Molecular mechanisms for the antiapoptotic action of gastrin

Gastrin (G17) has a CCK-B receptor-mediated growth-promoting effect on the AR42J rat acinar cell line. We examined whether G17 inhibits apoptosis induced by serum withdrawal of AR42J cells and CHO-K1 cells stably expressing CCK-B receptors (CHO-K1/CCK-B cells). Cellular apoptosis was measured by flow cytometry and the terminal deoxynucleotidyltransferase-mediated dUTP-FITC nick end-labeling method. Serum withdrawal induced AR42J and CHO-K1/CCK-B cell apoptosis. Addition of 10 nM G17 reversed these effects. We examined the action of G17 (10 nM) on phosphorylation and activation of protein kinase B/Akt, a kinase known to promote cell survival. Akt phosphorylation and activation were measured by kinase assays and Western blots with an anti-phospho-Akt antibody. G17 stimulated Akt phosphorylation and activation. G17 induction of Akt phosphorylation was inhibited by the phosphoinositide 3-kinase (PI 3-kinase) inhibitors LY-294002 (10 microM) and wortmannin (200 nM) but not by the mitogen-activated protein kinase kinase 1 inhibitor PD-98059 (50 microM). To study the role of p38 kinase in G17 signaling to Akt, we examined the effect of G17 on p38 kinase activation and phosphorylation using kinase assays and Western blots with an anti-phospho-p38 kinase antibody. G17 induced p38 kinase activity at doses and with kinetics similar to those observed for Akt induction. The p38 kinase inhibitor SB-203580 inhibited G17 induction of Akt phosphorylation and activation at a concentration (10 microM) 10-fold higher than necessary to block p38 kinase (1 microM), suggesting the possible involvement of kinase activities other than p38 kinase. Transduction of AR42J cells with the adenoviral vector Adeno-dn Akt, which overexpresses an inhibitor of Akt, reversed the antiapoptotic action of G17. In conclusion, G17 promotes AR42J cell survival through the induction of Akt via PI 3-kinase and SB-203580-sensitive kinase activities.

Cocaine and antidepressant-sensitive biogenic amine transporters exist in regulated complexes with protein phosphatase 2A

Presynaptic transporter proteins regulate the clearance of extracellular biogenic amines after release and are important targets for multiple psychoactive agents, including amphetamines, cocaine, and antidepressant drugs. Recent studies reveal that dopamine (DA), norepinephrine (NE), and serotonin (5-HT) transporters (DAT, NET, and SERT, respectively) are rapidly regulated by direct or receptor-mediated activation of cellular kinases, particularly protein kinase C (PKC). With SERTs, PKC activation results in activity-dependent transporter phosphorylation and sequestration. Protein phosphatase 1/2A (PP1/PP2A) inhibitors, such as okadaic acid (OA) and calyculin A, also promote SERT phosphorylation and functional downregulation. How kinase, phosphatase, and transporter activities are linked mechanistically is unclear. In the present study, we found that okadaic acid-sensitive phosphatase activity is enriched in SERT immunoprecipitates from human SERT stably transfected cells. Moreover, blots of these immunoprecipitates reveal the presence of PP2A catalytic subunit (PP2Ac), findings replicated using brain preparations. Whole-cell treatments with okadaic acid or calyculin A diminished SERT/PP2Ac associations. Phorbol esters, which trigger SERT phosphorylation, also diminish SERT/PP2Ac associations, effects that can be blocked by PKC antagonists as well as the SERT substrate 5-HT. Similar transporter/PP2Ac complexes were also observed in coimmunoprecipitation studies with NETs and DATs. Our findings provide evidence for the existence of regulated heteromeric assemblies involving biogenic amine transporters and PP2A and suggest that the dynamic stability of these complexes may govern transporter phosphorylation and sequestration.

