Increased protein level of PEPT1 intestinal H+-peptide cotransporter upregulates absorption of glycylsarcosine and ceftibuten in 5/6 nephrectomized rats

Yuriko Shimizu, Satohiro Masuda, Kumiko Nishihara, Lin Ji, Masahiro Okuda, and Ken-ichi Inui

Department of Pharmacy, Kyoto University Hospital, Faculty of Medicine, Kyoto University, Sakyo-ku, Kyoto, Japan

Submitted 22 June 2004 ; accepted in final form 3 November 2004


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In chronic renal failure (CRF), dietary protein is one of the factors that deteriorates residual renal functions. Numerous studies have indicated that the products of protein digestion are mainly absorbed as small peptides. However, how small peptides are absorbed in CRF remains poorly understood. H+-coupled peptide transporter (PEPT1/SLC15A1) plays an important role in the absorption of small peptides and peptide-like drugs in the small intestine. Because dietary protein intake is one of the risk factors for renal failure, the alteration of intestinal PEPT1 might have implications in the progression of renal disease as well as the pharmacokinetics of peptide-like drugs. In this study, we examined the alteration of intestinal PEPT1 in 5/6 nephrectomized (5/6 NR) rats, extensively used as a model of chronic renal failure. Absorption of [14C]glycylsarcosine and ceftibuten was significantly increased in 5/6 NR rats compared with sham-operated rats, without a change in intestinal protease activity. Western blot analysis indicated that the amount of intestinal PEPT1 protein in 5/6 NR rats was increased mainly at the upper region. On the other hand, the amount of intestinal PEPT1 mRNA was not significantly different from that of sham-operated rats. These findings indicate that the increase in absorption of small peptides and peptide-like drugs, caused by the upregulation of intestinal PEPT1 protein, might contribute to the progression of renal failure as well as the alteration of drug pharmacokinetics.

renal failure; H+-coupled peptide transporter; intestine


SMALL PEPTIDES, including di- and tripeptides, are the main products of protein digestion in the gut lumen (2, 13). The absorption of small peptides is mediated by H+-coupled peptide transporter (PEPT1), which is localized at the brush-border membranes of intestinal epithelial cells. Furthermore, PEPT1 mediates the absorption of a broad range of peptide-like drugs, such as {beta}-lactam antibiotics, the anti-cancer agent bestatin, and angiotensin-converting enzyme (ACE) inhibitors (10, 12, 21). Recently, Gangopadhyay et al. (8) reported that uncontrollable diabetes has a profound effect on the expression of intestinal PEPT1.

In chronic renal failure (CRF), morphological and enzymatic abnormalities have been found in the small intestinal mucosa of patients (7) and model rats at 12 wk after nephrectomy (9). For example, the activities of sucrase and maltase were reduced (9), and the expression of intestinal cytochrome P-450 was downregulated in rats at 6 wk after nephrectomy (11). However, the activity of some dipeptidases was significantly but weakly increased or unchanged in the isolated brush-border membranes from the nephrectomized rat intestinal mucosa at 8 wk after surgery (31). It seems plausible to presume that the absorptive function of the mucosa in CRF is disturbed. Recently, the impairment of intestinal P-glycoprotein function was reported in CRF rats (30). In CRF, dietary protein is considered to impair residual renal function, and therefore, patients with CRF are recommended to take a low-protein diet to prevent uremia (25). However, the regulation of intestinal PEPT1 in CRF remains unclear. On the basis of this background, we have hypothesized that the alteration of intestinal PEPT1 has implications not only in the pharmacokinetics of peptide-like drugs but also in the progression of renal failure in patients.

In the present study, we examined the functional and expressional changes of intestinal PEPT1 in 5/6 nephrectomized (5/6 NR) rats and demonstrated the effect of CRF on the intestinal absorptive rates of small peptides and peptide-like drugs. We report in this article that the activity of the intestinal peptide transporter was increased in CRF, which was caused by an upregulation of PEPT1 expression at the protein level.


    MATERIALS AND METHODS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
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Materials. [14C]glycylsarcosine (Gly-Sar; 1.78 GBq/mmol) was obtained from Daiichi Pure Chemicals (Ibaraki, Japan). Ceftibuten was from Shionogi (Osaka, Japan). All other chemicals were of the highest purity available.

