Novel role for Na,K-ATPase in phosphatidylinositol 3-kinase signaling and suppression of cell motility

Targeting the alpha 1 subunit of the sodium pump to combat glioblastoma cells

OBJECTIVE

Ion transporters play pivotal roles in cancer cell migration in general and in glioblastomas (GBMs) in particular. However, the specific role of Na/K-ATPase (the sodium pump) and, in particular, its alpha1 subunit, has remained unexplored in GBMs.

MATERIALS AND METHODS

The expression of Na+/K+ -ATPase alpha1 in GBM clinical samples, normal brain tissue, and a human GBM cell line has been investigated. Using the novel cardenolide UNBS1450 (Unibioscreen, Brussels, Belgium), which is a ligand of the sodium pump, we have characterized the effects of inhibiting Na+/K+ -ATPase alpha1 in human GBM cells with respect to cell proliferation; morphology; impact on intracellular Na+, Ca2+, and adenosine triphosphate; and changes in the actin cytoskeleton. We have investigated the mechanism by which UNBS1450 overcomes the apoptosis resistance of GBMs and determined its anti-tumor effects in comparative studies in vitro in GBM cell viability assays and in vivo using an orthotopic human GBM xenograft model.

RESULTS

Overall, the alpha1 subunit of Na+/K+ -ATPase is highly expressed in a majority of glioblastomas compared with normal brain tissues, and by binding to this subunit in human U373-MG GBM cells, UNBS1450 impairs cell proliferation and migration via an intracellular adenosine triphosphate decrease-mediated disorganization of the actin cytoskeleton and cytotoxic proautophagic effects. UNBS1450 also significantly increases the in vivo survival of mice orthotopically grafted with U373-MG GBM cells.

CONCLUSION

Inhibition of the Na+/K+ -ATPase alpha1 subunit in human GBM cells impairs both cell migration and cell proliferation.

Evidence for a potential tumor suppressor role for the Na,K-ATPase beta1-subunit

The Na,K-ATPase, consisting of two essential subunits (alpha, beta), plays a critical role in the regulation of ion homeostasis in mammalian cells. Recent studies indicate that reduced expression of the beta1 isoform (NaK-beta1) is commonly observed in carcinoma and is associated with events involved in cancer progression. In this study, we present evidence that repletion of NaK-beta1 in Moloney sarcoma virus-transformed Madin-Darby canine kidney cells (MSV-MDCK), a highly tumorigenic cell line, inhibits anchorage independent growth and suppresses tumor formation in immunocompromised mice. Additionally, using an in vitro cell-cell aggregation assay, we showed that cell aggregates of NaK-beta1 subunit expressing MSV-MDCK cells have reduced extracellular regulated kinase (ERK) 1/2 activity compared with parental MSV-MDCK cells. Finally, using immunohistochemistry and fully quantitative image analysis approaches, we showed that the levels of phosphorylated ERK 1/2 are inversely correlated to the NaK-beta1 levels in the tumors. These findings reveal for the first time that NaK-beta1 has a potential tumor-suppressor function in epithelial cells.

The sodium pump alpha1 subunit as a potential target to combat apoptosis-resistant glioblastomas

PURPOSE

To review the involvement of the ion transporter Na+/K+-ATPase (NaK) in the migration and proliferation of glioma cells. Preliminary studies indicate that NaK alpha1 subunits seem to be upregulated in a proportion of glioblastomas but not in normal brain tissues.

DESIGN

The present review focuses on (1) the natural resistance of migrating malignant glioma cells to apoptosis, (2) autophagic cell death as an alternative to combat malignant gliomas, (3) the fact that reducing the levels of malignant glioma cell motility can restore proapoptotic drug sensitivity,and (4) on the observation that inhibiting the NaK activity reduces both glioma cell proliferation and migration.