Regulation and function of p38 protein kinase in isolated canine gastric parietal cells

We examined the regulation and functional role of p38 kinase in gastric acid secretion. p38 kinase was immunoprecipitated from cell lysates of highly purified gastric parietal cells in primary culture, and its activity was quantitated by in vitro kinase assay. Carbachol effects were dose- and time-dependent, with a maximal 10-fold stimulatory effect detected after 30 min of incubation. SB-203580, a highly selective inhibitor of p38 kinase, blocked carbachol induction of p38 kinase activity, with maximal inhibition at 10 microM. Stimulation by carbachol was unaffected by preincubation of parietal cells with the intracellular Ca(2+) chelator BAPTA-AM, but incubation of cells in Ca(2+)-free medium led to a 50% inhibition of carbachol induction of p38 kinase activity. Because some of the effects of carbachol are mediated by the small GTP-binding protein Rho, we examined the role of Rho in carbachol induction of p38 kinase activity. We tested the effect of exoenzyme C3 from Clostridium botulinum (C3), a toxin known to ADP-ribosylate and specifically inactivate Rho. C3 led to complete ADP-ribosylation of Rho, and it inhibited carbachol induction of p38 kinase by 50%. We then tested the effect of SB-203580 and C3 on carbachol-stimulated uptake of [(14)C]aminopyrine (AP). Inhibition of p38 kinase by SB-203580 led to a dose-dependent increase in AP uptake induced by carbachol, with maximal (threefold) effect at 10 microM SB-203580. Similarly, preincubation of parietal cells with C3 led to a twofold increase in AP uptake induced by carbachol. Thus carbachol induces a cascade of events in parietal cells that results in activation of p38 kinase through signaling pathways that are at least in part dependent on Rho activation and on the presence of extracellular Ca(2+). p38 kinase appears to inhibit gastric acid secretion.

Carbachol activates IkappaB kinase in isolated canine gastric parietal cells

IkappaB kinase (IKK) is a recently discovered kinase complex composed of the kinases IKKalpha and beta, which plays a crucial role in the activation of NF-kappaB. In this study we examined the regulation of IKK by carbachol in isolated gastric parietal cells. IKKalpha and beta activities were measured by immune complex kinase assay. Carbachol induced both IKK alpha and beta in a time-dependent fashion, with a maximal stimulatory effect detected after 5 min of incubation. The action of carbachol was inhibited by the intracellular Ca(++) chelator BAPTA-AM, the PKC inhibitor GF109203X, and the NF-kappaB inhibitor PDTC. Carbachol also induced degradation of IkappaBalpha, which was reversed by addition of both GF109203X and PDTC and stimulated the activity of a NF-kappaB-luciferase reporter gene plasmid in COS-7 cells stably expressing the human M3 muscarinic receptor. In conclusion, carbachol induces IKK in the parietal cells via intracellular Ca(++)- and PKC-dependent signaling pathways. This observation represents a novel mechanism for the regulation of NF-kappaB through the activation of seven transmembrane G-protein-coupled receptors.

Phosphorylation and sequestration of serotonin transporters differentially modulated by psychostimulants

Many psychotropic drugs interfere with the reuptake of dopamine, norepinephrine, and serotonin. Transport capacity is regulated by kinase-linked pathways, particularly those involving protein kinase C (PKC), resulting in transporter phosphorylation and sequestration. Phosphorylation and sequestration of the serotonin transporter (SERT) were substantially impacted by ligand occupancy. Ligands that can permeate the transporter, such as serotonin or the amphetamines, prevented PKC-dependent SERT phosphorylation. Nontransported SERT antagonists such as cocaine and antidepressants were permissive for SERT phosphorylation but blocked serotonin effects. PKC-dependent SERT sequestration was also blocked by serotonin. These findings reveal activity-dependent modulation of neurotransmitter reuptake and identify previously unknown consequences of amphetamine, cocaine, and antidepressant action.

