Tyrosine phosphatase PTP1B interacts with TRPV6 in vivo and plays a role in TRPV6-mediated calcium influx in HEK293 cells

Calcium selective channel TRPV6: Structure, function, and implications in health and disease

Calcium-selective channel TRPV6 (Transient Receptor Potential channel family, Vanilloid subfamily member 6) belongs to the TRP family of cation channels and plays critical roles in transcellular calcium (Ca2+) transport, reuptake of Ca2+ into cells, and maintaining a local low Ca2+ environment for certain biological processes. Recent crystal and cryo-electron microscopy-based structures of TRPV6 have revealed mechanistic insights on how the protein achieves Ca2+ selectivity, permeation, and inactivation by calmodulin. The TRPV6 protein is expressed in a range of epithelial tissues such as the intestine, kidney, placenta, epididymis, and exocrine glands such as the pancreas, prostate and salivary, sweat, and mammary glands. The TRPV6 gene is a direct transcriptional target of the active form of vitamin D and is efficiently regulated to meet the body's need for Ca2+ demand. In addition, TRPV6 is also regulated by the level of dietary Ca2+ and under physiological conditions such as pregnancy and lactation. Genetic models of loss of function in TRPV6 display hypercalciuria, decreased bone marrow density, deficient weight gain, reduced fertility, and in some cases alopecia. The models also reveal that the channel plays an indispensable role in maintaining maternal-fetal Ca2+ transport and low Ca2+ environment in the epididymal lumen that is critical for male fertility. Most recently, loss of function mutations in TRPV6 gene is linked to transient neonatal hyperparathyroidism and early onset chronic pancreatitis. TRPV6 is overexpressed in a wide range of human malignancies and its upregulation is strongly correlated to tumor aggressiveness, metastasis, and poor survival in selected cancers. This review summarizes the current state of knowledge on the expression, structure, biophysical properties, function, polymorphisms, and regulation of TRPV6. The aberrant expression, polymorphisms, and dysfunction of this protein linked to human diseases are also discussed.

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Transient Receptor Potential (TRP) channels in T cells

The transient receptor potential (TRP) family of ion channels is widely expressed in many cell types and plays various physiological roles. Growing evidence suggests that certain TRP channels are functionally expressed in the immune system. Indeed, an increasing number of reports have demonstrated the functional expression of several TRP channels in innate and adaptive immune cells and have highlighted their critical role in the activation and function of these cells. However, very few reviews have been entirely dedicated to this subject. Here, we will summarize the recent findings with regards to TRP channel expression in T cells and discuss their emerging role as regulators of T cell activation and functions. Moreover, these studies suggest that beyond their pharmaceutical interest in pain management, certain TRP channels may represent potential novel therapeutic targets for various immune-related diseases.

Novel role of reactive oxygen species-activated Trp melastatin channel-2 in mediating angiogenesis and postischemic neovascularization

OBJECTIVE

Transient receptor potential melastatin-2 (TRPM2) channel is a nonselective cation channel that mediates influx of Ca(2+) and Na(+) with relative permeability of PCa:PNa ≈0.6 in response to cellular oxidative stress. As angiogenesis and ischemic neovascularization are both significantly dependent on oxidant signaling, here we investigated the possible role of vascular endothelial growth factor (VEGF)-induced reactive oxygen species production in activating TRPM2-dependent Ca(2+) signaling and in the mechanism of angiogenesis and ischemic neovascularization.

APPROACH AND RESULTS

We observed that VEGF stimulation rapidly induced the association of TRPM2 and cellular Src kinase with vascular endothelial-cadherin forming a signalplex at vascular endothelial-cadherin junctions in endothelial cells. Using endothelial cells isolated from TRPM2(-/-) mice or after small interfering RNA depletion of TRPM2, we demonstrated that TRPM2-activated Ca(2+) signaling was required for cellular Src kinase-induced phosphorylation of vascular endothelial-cadherin at Y658 and Y731, the crucial sites involved in vascular endothelial-cadherin internalization in response to VEGF. VEGF-induced reactive oxygen species generation activated TRPM2-induced Ca(2+) entry, whereas the reactive oxygen species-insensitive TRPM2 mutant (C1008→A) showed impaired Ca(2+) entry. Endothelial cells depleted of TRPM2 also displayed significantly perturbed migratory phenotype and impaired activation of cellular Src in response to VEGF. TRPM2(-/-) mice reconstituted with wild-type myeloid cells demonstrated aberrant angiogenesis and neovascularization in the hindlimb ischemia model as compared with wild-type mice.

