Abstract:PurposeThe forkhead transcription factor (FoxO1) is a master transcriptional regulator of fundamental cellular processes ranging from cell proliferation and differentiation to inflammation and metabolism. However, despite its relevance, the mechanism(s) underlying FoxO1 gene regulation are largely unknown. We have previously shown that the chromatin factor high-mobility group A1 (HMGA1) plays a key role in the transcriptional regulation of glucose-responsive genes, including some that are involved in FoxO1-med… Show more
“…In these experiments, mRNA expression of FoxO1 and Igfbp1 was measured in primary cultured cells that were pretreated with the casein kinase (CK) 2 inhibitor 4,5,6,7-tetrabromo-1H-benzimidazole (TBB), which selectively blocks insulin-induced HMGA1 phosphorylation, and the subsequent detachment of HMGA1 from DNA 23 , without affecting FoxO1, whose phosphorylation is, instead, dependent on CK1 and several other protein kinases that alter subcellular location of FoxO1, DNA binding and transcriptional activity 42 – 45 . Consistent with previous observations from our group 23 , 40 , 46 , mRNA expression of FoxO1 and Igfbp1 was lower in primary cultured Hmga1 −/− hepatocytes than in wild-type hepatocytes (Fig. 6 ).…”
Section: Resultssupporting
confidence: 92%
“…Consistent with the above observations, fasting glycemia was significantly lower in Hmga1 -null mice, compared to wild-type animals (Fig. 9 ), confirming the results obtained in previous studies using a large number of both wild-type and Hmga1 -mutant mice 40 , 46 , 47 . Figure 8 FoxO1 occupancy at the endogenous IGFBP1 locus in vivo , in normal and Hmga1 mutant mice.…”
Section: Resultssupporting
confidence: 91%
“…In this regard, we previously reported that the counter-regulatory hormone glucagon, which acts in opposition to insulin to maintain fasting euglycemia, upregulated HMGA1 expression via the cAMP pathway, both in vitro and in vivo in whole mice 47 , 48 . Consistent with these previous findings, and with the observation that HMGA1 activates FoxO1 gene expression 46 , upregulation of FoxO1 via the glucagon-cAMP-PKA signaling has been reported in liver of fasting mice to maintain fasting euglycemia 49 . Based on our findings here, it is likely that upregulation of HMGA1 during fasting (when glucagon peaks) may contribute to the maintenance of fasting euglycemia through two distinct but converging mechanisms: by increasing FoxO1 expression in response to glucagon, and by directly and dinamically interacting with FoxO1, thus playing an important role in fine-tuning the activation of FoxO1’s transcriptional activity.…”
As a mediator of insulin-regulated gene expression, the FoxO1 transcription factor represents a master regulator of liver glucose metabolism. We previously reported that the high-mobility group AT-hook 1 (HMGA1) protein, a molecular switch for the insulin receptor gene, functions also as a downstream target of the insulin receptor signaling pathway, representing a critical nuclear mediator of insulin function. Here, we investigated whether a functional relationship existed between FoxO1 and HMGA1, which might help explain insulin-mediated gene transcription in the liver. To this end, as a model study, we investigated the canonical FoxO1-HMGA1-responsive IGFBP1 gene, whose hepatic expression is regulated by insulin. By using a conventional GST-pull down assay combined with co-immunoprecipitation and Fluorescence Resonance Energy Transfer (FRET) analyses, we provide evidence of a physical interaction between FoxO1 and HMGA1. Further investigation with chromatin immunoprecipitation, confocal microscopy, and Fluorescence Recovery After Photobleaching (FRAP) technology indicated a functional significance of this interaction, in both basal and insulin-stimulated states, providing evidence that, by modulating FoxO1 transactivation, HMGA1 is essential for FoxO1-induced IGFBP1 gene expression, and thereby a critical modulator of insulin-mediated FoxO1 regulation in the liver. Collectively, our findings highlight a novel FoxO1/HMGA1-mediated mechanism by which insulin may regulate gene expression and metabolism.