Regulated phosphorylation and trafficking of antidepressant-sensitive serotonin transporter proteins

Presynaptic serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) mediate antidepressant-sensitive clearance of 5-HT following release. Although we have been aware for decades that SERT-mediated 5-HT clearance can be modulated by exogenous agents including serotonin-selective reuptake inhibitors, amphetamines, and cocaine, we have had little reason to speculate that SERT activity was actively controlled through endogenous pathways. Recent studies indicate that SERTs are likely to be trafficked to specific plasma membrane subdomains to achieve localized clearance of 5-HT, and that the number of SERTs resident in the plasma membrane is controlled through kinase- and phosphatase-linked pathways. In particular, roles for protein kinase C and phosphatase 2A become apparent through studies with enzyme activators and inhibitors in SERT-transfected cells, where SERT proteins are rapidly phosphorylated in parallel with transporter redistribution and loss of functional uptake capacity. Based on our findings, and the studies of others in native tissues and transfected cells, we propose a model whereby SERTs are organized in a macromolecular complex in the plasma membrane that may serve to locate reuptake activity near release sites. Although many elements of this model remain hypothetical, our findings suggest a much more dynamic picture of transporter-mediated 5-HT reuptake than typically described and suggest opportunities both for the development of new SERT regulatory agents and for the identification of regulatory pathways that may be compromised in mental illness.

A comparison between microsatellite and quantitative PCR analyses to detect frequent p16 copy number changes in primary bladder tumors

We tested 70 primary bladder tumors for altered copy number of p16 (D9S1752) by microsatellite analysis and by a quantitative PCR (QPCR) assay. These two approaches were fully concordant for 53 tumors, including all 39 tumors in which microsatellite analysis detected loss. In addition, the QPCR method detected useful anomalies in 17 additional cases, including those in which D9S1752 was uninformative. QPCR was abnormal in 56 of 70 (80%) cases, whereas microsatellite analysis was abnormal in 39 of 70 (56%) cases. Although QPCR uses more DNA than microsatellite analysis, it represents a rapid, informative technique that can readily detect both chromosome 9p21 deletions and amplifications in primary bladder tumors without the need for electrophoretic separation.

Phosphorylation and regulation of antidepressant-sensitive serotonin transporters

Antidepressant-sensitive serotonin (5-hydroxytryptamine, 5HT) transporters (SERTs) are responsible for efficient synaptic clearance of extracellular 5HT. Previously (Qian, Y., Galli, A., Ramamoorthy, S., Risso, S., DeFelice, L. J., and Blakely, R. D. (1997) J. Neurosci. 17, 45-47), we demonstrated that protein kinase (PKC)-linked pathways in transfected HEK-293 cells lead to the internalization of cell-surface human (h) SERT protein and a reduction in 5HT uptake capacity. In the present study, we report that PKC activators rapidly, and in a concentration-dependent manner, elevate the basal level of hSERT phosphorylation 5-6-fold. Similarly, protein phosphatase (PP1/PP2A) inhibitors down-regulate 5HT transport and significantly elevate hSERT 32P incorporation, effects that are additive with those of PKC activators. Moreover, hSERT phosphorylation induced by beta-phorbol 12-myristate 13-acetate is abolished selectively by the PKC inhibitors staurosporine and bisindolylmaleimide I, whereas hSERT phosphorylation induced by phosphatase inhibitors is insensitive to these agents at comparable concentrations. Protein kinase A and protein kinase G activators fail to acutely down-regulate 5HT uptake but significantly enhance hSERT phosphorylation. Basal hSERT and okadaic acid-induced phosphorylation were insensitive to chelation of intracellular calcium and Ca2+/calmodulin-dependent protein kinase inhibitors. Together these results reveal hSERT to be a phosphoprotein whose phosphorylation state is likely to be tightly controlled by multiple kinase and phosphatase pathways that may also influence the transporter's regulated trafficking.

Characterization of a sodium-dependent vitamin transporter mediating the uptake of pantothenate, biotin and lipoate in human placental choriocarcinoma cells

