Regulation of the Ca2+ Channel TRPV6 by the Kinases SGK1, PKB/Akt, and PIKfyve

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.

Discovery of Herbacetin as a Novel SGK1 Inhibitor to Alleviate Myocardial Hypertrophy

Cardiac hypertrophy is a pivotal pathophysiological step of various cardiovascular diseases, which eventually leads to heart failure and death. Extracts of Rhodiola species (Ext.R), a class of commonly used medicinal herbs in Europe and East Asia, can attenuate cardiac hypertrophy both in vitro and in vivo. Serum/glucocorticoid regulated kinase 1 (SGK1) is identified as a potential target of Ext. R. By mass spectrometry-based kinase inhibitory assay, herbacetin (HBT) from Ext.R is identified as a novel SGK1 inhibitor with IC50 of 752 nmol. Thermal shift assay, KINOMEscan in vitro assay combined with molecular docking proves a direct binding between HBT and SGK1. Site-specific mutation of Asp177 in SGK1 completely ablates the inhibitory activity of HBT. The presence of OH groups at the C-3, C-8, C-4' positions of flavonoids is suggested to be favorable for the inhibition of SGK1 activity. Finally, HBT significantly suppresses cardiomyocyte hypertrophy in vitro and in vivo, reduces reactive oxygen species (ROS) synthesis and calcium accumulation. HBT decreases phosphorylation of SGK1 and regulates its downstream forkhead box protein O1 (FoxO1) signaling pathway. Taken together, the findings suggest that a panel of flavonoids structurally related to HBT may be novel leads for developing new therapeutics against cardiac hypertrophy.

Overcoming Glucocorticoid Resistance in Acute Lymphoblastic Leukemia: Repurposed Drugs Can Improve the Protocol

Glucocorticoids (GCs) are a central component of multi-drug treatment protocols against T and B acute lymphoblastic leukemia (ALL), which are used intensively during the remission induction to rapidly eliminate the leukemic blasts. The primary response to GCs predicts the overall response to treatment and clinical outcome. In this review, we have critically analyzed the available data on the effects of GCs on sensitive and resistant leukemic cells, in order to reveal the mechanisms of GC resistance and how these mechanisms may determine a poor outcome in ALL. Apart of the GC resistance, associated with a decreased expression of receptors to GCs, there are several additional mechanisms, triggered by alterations of different signaling pathways, which cause the metabolic reprogramming, with an enhanced level of glycolysis and oxidative phosphorylation, apoptosis resistance, and multidrug resistance. Due to all this, the GC-resistant ALL show a poor sensitivity to conventional chemotherapeutic protocols. We propose pharmacological strategies that can trigger alternative intracellular pathways to revert or overcome GC resistance. Specifically, we focused our search on drugs, which are already approved for treatment of other diseases and demonstrated anti-ALL effects in experimental pre-clinical models. Among them are some “truly” re-purposed drugs, which have different targets in ALL as compared to other diseases: cannabidiol, which targets mitochondria and causes the mitochondrial permeability transition-driven necrosis, tamoxifen, which induces autophagy and cell death, and reverts GC resistance through the mechanisms independent of nuclear estrogen receptors (“off-target effects”), antibiotic tigecycline, which inhibits mitochondrial respiration, causing energy crisis and cell death, and some anthelmintic drugs. Additionally, we have listed compounds that show a classical mechanism of action in ALL but are not used still in treatment protocols: the BH3 mimetic venetoclax, which inhibits the anti-apoptotic protein Bcl-2, the hypomethylating agent 5-azacytidine, which restores the expression of the pro-apoptotic BIM, and compounds targeting the PI3K-Akt-mTOR axis. Accordingly, these drugs may be considered for the inclusion into chemotherapeutic protocols for GC-resistant ALL treatments.

The amino acid transporter SLC-36.1 cooperates with PtdIns3P 5-kinase to control phagocytic lysosome reformation

How lysosomes reform following phagolysosomal digestion of apoptotic cells is poorly understood. Gan et al. reveal that the amino acid transporter SLC-36.1 cooperates with PtdIns3P 5-kinase to control phagocygtic lysosome reformation in Caenorhabditis elegans embryos and autophagic lysosome reformation in adult animals.

