Low-voltage-activated calcium channels in human retinoblastoma cells

Identification of Promising Drug Candidates against Prostate Cancer through Computationally-Driven Drug Repurposing

Prostate cancer (PC) is one of the most common types of cancer in males. Although early stages of PC are generally associated with favorable outcomes, advanced phases of the disease present a significantly poorer prognosis. Moreover, currently available therapeutic options for the treatment of PC are still limited, being mainly focused on androgen deprivation therapies and being characterized by low efficacy in patients. As a consequence, there is a pressing need to identify alternative and more effective therapeutics. In this study, we performed large-scale 2D and 3D similarity analyses between compounds reported in the DrugBank database and ChEMBL molecules with reported anti-proliferative activity on various PC cell lines. The analyses included also the identification of biological targets of ligands with potent activity on PC cells, as well as investigations on the activity annotations and clinical data associated with the more relevant compounds emerging from the ligand-based similarity results. The results led to the prioritization of a set of drugs and/or clinically tested candidates potentially useful in drug repurposing against PC.

The T-type Calcium Channel Cav3.1 in Y79 Retinoblastoma Cells is Regulated by the Epidermal Growth Factor Receptor via the MAPK Signaling Pathway

PURPOSE

Retinoblastoma is the most frequent intraocular cancer in children. It is also one of the most common causes for enucleation and carries a significant morbidity rate in affected individuals. Hence, studies on its pathophysiological and growth regulatory mechanisms are urgently needed to identify more effective novel therapeutics.

METHODS

Using the Y79 retinoblastoma cell line, we investigated the electrophysiological and functional activities of the T-type voltage-gated calcium channel Cav3.1, that is constitutively expressed in these cells. We also analyzed the Akt and MAPK signaling pathways downstream of the epidermal growth factor receptor (EGFR) to understand the mechanism responsible for the inhibition of Cav3.1.

RESULTS

We demonstrate that the EGFR inhibitor Afatinib significantly reduced cell viability and Cav3.1 mRNA expression and electrophysiological activity. At low concentrations (1 µM), Afatinib reduced the amplitude of Cav3.1 current density, whereas at a high concentration (10 µM), it completely abolished the voltage-gated calcium current. Our results show that inhibition of the MAPK pathway by a specific inhibitor VX-11e affected the Cav3.1 current in a dose-dependent manner. VX-11e (50 nM-1 µM) treatment reduced Cav3.1 current densities in Y79 cells, with complete abolishment of Cav3.1 current at higher concentrations (5 µM). We also demonstrate that the specific inhibition of the Akt kinase (using MK-2206) had no effect on the Cav3.1 currents.

CONCLUSION

Our study provides a functional relationship between the MAPK pathway and EGFR signaling and indicates that the MAPK signaling pathway mediates the control of Cav3.1 by EGFR in retinoblastoma.

Giant Y79 retinoblastoma cells contain functionally active T-type calcium channels

Retinoblastoma is the most common malignant intraocular tumor in children. Y79 human retinoblastoma cells are in vitro models of retinal tumors used for drug screening. Undifferentiated Y79 cells originate from a primitive multi-potential neuroectodermal cell and express neuronal and glial properties. However, the nature of cellular heterogeneity in Y79 cells is unclear because functional methods to characterize neurons or glial cells have not been employed to Y79 cells. Here, we perform patch-clamp recordings to characterize electrophysiological properties in retinoblastoma cells. We identified a population of large-sized Y79 cells (i.e., giant cells, ~ 40-µm diameter), hyperpolarized resting membrane potential (-54 mV), and low input resistance (~ 600 MΩ), indicating electrically mature cells. We also found that giant Y79 cells contain increased density of T-type calcium channels. Finally, we found that T-type calcium channels are active only in giant cells suggesting that cancer treatments aimed to prevent calcium influx in retinoblastomas should be tested in giant cells.

Ascorbate-induced oxidative stress mediates TRP channel activation and cytotoxicity in human etoposide-sensitive and -resistant retinoblastoma cells

