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FoxO6 integrates insulin signaling with gluconeogenesis in the liver.

Kim DH, Perdomo G, Zhang T, Slusher S, Lee S, Phillips BE, Fan Y, Giannoukakis N, Gramignoli R, Strom S, Ringquist S, Dong HH - Diabetes (2011)

Bottom Line: This effect stems from inept insulin suppression of hepatic gluconeogenesis.FoxO6 stimulates gluconeogenesis, which is counteracted by insulin.Insulin inhibits FoxO6 activity via a distinct mechanism by inducing its phosphorylation and disabling its transcriptional activity, without altering its subcellular distribution in hepatocytes.

View Article: PubMed Central - PubMed

Affiliation: Division of Immunogenetics, Department of Pediatrics, Children’s Hospital of Pittsburgh of the University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania, USA. dongh@pitt.edu

ABSTRACT

Objective: Excessive endogenous glucose production contributes to fasting hyperglycemia in diabetes. This effect stems from inept insulin suppression of hepatic gluconeogenesis. To understand the underlying mechanisms, we studied the ability of forkhead box O6 (FoxO6) to mediate insulin action on hepatic gluconeogenesis and its contribution to glucose metabolism.

Research design and methods: We characterized FoxO6 in glucose metabolism in cultured hepatocytes and in rodent models of dietary obesity, insulin resistance, or insulin-deficient diabetes. We determined the effect of FoxO6 on hepatic gluconeogenesis in genetically modified mice with FoxO6 gain- versus loss-of-function and in diabetic db/db mice with selective FoxO6 ablation in the liver.

Results: FoxO6 integrates insulin signaling to hepatic gluconeogenesis. In mice, elevated FoxO6 activity in the liver augments gluconeogenesis, raising fasting blood glucose levels, and hepatic FoxO6 depletion suppresses gluconeogenesis, resulting in fasting hypoglycemia. FoxO6 stimulates gluconeogenesis, which is counteracted by insulin. Insulin inhibits FoxO6 activity via a distinct mechanism by inducing its phosphorylation and disabling its transcriptional activity, without altering its subcellular distribution in hepatocytes. FoxO6 becomes deregulated in the insulin-resistant liver, accounting for its unbridled activity in promoting gluconeogenesis and correlating with the pathogenesis of fasting hyperglycemia in diabetes. These metabolic abnormalities, along with fasting hyperglycemia, are reversible by selective inhibition of hepatic FoxO6 activity in diabetic mice.

Conclusions: Our data uncover a FoxO6-dependent pathway by which the liver orchestrates insulin regulation of gluconeogenesis, providing the proof-of-concept that selective FoxO6 inhibition is beneficial for curbing excessive hepatic glucose production and improving glycemic control in diabetes.

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CRM-1 mediates insulin-dependent FoxO1 trafficking. HepG2 cells were transduced with FoxO1 vector at a fixed dose (50 plaque forming units/cell). After 24-h incubation, cells were serum-starved for 6 h, followed by treatment with insulin (100 nmol/L) in the absence or presence of leptomycin B (10 nmol/L) in culture medium for 30 min, followed by immunohistochemistry using anti-FoxO1 and anti–CRM-1 antibodies. FoxO1 was localized predominantly within the nucleus in the absence of insulin (A–D). In response to insulin, FoxO1 and CRM-1 were translocated from the nucleus to cytoplasm (E–H). This insulin-stimulated FoxO1 trafficking was abolished by leptomycin B (I–L), an agent that binds specifically to CRM-1 and disables the ability of CRM-1 to transport its cargo protein from the nucleus to the cytoplasm. (A high-quality digital representation of this figure is available in the online issue.)
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Figure 5: CRM-1 mediates insulin-dependent FoxO1 trafficking. HepG2 cells were transduced with FoxO1 vector at a fixed dose (50 plaque forming units/cell). After 24-h incubation, cells were serum-starved for 6 h, followed by treatment with insulin (100 nmol/L) in the absence or presence of leptomycin B (10 nmol/L) in culture medium for 30 min, followed by immunohistochemistry using anti-FoxO1 and anti–CRM-1 antibodies. FoxO1 was localized predominantly within the nucleus in the absence of insulin (A–D). In response to insulin, FoxO1 and CRM-1 were translocated from the nucleus to cytoplasm (E–H). This insulin-stimulated FoxO1 trafficking was abolished by leptomycin B (I–L), an agent that binds specifically to CRM-1 and disables the ability of CRM-1 to transport its cargo protein from the nucleus to the cytoplasm. (A high-quality digital representation of this figure is available in the online issue.)

Mentions: To underpin these findings, we treated HepG2 cells with leptomycin B, an agent that binds specifically to CRM-1 and inhibits its cargo-trafficking activity. As shown in Fig. 5, FoxO1 and CRM-1 were colocalized in the nucleus in the absence of insulin. In response to insulin, FoxO1, along with CRM-1, was translocated from the nucleus to the cytoplasm. Leptomycin B treatment abrogated the ability of FoxO1 to undergo insulin-elicited nuclear export. Like its wild-type counterpart, the FoxO6-CA mutant remained in the nucleus irrespective of insulin action (Supplementary Fig. 7). These data indicate that the incapability of FoxO6 to undergo insulin-dependent trafficking lies in its inability to interact with CRM-1.


