Inhibition of glucose absorption by phlorizin affects intestinal functions in rats

Identification of key nodes of type 2 diabetes mellitus protein interactome and study of their interactions with phloridzin

Network biology-inspired approaches could be used effectively in probing regulatory processes by which small molecules intervene with disease mechanisms. The present study aims at identification of key targets of type 2 diabetes mellitus (T2DM) by network analysis of the underlying protein interactome, and probing for mechanisms by which phloridzin could be critical at altering the disease phenotype. Towards this goal, we constructed a protein-protein interaction network associated with T2DM, starting from candidate genes and systems-level interactions data available. The relevance of the network constructed was verified with the help of gene ontology, node deletion, and biological essentiality studies. Using a network analysis method, MAPK1, EP300, and SMAD2 were identified as the most central proteins of potential therapeutic value. Phloridzin, a known antidiabetic agent, potentially interacts with proteins central to T2DM mechanisms. The structural understanding of interaction of phloridzin with these proteins of relevance to T2DM could provide better insight into its regulatory mechanisms and help in developing better therapeutic agents. The molecular docking results suggest that phloridzin is potentially involved in making critical interactions with MAPK1. These results could further be validated by experimental studies and could be used to design therapeutic agents for T2DM intervention.

Analysis of different complexes of type IIa sodium-dependent phosphate transporter in rat renal cortex using blue-native polyacrylamide gel electrophoresis

Type IIa sodium-dependent phosphate transporter (NaPi-IIa) can be localized in the apical plasma membrane of renal proximal tubule to carry out a rate-limiting step of phosphate reabsorption. For the apical localization, NaPi-IIa is required to form a macromolecular complex with some adaptor proteins such as Na(+)/H(+) exchanger regulatory factor 1 (NHERF-1) and ezrin. However, the detail of macromolecular complex containing NaPi-IIa in the apical membrane of the renal proximal tubular cells has not been clarified. In this study, we identified at least four different complexes (220, 480, 920, 1,100 kDa) containing NaPi-IIa by using blue-native polyacrylamide gel electrophoresis. Interestingly, LC-MS/MS analysis and immunoprecipitation analysis reveal that megalin is a component of larger complexes (920 and 1,100 kDa). In addition, NaPi-IIa can be heterogeneously co-localized with ezrin and megalin on the apical membrane of renal proximal tubuler cells by fluorescence microscopy analysis. These results suggest that NaPi-IIa can form some different complexes on the apical plasma membrane of renal proximal tubular cells.

Thyroid hormones regulate phosphate homoeostasis through transcriptional control of the renal type IIa sodium-dependent phosphate co-transporter (Npt2a) gene

The type IIa renal sodium-dependent phosphate (Na/Pi) co-transporter Npt2a is implicated in the control of serum phosphate levels. It has been demonstrated previously that renal Npt2a protein and its mRNA expression are both up-regulated by the thyroid hormone T3 (3,3',5-tri-iodothyronine) in rats. However, it has never been established whether the induction was mediated by a direct effect of thyroid hormones on the Npt2a promoter. To address the role of Npt2a in T3-dependent regulation of phosphate homoeostasis and to identify the molecular mechanisms by which thyroid hormones modulate Npt2a gene expression, mice were rendered pharmacologically hypo- and hyper-thyroid. Hypothyroid mice showed low levels of serum phosphate and a marked decrease in renal Npt2a protein abundance. Importantly, we also showed that Npt2a-deficient mice had impaired serum phosphate responsiveness to T3 compared with wild-type mice. Promoter analysis with a luciferase assay revealed that the transcriptional activity of a reporter gene containing the Npt2a promoter and intron 1 was dependent upon TRs (thyroid hormone receptors) and specifically increased by T3 in renal cells. Deletion analysis and EMSAs (electrophoretic mobility-shift assays) determined that there were unique TREs (thyroid-hormone-responsive elements) within intron 1 of the Npt2a gene. These results suggest that Npt2a plays a critical role as a T3-target gene, to control phosphate homoeostasis, and that T3 transcriptionally activates the Npt2a gene via TRs in a renal cell-specific manner.