Phagocytic removal of apoptotic cells involves formation, maturation, and digestion of cell corpse–containing phagosomes. The retrieval of lysosomal components following phagolysosomal digestion of cell corpses remains poorly understood. Here we reveal that the amino acid transporter SLC-36.1 is essential for lysosome reformation during cell corpse clearance in Caenorhabditis elegans embryos. Loss of slc-36.1 leads to formation of phagolysosomal vacuoles arising from cell corpse–containing phagosomes. In the absence of slc-36.1, phagosome maturation is not affected, but the retrieval of lysosomal components is inhibited. Moreover, loss of PPK-3, the C. elegans homologue of the PtdIns3P 5-kinase PIKfyve, similarly causes accumulation of phagolysosomal vacuoles that are defective in phagocytic lysosome reformation. SLC-36.1 and PPK-3 function in the same genetic pathway, and they directly interact with one another. In addition, loss of slc-36.1 and ppk-3 causes strong defects in autophagic lysosome reformation in adult animals. Our findings thus suggest that the PPK-3–SLC-36.1 axis plays a central role in both phagocytic and autophagic lysosome formation.

Mechanisms Underpinning Adaptations in Placental Calcium Transport in Normal Mice and Those With Fetal Growth Restriction

Fetal delivery of calcium, via the placenta, is crucial for appropriate skeletal mineralization. We have previously demonstrated that maternofetal calcium transport, per gram placenta, is increased in the placental specific insulin-like growth factor 2 knockout mouse (P0) model of fetal growth restriction (FGR) compared to wild type littermates (WTL). This effect was mirrored in wild-type (WT) mice comparing lightest vs. heaviest (LvH) placentas in a litter. In both models increased placental calcium transport was associated with normalization of fetal calcium content. Despite this adaptation being observed in small normal (WT), and small dysfunctional (P0) placentas, mechanisms underpinning these changes remain unknown. Parathyroid hormone-related protein (PTHrP), elevated in cord blood in FGR and known to stimulate plasma membrane calcium ATPase, might be important. We hypothesized that PTHrP expression would be increased in LvH WT placentas, and in P0 vs. WTL. We used calcium pathway-focused PCR arrays to assess whether mechanisms underpinning these adaptations in LvH WT placentas, and in P0 vs. WTL, were similar. PTHrP protein expression was not different between LvH WT placentas at E18.5 but trended toward increased expression (139%; P = 0.06) in P0 vs. WTL. PCR arrays demonstrated that four genes were differentially expressed in LvH WT placentas including increased expression of the calcium-binding protein calmodulin 1 (1.6-fold; P < 0.05). Twenty-four genes were differentially expressed in placentas of P0 vs. WTL; significant reductions were observed in expression of S100 calcium binding protein G (2-fold; P < 0.01), parathyroid hormone 1 receptor (1.7-fold; P < 0.01) and PTHrP (2-fold; P < 0.05), whilst serum/glucocorticoid-regulated kinase 1 (SGK1), a regulator of nutrient transporters, was increased (1.4 fold; P < 0.05). Tartrate resistant acid phosphatase 5 (TRAP5 encoded by Acp5) was reduced in placentas of both LvH WT and P0 vs. WTL (1.6- and 1.7-fold, respectively; P < 0.05). Signaling events underpinning adaptations in calcium transport are distinct between LvH placentas of WT mice and those in P0 vs. WTL. Calcium binding proteins appear important in functional adaptations in the former whilst PTHrP and SGK1 are also implicated in the latter. These data facilitate understanding of mechanisms underpinning placental calcium transport adaptation in normal and growth restricted fetuses.

SGK1: The Dark Side of PI3K Signaling

Activation of the PI3K pathway is central to a variety of physiological and pathological processes. In these contexts, AKT is classically considered the de facto mediator of PI3K-dependent signaling. However, in recent years, accumulating data point to the existence of additional effectors of PI3K activity, parallel to and independent of AKT, that play critical and unique roles in mediating different developmental, homeostatic, and pathological processes. In this review, I summarize and discuss our current understanding of the function of the serine/threonine kinase SGK1 as a downstream effector of PI3K, and try to separate targets and pathways validated as uniquely SGK1-dependent from those shared with AKT.

