Interrelationships of platelet-derived growth factor isoform-induced changes in c-fos expression, intracellular free calcium, and mitogenesis

Heptachlor epoxide induces a non-capacitative type of Ca2+ entry and immediate early gene expression in mouse hepatoma cells

The effects of the organochlorine (OC) liver tumor promoter heptachlor epoxide (HE) and a related non-tumor promoting OC, delta-hexachlorocyclohexane (delta-HCH), on the dynamics of intracellular calcium (Ca2+) were investigated in mouse 1c1c7 hepatoma cells. HE induced a non-capacitative, Ca2+ entry-like phenomenon, which was transient and concentration-dependent with 10 and 50 microM HE. The plasma membrane Ca2+ channel blocker SKF-96365 antagonized this HE-induced Ca2+ entry. delta-HCH failed to induce Ca2+ entry, rather it antagonized the HE-induced Ca2+ entry. Both HE and delta-HCH induced Ca2+ release from endoplasmic reticulum (ER) at treatment concentrations as low as 10 microM; at 50 microM, the former induced 5x as much Ca2+ release as the latter. The HE-induced Ca2+ release from the ER was antagonized using the IP3 receptor/channel blocker xestospongin C, suggesting that HE induces ER Ca2+ release through the IP3 receptor/channel pore. These results show that the effect of HE on cellular Ca2+ mimics that of mitogens such as epidermal and hepatocyte growth factors. They also provide insight into the similarities and differences between tumorigenic and non-tumorigenic OCs, in terms of the mechanisms and the extent of the [Ca2+]i increased by these agents.

Calcium and growth responses of hyperresponsive airway smooth muscle to different isoforms of platelet-derived growth factor (PDGF)

Airway smooth muscle (ASM) mass is likely to be an important determinant of airway responsiveness. Highly inbred Fisher rats model innate hyperresponsiveness, and also have more ASM in vivo than control Lewis rats. Platelet derived growth factor (PDGF) is an important endogenous growth factor for ASM, and partially purified PDGF-AB causes enhanced growth of Fisher rat ASM cells, compared to Lewis cells. The aim of the present study was to determine the mitogenic effects of all three recombinant PDGF isoforms on ASM cells, and investigate the mechanisms of enhanced Fisher ASM growth responses. The potential mechanisms assessed include PDGF receptor expression and activation (tyrosine phoshorylation), and intracellular calcium (Ca2+) responses to PDGF isoforms. Fisher ASM cells had a greater mitogenic response to PDGF-AB and -AA, and a greater Ca2+ response to -BB than Lewis ASM cells. A Ca2+ response was not necessary for a mitogenic response, and the effects of PDGF isoforms on Ca2+ were not associated with their effects on growth. Therefore, we suggest that enhanced Fisher mitogenic response to PDGF-AA and -AB is not mediated by differences in Ca2+ signalling. Western analysis of the PDGF receptor indicated a similar expression of β-PDGF receptor in ASM cells from the two rat strains, but a greater expression of α-PDGF receptor in Fisher cells; however, phosphorylation of the PDGF receptor following growth stimulation did not differ between strains. This suggests a role for post-receptor signals, in addition to enhanced receptor expression, in the enhanced growth response of Fisher ASM cells to PDGF-AA and -AB.Key words: PDGF receptors, tyrosine phosphorylation, intracellular calcium, proliferation, airway smooth muscle cells.

Calcium channel antagonist effect on in vitro meningioma signal transduction pathways after growth factor stimulation

OBJECTIVE

We have previously demonstrated that calcium channel antagonists inhibit the growth of human meningiomas in culture after stimulation with growth factors. This study examined the effects of these drugs on signaling transduction pathways in an attempt to elucidate potential mechanisms by which this growth inhibition is mediated.

METHODS

Primary cell cultures from patients with intracranial meningiomas were established. Cell growth studies were performed with inhibitors and stimulators of tyrosine kinase signal transduction. Intracellular calcium changes and inositol phosphate production were measured after growth factor exposure, with or without pretreatment by calcium channel antagonists.

RESULTS

The growth of meningiomas in culture can be inhibited by tyrosine kinase receptor inhibitors. Inhibitors and stimulators of phospholipase C can stimulate or inhibit the growth of in vitro meningiomas, respectively. Calcium channel antagonists inhibit intracellular calcium changes induced by serum and epidermal growth factor. Inositol phosphate production is increased after growth factor stimulation, and calcium channel antagonists potentiate this effect.

CONCLUSION

Calcium channel antagonists interfere with intracellular signaling pathways of cultured meningioma cells. This inhibition is unrelated to voltage-sensitive calcium channels. The findings of this project may aid in the understanding of the signal transduction mechanisms involved in growth factor-mediated meningioma proliferation and may lead to clinically relevant strategies for growth inhibition.

