nutraceuticals/functional food and also as modulators of cellular processes connected to the expression of a cancer phenotype.

Chapter 4: Regulation of Clusterin activity by calcium

In this chapter, the attention is put on Ca(2+) effect on Clusterin (CLU) activity. We showed that two CLU forms (secreted and nuclear) are differently regulated by Ca(2+) and that Ca(2+) fluxes affect CLU gene expression. A secretory form (sCLU) protects cell viability whereas nuclear form (nCLU) is proapoptotic. Based on available data we suggest, that different CLU forms play opposite roles, depending on intracellular Ca(2+) concentration, time-course of Ca(2+) current, intracellular Ca(2+) compartmentalization, and final Ca(2+) targets. Discussion will be motivated on how CLU acts on cell in response to Ca(2+) waves. The impact of Ca(2+) on CLU gene activity and transcription, posttranscriptional modifications, translation of CLU mRNA, and posttranslational changes as well as biological effects of CLU will be discussed. We will also examine how Ca(2+) signal and Ca(2+)-dependent proteins are attributable to changes in CLU characteristics. Some elucidation of CLU gene activity, CLU protein formation, maturation, secretion, and intracellular translocations in response to Ca(2+) is presented. In response to cell stress (i.e., DNA damage) CLU gene is activated. We assume that commonly upregulated mRNA for nCLU versus sCLU and vice versa are dependent on Ca(2+) accessibility and its intracellular distribution. It looks as if at low intracellular Ca(2+) the delay in cell cycle allows more time for DNA repair; otherwise, cells undergo nCLU-dependent apoptosis. If cells are about to survive, intrinsic apoptosis is abrogated by sCLU interacting with activated Bax. In conclusion, a narrow range of intracellular Ca(2+) concentrations is responsible for the decision whether nCLU is mobilized (apoptosis) or sCLU is appointed to improve survival. Since the discovery of CLU, a huge research progress has been done. Nonetheless we feel that much work is left ahead before remaining uncertainties related to Ca(2+) signal and the respective roles of CLU proteins are unraveled.

Regulation of store-operated and voltage-operated Ca2+ channels in the proliferation and death of oligodendrocyte precursor cells by golli proteins

OPCs (oligodendrocyte precursor cells) express golli proteins which, through regulation of Ca²⁺ influx, appear to be important in OPC process extension/retraction and migration. The aim of the present study was to examine further the role of golli in regulating OPC development. The effects of golli ablation and overexpression were examined in primary cultures of OPCs prepared from golli-KO (knockout) and JOE (golli J37-overexpressing) mice. In OPCs lacking golli, or overexpressing golli, differentiation induced by growth factor withdrawal was impaired. Proliferation analysis in the presence of PDGF (platelet-derived growth factor), revealed that golli enhanced the mitogen-stimulated proliferation of OPCs through activation of SOCCs (store-operated Ca²⁺ channels). PDGF treatment induced a biphasic increase in OPC intracellular Ca²⁺, and golli specifically increased Ca²⁺ influx during the second SOCC-dependent phase that followed the initial release of Ca²⁺ from intracellular stores. This store-operated Ca²⁺ uptake appeared to be essential for cell division, since specific SOCC antagonists completely blocked the effects of PDGF and golli on OPC proliferation. Additionally, in OPCs overexpressing golli, increased cell death was observed after mitogen withdrawal. This phenomenon could be prevented by exposure to VOCC (voltage-operated Ca²⁺ channel) blockers, indicating that the effect of golli on cell death involved increased Ca²⁺ influx through VOCCs. The results showed a clear effect of golli on OPC development and support a role for golli in modulating multiple Ca²⁺-regulatory events through VOCCs and SOCCs. Our results also suggest that PDGF engagement of its receptor resulting in OPC proliferation proceeds through activation of SOCCs.

Enhanced contractile response of the basilar artery to platelet-derived growth factor in subarachnoid hemorrhage

BACKGROUND AND PURPOSE

The level of platelet-derived growth factor (PDGF) in cerebrospinal fluid is elevated in subarachnoid hemorrhage (SAH). Therefore, the contractile effect of PDGF on the basilar artery was examined in SAH.

METHODS

A rabbit double-hemorrhage SAH model was used. In the medial layers of the control basilar artery, PDGF had no effect on contraction up to 1 nmol/L, whereas 3 nmol/L PDGF induced slight contraction. In SAH, PDGF induced an enhanced contraction with an increase in [Ca(2+)](i) at 1 nmol/L and higher concentrations. The levels of [Ca(2+)](i) and tension induced by 1 nmol/L PDGF in SAH were 17% and 20%, respectively, of those obtained with 118 mmol/L K(+) depolarization. The PDGF-induced elevation of [Ca(2+)](i) and contraction seen in SAH were abolished in the absence of extracellular Ca(2+). In alpha-toxin-permeabilized strips of SAH animals, PDGF induced no further development of tension during contraction induced by 300 nmol/L Ca(2+), suggesting no direct effect on myofilament Ca(2+) sensitivity. Genistein at 10 micromol/L completely inhibited the tension induced by 1 nmol/L PDGF. The level of myosin light-chain phosphorylation was significantly increased by 1 nmol/L PDGF.