The characteristics of the uptake of the vitamin pantothenate into JAr human placental choriocarcinoma cells were investigated and these cells were found to accumulate the vitamin against a concentration gradient by a Na(+)-dependent process. Substitution of Na+ with over other monovalent cations abolished the uptake completely. The transport process showed no preference for any particular anion. Kinetic analysis indicated the presence of a single saturable transport system with a Michaelis-Menten constant of 2.1 +/- 0.2 microM and a maximal velocity of 341 +/- 12 pmol/mg of protein per 10 min. The dependence of the uptake rate of pantothenate on Na+ concentration exhibited sigmoidal kinetics, indicating interaction of more than one Na+ ion with the transporter. The Hill coefficient for this process was calculated to be 1.6. The Na+/pantothenate coupling ratio being greater than unity suggests that the transport process is electrogenic, resulting in net transfer of positive charge across the membrane. This was confirmed in plasma membrane vesicles prepared from JAr cells where the uptake of pantothenate was found to be significantly stimulated by valinomycin-induced inside-negative K(+)-diffusion potential. Substrate specificity studies showed that, in addition to pantothenate, the transporter interacts with two other vitamins, namely biotin and lipoate. The characteristics of pantothenate uptake in the placental cell line BeWo was also investigated. These cells were also found to express a pantothenate transport system similar to that expressed in the JAr cells.

Protein kinase C activation regulates human serotonin transporters in HEK-293 cells via altered cell surface expression

Antidepressant- and cocaine-sensitive serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) dictate clearance of extracellular 5-HT after release. To explore protein kinase C-mediated SERT regulation, we generated a stable human SERT (hSERT)-expressing cell line (293-hSERT) and evaluated modulation of 5-HT activity via studies of 5-HT flux, hSERT-mediated currents under voltage clamp, and surface distribution of SERT protein. 293-hSERT cells exhibit saturable, high-affinity, and antidepressant-sensitive 5-HT uptake as well as hSERT-dependent whole-cell currents. In these cells, the protein kinase C activator beta-PMA caused a time-dependent reduction in 5-HT uptake capacity (Vmax) after acute application and a reduction in SERT-mediated currents. Effects of beta-PMA were mimicked by the phorbol ester beta-PDBu, were not observed with the inactive alpha-isomers, and could be blocked by treatment of cells with the protein kinase C inhibitor staurosporine. Biotinylation/immunoblot analyses showed that activity reductions are paralleled by a staurosporine-sensitive loss of surface SERT protein. These data indicate that altered surface abundance, rather than reduced catalytic transport efficiency, mediates acute PKC-dependent modulation of 5-HT uptake.

Sodium-dependent carnitine transport in human placental choriocarcinoma cells

The JAR human placental choriocarcinoma cells were found to transport carnitine into the intracellular space by a Na(+)-dependent process. The transport showed no requirement for anions. The Na+-dependent process was saturable and the apparent Michaelis-Menten constant for carnitine was 12.3 +/- 0.5 microM. Na+ activated the transport by increasing the affinity of the transport system for carnitine. The transport system specifically interacted with L-carnitine, D-carnitine, acetyl-DL-carnitine and betaine. 6-N-Trimethyllysine and choline had little or no effect on carnitine transport. Of the total transport measured, transport into the intracellular space represented 90%. Plasma membrane vesicles prepared from JAR cells were found to bind carnitine in a Na(+)-dependent manner. The binding was saturable with an apparent dissociation constant of 0.66 +/- 0.08 microM. The binding process was specific for L-carnitine, D-carnitine, acetyl-DL-carnitine, and betaine. 6-N-Trimethyllysine and choline showed little or no affinity. It is concluded that the JAR cells express a Na(+)-dependent high-affinity system for carnitine transport and that the Na(+)-dependent high-affinity carnitine binding detected in purified JAR cell plasma membrane vesicles is possibly related to the transmembrane transport process.

Inability to N-glycosylate the human norepinephrine transporter reduces protein stability, surface trafficking, and transport activity but not ligand recognition

