Calcium regulation by temperature-sensitive transient receptor potential channels in human uveal melanoma cells

Thermosensitive TRP Channels Are Functionally Expressed and Influence the Lipogenesis in Human Meibomian Gland Cells

While the involvement of thermosensitive transient receptor potential channels (TRPs) in dry eye disease (DED) has been known for years, their expression in the meibomian gland (MG) has never been investigated. This study aims to show their expression and involvement in the lipogenesis of the MG, providing a possible new drug target in the treatment of DED. Our RT-PCR, Western blot and immunofluorescence analysis showed the expression of TRPV1, TRPV3, TRPV4 and TRPM8 in the MG at the gene and the protein level. RT-PCR also showed gene expression of TRPV2 but not TRPA1. Calcium imaging and planar patch-clamping performed on an immortalized human meibomian gland epithelial cell line (hMGECs) demonstrated increasing whole-cell currents after the application of capsaicin (TRPV1) or icilin (TRPM8). Decreasing whole-cell currents could be registered after the application of AMG9810 (TRPV1) or AMTB (TRPM8). Oil red O staining on hMGECs showed an increase in lipid expression after TRPV1 activation and a decrease after TRPM8 activation. We conclude that thermo-TRPs are expressed at the gene and the protein level in MGs. Moreover, TRPV1 and TRPM8's functional expression and their contribution to their lipid expression could be demonstrated. Therefore, TRPs are potential drug targets and their clinical relevance in the therapy of meibomian gland dysfunction requires further investigation.

Joint CB1 and NGF Receptor Activation Suppresses TRPM8 Activation in Etoposide-Resistant Retinoblastoma Cells

In childhood, retinoblastoma (RB) is the most common primary tumor in the eye. Long term therapeutic management with etoposide of this life-threatening condition may have diminishing effectiveness since RB cells can develop cytostatic resistance to this drug. To determine whether changes in receptor-mediated control of Ca2+ signaling are associated with resistance development, fluorescence calcium imaging, semi-quantitative RT-qPCR analyses, and trypan blue dye exclusion staining patterns are compared in WERI-ETOR (etoposide-insensitive) and WERI-Rb1 (etoposide-sensitive) cells. The cannabinoid receptor agonist 1 (CNR1) WIN55,212-2 (40 µM), or the transient receptor potential melastatin 8 (TRPM8) agonist icilin (40 µM) elicit similar large Ca2+ transients in both cell line types. On the other hand, NGF (100 ng/mL) induces larger rises in WERI-ETOR cells than in WERI-Rb1 cells, and its lethality is larger in WERI-Rb1 cells than in WERI-ETOR cells. NGF and WIN55,212-2 induced additive Ca2+ transients in both cell types. However, following pretreatment with both NGF and WIN55,212-2, TRPM8 gene expression declines and icilin-induced Ca2+ transients are completely blocked only in WERI-ETOR cells. Furthermore, CNR1 gene expression levels are larger in WERI-ETOR cells than those in WERI-Rb1 cells. Therefore, the development of etoposide insensitivity may be associated with rises in CNR1 gene expression, which in turn suppress TRPM8 gene expression through crosstalk.

Transient Receptor Potential Ankyrin 1 (TRPA1) Channel as a Sensor of Oxidative Stress in Cancer Cells

Moderate levels of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), fuel tumor metastasis and invasion in a variety of cancer types. Conversely, excessive ROS levels can impair tumor growth and metastasis by triggering cancer cell death. In order to cope with the oxidative stress imposed by the tumor microenvironment, malignant cells exploit a sophisticated network of antioxidant defense mechanisms. Targeting the antioxidant capacity of cancer cells or enhancing their sensitivity to ROS-dependent cell death represent a promising strategy for alternative anticancer treatments. Transient Receptor Potential Ankyrin 1 (TRPA1) is a redox-sensitive non-selective cation channel that mediates extracellular Ca²⁺ entry upon an increase in intracellular ROS levels. The ensuing increase in intracellular Ca²⁺ concentration can in turn engage a non-canonical antioxidant defense program or induce mitochondrial Ca²⁺ dysfunction and apoptotic cell death depending on the cancer type. Herein, we sought to describe the opposing effects of ROS-dependent TRPA1 activation on cancer cell fate and propose the pharmacological manipulation of TRPA1 as an alternative therapeutic strategy to enhance cancer cell sensitivity to oxidative stress.

