Nox enzymes and oxidative stress in the immunopathology of the gastrointestinal tract

Chronic inflammation caused by Helicobacter pylori infection or inflammatory bowel disease (IBD) is closely linked to cancer development. Innate immune abnormalities and enhanced production of reactive oxygen species through a phagocyte NADPH oxidase (Nox2) are key issues in understanding the pathogenesis of inflammation-dependent carcinogenesis. Besides Nox2, functionally distinct homologues (Nox1, Nox3, Nox4, Nox5, Duox1, and Duox2) have been identified. Nox1 and Duox2 are highly expressed in the gastrointestinal tract. Although the functional roles of Nox/Duox in the gastrointestinal tract are still unclear, we will review their potential roles in the gastrointestinal immunopathology, particularly in H. pylori-induced inflammation, IBD, and malignancy.     

Tanshinone IIA blocks dexamethasone-induced apoptosis in osteoblasts through inhibiting Nox4-derived ROS production

Apoptosis of osteoblasts caused by glucocorticoids has been identified as an important contributor to the development of osteoporosis. Tanshinone IIA (Tan), an active ingredient extracted from the rhizome of the Salvia miltiorrhiza Bunge (Danshen), has been reported to cast positive effects on osteoporosis. However, the precise mechanisms accounting this action remain elusive. In this study, by using osteoblastic MC3T3-E1 cells as a model, we confirmed the protective effects of Tan against dexamethasone (Dex)-induced cell apoptosis and further clarified its molecular mechanism of action. Our results showed that treatment with Dex caused cell injury, increased cytosol cytochrome c level and Nox expression, induced apoptosis in caspase-9-dependent manner, and enhanced reactive oxygen species (ROS) production. Tan attenuated these deleterious consequence triggered by Dex. Moreover, Dex-induced ROS production and cell injury were inhibited by antioxidant, NADPH oxidases inhibitors, Nox4 inhibitor, and Nox4 small interfering RNA (siRNA). Overexpression of Nox4 almost abolished the inhibitory effect of Tan on Dex-induced cell injury and apoptosis. The results also demonstrated significant involvement of Nox4 in the Dex-induced apoptosis. Nox4-derived ROS led to apoptosis through activation of intrinsic mitochondrial pathway. Additionally, we evidenced that Tan reversed Dex-induced apoptosis via inactivation of Nox4. The present findings suggest that inhibition of Nox4 may be a novel therapeutic approach of Tan to prevent against glucocorticoids-induced osteoblasts apoptosis and osteoporosis.     

Sed4p functions as a positive regulator of Sar1p probably through inhibition of the GTPase activation by Sec23p

BACKGROUND: Sar1p belongs to a unique subfamily of small GTPases and is essential for formation of the transport vesicles from the endoplasmic reticulum (ER) that are destined to the Golgi apparatus. To understand how the GTPase cycle of Sar1p is regulated, we screened for multicopy suppressors of sar1 ts mutants and identified SED4. RESULTS: Although deletion of sed4 alone shows no growth defect, sar1 delta(sed4) double mutant cells are inviable. In the delta(sed4) background, the suppression activity of SAR1 towards sec12 and sec16 is lost. These observations suggest that SED4 is a very close partner of SAR1 and imply that Sed4p may act to increase the active Sar1p in the cell. Over-expression of SEC12 does not remedy the lethality of sar1 delta(sed4). The purified cytoplasmic domain of Sed4p does not show a guanine nucleotide exchange (GEF) activity toward Sar1p nor increase the GEF activity of Sec12p. On the contrary, over-expression of SED4 aggravates the ts growth of sec23 cells. The cytoplasmic domain of Sed4p weakly inhibits the GTPase-activating (GAP) activity of Sec23p toward Sar1p. In a microsome-based COPII binding assay, the binding of the GDP-form mutant Sar1p (D32G) is lower on the delta(sed4) microsomes than on the wild-type membranes. CONCLUSION: We propose a model that Sed4p counteracts the GAP action of Sec23p on to Sar1p.     

