Novel variants of voltage-operated calcium channel alpha 1-subunit transcripts in a rat liver-derived cell line: deletion in the IVS4 voltage sensing region

Ca2+ Transportome and the Interorganelle Communication in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a type of liver cancer with a poor prognosis for survival given the complications it bears on the patient. Though damages to the liver are acknowledged prodromic factors, the precise molecular aetiology remains ill-defined. However, many genes coding for proteins involved in calcium (Ca²⁺) homeostasis emerge as either mutated or deregulated. Ca²⁺ is a versatile signalling messenger that regulates functions that prime and drive oncogenesis, favouring metabolic reprogramming and gene expression. Ca²⁺ is present in cell compartments, between which it is trafficked through a network of transporters and exchangers, known as the Ca²⁺ transportome. The latter regulates and controls Ca²⁺ dynamics and tonicity. In HCC, the deregulation of the Ca²⁺ transportome contributes to tumorigenesis, the formation of metastasizing cells, and evasion of cell death. In this review, we reflect on these aspects by summarizing the current knowledge of the Ca²⁺ transportome and overviewing its composition in the plasma membrane, endoplasmic reticulum, and the mitochondria.

Targeting microglia L-type voltage-dependent calcium channels for the treatment of central nervous system disorders

Calcium (Ca2+ ) is a ubiquitous mediator of a multitude of cellular functions in the central nervous system (CNS). Intracellular Ca2+ is tightly regulated by cells, including entry via plasma membrane Ca2+ permeable channels. Of specific interest for this review are L-type voltage-dependent Ca2+ channels (L-VDCCs), due to their pleiotropic role in several CNS disorders. Currently, there are numerous approved drugs that target L-VDCCs, including dihydropyridines. These drugs are safe and effective for the treatment of humans with cardiovascular disease and may also confer neuroprotection. Here, we review the potential of L-VDCCs as a target for the treatment of CNS disorders with a focus on microglia L-VDCCs. Microglia, the resident immune cells of the brain, have attracted recent attention for their emerging inflammatory role in several CNS diseases. Intracellular Ca2+ regulates microglia transition from a resting quiescent state to an "activated" immune-effector state and is thus a valuable target for manipulation of microglia phenotype. We will review the literature on L-VDCC expression and function in the CNS and on microglia in vitro and in vivo and explore the therapeutic landscape of L-VDCC-targeting agents at present and future challenges in the context of Alzheimer's disease, Parkinson's disease, Huntington's disease, neuropsychiatric diseases, and other CNS disorders.

Deranged hepatocyte intracellular Ca2+ homeostasis and the progression of non-alcoholic fatty liver disease to hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the second leading cause of cancer-related deaths in men, and the sixth in women. Non-alcoholic fatty liver disease (NAFLD) is now one of the major risk factors for HCC. NAFLD, which involves the accumulation of excess lipid in cytoplasmic lipid droplets in hepatocytes, can progress to non-alcoholic steatosis, fibrosis, and HCC. Changes in intracellular Ca²⁺ constitute important signaling pathways for the regulation of lipid and carbohydrate metabolism in normal hepatocytes. Recent studies of steatotic hepatocytes have identified lipid-induced changes in intracellular Ca²⁺, and have provided evidence that altered Ca²⁺ signaling exacerbates lipid accumulation and may promote HCC. The aims of this review are to summarise current knowledge of the lipid-induced changes in hepatocyte Ca²⁺ homeostasis, to comment on the mechanisms involved, and discuss the pathways leading from altered Ca²⁺ homeostasis to enhanced lipid accumulation and the potential promotion of HCC. In steatotic hepatocytes, lipid inhibits store-operated Ca²⁺ entry and SERCA2b, and activates Ca²⁺ efflux from the endoplasmic reticulum (ER) and its transfer to mitochondria. These changes are associated with changes in Ca²⁺ concentrations in the ER (decreased), cytoplasmic space (increased) and mitochondria (likely increased). They lead to: inhibition of lipolysis, lipid autophagy, lipid oxidation, and lipid secretion; activation of lipogenesis; increased lipid; ER stress, generation of reactive oxygen species (ROS), activation of Ca²⁺/calmodulin-dependent kinases and activation of transcription factor Nrf2. These all can potentially mediate the transition of NAFLD to HCC. It is concluded that lipid-induced changes in hepatocyte Ca²⁺ homeostasis are important in the initiation and progression of HCC. Further research is desirable to better understand the cause and effect relationships, the time courses and mechanisms involved, and the potential of Ca²⁺ transporters, channels, and binding proteins as targets for pharmacological intervention.

Signature Channels of Excitability no More: L-Type Channels in Immune Cells

Although the concept of Ca(2+) as a universal messenger is well established, it was assumed that the regulatory mechanisms of Ca(2+)-signaling were divided along the line of electric excitability. Recent advances in molecular biology and genomics have, however, provided evidence that non-excitable cells such as immunocytes also express a wide and diverse pool of ion channels that does not differ as significantly from that of excitable cells as originally assumed. Ion channels and transporters are involved in virtually all aspects of immune response regulation, from cell differentiation and development to activation, and effector functions such as migration, antibody-secretion, phagosomal maturation, or vesicular delivery of bactericidal agents. This comprises TRP channel family members, voltage- and Ca(2+)-gated K(+)- and Na(+)-channels, as well as unexpectedly, components of the CaV1-subfamily of voltage-gated L-type Ca(2+)-channels, originally thought to be signature molecules of excitability. This article provides an overview of recent observations made in the field of CaV1 L-type channel function in the immune context, as well as presents results we obtained studying these channels in B-lymphocytes.