RESULTS

The natural ligands of the NaK are the cardiotonic steroids. A hemisynthetic derivative of 2"-oxovoruscharin (UNBS1450), a novel cardenolide, displays unique structural features, making its binding affinity to NaK alpha subunits (including alpha1) 10 to 100 times higher than that of other cardenolides. UNBS1450 markedly decreases intracellular ATP concentration in glioma cells, disorganizes the actin cytoskeleton, and leads to autophagic cell death in NaK alpha1 over-expressing glioma cells.

CONCLUSIONS

Glioblastoma patients who do not respond to chemotherapy and whose tumors over-express NaK alpha1 subunits could benefit from a treatment using ligands with marked binding affinity for the NaK alpha1 subunit.

Interactions of tight junctions with membrane channels and transporters

Tight junctions are unique organelles in epithelial cells. They are localized to the apico-lateral region and essential for the epithelial cell transport functions. The paracellular transport process that occurs via tight junctions is extensively studied and is intricately regulated by various extracellular and intracellular signals. Fine regulation of this transport pathway is crucial for normal epithelial cell functions. Among factors that control tight junction permeability are ions and their transporters. However, this area of research is still in its infancy and much more needs to be learned about how these molecules regulate tight junction structure and functions. In this review we have attempted to compile literature on ion transporters and channels involved in the regulation of tight junctions.

17Beta-oestradiol regulates the expression of Na+/K+-ATPase beta1-subunit, sarcoplasmic reticulum Ca2+-ATPase and carbonic anhydrase iv in H9C2 cells

1. It is necessary to improve our understanding of the effect of 17beta-oestradiol (E2) on the heart at a molecular and cellular level. In the present study, the effects of E2 on Na(+)/K(+)-ATPase, sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA) and carbonic anhydrase IV (CAIV) in H9C2 cells were investigated. To identify the mechanism of action of E2 on these proteins, the oestrogen receptor (ER) antagonist tamoxifen was used. 2. The results indicated that 1 and 100 nmol/L E2 can enhance the activity of Na(+)/K(+)-ATPase and SERCA and upregulate the expression of the Na(+)/K(+)-ATPase beta1-subunit, SERCA2a and CAIV at both the mRNA and protein level compared with 0 and 0.01 nmol/L E2. 17beta-Oestradiol had the greatest effect at 100 nmol/L; 1 micromol/L E2 did not further protein expression compared with 100 nmol/L E2. 3. Tamoxifen (10 nmol/L) significantly decreased the activity of SERCA, as well as the expression of the Na(+)/K(+)-ATPase beta1-subunit and SERCA at the mRNA and protein level, in H9C2 cells cultured with 1 nmol/L E2. Tamoxifen alone had no significant effect on these proteins in H9C2 cells. 4. It may be hypothesized that a suitable E2 concentration has a protective effect on the heart and that the actual dose of E2 used in hormone-replacement therapy is important in menopausal women.

Association of PI3K-Akt signaling pathway with digitalis-induced hypertrophy of cardiac myocytes

Our previous studies on cardiac myocytes showed that positive inotropic concentrations of the digitalis drug ouabain activated signaling pathways linked to Na(+)-K(+)-ATPase through Src and epidermal growth factor receptor (EGFR) and led to myocyte hypertrophy. In view of the known involvement of phosphatidylinositol 3-kinase (PI3K)-Akt pathways in cardiac hypertrophy, the aim of the present study was to determine whether these pathways are also linked to cardiac Na(+)-K(+)-ATPase and, if so, to assess their role in ouabain-induced myocyte growth. In a dose- and time-dependent manner, ouabain activated Akt and phosphorylation of its substrates mammalian target of rapamycin and glycogen synthase kinase in neonatal rat cardiac myocytes. Akt activation by ouabain was sensitive to PI3K inhibitors and was also noted in adult myocytes and isolated hearts. Ouabain caused a transient increase of phosphatidylinositol 3,4,5-trisphosphate content of neonatal myocytes, activated class IA, but not class IB, PI3K, and increased coimmunoprecipitation of the alpha-subunit of Na(+)-K(+)-ATPase with the p85 subunit of class IA PI3K. Ouabain-induced activation of ERK1/2 was prevented by Src, EGFR, and MEK inhibitors, but not by PI3K inhibitors. Activation of Akt by ouabain, however, was sensitive to inhibitors of PI3K and Src, but not to inhibitors of EGFR and MEK. Similarly, ouabain-induced myocyte hypertrophy was prevented by PI3K and Src inhibitors, but not by an EGFR inhibitor. These findings 1) establish the linkage of the class IA PI3K-Akt pathway to Na(+)-K(+)-ATPase and the essential role of this linkage to ouabain-induced myocyte hypertrophy and 2) suggest cross talk between these PI3K-Akt pathways and the signaling cascades previously identified to be associated with cardiac Na(+)-K(+)-ATPase.

