Abstract
Background/Aims: Janus-activated kinase-2 JAK2 participates in the signaling of several hormones including growth hormone, fosters tumor growth and modifies the activity of several Na+ coupled nutrient transporters. Peptide uptake into intestinal and tumor cells is accomplished by electrogenic peptide transporters PEPT1 and PEPT2. The present study thus explored whether JAK2 contributes to the regulation of PEPT1 and PEPT2 activity. Methods: cRNA encoding either PEPT1 or PEPT2 was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild type JAK2, constitutively active V617FJAK2 or inactive K882EJAK2. The current created by the dipeptide glycine-glycine (Igly-gly) was determined by dual electrode voltage clamp and taken as measure for electrogenic peptide transport. Results: No appreciable Igly-gly was observed in water injected oocytes. In PEPT1 or PEPT2 expressing oocytes Igly-gly was significantly increased by additional coexpression of JAK2. As shown in PEPT1 expressing oocytes, Igly-gly without significantly modifying the concentration required for halfmaximal Igly-gly (KM). Following disruption of carrier insertion with brefeldin A (5 µM) Igly-gly declined similarly fast in Xenopus oocytes expressing PEPT1 with JAK2 and in Xenopus oocytes expressing PEPT1 alone. In oocytes expressing both, PEPT1 and V617FJAK2, Igly-gly was gradually decreased by JAK2 inhibitor AG490 (40 µM). According to Ussing chamber experiments pharmacological JAK2 inhibition similarly decreased Igly-gly in mouse intestine. Conclusion: Regulation of the peptide transporters PEPT and PEPT2 does involve the Janus-activated kinase-2 JAK2.
Introduction
The janus-activated kinase-2 JAK2 is involved in the signaling of a variety of hormones and cytokines [1,2,3], such as leptin [4], growth hormone [5,6], erythropoietin [2], thrombopoietin [2] and granulocyte colony-stimulating factor [2]. Moreover, JAK2 is activated by oxidative stress [7], ischemia [7] and hypertonicity [8,9]. Excessive JAK2 activity may lead to development of malignancy and the gain of function mutation V617FJAK2 presumably predisposes to the development of myeloproliferative disease [10,11,12,13]. Conversely JAK2 inhibitors are considered for the treatment of myeloproliferative disorders [14,15,16,17,18,19].
JAK2 has most recently been shown to be involved in the regulation of several transporters including the betaine/GABA transporter BGT1 [20], the creatine transporter CreaT [21], the glucose transporter SGLT1 [22] and several amino acid transporters [23,24]. Transporters contributing to intestinal nutrient uptake and expressed in tumor cells include the peptide transporters 1 (PEPT1) and 2 (PEPT2), carriers mediating the cellular uptake of di- and tripeptides [25,26,27]. The carriers further mediate the cellular uptake of peptide-like drugs [25,26]. Thus, regulation of PEPT1/PEPT2 activity may impact on the sensitivity of tumor cells to those drugs.
Previous experiments revealed that intestinal peptide transport is up-regulated by the PI3kinase pathway [28] and by glucocortioids [29], both effects at least partially mediated by the serum & glucocorticoid inducible kinase SGK1 [30]. PEPT1 is further regulated by AMP activated kinase [31]. Peptide transport is stimulated by leptin [32] and growth hormone [33]. The signaling linking the leptin receptor to peptide transport is not known but could, at least in theory, involve JAK2.
The present study thus explored whether JAK2 influences the peptide transporters PEPT1 and/or PEPT2. To this end, cRNA encoding the peptide transporters was injected into Xenopus oocytes with or without cRNA encoding wild type JAK2, constitutively active V617FJAK2 or inactive K882EJAK2 and peptide transport estimated from peptide induced current. To explore whether JAK2 sensitivity of electrogenic peptide transport may be relevant in vivo, additional Ussing experiments were performed elucidating the effect of pharmacological JAK2 inhibition on peptide induced current in intestine.
