PI3-kinase-dependent electrogenic intestinal transport of glucose and amino acids

Biology of Peptide Transporter 2 in Mammals: New Insights into Its Function, Structure and Regulation

Peptide transporter 2 (PepT2) in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also of significant medical and pharmacological significance in the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. In this review, we provide a comprehensive overview of the structure, substrate preferences and localization of PepT2 in mammals. As PepT2 is expressed in various organs, its function in the liver, kidney, brain, heart, lung and mammary gland has also been addressed. Finally, the regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed.

Ca2+-Permeable Channels/Ca2+ Signaling in the Regulation of Ileal Na+/Gln Co-Transport in Mice

Oral glutamine (Gln) has been widely used in gastrointestinal (GI) clinical practice, but it is unclear if Ca²⁺ regulates intestinal Gln transport, although both of them are essential nutrients for mammals. Chambers were used to determine Gln (25 mM)-induced I_sc through Na⁺/Gln co-transporters in the small intestine in the absence or the presence of selective activators or blockers of ion channels and transporters. Luminal but not serosal application of Gln induced marked intestinal I_sc, especially in the distal ileum. Lowering luminal Na⁺ almost abolished the Gln-induced ileal I_sc, in which the calcium-sensitive receptor (CaSR) activation were not involved. Ca²⁺ removal from both luminal and serosal sides of the ileum significantly reduced Gln- I_sc. Blocking either luminal Ca²⁺ entry via the voltage-gated calcium channels (VGCC) or endoplasmic reticulum (ER) release via inositol 1,4,5-triphosphate receptor (IP₃R) and ryanodine receptor (RyR) attenuated the Gln-induced ileal I_sc, Likewise, blocking serosal Ca²⁺ entry via the store-operated Ca²⁺ entry (SOCE), TRPV1/2 channels, and Na⁺/Ca²⁺ exchangers (NCX) attenuated the Gln-induced ileal I_sc. In contrast, activating TRPV1/2 channels enhanced the Gln-induced ileal I_sc. We concluded that Ca²⁺ signaling is critical for intestinal Gln transport, and multiple plasma membrane Ca²⁺-permeable channels and transporters play roles in this process. The Ca²⁺ regulation of ileal Na⁺/Gln transport expands our understanding of intestinal nutrient uptake and may be significant in GI health and disease.

Segment-specific effects of resveratrol on porcine small intestinal dipeptide absorption depend on the mucosal pH and are due to different mechanisms: potential roles of different transport proteins and protein kinases

Numerous beneficial features of the polyphenol resveratrol (RSV) have been demonstrated in several tissues and cell culture models. There is also evidence, that RSV impairs intestinal nutrient transport but the underlying mechanisms are not understood. The aim of the present study was to evaluate whether RSV has also an impact on the H⁺-coupled transport of peptides via the peptide transporter 1 (PepT1) and to characterize RSV mediated changes in the apical abundance of nutrients transport proteins and protein kinases that may be involved. RSV decreased the H⁺-coupled transport of peptides in the porcine small intestines in a pH and location specific manner (jejunum vs ileum) as measured in Ussing chamber experiments. The comparison of the effects of RSV with the effects of the cAMP/PKA-activating agent forskolin indicates that different mechanisms may be responsible in the intestinal segments. Additionally, it seems that the transport of peptides and glucose in the jejunum are inhibited via the same mechanism while there might be two mechanisms involved in the ileum. Functional data and protein expression data indicate, that, besides PepT1, the activity of the Na⁺/H⁺-exchanger 3 (NHE3) may be involved. Protein kinase A (PKA) and AMP-activated kinase (AMPK) are both activated by RSV while the extracellular regulated kinase (ERK) and the serum and glucocorticoid induced kinase (SGK) are widely unaffected. Although PKA and AMPK are activated, AMPK seems not to be related to the effects of RSV. Additionally, both the functional data and the protein expression data reveal some interesting pH- and segment-specific differences.