Beyond ion translocation: structural functions of the sodium-hydrogen exchanger isoform-1

PURPOSE OF REVIEW

The sodium-hydrogen exchanger isoform-1 (NHE1) functions in intracellular pH and cell volume homeostasis by catalyzing an electroneutral exchange of extracellular sodium and intracellular hydrogen. Recent studies have revealed the structural functions of NHE1 as an anchor for actin filaments and a scaffold for an ensemble of signaling proteins. This review highlights how these functions contribute to NHE1 regulation of biochemical events and cell behaviors.

RECENT FINDINGS

New data confirming nontransport structural functions of NHE1 suggest reexamining how NHE1 regulates cell functions. Cell survival, cell substrate adhesion, and organization of the actin cytoskeleton are confirmed to be regulated through actin anchoring by NHE1 and likely by NHE1-dependent scaffolding of signaling proteins. A role for NHE1 in mechanotransduction is emerging and a challenge of future studies is to determine whether structural functions of NHE1 are important for mechanoresponsiveness.

SUMMARY

This review highlights evidence for the nontransport functions of NHE1 and describes how the structural functions are integrated with ion translocation to regulate a range of cellular processes. Nontransporting features of NHE1 are analogous to recently observed nonconducting actions of ion channels in regulating cell behaviors and represent an emerging paradigm of ion transporters as multifunctional proteins.

Caveolins bind to (Na+, K+)/H+ exchanger NHE7 by a novel binding module

NHE7 was identified as the first mammalian organelle-membrane type (Na+, K+)/H+ exchanger that may contribute to the ion homeostasis in the trans-Golgi network (TGN) and endosomes. Here we show that caveolins directly bind to the C-terminal extension of NHE7 by an unconventional binding-module. NHE7 is partly associated with caveolae/lipid raft fractions, and heterologous expression of caveolin dominant-negative mutants as well as cholesterol depriving drugs diminished such associations. In contrast to the wild type NHE7, a deletion mutant lacking the C-terminal extension was predominantly detected in non-caveolae/lipid rafts. We further show that a small fraction of NHE7 is targeted to the cell surface and subsequently internalized. Endocytosis of NHE7 was efficiently inhibited by pharmacological maneuvers that block clathrin-dependent endocytosis, whereas dominant-negative caveolin mutants or methyl beta-cyclodextrin did not affect NHE7-internalization. Thus, NHE7 associates with both caveolae/lipid rafts and non-caveolae/lipid raft, and the two pools likely exhibit separate dynamics.

Annexin 2 regulates intestinal epithelial cell spreading and wound closure through Rho-related signaling

Epithelial cell migration is a critical event in gastrointestinal mucosal wound healing and is dependent on actin cytoskeletal reorganization. We observed increased expression of an actin regulatory protein, annexin 2, in migrating intestinal epithelial cells. Small interfering RNA (siRNA)-mediated knockdown of annexin 2 expression in Caco-2 epithelial cells resulted in significant reductions in cell spreading and wound closure associated with decreased formation of filamentous actin bundles along the base of migrating cells. Because annexin 2 has been shown to influences actin cytoskeletal remodeling through targeting signaling molecules to membrane domains, we examined the membrane association and activation status of Rho GTPases after annexin 2 knockdown. We observed Rho dissociation from membranes and decreased Rho activity following annexin 2 siRNA transfection. Inhibition of cell spreading and wound closure in annexin 2 siRNA-transfected cells was prevented by expression of constitutively active RhoA. Rho colocalized with annexin 2 in lamellipodia and along the cytoplasmic face of the plasma membrane. In addition, annexin 2 was observed to co-immunoprecipitate with endogenous Rho and constitutively active RhoA. These findings suggest that annexin 2 plays a role in targeting Rho to cellular membranes, thereby modulating Rho-related signaling events regulating cytoskeletal reorganization during epithelial cell migration.

