Nutrient regulation of human intestinal sugar transporter (SGLT1) expression

BACKGROUND

The activity of most intestinal nutrient transporters is adaptively regulated by the type and amounts of nutrients entering the intestinal lumen. The concentration and activity of the intestinal Na+/glucose cotransporter (SGLT1) are regulated by dietary sugars in most animal species. The activity and abundance of SGLT1 in biopsy specimens removed from human jejunal regions exposed to, and having limited access to, luminal nutrients have been measured and compared.

AIMS

To study the effects of luminal nutrients on the expression of SGLT1 in the human intestine.

PATIENT AND METHODS

Brush border membrane vesicles (BBMV) were prepared from biopsy specimens removed from the intestine of a 50 year old man who had developed a high output jejunal fistula, and adjacent mucosal fistula, a condition present for 12 months after surgery for a strangulated hernia. BBMV prepared from intestine exposed to luminal nutrients, and from dysfunctional intestine with a limited exposure to nutrients, were used to measure Na+ dependent glucose transport and abundance of SGLT1 protein.

RESULTS

The levels of SGLT1 activity and abundance in the BBMV prepared from control biopsy specimens were similar to those found in BBMV prepared from the intestine of healthy individuals. BBMV from the dysfunctional intestine, exposed to limited levels of luminal nutrients, had reduced levels of SGLT1 activity. This reduction in SGLT1 activity and abundance was above that associated with any villus atrophy, as assessed by the abundance/activity of lactase and villin concentrations.

CONCLUSIONS

These data indicate that the activity and expression of SGLT1 in human intestine is maintained by the presence of luminal nutrients.

Nutrient regulation of the intestinal Na+/glucose co-transporter (SGLT1) gene expression

It is known that dietary carbohydrates regulate the activity of the intestinal SGLT1. We have demonstrated that modifications in SGLT1 activity are due to alterations in SGLT1 expression in response to the sugar content of the diet. To assess the correlation between changes in the activity of SGLT1 and the abundance of SGLT1 protein, we have employed a method for the quantitative measurement of immunoreactive proteins. A calibration curve has been constructed using either a nonadecapeptide (amino acids 402-420), or a recombinant protein corresponding to amino acids 554-640 of the SGLT1 sequence. Immunoblotting the protein samples concurrently with specific quantities of either the peptide or recombinant standard, using antibodies raised against these antigens, enabled accurate quantification of the absolute amounts of immunoreactive protein in the samples. The amount of SGLT1 protein correlates well with measurements of SGLT1 activity. The modulation of the activity of SGLT1 in response to lumenal sugars is due to corresponding changes in the absolute levels of SGLT1 protein.

Amino acid sequence and the cellular location of the Na(+)-dependent D-glucose symporters (SGLT1) in the ovine enterocyte and the parotid acinar cell

The Na(+)-dependent D-glucose symporter has been shown to be located on the basolateral domain of the plasma membrane of ovine parotid acinar cells. This is in contrast to the apical location of this transporter in the ovine enterocyte. The amino acid sequences of these two proteins have been determined. They are identical. The results indicated that the signals responsible for the differential targeting of these two proteins to the apical and the basal domains of the plasma membrane are not contained within the primary amino acid sequence.

Preparation and characterization of basolateral membrane vesicles from pig and human colonocytes: the mechanism of glucose transport

Membrane vesicles were isolated from the basolateral domains of pig and normal human colonocytes. The activity of the ouabain-sensitive K(+)-activated phosphatase, the basolateral membrane marker, was enriched 13-fold in these membrane vesicles over the original homogenate. The membranes displayed cross-reactions with antibodies to the (Na+/K+)ATPase and the RLA class I major histocompatibility antigen, both known indicators of the basolateral membrane. There was negligible contamination by other organelles and the luminal membrane, as revealed by marker-enzyme analysis and Western blotting, using an antibody to villin. The vesicles transported D-glucose in a cytochalasin B-inhibitable Na(+)-independent manner, with a Km of 28.1 +/- 0.8 mM and Vmax. of 3.1 +/- 0.4 nmol/s per mg of protein. The transport was inhibited by 2-deoxy-D-glucose and 3-O-methyl-D-glucose, but not by L-glucose or methyl-alpha-D-glucose. Probing the colonocyte basolateral membranes with an antibody against the C-terminus of the human liver GLUT 2 produced a cross-reaction at 52 kDa. These properties indicate the presence of a GLUT 2 isoform on the basolateral membranes of human and pig colonocytes.

