307 INVOLVEMENT OF THE CALCIUM SENSING RECEPTOR IN GROWTH AND PROLIFERATION OF STEM CELLS FROM EQUINE UMBILICAL CORD MATRIX

The calcium sensing receptor (CaSR) plays a key role in cells involved in calcium (Ca2+) homeostasis by directly sensing changes in extracellular Ca2+ concentration ([Ca2+]o), and external Ca2+ is a potent mediator of cell proliferation. The present study investigated the effects of high [Ca2+]o and of the CaSR agonist NPS R-467 on growth and proliferation of equine size-sieved umbilical cord matrix mesenchymal stem cells (UCM-MSC). The involvement of CaSR on observed cell response was analysed at the mRNA and protein level. Two subpopulations of UCM-MSC, isolated using multi-dishes with transwell inserts of 8-μm pores and expressing MSC markers (CD105, CD44, CD29; Corradetti et al. 2010 Reprod. Fertil. Dev. 22, 347–348), were analysed. Cells were cultured in medium containing: (A) low [Ca2+]o (0.37 mM), (B) high [Ca2+]o (2.87 mM), (C) NPS R-467 (3 μm) in the presence of high [Ca2+]o, and (D) the CaSR antagonist NPS 2390 (10 μm for 30′) followed by NPS R-467 in the presence of high [Ca2+]o. Growth and proliferation rates were compared among treatments (Student’s t-test). The CaSR expression and subcellular localization were investigated by real-time quantitative RT-PCR, immunofluorescence, and confocal microscopy. In the >8-μm cell line, the addition of NPS R-467, in the presence of [Ca2+]o, significantly increased cell growth after day 7 of culture (C v. A and B; P < 0.001). Increasing [Ca2+]o was not effective in this cell line (B v. A; not significant). In the <8-μm cell line, NPS R-467 increased cell growth, even at a lower extent (C v. A; P < 0.05), as observed on day 9 of culture. In this cell line, an increased proliferation rate was observed upon [Ca2+]o increase (B v. A; P < 0.05). In both cell lines, preincubation with NPS 2390 significantly inhibited the agonistic effect of NPS R-467. In both cell lines, a stimulatory effect of additional calcium and NPS R-467 on cell proliferation, in terms of reduced DT values, was observed. In the 2 cell lines, CaSR expression was down-regulated in the presence of high calcium and in NPS R-467-treated cells compared with controls (B and C v. A cells; P < 0.001). Treatment with high calcium or NPS R-467 reduced CaSR labelling in the cytosol and increased it at the cortical level. We found that CaSR is expressed at mRNA and protein levels in equine UCM-MSC, and it is functionally active because the selective CaSR agonist NPS R-467 induced a stimulatory effect on cell growth and proliferation, which was reversed by the CaSR antagonist NPS 2390. The different responses to treatments between the 2 UCM-MSC subpopulations suggest that CaSR could be differentially activated in these cell lines. The calcimimetic NPS R-467 might be useful as an adjunctive component of media for UCM-MSC culture to obtain enough cells for down-stream purposes. Financial support was provided by Fondi di Ateneo 2009; University of Bari Aldo Moro (COD. ORBA09UDWX) (Resp. Sci. Maria Elena Dell’Aquila).

NHE3 inhibits PKA-dependent functional expression of CFTR by NHERF2 PDZ interactions

It has been shown that when CFTR and NHE3 are co-expressed on the apical membrane of the A6-NHE3 cell monolayers, the two transporters interact via a shared regulatory complex composed of NHERF2, ezrin, and PKA. We observe here that co-expression of NHE3 reduced both PKA-dependent apical CFTR expression and its activation once in place by approximately 50%. To analyze the role of NHERF2 in this process, we transfected NHE3 expressing and non-expressing A6 monolayers with NHERF2 cDNA in which its binding domains had been deleted. When only CFTR is expressed on the apical membrane, deletion of any of the NHERF2 binding domains inhibited both PKA-dependent apical CFTR expression and its activation, while when NHE3 was co-expressed with CFTR PDZ2 deletion was without effect on CFTR sorting and activity. This suggests that when the PDZ2 domain is "sequestered" by interacting with NHE3 it can no longer participate in CFTR functional expression.

Stimulation of Xenopus P2Y1 receptor activates CFTR in A6 cells

Nucleotide binding to purinergic P2Y receptors contributes to the regulation of a variety of physiological functions in renal epithelial cells. Here, we investigate the regulatory mechanism of the P2Y1 receptor agonist 2-methylthioadenosine diphosphate (2-MeSADP) on Cl- transport in A6 cells, a commonly used model of the distal section of the Xenopus laevis nephron. Protein and mRNA expression analysis together with functional measurements demonstrated the basolateral location of the Xenopus P2Y1 receptor. 2-MeSADP increased intracellular [Ca2+] and cAMP and Cl- efflux, responses that were all inhibited by the specific P2Y1 receptor antagonist MRS 2179. Cl- efflux was also inhibited by the cystic fibrosis transmembrane conductance regulator (CFTR) blocker glibenclamide. Inhibition of either protein kinase A (PKA) or the binding between A-kinase-anchoring proteins (AKAPs) and the regulatory PKA RII subunit blocked the 2-MeSADP-induced activation of CFTR, suggesting that PKA mediates P2Y1 receptor regulation of CFTR through one or more AKAPs. Further, the truncation of the PDZ1 domain of the scaffolding protein Na+/H+ exchanger regulatory factor-2 (NHERF-2) inhibited 2-MeSADP-dependent stimulation of Cl- efflux, suggesting the involvement of this scaffolding protein. Activation or inhibition of PKC had no effect per se on basal Cl- efflux but potentiated or reduced the 2-MeSADP-dependent stimulation of Cl- efflux, respectively. These data suggest that the X laevis P2Y1 receptor in A6 cells can increase both cAMP/PKA and Ca2+/PKC intracellular levels and that the PKC pathway is involved in CFTR activation via potentiation of the PKA pathway.

Hydrogen ion dynamics and the Na+/H+ exchanger in cancer angiogenesis and antiangiogenesis

Tumour angiogenesis and cellular pH regulation, mainly represented by Na(+)/H(+) antiporter exchange, have been heretofore considered unrelated subfields of cancer research. In this short review, the available experimental evidence relating these areas of modern cancer research is introduced. This perspective also helps to design a new approach that facilitates the opening and development of novel research lines oriented towards a rational incorporation of anticancer drugs into more selective and less toxic therapeutic protocols. The final aim of these efforts is to control cancer progression and dissemination through the control of tumour angiogenesis. Finally, different antiangiogenic drugs that can already be clinically used to this effect are briefly presented.