Na/K-ATPase beta1 subunit expression is required for blastocyst formation and normal assembly of trophectoderm tight junction-associated proteins

Na/K-ATPase plays an important role in mediating blastocyst formation. Despite the expression of multiple Na/K-ATPase alpha and beta isoforms during mouse preimplantation development, only the alpha1 and beta1 isoforms have been localized to the basolateral membrane regions of the trophectoderm. The aim of the present study was to selectively down-regulate the Na/K-ATPase beta1 subunit employing microinjection of mouse 1 cell zygotes with small interfering RNA (siRNA) oligos. Experiments comprised of non-injected controls and two groups microinjected with either Stealthtrade mark Na/K-ATPase beta1 subunit oligos or nonspecific Stealthtrade mark siRNA as control. Development to the 2-, 4-, 8-, and 16-cell and morula stages did not vary between the three groups. However, only 2.3% of the embryos microinjected with Na/K-ATPase beta1 subunit siRNA oligos developed to the blastocyst stage as compared with 73% for control-injected and 91% for non-injected controls. Na/K-ATPase beta1 subunit down-regulation was validated by employing reverse transcription-PCR and whole-mount immunofluorescence methods to demonstrate that Na/K-ATPase beta1 subunit mRNAs and protein were not detectable in beta1 subunit siRNA-microinjected embryos. Aggregation chimera experiments between beta1 subunit siRNA-microinjected embryos and controls demonstrated that blockade of blastocyst formation was reversible. The distribution of Na/K-ATPase alpha1 and tight junction-associated proteins occludin and ZO-1 were compared among the three treatment groups. No differences in protein distribution were observed between control groups; however, all three polypeptides displayed an aberrant distribution in Na/K-ATPase beta1 subunit siRNA-microinjected embryos. Our results demonstrate that the beta1 subunit of the Na/K-ATPase is required for blastocyst formation and that this subunit is also required to maintain a normal Na/K-ATPase distribution and localization of tight junction-associated polypeptides during preimplantation development.

Na-K-ATPase regulates tight junction permeability through occludin phosphorylation in pancreatic epithelial cells

Tight junctions are crucial for maintaining the polarity and vectorial transport functions of epithelial cells. We and others have shown that Na-K-ATPase plays a key role in the organization and permeability of tight junctions in mammalian cells and analogous septate junctions in Drosophila. However, the mechanism by which Na-K-ATPase modulates tight junctions is not known. In this study, using a well-differentiated human pancreatic epithelial cell line HPAF-II, we demonstrate that Na-K-ATPase is present at the apical junctions and forms a complex with protein phosphatase-2A, a protein known to be present at tight junctions. Inhibition of Na-K-ATPase ion transport function reduced protein phosphatase-2A activity, hyperphosphorylated occludin, induced rearrangement of tight junction strands, and increased permeability of tight junctions to ionic and nonionic solutes. These data suggest that Na-K-ATPase is required for controlling the tight junction gate function.

New roles for an old enzyme: Na,K-ATPase emerges as an interesting drug target

Na,K-ATPase (NKA) is well known for its role as a maintainer of electrolyte and fluid balance in cells, organs and whole body. Exciting new findings have revealed additional fundamental roles for NKA as a signal transducer and modulator of growth, apoptosis, cell adhesion and motility. The signal transduction function can be triggered by the binding of ouabain, the mammalian analogue of digitalis to NKA. The catalytic subunit of NKA exists in different forms and mutations in two of the forms that are expressed in brain can give rise to migraine, epilepsy and Parkinsonism-like symptoms. This review will present these new aspects of NKA and their clinical implications.

