Alessandra Fiorio Pla

Canonical Transient Receptor Potential 1 Plays a Role in Basic Fibroblast Growth Factor (bFGF)/FGF Receptor-1-Induced Ca2+ Entry and Embryonic Rat Neural Stem Cell Proliferation

Basic fibroblast growth factor (bFGF) and its major receptor FGF receptor-1 (FGFR-1) play an important role in the development of the cortex. The mechanisms underlying the mitogenic role of bFGF/FGFR-1 signaling have not been elucidated. Intracellular Ca2+ concentrations ([Ca2+]i) in proliferating cortical neuroepithelial cells are markedly dependent on Ca2+ entry (Maric et al., 2000a). The absence of voltage-dependent Ca2+ entry channels, which emerge later, indicates that other membrane mechanisms regulate [Ca2+]i during proliferation. Canonical transient receptor potential (TRPC) family channels are candidates because they are voltage independent and are expressed during CNS development (Strübing et al., 2003). Here, we investigated the involvement of TRPC1 in bFGF-mediated Ca2+ entry and proliferation of embryonic rat neural stem cells (NSCs). Both TRPC1 and FGFR-1 are expressed in the embryonic rat telencephalon and coimmunoprecipitate. Quantitative fluorescence-activated cell sorting analyses of phenotyped telencephalic dissociates show that ∼80% of NSCs are TRPC1+, proliferating, and express FGFR-1. Like NSCs profiled ex vivo, NSC-derived progeny proliferating in vitro coexpress TRPC1 and FGFR1. Antisense knock-down of TRPC1 significantly decreases bFGF-mediated proliferation of NSC progeny, reduces the Ca2+ entry component of the Cai2+ response to bFGF without affecting Ca2+ release from intracellular stores or 1-oleoyl-2-acetyl-sn-glycerol-induced Ca2+ entry, and significantly blocks an inward cation current evoked by bFGF in proliferating NSCs. Both Ca2+ influx evoked by bFGF and NSC proliferation are attenuated by Gd3+ and SKF96365, two antagonists of agonist-stimulated Ca2+ entry. Together, these results show that TRPC1 contributes to bFGF/FGFR-1-induced Ca2+ influx, which is involved in self-renewal of embryonic rat NSCs.

Self-Renewing and Differentiating Properties of Cortical Neural Stem Cells Are Selectively Regulated by Basic Fibroblast Growth Factor (FGF) Signaling via Specific FGF Receptors

Developmental processes mediating the initiation of lineage commitment from self-renewing neural stem cells (NSCs) remain mostly unclear because of the persisting ambiguity in identifying true NSCs from proliferative lineage-restricted progenitors (LRPs), which are directly or indirectly derived from NSCs. Our multilineage immunohistochemical analyses of early embryonic rat telencephalon at the onset of neurogenesis revealed clear dorsoventral gradients in the emergence of two types of neuronal progenitors (NPs) from multilineage-negative NSCs. Enumeration of NSCs using comprehensive flow cytometric analysis demonstrated that their precipitous decline in vivo involved both active differentiation into NPs and an increased propensity toward apoptosis. Both processes paralleled the dorsoventral changes in fibroblast growth factor receptor (FGFR) expressions. NSCs residing in the dorsal telencephalon coexpressed FGFR1 and FGFR3, whereas those residing in the ventral telencephalon also expressed FGFR2. NSCs exposed to basic fibroblast growth factor (bFGF) in vitro generated four stereotypical clonal expansion states: efficiently self-renewing, inefficiently self-renewing limited by apoptosis, exclusively neurogenic, and multipotential, generating up to five types of LRPs. The plasticity among these expansion states depended on ambient [bFGF], telencephalic developmental stage, and differential activation/inactivation of specific FGFRs. Coactivation of FGFR1 and FGFR3 promoted symmetrical divisions of NSCs (self-renewal), whereas inactivation of either triggered asymmetrical divisions and neurogenesis from these cells. Developmental upregulation of FGFR2 expression correlated with a shift of NSCs into a multipotential state or apoptosis. These results provide new insights regarding the roles of FGFRs in diversification of NSC properties and initiation of neural lineage-restricted differentiation.