Association of pS2 (TFF1) release with breast tumour proliferative rate: in vitro and in vivo studies

Although cytosolic expression of the protein pS2 (TFF1) is considered to be a marker of oestrogen receptor (OR) function, there exists some clinical data to suggest an inverse relationship of cytosolic pS2 to tumour proliferation. Although secreted from breast cancer cells, the relationship of pS2 secretion to tumour natural history has been little studied. The mechanisms and kinetics of pS2 release and its relation to tumour cell proliferation were studied in a human breast cancer cell line MCF-7 and verified in a preliminary clinical study. Stimulation by stripped serum or oestradiol resulted in parallel increases of proliferation and pS2 release in both time course and dose-response experiments. Direct pharmacological alterations of proliferation were followed by identical changes in pS2 release. The relationship between serum pS2 levels and tumour proliferative activity when analysed as a function of steroid status showed a slope of 0.56 in OR+ vs. 0.19 in OR- tumours. It is concluded that pS2 release from breast cancer cells is associated with their proliferation and measurement of serum pS2 levels might be a good predictor of tumour proliferative state and could permit noninvasive monitoring of this tumour parameter.

Effects of beta-endorphin and Naloxone on in vitro maturation of bovine oocytes

Bovine cumulus-oocyte complexes (COCs) and mural granulosa cells express the mRNA coding for the micro-opioid receptor. The addition of beta-endorphin (beta-end) to oocytes cultured in hormonally-supplemented in vitro maturation (IVM) medium had no effect on the rates of oocytes reaching the metaphase II (MII) stage, but significantly decreased the maturation rate (P < 0.05) and arrested oocytes at metaphase I (MI) after culture in hormone-free medium (P < 0.001). Naloxone (Nx) reverted this inhibitory effect of beta-end. Moreover, Nx "per se" showed a dose-dependent dual effect. When added at high concentration (10 x (-3) M), it significantly reduced the rate of oocytes in MII (P < 0.001), thus increasing the rate of oocytes arrested in MI. However, Nx added at low concentration (10 x (-8) M) significantly increased oocyte maturation (P < 0.001). High concentration of Nx induced an increase in both intracellular calcium concentration ([Ca(2+)](i)) and in the activity of the mitogen-activated protein kinase (MAPK) also called extracellular-regulated kinase (ERK) in cumulus cells of bovine COCs. Blocking the rise in [Ca(2+)](i) with the calcium chelator acetoxymethylester-derived form of bis (o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) reversed the Nx-dependent inhibition of meiotic maturation observed at high Nx concentrations. Whereas blocking ERK with the MAPK/ERK kinase (MEK) inhibitor, PD98059, had no effect on this process. Therefore, we concluded that the mocro-opioid receptor, by inducing [Ca(2+)](i) increase, participates in the cumulus-oocyte coupled signaling associated with oocyte maturation.

Extracellular adenine nucleotides regulate Na+/H+ exchanger NHE3 activity in A6-NHE3 transfectants by a cAMP/PKA-dependent mechanism

As potential autocrine or paracrine factors, extracellular nucleotides are known to be important regulators of renal ion transporters by activating cell surface receptors and intracellular signaling pathways. We investigated the influence of extracellular adenine nucleotides on Na+/H+ exchanger isoform 3 (NHE3) activity in A6-NHE3 cells. This is a polarized cell line obtained by stable transfection of A6 cells with the cDNA encoding the rat isoform of NHE3, which is expressed on the apical membrane. Basolateral addition of the P2Y(1) agonist, 2-MeSADP, induced an inhibition of NHE3 activity, which was prevented by preincubation with selective P2Y(1) antagonists, MRS 2179 (N6-methyl-2'-deoxyadenosine-3',5'-bisphosphate) and MRS 2286 (2-[2-(2-chloro-6-methylamino-purin-9-yl)-ethyl]-propane-1,3-bisoxy(diammoniumphosphate)). NHE3 activity was also significantly inhibited by ATP and ATP-gamma-S but not by UTP. 2-MeSADP induced a P2Y(1) antagonist-sensitive increase in both [Ca2+]i and cAMP production. Pre-incubation with a PKC inhibitor, Calphostin C, or the calcium chelator BAPTA-AM, had no effect on the 2-MeSADP-dependent inhibition of NHE3 activity, whereas this inhibition was reversed by either incubation with the PKA inhibitor H89 or by mutation of two PKA target serines (S552 and S605) on NHE3. Pre-incubation of the A6-NHE3 cells with the synthetic peptide, Ht31, which prevents the binding between AKAPs and the regulatory PKA subunits RII, also prevented the 2-MeSADP-induced inhibition of NHE3. We conclude that only the cAMP/PKA pathway is involved in the inhibition of NHE3 activity.

Naloxone inhibits A6 cell Na(+)/H(+) exchange by activating protein kinase C via the mobilization of intracellular calcium

The opioid receptor antagonist, naloxone, has been shown to have beneficial effects in the kidney and to be implicated in renal salt and water balance. In the present study the signal transduction pathways utilized by naloxone were studied in an epithelial cell line model of the cortical collecting duct, A6 cells. We found that naloxone has a dual effect depending on the concentration used: at a low concentration (10(-6) M) it antagonized the beta-endorphin-dependent increase in cytoplasmic calcium [Ca(2+)](i), while at higher concentrations (>10(-5) M) it increased [Ca(2+)](i) and intracellular inositol phosphate levels. While naloxone-induced increases in [Ca(2+)](i) occurred in the absence of external calcium, it was significantly stimulated by increasing the external calcium concentration, suggesting that naloxone increases [Ca(2+)](i) via both calcium release and calcium influx. In polarized A6 cell monolayers naloxone inhibited the activity of the Na(+)/H(+) exchanger (NHE) only when added to the basolateral cell surface. This inhibition of the NHE was prevented by pretreatment of the cells with either the intracellular calcium chelator, BAPTA or with the protein kinase C inhibitor, calphostin C. These findings demonstrate that naloxone induces a rapid increase in intracellular calcium which inhibits the NHE via the calcium-dependent protein kinase C regulatory pathway.