Animals and their treatment. Male Wistar albino rats (200–220 g) were nephrectomized as described previously (16, 26). Briefly, the right kidney was removed, and the posterior and anterior apical segmental branches of the left renal artery were individually ligated. Sham-operated (sham) animals were used as controls. After surgery, animals were allowed to recover from anesthesia and surgery in cages, with free access to water and standard rat chow containing 23.6% (wt/wt) of protein.

To examine renal function, we maintained rats in metabolic cages for 24 h before the experiments. The blood urea nitrogen (BUN) was determined using the urease-indophenol method. The levels of creatinine in plasma and urine were determined using the Jaffé reaction. For these measurements, we used assay kits from Wako Pure Chemical Industries (Osaka, Japan). The concentration of urinary albumin was measured using an ELISA kit (NEPHRAT II; Exocell, Philadelphia, PA). The intestinal protease activity in the mucosal homogenate of duodenum, jejunum, or ileum was determined using a protease assay kit (Calbiochem, La Jolla, CA) according to the manufacturer's instructions. Data are expressed as percent activity by using trypsin (10 µg) as a positive control. The animal experiments were performed in accordance with the "Guidelines for Animal Experiments of Kyoto University." All protocols were previously approved by the Animal Research Committee, Graduate School of Medicine, Kyoto University.

Morphological studies. The intestinal segments (duodenum, jejunum, and ileum) were fixed in 10% phosphate-buffered saline and stained with hematoxylin-eosin (HE). These stained sections were evaluated using a light microscope, and villous height and crypt depth of only well-orientated sections were measured with IP Lab Spectrum image analysis software (Signal Analytics, Vienna, VA).

In situ loop technique. We examined [14C]Gly-Sar and ceftibuten transport activity by using the in situ loop technique. A cannula with a polyethylene tube was inserted in the portal vein. A loop was set from the duodenum to ileum, and then [14C]Gly-Sar (29.2 µg/kg) with [3H]inulin (14 µg/kg) or ceftibuten (1.5 mg/kg) was introduced into the loop (1 ml/kg). Blood was withdrawn from the portal vein at designated times, and the plasma was immediately separated from erythrocytes by centrifugation. To determine the concentration of [14C]Gly-Sar, we solubilized plasma samples in 0.5 ml of NCS II tissue solubilizer (Amersham Pharmacia Biotech, Uppsala, Sweden) and determined radioactivity in 5 ml of ACS II scintillation cocktail (Amersham Pharmacia Biotech) using liquid scintillation counting. The plasma concentration of ceftibuten was determined using high-performance liquid chromatography as described previously (14).

Western blot analysis. The rabbit anti-PEPT1 antibody was raised against the 15 COOH-terminal amino acids of rat PEPT1 (21). Goat anti-villin polyclonal IgG was obtained from Santa Cruz Biotechnology (Santa Cruz, CA). While animals were under anesthesia, small intestinal tissue was removed and flushed with cold PBS, and then the mucosa was scraped. A portion of the mucosa was rapidly frozen in liquid nitrogen for the later preparation of crude plasma membrane fractions. Crude plasma membrane fractions were prepared as described previously (18) and were separated by 10% SDS-PAGE and analyzed by immunoblotting with each antibody as reported previously (21). The relative amounts of the bands in each lane were determined densitometrically using NIH Image 1.61 (National Institutes of Health, Bethesda, MD).

Competitive PCR analysis. Competitive PCR was performed according to the method of Siebert and Larrick (22) with some modifications as described previously (16, 26). Briefly, total RNA (1 µg), isolated from rat small intestine using RNeasy mini kit (Qiagen, Hilden, Germany), was reverse transcribed with random hexamers, using Superscript II reverse transcriptase (Invitrogen, Carlsbad, CA), and digested by RNase H (Invitrogen). After a 10-fold dilution of the reaction mixture, 5-µl aliquots were used for subsequent PCR (20 µl). After denaturation of the first-strand DNA at 95°C for 3 min, PCR was performed at 94°C for 1 min, 60°C for 1 min, and 72°C for 1 min for 34 cycles with competitor DNAs (2.5 x 10–19 mol/reaction). Primer sets specific for PEPT1 were as follows: sense primer, 5'-GTGTGGGGCCCCAATCTATACCGT-3', corresponding to bases 1442–1465, and antisense primer, 5'-GTTTGTCTGTGAGACAGGTTCCAA-3', corresponding to bases 2153–2176. The expected size of the amplified products derived from the mRNA was 735 bp, and that from the mimic competitor was 607 bp. The amplified PCR products were separated by electrophoresis on 1.7% agarose gels and stained with ethidium bromide. The reactive amounts of bands in each reaction were determined densitometrically using NIH Image 1.61. The densitometric data were normalized for each batch of RNA by correcting the amount of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA as an internal control.