Therapeutic potential of TRPM8 channels in cancer treatment

Cancer is a multifactorial process associated with changes in signaling pathways leading to cell cycle variations and gene expression. The transient receptor potential melastatin 8 (TRPM8) channel is a non-selective cation channel expressed in neuronal and non-neuronal tissues, where it is involved in several processes, including thermosensation, differentiation, and migration. Cancer is a multifactorial process associated with changes in signaling pathways leading to variations in cell cycle and gene expression. Interestingly, it has been shown that TRPM8 channels also participate in physiological processes related to cancer, such as proliferation, survival, and invasion. For instance, TRPM8 channels have an important role in the diagnosis, prognosis, and treatment of prostate cancer. In addition, it has been reported that TRPM8 channels are involved in the progress of pancreatic, breast, bladder, colon, gastric, and skin cancers, glioblastoma, and neuroblastoma. In this review, we summarize the current knowledge on the role of TRPM8 channels in cancer progression. We also discuss the therapeutic potential of TRPM8 in carcinogenesis, which has been proposed as a molecular target for cancer therapy.

The TRPA1 Channel Amplifies the Oxidative Stress Signal in Melanoma

Macrophages (MΦs) and reactive oxygen species (ROS) are implicated in carcinogenesis. The oxidative stress sensor, transient receptor potential ankyrin 1 (TRPA1), activated by ROS, appears to contribute to lung and breast cancer progression. Although TRPA1 expression has been reported in melanoma cell lines, and oxidative stress has been associated with melanocytic transformation, their role in melanoma remains poorly known. Here, we localized MΦs, the final end-product of oxidative stress, 4-hydroxynonenal (4-HNE), and TRPA1 in tissue samples of human common dermal melanocytic nevi, dysplastic nevi, and thin (pT1) and thick (pT4) cutaneous melanomas. The number (amount) of intratumoral and peritumoral M2 MΦs and 4-HNE staining progressively increased with tumor severity, while TRPA1 expression was similar in all samples. Hydrogen peroxide (H₂O₂) evoked a TRPA1-dependent calcium response in two distinct melanoma cell lines (SK-MEL-28 and WM266-4). Furthermore, H₂O₂ induced a TRPA1-dependent H₂O₂ release that was prevented by the TRPA1 antagonist, A967079, or Trpa1 gene silencing (siRNA). ROS release from infiltrating M2 MΦs may target TRPA1-expressing melanoma cells to amplify the oxidative stress signal that affects tumor cell survival and proliferation.

TRPM8 Channel Promotes the Osteogenic Differentiation in Human Bone Marrow Mesenchymal Stem Cells

Various families of ion channels have been characterized in mesenchymal stem cells (MSCs), including some members of transient receptor potential (TRP) channels family. TRP channels are involved in critical cellular processes as differentiation and cell proliferation. Here, we analyzed the expression of TRPM8 channel in human bone marrow MSCs (hBM-MSCs), and its relation with osteogenic differentiation. Patch-clamp recordings showed that hBM-MSCs expressed outwardly rectifying currents which were increased by exposure to 500 μM menthol and were partially inhibited by 10 μM of BCTC, a TRPM8 channels antagonist. Additionally, we have found the expression of TRPM8 by RT-PCR and western blot. We also explored the TRPM8 localization in hBM-MSCs by immunofluorescence using confocal microscopy. Remarkably, hBM-MSCs treatment with 100 μM of menthol or 10 μM of icilin, TRPM8 agonists, increases osteogenic differentiation. Conversely, 20 μM of BCTC, induced a decrease of osteogenic differentiation. These results suggest that TRPM8 channels are functionally active in hBM-MSCs and have a role in cell differentiation.