“…In these experiments, mRNA expression of FoxO1 and Igfbp1 was measured in primary cultured cells that were pretreated with the casein kinase (CK) 2 inhibitor 4,5,6,7-tetrabromo-1H-benzimidazole (TBB), which selectively blocks insulin-induced HMGA1 phosphorylation, and the subsequent detachment of HMGA1 from DNA 23 , without affecting FoxO1, whose phosphorylation is, instead, dependent on CK1 and several other protein kinases that alter subcellular location of FoxO1, DNA binding and transcriptional activity 42 – 45 . Consistent with previous observations from our group 23 , 40 , 46 , mRNA expression of FoxO1 and Igfbp1 was lower in primary cultured Hmga1 −/− hepatocytes than in wild-type hepatocytes (Fig. 6 ).…”
Section: Resultssupporting
confidence: 92%
“…Consistent with the above observations, fasting glycemia was significantly lower in Hmga1 -null mice, compared to wild-type animals (Fig. 9 ), confirming the results obtained in previous studies using a large number of both wild-type and Hmga1 -mutant mice 40 , 46 , 47 . Figure 8 FoxO1 occupancy at the endogenous IGFBP1 locus in vivo , in normal and Hmga1 mutant mice.…”
Section: Resultssupporting
confidence: 91%
“…In this regard, we previously reported that the counter-regulatory hormone glucagon, which acts in opposition to insulin to maintain fasting euglycemia, upregulated HMGA1 expression via the cAMP pathway, both in vitro and in vivo in whole mice 47 , 48 . Consistent with these previous findings, and with the observation that HMGA1 activates FoxO1 gene expression 46 , upregulation of FoxO1 via the glucagon-cAMP-PKA signaling has been reported in liver of fasting mice to maintain fasting euglycemia 49 . Based on our findings here, it is likely that upregulation of HMGA1 during fasting (when glucagon peaks) may contribute to the maintenance of fasting euglycemia through two distinct but converging mechanisms: by increasing FoxO1 expression in response to glucagon, and by directly and dinamically interacting with FoxO1, thus playing an important role in fine-tuning the activation of FoxO1’s transcriptional activity.…”
As a mediator of insulin-regulated gene expression, the FoxO1 transcription factor represents a master regulator of liver glucose metabolism. We previously reported that the high-mobility group AT-hook 1 (HMGA1) protein, a molecular switch for the insulin receptor gene, functions also as a downstream target of the insulin receptor signaling pathway, representing a critical nuclear mediator of insulin function. Here, we investigated whether a functional relationship existed between FoxO1 and HMGA1, which might help explain insulin-mediated gene transcription in the liver. To this end, as a model study, we investigated the canonical FoxO1-HMGA1-responsive IGFBP1 gene, whose hepatic expression is regulated by insulin. By using a conventional GST-pull down assay combined with co-immunoprecipitation and Fluorescence Resonance Energy Transfer (FRET) analyses, we provide evidence of a physical interaction between FoxO1 and HMGA1. Further investigation with chromatin immunoprecipitation, confocal microscopy, and Fluorescence Recovery After Photobleaching (FRAP) technology indicated a functional significance of this interaction, in both basal and insulin-stimulated states, providing evidence that, by modulating FoxO1 transactivation, HMGA1 is essential for FoxO1-induced IGFBP1 gene expression, and thereby a critical modulator of insulin-mediated FoxO1 regulation in the liver. Collectively, our findings highlight a novel FoxO1/HMGA1-mediated mechanism by which insulin may regulate gene expression and metabolism.