CONCLUSIONS

VEGF-induced angiogenesis and postischemic neovascularization in mice required reactive oxygen species generation in endothelial cells and resultant TRPM2 activation. Thus, our findings provide novel insight into the role of TRPM2 in mechanism of angiogenesis and ischemic neovascularization.

Novel calcium ion channels TRPV5 and TRPV6 and gastrointestinal tumors

Transient receptor potential cation channel, subfamily V, member 5 (TRPV5) and TRPV6 are the subfamily members of the transient receptor potential (TRP), representing new highly selective Ca2+ membrane transport channels, which are mainly responsible for active transport of Ca2+ across the cell membrane and participate in regulation of many physiological activities in the body. This paper discusses the structures and electrophysiological properties of TRPV5 and TRPV6, their related factors and their relationship with gastrointestinal tumors, highlighting the role of TRPV5 and TRPV6 in the formation of gastrointestinal tumors.

Thioredoxin-mediated redox regulation of resistance to endocrine therapy in breast cancer

Resistance to endocrine therapy in breast carcinogenesis due to the redox regulation of the signal transduction system by reactive oxygen species (ROS) is the subject of this review article. Both antiestrogens and aromatase inhibitors are thought to prevent cancer through modulating the estrogen receptor function, but other mechanisms cannot be ruled out as these compounds also block metabolism and redox cycling of estrogen and are free radical scavengers. Endocrine therapeutic agents, such as, tamoxifen and other antiestrogens, and the aromatase inhibitor, exemestane, are capable of producing ROS. Aggressive breast cancer cells have high oxidative stress and chronic treatment with exemestane, fulvestrant or tamoxifen may add additional ROS stress. Breast cancer cells receiving long-term antiestrogen treatment appear to adapt to this increased persistent level of ROS. This, in turn, may lead to the disruption of reversible redox signaling that involves redox-sensitive phosphatases, protein kinases, such as, ERK and AKT, and transcription factors, such as, AP-1, NRF-1 and NF-κB. Thioredoxin modulates the expression of estrogen responsive genes through modulating the production of H2O2 in breast cancer cells. Overexpressing thioredoxine reductase 2 and reducing oxidized thioredoxin restores tamoxifen sensitivity to previously resistant breast cancer cells. In summary, it appears that resistance to endocrine therapy may be mediated, in part, by ROS-mediated dysregulation of both estrogen-dependent and estrogen-independent redox-sensitive signaling pathways. Further studies are needed to define the mechanism of action of thioredoxin modifiers, and their effect on the redox regulation that contributes to restoring the antiestrogen-mediated signal transduction system and growth inhibitory action.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Store-operated Ca2+ entry in hippocampal neurons: Regulation by protein tyrosine phosphatase PTP1B

Store operated Ca(2+) entry (SOCE) replenishes intracellular Ca(2+) stores and activates a number of intracellular signalling pathways. Whilst several molecular components forming store operated Ca(2+) channels (SOCC) have been identified, their modulation in neurons remains poorly understood. Here, we extend on our previous findings and show that neuronal SOCE is modulated by tyrosine phosphorylation. Cyclopiazonic acid induced SOCE was characterised in hippocampal cultures derived from forebrain specific protein tyrosine phosphatase 1B knockout (PTP1B KO) mice and wild type (WT) litter mates using Fura-2 Ca(2+) imaging. PTP1B KO cultures expressed elevated SOCE relative to WT cultures without changes in cytoplasmic Ca(2+) homeostasis or depolarisation-induced Ca(2+) influx. WT and PTP1B KO cultures displayed similar pharmacological sensitivities towards the SOCE inhibitors gadolinium and 2-aminoethoxydiphenyl borate, as well as the tyrosine kinase inhibitor Ag126 indicating an augmentation of native SOCCs by PTP1B. Following store depletion WT culture homogenates showed heightened phospho-tyrosine levels, an increase in Src tyrosine kinase activation and two minor PTP1B species. These data suggest tyrosine phosphorylation gating SOCE, and implicate PTP1B as a key regulatory enzyme. The involvement of PTP1B in SOCE and its relation to SOCC components and mechanism of regulation are discussed.