CONCLUSIONS

These results show that the contractile response to PDGF of the basilar artery was enhanced in SAH. The PDGF-induced contraction depended mostly on tyrosine phosphorylation and Ca(2+)-dependent myosin light-chain phosphorylation. The enhancement of the responsiveness to PDGF may therefore contribute to the development of cerebral vasospasm after SAH.

Association of VASP with TRPC4 in PKG-mediated inhibition of the store-operated calcium response in mesangial cells

We tested the hypotheses that the NO-cGMP-PKG pathway mediates inhibition of the store-operated cation channel (SOC) in human glomerular mesangial cells (HMC) and that TRPC4, a molecular component of SOC in HMC, is associated with PKG-phosphorylated vasodilator-stimulated phosphoprotein (VASP). Using fura 2 ratiometry, we measured intracellular Ca(2+) concentration [Ca(2+)](i) to determine whether sodium nitroprusside (SNP), an NO donor, and 8-Br-cGMP affected SOC-TRPC4 via PKG. We found that the SOC response in HMC was attenuated in the presence of 100 microM SNP, an NO donor, or 100 microM 8-Br-cGMP. Addition of DT-3 (2.5 microM), a specific PKG-1alpha inhibitor, reversed the effects of 8-Br-cGMP on the SOC response. Application of 100 microM cAMP did not significantly inhibit the SOC response. RT-PCR and Western blotting revealed PKG-1alpha transcript and protein in HMC. Immunocytochemical analysis localized PKG-1alpha to the cytoplasm and plasma membrane of HMC. Previous studies have shown that PKG-mediated phosphorylation of VASP attenuates cellular Ca(2+) entry, resulting in altered growth and proliferation. Therefore, we used Western blotting and immunocytochemistry to determine whether PKG-phosphorylated VASP associates with TRPC4. Western blot analysis revealed that 8-Br-cGMP enhanced the phosphorylation of VASP at serine 239 (Ser239), a known PKG phosphorylation site, in HMC within 5 min. Coimmunoprecipitation and coimmunostaining showed that P-Ser239-VASP associated with TRPC4. However, VASP that was unphosphorylated at Ser239 was not associated with TRPC4. These results indicate that VASP has a role in the NO/PKG-1alpha-mediated inhibition of the TRPC4-SOC response in HMC.

Effect of thiazolidinedione on the proliferation of bovine retinal endothelial cells stimulated by vascular endothelial cell growth factor

PURPOSE

To investigate the effect of troglitazone, an antidiabetic drug, on the cytosolic Ca(2+) concentrations ([Ca(2+)]i) and the cell cycles of bovine retinal endothelial cells (RECs) stimulated with vascular endothelial growth factor (VEGF).

METHODS

The changes in [Ca(2+)]i were monitored using microfluorometry with Fura-2. The phase of the cell cycle was examined by an immunocytochemical analysis using monoclonal antibodies against cell cycle-specific nuclear antigens.

RESULTS

In the presence of extracellular Ca(2+), VEGF-induced transient [Ca(2+)]i elevation followed by continuous steady-state elevation resulted in cell cycle progression in RECs. The removal of extracellular Ca(2+) inhibited the continuous component, but transient [Ca(2+)]i elevation was still observed. These results are compatible with the hypothesis that a continuous steady-state elevation of [Ca(2+)]i may be mediated mainly through the influx of extracellular Ca(2+). Pretreatment with 10 microM troglitazone prevented the transient and continuous steady-state elevation of [Ca(2+)]i, resulting in an inhibition of the cell cycle in RECs stimulated with VEGF.

CONCLUSIONS

These data suggest that troglitazone inhibits VEGF-induced cell cycle progression through the inhibition of [Ca(2+)]i in RECs.

Anticancer activity of the lanthanum compound [tris(1,10-phenanthroline)lanthanum(III)]trithiocyanate (KP772; FFC24)

Aim of this study was to investigate the anticancer properties of the new lanthanum compound [tris(1,10-phenanthroline)lanthanum(III)]trithiocyanate (KP772; FFC24). In vitro, growth inhibition by KP772 was comparable for >60 tumour cell models with IC50 values generally in the low microM range. KP772 induced tumour cell apoptosis indicated by chromatin condensation, caspase substrate cleavage and mitochondrial membrane depolarisation. DNA is unlikely to represent the primary molecular target of KP772, as no significant interaction or damage of DNA was detectable both in vitro and in living cells. Moreover, we found no evidence for induction of radical species. In contrast, KP772 potently inhibited DNA synthesis paralleled by a massive block of cell cycle in G0/G1 phase and a selective decrease of cyclin B1. Although treatment with KP772 induced expression of p53 and p21Waf1, transfection of wild-type p53 into knock-out cells only marginally enhanced the cytostatic activity of KP772. In vivo, the anticancer activity of KP772 against human DLD-1 colon carcinoma xenografts was comparable to that of cisplatin and methotrexate at doses not causing significant adverse effects. With regard to toxicity, the LD50 and no-observed-adverse-effect levels (NOAEL) of KP772 in Sprague-Dawley rats were 21.6 and 7.5 mg/kg, in outbred albino mice 62 and 10 mg/kg, respectively. In summary, KP772 exerts anticancer activity via potent induction of cell cycle arrest and/or apoptosis and has promising in vivo anticancer activity against a human colon cancer xenograft. Together, these data suggest further development of KP772 as a new anticancer metal-drug.

Intracellular calcium signals and control of cell proliferation: how many mechanisms?