Reciprocal positive regulation between TRPV6 and NUMB in PTEN-deficient prostate cancer cells

Calcium acts as a second messenger and plays a crucial role in signaling pathways involved in cell proliferation. Recently, calcium channels related to calcium influx into the cytosol of epithelial cells have attracted attention as a cancer therapy target. Of these calcium channels, TRPV6 is overexpressed in prostate cancer and is considered an important molecule in the process of metastasis. However, its exact role and mechanism is unclear. NUMB, well-known tumor suppressor gene, is a novel interacting partner of TRPV6. We show that NUMB and TRPV6 have a reciprocal positive regulatory relationship in PC-3 cells. We repeated this experiment in two other prostate cancer cell lines, DU145 and LNCaP. Interestingly, there were no significant changes in TRPV6 expression following NUMB knockdown in DU145. We revealed that the presence or absence of PTEN was the cause of NUMB-TRPV6 function. Loss of PTEN caused a positive correlation of TRPV6-NUMB expression. Collectively, we determined that PTEN is a novel interacting partner of TRPV6 and NUMB. These results demonstrated a novel relationship of NUMB-TRPV6 in prostate cancer cells, and show that PTEN is a novel regulator of this complex.

AMPK-sensitive cellular transport

The energy sensing AMP-activated protein kinase (AMPK) regulates cellular and whole-body energy balance through stimulating catabolic ATP-generating and suppressing anabolic ATP-consuming pathways thereby helping cells survive during energy depletion. The kinase has previously been reported to be either directly or indirectly involved in the regulation of several carriers, channels and pumps of high significance in cellular physiology. Thus AMPK provides a necessary link between cellular energy metabolism and cellular transport activity. Better understanding of the AMPK role in cellular transport offers a potential for improved therapies in various human diseases and disorders. In this review, we discuss recent advances in understanding the role and function of AMPK in transport regulation under physiological and pathological states.

Regulation of ion channels and transporters by AMP-activated kinase (AMPK)

The energy-sensing AMP-activated kinase AMPK ensures survival of energy-depleted cells by stimulating ATP production and limiting ATP utilization. Both energy production and energy consumption are profoundly influenced by transport processes across the cell membane including channels, carriers and pumps. Accordingly, AMPK is a powerful regulator of transport across the cell membrane. AMPK regulates diverse K(+) channels, Na(+) channels, Ca(2+) release activated Ca(2+) channels, Cl(-) channels, gap junctional channels, glucose carriers, Na(+)/H(+)-exchanger, monocarboxylate-, phosphate-, creatine-, amino acid-, peptide- and osmolyte-transporters, Na(+)/Ca(2+)-exchanger, H(+)-ATPase and Na(+)/K(+)-ATPase. AMPK activates ubiquitin ligase Nedd4-2, which labels several plasma membrane proteins for degradation. AMPK further regulates transport proteins by inhibition of Rab GTPase activating protein (GAP) TBC1D1. It stimulates phosphatidylinositol 3-phosphate 5-kinase PIKfyve and inhibits phosphatase and tensin homolog (PTEN) via glycogen synthase kinase 3β (GSK3β). Moreover, it stabilizes F-actin as well as downregulates transcription factor NF-κB. All those cellular effects serve to regulate transport proteins.

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.

Up-regulation of amino acid transporter SLC6A19 activity and surface protein abundance by PKB/Akt and PIKfyve

BACKGROUND

The amino acid transporter B0AT1 (SLC6A19) accomplishes concentrative cellular uptake of neutral amino acids. SLC6A19 is stimulated by serum- &amp; glucocorticoid-inducible kinase (SGK) isoforms. SGKs are related to PKB/Akt isoforms, which also stimulate several amino acid transporters. PKB/Akt modulates glucose transport in part by phosphorylating and thus activating phosphatidylinositol-3-phosphate-5-kinase (PIKfyve), which fosters carrier protein insertion into the cell membrane. The present study explored whether PKB/Akt and/or PIKfyve stimulate SLC6A19.