Functional Characterization of Peptide Transporters in Bovine Mammary Epithelial Cells

The objective of this study was to characterize the expression profile, transport kinetics, and regulation of peptide transporters in bovine mammary epithelial cells (BMECs). Quantitative reverse-transcription real-time PCR, Western blotting, and immunofluorescence staining were used to investigate the expression of peptide transporters in bovine mammary tissues. The effects of time, pH, concentration, and specific inhibitors on β-alanyl-l-lysyl- Nε-7-amino-4-methyl-coumarin-3-acetic acid (β-Ala-Lys-AMCA) uptake in BMECs were also studied. The results showed that the peptide transporters PepT2 and PhT1 are both expressed in bovine mammary glands. The optimal pH for the uptake of β-Ala-Lys-AMCA in BMECs was 6.5. The transport-kinetics study suggested that the uptake of β-Ala-Lys-AMCA in BMECs is saturable over the tested concentration, with a K_m value of 82 ± 18 μM and a V_max of 124 ± 11 pmol/min per milligram of protein. Other dipeptides, including Gly-Sar, Met-Gly, and Met-Met, competitively inhibited β-Ala-Lys-AMCA uptake in BMECs. However, histidine had no effect on β-Ala-Lys-AMCA uptake. Furthermore, knocking down PepT2 could significantly reduce β-Ala-Lys-AMCA uptake, but PhT1 interference had no effect on peptide uptake in BMECs. The inhibition of PI3K and Akt decreased the uptake of β-Ala-Lys-AMCA. The above results revealed functional characteristics of peptide transporters and demonstrated that PepT2 may play a major role in β-Ala-Lys-AMCA uptake in BMECs. Moreover, the PI3K-Akt signaling pathway may regulate the uptake of β-Ala-Lys-AMCA in BMECs.

RNA sequencing reveals widespread transcriptome changes in a renal carcinoma cell line

We used RNA sequencing (RNA-Seq) technology to investigate changes in the transcriptome profile in the Caki-1 clear cell renal cell carcinoma (ccRCC) cells, which overexpress monocyte chemoattractant protein-induced protein 1 (MCPIP1). RNA-Seq data showed changes in 11.6% and 41.8% of the global transcriptome of Caki-1 cells overexpressing wild-type MCPIP1 or its D141N mutant, respectively. Gene ontology and KEGG pathway functional analyses showed that these transcripts encoded proteins involved in cell cycle progression, protein folding in the endoplasmic reticulum, hypoxia response and cell signalling. We identified 219 downregulated transcripts in MCPIP1-expressing cells that were either unchanged or upregulated in D141N-expressing cells. We validated downregulation of 15 transcripts belonging to different functional pathways by qRT-PCR. The growth and viability of MCPIP1-expressing cells was reduced because of elevated p21Cip1 levels. MCPIP1-expressing cells also showed reduced levels of DDB1 transcript that encodes component of the E3 ubiquitin ligase that degrades p21Cip1. These results demonstrate that MCPIP1 influences the growth and viability of ccRCC cells by increasing or decreasing the transcript levels for proteins involved in cell cycle progression, protein folding, hypoxia response, and cell signaling.

Cardiotrophin-1 decreases intestinal sugar uptake in mice and in Caco-2 cells

AIM

Cardiotrophin-1 (CT-1) is a member of the IL-6 family of cytokines with a key role in glucose and lipid metabolism. In the current investigation, we examined the in vivo and in vitro effects of CT-1 treatment on intestinal sugar absorption in different experimental models.

METHODS

rCT-1 effects on α-Methyl-D-glucoside uptake were assessed in everted intestinal rings from wild-type and CT-1(-/-) mice and in Caco-2 cells. rCT-1 actions on SGLT-1 expression in brush border membrane vesicles and the identification of the potential signalling pathways involved were determined by Western blot.

RESULTS

In vivo administration (0.2 mg kg(-1) ) of rCT-1 caused a significant decrease on α-Methyl-D-glucoside uptake in everted intestinal rings from wild-type and CT-1(-/-) mice after short-term and long-term treatments. Similarly, in vitro treatment (1-50 ng mL(-1) ) with rCT-1 reduced α-Methyl-D-glucoside uptake in everted intestinal rings. In Caco-2 cells, rCT-1 treatment (20 ng mL(-1) , 1 and 24 h) lowered apical uptake of α-Methyl-D-glucoside in parallel with a decrease on SGLT-1 protein expression. rCT-1 promoted the phosphorylation of STAT-3 after 5 and 15 min treatment, but inhibited the activation by phosphorylation of AMPK after 30 and 60 min. Interestingly, pre-treatment with the JAK/STAT inhibitor (AG490) and with the AMPK activator (AICAR) reversed the inhibitory effects of rCT-1 on α-Methyl-D-glucoside uptake. AICAR also prevented the inhibition of SGLT-1 observed in rCT-1-treated cells.