Activation of A(3) adenosine receptor induces calcium entry and chloride secretion in A(6) cells

We have previously demonstrated that in A(6) renal epithelial cells, a commonly used model of the mammalian distal section of the nephron, adenosine A(1) and A(2A) receptor activation modulates sodium and chloride transport and intracellular pH (Casavola et al., 1997). Here we show that apical addition of the A(3) receptor-selective agonist, 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-methyluronamide (Cl-IB-MECA) stimulated a chloride secretion that was mediated by calcium- and cAMP-regulated channels. Moreover, in single cell measurements using the fluorescent dye Fura 2-AM, Cl-IB-MECA caused an increase in Ca(2+) influx. The agonist-induced rise in [Ca(2+)](i) was significantly inhibited by the selective adenosine A(3) receptor antagonists, 2,3-diethyl-4, 5-dipropyl-6-phenylpyridine-3-thiocarboxylate-5-carboxylate (MRS 1523) and 3-ethyl 5-benzyl 2-methyl-6-phenyl-4-phenylethynyl-1, 4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS 1191) but not by antagonists of either A(1) or A(2) receptors supporting the hypothesis that Cl-IB-MECA increases [Ca(2+)](i) by interacting exclusively with A(3) receptors. Cl-IB-MECA-elicited Ca(2+) entry was not significantly inhibited by pertussis toxin pretreatment while being stimulated by cholera toxin preincubation or by raising cellular cAMP levels with forskolin or rolipram. Preincubation with the protein kinase A inhibitor, H89, blunted the Cl-IB-MECA-elicited [Ca(2+)](i) response. Moreover, Cl-IB-MECA elicited an increase in cAMP production that was inhibited only by an A(3) receptor antagonist. Altogether, these data suggest that in A(6) cells a G(s)/protein kinase A pathway is involved in the A(3) receptor-dependent increase in calcium entry.

Na+/H+ exchanger-dependent intracellular alkalinization is an early event in malignant transformation and plays an essential role in the development of subsequent transformation-associated phenotypes

In this study we investigate the mechanism of intracellular pH change and its role in malignant transformation using the E7 oncogene of human papillomavirus type 16 (HPV16). Infecting NIH3T3 cells with recombinant retroviruses expressing the HPV16 E7 or a transformation deficient mutant we show that alkalinization is transformation specific. In NIH3T3 cells in which transformation can be turned on and followed by induction of the HPV16 E7 oncogene expression, we demonstrate that cytoplasmic alkalinization is an early event and was driven by stimulation of Na+/H+ exchanger activity via an increase in the affinity of the intracellular NHE-1 proton regulatory site. Annulment of the E7-induced cytoplasmic alkalinization by specific inhibition of the NHE-1, acidification of culture medium, or clamping the pHi to nontransformed levels prevented the development of later transformed phenotypes such as increased growth rate, serum-independent growth, anchorage-independent growth, and glycolytic metabolism. These findings were verified in human keratinocytes (HPKIA), the natural host of HPV. Results from both NIH3T3 and HPKIA cells show that alkalinization acts on pathways that are independent of the E2F-mediated transcriptional activation of cell cycle regulator genes. Moreover, we show that the transformation-dependent increase in proliferation is independent of the concomitant stimulation of glycolysis. Finally, treatment of nude mice with the specific inhibitor of NHE-1, DMA, delayed the development of HPV16-keratinocyte tumors. Our data confirm that activation of the NHE-1 and resulting cellular alkalinization is a key mechanism in oncogenic transformation and is necessary for the development and maintenance of the transformed phenotype.

Phosphoinositide 3-kinase is involved in the tumor-specific activation of human breast cancer cell Na(+)/H(+) exchange, motility, and invasion induced by serum deprivation

Whereas the tumor acidic extracellular pH plays a crucial role in the invasive process, the mechanism(s) behind this acidification, especially in low nutrient conditions, are unclear. The regulation of the Na(+)/H(+) exchanger (NHE) and invasion by serum deprivation were studied in a series of breast epithelial cell lines representing progression from non-tumor to highly metastatic cells. Whereas serum deprivation reduced lactate production in all three cells lines, it inhibited NHE activity in the non-tumor cells and stimulated it in the tumor cells with a larger stimulation in the metastatic cells. The stimulation of NHE in the tumor cell lines was the result of an increased affinity of the internal H(+) regulatory site of the NHE without changes in sodium kinetics or expression. Serum deprivation conferred increased cell motility and invasive ability that were abrogated by specific inhibition of the NHE. Inhibition of phosphoinositide 3-kinase by overexpression of a dominant-negative mutant or wortmannin incubation inhibited NHE activity and invasion in serum replete conditions while potentiating the serum deprivation-dependent activation of the NHE and invasion. These results indicate that the up-regulation of the NHE by a phosphoinositide 3-kinase-dependent mechanism plays an essential role in increased tumor cell invasion induced by serum deprivation.

GnRH receptors in human breast cancer and its contiguous not-involved breast tissue

The present study was undertaken to evaluate the presence of GnRH receptors (GnRH-R) in breast cancer and not-involved breast tissue, and the relationships between GnRH-R and receptors for estrogen (ER) and progesterone (PgR) in the same tissues. Utilizing a tritiated natural GnRH in order to assay the native receptor binding we analyzed the level of binding sites for GnRH in membranes derived from 90 breast tumors and in 40 cases from neighboring, not-involved breast tissue. GnRH-R was found both in cancer and normal tissues. The prevalence for GnRH-R was higher in tumor than in not-tumor tissue (45% vs 39%, respectively), but the overall levels were not significantly different (15.9+/-24 fmol/mg protein vs 18.2+/-39 fmol/mg protein, respectively). The only statistically different content of GnRH-R we found concerned PgR negative vs PgR positive tumor tissues (mean content: 23 vs 11 fmol/mg protein, respectively in PgR- and PgR+ tumors, p=0.03 by t test); furthermore the proportion of GnRH-R positive cases in the tumor resulted significantly higher in premenopausal patients vs postmenopausal (56% vs 32%, by Chi square test, p<0.05). The GnRH receptors status of primary tumor and contiguous not-involved breast tissue resulted associated (overall agreement: 63%, p<0.05) but no specific steroid patterns for GnRH-R positivity was observed.

TGF-beta1 and IGF-1 expression are differently regulated by serum in metastatic and non-metastatic human breast cancer cells

Transforming growth factor-beta (TGF-beta) exerts an inhibitory effect on epithelial cell proliferation while insulin-like growth factor-1 (IGF-1) is a positive regulator of proliferation and together they may participate in driving neoplastic progression. The regulation of TGF-beta1 and IGF-1 gene expression was analyzed in an in vitro model of an estrogen receptor positive (ER+), non-metastatic (MCF-7) and an (ER-), metastatic (MDA-MB-435) breast cancer cell line, respectively. Our results indicate a loss of the regulation of TGF-beta1 and the gain of the expression and upregulation of IGF-1 pathways during malignant progression. These data demonstrate that two factors, convergent on cell growth, can have divergent roles in the regulation of the expression of TGF-beta1.