Protein kinase C-dependent activation of CaV1.2 channels selectively controls human TH2-lymphocyte functions

BACKGROUND

In addition to calcium release-activated calcium channel/ORAI calcium channels, the role of voltage-gated calcium (Cav1) channels in T-cell calcium signaling is emerging. Cav1 channels are formed by α1 (CaV1.1 to CaV1.4) and auxiliary subunits. We previously demonstrated that mouse TH2 cells selectively overexpressed CaV1.2 and CaV1.3 channels. Knocking down these channels with Cav1 antisense (AS) oligonucleotides inhibited TH2 functions and experimental asthma.

OBJECTIVE

We investigated the expression profile and role of Cav1 channels in human T-cell subsets, with a focus on TH2 cells.

METHODS

We compared the profile of CaV1 channel subunit expression in T-cell subsets isolated ex vivo from the blood of healthy donors, as well as in vitro-polarized T-cell subsets, and tested the effect of the Cav1 inhibitors nicardipine and Cav1.2AS on their functions.

RESULTS

CaV1.4 expression was detectable in CD4(+) T cells, ex vivo TH1 cells, and TH17 cells, whereas Cav1.2 channels predominated in TH2 cells only. T-cell activation resulted in Cav1.4 downregulation, whereas Cav1.2 expression was selectively maintained in polarized TH2 cells and absent in TH1 or TH9 cells. Nicardipine and CaV1.2AS decreased Ca(2+) and cytokine responses in TH2, but not TH1, cells. Protein kinase C (PKC) α/β inhibition decreased Ca(2+) and cytokine responses, whereas both calcium and cytokine responses induced by PKC activation were inhibited by nicardipine or Cav1.2AS in TH2 cells.

CONCLUSION

This study highlights the selective expression of Cav1.2 channels in human TH2 cells and the role of PKC-dependent Cav1.2 channel activation in TH2 cell function. Blocking PKC or Cav1.2 channel activation in TH2 cells might represent new strategies to treat allergic diseases in human subjects.

Effects and mechanisms of store-operated calcium channel blockade on hepatic ischemia-reperfusion injury in rats

AIM

To further investigate the important role of store-operated calcium channels (SOCs) in rat hepatocytes and to explore the effects of SOC blockers on hepatic ischemia-reperfusion injury (HIRI).

METHODS

Using freshly isolated hepatocytes from a rat model of HIRI (and controls), we measured cytosolic free Ca(2+) concentration (by calcium imaging), net Ca(2+) fluxes (by a non-invasive micro-test technique), the SOC current (I(SOC); by whole-cell patch-clamp recording), and taurocholate secretion [by high-performance liquid chromatography and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays].

RESULTS

Ca(2+) oscillations and net Ca(2+) fluxes mediated by Ca(2+) entry via SOCs were observed in rat hepatocytes. I(SOC) was significantly higher in HIRI groups than in controls (57.0 ± 7.5 pA vs 31.6 ± 2.7 pA, P < 0.05) and was inhibited by La(3+). Taurocholate secretion by hepatocytes into culture supernatant was distinctly lower in HIRI hepatocytes than in controls, an effect reversed by SOC blockers.

CONCLUSION

SOCs are pivotal in HIRI. SOC blockers protected against HIRI and assisted the recovery of secretory function in hepatocytes. Thus, they are likely to become a novel class of effective drugs for prevention or therapy of HIRI patients in the future.

Expression and function of TRP channels in liver cells

The liver plays a central role in whole body homeostasis by mediating the metabolism of carbohydrates, fats, proteins, drugs and xenobiotic compounds, and bile acid and protein secretion. Hepatocytes together with endothelial cells, Kupffer cells, smooth muscle cells, stellate and oval cells comprise the functioning liver. Many members of the TRP family of proteins are expressed in hepatocytes. However, knowledge of their cellular functions is limited. There is some evidence which suggests the involvement of TRPC1 in volume control, TRPV1 and V4 in cell migration, TRPC6 and TRPM7 in cell proliferation, and TRPPM in lysosomal Ca(2+) release. Altered expression of some TRP proteins, including TRPC6, TRPM2 and TRPV1, in tumorigenic cell lines may play roles in the development and progression of hepatocellular carcinoma and metastatic liver cancers. It is likely that future experiments will define important roles for other TRP proteins in the cellular functions of hepatocytes and other cell types of which the liver is composed.