Regulation of the ovine intestinal Na+/glucose co-transporter (SGLT1) is dissociated from mRNA abundance

We have investigated the mechanisms of regulation of the Na+/glucose co-transporter (SGLT1) in a ruminant animal, which is an exceptional model system for studying intestinal glucose transport. Pre-ruminant lambs absorb glucose, produced by hydrolysis of the milk sugar lactose, in the intestine via apical SGLT1 and basolateral facilitative glucose transporters (GLUT2). Weaning coincides with the development of the rumen, and consequently the amount of hexoses reaching the small intestine of the ruminant sheep is undetectable. During development, SGLT1 activity and abundance in intestinal brush-border membranes decreased by over 200-fold, and either maintaining lambs on a milk replacer diet or infusing sheep intestine with D-glucose restored co-transporter activity and expression. We have measured ovine intestinal SGLT1 mRNA levels during development, with changes in diet and after direct infusion of D-glucose or methyl alpha-D-glucopyranoside into the intestinal lumen, in order to determine the level of regulation. During development, mRNA levels decreased only 4-fold. Lambs maintained on a milk replacer diet showed no change in mRNA levels relative to age-matched controls. Finally, upon infusion of the intestine of the ruminant sheep with sugars, D-glucose infusion increased SGLT1 mRNA, but only by 2-fold, compared with a 60-90-fold increase in co-transporter number and activity. Since the change in Na(+)-dependent glucose transport activity is correlated with SGLT1 protein abundance, and since changes in mRNA levels do not account for the dramatic changes in protein abundance, we conclude that the principal level of SGLT1 regulation by luminal sugar is translational or post-translational.

The expression of the Na+/glucose cotransporter (SGLT1) gene in lamb small intestine during postnatal development

We have shown previously that the activity and abundance of the intestinal Na+/glucose cotransporter (SGLT1) declines dramatically during the postnatal development of lambs, and that it can be restored in the intestine of ruminant sheep by intra-luminal infusion of D-glucose. The work presented in this paper has followed the expression of the SGLT1 gene along the vertical and horizontal axes of the ovine small intestine during early development, using quantitative in situ hybridisation histochemistry. Along the vertical axis, SGLT1 mRNA was first detectable just below the crypt-villus junction and rose rapidly to a peak level approx. 150 microns above this point. After reaching a maximum, the amount of message gradually declined towards the villus tip. This pattern of mRNA accumulation along the crypt-villus axis was similar in all intestinal positions and age groups. Along the length of the small intestine (horizontal axis), a decline in the level of SGLT1 mRNA was observed first in the distal intestine. This decrease in SGLT1 mRNA was significant in the intestine (75% of length) of 5-week-old lambs when compared to tissue taken from 25 and 50% of length (P < 0.01 and P < 0.02, respectively). However, the observed fall in the expression of this gene during weaning did not coincide with the fall in activity and amount of SGLT1. In adult animals, where the activity of SGLT1 is very low, the amount of message was greatly reduced. This work supports the finding that the expression of SGLT1 is primarily controlled at the post-transcriptional level during the postnatal development of ovine intestine.

Characterization of the ileal Na+/bile salt co-transporter in brush border membrane vesicles and functional expression in Xenopus laevis oocytes

The Na+/bile salt co-transporter of the pig ileal brush border membrane has been expressed in Xenopus laevis oocytes. Injection of pig ileal poly (A)+ RNA into oocytes resulted in the functional expression of an Na(+)-gradient-stimulated taurocholate uptake within 2-5 days. The expressed Na(+)-dependent taurocholate uptake exhibited saturation kinetics (apparent Km of 48 microM), and displayed similar competitive substrate inhibition by taurodeoxycholate as the native brush border Na+/bile salt co-transporter studied in pig ileal brush border membrane vesicles. Interestingly, injection of pig proximal and mid intestinal poly (A)+ RNA into oocytes also resulted in the expression of the Na+/bile salt co-transporter, though the Na(+)-dependent transport of bile salts does not occur in brush border membrane vesicles (BBMV) isolated from pig proximal and mid intestine. This suggests that the mRNA coding for the co-transporter is present in the enterocytes lining the whole length of the small intestine, but that the function is only expressed in the brush border of the distal small intestine. The transport of D-glucose into BBMV, and the transport of methyl-alpha-D-glucopyranoside (a non-metabolizable hexose derivative) into oocytes were used throughout the study as methods of confirming the integrity of vesicles and oocytes.