Janus model of the Na,K-ATPase beta-subunit transmembrane domain: distinct faces mediate alpha/beta assembly and beta-beta homo-oligomerization

Na,K-ATPase is a hetero-oligomer of alpha and beta-subunits. The Na,K-ATPase beta-subunit (Na,K-beta) is involved in both the regulation of ion transport activity, and in cell-cell adhesion. By structure prediction and evolutionary analysis, we identified two distinct faces on the Na,K-beta transmembrane domain (TMD) that could mediate protein-protein interactions: a glycine zipper motif and a conserved heptad repeat. Here, we show that the heptad repeat face is involved in the hetero-oligomeric interaction of Na,K-beta with Na,K-alpha, and the glycine zipper face is involved in the homo-oligomerization of Na,K-beta. Point mutations in the heptad repeat motif reduced Na,K-beta binding to Na,K-alpha, and Na,K-ATPase activity. Na,K-beta TMD homo-oligomerized in biological membranes, and mutation of the glycine zipper motif affected oligomerization and cell-cell adhesion. These results provide a structural basis for understanding how Na,K-beta links ion transport and cell-cell adhesion.

The role of the beta1 subunit of the Na,K-ATPase and its glycosylation in cell-cell adhesion

Based on recent data showing that overexpression of the Na,K-ATPase beta(1) subunit increased cell-cell adhesion of nonpolarized cells, we hypothesized that the beta(1) subunit can also be involved in the formation of cell-cell contacts in highly polarized epithelial cells. In support of this hypothesis, in Madin-Darby canine kidney (MDCK) cells, the Na,K-ATPase alpha(1) and beta(1) subunits were detected as precisely co-localized with adherens junctions in all stages of the monolayer formation starting from the initiation of cell-cell contact. The Na,K-ATPase and adherens junction protein, beta-catenin, stayed partially co-localized even after their internalization upon disruption of intercellular contacts by Ca(2+) depletion of the medium. The Na,K-ATPase subunits remained co-localized with the adherens junctions after detergent treatment of the cells. In contrast, the heterodimer formed by expressed unglycosylated Na,K-ATPase beta(1) subunit and the endogenous alpha(1) subunit was easily dissociated from the adherens junctions and cytoskeleton by the detergent extraction. The MDCK cell line in which half of the endogenous beta(1) subunits in the lateral membrane were substituted by unglycosylated beta(1) subunits displayed a decreased ability to form cell-to-cell contacts. Incubation of surface-attached MDCK cells with an antibody against the extracellular domain of the Na,K-ATPase beta(1) subunit specifically inhibited cell-cell contact formation. We conclude that the Na,K-ATPase beta(1) subunit is involved in the process of intercellular adhesion and is necessary for association of the heterodimeric Na,K-ATPase with the adherens junctions. Further, normal glycosylation of the Na,K-ATPase beta(1) subunit is essential for the stable association of the pump with the adherens junctions and plays an important role in cell-cell contact formation.

Multiplicity of expression of FXYD proteins in mammalian cells: dynamic exchange of phospholemman and gamma-subunit in response to stress