Nox4在同型半胱氨酸介导的动脉粥样硬化中作用的研究

同型半胱氨酸（Hcy）是心血管疾病的独立危险因子之一。Hcy能够诱导细胞活性氧（ROS）的生成增多引起内皮损伤从而导致动脉粥样硬化（AS）发生发展。内皮细胞中ROS主要来源于Nox4（NADPH oxidases 4）。NLRP3在冠状动脉粥样硬化及主动脉硬化中高表达。NLRP3是AS中IL-1家族细胞因子产生的关键介质,特别是当细胞内产生的大量ROS,可导致caspase-1前体大量剪切成具有生物活性的caspase-1 P10片段,推动下游IL-1β和IL-18等炎症因子的成熟及分泌,使内皮细胞功能受损,引起细胞炎症损伤及凋亡,正反馈加重血管损伤。本文主要探讨Nox4在Hcy介导的内皮细胞炎症及凋亡的作用,Hcy导致的AS提供可能的机制。     

Nox2 contributes to cardiac fibrosis in diabetic cardiomyopathy in a transforming growth factor-β dependent manner

Purpose: This study aimed to investigate the effect of Nox2 on cardiac fibrosis and to elucidate the regulatory mechanism of Nox2 in the development of DCM. Methods: We established normal and insulin-resistant cellular model using neonatal rat cardiac fibroblasts. Then Nox2-specific siRNA were transfected into cardiac fibroblasts with Lipofectamine ® 2000 and crambled siRNA sequence was considered as control. Meanwhile, a part of cells were randomly selected to be treated with or without transforming growth factor-β (TGF-β). Moreover, quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot were respectively performed to determine the expression level of related molecules, such as Nox2, collagen type I and III (COL I and III) and PI3K/AKT and PKC/Rho signaling pathway-related proteins. Results: TGF-β stimulation significantly increased the expression level of Nox2 both in mRNA and protein levels. Suppression of the Nox2 markedly decreased the expression of COL I and COL III in normal and insulin-resistant cellular model with TGF-β stimulation. Moreover, suppression of the Nox2 significantly decreased the expression of PI3K/AKT and PKC/Rho signaling pathway-related proteins in insulin-resistant cellular model with TGF-β stimulation. However, suppression of Nox2 had no effects on these proteins without TGF-β stimulation. Conclusions: Our finding reveals that Nox2 may promote synthesis of COL I and III via involved in PI3K/AKT and PKC/Rho signaling pathway in a TGF-β dependent manner and consequently promote cardiac fibrosis in the development of DCM.     

Characterisation of a delta-COP homologue in the malaria parasite,Plasmodium falciparum

The mature human erythrocyte is a simple haemoglobin-containing cell with no internal organelles and no protein synthesis machinery. The malaria parasite invades this cell and develops inside a parasitophorous vacuole (PV). The parasite exports proteins into the erythrocyte to bring about extensive remodelling of its adopted cellular home. Plasmodial homologues of two COPII proteins, PfSar1p and PfSec31p, are exported to the erythrocyte cytosol where they appear to play a role in the trafficking of proteins across the erythrocyte cytoplasm [Eur. J. Cell Biol. 78 (1999) 453; J. Cell Sci. 114 (2001) 3377]. We have now characterised a homologue of the COPI protein, delta-COP. A recombinant protein corresponding to 90% of the Pfdelta-COP sequence was used to raise antibodies. The affinity-purified antiserum recognised a protein with an apparent M(r) of 58 x 10(3) on Western blots of malaria parasite-infected erythrocytes but not on blots of uninfected erythrocytes. Pfdelta-COP was shown to be largely insoluble in non-ionic detergent, possibly suggesting cytoskeletal attachment. Confocal immunofluorescence microscopy of parasitised erythrocytes was used to show that, in contrast to the COPII proteins, Pfdelta-COP is located entirely within the parasite. The location of Pfdelta-COP partly overlaps that of the endoplasmic reticulum (ER)-located protein, PfERC, and partly that of the trans-Golgi-associated protein, PfRab6. Treatment of ring-stage plasmodium-infected erythrocytes with brefeldin A (BFA) inhibited development of the ER structure within the parasite cytosol and prevented the trafficking of the P. falciparum erythrocyte membrane protein-1, PfEMP1, to the erythrocyte cytosol. The Pfdelta-COP and PfSec31p populations each appear to be associated with the restricted ER structure in brefeldin-treated rings. When more mature stage parasites were treated with BFA, erythrocyte cytosol-located populations of parasite proteins were not reorganised, however, the overlap between Pfdelta-COP and PfERC in parasite cytosol was more complete suggesting a possible redistribution of the Golgi compartment into the ER. These data support the suggestion that both COPI and COPII proteins are involved in the trafficking of proteins within the parasite cytoplasm. However, only COPII proteins are exported to the erythrocyte cytosol to establish a vesicle-mediated protein trafficking pathway to the erythrocyte membrane.     

