Loss-of-function mutation of the GPR40 gene associates with abnormal stimulated insulin secretion by acting on intracellular calcium mobilization.
BACKGROUND
Free fatty acids (FFAs) acutely stimulate but chronically impair glucose-stimulated insulin secretion from beta-cells. The G protein-coupled transmembrane receptor 40 (GPR40) mediates both acute and chronic effects of FFAs on insulin secretion and plays a role in glucose homeostasis. Limited information is available on the effect of GPR40 genetic abnormalities on insulin secretion and metabolic regulation in human subjects.


RESULTS
For in vivo studies, we screened 734 subjects for the coding region of GPR40 and identified a new single-nucleotide mutation (Gly180Ser). The mean allele frequency was 0.75%, which progressively increased (P < 0.05) from nonobese subjects (0.42%) to moderately obese (body mass index = 30-39.9 kg/m2, 1.07%) and severely obese patients (body mass index > or = 40 kg/m2, 2.60%). The relationship between the GPR40 mutation, insulin secretion, and metabolic alterations was studied in 11 Gly/Ser mutation carriers. In these subjects, insulin secretion (insulinogenic index derived from oral glucose tolerance test) was significantly lower than in 692 Gly/Gly carriers (86.0 +/- 48.2 vs. 183.7 +/- 134.4, P < 0.005). Moreover, a case-control study indicated that plasma insulin and C-peptide responses to a lipid load were significantly (P < 0.05) lower in six Gly/Ser than in 12 Gly/Gly carriers. In vitro experiments in HeLa cells cotransfected with aequorin and the mutated Gly/Ser GPR40 indicated that intracellular Ca2+ concentration increase after oleic acid was significantly lower than in Gly/Gly GPR40-transfected cells. This fact was confirmed using fura-2 acetoxymethyl ester.


CONCLUSIONS
This newly identified GPR40 variant results in a loss of function that prevents the beta-cell ability to adequately sense lipids as an insulin secretory stimulus because of impaired intracellular Ca2+ concentration increase.
Vettor
Roberto
R
Endocrine-Metabolic Laboratory, Internal Medicine, Department of Medical and Surgical Sciences, University of Padova, via Ospedale, 105, I-35128 Padova, Italy. roberto.vettor@unipd.it
Granzotto
Marnie
M
De Stefani
Diego
D
Trevellin
Elisabetta
E
Rossato
Marco
M
Farina
Maria Grazia
MG
Milan
Gabriella
G
Pilon
Catia
C
Nigro
Angela
A
Federspil
Giovanni
G
Vigneri
Riccardo
R
Vitiello
Libero
L
Rizzuto
Rosario
R
Baratta
Roberto
R
Frittitta
Lucia
L
D000328:Adult; D002118:Calcium / Q000378:metabolism*; D004252:DNA Mutational Analysis; D005260:Female; D005787:Gene Frequency; D008040:Genetic Linkage; D005838:Genotype; D005951:Glucose Tolerance Test; D006367:HeLa Cells; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000378:metabolism / Q000473:pathology; D007424:Intracellular Fluid / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D020125:Mutation, Missense / Q000502:physiology; D009765:Obesity / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D043562:Receptors, G-Protein-Coupled / Q000235:genetics* / Q000502:physiology; D014162:Transfection
C473034:FFAR1 protein, human; D007328:Insulin; D043562:Receptors, G-Protein-Coupled; D002118:Calcium