The progression through the cell cycle in non-transformed cells is under the strict control of extracellular signals called mitogens, that act by eliciting complex cascades of intracellular messengers. Among them, increases in cytosolic free calcium concentration have been long realized to play a crucial role; however, the mechanisms coupling membrane receptor activation to calcium signals are still only partially understood, as are the pathways of calcium entry in the cytosol. This article centers on the role of calcium influx from the extracellular medium in the control of proliferative processes, and reviews the current understanding of the pathways responsible for this influx and of the second messengers involved in their activation.

Biphasic calcium response of platelet-derived growth factor stimulated glioblastoma cells is a function of cell confluence

BACKGROUND

Previous reports have linked the spiking or two-phased character of calcium transients evoked by platelet-derived growth factor (PDGF) to the position of cells in the cell cycle without regard to cell-cell contact and communication. Because cell confluence can regulate growth factor receptor expression and dephosphorylation, we investigated the effect of cell culture confluence and cell cycle on calcium responses of PDGF-BB-stimulated A172 glioblastoma cells.

METHODS

Digital imaging cytometry was used to correlate the peak and duration of calcium response with bromodeoxyuridine positivity and DNA content and with culture confluence on a cell-by-cell basis.

RESULTS

In serum-starved cultures, complete two-phase calcium signals and shorter, lower spikes occurred independent of cell cycle phase. However, the confluence of cell culture seemed essential for inducing a complete response because cells in sparse cultures exhibited mostly short spikes with lower peaks or no transients at all.

CONCLUSION

Because cell confluence, by virtue of cell-cell contacts, is assumed to be an important regulator of proliferation, one is tempted to speculate that in transformed cells the ability to produce stronger growth signals upon reaching confluence and facing contact inhibition could provide a proliferative advantage.

Increased calcium influx and ribosomal content correlate with resistance to endoplasmic reticulum stress-induced cell death in mutant leukemia cell lines

Cell clones were derived by treatment of HL-60 cells with stepwise increasing concentrations of econazole (Ec), an imidazole antifungal that blocks Ca2+ influx and induces endoplasmic reticulum (ER) stress-related cell death in multiple mammalian cell types. Clones exhibit 20- to more than 300-fold greater resistance to Ec. Unexpectedly, they also display stable cross-resistance to tunicamycin, thapsigargin, dithiothreitol, and cycloheximide but not doxorubicin, etoposide, or Fas ligand. Phenotypic analysis indicates that the cells display increased store-operated calcium influx and resistance to ER Ca2+ store depletion by Ec. E2R2, the most resistant clone, was observed to maintain protein synthesis levels after treatment with Ec or thapsigargin. Expression of GRP78, an ER-based chaperone, was induced by these ER stress treatments but to equal degrees in HL-60 and E2R2 cells. By using microarray analysis, at least 15 ribosomal protein genes were found to be overexpressed in E2R2 compared with HL-60 cells. We also found that ribosomal protein content was increased by 30% in E2R2 as well as other clones. The resistance phenotype was partially reversed by the ribosome-inactivating protein saporin. Therefore, increased store-operated calcium influx, resistance to ER Ca2+ store depletion, and overexpression of ribosomal proteins define a novel phenotype of ER stress-associated multidrug resistance.

Inositol 1,4,5-trisphosphate-independent calcium signalling by platelet-derived growth factor in the human SH-SY5Y neuroblastoma cell

In adherent SH-SY5Y human neuroblastoma cells, activation of G-protein-coupled muscarinic M3 receptors evoked a biphasic elevation of both intracellular [Ca(2+)] ([Ca(2+)]i) and inositol-1,4,5-trisphosphate (D-Ins(1,4,5)P3) mass. In both cases, temporal profiles consisted of rapid transient elevations followed by a decline to a lower, yet sustained level. In contrast, platelet-derived growth factor (PDGF), a receptor tyrosine kinase agonist acting via PDGF receptor b chains in these cells, elicited a slow and transient elevation of [Ca(2+)]i that returned to basal levels within 5 to 10 min with no evidence of inositol phosphate generation. Full responses for either receptor type required intracellular and extracellular Ca(2+) and mobilization of a shared thapsigargin-sensitive intracellular Ca(2+) store. Strategies that affected the ability of D-Ins(1,4,5)P3 to interact with the Ins(1,4,5)P3-receptor demonstrated an Ins(1,4,5)P3-dependency of the muscarinic receptor-mediated elevation of [Ca(2+)]i but showed that PDGF-mediated elevations of [Ca(2+)]i are Ins(1,4,5)P3-independent in these cells.

17 beta-estradiol increases Ca(2+) influx and down regulates interleukin-2 receptor in mouse thymocytes

The influx of Ca(2+) across the T lymphocyte membrane is an essential triggering signal for activation and proliferation by an antigen. The aim of this study was to determine if Ca(2+) influx through estradiol receptor (ER) operated channels of Ca(2+) entry induced activation of lymphoid cells. Mouse thymocytes were incubated with 17 beta-estradiol (E) and in the presence or absence of the mitogen, phytohemagglutinin (PHA). Despite evidence of an enhanced binding of E to ER on thymocyte membranes, and an E dose-related influx of Ca(2+), there was a consistent down regulation of IL-2 receptor expression (P < 0.001). Incubation of thymocytes with PHA enhanced IL-2 receptor expression although the down regulatory effect of E was still evident. The results suggest that the Ca(2+) channel activated by E may have a down regulatory effect on the IL-2 receptor in thymus cells leading to the dampening of cell activation process.