Polarized distribution of Na+/H+ exchanger isoforms in rabbit collecting duct cells

The present study describes two Na+/H+ exchanger (NHE) isoforms in an immortalized rabbit renal cortical collecting tubule cell line (RC.SV3). Na+/H+ exchange activity was assayed using fluorescence measurements of intracellular pH (pHi) in monolayers mounted in a cuvette containing two fluid compartments, making it possible to independently measure Na+/H+ exchange activity on either the apical or basolateral surface. RC.SV3 monolayers express Na+/H+ exchange activities in both the apical and basolateral membrane domains. The two exchangers have half-saturation constants (Km) for external sodium and sensitivities to dimethylamiloride, to HOE-694 and to cimetidine and clonidine consistant with the NHE-1 isoform on the basolateral cell surface and the NHE-2 isoform on the apical surface. Protein kinase A inhibition of basolateral exchanger activity was significantly higher than that of the apical exchanger. Protein kinase C significantly stimulated both exchangers equally. RT-PCR analysis found RNA for only NHE-1 and NHE-2, and immunofluorescence with an antibody against NHE-1 demonstrated a basolateral location for this isoform. The results suggest that RC.SV3 cells have two Na+/H+ exchange activities separated spatially to the two cellular membranes, with the NHE-1 and the NHE-2 isoforms located on the basolateral and the apical membranes, respectively.

Release of the aspartyl protease cathepsin D is associated with and facilitates human breast cancer cell invasion

Data concerning the hormone sensitivity of the release and role of the aspartyl protease cathepsin D in tumor proliferative and invasive processes have been contradictory. To clarify the mechanisms of its release and role we first studied the contribution of estradiol and stripped serum to the time course and kinetics of cathepsin D release, proliferation, and invasion in parallel in the MCF-7 in vitro breast cancer cell culture model. Both estradiol and stripped serum independently stimulated both proliferation and cathepsin D release. However, the dose-response of estradiol and stripped serum-dependent stimulated release were similar to those for invasion and differed from those for proliferation: cathepsin D release and invasion were first stimulated at a stripped serum concentration more than 10-fold lower than that which initiated proliferation and had half stimulation constants almost 10-fold lower than those for proliferation. These results demonstrate that cathepsin D release is not related in any direct way to proliferation. The effect of the reduction of cathepsin D activity or release on in vitro invasion was also measured: both the inhibition of secreted cathepsin D activity by a specific inhibitor, diazoacetyl-DL-Nle-OMe, and the reduction of cathepsin D release by antisense oligonucleotides against its translation start site reduced cellular in vitro invasion without affecting proliferation. Cathepsin D release and activity are concluded to be directly involved in the process of invasion.

Polarization of adenosine effects on intracellular pH in A6 renal epithelial cells

The effect of adenosine on Na+/H+ exchange activity was examined in cultured A6 renal epithelial cells. Adenosine and its analogue N6-cyclopentyladenosine (CPA) had different effects on Na+/H+ exchange activity depending on the side of addition. Basolateral CPA induced a stimulation of Na+/H+ exchange activity that was completely prevented by preincubation with an A2A-selective antagonist, 8-(3-chlorostyryl)caffeine, whereas apical CPA induced a slight but significant inhibition of Na+/H+ exchange activity that was significantly reduced by the A1-receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine. Protein kinase C activation may be involved in mediating the apical CPA inhibition of Na+/H+ exchange activity; this inhibition was prevented by the protein kinase C inhibitor calphostin C. Treatment with either forskolin or 8-bromo-cAMP significantly stimulated Na+/H+ exchange activity; only basolateral CPA addition induced an increase in cAMP level. These observations together with the finding that the CPA-dependent stimulation of exchange activity was prevented by the protein kinase A inhibitor H-89 support the hypothesis that basolateral CPA stimulates Na+/H+ exchange via adenylate cyclase/protein kinase A activation. Basolateral CPA also increased transepithelial Na+ transport, and this stimulation was prevented by the Na+/H+ exchange inhibitor HOE-694, suggesting that changes in pHi during hormone action can act as an intermediate in the second-messenger cascade.

Effect of adenosine on Na+ and Cl- currents in A6 monolayers. Receptor localization and messenger involvement

The effect of adenosine regulation on sodium and chloride transport was examined in cultured A6 renal epithelial cells. Adenosine and its analogue N6-cyclopentyladenosine (CPA) had different effects on short-circuit current (Isc) depending on the side of addition. Basolateral CPA addition induced an approximately threefold increase of the Isc that reached a maximum effect 20 min after addition and was completely inhibited by preincubation with either an A2 selective antagonist, CSC, or the sodium channel blocker, amiloride. Apical CPA addition induced a biphasic Isc response characterized by a rapid fourfold transient increase over its baseline followed by a decline and a plateau phase that were amiloride insensitive. The A1 adenosine antagonist, CPX, completely prevented this response. This Isc response to apical CPA was also strongly reduced in Cl--free media and was significantly inhibited either by basolateral bumetanide or apical DPC preincubation. Only basolateral CPA addition was able to induce an increase in cAMP level. CPA, added to cells in suspension, caused a rapid rise in [Ca2+]i that was antagonized by CPX, not affected by CSC and prevented by thapsigargin preincubation. These data suggest that basolateral CPA regulates active sodium transport via A2 adenosine receptors stimulating adenylate cyclase while apical CPA regulates Cl- secretion via A1 receptor-mediated changes in [Ca2+]i.

Differential responsiveness of proliferation and cytokeratin release to stripped serum and oestrogen in the human breast cancer cell line, MCF-7

In vitro research into hormone sensitivity and the relation to proliferation of cytokeratin release from cancer cells is scarce. Therefore, we examined the stimulation of proliferation and the release of cytokeratins in a breast cancer cell culture model. Cell growth was stimulated by 17 beta-oestradiol (10(-11) M), stripped serum (10%) and by the two together. Cytokeratin release was stimulated only by stripped serum, oestradiol having no effect. After long incubation periods (> 12 h), cytokeratin release also commenced in the control and oestradiol treatments. Release rate versus time analysis suggested that there are two different release processes. Cytokeratin release was first stimulated at a stripped serum concentration approximately 100 times lower than that which initiated proliferation. Pharmacological alteration of proliferation with cordyceptin resulted in growth changes without alterations in cytokeratin release. We conclude that cytokeratin release in these cells is unrelated to proliferation, independent of oestrogen action and probably in some way related to growth factor receptor function.