Functional properties of Na-K-ATPase can be modified by association with FXYD proteins, expressed in a tissue-specific manner. Here we show that expression of FXYDs in cell lines does not necessarily parallel the expression pattern of FXYDs in the tissue(s) from which the cells originate. While being expressed only in lacis cells in the juxtaglomerular apparatus and in blood vessels in kidney, FXYD1 was abundant in renal cell lines of proximal tubule origin (NRK-52E, LLC-PK1, and OK cells). Authenticity of FXYD1 as a part of Na-K-ATPase in NRK-52E cells was demonstrated by co-purification, co-immunoprecipitation, and co-localization. Induction of FXYD2 by hypertonicity (500 mosmol/kgH(2)O with NaCl for 48 h or adaptation to 700 mosmol/kgH(2)O) correlated with downregulation of FXYD1 at mRNA and protein levels. The response to hypertonicity was influenced by serum factors and entailed, first, dephosphorylation of FXYD1 at Ser(68) (1-5 h) and, second, induction of FXYD2a and a decrease in FXYD1 with longer exposure. FXYD1 was completely replaced with FXYD2a in cells adapted to 700 mosmol/kgH(2)O and showed a significantly decreased sodium affinity. Thus dephosphorylation of FXYD1 followed by exchange of regulatory subunits is utilized to make a smooth transition of properties of Na-K-ATPase. We also observed expression of mRNA for multiple FXYDs in various cell lines. The expression was dynamic and responsive to physiological stimuli. Moreover, we demonstrated expression of FXYD5 protein in HEK-293 and HeLa cells. The data imply that FXYDs are obligatory rather than auxiliary components of Na-K-ATPase, and their interchangeability underlies responses of Na-K-ATPase to cellular stress.

Connexin 26 expression prevents down-regulation of barrier and fence functions of tight junctions by Na+/K+-ATPase inhibitor ouabain in human airway epithelial cell line Calu-3

Gap junctions are considered to play a crucial role in differentiation of epithelial cells and to be associated with tight junction proteins. In this study, to investigate the role of gap junctions in regulation of the barrier function and fence function on the tight junctions, we introduced the Cx26 gene into human airway epithelial cell line Clau-3 and used a disruption model of tight junctions employing the Na(+)/K(+)-ATPase inhibitor ouabain. In parental Calu-3 cells, gap junction proteins Cx32 and Cx43, but not Cx26, and tight junction proteins occludin, JAM-1, ZO-1, claudin-1, -2, -3, -4, -5, -6, -7, -8, -9, and -14 were detected by RT-PCR. The barrier function and fence function of tight junctions were well maintained, whereas the GJIC was low level. Treatment with ouabain caused disruption of the barrier function and fence function of tight junctions together with down-regulation of occludin, JAM-1, claudin-2, and -4 and up-regulation of ZO-1 and claudin-14. In Cx26 transfectants, Cx26 protein was detected by Western blotting and immunocytochemistry, and many gap junction plaques were observed with well-developed tight junction strands. Expression of claudin-14 was significantly increased in Cx26 transfectants compared to parental cells, and in some cells, Cx26 was co-localized with claudin-14. Interestingly, transfection with Cx26 prevented disruption of both functions of tight junctions by treatment with ouabain without changes in the tight junction proteins. Pretreatment with the GJIC blockers 18beta-glycyrrhetinic acid and oleamide did not affect the changes induced by Cx26 transfection. These results suggest that Cx26 expression, but not the mediated intercellular communication, may regulate tight junction barrier and fence functions in human airway epithelial cell line Calu-3.

Distinct Role of the N-terminal Tail of the Na,K-ATPase Catalytic Subunit as a Signal Transducer

Mounting evidence suggests that the ion pump, Na,K-ATPase, can, in the presence of ouabain, act as a signal transducer. A prominent binding motif linking the Na,K-ATPase to intracellular signaling effectors has, however, not yet been identified. Here we report that the N-terminal tail of the Na,K-ATPase catalytic alpha-subunit (alphaNT-t) binds directly to the N terminus of the inositol 1,4,5-trisphosphate receptor. Three amino acid residues, LKK, conserved in most species and most alpha-isoforms, are essential for the binding to occur. In wild-type cells, low concentrations of ouabain trigger low frequency calcium oscillations that activate NF-kappaB and protect from apoptosis. All of these effects are suppressed in cells overexpressing a peptide corresponding to alphaNT-t but not in cells overexpressing a peptide corresponding to alphaNT-t deltaLKK. Thus we have identified a well conserved Na,K-ATPase motif that binds to the inositol 1,4,5-trisphosphate receptor and can trigger an anti-apoptotic calcium signal.