NSAIDs, Mitochondria and Calcium Signaling: Special Focus on Aspirin/Salicylates

Aspirin (acetylsalicylic acid) is a well-known nonsteroidal anti-inflammatory drug (NSAID) that has long been used as an anti-pyretic and analgesic drug. Recently, much attention has been paid to the chemopreventive and apoptosis-inducing effects of NSAIDs in cancer cells. These effects have been thought to be primarily attributed to the inhibition of cyclooxygenase activity and prostaglandin synthesis. However, recent studies have demonstrated unequivocally that certain NSAIDs, including aspirin and its metabolite salicylic acid, exert their anti-inflammatory and chemopreventive effects independently of cyclooxygenase activity and prostaglandin synthesis inhibition. It is becoming increasingly evident that two potential common targets of NSAIDs are mitochondria and the Ca²⁺ signaling pathway. In this review, we provide an overview of the current knowledge regarding the roles of mitochondria and Ca²⁺ in the apoptosis-inducing effects as well as some side effects of aspirin, salicylates and other NSAIDs, and introducing the emerging role of L-type Ca²⁺ channels, a new Ca²⁺ entry pathway in non-excitable cells that is up-regulated in human cancer cells.

L-type Ca2+ channels: a new player in the regulation of Ca2+ signaling, cell activation and cell survival in immune cells

Ca(2+) is a highly versatile intracellular second messenger in many cell types, and regulates many complicated cellular processes, including cell activation, proliferation and apoptosis. Influx of Ca(2+) from the extracellular fluid is required for sustained elevation of the cytosolic Ca(2+) concentration and full activation of Ca(2+)-dependent processes. It is widely accepted that Ca(2+) release-activated Ca(2+) channels are the major routes of Ca(2+) influx in electrically non-excitable cells, including hematopoietic cells, whereas voltage-gated Ca(2+) channels such as L-type Ca(2+) channels (LTCCs) serve as the principal routes of Ca(2+) entry into electrically excitable cells such as neurons and myocytes. However, recent pharmacological and molecular genetic studies have revealed the existence of functional LTCCs and/or LTCC-like channels in a variety of immune cells including mast cells. In this article, we review recent advances in our understanding of Ca(2+) signaling in immune cells with a special interest in mast cells. We highlight roles for LTCCs in antigen receptor-mediated mast cell activation and survival.

Roles of Ca(v) channels and AHNAK1 in T cells: the beauty and the beast

T lymphocytes require Ca2+ entry though the plasma membrane for their activation and function. Recently, several routes for Ca2+ entry through the T-cell plasma membrane after activation have been described. These include calcium release-activated channels (CRAC), transient receptor potential (TRP) channels, and inositol-1,4,5-trisphosphate receptors (IP3Rs). Herein we review the emergence of a fourth new route for Ca2+ entry, composed of Ca(v) channels (also known as L-type voltage-gated calcium channels) and the scaffold protein AHNAK1 (AHNAK/desmoyokin). Both helper (CD4+) and killer (CD8+) T cells express high levels of Ca(v)1 alpha1 subunits (alpha1S, alpha1C, alpha1D, and alpha1F) and AHNAK1 after their differentiation and require these molecules for Ca2+ entry during an immune response. In this article, we describe the observations and open questions that ultimately suggest the involvement of multiple consecutive routes for Ca2+ entry into lymphocytes, one of which may be mediated by Ca(v) channels and AHNAK1.

Measuring Ca2+ influxes of TRPC1-dependent Ca2+ channels in HL-7702 cells with non-invasive micro-test technique

AIM

To explore the possibility of using the Non-invasive Micro-test Technique (NMT) to investigate the role of Transient Receptor Potential Canonical 1 (TRPC1) in regulating Ca(2+) influxes in HL-7702 cells, a normal human liver cell line.

METHODS

Net Ca(2+) fluxes were measured with NMT, a technology that can obtain dynamic information of specific/selective ionic/molecular activities on material surfaces, non-invasively. The expression levels of TRPC1 were increased by liposomal transfection, whose effectiveness was evaluated by Western-blotting and single cell reverse transcription-polymerase chain reaction.

RESULTS

Ca(2+) influxes could be elicited by adding 1 mmol/L CaCl(2) to the test solution of HL-7702 cells. They were enhanced by addition of 20 micromol/L noradrenaline and inhibited by 100 micromol/L LaCl(3) (a non-selective Ca(2+) channel blocker); 5 micromol/L nifedipine did not induce any change. Overexpression of TRPC1 caused increased Ca(2+) influx. Five micromoles per liter nifedipine did not inhibit this elevation, whereas 100 micromol/L LaCl(3) did.

CONCLUSION

In HL-7702 cells, there is a type of TRPC1-dependent Ca(2+) channel, which could be detected via NMT and inhibited by La(3+).