Identification of membrane calcium channels essential for cytoplasmic and nuclear calcium elevations induced by vascular endothelial growth factor in human endothelial cells

Vascular endothelial growth factor (VEGF) is mitogenic for endothelial cells and has been shown to induce angiogenesis and endothelial cell migration through stimulation of endothelial tyrosine-kinase receptors. Here, using confocal microscopy and the patch-clamp technique on endothelial cells, membrane permeability to calcium as well as cytoplasmic and nuclear free calcium levels have been investigated in the first stages of tyrosine-kinase receptor activation by VEGF. VEGF (0.5nM) as well as inositol trisphosphate (IP3) induced an activation of membrane calcium-permeable channels exhibiting a similar low conductance in the range of 10 pS. The VEGF-triggered activation of these calcium channels, mediated by IP3 and involving the intracellular calcium stores, results in an increase in both cytoplasmic and nuclear calcium levels in endothelial cells, potentially modulating gene expression. Finally, the effect of Ni2+, a calcium channel blocker, on endothelial cell proliferation has been studied. The results show that inhibition of extracellular calcium influx significantly inhibits VEGF-induced cell proliferation. In the process of cell stimulation by VEGF, and possibly by other growth factors, activation of calcium channels could then be a key step in calcium-regulated gene expression and cell activation. These results suggest that the use of calcium channel blockers could be a novel way of prevention or reversion of VEGF-induced tumoral angiogenesis.

Rapid Ca2+ influx and diacylglycerol synthesis in growth hormone-mediated islet beta -cell mitogenesis

Growth hormone (GH) is an important mitogenic stimulus for the insulin-producing beta-cell. We investigated the effects of GH on Ca(2+) handling and diacylglycerol (DAG) and cAMP formation in the beta-cell. GH elicited a rapid increase in the cytoplasmic free [Ca(2+)], which required extracellular Ca(2+) and was also blocked by pertussis toxin or protein kinase C (PKC) inhibition. GH also elevated islet DAG content, which should lead to PKC activation. Pertussis toxin and PKC inhibitors obliterated the mitogenicity of GH, suggesting involvement of GTP-binding proteins. PKC activation stimulated beta-cell proliferation, and it also activated phospholipase D. Islet cAMP content was not elevated by GH. Addition of a specific protein kinase A antagonist failed to influence the mitogenicity of GH, whereas a stimulatory cAMP agonist stimulated beta-cell replication. We conclude that GH rapidly increases the beta-cell cytoplasmic free [Ca(2+)] and also evokes a similar increase in DAG content via a phosphatidylcholine-specific phospholipase C, but does not affect mitogen-activated protein kinases, phospholipase D, or the cAMP signaling pathway. This rise in DAG may be of importance in translation of the stimulatory signal of GH into a proliferative response by the beta-cell, which seems to occur through GTP-binding proteins and PKC-dependent mechanisms.

Nonsteroidal anti-inflammatory drugs attenuate proliferation of colonic carcinoma cells by blocking epidermal growth factor-induced Ca++ mobilization

Numerous studies suggest that nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit colorectal carcinogenesis. We have previously reported that NSAIDs, in human colonic carcinoma cells (Caco-2), attenuate epidermal growth factor (EGF)-induced cellular proliferation through a process independent of their inhibitory effects on prostaglandin synthesis. Furthermore, separate studies have also suggested that NSAIDs inhibit EGF-induced store-operated Ca++ influx. Thus we developed the hypothesis that NSAIDs may limit the activity of EGF by altering intracellular Ca++ ([Ca++]i) mobilization. Serum-deprived Caco-2 cells were employed for all experimentation. [Ca++]i was measured with Fluo-3 and extracellular Ca++ influx was monitored by quenching Fluo-3 fluorescence with Mn++. Proliferation was quantitated with two assays: cellular nucleic acid and total protein content. Caco-2 cells exposed to EGF demonstrated an initial increase in [Ca++]i which was blocked by neomycin, an inhibitor of IPsubscript 3 generation, and the phospholipase C inhibitor U73122 but not U73343 (inactive control). This was followed by sustained extracellular Ca++ influx, which was attenuated with calcium-free buffer (-Ca++), the store- operated Ca++ channel blocker lanthanum, indomethacin, ibuprofen, and aspirin. In subsequent studies, cells were treated with either serum-free media or EGF +/- the aforementioned inhibitors, and again serum starved. Cells exposed to EGF +/- the inactive phospholipase C inhibitor U73343 demonstrated a significant increase in nucleic acid and protein. However, proliferation induced by EGF was not observed when [Ca++]i elevation was prevented by blocking either internal Ca++ store release via phospholipase C/IPsubscript 3 or sustained Ca++ influx through store-operated Ca++ channels. Sustained [Ca++]i elevation, as induced by EGF, appears to be required for mitogenesis. These data support our premise that one mechanism whereby NSAIDs may attenuate colonic neoplasia is by blocking EGF-induced Ca++ mobilization.

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.