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.

An 11-gene-based prognostic signature for uveal melanoma metastasis based on gene expression and DNA methylation profile

Uveal melanoma (UM) is the most common intraocular tumor worldwide. We proposed to identify a vital gene signature that has prognostic value for UM metastasis. For this purpose, we obtained a published DNA methylation and gene expression data set associated with UM from the Gene Expression Omnibus. The genes whose aberrant expression significantly associated with UM patients' metastasis-free survival (MFS) were identified by applying a univariate Cox proportional hazards model to the gene expression data set followed by a robust likelihood-based survival analysis to screen the optimal prognostic gene signatures (PGS). A formula for calculating the risk score that represents UM metastasis risk was constructed by including the PGSs' expression values weighted by their regression coefficients, which were obtained by a multivariate Cox regression analysis. As a result, aberrant expression of 2884 genes were found to be significantly associated with UM patients' MFS, which were referred to as MFSGs, and 11 out of those MFSGs, GJC1, TCEA1, MFSD3, FAF2, TLCD1, GPAA1, CYC1, ASAP1, JPH1, LDB3, and KDELR3, were identified as PGSs through which we could accurately separate UM samples with shorter MFS from those with longer MFS. By combining the DNA methylation data set and MFSGs, we further identified 265 MFSGs, which contained CpG sites that significantly hyper- or hypo-methylated in UM samples compared with control samples. Functional enrichment analysis and pathway crosstalk analysis of those genes indicated significant enrichment of cancer-related pathways. In conclusion, we identified an 11-gene-based prognostic signature and several gene biomarkers for UM metastasis, which should be helpful for selecting an appropriate treatment method for specific patients with UM.

Mechanism for Regulation of Melanoma Cell Death via Activation of Thermo-TRPV4 and TRPV2

Background

Thermo-TRPs (temperature-sensitive transient receptor potential channels) belong to the TRP (transient receptor potential) channel superfamily. Emerging evidence implied that thermo-TRPs have been involved in regulation of cell fate in certain tumors. However, their distribution profiles and roles in melanoma remain incompletely understood.

Methods

Western blot and digital PCR approaches were performed to identify the distribution profiles of six thermo-TRPs. MTT assessment was employed to detect cell viability. Flow cytometry was applied to test cell cycle and apoptosis. Calcium imaging was used to determine the function of channels. Five cell lines, including one normal human primary epidermal melanocytes and two human malignant melanoma (A375, G361) and two human metastatic melanoma (A2058, SK-MEL-3) cell lines, were chosen for this research.

Results

In the present study, six thermo-TRPs including TRPV1/2/3/4, TRPA1, and TRPM8 were examined in human primary melanocytes and melanoma cells. We found that TRPV2/4, TRPA1, and TRPM8 exhibited ectopic distribution both in melanocytes and melanoma cells. Moreover, activation of TRPV2 and TRPV4 could lead to the decline of cell viability for melanoma A2058 and A375 cells. Subsequently, activation of TRPV2 by 2-APB (IC₅₀ = 150 μM) induced cell necrosis in A2058 cells, while activation of TRPV4 by GSK1016790A (IC₅₀ = 10 nM) enhanced apoptosis of A375 cells. Furthermore, TRPV4 mediated cell apoptosis of melanoma via phosphorylation of AKT and was involved in calcium regulation.

Conclusion

Overall, our studies revealed that TRPV4 and TRPV2 mediated melanoma cell death via channel activation and characterized the mechanism of functional TRPV4 ion channel in regulating AKT pathway driven antitumor process. Thus, they may serve as potential biomarkers for the prognosis and are targeted for the therapeutic use in human melanoma.