G protein-coupled receptor signaling via Src kinase induces endogenous human transient receptor potential vanilloid type 6 (TRPV6) channel activation

Ca(2+) homeostasis plays a critical role in a variety of cellular processes. We showed previously that stimulation of the prostate-specific G protein-coupled receptor (PSGR) enhances cytosolic Ca(2+) and inhibits proliferation of prostate cells. Here, we analyzed the signaling mechanisms underlying the PSGR-mediated Ca(2+) increase. Using complementary molecular, biochemical, electrophysiological, and live-cell imaging techniques, we found that endogenous Ca(2+)-selective transient receptor potential vanilloid type 6 (TRPV6) channels are critically involved in the PSGR-induced Ca(2+) signal. Biophysical characterization of the current activated by PSGR stimulation revealed characteristic properties of TRPV6. The molecular identity of the involved channel was confirmed using RNA interference targeting TrpV6. TRPV6-mediated Ca(2+) influx depended on Src kinase activity. Src kinase activation occurred independently of G protein activation, presumably by direct interaction with PSGR. Taken together, we report that endogenous TRPV6 channels are activated downstream of a G protein-coupled receptor and present the first physiological characterization of these channels in situ.

Pharmacology of vanilloid transient receptor potential cation channels

Depending on their primary structure, the 28 mammalian transient receptor potential (TRP) cation channels identified so far can be sorted into 6 subfamilies: TRPC ("Canonical"), TRPV ("Vanilloid"), TRPM ("Melastatin"), TRPP ("Polycystin"), TRPML ("Mucolipin"), and TRPA ("Ankyrin"). The TRPV subfamily (vanilloid receptors) comprises channels critically involved in nociception and thermosensing (TRPV1, TRPV2, TRPV3, and TRPV4), whereas TRPV5 and TRPV6 are involved in renal Ca(2+) absorption/reabsorption. Apart from TRPV1, the pharmacology of these channels is still insufficiently known. Furthermore, only few small-molecule ligands for non-TRPV1 vanilloid receptors have been identified, and little is known of their endogenous ligands, resulting in a substantial "orphan" state for these channels. In this review, we summarize the pharmacological properties of members of the TRPV subfamily, highlighting the critical issues and challenges facing their "deorphanization" and clinical exploitation.

Intracellular Ca2+ stores modulate SOCCs and NMDA receptors via tyrosine kinases in rat hippocampal neurons

The regulation of intracellular Ca(2+) signalling by phosphorylation processes remains poorly defined, particularly with regards to tyrosine phosphorylation. Evidence from non-excitable cells implicates tyrosine phosphorylation in the activation of so-called store-operated Ca(2+) channels (SOCCs), but their involvement in neuronal Ca(2+) signalling is still elusive. In the present study, we determined the role of protein tyrosine kinases (PTKs) and tyrosine phosphatases (PTPs) in the coupling between intracellular Ca(2+) stores and SOCCs in neonatal rat hippocampal neurons by Fura-2 Ca(2+) imaging. An early Ca(2+) response from intracellular stores was triggered with thapsigargin, and followed by a secondary plasma membrane Ca(2+) response. This phase was blocked by the non-specific Ca(2+) channel blocker NiCl and the SOCC blocker, 2-aminoethoxydiphenyl borate (2-APB). Interestingly, two structurally distinct PTK inhibitors, genistein and AG126, also inhibited this secondary response. Application of the PTP inhibitor sodium orthovanadate (OV) also activated a sustained and tyrosine kinase dependent Ca(2+) response, blocked by NiCl and 2-APB. In addition, OV resulted in a Ca(2+) store dependent enhancement of NMDA responses, corresponding to, and occluding the signalling pathway for group I metabotropic glutamate receptors (mGluRs). This study provides first evidence for tyrosine based phospho-regulation of SOCCs and NMDA signalling in neurons.

Steroid effects on ZAP-70 and SYK in relation to apoptosis in poor prognosis chronic lymphocytic leukemia

There is resurgent interest in glucocorticoids (GCs) in the treatment of poor prognosis chronic lymphocytic leukemia (CLL). Little is known however on how GCs induce apoptosis in CLL. Methylprednisolone (MP) induces apoptosis in ZAP-70 positive CLL more readily than in ZAP-70 negative CLL, which is in contrast to the effects of radiation and chemotherapy. The increased GC sensitivity of ZAP-70+ CLL was studied in relation to the expression status of ZAP-70 and the related signal transducing tyrosine kinase SYK. Both ZAP-70 and SYK were downregulated by GC treatment. Moreover, SYK was dephosphorylated by the phosphatase PTP1B of which the expression and translation levels were induced by GCs. Inhibition of PTP1B successfully restored ZAP-70 expression and SYK phosphorylation but did not interfere with GC-induced apoptosis. Therefore, the downregulation of ZAP-70 and P-SYK per se during treatment with GCs is not sufficient to induce apoptosis, and different mechanisms must therefore be responsible for the increased steroid sensitivity of ZAP-70+ CLL.

