Faculty Bibliography

Various transporters such as H+/peptide cotransporter PEPT1 are expressed in the intestine, and play important physiological and pharmacological roles in the body. Present study was performed to examine the expression profile of 20 kinds of transporters (PEPT1 and 2, P-glycoprotein, amino acid transporters and organic ion transporters) along the human digestive tract, especially focusing on PEPT1. Using normal mucosal specimens, real-time polymerase chain reactions were carried out. Immunoblot analyses were also performed for PEPT1 expression. PEPT1 mRNA was highly expressed in the small intestine (duodenum>jejunum>ileum) compared to other tissues, and some patients showed a significant level of expression in the stomach. The expressional pattern of PEPT1 in the stomach and histological diagnosis indicated that gastric PEPT1 originated from the intestinal metaplasia. The amino acid transporters showed unique mRNA expression levels and distributions in the digestive tract. For example, the expression levels of B(0)AT1, a Na+-dependent and chloride-independent neutral amino acid transporter, were increased from the duodenum to ileum, which pattern is completely inverted to that for PEPT1. There is little expression of organic ion transporters except for organic cation/carnitine transporter OCTN2. In conclusion, PEPT1 was abundantly expressed in the small intestine, and the reciprocal expression of PEPT1 and B(0)AT1 may serve for the efficient absorption of protein digestive products.

PURPOSE OF REVIEW/OBJECTIVE:The assembly of intestinal lipoproteins is critical for the transport of fat and fat-soluble vitamins. In this review we propose a nomenclature for these lipoproteins and have summarized recent data about their intracellular assembly and factors that modulate their secretion. RECENT FINDINGS/RESULTS:The assembly and secretion of intestinal lipoproteins increases with the augmented synthesis of apoB, apoAIV and lipids. Chylomicron assembly begins with the formation of primordial, phospholipid-rich particles in the membrane, and their conversion to large chylomicrons occurs in the lumen of the smooth endoplasmic reticulum. Chylomicrons are transported from the endoplasmic reticulum via specialized vesicles to the Golgi for secretion. The identification of genetic mutations in chylomicron retention disease indicates that Sar1b may play a critical role in this process. In addition to chylomicron assembly, intestinal cells have been shown to transport dietary cholesterol via apoB-independent pathways, such as efflux. SUMMARY/CONCLUSIONS:Understanding the mechanisms involved in the intracellular transport of chylomicrons and chylomicron-independent secretion pathways are expected to be the next frontiers in the field of intestinal lipoprotein assembly and secretion.

The intestinal Na(+)/glucose cotransporter 1 (SGLT1) and H(+)/peptide cotransporter 1 (PEPT1) play important roles in the absorption of carbohydrate and protein. Although they exhibit a diurnal rhythm in their expression and function, the factors responsible for this are unclear. In the present study, we examined the effects of various feeding conditions on the diurnal rhythm of intestinal SGLT1 and PEPT1. Rats were divided into 1 of 4 groups: group 1 was fed, group 2 was food deprived for 1-4 d, group 3 was food deprived for 4 d and then refed for 1 or 2 d, and group 4 was fed during the daytime (0900-1500 h) for 10 d. In fed rats, the SGLT1 protein level was significantly higher at 2000 h than at 0800 h. However, in rats deprived of food for 2-4 d, protein levels did not differ between 0800 and 2000 h. In contrast, the SGLT1 messenger RNA (mRNA) level was significantly higher at 2000 h than at 0800 h in rats deprived of food for 4 d. Refeeding for 2 d after 4 d of food deprivation returned the diurnal variation in SGLT1 and PEPT1 protein expressions to normal. Consuming food during the daytime only shifted the peaks of SGLT1 and PEPT1 mRNAs and protein expressions from the dark phase to the light phase. These findings suggest that food intake, rather than the light cycle, greatly affects the diurnal rhythm of SGLT1 and PEPT1 expressions.

We previously demonstrated that H+/peptide cotransporter PEPT1 shows a diurnal rhythm in the rat small intestine. In the present study, we examined the effect of food intake on the diurnal rhythm of intestinal PEPT1 using fed and fasted rats and also determined whether such variation affected the pharmacokinetics of peptide-like drugs. In fed rats, PEPT1 protein level was significantly higher at 8:00 PM than at 8:00 AM. However, during fasting for 2 to 4 days, the differences of PEPT1 protein levels between 8:00 AM and 8:00 PM gradually disappeared. Intestinal absorption of an oral antibiotic ceftibuten (CETB), a pharmacological substrate for PEPT1, was also greater at 8:00 PM than at 8:00 AM in fed rats, but not different in 4-day fasted rats. In contrast to PEPT1 protein levels, PEPT1 mRNA levels retained a diurnal rhythm after 4 days of fasting. Pharmacokinetic analyses of CETB after intraintestinal administration demonstrated that both Cmax and area under the plasma concentration-time curve from 0 to 3 h were greater at 8:00 PM than at 8:00 AM in fed rats. In contrast, pharmacokinetic parameters showed no significant difference between 8:00 AM and 8:00 PM for intraintestinal administration in 4-day fasted rats and for intravenous administration in fed and 4-day fasted rats. These findings suggested that the diurnal rhythm of intestinal PEPT1 transport activity was disrupted by fasting and that diurnal variation of intestinal PEPT1 functionality could influence the pharmacokinetics of peptide-like drugs such as CETB.

In mammals, most physiological, biochemical, and behavioral processes show a circadian rhythm. In the present study, we examined the diurnal rhythm of the H+-peptide cotransporter (PEPT1), which transports small peptides and peptide-like drugs in the small intestine and kidney, using rats maintained in a 12-h photoperiod with free access to chow. The transport of [14C]glycylsarcosine (Gly-Sar), a typical substrate for PEPT1 by in situ intestinal loop and everted intestine, was greater in the dark phase than the light phase. PEPT1 protein and mRNA levels varied significantly, with a maximum at 2000 and minimum at 800. Similar functional and expressional diurnal variations were observed in the intestinal Na+-glucose cotransporter (SGLT1). In contrast, renal PEPT1 and SGLT1 showed little diurnal rhythmicity in protein and mRNA expression. These findings indicate that the intestinal PEPT1 undergoes diurnal regulation in its activity and expression, and this could affect the intestinal absorption of dietary protein.