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The Polycomb protein, Bmi1, regulates insulin sensitivity.

Cannon CE, Titchenell PM, Groff DN, El Ouaamari A, Kulkarni RN, Birnbaum MJ, Stoffers DA - Mol Metab (2014)

Bottom Line: The Polycomb Repressive Complexes (PRC) 1 and 2 function to epigenetically repress target genes.The PRC1 component, Bmi1, plays a crucial role in maintenance of glucose homeostasis and beta cell mass through repression of the Ink4a/Arf locus.Glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamps were performed.

View Article: PubMed Central - PubMed

Affiliation: Institute for Diabetes, Obesity and Metabolism and the Division of Endocrinology, Diabetes and Metabolism, Department of Medicine, Perelman School of Medicine at the University of Pennsylvania, 3400 Civic Center Boulevard, Philadelphia, PA 19104, USA.

ABSTRACT

Objective: The Polycomb Repressive Complexes (PRC) 1 and 2 function to epigenetically repress target genes. The PRC1 component, Bmi1, plays a crucial role in maintenance of glucose homeostasis and beta cell mass through repression of the Ink4a/Arf locus. Here we have explored the role of Bmi1 in regulating glucose homeostasis in the adult animal, which had not been previously reported due to poor postnatal survival of Bmi1 (-/-) mice.

Methods: The metabolic phenotype of Bmi1 (+/-) mice was characterized, both in vivo and ex vivo. Glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamps were performed. The insulin signaling pathway was assessed at the protein and transcript level.

Results: Here we report a negative correlation between Bmi1 levels and insulin sensitivity in two models of insulin resistance, aging and liver-specific insulin receptor deficiency. Further, heterozygous loss of Bmi1 results in increased insulin sensitivity in adult mice, with no impact on body weight or composition. Hyperinsulinemic-euglycemic clamp reveals increased suppression of hepatic glucose production and increased glucose disposal rate, indicating elevated glucose uptake to peripheral tissues, in Bmi1 (+/-) mice. Enhancement of insulin signaling, specifically an increase in Akt phosphorylation, in liver and, to a lesser extent, in muscle appears to contribute to this phenotype.

Conclusions: Together, these data define a new role for Bmi1 in regulating insulin sensitivity via enhancement of Akt phosphorylation.

No MeSH data available.


Related in: MedlinePlus

Hepatic insulin signaling is enhanced by Bmi1 heterozygosity. (A–B) 15-week-old males fasted 5 h and then injected IP with saline or 1.125 U/kg insulin. Livers harvested 20 min later and lysates assessed by western blot. (A) Representative Western blots showing levels of phospho-Akt and total Akt. Ran immunoreactivity was used as the loading control. (B) Quantification of phospho- and total Akt levels. The ratio of phospho-Akt/total Akt is shown. n = 4–5 per genotype and treatment. *p < 0.05 vs Bmi1+/+ saline; **p < 0.05 vs Bmi1+/+ insulin. (C–D) Cultured hepatocytes isolated from 15-week-old males treated with indicated dose of insulin for 20 min prior to harvest. Lysates assessed by western blot. (C) Representative Western blots showing levels of phospho-Akt and total Akt. (D) Quantification of phospho- and total Akt levels. The ratio of phospho-Akt/total Akt is shown. Results shown obtained from n = 5 independent hepatocyte isolations per genotype. p < 0.001 by two-way ANOVA. *p < 0.05 vs WT; **p < 0.001 vs WT.
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fig4: Hepatic insulin signaling is enhanced by Bmi1 heterozygosity. (A–B) 15-week-old males fasted 5 h and then injected IP with saline or 1.125 U/kg insulin. Livers harvested 20 min later and lysates assessed by western blot. (A) Representative Western blots showing levels of phospho-Akt and total Akt. Ran immunoreactivity was used as the loading control. (B) Quantification of phospho- and total Akt levels. The ratio of phospho-Akt/total Akt is shown. n = 4–5 per genotype and treatment. *p < 0.05 vs Bmi1+/+ saline; **p < 0.05 vs Bmi1+/+ insulin. (C–D) Cultured hepatocytes isolated from 15-week-old males treated with indicated dose of insulin for 20 min prior to harvest. Lysates assessed by western blot. (C) Representative Western blots showing levels of phospho-Akt and total Akt. (D) Quantification of phospho- and total Akt levels. The ratio of phospho-Akt/total Akt is shown. Results shown obtained from n = 5 independent hepatocyte isolations per genotype. p < 0.001 by two-way ANOVA. *p < 0.05 vs WT; **p < 0.001 vs WT.

Mentions: After a 5 h fast to minimize stimulation by endogenous insulin, Bmi1+/+ and Bmi1+/− mice were injected IP with saline or insulin 20 min prior to sacrifice. Western blot analysis of liver lysates revealed no effect of either genotype or insulin treatment on total Akt levels (Figure 4A); however, Akt phosphorylation levels in heterozygous mice were lower at baseline as compared to controls, suggesting reduced basal insulin signaling in Bmi1+/− livers. This supports the observation from whole animal physiology that less insulin is needed to maintain normal glucose homeostasis in the Bmi1 heterozygous animals (Figure 2F). Upon insulin stimulation, Akt phosphorylation was higher in Bmi1+/− liver lysates, compared to controls, indicating enhanced signal transduction in response to insulin (Figure 4A–B; 2.2-fold change in Bmi1+/+ vs 8.9-fold in Bmi1+/−).


The Polycomb protein, Bmi1, regulates insulin sensitivity.