Carbohydrate/fat ratio in the diet alters histone acetylation on the sucrase-isomaltase gene and its expression in mouse small intestine

A diet with a high carbohydrate/fat ratio enhances jejunal SI gene expression. Using ChIP assay, we revealed that the acetylation of histone H3 on transcriptional region and H4 on promoter region, respectively, of mouse SI gene are high. The acetylation of histone H3 and H4 as well as binding of HNF-1 and Cdx-2 on SI gene, was enhanced by increase in carbohydrate/fat ratio in the diet. These suggest that induction of SI gene by the diet rich in carbohydrate is associated with acetylation of histone H3 and H4 as well as binding of HNF-1 and Cdx-2 on SI gene.

Interaction of a farnesylated protein with renal type IIa Na/Pi co-transporter in response to parathyroid hormone and dietary phosphate

Treatment with PTH (parathyroid hormone) or a high-P(i) diet causes internalization of the type IIa sodium-dependent phosphate (Na/P(i) IIa) co-transporter from the apical membrane and its degradation in the lysosome. A dibasic amino acid motif (KR) in the third intracellular loop of the co-transporter is essential for protein's PTH-induced retrieval. To elucidate the mechanism of internalization of Na/P(i) IIa, we identified the interacting protein for the endocytic motif by yeast two-hybrid screening. We found a strong interaction of the Na/P(i) IIa co-transporter with a small protein known as the PEX19 (human peroxisomal farnesylated protein; PxF, Pex19p). PEX19 can bind to the KR motif, but not to a mutant with this motif replaced with NI residues. PEX19 is highly expressed in mouse and rat kidney. Western blot analysis indicates that PEX19 is located in the cytosolic and brush-border membrane fractions (microvilli and the subapical component). Overexpression of PEX19 stimulated the endocytosis of the Na/P(i) IIa co-transporter in opossum kidney cells in the absence of PTH. In conclusion, the present study indicates that PEX19 may be actively involved in controlling the internalization and trafficking of the Na/P(i) IIa co-transporter.

Identification of three isoforms for the Na+-dependent phosphate cotransporter (NaPi-2) in rat kidney

We have isolated three unique NaPi-2-related protein cDNAs (NaPi-2alpha, NaPi-2beta, and NaPi-2gamma) from a rat kidney library. NaPi-2alpha cDNA encodes 337 amino acids which have high homology to the N-terminal half of NaPi-2 containing 3 transmembrane domains. NaPi-2beta encodes 327 amino acids which are identical to the N-terminal region of NaPi-2 containing 4 transmembrane domains, whereas the 146 amino acids in the C-terminal region are completely different. In contrast, NaPi-2gamma encodes 268 amino acids which are identical to the C-terminal half of NaPi-2. An analysis of phage and cosmid clones indicated that the three related proteins were produced by alternative splicing in the NaPi-2 gene. In a rabbit reticulocyte lysate system, NaPi-2 alpha, beta, and gamma were found to be 36, 36, and 29 kDa amino acid polypeptides, respectively. NaPi-2alpha and NaPi-2gamma were glycosylated and revealed to be 45- and 35-kDa proteins, respectively. In isolated brush-border membrane vesicles, an N-terminal antibody was reacted with 45- and 40-kDa, and a C-terminal antibody was reacted with 37-kDa protein. The sizes of these proteins corresponded to those in glycosylated forms. A functional analysis demonstrated that NaPi-2gamma and -2alpha markedly inhibited NaPi-2 activity in Xenopus oocytes. The results suggest that these short isoforms may function as a dominant negative inhibitor of the full-length transporter.

Effects of dietary Pi on the renal Na+-dependent Pi transporter NaPi-2 in thyroparathyroidectomized rats

Dietary Pi and parathyroid hormone (PTH) are two most important physiological and pathophysiological regulators of Pi re-absorption in the renal proximal tubule. Effects of dietary Pi on Na+/Pi co-transporter NaPi-2 were investigated in thyroparathyroidectomized (TPTX) rats. NaPi-2 protein and mRNA in the kidney cortex of TPTX rats were increased approximately 3.8- and 2.4-fold in amount respectively compared with those in the sham-operated animals. Administration of PTH to the TPTX rats resulted in a decrease in the amount of NaPi-2 protein, but not in the abundance of NaPi-2 mRNA. Deprivation of dietary Pi in the TPTX rats did not affect the amount of NaPi-2 mRNA and protein. In the Pi-deprived TPTX rats, feeding of a high-Pi diet resulted in marked decreases in Pi transport activity and the amount of NaPi-2 protein in the superficial nephrons. Immunohistochemical analysis demonstrated that administration of PTH to TPTX rats resulted in a decrease in NaPi-2 immunoreactivity from both superficial and juxtamedullary nephrons within 4 h. Switching TPTX animals from a low-Pi diet to the high-Pi diet decreased NaPi-2 immunoreactivity from superficial nephrons, but not from juxtamedullary nephrons, within 4 h. These results suggest that dietary Pi could regulate the amount of NaPi-2 protein in the superficial nephrons in a PTH-independent manner.