Ca(2+) -permeable channels in the hepatocyte plasma membrane and their roles in hepatocyte physiology

Hepatocytes are highly differentiated and spatially polarised cells which conduct a wide range of functions, including intermediary metabolism, protein synthesis and secretion, and the synthesis, transport and secretion of bile acids. Changes in the concentrations of Ca(2+) in the cytoplasmic space, endoplasmic reticulum (ER), mitochondria, and other intracellular organelles make an essential contribution to the regulation of these hepatocyte functions. While not yet fully understood, the spatial and temporal parameters of the cytoplasmic Ca(2+) signals and the entry of Ca(2+) through Ca(2+)-permeable channels in the plasma membrane are critical to the regulation by Ca(2+) of hepatocyte function. Ca(2+) entry across the hepatocyte plasma membrane has been studied in hepatocytes in situ, in isolated hepatocytes and in liver cell lines. The types of Ca(2+)-permeable channels identified are store-operated, ligand-gated, receptor-activated and stretch-activated channels, and these may vary depending on the animal species studied. Rat liver cell store-operated Ca(2+) channels (SOCs) have a high selectivity for Ca(2+) and characteristics similar to those of the Ca(2+) release activated Ca(2+) channels in lymphocytes and mast cells. Liver cell SOCs are activated by a decrease in Ca(2+) in a sub-region of the ER enriched in type1 IP(3) receptors. Activation requires stromal interaction molecule type 1 (STIM1), and G(i2alpha,) F-actin and PLCgamma1 as facilitatory proteins. P(2x) purinergic channels are the only ligand-gated Ca(2+)-permeable channels in the liver cell membrane identified so far. Several types of receptor-activated Ca(2+) channels have been identified, and some partially characterised. It is likely that TRP (transient receptor potential) polypeptides, which can form Ca(2+)- and Na(+)-permeable channels, comprise many hepatocyte receptor-activated Ca(2+)-permeable channels. A number of TRP proteins have been detected in hepatocytes and in liver cell lines. Further experiments are required to characterise the receptor-activated Ca(2+) permeable channels more fully, and to determine the molecular nature, mechanisms of activation, and precise physiological functions of each of the different hepatocyte plasma membrane Ca(2+) permeable channels.

A calcium flux is required for circadian rhythm generation in mammalian pacemaker neurons

Generation of mammalian circadian rhythms involves molecular transcriptional and translational feedback loops. It is not clear how membrane events interact with the intracellular molecular clock or whether membrane activities are involved in the actual generation of the circadian rhythm. We examined the role of membrane potential and calcium (Ca2+) influx in the expression of the circadian rhythm of the clock gene Period 1 (Per1) within the rat suprachiasmatic nucleus (SCN), the master pacemaker controlling circadian rhythmicity. Membrane hyperpolarization, caused by lowering the extracellular concentration of potassium or blocking Ca2+ influx in SCN cultures by lowering [Ca2+], reversibly abolished the rhythmic expression of Per1. In addition, the amplitude of Per1 expression was markedly decreased by voltage-gated Ca2+ channel antagonists. A similar result was observed for mouse Per1 and PER2. Together, these results strongly suggest that a transmembrane Ca2+ flux is necessary for sustained molecular rhythmicity in the SCN. We propose that periodic Ca2+ influx, resulting from circadian variations in membrane potential, is a critical process for circadian pacemaker function.

Deletions in L-type calcium channel α1 subunit testicular transcripts correlate with testicular cadmium and apoptosis in infertile men with varicoceles

OBJECTIVE

To identify and understand predictors of successful varicocelectomy.

DESIGN

Examination of testicular L-type voltage-dependent calcium channel (L-VDCC) mRNAs and proteins in testis biopsies and comparison of presence and absence of various mRNAs with testicular cadmium levels, with apoptosis, and with sperm count change after varicocelectomy.

SETTING

University clinical urology practice and research laboratories.

PATIENT(S)

Infertile men with varicocele (left varicocele only, n = 18; bilateral varicoceles, n = 26) and controls (men with obstructive azoospermia undergoing testicular sperm extraction before intracytoplasmic sperm injection; n = 7).

INTERVENTION(S)

Left testis biopsies by percutaneous needle aspiration biopsy. Varicocele repair by subinguinal approach.

MAIN OUTCOME MEASURE(S)

Calcium channel mRNA sequence by reverse transcription-polymerase chain reaction and amplicon analysis; calcium channel protein distribution by immunocytochemistry; cadmium levels by atomic absorption and apoptosis by deoxynucleotidyl transferase labeling; and sperm counts in the ejaculate before and after varicocelectomy.

RESULT(S)

Calcium channel mRNAs are polymorphic in human testis biopsies from different men. Proteins from sequence-deleted exons 7 and/or 8 localize to germ cell membranes. Expression of undeleted L-type calcium channel mRNAs correlates with normal testes cadmium and increased sperm count after varicocelectomy. Apoptosis is lower in such cases.

CONCLUSION(S)

Expression of normal testicular L-VDCC sequence in exons 7-8 predicts postvaricocelectomy sperm count increase. Deletions may alter calcium channel function and affect testicular cadmium and apoptosis.