ROS‐generating oxidases Nox1 and Nox4 contribute to oncogenic Ras‐induced premature senescence

Activated oncogenes induce premature cellular senescence, a permanent state of proliferative arrest in primary rodent and human fibroblasts. Recent studies suggest that generation of reactive oxygen species (ROS) is involved in oncogenic Ras‐induced premature senescence. However, the signaling mechanism controlling this oxidant‐mediated irreversible growth arrest is not fully understood. Here, we show that through the Ras/MEK pathway, Ras oncogene up‐regulated the expression of superoxide‐generating oxidases, Nox1 in rat REF52 cells and Nox4 in primary human lung TIG‐3 cells, leading to an increase in intracellular level of ROS. Ablation of Nox1 and Nox4 by small interfering RNAs (siRNAs) blocked the RasV12 senescent phenotype including β‐galactosidase activity, growth arrest and accumulation of tumor suppressors such as p53 and p16^Ink4a. This suggests that Nox‐generated ROS transduce senescence signals by activating the p53 and p16^Ink4a pathway. Furthermore, Nox1 and Nox4 siRNAs inhibited both Ras‐induced DNA damage response and p38MAPK activation, whereas overexpression of Nox1 and Nox4 alone was able to induce senescence. The involvement of Nox1 in Ras‐induced senescence was also confirmed with embryonic fibroblasts derived from Nox1 knockout mice. Together, these findings suggest that Nox1‐ and Nox4‐generated ROS play an important role in Ras‐induced premature senescence, which may involve DNA damage response and p38MAPK signaling pathways.     

Differential response of the human renal proximal tubular epithelial cell line HK-2 to Shiga toxin types 1 and 2

Shiga toxins (Stxs) are expressed by the enteric pathogens Shigella dysenteriae serotype 1 and certain serotypes of Escherichia coli. Stx-producing bacteria cause bloody diarrhea with the potential to progress to acute renal failure. Stxs are potent protein synthesis inhibitors and are the primary virulence factors responsible for renal damage that may follow diarrheal disease. We explored the use of the immortalized human proximal tubule epithelial cell line HK-2 as an in vitro model of Stx-induced renal damage. We showed that these cells express abundant membrane Gb(3) and are differentially susceptible to the cytotoxic action of Stxs, being more sensitive to Shiga toxin type 1 (Stx1) than to Stx2. At early time points (24 h), HK-2 cells were significantly more sensitive to Stxs than Vero cells; however, by 72 h, Vero cell monolayers were completely destroyed while some HK-2 cells survived toxin challenge, suggesting that a subpopulation of HK-2 cells are relatively toxin resistant. Fluorescently labeled Stx1 B subunits localized to both lysosomal and endoplasmic reticulum (ER) compartments in HK-2 cells, suggesting that differences in intracellular trafficking may play a role in susceptibility to Stx-mediated cytotoxicity. Although proinflammatory cytokines were not upregulated by toxin challenge, Stx2 selectively induced the expression of two chemokines, macrophage inflammatory protein-1α (MIP-1α) and MIP-1β. Stx1 and Stx2 differentially activated components of the ER stress response in HK-2 cells. Finally, we demonstrated significant poly(ADP-ribose) polymerase (PARP) cleavage after exposure to Stx1 or Stx2. However, procaspase 3 cleavage was undetectable, suggesting that HK-2 cells may undergo apoptosis in response to Stxs in a caspase 3-independent manner.     

M‐COPA,a novel Golgi system disruptor,suppresses apoptosis induced by Shiga toxin

Shiga toxin (Stx) is a main virulence factor of Stx‐producing Escherichia coli (STEC) that contributes to diarrhea and hemorrhagic colitis and occasionally to fatal systemic complications. Therefore, the development of an antidote to neutralize Stx toxicity is urgently needed. After internalization into cells, Stx is transferred to the Golgi apparatus via a retrograde vesicular transport system. We report here that 2‐methylcoprophilinamide (M‐COPA), a compound that induces disassembly of the Golgi apparatus by inactivating ADP‐ribosylation factor 1 (Arf1), suppresses Stx‐induced apoptosis. M‐COPA inhibited transport of Stx from the plasma membrane to the Golgi apparatus and suppressed degradation of anti‐apoptotic proteins and the activation of caspases. These findings suggest that inhibition of Stx retrograde transport by M‐COPA could be a novel approach to suppress Stx toxicity.     