TRPM8 Activation via 3-Iodothyronamine Blunts VEGF-Induced Transactivation of TRPV1 in Human Uveal Melanoma Cells

In human uveal melanoma (UM), tumor enlargement is associated with increases in aqueous humor vascular endothelial growth factor-A (VEGF-A) content that induce neovascularization. 3-Iodothyronamine (3-T₁AM), an endogenous thyroid hormone metabolite, activates TRP melastatin 8 (TRPM8), which blunts TRP vanilloid 1 (TRPV1) activation by capsaicin (CAP) in human corneal, conjunctival epithelial cells, and stromal cells. We compare here the effects of TRPM8 activation on VEGF-induced transactivation of TRPV1 in an UM cell line (92.1) with those in normal primary porcine melanocytes (PM) since TRPM8 is upregulated in melanoma. Fluorescence Ca²⁺-imaging and planar patch-clamping characterized functional channel activities. CAP (20 μM) induced Ca²⁺ transients and increased whole-cell currents in both the UM cell line and PM whereas TRPM8 agonists, 100 μM menthol and 20 μM icilin, blunted such responses in the UM cells. VEGF (10 ng/ml) elicited Ca²⁺ transients and augmented whole-cell currents, which were blocked by capsazepine (CPZ; 20 μM) but not by a highly selective TRPM8 blocker, AMTB (20 μM). The VEGF-induced current increases were not augmented by CAP. Both 3-T₁AM (1 μM) and menthol (100 μM) increased the whole-cell currents, whereas 20 μM AMTB blocked them. 3-T₁AM exposure suppressed both VEGF-induced Ca²⁺ transients and increases in underlying whole-cell currents. Taken together, functional TRPM8 upregulation in UM 92.1 cells suggests that TRPM8 is a potential drug target for suppressing VEGF induced increases in neovascularization and UM tumor growth since TRPM8 activation blocked VEGF transactivation of TRPV1.

Intracellular aggregated TRPV1 is associated with lower survival in breast cancer patients

Background

Breast cancer is a malignant disease that represents an important public health burden. The description of new molecular markers can be important to diagnosis, classification, and treatment. Transient receptor potential vanilloid 1 (TRPV1) polymodal channel is expressed in different neoplastic tissues and cell lines of breast cancer and associated with the regulation of tumor growth, tumor neurogenesis, cancer pain, and malignant progression of cancer. In primary and metastatic breast cancer tumors, TRPV1 is expressed during neoplastic transformation, invasive behavior, and resistance to cytotoxic therapy.

Objective

The objective of this study was to describe the subcellular distribution of TRPV1 in invasive breast carcinomas and its association with survival.

Methods

In 33 cases of invasive breast carcinomas, we identified immunohistochemical and immunofluorescent expression patterns of TRPV1 compared to healthy breast tissue. We characterized the expression of TRPV1 induced by estrogens in breast cancer cell lines MCF-7 and MDA to establish a model of the TRPV1-estrogen relationship regarding the malignant potential. We examined the association of TRPV1 patterns with patients' survival with the Kaplan-Meyer model, using the log-rank test at 5 years of follow-up. The relation of TRPV1 expression patterns to the St. Gallen breast cancer subtypes was also tested.

Results

Based on immunohistochemical expression pattern of TRPV1, we distinguished two main categories of breast cancer tissue, a "classical category" that exhibited diffuse expression of the channel and a "non-classical category" that expressed the channel in aggregates at the ER/Golgi and/or surrounding these structures. The classical pattern of TRPV1 was associated with a higher survival rate. In breast cancer cell lines, increasing doses of estrogens induced increased TRPV1 expression with nonclassical patterns at higher doses via a mechanism dependent on ER α.

Conclusion

The expression and distribution of TRPV1 in invasive breast carcinomas may be considered as a biomarker for prognosis of the disease and a probable therapeutic target.

