Transcriptional Regulation of the HMGA1 Gene by Octamer-Binding Proteins Oct-1 and Oct-2

Abstract

The High-Mobility Group AT-Hook 1 (HMGA1) protein is an architectural transcription factor that binds to AT-rich sequences in the promoter region of DNA and functions as a specific cofactor for gene activation. Previously, we demonstrated that HMGA1 is a key regulator of the insulin receptor (INSR) gene and an important downstream target of the INSR signaling cascade. Moreover, from a pathogenic point of view, overexpression of HMGA1 has been associated with human cancer, whereas functional variants of the HMGA1 gene have been recently linked to type 2 diabetes mellitus and metabolic syndrome. However, despite of this biological and pathological relevance, the mechanisms that control HMGA1 gene expression remain unknown. In this study, to define the molecular mechanism(s) that regulate HMGA1 gene expression, the HMGA1 gene promoter was investigated by transient transfection of different cell lines, either before or after DNA and siRNA cotransfections. An octamer motif was identified as an important element of transcriptional regulation of this gene, the interaction of which with the octamer transcription factors Oct-1 and Oct-2 is crucial in modulating HMGA1 gene and protein expression. Additionally, we demonstrate that HMGA1 binds its own promoter and contributes to its transactivation by Oct-2 (but not Oct-1), supporting its role in an auto-regulatory circuit. Overall, our results provide insight into the transcriptional regulation of the HMGA1 gene, revealing a differential control exerted by both Oct-1 and Oct-2. Furthermore, they consistently support the hypothesis that a putative defect in Oct-1 and/or Oct-2, by affecting HMGA1 expression, may cause INSR dysfunction, leading to defects of the INSR signaling pathway.

Transcriptional Regulation of Glucose Metabolism: The Emerging Role of the HMGA1 Chromatin Factor

“…Also, the HMGA1 promoter is activated by the transforming growth factor-β1 (TGF-β1) ( 29 ). Recently, we identified an octamer motif ATGCAAAT at the beginning of exon 1, where the octamer transcription factors Oct-1 and Oct-2 exert a differential regulation of HMGA1 gene transcription, and demonstrated that, by binding its own promoter, HMGA1 can contribute to its transactivation by Oct-2 ( 30 ), thus supporting a previous observation about the role of HMGA1 in an auto-regulatory circuit ( 31 ). More recently, it has been reported that G9a, an activator of gene transcription and a histone methyltransferase, positively regulates the expression of the HMGA1 gene in hepatic cells ( 32 ).…”

Different N-terminal isoforms of Oct-1 control expression of distinct sets of genes and their high levels in Namalwa Burkitt's lymphoma cells affect a wide range of cellular processes

“…The Oct-1X is expressed at a low level comparatively to Oct-1A. This is an important fact, since at present time this isoform is considered as a ‘canonical’ Oct-1 sequence and is used in the majority of studies devoted to investigation of Oct-1 functions (14,23,25,40–46). …”

HMGA1 is a novel transcriptional regulator of the FoxO1 gene

“…Nuclear protein extracts were prepared as previously [ 29 ] and final protein concentration in the extracts was determined by the modified Bradford method (Bio-Rad Laboratories, Hercules, CA, USA). The antibodies used for Western blot studies were: anti-HMGA1 [ 41 ], anti-FoxO1 (Santa Cruz Biotechnology, Santa Cruz, CA) and anti Sp1 [ 42 ].…”