Functional Characterization of Src-interacting Na/K-ATPase Using RNA Interference Assay

We have shown that the Na/K-ATPase and Src form a signaling receptor complex. Here we determined how alterations in the amount and properties of the Na/K-ATPase affect basal Src activity and ouabain-induced signal transduction. Several alpha1 subunit knockdown cell lines were generated by transfecting LLC-PK1 cells with a vector expressing alpha1-specific small interference RNA. Although the alpha1 knockdown resulted in significant decreases in Na/K-ATPase activity, it increased the basal Src activity and tyrosine phosphorylation of focal adhesion kinase, a Src effector. Concomitantly it also abolished ouabain-induced activation of Src and ERK1/2. When the knockdown cells were rescued by a rat alpha1, both Na/K-ATPase activity and the basal Src activity were restored. In addition, ouabain was able to stimulate Src and ERK1/2 in the rescued cells at a much higher concentration, consistent with the established differences in ouabain sensitivity between pig and rat alpha1. Finally both fluorescence resonance energy transfer analysis and co-immunoprecipitation assay indicated that the pumping-null rat alpha1 (D371E) mutant could also bind Src. Expression of this mutant restored the basal Src activity and focal adhesion kinase tyrosine phosphorylation. Taken together, the new findings suggest that LLC-PK1 cells contain a pool of Src-interacting Na/K-ATPase that not only regulates Src activity but also serves as a receptor for ouabain to activate protein kinases.

Association of prostate-specific membrane antigen with caveolin-1 and its caveolae-dependent internalization in microvascular endothelial cells: Implications for targeting to tumor vasculature

Prostate-specific membrane antigen (PSMA) is a transmembrane protein with a highly restricted profile of expression. Expression is primarily limited to secretory cells of the prostatic epithelium, with elevated levels observed in prostate cancer. As an integral membrane protein correlated with prostate cancer, PSMA offers a potentially valuable target for immunotherapy. PSMA is also detected in the neovasculature of a variety of solid tumors but not in the endothelial cells of preexisting blood vessels. Although the significance of PSMA expression in these cells remains elusive, this pattern of expression implies that PSMA may perform a functional role in angiogenesis and may offer a therapeutic target for the treatment of a broad spectrum of solid tumors. In this study, we have expressed PSMA in human microvascular endothelial cells and demonstrate that PSMA binds to caveolin-1 and undergoes internalization via a caveolae-dependent mechanism. The association between PSMA and caveolae in endothelial cells may provide important insight into PSMA function and ways to best exploit this protein for therapeutic benefit.

Sodium/potasium ATPase (Na+, K+-ATPase) and ouabain/related cardiac glycosides: a new paradigm for development of anti- breast cancer drugs?

Prolonged exposure to 17beta-estradiol (E2) is a key etiological factor for human breast cancer. The biological effects and carcinogenic effects of E2 are mediated via estrogen receptors (ERs), ERalpha and ERbeta. Anti-estrogens, e.g. tamoxifen, and aromatase inhibitors have been used to treat ER-positive breast cancer. While anti-estrogen therapy is initially successful, a major problem is that most tumors develop resistance and the disease ultimately progresses, pointing to the need of developing alternative drugs targeting to other critical targets in breast cancer cells. We have identified that Na+, K+-ATPase, a plasma membrane ion pump, has unique/valuable properties that could be used as a potentially important target for breast cancer treatment: (a) it is a key player of cell adhesion and is involved in cancer progression; (b) it serves as a versatile signal transducer and is a target for a number of hormones including estrogens and (d) its aberrant expression and activity are implicated in the development and progression of breast cancer. There are several lines of evidence indicating that ouabain and related digitalis (the potent inhibitors of Na+, K+-ATPase) possess potent anti-breast cancer activity. While it is not clear how the suggested anti-cancer activity of these drugs work, several observations point to ouabain and digitalis as being potential ER antagonists. We critically reviewed many lines of evidence and postulated a novel concept that Na+, K+-ATPase in combination with ERs could be important targets of anti-breast cancer drugs. Modulators, e.g. ouabain and related digitalis could be useful to develop valuable anti-breast cancer drugs as both Na+, K+-ATPase inhibitors and ER antagonists.