There are indications that pharmacological doses of ascorbate (Asc) used as an adjuvant improve the chemotherapeutic management of cancer. This favorable outcome stems from its cytotoxic effects due to prooxidative mechanisms. Since regulation of intracellular Ca2+ levels contributes to the maintenance of cell viability, we hypothesized that one of the effects of Asc includes disrupting regulation of intracellular Ca2+ homeostasis. Accordingly, we determined if Asc induced intracellular Ca2+ influx through activation of pertussis sensitive Gi/o-coupled GPCR which in turn activated transient receptor potential (TRP) channels in both etoposide-resistant and -sensitive retinoblastoma (WERI-Rb1) tumor cells. Ca2+ imaging, whole-cell patch-clamping, and quantitative real-time PCR (qRT-PCR) were performed in parallel with measurements of RB cell survival using Trypan Blue cell dye exclusion. TRPM7 gene expression levels were similar in both cell lines whereas TRPV1, TRPM2, TRPA1, TRPC5, TRPV4, and TRPM8 gene expression levels were downregulated in the etoposide-resistant WERI-Rb1 cells. In the presence of extracellular Ca2+, 1 mM Asc induced larger intracellular Ca2+ transients in the etoposide-resistant WERI-Rb1 than in their etoposide-sensitive counterpart. With either 100 µM CPZ, 500 µM La3+, 10 mM NAC, or 100 µM 2-APB, these Ca2+ transients were markedly diminished. These inhibitors also had corresponding inhibitory effects on Asc-induced rises in whole-cell currents. Pertussis toxin (PTX) preincubation blocked rises in Ca2+ influx. Microscopic analyses showed that after 4 days of exposure to 1 mM Asc cell viability fell by nearly 100% in both RB cell lines. Taken together, one of the effects underlying oxidative mediated Asc-induced WERI-Rb1 cytotoxicity stems from its promotion of Gi/o coupled GPCR mediated increases in intracellular Ca2+ influx through TRP channels. Therefore, designing drugs targeting TRP channel modulation may be a viable approach to increase the efficacy of chemotherapeutic treatment of RB. Furthermore, Asc may be indicated as a possible supportive agent in anti-cancer therapies.

Drug Repurposing for Retinoblastoma: Recent Advances

Retinoblastoma is the intraocular malignancy that occurs during early childhood. The current standard of care includes chemotherapy followed by focal consolidative therapies, and enucleation. Unfortunately, these are associated with many side and late effects. New drugs and/or drug combinations need to be developed for safe and effective treatment. This compelling need stimulated efforts to explore drug repurposing for retinoblastoma. While conventional drug development is a lengthy and expensive process, drug repurposing is a faster, alternate approach, where an existing drug, not meant for treating cancer, can be repurposed to treat retinoblastoma. The present article reviews various attempts to test drugs approved for different purposes such as calcium channels blockers, non-steroidal antiinflammatory drugs, cardenolides, antidiabetic, antibiotics and antimalarial for treating retinoblastoma. It also discusses other promising candidates that could be explored for repurposing for retinoblastoma.

Ocular transient receptor potential channel function in health and disease

Transient receptor potential (TRP) channels sense and transduce environmental stimuli into Ca(2+) transients that in turn induce responses essential for cell function and adaptation. These non-selective channels with variable Ca(2+) selectivity are grouped into seven different subfamilies containing 28 subtypes based on differences in amino acid sequence homology. Many of these subtypes are expressed in the eye on both neuronal and non-neuronal cells where they affect a host of stress-induced regulatory responses essential for normal vision maintenance. This article reviews our current knowledge about the expression, function and regulation of TRPs in different eye tissues. We also describe how under certain conditions TRP activation can induce responses that are maladaptive to ocular function. Furthermore, the possibility of an association between TRP mutations and disease is considered. These findings contribute to evidence suggesting that drug targeting TRP channels may be of therapeutic benefit in a clinical setting. We point out issues that must be more extensively addressed before it will be possible to decide with certainty that this is a realistic endeavor. Another possible upshot of future studies is that disease process progression can be better evaluated by profiling changes in tissue specific functional TRP subtype activity as well as their gene and protein expression.

Targeting ion channels for cancer therapy by repurposing the approved drugs

Ion channels have been shown to be involved in oncogenesis and efforts are being poured in to target the ion channels. There are many clinically approved drugs with ion channels as "off" targets. The question is, can these drugs be repurposed to inhibit ion channels for cancer treatment? Repurposing of drugs will not only save investors' money but also result in safer drugs for cancer patients. Advanced bioinformatics techniques and availability of a plethora of open access data on FDA approved drugs for various indications and omics data of large number of cancer types give a ray of hope to look for possibility of repurposing those drugs for cancer treatment. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Functional Expression of P2Y Receptors in WERI-Rb1 Retinoblastoma Cells