Evaluation, using targeted aequorins, of the roles of the endoplasmic reticulum and its (Ca2++Mg2+)ATP-ases in the activation of store-operated Ca2+ channels in liver cells

The process by which store-operated Ca2+ channels (SOCs) deliver Ca2+ to the endoplasmic reticulum (ER) and the role of (Ca2++Mg2+)ATP-ases of the ER in the activation of SOCs in H4-IIE liver cells were investigated using cell lines stably transfected with apo-aequorin targeted to the cytoplasmic space or the ER. In order to measure the concentration of Ca2+ in the ER ([Ca2+]er), cells were pre-treated with 2,5-di-tert-butylhydroquinone (DBHQ) to deplete Ca2+ in the ER before reconstitution of holo-aequorin. The addition of extracellular Ca2+ (Cao2+) to Ca2+-depleted cells induced refilling of the ER, which was complete within 5 min. This was associated with a sharp transient increase in the cytoplasmic Ca2+ concentration ([Ca2+]cyt) of about 15 s duration (a Cao2+-induced [Ca2+]cyt spike) after which [Ca2+]cyt remained elevated slightly above the basal value for a period of about 2 min (low [Ca2+]cyt plateau). The Cao2+-induced [Ca2+]cyt spike was inhibited by Gd3+, not affected by tetrakis-(2-pyridymethyl) ethylenediamine (TPEN), and broadened by ionomycin and the intracellular Ca2+ chelators BAPTA and EGTA. Refilling of the ER was inhibited by caffeine. Neither thapsigargin nor DBHQ caused a detectable inhibition or change in shape of the Cao2+-induced [Ca2+]cyt spike or the low [Ca2+]cyt plateau whereas each inhibited the inflow of Ca2+ to the ER by about 80%. Experiments conducted with carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) indicated that thapsigargin did not alter the amount of Ca2+ accumulated in mitochondria. The changes in [Ca2+]cyt reported by aequorin were compared with those reported by fura-2. It is concluded that (i) there are significant quantitative differences between the manner in which aequorin and fura-2 sense changes in [Ca2+]cyt and (ii) thapsigargin and DBHQ inhibit the uptake of Ca2+ to the bulk of the ER but this is not associated with inhibition of the activation of SOCs. The possible involvement of a small sub-region of the ER (or another intracellular Ca2+ store), which contains thapsigargin-insensitive (Ca2++Mg2+)ATP-ases, in the activation of SOCs is briefly discussed.

Non-voltage-gated L-type Ca2+ channels in human T cells: pharmacology and molecular characterization of the major alpha pore-forming and auxiliary beta-subunits

In T lymphocytes, engagement of the antigen receptor leads to a biphasic Ca2+ flux consisting of a mobilization of Ca2+ from intracellular stores followed by a lower but sustained elevation that is dependent on extracellular Ca2+. The prolonged Ca2+ flux is required for activation of transcription factors and for subsequent activation of the T cell. Ca2+ influx requires as yet unidentified Ca2+ channels, which potentially play a role in T cell activation. Here we present evidence that human T cells express a non-voltage-gated Ca2+ channel related to L-type voltage-gated Ca2+ channels. Drugs that block classical L-type channels inhibited the initial phase of the antigen receptor-induced Ca2+ flux and could also inhibit the sustained phase of the Ca2+ signal suggesting a role for the L-type Ca2+ channel in antigen receptor signaling. T cells expressed transcripts for the alpha(1) 1.2 and alpha(1) 1.3 pore-forming subunits of L-type voltage-gated Ca2+ channels and transcripts for all four known beta-subunits including several potential new splice variants. Jurkat T leukemia cells expressed a small amount of full-length alpha(1)1.2 protein but the dominant form was a truncated protein identical in size to a truncated alpha(1) 1.2 protein known to be expressed in B lymphocytes. They further expressed a truncated form of the alpha(1) 1.3 subunit and auxiliary beta1- and beta3-subunit proteins. Our data strongly suggest that functional but non-voltage-gated L-type Ca2+ channels are expressed at the plasma membrane in T cells and play a role in the antigen receptor-mediated Ca2+ flux in these cells.

Identification and functional characterization of voltage-dependent calcium channels in T lymphocytes

In T lymphocytes, sustained calcium (Ca2+) influx through Ca2+ channels localized in the plasma membrane is critical for T cell activation and proliferation. Previous studies indicated that voltage-dependent Ca2+ channels (VDCCs) play a role in Ca2+ mobilization during T lymphocyte activation. However, the role of VDCCs in otherwise nonexcitable cells is still poorly understood. We used RT-PCR to identify a transcript encoding the pore-forming alpha1F-subunit of an L-type Ca2+ channel in T lymphocytes. Its identity was confirmed by DNA sequencing. To further investigate the contribution of Ca2+ influx through VDCCs, we assessed the effects of the 1,4-dihydropyridine L-type Ca2+ channel agonist, (+/-) Bay K 8644, and antagonist, nifedipine, on the human Jurkat T cell leukemia line, human peripheral blood T lymphocytes and mouse splenocytes. We found that treatment of T lymphocytes with (+/-) Bay K 8644 increased intracellular Ca2+ and induced the activation of phosphoextracellular-regulated kinase 1/2 (Erk1/2), whereas nifedipine blocked Ca2+ influx, the activity of Erk1/2 and nuclear factor of activated T cells (NFAT), interleukin-2 (IL-2) production, and IL-2 receptor expression. Nifedipine also significantly suppressed splenocyte proliferation in an in vitro mixed lymphocyte reaction and the proliferation of male antigen (H-Y)-specific T cell receptor-transgenic CD8+ T cells in transplanted male mice in vivo. Taken together these novel findings indicate that an L-type Ca2+ channel plays a significant role in the Ca2+ influx pathways mediating T lymphocyte activation and proliferation in vitro and in vivo.