Ontogenic development of lamb intestinal sodium-glucose co-transporter is regulated by diet

1. The ontogenic development of the intestinal Na(+)-glucose co-transporter was measured in lambs as a function of diet. Transport activity was assayed in brush-border membrane vesicles and the expression of transport protein in the brush-border membrane determined by Western analysis. 2. Na(+)-dependent D-glucose transport increased to a maximum (300-700 pmol mg-1 s-1) within the first 2 weeks of birth and then declined to negligible amounts (less than 10 pmol mg-1 s-1) over the next 8 weeks. There was no further change over the next 2-3 years. Early changes were associated with modifications in both the maximum velocity Vmax for transport and expression of carrier protein in the brush-border plasma membrane. 3. Maintaining lambs on a milk replacer diet beyond the normal weaning period prevented the normal decline in the expression of Na(+)-glucose co-transport. At 5 weeks the transport rate was 433 +/- 150 pmol mg-1 s-1 in lambs maintained on milk replacer, but only 79 +/- 40 pmol mg-1 s-1 in normally reared control lambs. 4. Infusing the proximal intestine of 2- to 3-year-old sheep with 30 mM-D-glucose for four days increased the rate of transport 40- to 80-fold above that found in control animals perfused with mannitol. A similar but smaller increase was observed in one animal perfused with the non-metabolizable sugar alpha-methyl-D-glucopyranoside. The induced increase in glucose transport was correlated with the expression of the co-transporter protein in the brush-border plasma membrane. 5. It is concluded that the age-related decline in Na(+)-glucose co-transport in the sheep intestine is directly due to the decrease in D-glucose (and D-galactose) reaching the small intestine after development of the rumen. These results further suggest that luminal sugar substrates for the co-transporter promote both the maintenance and the up-regulation of the brush-border transport protein and it is the intact sugar itself which controls gene expression during enterocyte maturation.

Postnatal development of lamb intestinal digestive enzymes is not regulated by diet

1. Change in digestive enzyme activities determined biochemically in brush-border membrane vesicles and cytochemically in isolated villi of lamb proximal intestine has been related to diet, intestinal structure and rumen development during the first 10 weeks of postnatal life. 2. Lactase activity halved, dipeptidylpeptidase IV activity doubled and aminopeptidase N and alkaline phosphatase activities remained constant during this period of development. Maintaining lambs on a milk replacer diet for 5 weeks after birth had no effect on this pattern of postnatal change in digestive enzyme activities. 3. Structural changes accompanying these selective effects on enzyme expression included a halving of villus height and a doubling of villus width. Villus surface area remained unaffected by these changes in height and width of villi. Crypt depth doubled during the first 10 weeks of postnatal life. Maintaining lambs on a milk replacer diet for 5 weeks did not affect this pattern of change in intestinal structure. 4. It appears from these results that postnatal decrease in lactase and increase in dipeptidylpeptidase IV activities are not regulated by factors such as diet, rumen development, or changes in intestinal structure. Attention is drawn to differences encountered between these results and a postnatal modification of glucose transport which clearly is dependent on diet.

Mechanisms of phosphate transport in sheep intestine and parotid gland: response to variation in dietary phosphate supply

The transport of phosphate in intestinal brush-border membrane and parotid basolateral membrane vesicles isolated from sheep maintained on high and low phosphate diets have been studied. The mechanism of the transport of phosphate in the intestine is via a proton symporter whilst in the parotid gland it is effected by a Na+ coupled transporter. In sheep fed a low-P diet there is no change in the capacity of the parotid basolateral membrane to transport phosphate into the parotid end piece cells. This is in marked contrast to the response of the enterocyte brush-border membrane, where there is a significant enhancement of the capacity of the membrane to transport phosphate. We conclude that in sheep the gut appears to play a major role in response to phosphate deprivation, by increasing the capacity to transport phosphate. This enhancement is not achieved by increases in the levels of circulating 1,25-dihydroxycholecalciferol.