P2Y receptors are metabotropic G-protein-coupled receptors, which are involved in many important biologic functions in the central nervous system including retina. Subtypes of P2Y receptors in retinal tissue vary according to the species and the cell types. We examined the molecular and pharmacologic profiles of P2Y purinoceptors in retinoblastoma cell, which has not been identified yet. To achieve this goal, we used Ca(2+) imaging technique and western blot analysis in WERI-Rb-1 cell, a human retinoblastoma cell line. ATP (10 µM) elicited strong but transient [Ca(2+)](i) increase in a concentration-dependent manner from more than 80% of the WERI-Rb-1 cells (n=46). Orders of potency of P2Y agonists in evoking [Ca(2+)](i) transients were 2MeS-ATP>ATP>>UTP=αβ-MeATP, which was compatible with the subclass of P2Y(1) receptor. The [Ca(2+)](i) transients evoked by applications of 2MeS-ATP and/or ATP were also profoundly suppressed in the presence of P2Y(1) selective blocker (MRS 2179; 30 µM). P2Y(1) receptor expression in WERI-Rb-1 cells was also identified by using western blot. Taken together, P2Y(1) receptor is mainly expressed in a retinoblastoma cell, which elicits Ca(2+) release from internal Ca(2+) storage sites via the phospholipase C-mediated pathway. P2Y(1) receptor activation in retinoblastoma cell could be a useful model to investigate the role of purinergic [Ca(2+)](i) signaling in neural tissue as well as to find a novel therapeutic target to this lethal cancer.

Calcium mobilization by activation of M(3)/M(5) muscarinic receptors in the human retinoblastoma

Activation of muscarinic acetylcholine receptors (mAChR) is one of the most important signal transduction pathways in the human body. In this study, we investigated the role of mAChR activation in relation to its subtypes in human retinoblastoma cell-lines (WERI-Rb-1) using Ca(2+) measurement, real-time PCR, and Western Blot techniques. Acetylcholine (ACh) produced prominent [Ca(2+)](i) transients in a repeated manner in WERI-Rb-1 cells. The maximal amplitude of the [Ca(2+)](i) transient was almost completely suppressed by 97.3 +/- 0.8% after atropine (1 microM) pretreatment. Similar suppressions were noted after pretreatments with thapsigargin (1 microM), an ER Ca(2+)-ATPase (SERCA) inhibitor, whereas the ACh-induced [Ca(2+)](i) transient was not affected even in the absence of extracellular calcium. U-73122 (1 microM), a PLC inhibitor, and xestospongin C (2 microM), an IP(3)-receptor antagonist, elicited 11.5 +/- 2.9% and 17.8 +/- 1.9% suppressions, respectively. The 50% inhibitory concentration of (IC(50)) values for blockade of a 100 microM ACh response by pirenzepine and 4-DAMP were 315.8 and 9.1 nM, respectively. Moreover, both M(3) and M(5) mAChRs were prominent in quantitative real-time-PCR. Taken together, the M(3)/M(5) subtypes appear to be the major contributor, leading to intracellular calcium mobilization from the internal store via an IP(3)-dependent pathway in the undifferentiated retinoblastoma cells.

Cav3.1 splice variant expression during neuronal differentiation of Y-79 retinoblastoma cells

A decrease in transient-type calcium channel current, Ca(v)3.1 protein and the mRNA encoding these channels has been reported during differentiation of human retinoblastoma cells. In this study, we examined splice variants of Ca(v)3.1 before and after neuronal differentiation of the Y-79 retinoblastoma cell line to investigate the potential contribution of Ca(v)3.1 to Y-79 differentiation. In Ca(v)3.1, alternative splicing induces variations in three cytoplasmic regions, e.g. the link between domains II and III (producing isoforms e+ and e-), the link between domains III and IV (producing isoforms a, b, ac and bc) and the carboxy terminal region (producing isoforms f and d). Our results demonstrate that Ca(v)3.1e was not expressed in either undifferentiated or differentiated retinoblastoma cells. Splice variants Ca(v)3.1ac; Ca(v)3.1bc and Ca(v)3.1b were all identified in undifferentiated retinoblastoma cells, while expression of these variants in differentiated cells was restricted to the Ca(v)3.1bc isoform. The carboxy terminal variant Ca(v)3.1f is expressed independently of the differentiation status of retinoblastoma cells with or without Ca(v)3.1d. Examination of the functional contribution of Ca(v)3.1 protein to Y-79 cell differentiation revealed that in Y-79 cells transfected with Ca(v)3.1 antisense oligodeoxynucleotides, knockdown of Ca(v)3.1 did not alter the time-course of differentiation or neuritogenesis. The changes in Ca(v)3.1 splice variants were not required for the initiation of differentiation but may be associated with tissue-specific expression or localized alterations in Ca(2+) signaling that are essential for establishment of the mature differentiated phenotype.