Materials and Methods
Constructs
Voltage clamp in Xenopus oocytes
Xenopus oocytes were prepared as previously described [37,38]. Where not indicated otherwise, 10 ng PEPT1 and PEPT2 cRNA were injected on the first day and 10 ng of wild type JAK2 cRNA injected on the second day or same day after preparation of the oocytes [39]. The oocytes were maintained at 17°C in ND96 solution containing (in mM): 96 NaCl, 4 KCl, 1.8 MgC12, 0.1 CaC12, 5 HEPES, pH7.4, gentamycin (50 mg/l), tetracycline (50 mg/l), ciprofloxacin (1.6 mg/l), refobacin (100 mg/l), theophiline (90 mg/l). Where indicated, the JAK2 inhibitor AG490 (40 µM) or brefeldin A (5 µM) were added to the respective solutions. The voltage clamp experiments were performed at room temperature 3 days after injection [40,41]. Two-electrode voltage-clamp recordings were performed at a holding potential of -60 mV. The data were filtered at 10 Hz and recorded with a Digidata A/D-D/A converter and Clampex 9.2 software for data acquisition and analysis (Axon Instruments) [24]. The control superfusate (ND96) contained 96 mM NaCl, 2 mM KCl, 1.8 mM CaCl2, 1 mM MgCl2 and 5 mM HEPES, pH 7.4. Glycine-glycine was added to the solutions at a concentration of 1 mM, unless otherwise stated. The flow rate of the superfusion was approx. 20 ml/min, and a complete exchange of the bath solution was reached within about 10 s.
Ussing chamber experiments
All animal experiments were conducted according to the German law for the welfare of animals and were approved by local authorities. Experiments have been performed using intestinal segments from mice, fed a control diet (1314, Altromin, Heidenau, Germany) and having free access to tap drinking water.
For analysis of electrogenic intestinal peptide transport, jejunal segments were mounted into a custom made mini-Ussing chamber with an opening of 0.00769 cm2. Under control conditions, the serosal and luminal perfusate contained (in mM): 115 NaCl, 2 KCl, 1 MgCl2, 1.25 CaCl2, 0.4 KH2PO4, 1.6 K2HPO4, 5 Na pyruvate, 25 NaHCO3 (pH 7.4, NaOH). Where indicated, the dipeptide glycine-glycine (5 mM) was added to the luminal perfusate at the expense of mannitol (10 mM) (all substances were from Sigma, Schnelldorf, Germany, or from Roth, Karlsruhe, Germany). In all Ussing chamber experiments the transepithelial potential difference (Vt) was determined continuously and the transepithelial resistance (Rt) was estimated from the voltage deflections (ΔVt) elicited by imposing test currents (It). The resulting Rt was calculated according to Ohm's law [29].
Statistical analysis
Data are provided as means ± SEM, n represents the number of oocytes or intestinal segments investigated. All voltage clamp experiments were repeated with at least 2-3 batches of oocytes; in all repetitions qualitatively similar data were obtained. Data were tested for significance using ANOVA or t-test, as appropriate. Results with p < 0.05 were considered statistically significant.
Results
The present study explored, whether Janus kinase-2 (JAK2) participates in the regulation of the peptide transporters PEPT1 and PEPT2. To this end, cRNA encoding PEPT1 or PEPT2 were injected into Xenopus oocytes with or without additional injection of cRNA encoding JAK2. As PEPT1 and PEPT2 are electrogenic transporters, the peptide transport was estimated from the current generated upon addition of the dipeptide glycine-glycine (Igly-gly). No appreciable current was observed following exposure of non-injected or water-injected Xenopus oocytes to 1 mM glycine-glycine. Thus, Xenopus oocytes do not express appreciable endogenous electrogenic glycine-glycine transport (Fig. 1A,B). Moreover, no appreciable Igly-gly was observed in Xenopus oocytes expressing wild type JAK2 alone (Fig. 1A,B). Igly-gly was, however, observed in Xenopus oocytes injected with cRNA encoding PEPT1. The additional injection of cRNA encoding wild type JAK2 was followed by a significant increase of Igly-gly in PEPT1 expressing Xenopus oocytes (Fig. 1A,B).