Identification of tyrosines in the putative regulatory site of the Ca2+ channel TRPV6

In HEK293 cells, transfected with the Ca2+ channel protein TRPV6, Ca2+ influx is increased and TRPV6 is tyrosine phosphorylated following addition of the tyrosine phosphatase inhibitor N,N-dimethyl-hydroxamido hydroxovanadate to cells. This effect of DMHV is enhanced by co-transfection of cells with the tyrosine kinase Src and the tyrosine phosphatase 1B. It is abolished when cells had been treated with PP1, an inhibitor of Src family tyrosine kinases. PTP1B interacts with the N-terminal domain of TRPV6 within a region of amino acids 1-191 as shown by co-immunoprecipitation, bimolecular fluorescence complementation and the yeast 2-hybrid system. Point mutation of both tyrosines 161 and 162 in the TRPV6 protein abolishes the DMHV-effect on Ca2+ influx and tyrosine phosphorylation by Src. Single mutations of Y161 or Y162 shows that each of both tyrosines alone is sufficient for the DMHV-effect. We conclude that phosphorylation/dephosphorylation of tyrosines in position 161 and 162 is essential for regulation of Ca2+ influx through TRPV6 Ca2+ channels in HEK293 cells.

TRPV6

The ion channel TRPV6 is likely to function as an epithelial calcium channel in organs with high calcium transport requirements such as the intestine, kidney, and placenta. Transcriptional regulation of TRPV6 messenger RNA (mRNA) is controlled by 1,25-dihydroxyvitamin D, which is the active hormonal form of vitamin D3, and by additional calcium-dependent and vitamin D3-independent mechanisms. Under physiological conditions, the conductance of the channel itself is highly calcium-selective and underlies complex inactivation mechanisms triggered by intracellular calcium and magnesium ions. There is growing evidence that transcriptional regulation of TRPV6 in certain tissues undergoing malignant transformation, such as prostate cancer, is linked to cancer progression.

Regulation of TRP channels by phosphorylation

The transient receptor potential (TRP) channels are a group of Ca2+-permeable cation channels (except TRPM4 and TRPM5) that function as cellular sensors of various internal and external stimuli. Most of these channels are expressed in the nervous system and they play a key role in sensory physiology. They may respond to temperature, pressure, inflammatory agents, pain, osmolarity, taste and many other stimuli. Recent development indicates that the activity of these channels is regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. In this review, we present a comprehensive summary of the literature regarding the TRP channel regulation by different protein kinases.

Inhibition of protein tyrosine phosphatase 1B by reactive oxygen species leads to maintenance of Ca2+ influx following store depletion in HEK 293 cells

Depletion of inositol 1,4,5 trisphosphate-sensitive Ca2+ stores generates a yet unknown signal, which leads to increase in Ca2+ influx in different cell types [J.W. Putney Jr., A model for receptor-regulated calcium entry, Cell Calcium 7 (1986) 1-12]. Here, we describe a mechanism that modulates this store-operated Ca2+ entry (SOC). Ca2+ influx leads to inhibition of protein tyrosine phosphatase 1B (PTP1B) activity in HEK 293 cells [L. Sternfeld, et al., Tyrosine phosphatase PTP1B interacts with TRPV6 in vivo and plays a role in TRPV6-mediated calcium influx in HEK293 cells, Cell Signal 17 (2005) 951-960]. Since Ca2+ does not directly inhibit PTP1B, we assumed an intermediate signal, which links the rise in cytosolic Ca2+ concentration and PTP1B inhibition. We now show that Ca2+ influx is followed by generation of reactive oxygen species (ROS) and that it is reduced in cells preincubated with catalase. Furthermore, Ca2+-dependent inhibition of PTP1B can be abolished in the presence of catalase. H2O2 (100 microM) directly added to cells inhibits PTP1B and leads to increase in Ca2+ influx after store depletion. PP1, an inhibitor of the Src family tyrosine kinases, prevents H2O2-induced Ca2+ influx. Our results show that ROS act as fine tuning modulators of Ca2+ entry. We assume that the Ca2+ influx channel or a protein involved in its regulation remains tyrosine phosphorylated as a consequence of PTP1B inhibition by ROS. This leads to maintained Ca2+ influx in the manner of a positive feedback loop.