Mapacalcine specifically blocks hypoxia-induced calcium influx in rat hepatocytes

Post ischaemic cell calcium invasion has been described as one of the main causes of graft failure. Protective effects of calcium antagonists have been investigated but are not convincing and their mechanisms of action remain unclear. In this work we tested the protective effect of a new calcium inhibitor described to block a calcium current insensitive to all known calcium blockers. Specific mapacalcine receptors were first characterized on rat hepatocytes membranes using the 125I-labeled mapacalcine. 45Ca fluxes were then measured on cultured hepatocytes submitted (or not) to an hypoxic period. The action of mapacalcine was investigated on the ischaemia-induced calcium influx. We demonstrate here that: (a) there are specific receptors for mapacalcine in rat hepatocytes; (b) Mapacalcine is able to specifically block ischaemia-induced calcium influx with an IC50 of 0.3 micro m and does not significantly interact with the basal calcium flux. Our work demonstrates that the mapacalcine receptor is a cellular structure directly involved in the phenomenon of postischaemic cell invasion by calcium. Specific block of ischaemia-induced Ca2+ influx by mapacalcine suggests that the development of a panel of pharmacological drugs acting on this receptor could lead to the discovery of therapeutic agents able to protect cells against one of the events responsible for organ failure after transplantation or simply after an ischaemic period. Moreover, identification of the cellular protein which binds mapacalcine may become an important step in the research of mechanisms involved in postischaemic cell invasion by calcium.

Effects of tetrandrine on calcium and potassium currents in isolated rat hepatocytes

AIM: To study the effects of tetrandrine (Tet) on calcium release-activated calcium current (I_CRAC), delayed rectifier potassium current (I_K), and inward rectifier potassium currents (I_K1) in isolated rat hepatocytes.

METHODS: Hepatocytes of rat were isolated by using perfusion method. Whole cell patch-clamp techniques were used in our experiment.

RESULTS: The peak amplitude of I_CRAC was -508 ± 115 pA (n = 15), its reversal potential of I_CRAC was about 0 mV. At the potential of -100 mV, Tet inhibited the peak amplitude of I_CRAC from -521 ± 95 pA to -338 ± 85 pA (P < 0.01 vs control, n = 5), with the inhibitory rate of 35% at 10 µmol/L and from -504 ± 87 pA to -247 ± 82 pA (P < 0.01 vs control, n = 5), with the inhibitory rate of 49% at 100 µmol/L, without affecting its reversal potential. The amplitude of I_CRAC was dependent on extracellular Ca²⁺ concentration. The peak amplitude of I_CRAC was -205 ± 105 pA (n = 3) in tyrode’s solution with Ca²⁺ 1.8 mmol/L (P < 0.01 vs the peak amplitude of I_CRAC in external solution with Ca²⁺ 10 mmol/L). Tet at the concentration of 10 and 100 µmol/L did not markedly change the peak amplitude of delayed rectifier potassium current and inward rectifier potassium current (P > 0.05 vs control).

CONCLUSION: Tet protects hepatocytes by inhibiting I_CRAC, which is not related to I_K and I_K1.

A diacylglycerol-activated Ca2+ channel in PC12 cells (an adrenal chromaffin cell line) correlates with expression of the TRP-6 (transient receptor potential) protein

The structures, and mechanisms of activation, of plasma membrane intracellular-messenger-activated, non-selective cation channels in animal cells are not well understood. The PC12 adrenal chromaffin cell line is a well-characterized example of a nerve cell. In PC12 cells, 1-oleolyl-2-acetyl-sn-glycerol (OAG), a membrane-permeant analogue of diacylglycerol, initiated the inflow of Ca(2+), Mn(2+) and Sr(2+). Acetylcholine and thapsigargin initiated the inflow of Ca(2+) and Mn(2+), but not of Sr(2+). The activation of bivalent cation inflow by OAG: (i) was mimicked by another membrane-permeant diacylglycerol analogue, 1,2-dioctanoyl-sn-glycerol, but not by the membrane-impermeant analogue 1-stearoyl-2-arachidonyl-sn-glycerol; (ii) was not blocked by staurosporin or chelerythrine, inhibitors of protein kinase C; (iii) was enhanced by RHC80267 and R50922, inhibitors of diacylglycerol lipase and diacylglycerol kinase respectively; and (iv) was inhibited by extracellular Ca(2+). When OAG was added after acetylcholine, the effect of OAG on Ca(2+) inflow was over-and-above that induced by acetylcholine. 2-Aminoethyl diphenylborate (2-APB) inhibited Ca(2+) inflow initiated by either acetylcholine or thapsigargin, but not that initiated by OAG. Flufenamic acid inhibited OAG-initiated, but not acetylcholine-initiated, Ca(2+) and Mn(2+) inflow. OAG-initiated Ca(2+) inflow was less sensitive to inhibition by SK&F96365 than acetylcholine-initiated Ca(2+) inflow. In polyadenylated RNA prepared from PC12 cells, mRNA encoding TRP (transient receptor potential) proteins 1-6 was detected by reverse transcriptase (RT)-PCR, and in lysates of PC12 cells the endogenous TRP-6 protein was detected by Western blot analysis. It is concluded that PC12 cells express a diacylglycerol-activated, non-selective cation channel. Expression of this channel function correlates with expression of the TRP-3 and TRP-6 proteins, which have been shown previously to be activated by diacylglycerol when expressed heterologously in animal cells [Hofmann, Obukhov, Schaefer, Harteneck, Gudermann, and Schultz (1999) Nature (London) 397, 259-263].

