Tài liệu Báo cáo khoa học: Glucose sensing in the intestinal epithelium pdf

Glucose sensing in the intestinal epithelium

Jane Dyer1,*, Steven Vayro1,*, Timothy P. King2 and Soraya P. Shirazi-Beechey1

1

Epithelial Function and Development Group, Department of Veterinary Preclinical Sciences, University of Liverpool, England, UK;

2

Rowett Research Institute, Aberdeen, Scotland, UK

Dietary sugars regulate expression of the intestinal Na+/

glucose cotransporter, SGLT1, in many species. Using sheep

intestine as a model, we showed that lumenal monosaccha￾rides, both metabolisable and nonmetabolisable, regulate

SGLT1 expression. This regulation occurs not only at the

level of transcription, but also at the post-transcriptional

level. Introduction of D-glucose andsome D-glucose ana￾logues into ruminant sheep intestine resultedin > 50-fold

enhancement of SGLT1 expression. We aimedto determine

if transport of sugar into the enterocytes is requiredfor

SGLT1 induction, and delineate the signal-transduction

pathways involved.

A membrane impermeable D-glucose analogue, di(glucos￾6-yl)poly(ethylene glycol) 600, was synthesizedandinfused

into the intestines of ruminant sheep. SGLT1 expression was

determined using transport studies, Northern and Western

blotting, andimmunohistochemistry. An intestinal cell line,

STC-1, was usedto investigate the signalling pathways.

Intestinal infusion with di(glucos-6-yl)poly(ethylene gly￾col) 600 ledto induction of functional SGLT1, but the

compounddidnot inhibit Na+/glucose transport into

intestinal brush-border membrane vesicles. Studies using

cells showedthat increasedmedium glucose up-regulated

SGLT1 abundance and SGLT1 promoter activity, and

increasedintracellular cAMP levels. Glucose-inducedacti￾vation of the SGLT1 promoter was mimickedby the protein

kinase A (PKA) agonist, 8Br-cAMP, andwas inhibited

by H-89, a PKA inhibitor. Pertussis toxin, a G-protein

(Gi

)-specific inhibitor, enhancedSGLT1 protein abundance

to levels observedin response to glucose or 8Br-cAMP.

We conclude that lumenal glucose is sensed by a glucose

sensor, distinct from SGLT1, residing on the external face of

the lumenal membrane. The glucose sensor initiates a sig￾nalling pathway, involving a G-protein-coupledreceptor

linkedto a cAMP–PKA pathway resulting in enhancement

of SGLT1 expression.

Keywords: intestine; Na+/glucose cotransport; nutrient

transport; sugar sensing.

The dietary monosaccharides, D-glucose and D-galactose,

are transportedacross the brush-border membrane of

intestinal absorptive cells (enterocytes) by the Na+/glucose

cotransporter, SGLT1. It has been demonstrated that

lumenal glucose enhances the number of functional SGLT1

molecules in the intestinal brush-border membrane, and

that the metabolism of glucose is not requiredfor the

induction [1–5].

We have usedsheep intestine, which is an excellent model

system, for the study of monosaccharide regulation of

intestinal sugar transport [3,6]. We have shown that dietary

monosaccharides regulate the expression of intestinal brush￾border membrane Na+/glucose cotransporter at both the

transcriptional andpost-transcriptional levels [3,7,8].

In preruminant lambs (birth to 3 weeks), milk sugar lactose

is hydrolysed by the intestinal lactase into D-glucose and

D-galactose, andthese sugars are transportedby SGLT1.

Lambs are normally weanedat 3–10 weeks of age and, as

the diet changes from milk to grass, the rumen develops.

Dietary carbohydrates are fermented by rumen microflora

to short chain fatty acids, and under these conditions

negligible levels of monosaccharides reach the small intes￾tine [9,10]. Associatedwith the decline in lumenal sugars,

there is a decrease of over 50-fold in the levels of SGLT1

protein andmRNA [8]. Introduction of either D-glucose

or nonmetabolisable analogues of D-glucose, via duodenal

cannulae, into the intestinal lumenal contents of ruminant

sheep enhances the levels of functional SGLT1 protein and

mRNA to those detected in the preruminant state [4,8,11].

Intestinal infusions of D-glucose induced SGLT1 expression

in the brush-border membrane of enterocytes just below the

crypt–villus junction, with SGLT1 expression spreading to

the villus tip, with cell migration along the crypt–villus axis

[4,12]. We clonedandcharacterizedthe ovine SGLT1

promoter [13], andusing intestinal STC-1 cells as a suitable

in vitro model [8], we identified (a) the basal SGLT1

promoter, (b) a glucose-responsive element within the

promoter, and(c) a sugar-inducedtranscription factor

involvedin the transcriptional regulation of SGLT1 [8].

In this study, we set out to assess if the transport of sugar

across the brush-border membrane into the enterocyte is

requiredfor enhancement in the expression of intestinal

Correspondence to S. P. Shirazi-Beechey, Epithelial Function and

Development Group, Department of Veterinary Preclinical Sciences,

University of Liverpool, Brownlow Hill, Liverpool L69 7ZJ, UK.

Fax: + 44 (0) 151 794 4244, Tel.: + 44 (0) 151 794 4255,

E-mail: [email protected]

Abbreviations: GPCR, G-protein coupledreceptor; H-89, N-[2-

(p-bromocinnamylamino)-ethyl]-5-isoquinolinesulfonamide; PKA,

protein kinase A; SGLT1, Na+/glucose cotransporter; BBMV,

brush-border membrane vesicle; PEG, poly(ethylene glycol).

*Note: These two authors contributedequally to this work.

Note: A web site is available at http://www.liv.ac.uk/efdg

(Received11 April 2003, accepted16 June 2003)

Eur. J. Biochem. 270, 3377–3388 (2003)
FEBS 2003 doi:10.1046/j.1432-1033.2003.03721.x