Blocking Ca2+ Entry: A Way to Control Cell Proliferation

Ca(2+) signalling is involved in virtually all cellular processes: among the others, it controls cell survival, proliferation and death regulating a plethora of intracellular enzymes located in the cytoplasm, nucleus and organelles. Changes in the cytosolic free Ca(2+) concentration may be due either to release from the intracellular Ca(2+) stores or to influx from the extracellular medium, through the opening of plasma membrane calcium-permeable channels. In particular, Ca(2+) entry from the extracellular space is a mechanism able to sustain long lasting intracellular Ca(2+) elevations: this signal, activated by many growth factors and mitogens in normal and tumoral tissues, is linked to DNA transcription and duplication, finally leading to cell proliferation. In the last years many informations have been provided about the transduction mechanisms related to Ca(2+) entry induced by mitogenic factors, mostly binding to tyrosine kinase receptors, but also to G-protein coupled ones. Nevertheless, some key points remain to be fully clarified: among them, the molecular structure of the Ca(2+) channels involved, their regulation by intracellular messengers, and the modes through which specificity is achieved. The increasing knowledge on Ca(2+) entry-dependent control of proliferation may provide a more satisfactory understanding of pathological alterations, including cancer progression and angiogenesis. A detailed description of the mechanisms that trigger Ca(2+) entry, and in particular the definition of calcium-permeable channels and their modulators at the molecular levels, will greatly improve our possibility to take advantage of Ca(2+) entry regulation as a therapeutic approach for the control of cell proliferation, designing antibodies or molecules with low side effects and specific channel blocker functions. The review will focus on this topic.

Control of Endothelial Cell Proliferation by Calcium Influx and Arachidonic Acid Metabolism: A Pharmacological Approach

In physiological conditions, endothelial cell proliferation is strictly controlled by several growth factors, among which bFGF and VEGF are the most effective. Both bind to specific tyrosine kinase receptors and trigger intracellular signal cascades. In particular, bFGF stimulates the release of arachidonic acid (AA) and its metabolites in many types of endothelial cells in culture. In bovine aortic endothelial cells, it has been suggested that AA is released by the recruitment of cytosolic phospholipase A2 (cPLA2). AA metabolites are involved in the control of both endothelial cell motility (mostly via the cyclooxygenase pathway) and proliferation (via the lipoxygenase (LOX) cascade). On the other hand, evidence has been provided for a proliferative role of AA‐induced calcium influx. By using a pharmacological approach, we have tried to elucidate the contribution to bovine aortic endothelial proliferation of the different pathways leading to production of AA and its metabolites. Two main informations were obtained by our experiments: first, AA release is not entirely due to cPLA2 involvement, but also to DAG lipase recruitment; second, cyclooxygenase derivatives play a role in the control of cell proliferation, and not only of motility. Moreover, by combining proliferation assays and single cell calcium measurements, we show that the blocking effect of carboxyamido‐triazole (CAI), an inhibitor of tumor growth and angiogenesis acting on calcium influx‐dependent pathways, including AA metabolism, is at least in part due to a direct effect on AA‐induced calcium influx. J. Cell. Physiol. 197: 370–378, 2003© 2003 Wiley‐Liss, Inc.

Expression and functional role of bTRPC1 channels in native endothelial cells.

We have analyzed the expression and localization of bovine transient receptor potential‐C1 (bTRPC1) in bovine aortic endothelial cells, and its possible involvement in the store‐independent calcium influx induced by basic fibroblast growth factor (bFGF). RT‐PCR experiments confirmed the existence of two btrpc1 mRNA isoforms; conversely, the btrpc3 gene was not transcribed. Anti‐TRPC1 antibody revealed the presence of the protein in the membrane‐rich compartment only. Application of anti‐TRPC1 during the response to bFGF caused a partial but significant reduction of calcium entry. This is the first evidence of TRP channel involvement in a non‐capacitative calcium influx induced by a biologically relevant agonist such as the angiogenic factor bFGF in native endothelial cells.