The role of N-glycosylation in the expression, stability, and ligand recognition by the cocaine- and antidepressant-sensitive human norepinephrine transporter (hNET) was assessed in stably and transiently transfected cell lines. The use of hNET-specific antibodies and the membrane-impermeant biotinylating reagent sulfosuccinimidobiotin establishes that treatment of stably transfected LLC-PK1 cells with tunicamycin depletes surface membranes of mature hNET glycoproteins, which is consistent with a failure of less stable, nonglycosylated subunits to replenish surface compartments. To determine whether N-glycosylation plays a direct role in hNET stability, surface expression, and ligand recognition, we mutated the three hNET canonical N-glycosylation sites (hNETN184, 192, 198Q) and transiently expressed the mutant cDNA in parallel with the parental hNET construct in HeLa and COS cells. hNETN184, 192, 198Q protein exhibited increased electrophoretic mobility (approximately 46 kDa), similar to that of enzymatically N-deglycosylated hNET protein, which confirms the use of canonical sites in the second extracellular loop of the transporter. hNETN184, 192, 198Q protein in HeLa and COS extracts was reduced approximately 50% relative to hNET protein in parallel transfections, demonstrated to arise from a reduction in transporter half-life, which is consistent with the proposed role of N-glycosylation in hNET stability. Both HeLa and COS cells transfected with hNETN184, 192, 198Q exhibit a significantly greater reduction in transport activity than can be accounted for by losses in either total or surface NET protein. Furthermore, sensitivity of catecholamine transport to unlabeled substrate and antagonists was unchanged in the mutant, suggesting that residual nonglycosylated surface hNETs execute a key step in the transport cycle after ligand recognition with reduced efficiency.

Isolation and characterization of the human and mouse homologues (SUPT4H and Supt4h) of the yeast SPT4 gene

To study gene regulation mediated by chromatin in mammals, we isolated the human (SUPT4H) and murine (Supt4h) counterparts of the yeast gene encoding SPT4; the product of this gene presumably interacts with the products of the mammalian homologues (which we have also cloned) of yeast SPT5 and SPT6, thereby modulating chromatin formation and activity. We isolated two different sized human SUPT4H cDNA clones (1464 and 728 nt) and one murine Supt4h (688 nt) cDNA clone; all three encode the same 117-amino-acid protein with conservation of the zinc finger motif found in SPT4. Conservation of this zinc finger motif from yeast to mouse and human implies functional importance. Although the overall sequence homology at the DNA level between the human 728-nt transcript and the murine 688-nt transcript is only 78.4%, the DNA sequence homology is 97.7% within the coding region. At the protein level, the amino acid sequences of the translated murine Supt4h and the human SUPT4H gene products are identical. The likely functional copy of SUPT4H, which has at least two introns, maps to human chromosome 17, with candidate intronless pseudogenes on chromosomes 2, 12, and 20. Buttressing the hypothesis that this is a gene required constitutively, both the human SUPT4H transcripts and the murine Supt4h transcript are expressed widely, although not at equal levels (e.g., such as most histones), in all fetal and adult tissues that we examined.

Identification and analysis of the human and murine putative chromatin structure regulator SUPT6H and Supt6h

We have isolated and sequenced SUPT6H and Supt6h, the human and murine homologues of the Saccharomyces cerevisiae and Caenorhabditis elegans genes SPT6 (P using 1603 aa = 6.7 e-95) and emb-5 (P using 1603 aa = 7.0 e-288), respectively. The human and murine SPT6 homologues are virtually identical, as they share >98% identity and >99% similarity at the protein level. The derived amino acid sequences of these two genes predict a 1603-aa protein (human) and a 1726-bp protein (mouse), respectively. There were several known features, including a highly acidic 5'-region, a degenerate SH2 domain, and a leucine zipper. These features are consistent with a nuclear protein that regulates transcription, whose extreme conservation underscores the likely importance of this gene in mammalian development. Expression of human and murine SPT6 homologues was analyzed by Northern blotting, which revealed a 7. 0-kb transcript that was expressed constitutively. The SPT6 homologue was mapped to chromosome 17q11.2 in human by somatic cell hybrid analysis and in situ hybridization. These data indicate that SUPT6H and Supt6h are functionally analogous to SPT6 and emb-5 and may therefore regulate transcription through establishment or maintenance of chromatin structure.

Human placental monoamine transporters as targets for amphetamines

OBJECTIVES

The use of amphetamine and its derivatives during pregnancy is known to have adverse effects on the outcome of pregnancy. These effects are at least partly a result of impairment of placental function caused by these abusable drugs. We hypothesized that the two monoamine transporters, namely, the serotonin transporter and the norepinephrine transporter, that are expressed in the human placenta are direct targets for these drugs.