Sequential activation of different Ca2+ entry pathways upon cholinergic stimulation in mouse pancreatic acinar cells

1. We have studied capacitative calcium entry (CCE) under different experimental conditions in fura-2-loaded mouse pancreatic acinar cells by digital microscopic fluorimetry. CCE was investigated during [Ca2+]i decay after cell stimulation with a supramaximal concentration of ACh (10 microM) or during Ca2+ readmission in Ca2+-depleted cells (pretreated with thapsigargin or ACh). 2. La3+ and Zn2+ (100 microM) inhibited CCE during Ca2+ readmission but had negligible effects during ACh decay. In contrast flufenamic acid (100 microM), an inhibitor of non-selective cation channels, genistein (10 microM), a broad-range tyrosine kinase inhibitor, and piceatannol (10 microM), an inhibitor specific for non-receptor Syk tyrosine kinase, inhibited CCE during ACh decay but not during Ca2+ reintroduction. 3. Simultaneous detection of Mn2+ entry and [Ca2+]i measurement showed that, in the presence of extracellular calcium, application of 100 microM Mn2+ during ACh decay resulted in manganese influx without alteration of calcium influx, whilst when applied during Ca2+ readmission, Mn2+ entry was significantly smaller and induced a clear inhibition of CCE. 4. Application of the specific protein kinase C inhibitor GF109293X (3 microM) reduced CCE in Ca2+-depleted cells, whereas the activator phorbol 12-myristate, 13-acetate (3 microM) increased Ca2+ entry. 5. Based on these results we propose that cholinergic stimulation of mouse pancreatic acinar cells induces Ca2+ influx with an initial phase operated by a non-specific cation channel, sensitive to flufenamic acid and tyrosine kinase inhibitors but insensitive to lanthanum and divalent cations, followed by a moderately Ca2+-selective conductance inhibited by lanthanum and divalent cations.

Cyclic AMP regulates PDGF-stimulated signal transduction and differentiation of an immortalized optic-nerve-derived cell line

To facilitate the study of the molecular events underlying the development of optic-nerve-derived oligodendrocytes and their growth-factor-related signal transduction events, we immortalized perinatal rat optic nerve cells with a temperature-sensitive simian virus 40 large T-antigen, carrying the tsA58 and U19 mutations, via a retrovirus vector. The line, tsU19-9, was selected on the basis of the expression of the neural precursor marker nestin. At the permissive temperature, 33 degreesC, tsU19-9 cells had a flat epithelial morphology. In contrast, following exposure to platelet-derived growth factor (PDGF), a factor important in the lineage progression of oligodendrocytes, or in the presence of dibutyryl cyclic AMP at 39 degreesC (the non-permissive temperature), the cells underwent morphological and antigenic differentiation to cells characteristic of the oligodendrocyte lineage. We used this cell line to investigate the binding characteristics of PDGF and related signalling cascades. Competition binding, phosphoinositide hydrolysis and intracellular Ca2+ mobilization assays all demonstrated that the three different isoforms of PDGF (AA, AB and BB) bound to and acted on the cell line. Overnight exposure to forskolin, a treatment that initiated morphological and phenotypic progression into an oligodendrocyte lineage, decreased PDGF-BB-induced intracellular Ca2+ mobilization and inhibited basal and PDGF-stimulated [3H]thymidine incorporation. Our results demonstrate that tsU19-9 may serve as a resource to study early optic-nerve oligodendrocyte development.

Protection From Apoptosis by Steel Factor But Not Interleukin-3 Is Reversed Through Blockade of Calcium Influx

Steel factor (SLF), the ligand for the c-Kit receptor, protects hemopoietic progenitors and mast cells from apoptosis. We show here that protection of 32D-Kit cells or mast cells from apoptosis by SLF is abrogated through concurrent inhibition of Ca2+influx. In contrast, cell survival promoted by interleukin-3 is not affected by Ca2+ influx blockers. In the presence of blockers, increasing stimulation by SLF leads to greater levels of cell death in the population, indicating that it is the combination of activation by SLF with concurrent blockade of Ca2+ influx that results in apoptosis. The p815 mastocytoma, which expresses a mutated, constitutively active c-kit receptor, dies apoptotically in the presence of Ca2+ influx blockers alone. Ionomycin protects cells from SLF plus blocker-induced apoptosis, confirming specificity for Ca2+ ion blockade in cell death induction. Overexpression of bcl-2, which protects 32D-Kit cells from factor withdrawal, does not protect cells from apoptosis by SLF plus blocker. In contrast, caspase inhibitors YVAD-CHO, DEVD-FMK, and Boc-Asp-FMK protect cells from SLF plus blocker-induced death. These observations highlight the importance of SLF-stimulated Ca2+ influx in the protection of cells from apoptosis and demonstrate a new mechanism for inducing bcl-2 insensitive, caspase-dependent apoptosis through the combination of SLF stimulation with Ca2+ influx blockade.