3-Iodothyronamine Activates a Set of Membrane Proteins in Murine Hypothalamic Cell Lines

3-Iodothyronamine (3-T1AM) is an endogenous thyroid hormone metabolite. The profound pharmacological effects of 3-T1AM on energy metabolism and thermal homeostasis have raised interest to elucidate its signaling properties in tissues that pertain to metabolic regulation and thermogenesis. Previous studies identified G protein-coupled receptors (GPCRs) and transient receptor potential channels (TRPs) as targets of 3-T1AM in different cell types. These two superfamilies of membrane proteins are largely expressed in tissue which influences energy balance and metabolism. As the first indication that 3-T1AM virtually modulates the function of the neurons in hypothalamus, we observed that intraperitoneal administration of 50 mg/kg bodyweight of 3-T1AM significantly increased the c-FOS activation in the paraventricular nucleus (PVN) of C57BL/6 mice. To elucidate the underlying mechanism behind this 3-T1AM-induced signalosome, we used three different murine hypothalamic cell lines, which are all known to express PVN markers, GT1-7, mHypoE-N39 (N39) and mHypoE-N41 (N41). Various aminergic GPCRs, which are the known targets of 3-T1AM, as well as numerous members of TRP channel superfamily, are expressed in these cell lines. Effects of 3-T1AM on activation of GPCRs were tested for the two major signaling pathways, the action of Gαs/adenylyl cyclase and Gi/o. Here, we demonstrated that this thyroid hormone metabolite has no significant effect on Gi/o signaling and only a minor effect on the Gαs/adenylyl cyclase pathway, despite the expression of known GPCR targets of 3-T1AM. Next, to test for other potential mechanisms involved in 3-T1AM-induced c-FOS activation in PVN, we evaluated the effect of 3-T1AM on the intracellular Ca2+ concentration and whole-cell currents. The fluorescence-optic measurements showed a significant increase of intracellular Ca2+ concentration in the three cell lines in the presence of 10 μM 3-T1AM. Furthermore, this thyroid hormone metabolite led to an increase of whole-cell currents in N41 cells. Interestingly, the TRPM8 selective inhibitor (10 μM AMTB) reduced the 3-T1AM stimulatory effects on cytosolic Ca2+ and whole-cell currents. Our results suggest that the profound pharmacological effects of 3-T1AM on selected brain nuclei of murine hypothalamus, which are known to be involved in energy metabolism and thermoregulation, might be partially attributable to TRP channel activation in hypothalamic cells.

Mitochondrial Ca2+ removal amplifies TRAIL cytotoxicity toward apoptosis-resistant tumor cells via promotion of multiple cell death modalities

Ca2+ has emerged as a new target for cancer treatment since tumor-specific traits in Ca2+ dynamics contributes to tumorigenesis, malignant phenotypes, drug resistance, and survival in different tumor types. However, Ca2+ has a dual (pro-death and pro-survival) function in tumor cells depending on the experimental conditions. Therefore, it is necessary to minimize the onset of the pro-survival Ca2+ signals caused by the therapy. For this purpose, a better understanding of pro-survival Ca2+ pathways in cancer cells is critical. Here we report that Ca2+ protects malignant melanoma (MM) and osteosarcoma (OS) cells from tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) cytotoxicity. Simultaneous measurements using the site-specific Ca2+ probes showed that acute TRAIL treatment rapidly and dose-dependently increased the cytosolic Ca2+ concentration ([Ca2+]cyt) and mitochondrial Ca2+ concentration ([Ca2+]mit) Pharmacological analyses revealed that the [Ca2+]mit remodeling was under control of mitochondrial Ca2+ uniporter (MCU), mitochondrial permeability transition pore (MPTP), and a Ca2+ transport pathway sensitive to capsazepine and AMG9810. Ca2+ chelators and the MCU inhibitor ruthenium 360, an MPTP opener atractyloside, capsazepine, and AMG9810 all decreased [Ca2+]mit and sensitized these tumor cells to TRAIL cytotoxicity. The Ca2+ modulation enhanced both apoptotic and non-apoptotic cell death. Although the [Ca2+]mit reduction potentiated TRAIL-induced caspase-3/7 activation and cell membrane damage within 24 h, this potentiation of cell death became pronounced at 72 h, and not blocked by caspase inhibition. Our findings suggest that in MM and OS cells mitochondrial Ca2+ removal can promote apoptosis and non-apoptotic cell death induction by TRAIL. Therefore, mitochondrial Ca2+ removal can be exploited to overcome the resistance of these cancers to TRAIL.