Coexpression of JAK2 increases electrogenic peptide transport in PEPT1-expressing Xenopus oocytes. A: Representative original tracings showing glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (a), expressing wild type JAK2 alone (b), or expressing PEPT1 without (c) or with (d) additional coexpression of wild type JAK2. B: Arithmetic means ± SEM (n = 15) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (H2O, light grey bar), expressing JAK2 alone (JAK2, dark grey bar), or expressing PEPT1 without (PEPT1, white bar) or with (PEPT1+JAK2, black bar) additional coexpression of wild type JAK2. The respective values ± standard deviation are (1.8 ± 1.1, 2.2 ± 1.3, 41 ± 8, 52 ± 10, respectively). *** indicates statistically significant (p<0.001) difference from the absence of PEPT1. # indicates statistically significant (p<0.05) difference from the absence of JAK2. C: Arithmetic means ± SEM (n = 15) of glycine-glycine-induced current (Igly-gly) as a function of glycine-glycine concentration in Xenopus oocytes expressing PEPT1 without (open circles, dashed line), or with (closed circles, solid line) additional coexpression of wild type JAK2.
Coexpression of JAK2 increases electrogenic peptide transport in PEPT1-expressing Xenopus oocytes. A: Representative original tracings showing glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (a), expressing wild type JAK2 alone (b), or expressing PEPT1 without (c) or with (d) additional coexpression of wild type JAK2. B: Arithmetic means ± SEM (n = 15) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (H2O, light grey bar), expressing JAK2 alone (JAK2, dark grey bar), or expressing PEPT1 without (PEPT1, white bar) or with (PEPT1+JAK2, black bar) additional coexpression of wild type JAK2. The respective values ± standard deviation are (1.8 ± 1.1, 2.2 ± 1.3, 41 ± 8, 52 ± 10, respectively). *** indicates statistically significant (p<0.001) difference from the absence of PEPT1. # indicates statistically significant (p<0.05) difference from the absence of JAK2. C: Arithmetic means ± SEM (n = 15) of glycine-glycine-induced current (Igly-gly) as a function of glycine-glycine concentration in Xenopus oocytes expressing PEPT1 without (open circles, dashed line), or with (closed circles, solid line) additional coexpression of wild type JAK2.
Kinetic analysis was employed for the determination of maximal transport rate and affinity of the carrier. As illustrated in Fig. 1C, in PEPT1-expressing Xenopus oocytes Igly-gly increased following increase of substrate concentration. The currents allowed calculating a maximal current, which was significantly higher in Xenopus oocytes expressing PEPT1 together with JAK2 (83.7 ± 4.5 nA, n = 15), than in Xenopus oocytes expressing PEPT1 without JAK2 (65.2 ± 3.4 nA, n = 15). Accordingly, JAK2 enhanced the maximal transport rate. The glycine-glycine concentration required for halfmaximal current (KM) approached 364 ± 93 µM (n = 15) in Xenopus oocytes expressing PEPT1 alone, and 486 ± 181 µM (n = 15) in Xenopus oocytes expressing PEPT1 together JAK2, values not significantly different.
Similar to its effect on PEPT1, JAK2 up-regulated the peptide transporter isoform PEPT2. Similar to what was observed following expression of PEPT1, injection of cRNA encoding PEPT2 resulted in the appearance of glyine-glycine-induced currents. Igly-gly was higher in Xenopus oocytes injected with cRNA encoding PEPT2 together with JAK2 than in Xenopus oocytes injected with cRNA encoding PEPT2 alone (Fig. 2A,B).
Coexpression of JAK2 increases electrogenic peptide transport in PEPT2-expressing Xenopus oocytes. A: Representative original tracings showing glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (a), expressing wild type JAK2 alone (b), or expressing PEPT2 without (c) or with (d) additional coexpression of wild type JAK2. B: Arithmetic means ± SEM (n = 14) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (H2O, light grey bar), expressing JAK2 alone (JAK2, dark grey bar) or expressing PEPT2 without (PEPT2, white bar) or with (PEPT2+JAK2, black bar) additional coexpression of wild type JAK2. *** indicates statistically significant (p<0.001) difference from the absence of PEPT2. ### indicates statistically significant (p<0.001) difference from the absence of JAK2.
