Predominant role of active versus facilitative glucose transport for glucagon-like peptide-1 secretion.
OBJECTIVE
Several glucose-sensing pathways have been implicated in glucose-triggered secretion of glucagon-like peptide-1 (GLP-1) from intestinal L cells. One involves glucose metabolism and closure of ATP-sensitive K(+) channels, and another exploits the electrogenic nature of Na(+)-coupled glucose transporters (SGLTs). This study aimed to elucidate the role of these distinct mechanisms in glucose-stimulated GLP-1 secretion.


METHODS
Glucose uptake into L cells (either GLUTag cells or cells in primary cultures, using a new transgenic mouse model combining proglucagon promoter-driven Cre recombinase with a ROSA26tdRFP reporter) was monitored with the FLII(12)Pglu-700 μδ6 glucose sensor. Effects of pharmacological and genetic interference with SGLT1 or facilitative glucose transport (GLUT) on intracellular glucose accumulation and metabolism (measured by NAD(P)H autofluorescence), cytosolic Ca(2+) (monitored with Fura2) and GLP-1 secretion (assayed by ELISA) were assessed.


RESULTS
L cell glucose uptake was dominated by GLUT-mediated transport, being abolished by phloretin but not phloridzin. NAD(P)H autofluorescence was glucose dependent and enhanced by a glucokinase activator. In GLUTag cells, but not primary L cells, phloretin partially impaired glucose-dependent secretion, and suppressed an amplifying effect of glucose under depolarising high K(+) conditions. The key importance of SGLT1 in GLUTag and primary cells was evident from the impairment of secretion by phloridzin or Sglt1 knockdown and failure of glucose to trigger cytosolic Ca(2+) elevation in primary L cells from Sglt1 knockout mice.


CONCLUSIONS
SGLT1 acts as the luminal glucose sensor in L cells, but intracellular glucose concentrations are largely determined by GLUT activity. Although L cell glucose metabolism depends partially on glucokinase activity, this plays only a minor role in glucose-stimulated GLP-1 secretion.
Cambridge Institute for Medical Research, Wellcome Trust/MRC Building, Addenbrooke's Hospital, Box 139, Hills Road, Cambridge CB2 0XY, UK.
H E
HE
Parker
A
A
Adriaenssens
G
G
Rogers
P
P
Richards
H
H
Koepsell
F
F
Reimann
F M
FM
Gribble
D000818:Animals; D001692:Biological Transport; D002460:Cell Line; D002478:Cells, Cultured; D019858:Enteroendocrine Cells / Q000378:metabolism*; D005434:Flow Cytometry; D005786:Gene Expression Regulation; D005934:Glucagon / Q000378:metabolism; D052216:Glucagon-Like Peptide 1 / Q000378:metabolism*; D007150:Immunohistochemistry; D007422:Intestines / Q000473:pathology*; D054086:KATP Channels / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D010693:Phloretin / Q000494:pharmacology*; D051269:Sodium-Glucose Transporter 1 / Q000378:metabolism*
D054086:KATP Channels; C495675:Slc5a1 protein, mouse; D051269:Sodium-Glucose Transporter 1; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D010693:Phloretin