Regulation of type II renal Na+-dependent inorganic phosphate transporters by 1,25-dihydroxyvitamin D3. Identification of a vitamin D-responsive element in the human NAPi-3 gene

Vitamin D is an important regulator of phosphate homeostasis. The effects of vitamin D on the expression of renal Na+-dependent inorganic phosphate (Pi) transporters (types I and II) were investigated. In vitamin D-deficient rats, the amounts of type II Na+-dependent Pi transporter (NaPi-2) protein and mRNA were decreased in the juxtamedullary kidney cortex, but not in the superficial cortex, compared with control rats. The administration of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) to vitamin D-deficient rats increased the initial rate of Pi uptake as well as the amounts of NaPi-2 mRNA and protein in the juxtamedullary cortex. The transcriptional activity of a luciferase reporter plasmid containing the promoter region of the human type II Na+-dependent Pi transporter NaPi-3 gene was increased markedly by 1,25-(OH)2D3 in COS-7 cells expressing the human vitamin D receptor. A deletion and mutation analysis of the NaPi-3 gene promoter identified the vitamin D-responsive element as the sequence 5'-GGGGCAGCAAGGGCA-3' nucleotides -1977 to -1963 relative to the transcription start site. This element bound a heterodimer of the vitamin D receptor and retinoid X receptor, and it enhanced the basal transcriptional activity of the promoter of the herpes simplex virus thymidine kinase gene in an orientation-independent manner. Thus, one mechanism by which vitamin D regulates Pi homeostasis is through the modulation of the expression of type II Na+-dependent Pi transporter genes in the juxtamedullary kidney cortex.

Regulation of the PepT1 peptide transporter in the rat small intestine in response to 5-fluorouracil-induced injury

BACKGROUND & AIMS

The oligopeptide transport system of the small intestine is resistant to mucosal injury. The mechanism of this resistance was investigated by examining the activity level and expression of the peptide transporter PepT1 in the intestine of rats treated with 5-fluorouracil.

METHODS

The expression and localization of PepT1 were examined by immunoblot analysis of brush border membrane vesicles and immunohistochemical analysis of intestinal sections with PepT1-specific rabbit polyclonal antibodies. Also, Northern blot analysis was used for the expression of PepT1 messenger RNA (mRNA).

RESULTS

Although the amounts of sucrase and an Na+-dependent glucose transporter protein in intestinal vesicles decreased markedly after 5-fluorouracil treatment, the amount of PepT1 protein remained largely unaffected. Immunohistochemical analysis also showed that the PepT1 immunoreactivity level was preserved in the brush border membrane of the remaining villi of 5-fluorouracil-treated rats. Levels of amino acid, glucose, and phosphate transporter mRNAs were profoundly depressed in 5-fluorouracil-treated animals, whereas the level of PepT1 mRNA conversely increased.

CONCLUSIONS

The resistance of intestinal peptide transport to tissue injury may be attributable to increased synthesis of PepT1 rather than to a change in the kinetic properties of the residual absorbing cells.

Cloning, functional expression and dietary regulation of the mouse neutral and basic amino acid transporter (NBAT)

The Na+-independent dibasic and neutral amino acid transporter NBAT is among the least hydrophobic of mammalian amino acid transporters. The transporter contains one to four transmembrane domains and induces amino acid transport activity via a b0,+-like system when expressed in Xenopus oocytes. However, the physiological role of NBAT remains unclear. Complementary DNA clones encoding mouse NBAT have now been isolated. The expression of mouse NBAT in Xenopus oocytes also induced an obligatory amino acid exchange activity similar to that of the b0,+-like system. The amount of NBAT mRNA in mouse kidney increased during postnatal development, consistent with the increase in renal cystine and dibasic transport activity. Dietary aspartate induced a marked increase in cystine transport via the b0,+ system in mouse ileum. A high-aspartate diet also increased the amount of NBAT mRNA in mouse ileum. In the ileum of mice fed on the aspartate diet, the extent of cystine transport was further increased by preloading brush border membrane vesicles with lysine. Hybrid depletion of NBAT mRNA from ileal polyadenylated RNA revealed that the increase in cystine transport activity induced by the high-aspartate diet, as measured in Xenopus oocytes, was attributable to NBAT. These results demonstrate that mouse NBAT has an important role in cystine transport.