CONCLUSIONS

CT-1 inhibits intestinal sugar absorption by the reduction of SGLT-1 levels through the AMPK pathway, which could also contribute to explain the hypoglycaemic and anti-obesity properties of CT-1.

Sodium-glucose cotransport

PURPOSE OF REVIEW

Sodium-glucose cotransporters (SGLTs) are important mediators of glucose uptake across apical cell membranes. SGLT1 mediates almost all sodium-dependent glucose uptake in the small intestine, while in the kidney SGLT2, and to a lesser extent SGLT1, account for more than 90% and nearly 3%, respectively, of glucose reabsorption from the glomerular ultrafiltrate. Although the recent availability of SGLT2 inhibitors for the treatment of diabetes mellitus has increased the number of clinical studies, this review has a focus on mechanisms contributing to the cellular regulation of SGLTs.

RECENT FINDINGS

Studies have focused on the regulation of SGLT expression under different physiological/pathophysiological conditions, for example diet, age or diabetes mellitus. Several studies provide evidence of SGLT regulation via cyclic adenosine monophosphate/protein kinase A, protein kinase C, glucagon-like peptide 2, insulin, leptin, signal transducer and activator of transcription-3 (STAT3), phosphoinositide-3 kinase (PI3K)/Akt, mitogen-activated protein kinases (MAPKs), nuclear factor-kappaB (NF-kappaB), with-no-K[Lys] kinases/STE20/SPS1-related proline/alanine-rich kinase (Wnk/SPAK) and regulatory solute carrier protein 1 (RS1) pathways.

SUMMARY

SGLT inhibitors are important drugs for glycemic control in diabetes mellitus. Although the contribution of SGLT1 for absorption of glucose from the intestine as well as SGLT2/SGLT1 for renal glucose reabsorption has been comprehensively defined, this review provides an up-to-date outline for the mechanistic regulation of SGLT1/SGLT2.

SPAK-sensitive regulation of glucose transporter SGLT1

The WNK-dependent STE20/SPS1-related proline/alanine-rich kinase SPAK is a powerful regulator of ion transport. The study explored whether SPAK similarly regulates nutrient transporters, such as the Na(+)-coupled glucose transporter SGLT1 (SLC5A1). To this end, SGLT1 was expressed in Xenopus oocytes with or without additional expression of wild-type SPAK, constitutively active (T233E)SPAK, WNK-insensitive (T233A)SPAK or catalytically inactive (D212A)SPAK, and electrogenic glucose transport determined by dual-electrode voltage-clamp experiments. Moreover, Ussing chamber was employed to determine the electrogenic glucose transport in intestine from wild-type mice (spak(wt/wt)) and from gene-targeted mice carrying WNK-insensitive SPAK (spak(tg/tg)). In SGLT1-expressing oocytes, but not in water-injected oocytes, the glucose-dependent current (I(g)) was significantly decreased following coexpression of wild-type SPAK and (T233E)SPAK, but not by coexpression of (T233A)SPAK or (D212A)SPAK. Kinetic analysis revealed that SPAK decreased maximal I(g) without significantly modifying the glucose concentration required for halfmaximal I(g) (K(m)). According to the chemiluminescence experiments, wild-type SPAK but not (D212A)SPAK decreased SGLT1 protein abundance in the cell membrane. Inhibition of SGLT1 insertion by brefeldin A (5 μM) resulted in a decline of I(g), which was similar in the absence and presence of SPAK, suggesting that SPAK did not accelerate the retrieval of SGLT1 protein from the cell membrane but rather down-regulated carrier insertion into the cell membrane. Intestinal electrogenic glucose transport was significantly lower in spak(wt/wt) than in spak(tg/tg) mice. In conclusion, SPAK is a powerful negative regulator of SGLT1 protein abundance in the cell membrane and thus of electrogenic glucose transport.