Coexpression of JAK2 increases electrogenic peptide transport in PEPT2-expressing Xenopus oocytes. A: Representative original tracings showing glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (a), expressing wild type JAK2 alone (b), or expressing PEPT2 without (c) or with (d) additional coexpression of wild type JAK2. B: Arithmetic means ± SEM (n = 14) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (H2O, light grey bar), expressing JAK2 alone (JAK2, dark grey bar) or expressing PEPT2 without (PEPT2, white bar) or with (PEPT2+JAK2, black bar) additional coexpression of wild type JAK2. *** indicates statistically significant (p<0.001) difference from the absence of PEPT2. ### indicates statistically significant (p<0.001) difference from the absence of JAK2.
The effect of wild type JAK2 on Igly-gly was mimicked by the gain of function mutant V617FJAK2, but not by the inactive mutant K882EJAK2 (Fig. 3A,B). Accordingly, Igly-gly was significantly higher in Xenopus oocytes expressing PEPT1 together with V617FJAK2 than in Xenopus oocytes expressing PEPT1 alone. In contrast, Igly-gly was similar in Xenopus oocytes expressing PEPT1 together with K882EJAK2 and in Xenopus oocytes expressing PEPT1 alone.
The effect of JAK2 is mimicked by V617FJAK2 but not by the inactive mutant K882EJAK2. A: Representative original tracings showing glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (a), expressing PEPT1 alone (b), or expressing PEPT1 with JAK2 (c), with constitutively active V617FJAK2 (d), or with the inactive mutant K882EJAK2 (e). B. Arithmetic means ± SEM (n = 20-22) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (H2O, light grey bar), expressing PEPT1 without (PEPT1, white bar) or with wild type JAK2 (PEPT + JAK2, dark grey bar), with constitutively active V617FJAK2 (PEPT1+V617FJAK2, black bar) or with the inactive mutant K882EJAK2 (PEPT1+K882EJAK2, light grey bar) *, ** indicates statistically significant (p<0.05, p<0.01) difference to expression of PEPT1 alone.
The effect of JAK2 is mimicked by V617FJAK2 but not by the inactive mutant K882EJAK2. A: Representative original tracings showing glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (a), expressing PEPT1 alone (b), or expressing PEPT1 with JAK2 (c), with constitutively active V617FJAK2 (d), or with the inactive mutant K882EJAK2 (e). B. Arithmetic means ± SEM (n = 20-22) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (H2O, light grey bar), expressing PEPT1 without (PEPT1, white bar) or with wild type JAK2 (PEPT + JAK2, dark grey bar), with constitutively active V617FJAK2 (PEPT1+V617FJAK2, black bar) or with the inactive mutant K882EJAK2 (PEPT1+K882EJAK2, light grey bar) *, ** indicates statistically significant (p<0.05, p<0.01) difference to expression of PEPT1 alone.
At least in theory, the increase of Igly-gly in PEPT1 expressing Xenopus oocytes by coexpression of V617FJAK2 could have been due to accelerated clearance of carrier protein from the cell membrane. In order to estimate the stability of Igly-gly, the PEPT1-expressing Xenopus oocytes were treated with 5 µM brefeldin A, a substance blocking the insertion of new carrier protein into the cell membrane. As shown in Fig. 4A, in the presence of brefeldin A Igly-gly declined similarly fast in Xenopus oocytes expressing PEPT1 together with V617FJAK2 and in Xenopus oocytes expressing PEPT1 alone. The observation suggests that V617FJAK2 increases Igly-gly by a mechanism other than accelerating carrier clearance from the cell membrane.
Effects of brefeldin A on electrogenic peptide transport in Xenopus oocytes expressing PEPT1 with or without JAK2. A. Arithmetic means ± SEM (n = 18) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected PEPT1 alone (white bars) or expressing PEPT1 together with V617FJAK2 and subsequently incubated with or without 5 µM brefeldin A (black bars). ** indicates statistically significant (p<0.05, p<0.01) difference from the absence of V617FJAK2; #, ## indicates statistically significant (p<0.05, p<0.01) difference from the absence of brefeldin A.
Effects of brefeldin A on electrogenic peptide transport in Xenopus oocytes expressing PEPT1 with or without JAK2. A. Arithmetic means ± SEM (n = 18) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected PEPT1 alone (white bars) or expressing PEPT1 together with V617FJAK2 and subsequently incubated with or without 5 µM brefeldin A (black bars). ** indicates statistically significant (p<0.05, p<0.01) difference from the absence of V617FJAK2; #, ## indicates statistically significant (p<0.05, p<0.01) difference from the absence of brefeldin A.