Relative contributions of Na+-dependent phosphate co-transporters to phosphate transport in mouse kidney: RNase H-mediated hybrid depletion analysis

Reabsorption of Pi in the proximal tubule of the kidney is an important determinant of Pi homoeostasis. At least three types (types I-III) of high-affinity Na+-dependent Pi co-transporters have been identified in mammalian kidneys. The relative roles of these three types of Na+/Pi co-transporters in Pi transport in mouse kidney cortex have now been investigated by RNase H-mediated hybrid depletion. Whereas isolated brush-border membrane vesicles showed the presence of two kinetically distinct Na+/Pi co-transport systems (high Km-low Vmax and low Km-high Vmax), Xenopus oocytes, microinjected with polyadenylated [poly(A)+] RNA from mouse kidney cortex, showed only the high-affinity Pi uptake system. Kidney poly(A)+ RNA was incubated in vitro with antisense oligonucleotides corresponding to Npt-1 (type I), NaPi -7 (type II) or Glvr-1 (type III) Na+/Pi co-transporter mRNAs, and then with RNase H. Injection of such treated RNA preparations into Xenopus oocytes revealed that an NaPi-7 antisense oligonucleotide that resulted in complete degradation of NaPi-7 mRNA (as revealed by Northern blot analysis), also induced complete inhibition of Pi uptake. Degradation of Npt-1 or Glvr-1 mRNAs induced by corresponding antisense oligonucleotides had no effect on Pi transport, which was subsequently measured in oocytes. These results indicate that the type II Na+/Pi co-transporter NaPi-7 mediated most Na+-dependent Pi transport in mouse kidney cortex.

Characterization of the rabbit intestinal fructose transporter (GLUT5)

Recent studies suggest that the jejunal/kidney-type facilitative glucose transporter (GLUT5) functions as a high-affinity D-fructose transporter. However, its precise role in the small intestine is not clear. In an attempt to identify the fructose transporter in the small intestine, we measured fructose uptake in Xenopus oocytes expressing jejunal mRNA from five species (rat, mouse, rabbit, hamster and guinea-pig). Only jejunal mRNA from the rabbit significantly increased fructose uptake. We also cloned a rabbit GLUT5 cDNA from a jejunal library The predicted amino acid sequence of the 487-residue rabbit GLUT5 showed 72.3 and 67.1% identity with human and rat GLUT5 respectively. Northern-blot analysis revealed GLUT5 transcripts in rabbit duodenum, jejunum and, to a lesser extent, kidney. After separation of rabbit jejunal mRNA on a sucrose density gradient, the fractions that conferred D-fructose transport activity in oocytes also hybridized with rabbit GLUT5 cDNA. Hybrid depletion of jejunal mRNA with a GLUT5 antisense oligonucleotide markedly inhibited the mRNA-induced fructose uptake in oocytes. Immunoblot analysis indicated that GLUT5 (49 kDa) is located in the brush-border membrane of rabbit intestinal epithelial cells. Xenopus oocytes injected with rabbit GLUT5 cRNA exhibited fructose uptake activity with a Km of 11 mM for D-fructose. D-Fructose transport by GLUT5 was significantly inhibited by D-glucose and D-galactose. D-Fructose uptake in brush-border membrane vesicles shows a Km similar to that of GLUT5, but was not inhibited by D-glucose or D-galactose. Finally, cytochalasin B photolabelled a 49 kDa protein in rabbit brush-border-membrane preparations that was immunoprecipitated by antibodies to GLUT5. Our results suggest that GLUT5 functions as a fructose transporter in rabbit small intestine. However, biochemical properties of fructose transport in Xenopus oocytes injected with GLUT5 cRNA differed from those in rabbit jejunal vesicles.