siRNA沉默Nox1基因抑制胃癌细胞的生长

目的: 探讨siRNA沉默NADPH氧化酶1(Nox1)基因表达对胃癌细胞的生长抑制作用。方法: 合成Nox1 siRNA, 采用实时定量PCR 和Western blotting观察Nox1 siRNA 转染后胃癌SGC-7901细胞Nox1 mRNA及相应蛋白表达的变化; 用CCK-8细胞增殖实验、流式细胞检测技术分别检测胃癌SGC-7901细胞增殖及凋亡的变化。结果: 转染Nox1 siRNA 后的胃癌SGC-7901细胞Nox1 mRNA及蛋白表达明显受抑制。与对照组相比, 干扰组SGC-7901细胞生长明显变缓(P<0.05), 细胞凋亡率明显增高(P<0.01)。结论: Nox1 siRNA 可明显下调靶基因Nox1的表达, 在体外可抑制胃癌SGC-7901细胞的生长并促进其凋亡。     

Riboflavin (vitamin B2) deficiency impairs NADPH oxidase 2 (Nox2) priming and defense against Listeria monocytogenes

Riboflavin, also known as vitamin B₂, is converted by riboflavin kinase (RFK) into flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which are essential cofactors of dehydrogenases, reductases, and oxidases including the phagocytic NADPH oxidase 2 (Nox2). Riboflavin deficiency is common in young adults and elderly individuals, who are at the coincidental risk for listeriosis. To address the impact of acute riboflavin deficiency on host defense against Listeria monocytogenes (L.m.), we generated conditional RFK knockout (KO) strains of mice. Phagocyte‐specific RFK KO impaired the capability of phagocytes to control intracellular L.m., which corresponded to a greater susceptibility of mice to in vivo challenge with L.m. The oxidative burst of RFK‐deficient phagocytes in response to L.m. infection was significantly reduced. Mechanistically, TNF‐induced priming of Nox2, which is needed for oxidative burst, was defective in RFK‐deficient phagocytes. Lack of riboflavin in wild‐type macrophages for only 6 h shut down TNF‐induced, RFK‐mediated de novo FMN/FAD generation, which was accompanied by diminished ROS production and impaired anti‐listerial activity. Vice versa, ROS production by riboflavin‐deprived macrophages was rapidly restored by riboflavin supplementation. Our results suggest that acute riboflavin deficiency immediately impairs priming of Nox2, which is of crucial relevance for an effective phagocytic immune response in vivo.     

Identification and characterization of small molecules that inhibit intracellular toxin transport

Shiga toxin (Stx), cholera toxin (Ctx), and the plant toxin ricin are among several toxins that reach their intracellular destinations via a complex route. Following endocytosis, these toxins travel in a retrograde direction through the endosomal system to the trans-Golgi network, Golgi apparatus, and endoplasmic reticulum (ER). There the toxins are transported across the ER membrane to the cytosol, where they carry out their toxic effects. Transport via the ER from the cell surface to the cytosol is apparently unique to pathogenic toxins, raising the possibility that various stages in the transport pathway can be therapeutically targeted. We have applied a luciferase-based high-throughput screen to a chemical library of small-molecule compounds in order to identify inhibitors of Stx. We report two novel compounds that protect against Stx and ricin inhibition of protein synthesis, and we demonstrate that these compounds reversibly inhibit bacterial transport at various stages in the endocytic pathway. One compound (compound 75) inhibited transport at an early stage of Stx and Ctx transport and also provided protection against diphtheria toxin, which enters the cytosol from early endosomes. In contrast, compound 134 inhibited transport from recycling endosomes through the Golgi apparatus and protected only against toxins that access the ER. Small-molecule compounds such as these will provide insight into the mechanism of toxin transport and lead to the identification of compounds with therapeutic potential against toxins routed through the ER.     

A mutation in the ectodomain of herpes simplex virus 1 glycoprotein B causes defective processing and retention in the endoplasmic reticulum.