METHODS

SLC6A19 was expressed in Xenopus oocytes with or without wild-type PKB/Akt or inactive (T308A/S473A)PKB/Akt without or with additional expression of wild-type PIKfyve or PKB/Akt-resistant (S318A)PIKfyve. Electrogenic amino acid transport was determined by dual electrode voltage clamping.

RESULTS

In SLC6A19-expressing oocytes but not in water-injected oocytes, the addition of the neutral amino acid L-leucine (2 mM) to the bath generated a current (I(le)), which was significantly increased following coexpression of PKB/Akt, but not by coexpression of (T308A/S473A)PKB/Akt. The effect of PKB/Akt was augmented by additional coexpression of PIKfyve but not of (S318A)PIKfyve. Coexpression of PKB/Akt enhanced the maximal transport rate without significantly modifying the affinity of the carrier. The decline of I(le) following inhibition of carrier insertion by brefeldin A (5 µM) was similar in the absence and presence of PKB/Akt indicating that PKB/Akt stimulated carrier insertion into rather than inhibiting carrier retrieval from the cell membrane.

CONCLUSION

PKB/Akt up-regulates SLC6A19 activity, which may foster amino acid uptake into PKB/Akt-expressing epithelial and tumor cells.

Regulation of ion channels by the serum- and glucocorticoid-inducible kinase SGK1

The ubiquitously expressed serum- and glucocorticoid-inducible kinase-1 (SGK1) is genomically regulated by cell stress (including cell shrinkage) and several hormones (including gluco- and mineralocorticoids). SGK1 is activated by insulin and growth factors through PI3K and 3-phosphoinositide-dependent kinase PDK1. SGK1 activates a wide variety of ion channels (e.g., ENaC, SCN5A, TRPV4-6, ROMK, Kv1.3, Kv1.5, Kv4.3, KCNE1/KCNQ1, KCNQ4, ASIC1, GluR6, ClCKa/barttin, ClC2, CFTR, and Orai/STIM), which participate in the regulation of transport, hormone release, neuroexcitability, inflammation, cell proliferation, and apoptosis. SGK1-sensitive ion channels participate in the regulation of renal Na(+) retention and K(+) elimination, blood pressure, gastric acid secretion, cardiac action potential, hemostasis, and neuroexcitability. A common (∼3-5% prevalence in Caucasians and ∼10% in Africans) SGK1 gene variant is associated with increased blood pressure and body weight as well as increased prevalence of type II diabetes and stroke. SGK1 further contributes to the pathophysiology of allergy, peptic ulcer, fibrosing disease, ischemia, tumor growth, and neurodegeneration. The effect of SGK1 on channel activity is modest, and the channels do not require SGK1 for basic function. SGK1-dependent ion channel regulation may thus become pathophysiologically relevant primarily after excessive (pathological) expression. Therefore, SGK1 may be considered an attractive therapeutic target despite its broad range of functions.

The negative feedback regulation of TRPV4 Ca2+ ion channel function by its C-terminal cytoplasmic domain

The transient receptor potential vanilloid 4 (TRPV4) cation channel, a member of the TRP vanilloid subfamily, is expressed in a broad range of tissues where it participates in the generation of a Ca(2+) signal and/or depolarization of the membrane potential. Regulation of the abundance of TRPV4 at the cell surface is critical in osmo- and mechanotransduction. In this review, we discussed that the potential effect of Ca(2+) occurs via its action at an intracellular site in the C-terminus of the channel protein by the effect of the modulation on TRPV4 (such as 824 Ser residue phosphorylation), and its regulation for TRPV4 functions related with cell surface spread, wound healing or its polarity reorientation through its differential affinity with actin or tubulin.