TRP Channels in Skin Biology and Pathophysiology

Ion channels of the Transient Receptor Potential (TRP) family mediate the influx of monovalent and/or divalent cations into cells in response to a host of chemical or physical stimuli. In the skin, TRP channels are expressed in many cell types, including keratinocytes, sensory neurons, melanocytes, and immune/inflammatory cells. Within these diverse cell types, TRP channels participate in physiological processes ranging from sensation to skin homeostasis. In addition, there is a growing body of evidence implicating abnormal TRP channel function, as a product of excessive or deficient channel activity, in pathological skin conditions such as chronic pain and itch, dermatitis, vitiligo, alopecia, wound healing, skin carcinogenesis, and skin barrier compromise. These diverse functions, coupled with the fact that many TRP channels possess pharmacologically accessible sites, make this family of proteins appealing therapeutic targets for skin disorders.

Calcium Entry Through Thermosensory Channels

ThermoTRPs are unique channels that mediate Na(+) and Ca(2+) currents in response to changes in ambient temperature. In combination with their activation by other physical and chemical stimuli, they are considered key integrators of environmental cues into neuronal excitability. Furthermore, roles of thermoTRPs in non-neuronal tissues are currently emerging such as insulin secretion in pancreatic β-cells, and links to cancer. Calcium permeability through thermoTRPs appears a central hallmark for their physiological and pathological activities. Moreover, it is currently being proposed that beyond working as a second messenger, Ca(2+) can function locally by acting on protein complexes near the membrane. Interestingly, thermoTRPs can enhance and expand the inherent plasticity of signalplexes by conferring them temperature, pH and lipid regulation through Ca(2+) signalling. Thus, unveiling the local role of Ca(2+) fluxes induced by thermoTRPs on the dynamics of membrane-attached signalling complexes as well as their significance in cellular processes, are central issues that will expand the opportunities for therapeutic intervention in disorders involving dysfunction of thermoTRP channels.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

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.

3-Iodothyronamine increases transient receptor potential melastatin channel 8 (TRPM8) activity in immortalized human corneal epithelial cells

3-Iodothyronamine (3T1AM) is an endogenous thyroid hormone metabolite that interacts with the human trace amine-associated receptor 1 (hTAAR1), a G-protein-coupled receptor, to induce numerous physiological responses including dose-dependent body temperature lowering in rodents. 3T1AM also directly activates cold-sensitive transient receptor potential melastatin 8 (TRPM8) channels in human conjunctival epithelial cells (HCjEC) at constant temperature as well as reducing rises in IL-6 release induced by transient receptor potential vanilloid 1 (TRPV1) activation by capsaicin (CAP). Here, we describe that 3T1AM-induced TRPM8 activation suppresses through crosstalk TRPV1 activation in immortalized human corneal epithelial cells (HCEC). RT-PCR and immunofluorescent staining identified TRPM8 gene and protein expression. Increases in Ca(2+) influx induced by the TRPM8 agonists either 3T1AM (0.1-10 μM), menthol (500 μM), icilin (15-60 μM) or temperature lowering (either <17°C or >17°C) were all blocked by 10-20 μM BCTC, a mixed TRPV1/TRPM8 antagonist. BCTC blocked 3T1AM-induced recombinant TRPM8 activation of Ca(2+) transients in an osteosarcoma heterologous expression system. The effects of BCTC in HCEC were attributable to selective TRPM8 inhibition since whole-cell patch-clamp currents underlying Ca(2+) rises induced by 20 μM CAP were BCTC insensitive. On the other hand, Ca(2+) transients induced by activating TRPV1 with either CAP or a hyperosmolar medium were suppressed during exposure to either 1 μM 3T1AM or 15 μM icilin. All of these modulatory effects on intracellular Ca(2+) regulation induced by the aforementioned agents were attributable to changes in underlying inward and outward current. Taken together, TRPM8 activation by 3T1AM markedly attenuates and even eliminates hyperosmolar and CAP induced TRPV1 activation through crosstalk.