Protection From Apoptosis by Steel Factor But Not Interleukin-3 Is Reversed Through Blockade of Calcium Influx

Steel factor (SLF), the ligand for the c-Kit receptor, protects hemopoietic progenitors and mast cells from apoptosis. We show here that protection of 32D-Kit cells or mast cells from apoptosis by SLF is abrogated through concurrent inhibition of Ca2+influx. In contrast, cell survival promoted by interleukin-3 is not affected by Ca2+ influx blockers. In the presence of blockers, increasing stimulation by SLF leads to greater levels of cell death in the population, indicating that it is the combination of activation by SLF with concurrent blockade of Ca2+ influx that results in apoptosis. The p815 mastocytoma, which expresses a mutated, constitutively active c-kit receptor, dies apoptotically in the presence of Ca2+ influx blockers alone. Ionomycin protects cells from SLF plus blocker-induced apoptosis, confirming specificity for Ca2+ ion blockade in cell death induction. Overexpression of bcl-2, which protects 32D-Kit cells from factor withdrawal, does not protect cells from apoptosis by SLF plus blocker. In contrast, caspase inhibitors YVAD-CHO, DEVD-FMK, and Boc-Asp-FMK protect cells from SLF plus blocker-induced death. These observations highlight the importance of SLF-stimulated Ca2+ influx in the protection of cells from apoptosis and demonstrate a new mechanism for inducing bcl-2 insensitive, caspase-dependent apoptosis through the combination of SLF stimulation with Ca2+ influx blockade.

Non-capacitative calcium entry in Chinese hamster ovary cells expressing the platelet-derived growth factor receptor

Platelet-derived growth factor (PDGF) is believed to produce intracellular calcium (Ca2+i) transients by inositol trisphosphate (InsP3)-mediated release of intracellular Ca2+ stores followed by "capacitative" Ca2+ entry due to emptying of these stores. We examined the roles for the phospholipase Cgamma-InsP3 pathway and the emptying of InsP3-dependent intracellular Ca2+ stores in PDGF-mediated Ca2+ entry. Intracellular Ca2+ release and Ca2+ entry were measured with fluorometric methods in Chinese hamster ovary cells expressing wild type or mutant PDGF receptors. Activation of the wild type PDGF receptor caused both intracellular "Ca2+ release, " measured in nominally 0 Ca2+ extracellular medium, and "Ca2+ entry, " measured upon addition of 2 mM Ca2+ medium. Both phases were absent in Chinese hamster ovary cells expressing a PDGF receptor mutant (Y977F,Y989F) that fails to bind phospholipase Cgamma. Blockade of the InsP3 receptor, by microinjection of single cells with low molecular weight heparin (5-50 mg/ml), blocked only Ca2+i release (following PDGF or flash photolysis of caged InsP3) and had no effect on PDGF-induced Ca2+ entry. In whole cell patch-clamp experiments, intracellular heparin also failed to block PDGF-evoked ion currents. Release of InsP3-dependent intracellular Ca2+ stores, by flash photolysis of caged InsP3, was apparently not sufficient to maximally activate Ca2+ entry. Intracellular InsP3 caused significantly less Ca2+ entry than PDGF alone. These data suggest that InsP3 alone is not sufficient to maximally activate Ca2+ entry by the capacitative pathway and that products of phosphatidylinositol 4,5-bisphosphate breakdown other than InsP3 probably play a role in PDGF-mediated Ca2+ entry.

PDGF-stimulated calcium influx changes during in vitro cell transformation

Platelet Derived Growth Factor (PDGF)-stimulated intracellular calcium signals were obtained from preneoplastic clones derived from C3H 10T1/2 mouse fibroblasts during a four-week transformation assay. By the end of the assay, when transformed colonies of cells were becoming apparent, the PDGF-stimulated calcium signal had reduced significantly. In addition, whole-cell patch-clamp electrophysiology measurements indicated a marked reduction in expression of T-type calcium channels. These observations demonstrate that PDGF-induced calcium metabolism changes as initiated cells progress to the transformed phenotype.

In vitro evaluation of antiproliferative potential of calcium channel blockers in human Tenon's fibroblasts

The calcium cation has been known to play a central and diverse role in the proliferation of numerous types of cells including fibroblasts. Therefore, ib1p4llows that calcium channel blockers (CCBs) may potentially inhibit fibroblast attachment and proliferation. The time and dose-related effects of five commonly used CCBs-verapamil, diltiazem, nicardipine, trifluoperazine, and dantrolene-on human Tenon's fibroblast attachment and proliferation were studied. Fibroblasts were incubated with different concentrations of each drug. To evaluate the effect of each drug on fibroblast attachment, cell density was quantified by Coulter counter and hexosaminidase assays after 24 hours of incubation. To evaluate the effect of each drug on fibroblast proliferation, cell density was quantified by Coulter counter, hexosaminidase, and 3H-thymidine uptake assays on days 1, 3, and 7. Dantrolene had minimal effect, failing to cause even 20% inhibition at 10(-5) M, the highest concentration tested. Verapamil, diltiazem, nicardipine, and trifluoperazine all inhibited fibroblast attachment and proliferation. Trifluoperazine was the most potent inhibitor of attachment and proliferation with ID50S in the 10(-5) M range. Verapamil, diltiazem, and nicardipine had similar potency in inhibiting attachment and proliferation with ID50S in the 10(-4) M range. Because CCBs seem to inhibit fibroblast attachment and proliferation, future clinical studies may show that these agents reduce collagen production, scar formation, and bleb failure following glaucoma filtration surgery.

Calcium is permeable through a maitotoxin-activated nonselective cation channel in mouse L cells

The shellfish poison maitotoxin causes the irreversible opening of nonselective cation channels in mouse L cell fibroblasts, consistent with the action of this toxin in other cell types and the previously demonstrated existence of 28-pS voltage-insensitive nonselected cation channels that are activated by platelet-derived growth factor in these cells. Toxin-induced opening of these nonselective cation channels led to increases of intracellular calcium and secondary activation of calcium-activated potassium channel. These effects were completely dependent on influx of extracellular calcium, supporting the conclusion that the maitotoxin-activated nonselective cation channels are permeable to calcium as well as to sodium and potassium. The implication of this finding is that calcium signaling through this channel underlies its links into the growth factor response.