Measurement of plasma T3 concentration. The plasma concentration of thyroid hormone 3,5,3'-L-triiodothyronine (T3) was measured using an enzyme immunoassay method (IMx; Dainabot, Tokyo, Japan).

Statistical analysis. Data were analyzed statistically with the unpaired t-test. Probability values <5% were considered significant.


    RESULTS
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Renal functional data of 5/6 NR rats. As shown in Table 1, body weight tended to decrease in 5/6 NR rats. The levels of BUN and urinary albumin excretion were significantly increased, and the creatinine clearance was markedly decreased in 5/6 NR rats compared with sham rats. Therefore, the marked renal dysfunction was confirmed. In addition, the progressive renal failure in 5/6 NR rats in the postoperative period was also shown at 4 and 8 wk after nephrectomy.


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Table 1. Body weight, food intake and renal functional data in sham and 5/6 NR rats

 
Morphological studies. Investigators in our laboratory (27) previously reported that tubular damage was minimal at 2 wk after 5/6 NR, although glomerular hypertrophy was observed. We expected little histological alteration in the small intestine at 2 wk after the 5/6 NR. Histological examination of the small intestine in 5/6 NR rats showed no abnormalities at 2 wk after nephrectomy (Fig. 1A). We then evaluated the villous height and crypt depth by quantitative analysis. As shown in Fig. 1B, both villous height and crypt depth were comparable in sham and 5/6 NR rats. In addition, no abnormality in HE staining, villous height, and crypt depth was found in the 5/6 NR rats at 4 and 8 wk after surgery (data not shown). In addition, we examined the intestinal protease activity by using the trypsin as a positive control. Although the mucosal protease activity tended to be higher in the duodenum than in jejunum or ileum, there was no significant difference between sham and 5/6 NR rats at 2 wk after surgery (Fig. 1C).



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Fig. 1. Histological analysis of intestine in rats after 5/6 nephrectomy (5/6 NR). A: small intestinal hematoxylin-eosin staining of sham and 5/6 NR rats at 2 wk after surgery. Magnification, x40; bar, 20 µm. B: small intestinal villous height and crypt depth of sham (open bars) and 5/6 NR rats (filled bars) at 2 wk after surgery. Each value represents the mean ± SE of 8 rats. Six villous height and crypt depth measurements were made for each rat. C: mucosal protease activities of sham (open bars) and 5/6 NR rats (filled bars) at 2 wk after surgery. Each value represents the mean ± SE of 5 rats. Data are expressed as percentages of trypsin activity (10 µg) as a positive control.

 
Gly-Sar and ceftibuten absorption by in situ intestinal loops. We examined the change in intestinal absorption of [14C]Gly-Sar at 2 wk after 5/6 nephrectomy using the in situ intestinal loop technique. Plasma concentration profiles of [14C]Gly-Sar and [3H]inulin in the portal vein after intraintestinal administration are shown in Fig. 2. Although the absorption rate of [3H]inulin did not change throughout the period, the initial absorption rate of [14C]Gly-Sar was markedly increased in the 5/6 NR rats compared with the sham rats. The area under the concentration-time curve (AUC0–9 min) for [14C]Gly-Sar was 0.25 ± 0.06 µg·min·ml–1 in sham rats and 0.57 ± 0.05 µg·min·ml–1 in 5/6 NR rats (mean ± SE of 6 rats; P < 0.05 vs. sham rats) (Fig. 2A). Next, the alteration to the absorptive rate of a peptide-like drug, ceftibuten, was examined (Fig. 3). Because ceftibuten was a good substrate of apical PEPT1 but not of basolateral peptide transporter (28), the transcellular transport of ceftibuten was considered to be delayed compared with Gly-Sar. Therefore, the time course of ceftibuten absorption was set up to 30 min. Similarly, the absorptive rate of ceftibuten also was enhanced in the 5/6 NR rats compared with the sham rats. The AUC0–30 min for ceftibuten was 26.2 ± 4.5 µg·min·ml–1 in sham rats and 46.3 ± 6.4 µg·min·ml–1 in 5/6 NR rats (mean ± SE of 6 rats; P < 0.05 vs. sham rats).