Endothelin-1 and angiotensin II act as progression but not competence growth factors in vascular smooth muscle cells

The direct effects of endothelin-1 and angiotensin II on cell cycle progression were investigated in rat aorta smooth muscle cells in primary culture. The phase of the cell cycle was determined by an immunocytochemical analysis of cell cycle-specific nuclear antigens. The primary cultured cells were synchronized in the G0 phase (100%) by serum deprivation for 24 h. Endothelin-1 (0.1 microM) or angiotensin II (1 microM) had no effect on the cell cycle of G0 cells, whereas platelet-derived growth factor (PDGF) stimulated the entry of the G0 cells into the G1 phase (100%) without a further progression to the S and M phases. Endothelin-1 or angiotensin II stimulated the progression of the PDGF-pretreated G1 cells to the S and M phases. Fura-2 microfluorometry revealed that, between the G0 and G1 cells, there were no differences in the extent and time course of cytosolic Ca2+ elevations induced by endothelin-1 or angiotensin II, which suggested that endothelin-1 and angiotensin II receptors and their signaling pathways regulating cytosolic Ca2+ remained intact in these cell phases. We thus conclude that endothelin-1 and angiotensin II require the prior G0/G1 transition induced by a competence growth factor such as PDGF to exert their mitogenic effects. These results suggest the important role of endothelin-1 and angiotensin II in atherosclerosis as promoters (progression growth factors), but not as initiators.

Inhibition of in vitro meningioma proliferation after growth factor stimulation by calcium channel antagonists: Part II--Additional growth factors, growth factor receptor immunohistochemistry, and intracellular calcium measurements

We have previously reported that calcium channel antagonists can block both the growth of meningiomas in culture and the potent growth stimulation of meningioma cells by epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). This study further defines the nature of this growth inhibition. Primary meningioma cultures were established, and cells were characterized. Fibroblast growth factor or insulin-like growth factor-I growth stimulation in the presence of calcium channel antagonists was examined. In addition, the effects of ethylene glycol-bis-(aminoethylether) N,N,N',N"-tetraacetic acid and Bay K 8644, a calcium channel agonist, on the growth factors were analyzed. Growth factor receptor immunohistochemistry was performed on the original tumors and the in vitro meningioma cells. Twelve of 17 (71%) meningiomas in this study were positive for the EGF receptor, and 14 of 17 (82%) were positive for the PDGF receptor. Five of six (83%) of the culture cells were positive for the EGF receptor, and four of five (80%) were positive for the PDGF receptor. Intracellular calcium changes were quantified using the intracellular calcium-chelating, fluorescent dye, Fura-2. The growth stimulation of fibroblast growth factor and insulin-like growth factor-I on meningioma cells in culture was decreased in a dose-dependent manner by calcium channel antagonists. The growth stimulation of fibroblast growth factor and insulin-like growth factor-I was not affected by a reduction of extracellular calcium, whereas the growth stimulation of EGF and PDGF was. Interestingly, intracellular calcium was not increased after exposure to growth factors but was increased after serum stimulation. This increase could be blocked by preincubation with verapamil. Calcium channel antagonists can inhibit proliferation of meningioma cells in culture after stimulation with a number of growth factors. These drugs might disrupt intracellular calcium homeostasis or interfere with key elements of the growth factor signal transduction pathways. These mechanisms as well as the potential clinical relevance of these findings are discussed.

Growth factor-induced phosphorylation and activation of aortic smooth muscle Na+/Ca2+ exchanger

Although the Na+/Ca2+ exchanger is one of the major Ca2+ extrusion systems in excitable tissues, little is known about its regulation via protein phosphorylation. We now present evidence that the Na+/Ca2+ exchanger is phosphorylated in quiescent and growth factor-stimulated cultured aortic smooth muscle cells. The Na+/Ca2+ exchanger was isolated from 32P-labeled cells by immunoprecipitation with a specific polyclonal antibody. Phosphorylation of the exchanger was increased by up to 1.7-fold in response to platelet-derived growth factor-BB (PDGF-BB), alpha-thrombin, or phorbol 12-myristate 13-acetate (PMA). However, angiotensin II did not enhance the phosphorylation significantly. The extent of phosphorylation appeared to correlate with the growth factor-induced increase in cell 1,2-diacylglycerol. At least four phosphopeptides (P1 to P4) were detected by tryptic phosphopeptide map analysis of the phosphorylated exchanger, suggesting that phosphorylation occurred at multiple sites. PDGF-BB and PMA increased phosphorylation of the same phosphopeptides (in particular P1). Phosphorylated amino acids were exclusively serine residues in both quiescent and stimulated cells. We found that growth factors enhanced Na+/Ca2+ exchange activity and that there was a good correlation between the growth factor-induced stimulations of phosphorylation and exchange activity. PDGF-BB-induced activation of the exchanger was abolished by prior long treatment of cells with PMA. These results suggest that the Na+/Ca2+ exchanger is activated by protein kinase C-dependent phosphorylation in response to growth factors in vascular smooth muscle cells.