Serum- and glucocorticoid-regulated kinase 2 determines drug-activated pregnane X receptor to induce gluconeogenesis in human liver cells

Drug activation of the human nuclear pregnane X receptor (PXR) induced gluconeogenic genes and increased glucose production. In this study, we have determined that serum- and glucocorticoid-regulated kinase 2 (SGK2) is an essential factor that mediates this PXR-regulated glucose 6-phosphatase (G6Pase) induction and glucose production. Both SGK2 and G6Pase mRNAs were increased in rifampicin-treated HepG2 cells stably expressing human PXR. Reporter and chromatin immunoprecipitation assays delineated PXR activation of the SGK2 gene to a distal and proximal DNA sequence within its promoter: distal PXR response element (-2587/-2209) and proximal PXR response element (-115/-75), respectively. Small interfering RNA (siRNA) knockdown of SGK2 severely attenuated PXR-regulated induction of G6Pase as well as glucose production. SGK2 constitutes an insulin-independent signal pathway to regulate gluconeogenesis because siRNA knockdown of the insulin-responsive transcription factor forkhead box protein O1 did not affect rifampicin induction of G6Pase. Rifampicin treatment of two different samples of human primary hepatocytes revealed that PXR induces G6Pase in the presence of high levels of SGK2, whereas PXR represses G6Pase in its absence. Mediating PXR activation of the G6Pase gene is the first biological role found for hepatic SGK2 and might have therapeutic implications for side effects, such as diabetes, caused by drugs that activate PXR.

Loss of enteral nutrition in a mouse model results in intestinal epithelial barrier dysfunction

Total parenteral nutrition (TPN) administration in a mouse model leads to a local mucosal inflammatory response, resulting in a loss of epithelial barrier function (EBF). Although, the underlying mechanisms are unknown, a major contributing factor is a loss of growth factors and subsequent critical downstream signaling. An important component of these is the p-Akt pathway. An additional contributing factor to the loss of EBF with TPN is an increase in proinflammatory cytokine abundance within the mucosal epithelium, including TNF-α and IFN-γ. Loss of critical nutrients, including glutamine and glutamate, may affect EBF, contributing to the loss of tight junction proteins. Finding protective modalities for the small intestine during TPN administration may have important clinical applications. Supplemental glutamine and glutamate may be examples of such agents.

Enhancement of calcium transport in Caco-2 monolayer through PKCzeta-dependent Cav1.3-mediated transcellular and rectifying paracellular pathways by prolactin

Previous investigations suggested that prolactin (PRL) stimulated the intestinal calcium absorption through phosphoinositide 3-kinase (PI3K), protein kinase C (PKC), and RhoA-associated coiled-coil forming kinase (ROCK) signaling pathways. However, little was known regarding its detailed mechanisms for the stimulation of transcellular and voltage-dependent paracellular calcium transport. By using Ussing chamber technique, we found that the PRL-induced increase in the transcellular calcium flux and decrease in transepithelial resistance of intestinal-like Caco-2 monolayer were not abolished by inhibitors of gene transcription and protein biosynthesis. The PRL-stimulated transcellular calcium transport was completely inhibited by the L-type calcium channel blockers (nifedipine and verapamil) and plasma membrane Ca(2+)-ATPase (PMCA) inhibitor (trifluoperazine) as well as small interfering RNA targeting voltage-dependent L-type calcium channel Ca(v)1.3, but not TRPV6 or calbindin-D(9k). As demonstrated by (45)Ca uptake study, PI3K and PKC, but not ROCK, were essential for the PRL-enhanced apical calcium entry. In addition, PRL was unable to enhance the transcellular calcium transport after PKC(zeta) knockdown or exposure to inhibitors of PKC(zeta), but not of PKC(alpha), PKC(beta), PKC(epsilon), PKC(mu), or protein kinase A. Voltage-clamping experiments further showed that PRL markedly stimulated the voltage-dependent calcium transport and removed the paracellular rectification. Such PRL effects on paracellular transport were completely abolished by inhibitors of PI3K (LY-294002) and ROCK (Y-27632). It could be concluded that the PRL-stimulated transcellular calcium transport in Caco-2 monolayer was mediated by Ca(v)1.3 and PMCA, presumably through PI3K and PKC(zeta) pathways, while the enhanced voltage-dependent calcium transport occurred through PI3K and ROCK pathways.