Induction of c-Myc expression suppresses insulin gene transcription by inhibiting NeuroD/BETA2-mediated transcriptional activation.
Insulin biosynthesis and secretion are critical for pancreatic beta-cell function, but both are impaired under diabetic conditions. We have found that hyperglycemia induces the expression of the basic helix-loop-helix transcription factor c-Myc in islets in several different diabetic models. To examine the possible implication of c-Myc in beta-cell dysfunction, c-Myc was overexpressed in isolated rat islets using adenovirus. Adenovirus-mediated c-Myc overexpression suppressed both insulin gene transcription and glucose-stimulated insulin secretion. Insulin protein content, determined by immunostaining, was markedly decreased in c-Myc-overexpressing cells. In gel-shift assays c-Myc bound to the E-box in the insulin gene promoter region. Furthermore, in betaTC1, MIN6, and HIT-T15 cells and primary rat islets, wild type insulin gene promoter activity was dramatically decreased by c-Myc overexpression, whereas the activity of an E-box mutated insulin promoter was not affected. In HeLa and HepG2 cells c-Myc exerted a suppressive effect on the insulin promoter activity only in the presence of NeuroD/BETA2 but not PDX-1. Both c-Myc and NeuroD can bind the E-box element in the insulin promoter, but unlike NeuroD, the c-Myc transactivation domain lacked the ability to activate insulin gene expression. Additionally p300, a co-activator of NeuroD, did not function as a co-activator of c-Myc. In conclusion, increased expression of c-Myc in beta-cells suppresses the insulin gene transcription by inhibiting NeuroD-mediated transcriptional activation. This mechanism may explain some of the beta-cell dysfunction found in diabetes.
Kaneto
Hideaki
H
Section on Islet Transplantation and Cell Biology, Joslin Diabetes Center, One Joslin Place, Boston, MA 02215, USA. kaneto@osb.att.ne.jp
Sharma
Arun
A
Suzuma
Kiyoshi
K
Laybutt
D Ross
DR
Xu
Gang
G
Bonner-Weir
Susan
S
Weir
Gordon C
GC
D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D001483:Base Sequence; D051792:Basic Helix-Loop-Helix Proteins; D017931:DNA Primers; D004268:DNA-Binding Proteins / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D007328:Insulin / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism; D008297:Male; D011401:Promoter Regions, Genetic; D016271:Proto-Oncogene Proteins c-myc / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015534:Trans-Activators / Q000502:physiology*; D014157:Transcription Factors; D015533:Transcriptional Activation / Q000502:physiology*
D051792:Basic Helix-Loop-Helix Proteins; D017931:DNA Primers; D004268:DNA-Binding Proteins; D007328:Insulin; D016271:Proto-Oncogene Proteins c-myc; D015534:Trans-Activators; D014157:Transcription Factors; C093970:NEUROD1 protein, human; C490555:Neurod1 protein, rat