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Fig. 2. Plasma concentration of [14C]glycylsarcosine (A) and inulin (B) in portal vein of sham ({circ}) and 5/6 NR rats ({bullet}) after intraintestinal administration, at 2 wk after surgery. Each data point represents the mean ± SE of 6 rats. *P < 0.05, significantly different from sham rats.

 


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Fig. 3. Plasma concentration of ceftibuten in portal vein of sham ({circ}) and 5/6 NR ({bullet}) rats after intraintestinal administration, at 2 wk after surgery. Each data point represents the mean ± SE of 6 rats. *P < 0.05, significantly different from sham rats.

 
Detection of intestinal PEPT1 protein. Western blot analysis was performed to detect the change in the expression level of intestinal PEPT1 protein after 5/6 nephrectomy. At 2 wk after nephrectomy, the expression level of PEPT1 protein in the duodenum was significantly increased in 5/6 NR rats compared with sham rats (Fig. 4). In contrast, in the jejunum and ileum, the expression level of PEPT1 protein in 5/6 NR rats was comparable to that in sham rats. There were no differences in the expression level of villin protein between sham and 5/6 NR rats throughout any segment of the small intestine (Fig. 4).



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Fig. 4. Western blot analysis of intestinal H+-coupled peptide transporter (PEPT1) and villin proteins in sham and 5/6 NR rats at 2 wk after surgery. A: crude membrane fractions (25 µg) from each intestinal segment were separated by SDS-PAGE (10%) and blotted onto Immobilon P membranes. Antisera specific for PEPT1 and villin (1:1,000 dilution) were used as primary antibodies. Horseradish peroxidase-conjugated anti-rabbit (for PEPT1) or anti-goat (for villin) IgG antibody was used for detection of bound antibodies, and strips of blots were visualized by chemiluminescence on X-ray film. B: protein bands are expressed in densitometry units for sham (open bars) and 5/6 NR rats (filled bars). Values for sham rats have been arbitrarily defined as 100%. Each value represents the mean ± SE of 5 rats. *P < 0.05, significantly different from sham rats.

 
Detection of intestinal PEPT1 mRNA. To examine the regulation of intestinal PEPT1 mRNA in 5/6 NR rats, we carried out a semiquantitative PCR analysis. The data obtained by competitive PCR were normalized to the data from the competitive PCR for GAPDH. As shown in Fig. 5, the expression level of intestinal PEPT1 mRNA in 5/6 NR rats was not significantly different from that in sham rats at 2 wk after surgery.



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Fig. 5. Detection of PEPT1 and glyceraldehyde-3-dehydrogenase (GAPDH) mRNA in intestinal total RNA isolated from sham and 5/6 NR rats by competitive PCR. A: PCR amplification was carried out as described in MATERIALS AND METHODS. Gels show results of representative experiments; arrowheads indicate positions of each protein. CPT, competitor; D, duodenum; J, jejunum; I, ileum. B: densitometric determination of PEPT1 mRNA. Data were derived by dividing the ratio between densitometric data for PEPT1 and competitor for PEPT1 by the ratio between densitometric data for GAPDH and competitor for GAPDH, as an internal control. Each value represents the mean ± SE of 5 rats.

 
Plasma concentration of T3 in 5/6 NR rats. Investigators in our laboratory (3) previously reported that T3 treatment downregulated the activity and expression of the intestinal PEPT1. To examine the effect of T3 with the upregulation of PEPT1 protein in 5/6 NR rats, we measured plasma T3 concentrations in 5/6 NR rats. As a result, no significant difference was observed in plasma T3 levels between sham and 5/6 NR rats at 2 wk after surgery (sham, 0.49 ± 0.03 ng/ml vs. 5/6 NR, 0.40 ± 0.05 ng/ml; mean ± SE of 6 rats).

Effect of renal failure progression on expression of intestinal PEPT1 protein. Because the upregulation of intestinal PEPT1 was caused by the protein level, not the mRNA level, the increased level of PEPT1 protein might be a transient phenomenon. Therefore, we have further examined the regulation of intestinal PEPT1 in 5/6 NR rats concerning the time after nephrectomy, i.e., progression of renal impairment. The enterocyte PEPT1 proteins in 5/6 NR rats were detected at 4 and 8 wk after surgery by using Western blot analysis. As shown in Fig. 6, duodenal upregulation of PEPT1 was maintained at 4 and 8 wk after 5/6 nephrectomy. In addition, the expression of PEPT1 in the jejunum was significantly increased in 5/6 NR rats compared with sham rats at 8 wk after surgery. In contrast, the expression level in the ileum was comparable to that in sham rats for at least 8 wk after surgery.