Investigations on the mechanism of action of the antiproliferant and ion channel antagonist flufenamic acid

The compound flufenamic acid has been previously described as an inhibitor of chloride- and non-selective cation channels. Moreover, this compound showed antiproliferative effects in the mouse fibroblast cell line LM(TK-). In this study, we investigated the effects of this compound on cell proliferation and membrane currents induced by mitogens (such as fetal calf serum, FCS) or platelet-derived growth factor (PDGF) in LM(TK-) cells. After a brief application of FCS or PDGF (5-15 s), the electrical response of the cells was biphasic: First, a transient potassium conductance was activated, which appeared 8.3 +/- 0.7 s after the onset of stimulation and lasted for 30.1 +/- 2.9 s. The corresponding single channel currents in cell-attached patches had an amplitude of 3-4 pA (at a holding potential of +60 mV). The second effect of serum or PDGF was the occurrence of a cation conductance for monovalent ions (sodium, potassium and cesium) and calcium. In contrast to the potassium current, this conductance activated later (11.8 +/- 1.6 s after onset of fetal calf serum stimulation) and remained activated for minutes. Flufenamic acid inhibited the proliferation of LM(TK-) cells reversibly and in a concentration-dependent manner. This effect can be correlated with the inhibitory effects of flufenamic acid on mitogen-induced membrane currents: The compound inhibited the non-selective cation current with an IC50 of 38 microM, whereas 135 microM were necessary for halfmaximal inhibition of the potassium current; this is very close to the concentration for halfmaximal inhibition of cell proliferation (120 microM). Hence, on the grounds of this comparison the blockade of the non-selective cation current appears to be of only minor importance for the blockade of cell proliferation.

Role of mitogen-induced calcium influx in the control of the cell cycle in Balb-c 3T3 fibroblasts

The role of mitogen-activated calcium influx from the extracellular medium in the control of cell proliferation was studied in Balb-c 3T3 fibroblasts. Stimulation of serum-deprived, quiescent cells with 10% foetal calf serum (FCS) induced a long-lasting (up to 70 min) elevation of intracellular free calcium concentration ([Ca2+]i). Both the sustained [Ca2+]i increase and the related inward current, described in a previous paper [Lovisolo D. Munaron L. Baccino FM. Bonelli G. (1992) Potassium and calcium currents activated by foetal calf serum in Balb-c 3T3 fibroblasts. Biochim. Biophys. Acta, 1104, 73-82], could be abolished either by chelation of extracellular calcium with EGTA or by SK&F 96365, an imidazole derivative that can block receptor-activated calcium channels. The effect of the abolition of these ionic signals on FCS-induced proliferation was investigated by adding either EGTA or SK&F 96365 to the culture medium during the first hours of stimulation of quiescent cells with 10% FCS. As measured after 24 h, a 22% inhibition of growth was observed when SK&F 96365 was added for the first hour, and stronger inhibitions, up to 56%, were obtained by adding the blocker for the first 2 or 4 h. Similar effects were observed with addition of 3 mM EGTA, though the inhibition was less marked for the 4 h treatment. By contrast, incubation with either substance in the next 4 h of serum stimulation did not influence cell growth, except for a slight inhibition observed when SK&F 96365 was applied from the 4th to the 8th hour. The reduction in growth resulting from the abolition of the early calcium influx was paralleled by an accumulation of cells in the G2/M phase. Both growth inhibition and G2/M accumulation were reversible, since after further 24 h in 10% FCS cells had fully recovered the exponential growth. These data indicate that the early calcium influx seen in response to mitogen stimulation develops on a timescale long enough to play a significant role in cell cycle progression, and that its block in the early G1 phase can lead to a reduction of proliferation by arresting cells in later stages of the cycle.

The mitogenic effect of platelet-derived growth factor in human hepatic stellate cells requires calcium influx

Platelet-derived growth factor (PDGF) is a key mitogen for hepatic stellate cells (HSC) and has been shown to be implicated in liver tissue repair and fibrogenesis. In this study the relationship between PDGF-induced intracellular Ca2+ concentration ([Ca2+]i) increase and mitogenesis in cultured human HSC was evaluated. In high-density cell cultures (80-90% subconfluence), PDGF induced a significant increase in [Ca2+]i, characterized by a short-lasting peak phase, which was followed by a long-lasting plateau phase. The plateau phase was abolished in the absence of extracellular Ca2+. However, in low-density cell cultures (30-40% subconfluence), the plateau phase was absent or markedly less pronounced. In parallel sets of experiments, PDGF was significantly less effective in inducing mitogenesis in low-density cell cultures than in high-density cell cultures and was totally ineffective in the absence of extracellular Ca2+. These results suggest that 1) spatial and time dynamics of PDGF-induced [Ca2+]i increase are dependent on cell density and 2) PDGF-induced mitogenesis requires extracellular Ca2+ influx.