“…As a downstream target of the PI3K/Akt insulin signaling pathway [20] , HMGA1 is expected to undergo serine phosphorylation and a reduced ability to bind to IR DNA in vanadate-treated hepatoma cells. Interestingly by modulating FoxO1 binding to DNA and FoxO1 transactivation, HMGA1 has recently been shown to be essential for FoxO1-induced expression of the IGFBP-1 gene in liver cells, and thus identified as an important factor in modulating FoxO1 activity in the context of insulin signaling [37] , [38] . Also supporting a potential involvement of C/EBPβ, vanadate treatment has been shown to decrease, in vitro , C/EBPβ mRNA in Fao hepatoma cells (Bortoli S, unpublished observations) and to reverse, in vivo, the overexpression of C/EBPβ mRNA in liver of STZ-induced diabetic rats [39] .…”
Vanadate, a protein tyrosine phosphatase inhibitor which elicits insulin-like effects, has previously been shown to inhibit expression of the insulin receptor gene at the transcriptional level in rat hepatoma cells. In an attempt to identify the DNA sequence and transcription factors potentially involved in this effect, a fragment of the proximal 5′flanking region of the IR gene (−1143/−252 upstream the ATG codon) has been cloned and functionally characterized. RNase protection allowed the identification of several transcription start sites in the conserved region of the gene, among which two major sites at −455 and −396. Upon fusion to the luciferase gene and transient transfection into hepatoma cells, the −1143/−252 fragment showed promoter activity. This was unaffected by deletion of the −1143/−761 sequence, but markedly decreased (90%) by additional deletion of the −760/−465 sequence. Treatment of hepatoma cells with vanadate led to a dose-dependent decrease in promoter activity of the 1143/−252, −760/−252 and −464/−252 constructs (change relative to untreated cells, 40, 55 and 23% at 125 μM, and 70, 85 and 62% at 250 μM, respectively). These data suggest that although the entire DNA sequence upstream the transcription start sites is probably involved in vanadate-induced inhibition, the short sequence downstream of position −464 and is sufficient for inhibition. Potential targets of vanadate are the transcription factors FoxO1 and HMGA1, two downstream targets of the insulin signaling pathway which have been shown to mediate the inhibitory effect of insulin on IR gene expression.
“…However, given that substantial interspecies differences exist in pancreatic islet development and function ( 19 , 97 , 98 ), any parallelism between human and mouse at this level must be considered carefully and further details on this should be provided. For example, based on our recent observations highlighting a novel relationship between HMGA1 and FoxO1 ( 99 ), further investigation in this field could deliver deeper information on the possibility that an interplay among HMGA1 and FoxO1 can be a component of this regulation, as an overarching role of FoxO1 in pancreatic beta cell function has been already outlined ( 6 , 100 – 103 ).…”
HMGA1 (high mobility group A1) is a nonhistone architectural chromosomal protein that functions mainly as a dynamic regulator of chromatin structure and gene transcription. As such, HMGA1 is involved in a variety of fundamental cellular processes, including gene expression, epigenetic regulation, cell differentiation and proliferation, as well as DNA repair. In the last years, many reports have demonstrated a role of HMGA1 in the transcriptional regulation of several genes implicated in glucose homeostasis. Initially, it was proved that HMGA1 is essential for normal expression of the insulin receptor (INSR), a critical link in insulin action and glucose homeostasis. Later, it was demonstrated that HMGA1 is also a downstream nuclear target of the INSR signaling pathway, representing a novel mediator of insulin action and function at this level. Moreover, other observations have indicated the role of HMGA1 as a positive modulator of the Forkhead box protein O1 (FoxO1), a master regulatory factor for gluconeogenesis and glycogenolysis, as well as a positive regulator of the expression of insulin and of a series of circulating proteins that are involved in glucose counterregulation, such as the insulin growth factor binding protein 1 (IGFBP1), and the retinol binding protein 4 (RBP4). Thus, several lines of evidence underscore the importance of HMGA1 in the regulation of glucose production and disposal. Consistently, lack of HMGA1 causes insulin resistance and diabetes in humans and mice, while variations in the HMGA1 gene are associated with the risk of type 2 diabetes and metabolic syndrome, two highly prevalent diseases that share insulin resistance as a common pathogenetic mechanism. This review intends to give an overview about our current knowledge on the role of HMGA1 in glucose metabolism. Although research in this field is ongoing, many aspects still remain elusive. Future directions to improve our insights into the pathophysiology of glucose homeostasis may include epigenetic studies and the use of “omics” strategies. We believe that a more comprehensive understanding of HMGA1 and its networks may reveal interesting molecular links between glucose metabolism and other biological processes, such as cell proliferation and differentiation.
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