Involvement of direct phosphorylation in the regulation of the rat parotid Na(+)-K(+)-2Cl- cotransporter

We identify a 175-kDa membrane phosphoprotein (pp175) in rat parotid acini whose properties correlate well with the Na(+)-K(+)-2Cl- cotransporter previously characterized functionally and biochemically in this tissue. pp175 was the only phosphoprotein immunoprecipitated by an anti-Na(+)-K(+)-2Cl- cotransporter antibody and the only membrane protein whose phosphorylation state was conspicuously altered after a brief (45-s) exposure of acini to the beta-adrenergic agonist isoproterenol. Phosphopeptide mapping provided evidence for three phosphorylation sites on pp175, only one of which was labeled in response to isoproterenol treatment. The half-maximal effect of isoproterenol on phosphorylation of pp175 (approximately 20 nM) was in excellent agreement with its previously demonstrated up-regulatory effect on cotransport activity. Increased phosphorylation of pp175 was also seen following acinar treatment with a permeant cAMP analogue and with forskolin, conditions that have likewise been shown to up-regulate the cotransporter. Combined with earlier results from our laboratory, these data provide strong evidence that the up-regulation of the cotransporter by these agents is due to direct phosphorylation mediated by protein kinase A. AlF(-)4 treatment, which results in an up-regulation of cotransport activity comparable with that observed with isoproterenol (approximately 6-fold), caused a similar increase in phosphorylation of pp175. However, hypertonic shrinkage and treatment with the protein phosphatase inhibitor calyculin A, which also up-regulate the cotransporter (approximately 3-fold and approximately 6-fold, respectively) caused no change in the phosphorylation level. Furthermore, although acinar treatment with the muscarinic agonist carbachol results in a dramatic up-regulation of cotransport activity and a concomitant phosphorylation of pp175, no phosphorylation of pp175 was seen with the Ca(2+)-mobilizing agent thapsigargin, which is able to fully mimic the up-regulatory effect of carbachol on transport activity. Taken together, these results indicate that direct phosphorylation is only one of the mechanisms involved in secretagogue-induced regulation of the rat parotid Na(+)-K(+)-2Cl- cotransporter.

Identification, characterization and purification of a 160 kD bumetanide-binding glycoprotein from the rabbit parotid

We demonstrate the presence of a 160 kD protein in rabbit parotid basolateral membranes that can be labeled with the irreversible sulfhydryl reagent [14C]-N-ethylmaleimide in a bumetanide-protectable fashion. The specificity of this labeling, and our previous evidence for the existence of an essential sulfhydryl group closely associated with the bumetanide-binding site on the parotid Na(+)-K(+)-Cl-cotransporter (J. Membrane Biol. 112:51-58, 1989), provide strong evidence that this protein is a part or all of the parotid bumetanide-binding site. When this protein is treated with endoglycosidase F/N-glycosidase F to remove N-linked oligosaccharides, its apparent molecular weight decreases to 135 kD. The pI of this deglycosylated protein is approximately 6.4. The bumetanide-binding protein was purified using two preparative electrophoresis steps. First, a Triton X-100 extract enriched in this protein was run on preparative electrophoresis to obtain fractions containing proteins in the 160 kD range. These were then deglycosylated with endoglycosidase F/N-glycosidase F and selected fractions were pooled and rerun on preparative electrophoresis to obtain a final 135 kD fraction. The enrichment of the bumetanide-binding protein in this final 135 kD fraction estimated from [14C]-N-ethylmaleimide labeling was approximately 48 times relative to the starting membrane extract. Since the bumetanide-binding site represents approximately 2% of the total protein in this starting extract, this enrichment indicates a high degree of purity of this protein in the 135 kD fraction.

Na+/H(+)-exchange in A6 cells: polarity and vasopressin regulation

We have analyzed the mechanism of Na(+)-dependent pHi recovery from an acid load in A6 cells (an amphibian distal nephron cell line) by using the intracellular pH indicator 2'7'-bis(2-carboxyethyl)5,6 carboxyfluorescein (BCECF) and single cell microspectrofluorometry. A6 cells were found to express Na+/H(+)-exchange activity only on the basolateral membrane: Na+/H(+)-exchange activity follows simple saturation kinetics with an apparent Km for Na+ of approximately 11 mM; it is inhibited in a competitive manner by ethylisopropylamiloride (EIPA). This Na+/H(+)-exchange activity is inhibited by pharmacological activation of protein kinase A (PKA) as well as of protein kinase C (PKC). Addition of arginine vasopressin (AVP) either at low (subnanomolar) or at high (micromolar) concentrations inhibits Na+/H(+)-exchange activity; AVP stimulates IP3 production at low concentrations, whereas much higher concentrations are required to stimulate cAMP formation. These findings suggest that in A6 cells (i) Na+/H(+)-exchange is located in the basolateral membrane and (ii) PKC activation (heralded by IP3 turnover) is likely to be the mediator of AVP action at low AVP concentrations.

Involvement of C3 exotoxin-sensitive G proteins (rho/rac) in PTH signal transduction in OK cells

Parathyroid hormone (PTH) in opossum kidney (OK) cells leads to inhibition of Na-Pi cotransport, to the generation of inositol trisphosphate (IP3) and adenosine 3',5'-cyclic monophosphate (cAMP) and to a phosphorylation of proteins present in an enriched apical membrane fraction (27, 28; for review see Ref. 23). In the present report we have identified two of these phosphoproteins with molecular weights of approximately 22,000 and approximately 24,000, respectively, as guanosine 5'-triphosphate (GTP)-binding proteins, ADP-ribosylated by the Clostridium botulinum exotoxin C3 and recognized by an anti-rho polyclonal antibody but not by pan-ras monoclonal antibody; as suggested by Western-blot analysis the content of the proteins recognized by the anti-rho antibody did not alter in the membrane fraction as a function of treatment with PTH. Transient permeabilization of OK cells using streptolysin O and including the C3 exotoxin attenuated PTH-dependent inhibition of Na-Pi cotransport at hormone concentrations higher than 10(-10) M; residual PTH-dependent inhibition is equal to that observed after pharmacological activation of protein kinase A and protein kinase C, respectively. C3 exotoxin did not alter PTH-dependent generation of cAMP but modified production of IP3; it was increased at 10(-11) M and reduced at 10(-8) M PTH, respectively. It is suggested that protein kinase A may be involved in the phosphorylation of C3 exotoxin-sensitive G proteins (rho/rac). These proteins could be involved in PTH signal transduction.