STUDY DESIGN

The interaction of amphetamine and methamphetamine with human placental serotonin and norepinephrine transporters was examined. Activity of the serotonin transporter was assessed by serotonin uptake in both maternal-facing brush border membrane vesicles isolated from normal term human placentas and in JAR choriocarcinoma cells. Activity of the norepinephrine transporter was assessed by dopamine uptake and nisoxetine binding in placental brush border membrane vesicles.

RESULTS

Amphetamine and methamphetamine are potent inhibitors of the serotonin and norepinephrine transporters expressed in the human placenta. The inhibitory potency of amphetamine is greater than that of methamphetamine. In each case, the S(+)diastereoisomer is more potent than the corresponding R(-)diastereoisomer. The sensitivity of the norepinephrine transporter to inhibition by these drugs is at least two orders of magnitude greater than that of the serotonin transporter. At concentrations known to occur in the plasma of users, these drugs cause a marked inhibition of the norepinephrine transporter.

CONCLUSIONS

The results show that the norepinephrine transporter and, to a lesser extent, the serotonin transporter are cellular targets in the human placenta for the abusable drugs amphetamine and methamphetamine.

Proton/peptide cotransporter (PEPT 2) from human kidney: functional characterization and chromosomal localization

We report here on the functional characterization of the H+/peptide cotransporter PEPT 2 cloned from human kidney and on the chromosomal localization of the PEPT 2 gene. PEPT 2, when functionally expressed in HeLa cells, induces the transport of the neutral dipeptide glycylsarcosine. The induced transport activity is markedly influenced by extracellular pH. The optimum pH for the transport process is 6.0-7.0. Kinetic analysis has revealed that PEPT 2 is a high-affinity transporter, the Michaelis-Menten constant for glycylsarcosine being 74 +/- 14 microM. The human intestinal H+/peptide cotransporter PEPT 1 has 4-fold less affinity for the dipeptide under identical experimental conditions. Studies with other chemically diverse dipeptides have established that PEPT 2 possesses higher affinity than PEPT 1 not only for neutral peptides but also for peptides consisting of anionic and/or cationic amino acids. Somatic cell hybrid analysis and in situ hybridization have shown that the gene encoding PEPT 2 maps to human chromosome 3q13.3-q21.

Regulation of the human serotonin transporter by interleukin-1 beta

The serotonin transporter expressed in brain, platelets, and placenta is a primary target for antidepressants, cocaine, and amphetamines. Here we report that interleukin-1 beta is a potent regulator of the expression of this transporter. The activity of the serotonin transporter in human JAR choriocarcinoma cells is stimulated by this cytokine and the stimulation is accompanied by an increase in the steady state levels of the transporter mRNAs and in the transporter density. The increase in mRNA levels is effectively blocked by actinomycin D. The stimulatory effect of interleukin-1 beta is not associated with any change in the cellular levels of cAMP, indicating involvement of a cAMP-independent pathway. Even though cholera toxin, which is known to increase cAMP levels, also upregulates the serotonin transporter in JAR cells, the effects of interleukin-1 beta and cholera toxin on the transporter activity are additive. This constitutes the first report on the identification of a physiologically occurring hormone/cytokine which upregulates the expression of the human serotonin transporter gene.

Cyclic AMP-dependent up-regulation of the taurine transporter in a human retinal pigment epithelial cell line

This investigation was undertaken to study the role of cAMP in the regulation of the taurine transporter expressed in a human retinal pigment epithelial (HRPE) cell line. Treatment of the HRPE cells with cholera toxin for 24 h was found to stimulate the taurine transporter activity, as measured by taurine transport into the cells in the presence of NaCl, to a significant extent. The stimulation was 50-60% at 100 ng/ml cholera toxin. This stimulation was specific to the taurine transporter since the transport of two other amino acids (leucine and alanine), which are not substrates for the taurine transporter, was not affected by cholera toxin under similar conditions. Exposure of the cells to cholera toxin for a time period > 4 h was needed to elicit the stimulatory effect. The cholera toxin-induced stimulation of the taurine transporter activity was associated with an increase in the maximal velocity of the transport system. The affinity of the transporter for taurine was not altered by the treatment. The stimulatory effect was markedly blunted when the treatment of the cells with cholera toxin was done in the presence of actinomycin D, an inhibitor of transcription, or cycloheximide, an inhibitor of translation. The increase in the taurine transporter activity induced by cholera toxin was associated with a 2.6-fold increase in the steady state levels of the transporter mRNA. Measurement of cyclic nucleotides in control and cholera toxin-treated cells revealed that the toxin caused a 20-fold increase in the cellular levels of cAMP, the levels of cGMP remaining unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic AMP-independent up-regulation of the human serotonin transporter by staurosporine in choriocarcinoma cells