Glycyl-L-proline transport in rabbit enterocyte basolateral-membrane vesicles

The properties of a peptide-transport system in rabbit enterocyte basolateral membrane were examined with glycyl-L-proline as the substrate. Basolateral-membrane vesicles prepared from rabbit proximal intestine were characterized in terms of both purity and orientation. Marker-enzyme assays show that the basolateral-membrane marker, ouabain-sensitive K(+)-activated phosphatase, is enriched 17-fold with respect to the initial homogenate. The activities of enzymes used as markers for other membranes and organelles are low, and contamination of the final membrane fraction with these is minimal. The use of immunoblotting techniques further confirms the absence of brush-border-membrane contamination. Proteins in the basolateral-membrane vesicle preparation gave no cross-reaction with antibodies against the 140 kDa antigen and the Na+/glucose-symport protein, markers specific to the brush-border membrane of the enterocyte. Conversely, antibodies raised against the classical basolateral-membrane marker, the RLA class I histocompatibility complex, reacted strongly with a 43 kDa basolateral-membrane protein. The orientation of the basolateral-membrane vesicles was shown to be predominantly inside-out on determination by two independent criteria. The uptake of [1-14C]glycyl-L-proline by these vesicles is stimulated by the presence of an inwardly directed pH gradient, and this stimulation can be abolished by the proton ionophores carbonyl cyanide p-trichloromethoxyphenylhydrazone (CCCP) and tetrachlorotrifluoromethylbenzimidazole (TTFB). Transport is also inhibited by HgCl2, thimerosal, Na+ and other glycyl dipeptides.

Preparation and properties of brush-border membrane vesicles from human small intestine

This study describes a simple and rapid method for the preparation of brush-border membrane vesicles from intestinal biopsies. The specific activities of sucrase, amino peptidase N, and alkaline phosphatase in these vesicles were the same as those in vesicles prepared from intestinal segments. The vesicles from all the regions of the small intestine can transport D-glucose in an Na+-dependent manner. The rates of transport of D-glucose presented here are far higher than previously reported. The method should have a wide applicability to studies of transport mechanisms and the distribution of transport processes within the intestine.

Changes in the functions of the intestinal brush border membrane during the development of the ruminant habit in lambs

1. Brush border membrane vesicles were prepared from lamb enterocytes. These were used to study the changes in the enzyme contents and the transport capacities which occur during the change from a milk to a roughage diet. 2. Na+-dependent transport of D-glucose was present in all regions of the small intestine of pre-ruminant lambs and absent in ruminants. 3. Na+-dependent transport of L-proline was present in all regions of the small intestine irrespective of the age of the animal. 4. Phosphate transport was seen only in the presence of a transmembrane pH gradient (acid outside). The transport was not stimulated by either Na+ or K+. The transport capacity increases 2-fold as the animal becomes ruminant. 5. The activities of lactase and maltase diminished with age. Alkaline phosphatase and aminopeptidase N activities remain constant. Sucrase activity cannot be detected in lambs of any age.

Phosphate transport in intestinal brush-border membrane

In the small intestine of the rabbit the process of Na+-dependent uptake of phosphate occurs only at the brush-border of duodenal enterocytes. Li+ can replace Na+. The process is activated when either K+, Cs+, Rb+, or choline is present in the intravesicular space. The presence of membrane-permeable anions is essential for maximum rates of phosphate transport. We conclude that the mechanism of the phosphate carrier is electrogenic at pH 6-8, probably two Na+ moving with each H2PO4-. This will lead to the development of a positive charge within the vesicle. The variation of the Km for H2PO4- with pH is thought to be the consequence of the affinity of the carrier protein for H2PO4- increasing as the pH increases. Polyclonal antibodies against membrane vesicles isolated from rabbit duodenum, jejunum, and ileum were prepared. The antibodies raised against the ileum and jejunum both activated the phosphate transport process, while the anti-duodenum antibody preparation inhibited phosphate transport.