Roles of TRPM8 Ion Channels in Cancer: Proliferation, Survival, and Invasion

The goal of this article is to provide a critical review of the transient receptor potential melastatin-subfamily member 8 (TRPM8) in cancers, with an emphasis on its roles in cellular proliferation, survival, and invasion. The TRPM8 ion channels regulate Ca²⁺ homeostasis and function as a cellular sensor and transducer of cold temperature. Accumulating evidence has demonstrated that TRPM8 is aberrantly expressed in a variety of malignant solid tumors. Clinicopathological analysis has shown that over-expression of TRPM8 correlates with tumor progression. Experimental data have revealed important roles of TRPM8 channels in cancer cells proliferation, survival, and invasion, which appear to be dependent on the cancer type. Recent reports have begun to reveal the signaling mechanisms that mediate the biological roles of TRPM8 in tumor growth and metastasis. Determining the mechanistic roles of TRPM8 in cancer is expected to elucidate the impact of thermal and chemical stimuli on the formation and progression of neoplasms. Translational research and clinical investigation of TRPM8 in malignant diseases will help exploit these ion channels as molecular biomarkers and therapeutic targets for developing precision cancer medicine.

Redox-sensitive transient receptor potential channels in oxygen sensing and adaptation

Regulation of ion channels is central to the mechanisms that underlie immediate acute physiological responses to changes in the availability of molecular oxygen (O₂). A group of cation-permeable channels that are formed by transient receptor potential (TRP) proteins have been characterized as exquisite sensors of redox reactive species and as efficient actuators of electric/ionic signals in vivo. In this review, we first discuss how redox-sensitive TRP channels such as TRPA1 have recently emerged as sensors of the relatively inert oxidant O₂. With regard to the physiological significance of O₂ sensor TRP channels, vagal TRPA1 channels are mainly discussed with respect to their role in respiratory regulation in comparison with canonical pathways in glomus cells of the carotid body, which is a well-established O₂-sensing organ. TRPM7 channels are discussed regarding hypoxia-sensing function in ischemic cell death. Also, ubiquitous expression of TRPA1 and TRPM7 together with their physiological relevance in the body is examined. Finally, based upon these studies on TRP channels, we propose a hypothesis of “O₂ remodeling.” The hypothesis is that cells detect deviation of O₂ availability from appropriate levels via sensors and adjust local O₂ environments in vivo by controlling supply and consumption of O₂ via pathways comprising cellular signals and transcription factors downstream of sensors, which consequently optimize physiological functions. This new insight into O₂ adaptation through ion channels, particularly TRPs, may foster a paradigm shift in our understanding in the biological significance of O₂.

Polymodal roles of transient receptor potential channels in the control of ocular function

Maintenance of intracellular Ca²⁺ levels at orders of magnitude below those in the extracellular environment is a requisite for preserving cell viability. Membrane channels contribute to such control through modulating their time-dependent opening and closing behaviour. Such regulation requires Ca²⁺ to serve as a second messenger mediating receptor control of numerous life-sustaining responses. Transient receptor potential (TRP) channels signal transduce a wide variety of different sensory stimuli to induce responses modulating cellular function. These channels are non-selective cation channels with variable Ca²⁺ selectivity having extensive sequence homology. They constitute a superfamily made up of 28 different members that are subdivided into 7 different subfamilies based on differences in sequence homology. Some of these TRP channel isotypes are expressed in the eye and localized to both neuronal and non-neuronal cell membranes. Their activation generates intracellular Ca²⁺ transients and other downstream-linked signalling events that affect numerous responses required for visual function. As there is an association between changes in functional TRP expression in various ocular diseases, there are efforts underway to determine if these channels can be used as drug targets to reverse declines in ocular function. We review here our current knowledge about the expression, function and regulation of TRPs in different eye tissues in health and disease. Furthermore, some of the remaining hurdles are described to developing safe and efficacious TRP channel modulators for use in a clinical setting.