Regulation of KCNQ1/KCNE1 by β-catenin

β-catenin, a multifunctional protein expressed in all tissues including the heart stimulates the expression of several genes important for cell proliferation. Signaling involving ß-catenin participates in directing cardiac development and in the pathophysiology of cardiac hypertrophy. Nothing is known, however, on the role of β-catenin in the regulation of cardiac ion channels. The present study explored the functional interaction of β-catenin and KCNE1/KCNQ1, the K⁺ channel complex underlying the slowly activating outwardly rectifying K⁺ current. To this end, KCNE1/KCNQ1 was expressed in Xenopus oocytes with and without β-catenin and the depolarization (up to + 80 mV) induced current (I(Ks)) was determined using the two-electrode voltage clamp. As a result, β-catenin enhanced I(Ks) by 30%. The effect of β-catenin on I(Ks) was not affected by actinomycin D (10 μM), an inhibitor of transcription, indicating that β-catenin was not effective as transcription factor. Confocal microscopy revealed that β-catenin enhanced the KCNE1/KCNQ1 protein abundance in the cell membrane. Exposure of the oocytes to brefeldin A (5 μM), an inhibitor of vesicle insertion, was followed by a decline of I(Ks), which was then similar in oocytes expressing KCNE1/KCNQ1 together with β-catenin and in oocytes expressing KCNE1/KCNQ1 alone. In conclusion, β-catenin enhances I(Ks) by increasing the KCNE1/KCNQ1 protein abundance in the cell membrane, an effect requiring vesicle insertion into the cell membrane.

Calcium transport genes are differently regulated in maternal and fetal placenta in the knockout mice of calbindin-D(9k) and -D(28k)

Calbindin-D(9k) (CaBP-9k) and -D(28k) (CaBP-28k) are cytosolic proteins with EF-hand motifs that have a high affinity for calcium ions. Many types of calcium channels and intracellular calcium binding proteins, such as sodium/calcium exchangers (NCXs) and transient receptor potential cation channels (TRPVs), have been detected in the placenta. In this study, the expression of calcium channels involved in maternal-fetal calcium transport were investigated in wild-type mice versus CaBP-9k, CaBP-28k, and CaBP-9k/28k double knockout (KO) mouse models. The expression of calcium transport genes in three dissected sections of the placenta (maternal, central, and fetal) was examined on gestational day 19 (GD 19). The expression of CaBP-9k, TRPV6, TRPV5, and NCX1 mRNA was high in fetal compared to maternal placenta, while CaBP-28k was abundant in the maternal placenta. CaBP-9k was enhanced in all sections of placenta in CaBP-28k KO mice, whereas CaBP-28k was reduced in CaBP-9k KO mice. The expression of TRPV6, TRPV5, and NCX1 were induced in both maternal and fetal placentas in CaBP-9k KO mice, but were upregulated in maternal and central placentas of CaBP-28k KO mice. The levels of these proteins showed similar patterns with those of their mRNA. Placental CaBP-9k, TRPV6, TRPV5, and NCX1 proteins were abundantly expressed in the intraplacental yolk sac located in the fetal placenta. CaBP-28k did not colocalize with other calcium transport genes, although it was enriched in the placental trophoblasts of the decidual zone in the maternal placenta. These results indicate that placental TRPV6, TRPV5, and NCX1 compensate for CaBPs in CaBP-9k and/or CaBP-28k KO mice, and may take over the roles of CaBP-9k and CaBP-28k to transfer calcium ions in the placenta. Taken together, these results indicate that TRPV6, NCX1, and CaBP-9k in the fetal placenta and CaBP-28k in the maternal placenta may play key roles in controlling calcium transport across the placenta during pregnancy.

PIKfyve and its Lipid products in health and in sickness

PIKfyve, a phosphoinositide 5-kinase synthesizing PtdIns(3,5)P₂ and PtdIns5P in a cellular context, belongs to an evolutionarily ancient gene family of PtdIns(3,5)P₂-synthesizing enzymes that, except for plants, are products of a single-copy gene across species. In the dozen years after its discovery, enormous progress has been made in characterizing the numerous PIKfyve cellular functions and the regulatory mechanisms that govern these functions. It became clear that PIKfyve does not act alone but, rather, it engages the scaffolding regulator ArPIKfyve and the phosphatase Sac3 to make a multiprotein "PAS" complex, so called for the first letters of the protein names. This complex relays antagonistic signals, one for synthesis, another for turnover of PtdIns(3,5)P₂, whose dysregulated coordination is linked to several human diseases. The physiological significance for each protein in the PAS complex is underscored by the early lethality of the mouse models with disruption in any of the three genes. This chapter summarizes our current knowledge of the diverse and complex functionality of PIKfyve and PtdIns(3,5)P₂/PtdIns5P products with particular highlights on recent discoveries of inherited or somatic mutations in PIKfyve and Sac3 linked to human disorders.

