Profound defects in pancreatic β-cell function in mice with combined heterozygous mutations in Pdx-1 , Hnf-1 α, and Hnf-3 β

Abstract

Defects in pancreatic ␤-cell function contribute to the development of type 2 diabetes, a polygenic disease that is characterized by insulin resistance and compromised insulin secretion. Hepatocyte nuclear factors (HNFs) -1␣, -3␤, -4␣, and Pdx-1 contribute in the complex transcriptional circuits within the pancreas that are involved in ␤-cell development and function. In mice, a heterozygous mutation in Pdx-1 alone, but not Hnf-1␣ ؉/؊ , Hnf-3␤ ؉/؊ , or Hnf-4␣ ؉/؊ , causes impaired glucose-stimulated insulin secretion in mice. To investigate the possible functional relationships between these transcription factors on ␤-cell activity in vivo, we generated mice with the following combined heterozygous mutations: Pdx-, and Hnf-3␤ ؉/؊ ͞Hnf-4␣ ؉/؊ . The greatest loss in function was in combined heterozygous null alleles of Pdx-1 and Hnf-1␣ (Pdx-1 ؉/؊ ͞Hnf-1␣ ؉/؊ ), or Pdx-1 and Hnf-3␤ (Pdx-1 ؉/؊ ͞Hnf-3␤ ؉/؊ ). Both double mutants develop progressively impaired glucose tolerance and acquire a compromised first-and second-phase insulin secretion profile in response to glucose compared with Pdx-1 ؉/؊ mice alone. The loss in ␤-cell function in Pdx-1 ؉/؊ ͞Hnf-3␤ ؉/؊ mice was associated with decreased expression of Nkx-6.1, glucokinase (Gck), aldolase B (aldo-B), and insulin, whereas Nkx2.2, Nkx-6.1, Glut-2, Gck, aldo-B, the liver isoform of pyruvate kinase, and insulin expression was reduced in Pdx-1 ؉/؊ ͞Hnf-1␣ ؉/؊ mice. The islet cell architecture was also abnormal in Pdx-1 ؉/؊ ͞Hnf-3␤ ؉/؊ and Pdx-1 ؉/؊ ͞Hnf-1␣ ؉/؊ mice, with glucagon-expressing cells scattered throughout the islet, a defect that may be connected to decreased E-cadherin expression. Our data suggest that functional interactions between key islet regulatory factors play an important role in maintaining islet architecture and ␤-cell function. These studies also established polygenic mouse models for investigating the mechanisms contributing to ␤-cell dysfunction in diabetes.hepatocyte nuclear factors ͉ diabetes mellitus ͉ insulin secretion ͉ pancreatic islets G enetic and biochemical studies in humans and mice have identified many key transcription factors that are involved in controlling pancreas development and ␤Ϫcell function in the adult islet (see ref. 1 for review). These factors have in common an islet-enriched expression pattern during pancreatic development, although each [e.g., hepatocyte nuclear factors (HNFs)] are also expressed in extrapancreatic tissues (e.g., intestine, liver, brain͞neurons, and kidney). Heterozygous mutations in a subset of these genes, including HNF-4␣, HNF-1␣, PDX-1, HNF-1␤, and BETA2, are specifically associated with a form of type 2 diabetes, termed maturity-onset diabetes of the young (MODY), that is characterized by an early disease onset (usually Ͻ25 years), autosomal dominant inheritance, and a primary defect in insulin secretion (see ref. 2 for review). There is increasing evidence that these and other islet-enriched transcription factors (e.g., Nkx2.2, Nkx6

Pdx1 Expression in Irs2-deficient Mouse β-Cells Is Regulated in a Strain-dependent Manner

“…Expression of Pdx-1-regulating Transcription Factor Genes in Irs2 Ϫ/Ϫ Murine ␤-Cells-HNF3␤ and HNF1␣ reportedly regulate the expression of Pdx-1 in vivo and in vitro (33). Contrary to a previous report (37), real time quantitative PCR study revealed Hnf3␤ and Hnf1␣ mRNA expressions to not be decreased when ␤-actin was selected as an internal control (Fig.…”

“…Ϫ/Ϫ Murine ␤-Cells-Pdx-1 regulates the expressions of various pancreatic genes, including insulin, glucose transporter type 2 (Glut2) (29,41,42), and Nkx6.1 (32,33). The immunohistochemical results with anti-insulin, anti-Glut2, and anti-Nkx6.1 antibodies showed expression of these three proteins to be well preserved in Irs2 Ϫ/Ϫ murine islets, consistent with our previous observation that both glucose-induced insulin secretion and insulin contents were preserved (11) (Figs.…”

Control of beta cell function and proliferation in mice stimulated by small-molecule glucokinase activator under various conditions

“…Since Kushner et al reported that transgenic overexpression of Pdx1 restored beta cell mass in Irs2 −/− mice [28], Pdx1 may play a role in regulating beta cell mass. Nevertheless, because a haploinsufficiency of Pdx1 led to impaired beta cell function, but not to decreased beta cell mass [30], and because beta cell mass was smaller in our Irs2 −/− mice than in wild-type mice despite the Pdx1 expression level being maintained [29], two different pathways are involved in the stimulation of beta cell function and proliferation by a GKA.…”

Glucose Regulates Insulin Gene Transcription by Hyperacetylation of Histone H4

“…The expression of GLUT-2 has been shown to be up-regulated by glucose in pancreatic ␤ cells (4 -6). The regulation of glucokinase gene expression by glucose remains unclear (7,8); however, several of the transcription factors required for its expression are also involved in glucose regulation of gene expression (9).…”