Thyronamine induces TRPM8 channel activation in human conjunctival epithelial cells

3-Iodothyronamine (T1AM), an endogenous thyroid hormone (TH) metabolite, induces numerous responses including a spontaneously reversible body temperature decline. As such an effect is associated in the eye with increases in basal tear flow and thermosensitive transient receptor potential melastatin 8 (TRPM8) channel activation, we determined in human conjunctival epithelial cells (IOBA-NHC) if T1AM also acts as a cooling agent to directly affect TRPM8 activation at a constant temperature. RT-PCR and quantitative real-time PCR (qPCR) along with immunocytochemistry probed for TRPM8 gene and protein expression whereas functional activity was evaluated by comparing the effects of T1AM with those of TRPM8 mediators on intracellular Ca(2+) ([Ca(2+)]i) and whole-cell currents. TRPM8 gene and protein expression was evident and icilin (20μM), a TRPM8 agonist, increased Ca(2+) influx as well as whole-cell currents whereas BCTC (10μM), a TRPM8 antagonist, suppressed these effects. Similarly, either temperature lowering below 23°C or T1AM (1μM) induced Ca(2+) transients that were blocked by this antagonist. TRPM8 activation by both 1µM T1AM and 20μM icilin prevented capsaicin (CAP) (20μM) from inducing increases in Ca(2+) influx through TRP vanilloid 1 (TRPV1) activation, whereas BCTC did not block this response. CAP (20μM) induced a 2.5-fold increase in IL-6 release whereas during exposure to 20μM capsazepine this rise was completely blocked. Similarly, T1AM (1μM) prevented this response. Taken together, T1AM like icilin is a cooling agent since they both directly elicit TRPM8 activation at a constant temperature. Moreover, there is an inverse association between changes in TRPM8 and TRPV1 activity since these cooling agents blocked both CAP-induced TRPV1 activation and downstream rises in IL-6 release.

Temperature-sensitive transient receptor potential channels in corneal tissue layers and cells

We here provide a brief summary of the characteristics of transient receptor potential channels (TRPs) identified in corneal tissue layers and cells. In general, TRPs are nonselective cation channels which are Ca(2+) permeable. Most TRPs serve as thermosensitive molecular sensors (thermo-TRPs). Based on their functional importance, the possibilities are described for drug-targeting TRP activity in a clinical setting. TRPs are expressed in various tissues of the eye including both human corneal epithelial and endothelial layers as well as stromal fibroblasts and stromal nerve fibers. TRP vanilloid type 1 (TRPV1) heat receptor, also known as capsaicin receptor, along with TRP melastatin type 8 (TRPM8) cold receptor, which is also known as menthol receptor, are prototypes of the thermo-TRP family. The TRPV1 functional channel is the most investigated TRP channel in these tissues, owing to its contribution to maintaining tissue homeostasis as well as eliciting wound healing responses to injury. Other thermo-TRP family members identified in these tissues are TRPV2, 3 and 4. Finally, there is the TRP ankyrin type 1 (TRPA1) cold receptor. All of these thermo-TRPs can be activated within specific temperature ranges and transduce such inputs into chemical and electrical signals. Although several recent studies have begun to unravel complex roles for thermo-TRPs such as TRPV1 in corneal layers and resident cells, additional studies are needed to further elucidate their roles in health and disease.