Insulin secretion profiles are modified by overexpression of glutamate dehydrogenase in pancreatic islets.
OBJECTIVE
Glutamate dehydrogenase (GDH) is a mitochondrial enzyme playing a key role in the control of insulin secretion. However, it is not known whether GDH expression levels in beta cells are rate-limiting for the secretory response to glucose. GDH also controls glutamine and glutamate oxidative metabolism, which is only weak in islets if GDH is not allosterically activated by L-leucine or (+/-)-2-aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH).


METHODS
We constructed an adenovirus encoding for GDH to overexpress the enzyme in the beta-cell line INS-1E, as well as in isolated rat and mouse pancreatic islets. The secretory responses to glucose and glutamine were studied in static and perifusion experiments. Amino acid concentrations and metabolic parameters were measured in parallel.


RESULTS
GDH overexpression in rat islets did not change insulin release at basal or intermediate glucose (2.8 and 8.3 mmol/l respectively), but potentiated the secretory response at high glucose concentrations (16.7 mmol/l) compared to controls (+35%). Control islets exposed to 5 mmol/l glutamine at basal glucose did not increase insulin release, unless BCH was added with a resulting 2.5-fold response. In islets overexpressing GDH glutamine alone stimulated insulin secretion (2.7-fold), which was potentiated 2.2-fold by adding BCH. The secretory responses evoked by glutamine under these conditions correlated with enhanced cellular metabolism.


CONCLUSIONS
GDH could be rate-limiting in glucose-induced insulin secretion, as GDH overexpression enhanced secretory responses. Moreover, GDH overexpression made islets responsive to glutamine, indicating that under physiological conditions this enzyme acts as a gatekeeper to prevent amino acids from being inappropriate efficient secretagogues.
Carobbio
S
S
Division of Clinical Biochemistry, University Medical Center, Geneva, Switzerland.
Ishihara
H
H
Fernandez-Pascual
S
S
Bartley
C
C
Martin-Del-Rio
R
R
Maechler
P
P
D000255:Adenosine Triphosphate / Q000378:metabolism; D000596:Amino Acids / Q000187:drug effects / Q000378:metabolism; D000598:Amino Acids, Cyclic / Q000494:pharmacology; D000818:Animals; D015153:Blotting, Western; D002460:Cell Line; D045744:Cell Line, Tumor; D005947:Glucose / Q000494:pharmacology; D005969:Glutamate Dehydrogenase / Q000378:metabolism*; D005973:Glutamine / Q000494:pharmacology; D006801:Humans; D007150:Immunohistochemistry; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D007930:Leucine / Q000031:analogs & derivatives / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008807:Mice, Inbred BALB C; D008928:Mitochondria / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism; D010084:Oxidation-Reduction / Q000187:drug effects; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar
D000596:Amino Acids; D000598:Amino Acids, Cyclic; D007328:Insulin; D005973:Glutamine; C006504:2-aminobicyclo(2,2,1)heptane-2-carboxylic acid; D011189:Potassium Chloride; D000255:Adenosine Triphosphate; D005969:Glutamate Dehydrogenase; D007930:Leucine; D005947:Glucose