Treatment of confluent cultures of JAR human placental choriocarcinoma cells with staurosporine caused a marked stimulation of serotonin transport activity in these cells. The stimulatory effect was noticeable at nanomolar concentrations of staurosporine, and a treatment time of > 4 h was required for staurosporine to elicit the effect. At 40 nM and with a treatment time of 16 h, the stimulation of the transport activity was 3.5-6.0-fold. None of the several other protein kinase inhibitors tested had similar effect except KT 5720, a protein kinase A inhibitor, which showed a small but significant (approximately 1.4-fold) stimulatory effect at a concentration of 5 microM. Blockade of RNA synthesis and protein synthesis in the cells prevented completely the stimulation of the transport activity induced by staurosporine. The stimulation was observed not only in intact cells but also in plasma membrane vesicles prepared from staurosporine-treated cells. The stimulation was accompanied by a 5-7-fold increase in the steady state levels of the transporter-specific mRNAs, by a 7-fold increase in the maximal velocity of the transport process, and by a 6-fold increase in the transporter density in the plasma membrane. Even though both staurosporine and cholera toxin had similar effects on the serotonin transport activity in these cells, the effect was not additive when the cells were treated with both reagents together. While treatment of the cells with cholera toxin markedly elevated intracellular levels of cAMP, staurosporine did not have any effect on the cellular levels of this cyclic nucleotide. It is concluded that staurosporine up-regulates the serotonin transport activity in JAR cells by increasing the steady state levels of the serotonin transporter mRNA and by the consequent increase in the transporter density in the plasma membrane and that the process involves a cAMP-independent signaling pathway.

Regulation of taurine transport by Escherichia coli heat-stable enterotoxin and guanylin in human intestinal cell lines

The human colon carcinoma cell lines Caco-2 and HT-29 take up taurine actively. Treatment of Caco-2 cells with Escherichia coli heat-stable enterotoxin (STa) or with guanylin inhibited taurine uptake by approximately 40%. In contrast, neither STa nor guanylin changed the uptake of taurine in HT-29 cells. The inhibition in Caco-2 cells was associated with a decrease in the maximal velocity as well as in the affinity of the transporter. STa caused a 21-fold increase in guanosine 3',5'-cyclic monophosphate (cGMP) levels in Caco-2 cells with no change in cAMP levels. Neither cGMP nor cAMP levels were affected by STa treatment in HT-29 cells. Experiments with protein kinase inhibitors suggested that protein kinase A may mediate the observed effects of STa on taurine uptake. In accordance with this suggestion, treatment of Caco-2 cells with cholera toxin, which elevated intracellular cAMP levels, was found to inhibit taurine uptake. The steady state levels of the taurine transporter mRNA transcripts were not altered as a result of STa treatment. Studies with Caco-2 cells grown on permeable filters revealed that STa acts from the apical side. The taurine uptake from the apical side was inhibited by STa, but the taurine uptake from the basolateral side remained unaffected. It is suggested that the activity of the intestinal taurine transporter may be regulated by protein kinase A at a posttranslational level and that the intestinal absorption of taurine may be impaired during infection with enterotoxigenic strains of E. coli.