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Fig. 6. Western blot analysis of PEPT1 and villin proteins in crude membrane fractions of small intestine isolated from sham and 5/6 NR rats at 4 (A and B) and 8 wk (C and D) after surgery. A and C: blots show results of representative experiments with crude plasma membrane fractions at 4 (A) and 8 wk (C) after 5/6 NR. B and D: protein bands are expressed in densitometry units for sham (open bars) and 5/6 NR rats (filled bars). Values for sham rats were arbitrarily defined as 100%. Each value represents the mean ± SE of 5 rats. *P < 0.05, significantly different from sham rats.

 

    DISCUSSION
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Intestinal PEPT1 has physiological and pharmacological roles, such as maintaining protein nutrition and the absorption of peptide-like drugs (6). In addition, two isoforms of peptide transporter (PEPT1 and PEPT2) are expressed in the kidney (10, 12). Recently, investigators in our laboratory (26) found that renal tubular PEPT2, but not PEPT1, was upregulated in 5/6 NR rats. Considering the intestinal localization of PEPT1 and the heightened risk from a protein diet in CRF patients, the functional and molecular regulation of the intestinal PEPT1 should be more important to prevent a protein diet-induced progressive renal failure and/or the adverse effects of peptide-like drugs. However, the regulation of intestinal PEPT1 under CRF has not been elucidated. In the present study, we found that the intestinal transport activity of [14C]Gly-Sar and ceftibuten as well as the expression of intestinal PEPT1 protein was upregulated in 5/6 NR rats at 2 wk after surgery (Figs. 2 and 3). Because the expression level of intestinal PEPT1 mRNA in 5/6 NR rats was not significantly different from that in sham rats, this upregulation of PEPT1 by 5/6 nephrectomy was likely caused by a posttranscriptional modification and/or stabilization of the PEPT1 protein. In contrast to the present results, Sterner et al. (23) showed that the small intestinal absorptive capacity of dipeptides was not affected by nephrectomy in rats. This discrepancy may be explained by the difference in the segments of in situ loops used. Whereas Sterner et al. (23) used jejunal segments, we examined the transport activity of dipeptides in the whole small intestine (from duodenum to ileum) (Fig. 2A). Because our results show an upregulation of intestinal PEPT1 with 5/6 nephrectomy mainly in the upper region (Figs. 4 and 6), they might have failed to observe this change.

Although morphological abnormalities are found in the small intestine of patients in CRF (7), Haines et al. (9) and Wizemann et al. (31) reported that no significant morphological change was observed in nephrectomized rats comparable with our results. The time span of renal disease in humans is not the same as that in the present animal model. Therefore, a histological evaluation of the small intestine should be made in the future, to confirm the morphological alterations that occurred in 5/6 NR rats at ~20 wk after surgery. In the present study, we examined the intestinal permeability of inulin in 5/6 NR rats and found no significant difference from sham rats (Fig. 2B). In addition, no morphological and enzymatic abnormalities were found in the intestinal mucosa of 5/6 NR rats at 2 wk after surgery (Fig. 1). Considering the present results as well as the previous findings (7, 9, 11, 31), the increase of the intestinal PEPT1 may have occurred before other morphological and enzymatic alteration after nephrectomy. These results support the hypothesis that the increase of absorption of [14C]Gly-Sar and ceftibuten in the early phase of CRF was a specific event caused by the upregulation of PEPT1.