Maitotoxin activates an endogenous non-selective cation channel and is an effective initiator of the activation of the heterologously expressed hTRPC-1 (transient receptor potential) non-selective cation channel in H4-IIE liver cells

The structures and mechanisms of activation of non-selective cation channels (NSCCs) are not well understood although NSCCs play important roles in the regulation of metabolism, ion transport, cell volume and cell shape. It has been proposed that TRP (transient receptor potential) proteins are the molecular correlates of some NSCCs. Using fura-2 and patch-clamp recording, it was shown that the maitotoxin-activated cation channels in the H4-IIE rat liver cell line admit Ca(2+), Mn(2+) and Na(+), have a high selectivity for Na(+) compared with Ca(2+), and are inhibited by Gd(3+) (half-maximal inhibition at 1 microM). Activation of the channels by maitotoxin was inhibited by increasing the extracellular Ca(2+) concentration or by inclusion of 10 mM EGTA in the patch pipette. mRNA encoding TRP proteins 1, 2 and 3 at levels comparable with those in brain was detected using reverse transcriptase-polymerase chain reaction in poly(A)(+) RNA prepared from H4-IIE cells and freshly-isolated rat hepatocytes. In H4-IIE cells transiently transfected with cDNA encoding hTRPC-1, the expressed hTRPC-1 protein was chiefly located at intracellular sites and at the plasma membrane. Cells expressing hTRPC-1 exhibited a substantial enhancement of maitotoxin-initiated Ca(2+) inflow and a modest enhancement of thapsigargin-initiated Ca(2+) inflow (measured using fura-2) and no enhancement of the highly Ca(2+)-selective store-operated Ca(2+) current (measured using patch-clamp recording). In cells expressing hTRPC-1, maitotoxin activated channels which were not found in untransfected cells, have an approximately equal selectivity for Na(+) and Ca(2+), and are inhibited by Gd(3+) (half-maximal inhibition at 3 microM). It is concluded that in liver cells (i) maitotoxin initiates the activation of endogenous NSCCs with a high selectivity for Na(+) compared with Ca(2+); (ii) TRP proteins 1, 2 and 3 are expressed; (iii) maitotoxin is an effective initiator of activation of heterologously expressed hTRPC-1 channels; and (iv) the endogenous TRP-1 protein is unlikely to be the molecular counterpart of the maitotoxin-activated NSCCs nor the highly Ca(2+)-selective store-operated Ca(2+) channels.

Evidence that the TRP-1 protein is unlikely to account for store-operated Ca2+ inflow in Xenopus laevis oocytes

The role of the TRP-1 protein, an animal cell homologue of the Drosophila transient receptor potential Ca2+ channel, in store-operated Ca2+ inflow in Xenopus laevis oocytes was investigated. A strategy involving RT-PCR and 3' and 5' rapid amplification of cDNA ends (RACE) was used to confirm and extend previous knowledge of the nucleotide and predicted amino acid sequences of Xenopus TRP-1 (xTRP-1). The predicted amino acid sequence was used to prepare an anti-TRP-l polyclonal antibody which detected the endogenous oocyte xTRP-1 protein and the human TRPC-1 protein expressed in Xenopus oocytes. Ca2+ inflow (measured using fura-2) initiated by 3-deoxy-3-fluoroinositol 1,4,5-trisphosphate (InsP3F) or lysophosphatidic acid (LPA) was completely inhibited by low concentrations of lanthanides (IC50 = 0.5 microM), indicating that InsP3F and LPA principally activate store-operated Ca2+ channels (SOCs). Antisense cRNA or antisense oligodeoxynucleotides, based on different regions of the xTRP-1 cDNA sequence, when injected into Xenopus oocytes, did not inhibit InsP3F-, LPA- or thapsigargin-stimulated Ca2+ inflow. Oocytes expressing the hTRPC-1 protein, which is 96% similar to xTRP-1, exhibited no detectable enhancement of either basal or InsP3F-stimulated Ca2+ inflow and only a very small enhancement of LPA-stimulated Ca2+ in-flow compared with control oocytes. It is concluded that the endogenous xTRP-1 protein is unlikely to be responsible for Ca2+ inflow through the previously-characterised Ca2+ -specific SOCs which are found in Xenopus oocytes. It is considered that xTRP-1 is likely to be a receptor-activated non-selective cation channel such as the channel activated by maitotoxin.