Variation of T-type calcium channel protein expression affects cell division of cultured tumor cells

In this study we investigated the T-type calcium channel and its involvement in the cell division of U87MG cultured glioma cells and N1E-115 neuroblastoma cells. Using Western blot analysis, we found that expression of both alpha1G and alpha1H subunits of the T-type calcium channel decreased during conditions associated with a decrease in proliferation as evidenced by increased expression of cyclin D1, a marker for non-proliferating cells. Both serum starvation and application of mibefradil, a selective T-type calcium channel antagonist, resulted in a 50% decrease in the expression of alpha1G and alpha1H and a 700-900% increase in levels of cyclin D1 in U87MG and N1E-115 cells, respectively. Furthermore, overexpression of the alpha1H subunit resulted in a two-fold increase in cell proliferation compared to control cultures or cultures receiving an empty vector. In contrast, blocking expression of the alpha1G subunit using antisense oligonucleotides lead to a 70% decrease in proliferation of U87MG and N1E-115 cells compared to control cultures or cultures receiving a scrambled oligonucleotide. Our findings suggest that proliferation of U87MG glioma cells and N1E-115 is regulated by T-type calcium channel expression.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

The Ca(2+) channel antagonists mibefradil and pimozide inhibit cell growth via different cytotoxic mechanisms

We show that mitogenic cells expressing T-type Ca(2+) channels (T-channels) are more sensitive to the antiproliferative effects of the drugs pimozide and mibefradil than cells without significant T-channel expression. The growth of Y79 and WERI-Rb1 retinoblastoma cells, as well as MCF7 breast cancer epithelial cells, all of which express T-channel current and mRNA for T-channel subunits, is inhibited by pimozide and mibefradil with IC(50) values between 0.6 and 1.5 microM. Proliferation of glioma C6 cells, which show little T-channel expression, is less sensitive to these drugs (IC(50) = 8 and 5 microM for pimozide and mibefradil, respectively). Neither drug seems to alter cell cycle or the expression of cyclins. Although this strong correlation between T-channel expression and growth inhibition exists, the following results suggest that the drugs inhibit cell growth via different cytotoxic pathways: 1) pimozide and mibefradil have additive effects on T-channel current inhibition, whereas the antiproliferative activity of the drugs together is synergistic; 2) an increase in the number of apoptotic Y79 and MCF7 cells and a decrease in the mRNA for the antiapoptotic gene Bcl-2 is detected only in pimozide-treated cells, whereas in mibefradil-treated cells, the toxicity is primarily necrotic; and 3) growth inhibition by mibefradil is reduced in Y79 cells transfected with T-channel antisense and in differentiated Y79 cells (which have decreased T-channel expression), but growth inhibition by pimozide is affected to a lesser extent. These results suggest that pimozide and mibefradil inhibit cell proliferation via different cytotoxic pathways and that in the case of pimozide, it is unlikely that this effect is mediated solely by T-channel inhibition.

T-Type calcium channel alpha1G and alpha1H subunits in human retinoblastoma cells and their loss after differentiation

Human retinoblastoma cells are multipotent retinal precursor cells capable of differentiating into photoreceptors, neurons, and glia. The current-voltage relation of the undifferentiated cells is dominated by a transient inward current that disappears shortly after differentiation. In 20 mM Ba(2+)-containing bath solutions, the current has an activation midpoint near -25 mV and appears to be fully inactivated at -20 mV. Sr(2+) and Ca(2+) are preferred charge carriers relative to Ba(2+), and the current vanishes in the absence of these divalent cations. Cd(2+) blocks the current with an IC(50) of 160 microM, and Ni(2+) blocks in a biphasic manner with IC(50)s of 22 and 352 microM. The current is unaffected when sodium is replaced with other monovalent cations, and it is insensitive to nifedipine, omega-conotoxin GVIA, omega-agatoxin IVA, and omega-conotoxin MVIIC. RT-PCR revealed the presence of alpha 1G and alpha 1H mRNA in undifferentiated cells, but following differentiation, a striking reduction of both alpha 1G and alpha 1H mRNA was found, and this was paralleled by the loss of T-type Ca channel currents. alpha 1I subunit mRNA levels were low in undifferentiated and differentiated cells. These results suggest that T-type Ca channels could play a role in undifferentiated retinoblastoma cell physiology since alpha 1G and alpha 1H Ca channel subunit expression is reduced in cells that have differentiated and exited the cell cycle.