Earlier investigations suggested that the activity of PEPT1 varies in response to several factors (1). Dietary proteins are considered as a regulatory factor for intestinal PEPT1. Pan et al. (19) reported that the expression of rat PEPT1 in the duodenum is increased by starvation. In the present study, upregulation of PEPT1 was observed mainly in the upper region of the small intestine of 5/6 NR rats. These results may be partly explained by the reduced food intake in CRF. We monitored the amount of food intake in sham and 5/6 NR rats. The intake was reduced at 2 wk after nephrectomy but recovered to a comparable level with that for sham rats at 4 wk after surgery (Table 1). We found that the expression level of PEPT1 in the duodenum was upregulated in 5/6 NR rats at 4 wk as well as 2 wk after surgery (Fig. 5). Haines et al. (9) also showed that food intake did not significantly differ between nephrectomized and sham rats. Therefore, the variation of food intake after nephrectomy would not relate to the upregulation of PEPT1 protein expression in the small intestine. Other possibilities including hormonal regulation of PEPT1 also have been studied. For example, insulin and leptin stimulated the uptake of dipeptides in human intestinal Caco-2 cells by increasing the translocation of PEPT1 protein from a preformed cytoplasmic pool (5, 29). The treatment of thyroid hormone T3 and epidermal growth factor (EGF) downregulated the activity and expression of PEPT1 in intestinal cells (3, 17). Moreover, it has been reported that cAMP and protein kinase C activation inhibited the activity of PEPT1 (4, 15). Because the plasma levels of many hormones appear abnormal in CRF (24), we examined the relationship between the plasma T3 level and the upregulation of PEPT1 protein in 5/6 NR rats. However, we did not find a significant difference in the plasma T3 level between sham and 5/6 NR rats. In addition, we could not detect the mRNA for gastrointestinal leptin in either rat, and there was no significant difference in the intestinal EGF mRNA level between sham and 5/6 NR rats (data not shown). It was suggested that other factors, including the plasma EGF or insulin level, or other unknown regulators might be related to the upregulation of PEPT1 in CRF. Further studies are required to determine the precise mechanism of upregulation of PEPT1 protein in progressive CRF.

In CRF, dietary protein is thought to impair the residual renal function (25). In the present study, we found that the activity of intestinal PEPT1 was upregulated in CRF, accompanied by an increase in the absorption of oligopeptides derived from digested dietary proteins. In addition, we found that the upregulation of intestinal PEPT1 in CRF spread from the upper to lower region with the progression of renal failure (Fig. 6). These results suggest that the upregulation of intestinal PEPT1 and subsequent heightened absorption rate of small peptides relate the escalation in the plasma peptide level and subsequent progressive renal dysfunction. On the other hand, PEPT1 mediates the absorption of peptide-like drugs such as {beta}-lactam antibiotics and ACE inhibitors as well as oligopeptides (10). It is well acknowledged that the urinary excretion rates of these drugs are markedly decreased in CRF. Investigators in our laboratory (27) previously demonstrated that the expression of renal PEPT2, but not PEPT1, was upregulated in CRF and would partly contribute to the delayed urinary excretion rate of peptide-like drugs. In addition to the upregulation of renal PEPT2, the upregulation of intestinal PEPT1 mediated the increased absorption rate of dipeptides and peptide-like drugs. Therefore, the exposure to peptide-like drugs would be prolonged by the sum of the upregulation of intestinal PEPT1 and renal PEPT2 as well as the decreased glomerular filtration rate in CRF. In patients with hypertension and diabetic nephropathy, ACE inhibitors and angiotensin receptor blockers are used to protect renal function (20). Therefore, a small oral dosage of ACE inhibitors in these patients would provide a sufficient blood level and pharmacological effects to prevent CRF through enhanced absorption via enterocyte PEPT1 and reabsorption via renal PEPT2. Taken together, these data suggest that a careful dosage adjustment according to the intestinal absorptive capacity as well as renal function is needed in patients in CRF.

In the present study, we have demonstrated the upregulation of intestinal PEPT1 expression, which is not regulated by the mRNA level in 5/6 NR rats. The increased expression level of the enterocyte PEPT1 would be a molecular mechanism of accumulation of peptide-like drugs in addition to the decreased glomerular filtration and upregulation of renal PEPT2. In addition, the enhanced absorptive rate or bioavailability of oligopeptides due to the heightened enterocyte PEPT1 expression would partially be a risk factor for the dietary protein-induced progression of CRF. The upregulation of intestinal PEPT1 in 5/6 NR rats was suggested to spread with the progression of renal failure. These results provide useful information for understanding the progressive mechanism of renal failure and for the appropriate usage of peptide-like drugs by considering intestinal function.


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This work was supported by a Grant-in-Aid from the Japan Health Sciences Foundation, by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and by the 21st Century Center of Excellence program Knowledge Information Infrastructure for Genome Science.


    FOOTNOTES
 

Address for reprint requests and other correspondence: K. Inui, Dept. of Pharmacy, Kyoto Univ. Hospital, Sakyo-ku, Kyoto 606-8507, Japan (E-mail: inui{at}kuhp.kyoto-u.ac.jp)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


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