Arachidonic Acid–Induced Ca2+ Entry Is Involved in Early Steps of Tumor Angiogenesis

Growth factor–induced intracellular calcium signals in endothelial cells regulate cytosolic and nuclear events involved in the angiogenic process. Among the intracellular messengers released after proangiogenic stimulation, arachidonic acid (AA) plays a key role and its effects are strictly related to calcium homeostasis and cell proliferation. Here, we studied AA-induced intracellular calcium signals in endothelial cells derived from human breast carcinomas (B-TEC). AA promotes B-TEC proliferation and organization of vessel-like structures in vitro. The effect is directly mediated by the fatty acid without a significant contribution of its metabolites. AA induces Ca2+i signals in the entire capillary-like structure during the early phases of tubulogenesis in vitro. No such responses are detectable in B-TECs organized in more structured tubules. In B-TECs growing in monolayer, AA induces two different signals: a Ca2+i increase due to Ca2+ entry and an inhibition of store-dependent Ca2+ entry induced by thapsigargin or ATP. An inhibitor of Ca2+ entry and angiogenesis, carboxyamidotriazole, significantly and specifically decreases AA-induced B-TEC tubulogenesis, as well as AA-induced Ca2+ signals in B-TECs. We conclude that (a) AA-activated Ca2+ entry is associated with the progression through the early phases of angiogenesis, mainly involving proliferation and tubulogenesis, and it is down-regulated during the reorganization of tumor-derived endothelial cells in capillary-like structures; and (b) inhibition of AA-induced Ca2+ entry may contribute to the antiangiogenic action of carboxyamidotriazole. (Mol Cancer Res 2008;6(4):535–45)

Ion channels and transporters in cancer. 6. Vascularizing the tumor: TRP channels as molecular targets

Tumor vascularization is a critical process that determines tumor growth and metastasis. In the last decade new experimental evidence obtained from in vitro and in vivo studies have challenged the classical angiogenesis model forcing us to consider new scenarios for tumor neovascularization. In particular, the genetic stability of tumor-derived endothelial cells (TECs) has been recently questioned in several studies, which show that TECs, as well as pericytes, differ significantly from their normal counterparts at genetic and functional levels. In addition to such an epigenetic action of tumor microenvironment on endothelial cells (ECs) commitment, the distinct characteristics of TECs could be due to differences in their origin compared with preexisting differentiated ECs. Intracellular Ca(2+) signals are involved at different critical phases in the regulation of the complex process of angiogenesis and tumor progression. These signals are generated by a wide variety of intrinsic and extrinsic factors. Several key components of Ca(2+) signaling including Ca(2+) channels in the plasma membrane, endoplasmic reticulum, calcium pumps, and mitochondria contribute to the generation, amplitude, and frequency of these Ca(2+) change. In particular, several members of the transient receptor potential (TRP) family of calcium-permeable channels have profound effects on the function of ECs. Because of its multifaceted role in the control of cell function, proliferation, and motility, TRP channels have been suggested as a potential molecular target for control of tumor neovascularization. Since plasma membrane Ca(2+) channels are easily and directly accessible via the bloodstream, they are potential targets for a number of pharmacological and antibody-targeted therapeutic strategies, with specificity being the main limitation. In this review we discuss recent advances in understanding the role of Ca(2+) channels, with specific reference to TRP channels, in tumor vascularization process.