Cannon CE, Titchenell PM, Groff DN, El Ouaamari A, Kulkarni RN, Birnbaum MJ, Stoffers DA - Mol Metab (2014)

Hepatic insulin signaling is enhanced by Bmi1 heterozygosity. (A–B) 15-week-old males fasted 5 h and then injected IP with saline or 1.125 U/kg insulin. Livers harvested 20 min later and lysates assessed by western blot. (A) Representative Western blots showing levels of phospho-Akt and total Akt. Ran immunoreactivity was used as the loading control. (B) Quantification of phospho- and total Akt levels. The ratio of phospho-Akt/total Akt is shown. n = 4–5 per genotype and treatment. *p < 0.05 vs Bmi1+/+ saline; **p < 0.05 vs Bmi1+/+ insulin. (C–D) Cultured hepatocytes isolated from 15-week-old males treated with indicated dose of insulin for 20 min prior to harvest. Lysates assessed by western blot. (C) Representative Western blots showing levels of phospho-Akt and total Akt. (D) Quantification of phospho- and total Akt levels. The ratio of phospho-Akt/total Akt is shown. Results shown obtained from n = 5 independent hepatocyte isolations per genotype. p < 0.001 by two-way ANOVA. *p < 0.05 vs WT; **p < 0.001 vs WT.
© Copyright Policy - CC BY-NC-ND
Related In: Results  -  Collection

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fig4: Hepatic insulin signaling is enhanced by Bmi1 heterozygosity. (A–B) 15-week-old males fasted 5 h and then injected IP with saline or 1.125 U/kg insulin. Livers harvested 20 min later and lysates assessed by western blot. (A) Representative Western blots showing levels of phospho-Akt and total Akt. Ran immunoreactivity was used as the loading control. (B) Quantification of phospho- and total Akt levels. The ratio of phospho-Akt/total Akt is shown. n = 4–5 per genotype and treatment. *p < 0.05 vs Bmi1+/+ saline; **p < 0.05 vs Bmi1+/+ insulin. (C–D) Cultured hepatocytes isolated from 15-week-old males treated with indicated dose of insulin for 20 min prior to harvest. Lysates assessed by western blot. (C) Representative Western blots showing levels of phospho-Akt and total Akt. (D) Quantification of phospho- and total Akt levels. The ratio of phospho-Akt/total Akt is shown. Results shown obtained from n = 5 independent hepatocyte isolations per genotype. p < 0.001 by two-way ANOVA. *p < 0.05 vs WT; **p < 0.001 vs WT.
Mentions: After a 5 h fast to minimize stimulation by endogenous insulin, Bmi1+/+ and Bmi1+/− mice were injected IP with saline or insulin 20 min prior to sacrifice. Western blot analysis of liver lysates revealed no effect of either genotype or insulin treatment on total Akt levels (Figure 4A); however, Akt phosphorylation levels in heterozygous mice were lower at baseline as compared to controls, suggesting reduced basal insulin signaling in Bmi1+/− livers. This supports the observation from whole animal physiology that less insulin is needed to maintain normal glucose homeostasis in the Bmi1 heterozygous animals (Figure 2F). Upon insulin stimulation, Akt phosphorylation was higher in Bmi1+/− liver lysates, compared to controls, indicating enhanced signal transduction in response to insulin (Figure 4A–B; 2.2-fold change in Bmi1+/+ vs 8.9-fold in Bmi1+/−).

Bottom Line: The Polycomb Repressive Complexes (PRC) 1 and 2 function to epigenetically repress target genes.The PRC1 component, Bmi1, plays a crucial role in maintenance of glucose homeostasis and beta cell mass through repression of the Ink4a/Arf locus.Glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamps were performed.

View Article: PubMed Central - PubMed

Affiliation: Institute for Diabetes, Obesity and Metabolism and the Division of Endocrinology, Diabetes and Metabolism, Department of Medicine, Perelman School of Medicine at the University of Pennsylvania, 3400 Civic Center Boulevard, Philadelphia, PA 19104, USA.

ABSTRACT

Objective: The Polycomb Repressive Complexes (PRC) 1 and 2 function to epigenetically repress target genes. The PRC1 component, Bmi1, plays a crucial role in maintenance of glucose homeostasis and beta cell mass through repression of the Ink4a/Arf locus. Here we have explored the role of Bmi1 in regulating glucose homeostasis in the adult animal, which had not been previously reported due to poor postnatal survival of Bmi1 (-/-) mice.

Methods: The metabolic phenotype of Bmi1 (+/-) mice was characterized, both in vivo and ex vivo. Glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamps were performed. The insulin signaling pathway was assessed at the protein and transcript level.

Results: Here we report a negative correlation between Bmi1 levels and insulin sensitivity in two models of insulin resistance, aging and liver-specific insulin receptor deficiency. Further, heterozygous loss of Bmi1 results in increased insulin sensitivity in adult mice, with no impact on body weight or composition. Hyperinsulinemic-euglycemic clamp reveals increased suppression of hepatic glucose production and increased glucose disposal rate, indicating elevated glucose uptake to peripheral tissues, in Bmi1 (+/-) mice. Enhancement of insulin signaling, specifically an increase in Akt phosphorylation, in liver and, to a lesser extent, in muscle appears to contribute to this phenotype.

Conclusions: Together, these data define a new role for Bmi1 in regulating insulin sensitivity via enhancement of Akt phosphorylation.

No MeSH data available.


Related in: MedlinePlus