Binding of Src to Na+/K+-ATPase forms a functional signaling complex

We have shown that ouabain activates Src, resulting in subsequent tyrosine phosphorylation of multiple effectors. Here, we tested if the Na+/K+-ATPase and Src can form a functional signaling complex. In LLC-PK1 cells the Na+/K+-ATPase and Src colocalized in the plasma membrane. Fluorescence resonance energy transfer analysis indicated that both proteins were in close proximity, suggesting a direct interaction. GST pulldown assay showed a direct, ouabain-regulated, and multifocal interaction between the 1 subunit of Na+/K+-ATPase and Src. Although the interaction between the Src kinase domain and the third cytosolic domain (CD3) of 1 is regulated by ouabain, the Src SH3SH2 domain binds to the second cytosolic domain constitutively. Functionally, binding of Src to either the Na+/K+-ATPase or GST-CD3 inhibited Src activity. Addition of ouabain, but not vanadate, to the purified Na+/K+-ATPase/Src complex freed the kinase domain and restored the Src activity. Consistently, exposure of intact cells to ouabain apparently increased the distance between the Na+/K+-ATPase and Src. Concomitantly, it also stimulated tyrosine phosphorylation of the proteins that are associated with the Na+/K+-ATPase. These new findings illustrate a novel molecular mechanism of signal transduction involving the interaction of a P-type ATPase and a nonreceptor tyrosine kinase.

A moving new role for the sodium pump in epithelial cells and carcinomas

The Na,K-ATPase, or sodium pump, is a ubiquitous plasma membrane protein in higher eukaryotes, including humans, that carries out the coupled active transport of Na+ ions out of the cell and of K+ ions into the cell, using the energy of hydrolysis of adenosine triphosphate. In recent years, it has been suggested that that this protein may also be involved in various other functions, such as transducing information from the extracellular milieu to intracellular signaling pathways, much like a growth factor receptor. It has also been suggested that the sodium pump may be essential to the formation and function of junctional complexes in epithelial cells, and, most recently, it has been shown to play a role in epithelial cell motility. Maloney sarcoma virus-transformed Madin Darby canine kidney cells have depressed Na,K-ATPase beta subunit abundance and enhanced motility as compared with untransformed cells. Repletion of Na,K-ATPase beta subunits in the transformed cells results in suppression of motility. The most recent work, discussed here, demonstrates that the Na,K-ATPase alpha and beta subunits play distinct and separate roles, interacting with proteins in the phosphatidylinositol 3-kinase pathway and leading to the remodeling of the cytoskeleton and lamellipodia formation. The sodium pump subunits thus seem to play a role in regulating carcinoma cell motility and may be involved in cell motility suppression in many epithelial cells.

Lack of high affinity fiber receptor activity explains the resistance of ciliated airway epithelia to adenovirus infection

Although recombinant adenoviruses are attractive vectors for gene transfer to airway epithelia, they have proven to be relatively inefficient. To investigate the mechanisms of adenovirus-mediated gene transfer to airway epithelia, we examined the role of adenovirus fiber and penton base, the two proteins involved in attachment to and entry of virus into the cell. We used human airway epithelia grown under conditions that allow differentiation and development of a ciliated apical surface that closely resembles the in vivo condition. We found that addition of fiber protein inhibited virus binding and vector-mediated gene transfer to immature airway epithelia, as well as to primary cultures of rat hepatocytes and HeLa cells. However, fiber protein had no effect on vector binding and gene transfer to ciliated airway epithelia. We obtained similar results with addition of penton base protein: the protein inhibited gene transfer to immature epithelia, whereas there was no effect with ciliated epithelia. Moreover, infection was not attenuated with an adenovirus containing a mutation in penton base that prevents the interaction with cell surface integrins. These data suggest that the receptors required for efficient infection by adenovirus are either not present or not available on the apical surface of ciliated human airway epithelia. The results explain the reason for inefficient gene transfer and suggest approaches for improvement.