As illustrated in Fig. 5, Igly-gly in Xenopus oocytes expressing both, PEPT1 and V617FJAK2 was decreased by treatment with the JAK2 inhibitor AG490 (40µM). The effect of the inhibitor on Igly-gly reached statistical significance within 24 hours of preincubation with AG490.
The effect of V617FJAK2 is reversed by the JAK2 inhibitor AG490. A. Representative original tracings showing glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (a), expressing PEPT1 (b), or expressing PEPT1 + V617FJAK2 without (c) or with a 3 hours (d) or 8 hours (e) or 24 hours (f) pretreatment with JAK2 inhibitor AG490 (40 µM). B. Arithmetic means ± SEM (n = 20) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes expressing PEPT1 without (PEPT1, white bar) or with constitutive active V617FJAK2 (PEPT1+V617FJAK2, black bars) or following pretreatment with the JAK2 inhibitor AG490 (40 µM, grey bars) for the indicated time periods. ## indicates statistically significant (p<0.01) difference from the absence of JAK2 inhibitor AG490. *,** indicates statistically significant (p<0.05, p<0.01) difference from PEPT1 alone (absence of JAK2).
The effect of V617FJAK2 is reversed by the JAK2 inhibitor AG490. A. Representative original tracings showing glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes injected with water (a), expressing PEPT1 (b), or expressing PEPT1 + V617FJAK2 without (c) or with a 3 hours (d) or 8 hours (e) or 24 hours (f) pretreatment with JAK2 inhibitor AG490 (40 µM). B. Arithmetic means ± SEM (n = 20) of glycine-glycine (1 mM) - induced current (Igly-gly) in Xenopus oocytes expressing PEPT1 without (PEPT1, white bar) or with constitutive active V617FJAK2 (PEPT1+V617FJAK2, black bars) or following pretreatment with the JAK2 inhibitor AG490 (40 µM, grey bars) for the indicated time periods. ## indicates statistically significant (p<0.01) difference from the absence of JAK2 inhibitor AG490. *,** indicates statistically significant (p<0.05, p<0.01) difference from PEPT1 alone (absence of JAK2).
In order to test, whether the observed regulation of PEPT1 by JAK2 is relevant for peptide transport in vivo, peptide induced current was determined in Ussing chamber experiments of mouse intestinal segments. As illustrated in Fig. 6, addition of glycine-glycine (5 mM) to the luminal perfusate generated a transepithelial current (Igly-gly) which was gradually decreased by treatment with the JAK2 inhibitor AG490 (40 µM). The effect of the inhibitor on Igly-gly reached statistical significance within 30 min of preincubation with AG490.
Effect JAK2 inhibitor AG490 and JAK2 inhibitor III on glycine-glycine-induced current in intestine. A: Original tracings from typical experiments illustrating the effect of 5 mM glycine-glycine on the transepithelial potential difference of intestinal segments without (a) or with (b, c) pretreatment with (b) JAK2 inhibitor III (20 μM) or (c) JAK2 inhibitor AG490 (40 μM). B: Arithmetic means ± SEM (n = 6) of glycine-glycine (5 mM) induced current (Igly - gly) in jejunum in abesence (control, white bar) or presence of JAK2 inhibitor III (grey bar) or JAK2 inhibitor AG490 (black bar). * indicates statistically significant (p<0.05) difference from absence of inhibitor.
Effect JAK2 inhibitor AG490 and JAK2 inhibitor III on glycine-glycine-induced current in intestine. A: Original tracings from typical experiments illustrating the effect of 5 mM glycine-glycine on the transepithelial potential difference of intestinal segments without (a) or with (b, c) pretreatment with (b) JAK2 inhibitor III (20 μM) or (c) JAK2 inhibitor AG490 (40 μM). B: Arithmetic means ± SEM (n = 6) of glycine-glycine (5 mM) induced current (Igly - gly) in jejunum in abesence (control, white bar) or presence of JAK2 inhibitor III (grey bar) or JAK2 inhibitor AG490 (black bar). * indicates statistically significant (p<0.05) difference from absence of inhibitor.