In vitro growth inhibition of growth factor-stimulated meningioma cells by calcium channel antagonists

Studies have shown that a majority of meningiomas contain receptors for platelet-derived growth factor and epidermal growth factor and that these growth factors promote the proliferation of meningioma cells in culture. Although the mechanism of action has not been elucidated, intracellular calcium appears to be part of the signal transduction mechanism. Because alterations in intracellular calcium could interrupt this pathway and decrease cellular proliferation, we investigated the effects of calcium channel-blocking agents on the growth of meningioma cells in vitro. Primary meningioma cell cultures were established, and the cells were characterized by light and electron microscopy and by immunohistochemical studies. Then, the cultures were given growth factors and/or various calcium channel antagonists, and growth rates were measured. A dose-response decrease in cell growth was seen when verapamil, nifedipine, or diltiazem (voltage-dependent calcium channel-blocking agents) was added to serum-containing media. Also, these drugs blocked the growth stimulation of epidermal growth factor and platelet-derived growth factor in a similar fashion. Dantrolene, which inhibits the release of sequestered intracellular calcium, was also an effective blocker of the mitogenic stimulation of these growth factors.

Platelet-derived growth factor (PDGF) receptors in the developing mouse optic pathway

The molecules which control the patterns of cell division, growth, and precise interconnections characteristic of the central nervous system still remain largely unidentified. The protein platelet-derived growth factor (PDGF) has been shown to mediate interactions among glial cells in vitro. More recent evidence has indicated that PDGF may also be involved in controlling communication between neurons and glial cells and among neurons. The presence of receptors for PDGF on neurons of the developing nervous system is an essential piece of evidence in this chain of events. Ganglion cells are labeled with antibodies to PDGF receptor only during the period of active process outgrowth. These findings suggest that PDGF is used as a mediator of intercellular signaling during neuronal development.

Expression of voltage-gated calcium channels correlates with PDGF-stimulated calcium influx and depends upon cell density in C3H 10T1/2 mouse fibroblasts

Platelet derived growth factor (PDGF) mobilizes multiple calcium pools in the C3H 10T1/2 mouse fibroblast, including a sustained influx of extracellular calcium. We have used the whole cell patch-clamp technique to directly test for a role of plasma membrane calcium channels in this influx. Whole-cell patch-clamp recordings revealed a voltage-gated calcium channel with gating properties consistent with a 'T-type' designation. This phenotype of the C3H 10T1/2 fibroblasts was dependent upon the cell density in culture. The fraction of cells expressing calcium channels was low (< 10%) in subconfluent culture but rose to approximately 50% as the cells established a confluent monolayer. The magnitude of the PDGF-stimulated sustained calcium influx component measured using Fura-2 increased in parallel with the expression of calcium current. We interpret these results to support the hypothesis that T-type voltage-gated calcium channels contribute to the PDGF-stimulated intracellular calcium signals in these cells.

PDGF-AA effectively stimulates early events but has no mitogenic activity in AKR-2B mouse fibroblasts

The response of AKR-2B mouse fibroblasts, which express approximately equal numbers of platelet-derived growth factor (PDGF)-alpha and -beta receptors on their surface (V. Hoppe et al. Eur. J. Biochem. 187, 207-214, 1990) to all three isoforms of PDGF, was studied. All isoforms stimulated early events, i.e., receptor autophosphorylation on tyrosine, total cellular phosphorylation, increase in 32P-labeled phospholipid content, but there was no correlation between the extents measured for the different effects. Although rPDGF-AA effectively stimulated these early events, it was unable to induce [3H]thymidine incorporation and cell growth whereas rPDGF-BB and -AB stimulated the division of more than 90% of the cells. This activity was restored by addition of insulin-like growth factor I (IGF-I), which itself exhibited only a low mitogenic activity. rPDGF-AB or -BB did not require the presence of IGF-I to fully stimulate cells for [3H]thymidine incorporation and cell division. Apparently, rPDGF-AA induced only a "competence" state of the cells whereas rPDGF-AB or -BB was also able to initiate "progression". It is speculated that some early events occurring during the competence phase might be part of a "maintenance" program elicited by growth factors.

Differential induction of the c-fos promoter through distinct PDGF receptor-mediated signaling pathways

The multiple isoforms of PDGF induce fibroblastic mitogenesis through two distinct PDGF receptors, alpha and beta. The molecular mechanisms by which these alpha and beta PDGF receptors regulate gene expression are poorly understood. We present data which indicates that differential induction of c-fos gene expression by PDGF isoforms occurs through distinct PDGF alpha and beta receptor-mediated signaling pathways. Comparison of PDGF-AA with PDGF-BB stimulation showed that PDGF-BB induced prolonged expression of the c-fos gene in BALB/c-3T3 cells, but that PDGF-AA induced more potent activation of the serum response element (SRE) in transient transfection assays. PDGF-AA, which binds alpha but not beta PDGF receptors, could only induce the SRE through a protein kinase C (PKC)-dependent pathway, whereas PDGF-BB, which binds both alpha and beta PDGF receptors, could also induce the SRE through a PKC-independent pathway. These results suggest that PDGF alpha receptors activate the PKC-dependent signaling pathway while PDGF beta receptors also activate a PKC-independent pathway. In addition, we found that PDGF-BB could induce another c-fos promoter element within the -90 to +10 region, suggesting that the more potent mitogenic effect and prolonged c-fos gene expression induced by PDGF-BB may result from cooperativity between more than one c-fos promoter elements.