title: Induction of Mad expression leads to augmentation of insulin gene transcription.
abstract: Insulin gene transcription is critical for the maintenance of pancreatic beta-cell differentiation and insulin production. In this study, we found that the basic helix-loop-helix transcription factor Mad, which usually acts as a repressor to c-Myc, enhances insulin gene transcription. In isolated rat islets adenoviral overexpression of Mad augmented insulin mRNA expression and insulin protein content, as well as glucokinase and GLUT2 mRNA expression. Also, Mad overexpression upregulated insulin promoter activity in beta-cell-derived cell lines, MIN6 and betaTC1, as well as in non-insulin producing liver cell line, HepG2. Mad overexpression in rat islets enhanced PDX-1 expression and its DNA binding activity. We found that Mad mediated increased PDX-1 expression by an E-box dependent transcriptional regulation of the PDX-1 gene. That the effects of Mad on insulin expression were mediated through PDX-1 was further substantiated by studies showing inhibition of insulin promoter activation by Mad in the presence of mutated PDX-1 binding site. Although Mad functions as a negative regulatory factor for multiple target genes, these studies establish the fact that Mad can also function as a positive regulatory factor for insulin gene transcription. Such regulation of insulin expression by Mad with modulation of PDX-1 expression and DNA binding activity could offer useful therapeutic and/or experimental tools to promote insulin production in appropriate cell types.
authors.affiliation: Section of Islet Transplantation and Cell Biology, Joslin Diabetes Center, Boston, MA 02215, USA.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Patané
authors.forename: Hideaki
authors.initials: H
authors.lastname: Kaneto
authors.forename: Elena
authors.initials: E
authors.lastname: Toschi
authors.forename: Arun
authors.initials: A
authors.lastname: Sharma
authors.forename: Sanjeev
authors.initials: S
authors.lastname: Gupta
authors.forename: Gordon C
authors.initials: GC
authors.lastname: Weir
mesh_terms: D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D002460:Cell Line; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D005822:Genetic Vectors; D018398:Homeodomain Proteins; D007328:Insulin / Q000096:biosynthesis / Q000235:genetics*; D007515:Islets of Langerhans / Q000166:cytology / Q000378:metabolism*; D008297:Male; D011401:Promoter Regions, Genetic; D051381:Rats; D017207:Rats, Sprague-Dawley; D012097:Repressor Proteins; D015534:Trans-Activators / Q000378:metabolism; D015533:Transcriptional Activation
chemical_list: D004268:DNA-Binding Proteins; D018398:Homeodomain Proteins; D007328:Insulin; D012097:Repressor Proteins; D015534:Trans-Activators; C084010:pancreatic and duodenal homeobox 1 protein