SGK1-dependent stimulation of intestinal SGLT1 activity by vitamin D

The serum- and glucocorticoid-inducible kinase SGK1 has previously been shown to mediate the glucocorticoid-dependent stimulation of several intestinal transport systems including the electrogenic glucose transporter SGLT1. In squamous carcinoma cells, SGK1 expression is stimulated by 1,25(OH)₂D₃, the biologically active metabolite of vitamin D. The present study explored whether vitamin D influences the intestinal SGLT1 activity. Jejunal SGLT1 activity was determined by Ussing chamber experiments. Under a normal diet, the electrogenic glucose transport was similar in SGK1 knockout (sgk1 ( -/- )) and wild type mice (sgk1 ( +/+ )). Following a vitamin D-rich diet (14 days 10,000 I.U. vitamin D), the SGK1 transcript levels as well as the SGLT1 protein abundance were increased in sgk1(+/+) mice. Moreover, SGLT1 activity was increased in sgk1(+/+) mice but not in sgk1(-/-) mice following a vitamin D-rich diet. Furthermore, an oral glucose load was followed by an increase in the plasma glucose concentration to significantly higher values in sgk1(+/+) mice treated with a vitamin D-rich diet than in untreated sgk1(+/+) mice. In conclusion, vitamin D treatment upregulates the expression of SGK1, which in turn enhances SGLT1 activity.

The nucleophosmin-anaplastic lymphoma kinase oncogene interacts, activates, and uses the kinase PIKfyve to increase invasiveness

NPM-ALK is a chimeric tyrosine kinase detected in most anaplastic large cell lymphomas that results from the reciprocal translocation t(2,5)(p23;q35) that fuses the N-terminal domain of nucleophosmin (NPM) to the catalytic domain of the anaplastic lymphoma kinase (ALK) receptor. The constitutive activity of the kinase is responsible for its oncogenicity through the stimulation of several downstream signaling pathways, leading to cell proliferation, migration, and survival. We demonstrated previously that the high level of phosphatidylinositol 5-phosphate measured in NPM-ALK-expressing cells is controlled by the phosphoinositide kinase PIKfyve, a lipid kinase known for its role in vesicular trafficking. Here, we show that PIKfyve associates with NPM-ALK and that the interaction involves the 181-300 region of the oncogene. Moreover, we demonstrate that the tyrosine kinase activity of the oncogene controls PIKfyve lipid kinase activity but is dispensable for the formation of the complex. Silencing or inhibition of PIKfyve using siRNA or the PIKfyve inhibitor YM201636 have no effect on NPM-ALK-mediated proliferation and migration but strongly reduce invasive capacities of NPM-ALK-expressing cells and their capacity to degrade the extracellular matrix. Accordingly, immunofluorescence studies confirm a perturbation of matrix metalloproteinase 9 localization at the cell surface and defect in maturation. Altogether, these results suggest a role for PIKfyve in NPM-ALK-mediated invasion.

Stimulation of Ca2+-channel Orai1/STIM1 by serum- and glucocorticoid-inducible kinase 1 (SGK1)

Ca(2+) signaling includes store-operated Ca(2+) entry (SOCE) following depletion of endoplasmic reticulum (ER) Ca(2+) stores. On store depletion, the ER Ca(2+) sensor STIM1 activates Orai1, the pore-forming unit of Ca(2+)-release-activated Ca(2+) (CRAC) channels. Here, we show that Orai1 is regulated by serum- and glucocorticoid-inducible kinase 1 (SGK1), a growth factor-regulated kinase. Membrane Orai1 protein abundance, I(CRAC), and SOCE in human embryonic kidney (HEK293) cells stably expressing Orai1 and transfected with STIM1 were each significantly enhanced by coexpression of constitutively active (S422D)SGK1 (by+81, +378, and+136%, respectively) but not by inactive (K127N)SGK1. Coexpression of the ubiquitin ligase Nedd4-2, an established negatively regulated SGK1 target, down-regulated SOCE (by -48%) and I(CRAC) (by -60%), an effect reversed by expression of (S422D)SGK1 (by +175 and +173%, respectively). Orai1 protein abundance and SOCE were significantly lower in mast cells from SGK1-knockout (sgk1(-/-)) mice (by -37% and -52%, respectively) than in mast cells from wild-type (sgk1(+/+)) littermates. Activation of SOCE by sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase-inhibitor thapsigargin (2 μM) stimulated migration, an effect significantly higher (by +306%) in (S422D)SGK1-expressing than in (K127N)SGK1-expressing HEK293 cells, and also significantly higher (by +108%) in sgk1(+/+) than in sgk1(-/-) mast cells. SGK1 is thus a novel key player in the regulation of SOCE.