Store-operated Ca(2+) inflow in Reuber hepatoma cells is inhibited by voltage-operated Ca(2+) channel antagonists and, in contrast to freshly isolated hepatocytes, does not require a pertussis toxin-sensitive trimeric GTP-binding protein

The treatment of H4-IIE cells (an immortalised liver cell line derived from the Reuber rat hepatoma) with thapsigargin, 2, 5-di-(tert-butyl)-1,4-benzohydroquinone, cyclopiazonic acid, or pretreatment with EGTA, stimulated Ca(2+) inflow (assayed using intracellular fluo-3 and a Ca(2+) add-back protocol). No stimulation of Mn(2+) inflow by thapsigargin was detected. Thapsigargin-stimulated Ca(2+) inflow was inhibited by Gd(3+) (maximal inhibition at 2 microM Gd(3+)), the imidazole derivative SK&F 96365, and by relatively high concentrations of the voltage-operated Ca(2+) channel antagonists, verapamil, nifedipine, nicardipine and the novel dihydropyridine analogues AN406 and AN1043. The calmodulin antagonists W7, W13 and calmidazolium also inhibited thapsigargin-induced Ca(2+) inflow and release of Ca(2+) from intracellular stores. No inhibition of either Ca(2+) inflow or Ca(2+) release was observed with calmodulin antagonist KN62. Substantial inhibition of Ca(2+) inflow by calmidazolium was only observed when the inhibitor was added before thapsigargin. Pretreatment of H4-IIE cells with pertussis toxin, or treatment with brefeldin A, did not inhibit thapsigargin-stimulated Ca(2+) inflow. Compared with freshly isolated rat hepatocytes, H4-IIE cells exhibited a more diffuse actin cytoskeleton, and a more granular arrangement of the endoplasmic reticulum (ER). In contrast to freshly isolated hepatocytes, the arrangement of the ER in H4-IIE cells was not affected by pertussis toxin treatment. Western blot analysis of lysates of freshly isolated rat hepatocytes revealed two forms of G(i2(alpha)) with apparent molecular weights of 41 and 43 kDa. Analysis of H4-IIE cell lysates showed only the 41 kDa form of G(i2(alpha)) and substantially less total G(i2(alpha)) than that present in rat hepatocytes. It is concluded that H4-IIE cells possess store-operated Ca(2+) channels which do not require calmodulin for activation and exhibit properties similar to those in freshly isolated rat hepatocytes, including susceptibility to inhibition by relatively high concentrations of voltage-operated Ca(2+) channel antagonists. In contrast to rat hepatocytes, SOCs in H4-IIE cells do not require G(i2(alpha)) for activation. Possible explanations for differences in the requirement for G(i2(alpha)) in the activation of Ca(2+) inflow are briefly discussed.

The N terminus of the cardiac L-type Ca(2+) channel alpha(1C) subunit. The initial segment is ubiquitous and crucial for protein kinase C modulation, but is not directly phosphorylated

The first 46 amino acids (aa) of the N terminus of the rabbit heart (RH) L-type cardiac Ca(2+) channel alpha(1C) subunit are crucial for the stimulating action of protein kinase C (PKC) and also hinder channel gating (Shistik, E., Ivanina, T., Blumenstein, Y., and Dascal, N. (1998) J. Biol. Chem. 273, 17901-17909). The mechanism of PKC action and the location of the PKC target site are not known. Moreover, uncertainties in the genomic sequence of the N-terminal region of alpha(1C) leave open the question of the presence of RH-type N terminus in L-type channels in mammalian tissues. Here, we demonstrate the presence of alpha(1C) protein containing an RH-type initial N-terminal segment in rat heart and brain by using a newly prepared polyclonal antibody. Using deletion mutants of alpha(1C) expressed in Xenopus oocytes, we further narrowed down the part of the N terminus crucial for both inhibitory gating and for PKC effect to the first 20 amino acid residues, and we identify the first 5 aa as an important determinant of PKC action and of N-terminal effect on gating. The absence of serines and threonines in the first 5 aa and the absence of phosphorylation by PKC of a glutathione S-transferase-fusion protein containing the initial segment suggest that the effect of PKC does not arise through a direct phosphorylation of this segment. We propose that PKC acts by attenuating the inhibitory action of the N terminus via phosphorylation of a remote site, in the channel or in an auxiliary protein, that interacts with the initial segment of the N terminus.

Phytohemagglutinin-stimulated calcium signal in lymphocytes of alcoholics before, during and after detoxification

Acute and chronic exposure to ethanol influences intracellular calcium homeostasis via NMDA receptors, direct regulation of calcium channels or the phosphoinositide pathway. To explore the influence of alcohol withdrawal on calcium metabolism, we have investigated the resting and phytohemagglutinin (PHA) stimulated [Ca2+]i in lymphocytes of 10 alcoholics before, during and after withdrawal. Our findings suggest that both compartments of the PHA-stimulated signal are affected in alcoholics, with flattening of the initial peak and sustained calcium influx, as long as severe vegetative signs are present. MANOVA results showed significant interaction effects for both measurement points, for the initial peak, 40 s after stimulation (P = 0.05), and especially for the sustained influx at the end of the observation period (P = 0.001). The detailed mechanisms of this disturbed calcium homeostasis need further investigation.