Polarized expression of Na+/H+ exchange activity in LLC-PK1/PKE20 cells: II. Hormonal regulation

LLC-PK1/PKE20 cells (a continuous epithelial cell line) has two different Na/H exchange activities: Na/H-1 located in the basolateral membrane and Na/H-2 located in the apical membrane [Casavola et al. (1989) Biochem Biophys Res Commun 165:833-837; Haggerty et al. (1988) Proc Natl Acad Sci USA 86:6797-6801]. In the present report we have studied hormone regulation of these exchange activities by measuring Na-dependent recovery of pHi from an acid load (by using microspectrofluorometry and 2,7-bis(carboxyethyl)-5,6-carboxyfluorescein) in response to activation of regulatory cascades by either pharmacological agents or by vasopressin or calcitonin. Agents leading to activation of protein kinase A (cAMP-dependent), such as forskolin (10 microM), 8-Br-cAMP (0.25 mM), and isobutylmethylxanthine (0.5 mM), inhibited Na/H-2 and Na/H-1 by an average of 49%. Stimulation of protein kinase C by a phorbol ester (phorbol 12-myristate 13-acetate, TPA, 100 nM) inhibited Na/H-2 (by an average of 48%) and stimulated Na/H-1 (by an average of 38%); these effects of TPA were also observed in the presence of forskolin (100 microM). Addition of either vasopressin (2 microM) or calcitonin (0.3 microM) onto both sides of the monolayer decreased the activity of Na/H-2 by an average of 26.3% and 27.7% respectively, and stimulated the activity of Na/H-1 by an average of 17.4% and 38.7% respectively; exposure of cells to either hormone stimulated production of cAMP and inositol trisphosphate, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Apical and basolateral effects of PTH in OK cells: transport inhibition, messenger production, effects of pertussis toxin, and interaction with a PTH analog

The cellular distribution (apical vs. basolateral) of parathyroid hormone (PTH) signal transduction systems in opossum kidney (OK) cells was evaluated by measuring the action of PTH on apically located transport processes (Na/Pi cotransport and Na/H exchange) and on the generation of intracellular messengers (cAMP and IP3). PTH application led to immediate inhibition of Na/H-exchange without a difference in dose/response relationships between apical and basolateral cell-surface hormone addition (half-maximal inhibition at approximately 5 x 10(-12) M). PTH required 2-3 hr for maximal inhibition of Na/Pi cotransport with a half-maximal inhibition occurring at approximately 5 x 10(-10) M PTH for basolateral application and approximately 5 x 10(-12) M for apical application. PTH addition to either side of the monolayer produced a dose-dependent production of both cAMP and IP3. Half-maximal activation of IP3 was at about 7 x 10(-12) M PTH and displayed no differences between apical and basolateral hormone addition, while cAMP was produced with a half maximal concentration of 7 x 10(-9) M for apical PTH application and 10(-9) M for basolateral administration. The PTH analog [nle8.18,tyr34]PTH(3-34), (nlePTH), produced partial inhibition of Na/Pi cotransport (agonism) with no difference between apical and basolateral application. When applied as a PTH antagonist, nlePTH displayed dose-dependent antagonism of PTH inhibition of Na/Pi cotransport on the apical surface, failing to have an effect on the basolateral surface. Independent of addition to the apical or basolateral cell surface, nlePTH had only weak stimulatory effect on production of cAMP, whereas high levels of IP3 could be measured after addition of this PTH analog to either cell surface. Also an antagonistic action of nlePTH on PTH-dependent generation of the internal messengers, cAMP and IP3, was observed; at the apical and basolateral cell surface nelPTH reduced PTH-dependent generation of cAMP, while PTH-dependent generation of IP3 was only reduced by nlePTH at the apical surface. Pertussis toxin (PT) preincubation produced an attenuation of both PTH-dependent inhibition of Na/Pi cotransport and 1P3 generation while producing an enhancement of PTH-dependent cAMP generation; these effects displayed no cell surface polarity, suggesting that PTH action through either adenylate cyclase or phospholipase C was transduced through similar sets of G-proteins at each cell surface.

Functional asymmetry of phosphate transport and its regulation in opossum kidney cells: phosphate "adaptation"

The polarity (apical vs basolateral cell surface) of the up-regulatory response ("adaptation") to low medium phosphate (Pi) concentration on apical and basolateral Pi transport systems was investigated in opossum kidney (OK) cell monolayers grown on permeant supports. Incubation of cultures in low-Pi medium, given either only to the apical or simultanously to the apical and basolateral compartments, increased the rate of transport of both the apical and the basolateral Na/Pi cotransport systems. The basolateral Na-independent, 4,4-diisothiocyanatostilbene-2,2'-disulphonic-acid-sensitive Pi transport system was unaffected by Pi deprivation. Incubation with low-Pi medium from only the basolateral side failed to elicit any "adaptive" response in Pi transport. When cells were Pi-limited either apically or on both sides for short periods of time, adaptation was apparent within 2 h and close to maximal by 6 h, and the alteration in Pi transport was consistant with an increase in Jmax for both the apical and basolateral Na/Pi cotransport systems. These data suggest that apical Na-dependent Pi influx is important in signalling the adaptive response to low extracellular Pi.

Intestinal glycyl-L-phenylalanine and L-phenylalanine transport in a euryhaline teleost

The transport mechanisms for the dipeptide glycyl-L-phenylalanine (Gly-Phe) and L-phenylalanine (Phe) were characterized in fish intestinal brush-border membrane vesicles (BBMV). Gly-Phe was rapidly hydrolyzed only intravesicularly with almost total hydrolysis occurring even at 10 s. Dipeptide uptake was not stimulated by an inward gradient of Na, K, or H. Phe uptake was stimulated by an inward gradient of either Na or K but displayed an overshoot phenomenon only in the presence of an Na gradient. Kinetic analysis of the effect of substrate concentration on transport rate revealed that transport of both Gly-Phe and Phe occurred by a saturable process conforming to Michaelis-Menten kinetics. The Km for Gly-Phe was 9.8 +/- 3.5 mM, whereas that for Phe in the presence of Na or K, respectively, was 0.74 +/- 0.13 and 1.1 +/- 0.37 mM. Maximum uptake for Gly-Phe and for Phe in the presence of Na and K was 5.1, 0.9, and 0.4 nmol.mg and protein-1.5 s-1, respectively. Gly-Phe and Phe transport displayed different patterns of inhibition by dipeptides and amino acids. These results suggest that Gly-Phe and Phe are transported via different mechanisms, with Gly-Phe being hydrolyzed during a carrier-mediated, cation-independent process and Phe being transferred via a Na+ cotransport process similar to that described in mammals. During conditions of high luminal dipeptide concentrations, the Gly-Phe pathway may make a significant contribution to total Phe uptake.