Herpes simplex virus 1 (HSV-1) glycoprotein B (gB) is one of several envelope glycoproteins required for virion infectivity and is the only one known to oligomerize into homodimers. To study the conformational constraints for translocation of HSV-1 gB to the surface of eukaryotic cells, we analyzed the transport through the exocytic pathway of the wild-type glycoprotein and of mutant forms with insertions in the ectodomain and intracellular carboxy terminus. Transient expression of the glycoproteins in COS-1 cells showed that an insertion at position 479 in the amino-terminal ectodomain of gB, shown previously by reactions with monoclonal antibodies to have altered the conformation of the molecule, also had a drastic effect on transport, precluding exit of the mutant from the endoplasmic reticulum (ER) and transport to the Golgi and the plasma membrane. The fact that the mutant, gB-(Lk479), formed dimers suggests that local changes in assembled regions caused the transport defect. Mutants containing insertions at residues 600 of the ectodomain and 810 in the intracellular domain were slightly retarded in their rate of transport from the ER to the Golgi. The glucose-regulated proteins GRP78 and GRP94, which are resident proteins of the ER, associated with partially glycosylated, faster-migrating forms of gB but not with the fully processed, more slowly migrating product. GRP78 and GRP94 formed complexes with the mutant gB-(Lk479), which was degraded in the ER. Our results indicate that GRP78, and perhaps also GRP94, acts as a chaperone in the assembly of native gB oligomers and also binds to aberrant forms of the molecule, arresting their transport from the ER and possibly serving as markers for protein degradation in this compartment of the exocytic pathway.     

Functional link between Rab GTPase‐mediated membrane trafficking and PI4,5P2 signaling

Fission yeast its3⁺ encodes an essential phosphatidylinositol‐4‐phosphate 5‐kinase (PI4P5K) that regulates cell integrity and cytokinesis. We performed a genetic screen to identify genes that function in PI4P5K‐mediated signaling, and identified gyp10⁺ encoding a Rab GTPase‐activating protein (GAP), a negative regulator for Rab GTPase signaling. Its3 overproduction caused growth defects and abnormal cytoplasmic accumulation of the Its3 protein, which can be stained by calcofluor. Notably, Its3 overproducing cells displayed abnormal membranous structures, multilamella Golgi and fragmented vacuoles showed by Electron microscopy. Furthermore, the excess cytoplasmic Its3 structure partly colocalized with the fluorescence of FM4‐64. Gyp10 rescued both growth defects and abnormal Its3 localization when it was over‐expressed. Gyp10 functionally interacted with the Rab GTPases Ypt3 and Ryh1, both of which regulate Golgi membrane trafficking. Consistently, mutation or deletion of Ypt3 and Ryh1 suppressed phenotypes associated with Its3 overproduction. Importantly, the plasma membrane localization of Its3 was also affected by the impairment of the Ypt3/Ryh1 Rab membrane trafficking, thus suggesting that membrane trafficking events regulated by two Rab GTPases functionally interacts with PI4,5P₂ signaling. These results suggest a mechanism whereby PI4P5K signaling/localization is affected by Golgi membrane trafficking, thus provide a functional link between the PI4,5P₂ signaling and Rab‐mediated trafficking.     

C-type lectin SIGNR1 enhances cellular oxidative burst response against C. albicans in cooperation with Dectin-1

We investigated the role of SIGNR1 in the recognition of Candida albicans and the subsequent cellular oxidative burst response. Soluble SIGNR1 (sSIGNR1) tetramer bound equally to zymosan and both heat‐killed (HK) and live C. albicans in an EDTA‐sensitive manner, whereas sDectin‐1 tetramer predominantly bound to zymosan and HK‐microbes in an EDTA‐independent manner. In cellular response, enhanced oxidative burst was observed in RAW264.7 cells expressing SIGNR1 (RAW‐SIGNR1) compared with RAW‐control cells upon stimulation with HK‐C. albicans and zymosan. This response was independent of TLR2 and the cytosolic portion of SIGNR1 but dependent on the recognition by SIGNR1 via carbohydrate recognition domain. Antagonistic laminarin and anti‐Dectin‐1 mAb cooperatively reduced the response with mannan and anti‐SIGNR1 mAb, respectively, although they had no effect by themselves. Moreover, oxidative response and bactericidal activity largely relied on Syk‐mediated signaling. RAW‐SIGNR1 cells not only captured microbes more efficiently but also showed higher responses than RAW‐control cells. Similar enhanced responses were observed in SIGNR‐1‐expressing resident peritoneal M?. Interestingly, Dectin‐1 was recruited to the phagosomal membrane upon the stimulation and physically associated with SIGNR1. These results suggest that SIGNR1 plays a significant role in inducing oxidative response to C. albicans by Syk‐dependent signaling, possibly through Dectin‐1.     