Substrate-specific regulation of the taurine transporter in human placental choriocarcinoma cells (JAR)

Exposure of the JAR human placental choriocarcinoma cells to taurine leads to a marked decrease in the activity of the taurine transporter in these cells. The ability to induce this adaptive response is not unique to taurine but is shared by other substrates of the transporter as well. Compounds such as betaine and alpha-aminoisobutyric acid which are not substrates for the transporter do not produce this effect. The change in the taurine transporter activity induced by taurine exposure is however unique to the taurine transporter because the activities of many other transport systems remain unaffected under these conditions. The adaptive regulation is not associated with any change in the dependence of the transporter activity on Na+ and Cl-, in the Na+/Cl-/taurine stoichiometry and in the affinities of the transporter for Na+ and Cl-. The decrease in the transporter activity caused by taurine exposure is due to a decrease in the maximal velocity of the transporter, and to a lesser extent, in the substrate affinity of the transporter. The decrease in the transporter activity observed in intact cells is demonstrable in plasma membrane vesicles after isolation from control and taurine-exposed cells. Cycloheximide and actinomycin D block the adaptive response in intact cells to a significant extent, but not completely. Northern blot analysis of mRNA from control and taurine-exposed cells shows that taurine exposure causes a significant decrease in the steady state levels of the taurine transporter mRNA. It is concluded that the activity of the taurine transporter in JAR cells is subject to substrate-specific adaptive regulation and that transcriptional as well as posttranscriptional events are involved in this regulatory process.

Molecular cloning of PEPT 2, a new member of the H+/peptide cotransporter family, from human kidney

Mammalian kidney is known to express a transport system specific for small peptides and pharmacologically active aminocephalosporins. This system is energized by a transmembrane electrochemical H+ gradient. Recently, a H(+)-coupled peptide transporter has been cloned from rabbit and human intestine (Fei et al. (1994) Nature 368, 563-566; Liang et al., J. Biol. Chem., in press). Functional studies have established that the renal peptide transport system is similar but not identical to its intestinal counterpart. Therefore, in an attempt to isolate the renal H+/peptide cotransporter cDNA, we screened a human kidney cDNA library with a probe derived from the rabbit intestinal H+/peptide cotransporter cDNA. This has resulted in the isolation of a positive clone with a 2190 bp long open reading frame. The predicted protein consists of 729 amino acids. Hydropathy analysis of the amino acid sequence indicates the presence of twelve putative transmembrane domains. The primary structure of this protein exhibits 50% identity and 70% similarity to the human intestinal H+/peptide cotransporter. Functional expression of the kidney cDNA in HeLa cells results in the induction of a H(+)-coupled transport system specific for small peptides and aminocephalosporins. Reverse transcription-coupled polymerase chain reaction demonstrates that the cloned transporter is expressed in human kidney but not in human intestine. This transporter, henceforth called PEPT 2, represents a new member in the growing family of H(+)-coupled transport systems in the mammalian plasma membrane.

Human intestinal H+/peptide cotransporter. Cloning, functional expression, and chromosomal localization

In mammalian small intestine, a H(+)-coupled peptide transporter is responsible for the absorption of small peptides arising from digestion of dietary proteins. Recently a cDNA clone encoding a H+/peptide cotransporter has been isolated from a rabbit intestinal cDNA library (Fei, Y.J., Kanai, Y., Nussberger, S., Ganapathy, V., Leibach, F.H., Romero, M.F., Singh, S.K., Boron, W. F., and Hediger, M. A. (1994) Nature 368, 563-566). Screening of a human intestinal cDNA library with a probe derived from the rabbit H+/peptide cotransporter cDNA resulted in the identification of a cDNA which when expressed in HeLa cells or in Xenopus laevis oocytes induced H(+)-dependent peptide transport activity. The predicted protein consists of 708 amino acids with 12 membrane-spanning domains and two putative sites for protein kinase C-dependent phosphorylation. The cDNA-induced transport process accepts dipeptides, tripeptides, and amino beta-lactam antibiotics but not free amino acids as substrates. The human H+/peptide cotransporter exhibits a high degree of homology (81% identity and 92% similarity) to the rabbit H+/peptide cotransporter. But surprisingly these transporters show only a weak homology to the H(+)-coupled peptide transport proteins present in bacteria and yeast. Chromosomal assignment studies with somatic cell hybrid analysis and in situ hybridization have located the gene encoding the cloned human H+/peptide cotransporter to chromosome 13 q33-->q34.