TRPV5 and TRPV6 in transcellular Ca(2+) transport: regulation, gene duplication, and polymorphisms in African populations

TRPV5 and TRPV6 are unique members of the TRP super family. They are highly selective for Ca(2+) ions with multiple layers of Ca(2+)-dependent inactivation mechanisms, expressed at the apical membrane of Ca(2+) transporting epithelia, and robustly responsive to 1,25-dihydroxivitamin D(3). These features are well suited for their roles as Ca(2+) entry channels in the first step of transcellular Ca(2+) transport pathways, which are involved in intestinal absorption, renal reabsorption of Ca(2+), placental transfer of Ca(2+) to fetus, and many other processes. While TRPV6 is more broadly expressed in a variety of tissues such as esophagus, stomach, small intestine, colon, kidney, placenta, pancreas, prostate, uterus, salivary gland, and sweat gland, TRPV5 expression is relatively restricted to the distal convoluted tubule and connecting tubule of the kidney. There is only one TRPV6-like gene in fish and birds in comparison to both TRPV5 and TRPV6 genes in mammals, indicating TRPV5 gene was likely generated from duplication of TRPV6 gene during the evolution of mammals to meet the needs of complex renal function. TRPV5 and TRPV6 are subjected to vigorous regulations under physiological, pathological, and therapeutic conditions. The elevated TRPV6 level in malignant tumors such as prostate and breast cancers makes it a potential therapeutic target. TRPV6, and to a lesser extent TRPV5, exhibit unusually high levels of single nucleotide polymorphisms (SNPs) in African populations as compared to other populations, indicating TRPV6 gene was under selective pressure during or after humans migrated out of Africa. The SNPs of TRPV6 and TRPV5 likely contribute to the Ca(2+) conservation mechanisms in African populations.

Colorectal carcinoma cells--regulation of survival and growth by SGK1

Colorectal carcinoma is among the most common malignancies. The tumour cells may arise from mutations in genes encoding proteins involved in the regulation of cell survival and proliferation. Recent evidence disclosed the sensitivity of colon carcinoma to the expression of ubiquitous serum and glucocorticoid inducible kinase-1 (SGK1). The kinase is activated by insulin and growth factors via the phosphatidylinositide-3-kinase (PI3K) and the 3-phosphoinositide dependent kinase (PDK1). SGK1 regulates channels, carriers and Na(+)/K(+)-ATPase, enzymes such as glycogen-synthase-kinase-3 (GSK3) and ubiquitin-ligase Nedd4-2, as well as several transcription factors. SGK1 regulates transport, hormone release, neuroexcitability, inflammation, cell proliferation and apoptosis. SGK1 contributes to metabolic syndrome and the pathophysiology of neurodegeneration, allergy, peptic ulcer, fibrosing disease and response to ischemia. SGK1 is upregulated in some tumours but downregulated in others. SGK1-sensitive mechanisms fostering tumour growth include activation of K(+) channels and Ca(2+) channels, Na(+)/H(+) exchanger, amino acid transporters and glucose transporters, upregulation of the nuclear factor NFkappaB and beta-catenin as well as downregulation of the transcription factors Foxo3a/FKHRL1 and p53. SGK1 enhances survival, invasiveness, motility, epithelial to mesenchymal transition and adhesiveness of tumour cells. Following deficiency of APC (adenoma polyposis coli) or chemical cancerogenesis, SGK1 knockout mice develop less intestinal tumours than their wild-type littermates and pharmacological SGK1 inhibition counteracts growth of prostate cancer cells.