Basic fibroblast growth factor activates calcium channels in neonatal rat cardiomyocytes

Basic fibroblast growth factor (bFGF) is a potent mitogen for many cell lineages including fetal cardiomyocytes. Furthermore, bFGF has been shown to modify gene expression, in vitro, in adult nonproliferative ventricular myocytes. This effect is suspected to be partly responsible for the genetic modifications that occur in vivo under pathophysiological conditions such as ischemia or pressure overload and that lead to myocardial hypertrophy. However, little is known about the first steps of the molecular mechanisms that take place soon after cell activation by bFGF. In this study, using biochemical and electrophysiological approaches, we have established, on cardiomyocytes cultured from neonatal rat ventricles, that (i) differentiated beating cells express at least two classes of bFGF-receptors having high and low affinity (Kd = 10 +/- 2 pM and 1 +/- 0.5 nM); (ii) the stimulation of these bFGF receptors promotes an increase in the beating frequencies of cultured cardiomyocytes (40 +/- 10%); (iii) bFGF provokes the activation of poorly specific and voltage-independent calcium channels (12pS); (iv) inositol 1,4,5-trisphosphate enhances similar bFGF-induced Ca2+ currents and is therefore suspected to be a second messenger triggering this activation. These results support the presence, in cultured cardiomyocytes, of new calcium channels whose activation after bFGF binding may be partly responsible for the cell response to this growth factor.

Calcium signalling and cell proliferation

The orderly sequence of events that constitutes the cell cycle is carefully regulated. A part of this regulation depends upon the ubiquitous calcium signalling system. Many growth factors utilize the messenger inositol trisphosphate (InsP3) to set up prolonged calcium signals, often organized in an oscillatory pattern. These repetitive calcium spikes require both the entry of external calcium and its release from internal stores. One function of this calcium signal is to activate the immediate early genes responsible for inducing resting cells (G0) to re-enter the cell cycle. It may also promote the initiation of DNA synthesis at the G1/S transition. Finally, calcium contributes to the completion of the cell cycle by stimulating events at mitosis. The role of calcium in cell proliferation is highlighted by the increasing number of anticancer therapies and immunosuppressant drugs directed towards this calcium signalling pathway.

Sustained calcium influx activated by basic fibroblast growth factor in Balb-c 3T3 fibroblasts

1. We have investigated the ionic events elicited in Balb-c 3T3 fibroblasts by basic fibroblast growth factor (bFGF), a peptide that binds to membrane receptors with tyrosine kinase activity and has a mitogenic action on many cell types. The peptide (0.2-100 ng ml-1) caused the appearance of an inward current, as observed in whole-cell patch-clamp experiments at a holding potential of -50 mV, that could last for tens of minutes and had a peak density of 4.6 +/- 2.6 pA pF-1. The reversal potential was 18.8 +/- 16.7 mV. 2. The current was reversibly abolished by removal of bFGF from the external bath. Inhibition of low-affinity FGF receptors had no effect on the activation of the inward current; it was completely abolished when cells were pre-incubated with tyrphostin or 5'-methylthioadenosine (MTA), two inhibitors of the tyrosine kinase activity of the high-affinity FGF receptors. The inward current was not activated by the emptying of internal calcium stores, as tested with 200 nM thapsigargin. 3. Values of peak current density comparable to control ones were obtained when either all Na+ ions or all Ca2+ ions were removed from the external solution; when both ions were completely removed, no inward current could be observed. The inward current was not affected by 2 microM nifedipine, and was reversibly blocked by the imidazole derivative SK&F 96365-A. 4. Measurements of free intracellular calcium concentration ([Ca2+]i) with the dye fura-2 showed that bFGF elicited sustained increases in [Ca2+]i that were completely dependent on external calcium and on the presence of the agonist and could last more than 1 h. 5. Single channel currents (conductance 7.9 pS) in response to bFGF stimulation could be recorded in the cell-attached configuration with 100 mM CaCl2 in the pipette. When the resting potential was brought near to 0 mV by external perfusion in a high-K+ solution, Vrev was about 0 mV. 6. We conclude that in Balb-c 3T3 cells bFGF induces an inward current that is carried at least partially by Ca2+ ions; this current in turn causes a long-lasting increase in intracellular Ca2+ concentration. The amplitude and time course of these bFGF-activated ionic events are compatible with their involvement in the control of cell proliferation.

Ca2+ influx via T-type channels modulates PDGF-induced replication of mouse fibroblasts

The role of low-threshold voltage-gated calcium channels (VGCC) in modulating extracellular calcium influx and proliferation was investigated in platelet-derived growth factor (PDGF)-stimulated C3H/10T1/2 mouse fibroblasts. Previous studies demonstrated that cell cycle progression after PDGF stimulation was dependent on extracellular calcium influx producing a sustained increase in the intracellular calcium concentration. In this study, PDGF-induced calcium influx, the sustained intracellular calcium increase, and progression to S phase were inhibited by nordihydroguariaretic acid (NDGA), an inhibitor of calcium influx through VGCC. With the use of the whole cell patch-clamp technique to measure calcium currents, NDGA inhibited inward calcium current through low-threshold VGCC, the only VGCC expressed in C3H/10T1/2 fibroblasts. The inhibitory effects of NDGA on calcium influx and cell proliferation each had a mean inhibitory dose of 2-3 microM. Although NDGA also effectively inhibits cyclooxygenase and lipoxygenase, the addition of prostaglandins or leukotrienes could not reverse this inhibition nor could it be replicated by other antioxidants. These data support the hypothesis that low-threshold VGCC can mediate extracellular calcium influx on the stimulation of cell proliferation by PDGF.