Parathyroid hormone-induced alterations of protein content and phosphorylation in enriched apical membranes of opossum kidney cells

Parathyroid hormone (PTH) reduces Na/Pi co-transport activity in opossum kidney (OK) cells in a process mediated by protein kinases A and C. Further, inactivation of Na/Pi transport involves irreversible inhibition, possibly via internalization, of the transport system. This study analyzed alterations of concentration and phosphorylation of membrane proteins of an apically enriched preparation induced by short (10 min) and long (3 h) term incubation with 10(-10) M PTH of monolayer cultures of the OK-cell line. To this end, an apically enriched membrane fraction was isolated from cells grown on Petri dishes and analyzed by two-dimensional gel electrophoresis. Long term exposure of the cells to PTH induced changes in apical protein concentration. Four proteins were found to be decreased and one protein was found to be increased in its concentration. Addition of 10(-10) M PTH to the cells led to transient phosphorylation of five proteins. In contrast to transient phosphorylation, phosphorylation of one protein increased over the time period of 3 h. Combined analysis of silver staining and autoradiography led to the detection of an acidic 35-kDa protein in which specific phosphorylation increased over a time period of hours. The results document for the first time alterations in apical membrane protein content and phosphorylation state mediated by PTH when added to an intact cellular system. It is concluded that the identified proteins represent possible candidates for being involved directly or indirectly in PTH alterations of membrane transport.

Functional asymmetry in phosphate transport and its regulation in opossum kidney cells: parathyroid hormone inhibition

The sidedness (apical vs basolateral) of the inhibitory of phosphate (Pi) transport by parathyroid hormone (PTH) was investigated in opossum kidney (OK)-cell monolayers grown on permeant support. PTH was found to regulate the activity of only the apical Na Pi cotransporter, having no effect on the basolateral transport systems. Transport inhibition was approximately 100-fold more sensitive to apical PTH application (Kd: 5 x 10(-12) M) than to basolateral application (Kd: 5 x 10(-10) M). The time-course of the inhibitory response was identical from the two cell surfaces, with half-maximum inhibition occurring at about 20 min and almost full inhibition by 90 min. Experiments on diffusion and degradation demonstrated that the difference in Kd at the two cell surfaces was not due to differential metabolism or diffusion. Tests of cooperativity between the apical and basolateral regulatory events at intermediate concentrations suggested that the presence of PTH on one side of the monolayer reduced the scope of response from the other side. At maximum doses of PTH (10(-7)-10(-8) M) the transport inhibition from either side was equal and not additive. We conclude that in OK-cell monolayers grown on permeant support only apical Na/Pi co-transport is sensitive to PTH inhibition and that PTH receptor properties may be different on the apical and basolateral surfaces.

Functional asymmetry of phosphate transport and its regulation in opossum kidney cells: phosphate transport

The polarized distribution of phosphate (Pi) transport systems in a continuous renal cell line derived from opossum kidney (OK) was measured in monolayers grown on permeant filter support. When cultured on collagen-coated nitrocellulose filters, OK cells formed tight, functionally polarized monolayers. Three Pi transport systems were identified in these monolayers: one apical sodium (Na)-dependent system and two systems on the basolateral surface, one Na-dependent and one Na-independent. The apical system was high-affinity (Km = 0.4 mM Pi), low-capacity (Jmax = 1100 pmol Pi/mg protein per minute) with a Na:Pi stoichiometry greater than 1 (n = 3) and a high interaction coefficient (KNa = 105 mM Na). On the basolateral surface the Na-independent system comprised about 30% of the total Pi transport at this surface. Both basolateral systems were of low affinity (Km: Na-independent, 2.6 mM; Na-dependent, 5.2 mM) and high capacity (Jmax: Na-independent, 2100; Na-dependent, 2400 pmol/mg protein per minute). The basolateral Na-dependent system had a Nai stoichiometry of 1 and a relatively low interaction coefficient (KNa = 25 mM Na). Only the basolateral Na-independent system was inhibitable by 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). These results are compatible with a net vectorial transcellular transport of Pi from the apical through the basolateral cell surfaces. The presence of a basolateral Na-dependent system may reflect additional metabolic requirements that cannot be met only by apical influx. Taken together, these results demonstrate the ability to grow cell monolayers successfully, displaying polarized transport activities similar to in situ.

Proline transport by brush-border membrane vesicles of lobster antennal glands

Purified brush-border membrane vesicles (BBMV) of lobster antennal gland labyrinth and bladder were separately formed by a magnesium precipitation technique. L-[3H]proline uptake was stimulated by a transmembrane NaCl gradient [outside (o) greater than inside (i)] to a greater extent in BBMV from labyrinth than those from the bladder. Detailed study of the labyrinth proline-transport processes revealed a specific dependence on NaCl, with negligible stimulatory effects by NaSCN, Na-gluconate, or KCl. A transmembrane proton gradient (o greater than i) was without stimulatory effect on proline transport. A transmembrane potential difference alone, in the presence of equilibrated NaCl and L-[3H]proline, led to net influx of the labeled amino acid, suggesting that the uptake process was electrogenic and capable of bringing about the net transfer of positive charge to the vesicle interior. Although a transmembrane Na gradient alone, in the presence of equilibrated Cl and L-[3H]proline, was able to bring about the net influx of the amino acid, a transmembrane Cl gradient alone under Na- and L-[3H]proline-equilibrated conditions was not, suggesting that only the Na gradient could energize the carrier process through cotransport, while the anion served an essential activating role. Proline influx by these vesicles occurred by the combination of at least one saturable Michaelis-Menten carrier system (apparent Kt = 0.37 mM; apparent JM = 1.19 nmol.mg protein-1.10 s-1) and apparent diffusion (P = 0.33 nmol.mg protein-1.10 s-1.mM-1). Static head analysis of the transport process suggested a cotransport stoichiometry of 2 Na:1 proline with essential activation by Cl ion.