Congenital dyserythropoietic anemia type II (CDAII) is caused by mutations in the SEC23B gene

Congenital dyserythropoietic anemia type II (CDAII) is an autosomal recessive disease characterized by ineffective erythropoiesis, hemolysis, erythroblast morphological abnormalities, and hypoglycosylation of some red blood cell (RBC) membrane proteins. Recent studies indicated that CDAII is caused by a defect disturbing Golgi processing in erythroblasts. A linkage analysis located a candidate region on chromosome 20, termed the CDAN2 locus, in the majority of CDAII patients but the aberrant gene has not so far been elucidated. We used a proteomic‐genomic approach to identify SEC23B as the candidate gene for CDAII by matching the recently published data on the cytoplasmic proteome of human RBCs with the chromosomic localization of CDAN2 locus. Sequencing analysis of SEC23B gene in 13 CDAII patients from 10 families revealed 12 different mutations: six missense (c.40C>T, c.325G>A, c.1043A>C, c.1489C>T, c.1808C>T, and c.2101C>T), two frameshift (c.428_428delAinsCG and c.1821delT), one splicing (c.689+1G>A), and three nonsense (c.568C>T, c.649C>T, and c.1660C>T). Mutations c.40C>T and c.325G>A were detected in unrelated patients. SEC23B is a member of the Sec23/Sec24 family, a component of the COPII coat protein complex involved in protein transport through membrane vesicles. Abnormalities in this gene are likely to disturb endoplasmic reticulum (ER)‐to‐Golgi trafficking, affecting different glycosylation pathways and ultimately accounting for the cellular phenotype observed in CDAII. Hum Mutat 30:1–7, 2009. © 2009 Wiley‐Liss, Inc.     

Epidermal growth factor and insulin short‐term increase hPepT1‐mediated glycylsarcosine uptake in Caco‐2 cells

Aims: Little is known about the physiological regulation of the human intestinal di/tri‐peptide transporter, hPepT1. In the present study we evaluated the effects of epidermal growth factor (EGF) and insulin on hPepT1‐mediated dipeptide uptake in the intestinal cell line Caco‐2. Methods: Caco‐2 cells were grown on filters for 23–27 days. Apical dipeptide uptake was measured using [¹⁴C]glycylsarcosine([¹⁴C]Gly‐Sar). HPepT1 mRNA levels were investigated using RT‐PCR, cytosolic pH was determined using the pH‐sensitive fluorescent probe BCECF. Results: Basolateral application of EGF increased [¹⁴C]Gly‐Sar uptake with an ED₅₀ value of 0.77 ± 0.25 ng mL^?1 (n = 3?6) and a maximal stimulation of 33 ± 2% (n = 3?6). Insulin stimulated [¹⁴C]Gly‐Sar uptake with an ED₅₀ value of 3.5 ± 2.0 ng mL^?1 (n = 3?6) and a maximal stimulation of approximately 18% (n = 3?6). Gly‐Sar uptake followed simple Michaelis‐Menten kinetics. K_m in control cells was 0.98 ± 0.11 mM (n = 8) and V_max was 1.86 ± 0.07 nmol cm^?2 min^?1 (n = 8). In monolayers treated with 200 ng mL^?1 of EGF, K_m was 1.11 ± 0.05 mM (n = 5) and V_max was 2.79 ± 0.05 nmol cm^?2 min^?1 (n = 5). In monolayers treated with 50 ng mL^?1 insulin, K_m was 1.03 ± 0.08 mM and V_max was 2.19 ± 0.06 nmol cm^?2 min^?1 (n = 5). Kinetic data thus indicates an increase in the number of active transporters, following stimulation. The incrased Gly‐Sar uptake was not accompanied by changes in hPepT1 mRNA, nor by measurable changes in cytosolic pH. Conclusions: Short‐term stimulation with EGF and insulin caused an increase in hPepT1‐mediated uptake of Gly‐Sar in Caco‐2 cell monolayers, which could not be accounted for by changes in hPepT1 mRNA or proton‐motive driving force.