Calcium influx induced by activation of tyrosine kinase receptors in cultured bovine aortic endothelial cells

We studied the ionic currents activated by basic fibroblast growth factor (bFGF) and insulin‐like growth factor‐I (IGF‐I) in cultured bovine aortic endothelial cells (BAE‐1) by using patch‐clamp and single‐cell fluorimetric calcium measurements. In whole‐cell, voltage‐clamp experiments at Vh = −50 mV, the addition of either bFGF (20 ng/ml) or IGF‐I (50 ng/ml) induced an inward current with similar amplitude, time course, and permeation properties. The response was dependent on receptor occupancy and showed a desensitisation in the continued presence of the factors. Ionic substitutions in whole‐cell experiments indicated that the current barely discriminated among Na+, Ca+, and K+ ions. Accordingly, stimulation with bFGF or IGF‐I induced a dose‐dependent [Ca2+]i elevation completely due to entry from the extracellular medium, whereas no detectable release from internal stores was observed. Calcium influx was dependent on protein tyrosine kinase (PTK) activity; it was significantly inhibited by treatment with genistein or tyrphostin 47, two PTK inhibitors, and not affected by inactive analogues, daidzein, and tyrphostin 1. Moreover, addition of 200 μM Na3VO4, an inhibitor of protein tyrosine phosphatase (PTP) activity, evoked the responses to the factors both in patch‐clamp and in fluorimetric measurements. Cell‐attached recordings using 100 mM CaCl2 in the pipette showed that bFGF and IGF‐I activate calcium‐permeable channels with similar properties. These results provide evidence for a calcium influx induced by two factors that bind to tyrosine kinase receptors (RTK) in endothelial cells. J. Cell. Physiol. 185:454–463, 2000.

Emerging role of TRP channels in cell migration: from tumor vascularization to metastasis

Transient Receptor Potential (TRP) channels modulate intracellular Ca2+ concentrations, controlling critical cytosolic and nuclear events that are involved in the initiation and progression of cancer. It is not, therefore, surprising that the expression of some TRP channels is altered during tumor growth and metastasis. Cell migration of both epithelial and endothelial cells is an essential step of the so-called metastatic cascade that leads to the spread of the disease within the body. It is in fact required for both tumor vascularization as well as for tumor cell invasion into adjacent tissues and intravasation into blood/lymphatic vessels. Studies from the last 15 years have unequivocally shown that the ion channles and the transport proteins also play important roles in cell migration. On the other hand, recent literature underlies a critical role for TRP channels in the migration process both in cancer cells as well as in tumor vascularization. This will be the main focus of our review. We will provide an overview of recent advances in this field describing TRP channels contribution to the vascular and cancer cell migration process, and we will systematically discuss relevant molecular mechanism involved.

Proteomics-Based Metabolic Modeling Reveals That Fatty Acid Oxidation (FAO) Controls Endothelial Cell (EC) Permeability

Endothelial cells (ECs) play a key role to maintain the functionality of blood vessels. Altered EC permeability causes severe impairment in vessel stability and is a hallmark of pathologies such as cancer and thrombosis. Integrating label-free quantitative proteomics data into genome-wide metabolic modeling, we built up a model that predicts the metabolic fluxes in ECs when cultured on a tridimensional matrix and organize into a vascular-like network. We discovered how fatty acid oxidation increases when ECs are assembled into a fully formed network that can be disrupted by inhibiting CPT1A, the fatty acid oxidation rate-limiting enzyme. Acute CPT1A inhibition reduces cellular ATP levels and oxygen consumption, which are restored by replenishing the tricarboxylic acid cycle. Remarkably, global phosphoproteomic changes measured upon acute CPT1A inhibition pinpointed altered calcium signaling. Indeed, CPT1A inhibition increases intracellular calcium oscillations. Finally, inhibiting CPT1A induces hyperpermeability in vitro and leakage of blood vessel in vivo, which were restored blocking calcium influx or replenishing the tricarboxylic acid cycle. Fatty acid oxidation emerges as central regulator of endothelial functions and blood vessel stability and druggable pathway to control pathological vascular permeability.