Mutations in the promoter of adenylyl cyclase (AC)-III gene, overexpression of AC-III mRNA, and enhanced cAMP generation in islets from the spontaneously diabetic GK rat model of type 2 diabetes.
Glucose-induced insulin release is decreased in the spontaneously diabetic GK rat, a nonobese rodent model of type 2 diabetes. Forskolin restores the impaired insulin release in both the isolated perfused pancreas and isolated islets from these rats (Abdel-Halim et al., Diabetes 45:934-940, 1996). We demonstrate here that the insulinotropic effect of forskolin in the GK rat is due to increased generation of cAMP and that it is associated with overexpression of adenylyl cyclase (AC)-III mRNA and gene mutations. The AC-III mRNA overexpression was demonstrated by in situ hybridization using oligonucleotide probes binding to different regions of the rat AC-III mRNA. It was associated with the presence of two point mutations identified at positions -28 bp (A --> G) and -358 bp (A --> C) of the promoter region of the AC-III gene and was demonstrable in both GK rat islets and peripheral blood cells. Transfection of COS cells with a luciferase reporter gene system revealed up to 25-fold increased promoter activity of GK AC-III promoter when compared with normal rat promoter (P < 0.0001). In conclusion, forskolin restores the impaired insulin release in islets of the GK rat through enhanced cAMP generation. This is linked to overexpression of AC-III mRNA in GK islets due to two functional point mutations in the promoter region of the AC-III gene.
Abdel-Halim
S M
SM
Department of Molecular Medicine, Karolinska Hospital, Stockholm, Sweden. samy.abdel-halim@molmed.ki.se
Guenifi
A
A
He
B
B
Yang
B
B
Mustafa
M
M
Höjeberg
B
B
Hillert
J
J
Bakhiet
M
M
Efendić
S
S
D000262:Adenylyl Cyclases / Q000235:genetics*; D000818:Animals; D001483:Base Sequence; D015331:Cohort Studies; D000242:Cyclic AMP / Q000096:biosynthesis*; D017931:DNA Primers / Q000737:chemistry; D003924:Diabetes Mellitus, Type 2 / Q000201:enzymology / Q000235:genetics* / Q000473:pathology; D004195:Disease Models, Animal; D015971:Gene Expression Regulation, Enzymologic / Q000235:genetics*; D017403:In Situ Hybridization; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000737:chemistry* / Q000378:metabolism* / Q000473:pathology; D008297:Male; D009154:Mutation / Q000235:genetics*; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic / Q000235:genetics*; D012333:RNA, Messenger / Q000032:analysis / Q000235:genetics; D051381:Rats; D011919:Rats, Inbred Strains; D017208:Rats, Wistar
D017931:DNA Primers; D007328:Insulin; D012333:RNA, Messenger; D000242:Cyclic AMP; D000262:Adenylyl Cyclases