Dietary hormonal modification of growth, intestinal ATPase, and glucose transport in tilapia

The effect of growth stimulatory and inhibitory dietary applications of hormones [3,5,3'-triiodo-L-thyronine (T3) and 17 alpha-methyltestosterone (MT)] on Na+-K+-adenosinetriphosphatase (ATPase) activity and glucose transport by upper and lower intestinal brush-border membrane vesicles of tilapia (Oreochromis mossambicus) were characterized. Both enzyme activity and glucose transport were greater in growth-stimulatory treatments and lower in growth-inhibitory treatments than in the control. Growth on stimulatory hormone treatments increased apparent glucose influx kinetics (one-half maximum glucose influx, maximum glucose influx, and apparent diffusion coefficient) in both intestinal segments, whereas inhibitory treatments reduced these parameters in upper intestine but had no effect on these parameters in lower intestine. All hormone treatments increased the stoichiometry of Na-glucose cotransport from 1:1 in the control to 2:1 under test conditions. It is suggested that observed patterns of altered growth are due, in part, to hormonally modified intestinal nutrient transport and Na+-K+-ATPase activities.

Brush-border inositol transport by intestines of carnivorous and herbivorous teleosts

Transport characteristics of myoinositol by isolated brush-border membrane vesicles of two fish, the herbivorous tilapia (Oreochromis mossambicus) and the carnivorous eel (Anguilla anguilla), were measured. [3H]myoinositol uptake by vesicles of both fish was stimulated by a transmembrane Na gradient, was electrogenic, and was inhibited by phloridzin. Kinetic analysis of myoinositol influx disclosed species differences (tilapia, K = 0.15 mM, Jmax = 0.2 nmol.mg protein-1.min-1; eel, K = 2.6 mM, Jmax = 0.8 nmol.mg protein-1.min-1). D-glucose inhibition of myoinositol influx was shown to be noncompetitive. Additional inhibition studies with a range of sugars demonstrated that aldohexoses in the C-1 chair conformation were preferred substrates. Myoinositol had no effect on D-glucose transport. Preloading vesicles with myoinositol transstimulated [3H]myoinositol uptake, while the use of internal D-glucose was without effect. These results suggest that the intestinal brush border may have a pathway for myoinositol transport entirely separate from that for D-glucose but inhibited by D-glucose via binding to a regulator site on the myoinositol transporter. Markedly dissimilar influx kinetic constants suggest possible differences in myoinositol needs by carnivorous and herbivorous fish.

Basolateral inositol transport by intestines of carnivorous and herbivorous teleosts

Myoinositol transport by isolated basolateral membrane vesicles was characterized from intestines of the herbivorous tilapia (Oreochromis mossambicus) and the carnivorous eel (Anguilla anguilla). [3H]myoinositol transport occurred by nonelectrogenic, facilitated diffusion independent of cation gradients and was inhibited by phloretin (Ki = 0.6 and 0.9 mM for tilapia and eel, respectively) but not by phloridzin. Kinetic analysis of myoinositol influx disclosed no differences in concentration yielding half-maximal influx or maximum influx between these species. D-Glucose inhibition of myoinositol influx was shown to be noncompetitive. Additional inhibition studies with a range of sugars demonstrated that aldohexoses in the C-1 chair conformation were preferred substrates. Myoinositol had no inhibitory effect on D-glucose transport. Preloading vesicles with myoinositol transstimulated [3H]myoinositol uptake, while the use of internal D-glucose was without effect. These basolateral data support the contention that epithelial myoinositol carriers are separate from D-hexose transport systems present on the same membrane but are modulated by hexose binding to a regulator site on the myoinositol transporter. Furthermore, the comparison of two fish species suggests that genetic dietary transport adaptation occurs on the brush-border membrane, while no such adaptation is present at the epithelial basolateral pole.

Basolateral amino acid and glucose transport by the intestine of the teleost, Anguilla anguilla

1. D-glucose transport into BLMV was osmotically reactive, sodium independent, and inhibited by phloretin but not by phloridzin. 2. The survey of 6 L-amino acids identified three groups with respect to transfer across the basolateral cell border. Transport of proline and glutamate occurred by Na-dependent carriers and by apparent simple diffusion. Alanine, lysine and phenylalanine were transported by Na-independent carriers and apparent simple diffusion. Glycine transport was stimulated above apparent simple diffusion only by a simultaneous inwardly-directed Na gradient and outwardly-directed K gradient. 3. Only proline and glutamate demonstrated the ability to depolarize the membrane potential, consistent with Na-dependent rheogenic transport.

Basolateral glucose transport by intestine of teleost, Oreochromis mossambicus

Transport characteristics of D-glucose by isolated basolateral membrane vesicles of the teleost fish, Oreochromis mossambicus, were measured. Specific activity of the vesicle Na-K-adenosinetriphosphatase was increased 11-fold, whereas specific activities of brush-border and organelle membrane enzymes were enriched only 0.3- to 0.8-fold. Vesicles had diameters of 0.1-0.4 micron, 70% of vesicles were leaky (unsealed), and 60% of sealed vesicles were inside out. D-Glucose transport occurred by stereospecific facilitated diffusion, independent of 120 mM gradients of either NaCl or KCl, and was inhibited by sulfhydryl reagents, phloretin, and cytochalasin B, but not by phloridzin. Competition studies with a range of sugars demonstrated that aldohexoses in the C-1 chair conformation were preferred substrates and probably share the same carrier. Kinetic analysis of glucose influx yielded a Kt of 10 mM and a Jmax of 3,910 pmol X mg protein-1 X min-1. Fish intestinal basolateral D-glucose transport closely resembles that of mammalian or avian intestinal epithelia and of red blood cell plasma membrane. The magnitude of transport is much lower in fish than in other vertebrates, which may be related to lower metabolic rates in these poikilotherms.

Intestinal glucose transport and salinity adaptation in a euryhaline teleost

Glucose transport by upper and lower intestinal brush-border membrane vesicles of the African tilapia (Oreochromis mossambicus) was characterized in fish acclimated to either freshwater or full-strength seawater. D-[3H]-glucose uptake by vesicles was stimulated by a transmembrane Na gradient, was electrogenic, and was enhanced by counter-transport of either D-glucose or D-galactose. Glucose transport was greater in the upper intestine than in the lower intestine and in seawater animals rather than in fish acclimated to freshwater. Glucose influx (10-s uptake) involved both saturable and nonsaturable transport components. Seawater adaptation increased apparent glucose influx Kt, Jmax, apparent diffusional permeability (P), and the apparent Na affinity of the cotransport system in both intestinal segments, but the stoichiometry of Na-glucose transfer (1:1) was unaffected by differential saline conditions or gut region. It is suggested that increased sugar transport in seawater animals is due to the combination of enhanced Na-binding properties and an increase in number or transfer rate of the transport proteins. Freshwater animals compensate for reduced Na affinity of the coupled process by markedly increasing the protein affinity for glucose.