Discussion
The present study identifies a completely novel regulator of the peptide transporters PEPT1 and PEPT2. Coexpression of the Janus Kinase 2 (JAK2) significantly increased the electrogenic transport of the dipeptide glycine-glycine in Xenopus oocytes expressing either PEPT1 or PEPT2. The effect of wild type JAK2 was mimicked by the gain of function mutant V617FJAK2 but not by the inactive K882EJAK2. The current in PEPT1 expressing oocytes tended to be lower following coexpression of K882EJAK2, a difference, however, not reaching statistical significance. An inhibitory effect may have pointed to a transdominant inhibitory action of the inactive kinase.
JAK2 coexpression was effective by increasing the maximal transport rate, an observation compatible with an increase of carrier protein within the cell membrane. In theory, the carrier protein abundance could be modified by alterations of protein insertion into or of protein retrieval from the cell membrane. As the decline of current following brefeldin A treatment was similar in oocytes coexpressing PEPT1 with JAK2 and oocytes expressing PEPT1 alone, JAK2 was apparently not effective by delaying protein retrieval from the cell membrane. Possibly, JAK2 increases PEPT1 activity by inserting carrier protein into the cell membrane.
The effect of JAK2 was reversed by JAK2 inhibitor AG490. Pharmacological JAK2 inhibition further decreased the peptide induced current in segments of mouse small intestine, an observation pointing to the in vivo significance of JAK2 sensitive regulation of peptide transporters. As selectivity of pharmacological inhibitors may be limited, it remains, however, uncertain whether JAK2 indeed participates in the regulation of intestinal peptide transport. The exposure time required for significant inhibition of peptide transport by AG490 was considerably longer in JAK2 and PEPT1 expressing oocytes than in intestinal segments. It should be kept in mind that the inhibitory effect on cytosolic kinases requires the uptake of the inhibitor across the cell membrane, which may be particularly slow in oocytes.
PEPT1 accomplishes the uptake of luminal di- and tripeptides across the apical enterocyte cell membrane [42,43] and bile duct epithelium [42]. PEPT2 is expressed mainly by glial cells and by epithelial cells in the kidney, choroid plexus, lung and mammary gland [42,44]. Peptide transporters are further expressed in tumor cells [25,42,45,46]. JAK2 sensitive peptide transport may be relevant for intestinal nutrient uptake [26,47], as PEPT1 absorbs the majority of dietary nitrogen [48]. Moreover, PEPT1 has been implicated in inflammatory bowel disease [27]. In healthy individuals the carrier is mainly expressed in small intestine but not in colon. In inflammatory bowel disease, colonic tissue expresses high levels of PEPT1, which participates in peptide trafficking and thus peptide interactions with innate immune receptors. Peptide transporters are further important carriers involved in drug transport, such as beta-lactam antibiotics, angiotensin-converting enzyme inhibitors, antiviral drugs and anti-cancer agents [26,44,47,49,50,51]. JAK2 sensitive nutrient and drug uptake by peptide transporters may be relevant for the growth, survival and drug sensitivity of tumor cells [25,31,52,53]. Peptide transporters have thus been considered as potential targets for tumor therapy [54].
According to previous observations JAK2 stimulates cellular glucose uptake by affecting both, facilitative glucose carriers [55,56] and Na+ coupled glucose transport [22] . JAK2 is further involved in the regulation of Na+ coupled neutral amino acid transporter B(0)AT (SLC6A19) [23], Na+ coupled glutamate transport [24], the Na+ and Cl- coupled transport of betaine and GABA [20] and the Na+/H+ exchanger [57]. Accordingly, the kinase is a broad regulator of transport.
In conclusion, the present paper reveals that JAK2 is a powerful regulator of the peptide transporters PEPT1 and PEPT2. The kinase up-regulates the carriers and thus may influence peptide and drug transport in epithelia and tumor cells.
Conflict of Interest
The authors state that they have no conflicts of interests to disclose.
Acknowledgements
The authors acknowledge the meticulous preparation of the manuscript by Lejla Subasic and technical support by Elfriede Faber. This study was supported by the Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of Tuebingen University.