FoxO6 integrates insulin signaling with gluconeogenesis in the liver.

Kim DH, Perdomo G, Zhang T, Slusher S, Lee S, Phillips BE, Fan Y, Giannoukakis N, Gramignoli R, Strom S, Ringquist S, Dong HH - Diabetes (2011)

CRM-1 mediates insulin-dependent FoxO1 trafficking. HepG2 cells were transduced with FoxO1 vector at a fixed dose (50 plaque forming units/cell). After 24-h incubation, cells were serum-starved for 6 h, followed by treatment with insulin (100 nmol/L) in the absence or presence of leptomycin B (10 nmol/L) in culture medium for 30 min, followed by immunohistochemistry using anti-FoxO1 and anti–CRM-1 antibodies. FoxO1 was localized predominantly within the nucleus in the absence of insulin (A–D). In response to insulin, FoxO1 and CRM-1 were translocated from the nucleus to cytoplasm (E–H). This insulin-stimulated FoxO1 trafficking was abolished by leptomycin B (I–L), an agent that binds specifically to CRM-1 and disables the ability of CRM-1 to transport its cargo protein from the nucleus to the cytoplasm. (A high-quality digital representation of this figure is available in the online issue.)
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Related In: Results  -  Collection

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Figure 5: CRM-1 mediates insulin-dependent FoxO1 trafficking. HepG2 cells were transduced with FoxO1 vector at a fixed dose (50 plaque forming units/cell). After 24-h incubation, cells were serum-starved for 6 h, followed by treatment with insulin (100 nmol/L) in the absence or presence of leptomycin B (10 nmol/L) in culture medium for 30 min, followed by immunohistochemistry using anti-FoxO1 and anti–CRM-1 antibodies. FoxO1 was localized predominantly within the nucleus in the absence of insulin (A–D). In response to insulin, FoxO1 and CRM-1 were translocated from the nucleus to cytoplasm (E–H). This insulin-stimulated FoxO1 trafficking was abolished by leptomycin B (I–L), an agent that binds specifically to CRM-1 and disables the ability of CRM-1 to transport its cargo protein from the nucleus to the cytoplasm. (A high-quality digital representation of this figure is available in the online issue.)
Mentions: To underpin these findings, we treated HepG2 cells with leptomycin B, an agent that binds specifically to CRM-1 and inhibits its cargo-trafficking activity. As shown in Fig. 5, FoxO1 and CRM-1 were colocalized in the nucleus in the absence of insulin. In response to insulin, FoxO1, along with CRM-1, was translocated from the nucleus to the cytoplasm. Leptomycin B treatment abrogated the ability of FoxO1 to undergo insulin-elicited nuclear export. Like its wild-type counterpart, the FoxO6-CA mutant remained in the nucleus irrespective of insulin action (Supplementary Fig. 7). These data indicate that the incapability of FoxO6 to undergo insulin-dependent trafficking lies in its inability to interact with CRM-1.

Bottom Line: This effect stems from inept insulin suppression of hepatic gluconeogenesis.FoxO6 stimulates gluconeogenesis, which is counteracted by insulin.Insulin inhibits FoxO6 activity via a distinct mechanism by inducing its phosphorylation and disabling its transcriptional activity, without altering its subcellular distribution in hepatocytes.

View Article: PubMed Central - PubMed

Affiliation: Division of Immunogenetics, Department of Pediatrics, Children’s Hospital of Pittsburgh of the University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania, USA. dongh@pitt.edu

ABSTRACT

Objective: Excessive endogenous glucose production contributes to fasting hyperglycemia in diabetes. This effect stems from inept insulin suppression of hepatic gluconeogenesis. To understand the underlying mechanisms, we studied the ability of forkhead box O6 (FoxO6) to mediate insulin action on hepatic gluconeogenesis and its contribution to glucose metabolism.

Research design and methods: We characterized FoxO6 in glucose metabolism in cultured hepatocytes and in rodent models of dietary obesity, insulin resistance, or insulin-deficient diabetes. We determined the effect of FoxO6 on hepatic gluconeogenesis in genetically modified mice with FoxO6 gain- versus loss-of-function and in diabetic db/db mice with selective FoxO6 ablation in the liver.

Results: FoxO6 integrates insulin signaling to hepatic gluconeogenesis. In mice, elevated FoxO6 activity in the liver augments gluconeogenesis, raising fasting blood glucose levels, and hepatic FoxO6 depletion suppresses gluconeogenesis, resulting in fasting hypoglycemia. FoxO6 stimulates gluconeogenesis, which is counteracted by insulin. Insulin inhibits FoxO6 activity via a distinct mechanism by inducing its phosphorylation and disabling its transcriptional activity, without altering its subcellular distribution in hepatocytes. FoxO6 becomes deregulated in the insulin-resistant liver, accounting for its unbridled activity in promoting gluconeogenesis and correlating with the pathogenesis of fasting hyperglycemia in diabetes. These metabolic abnormalities, along with fasting hyperglycemia, are reversible by selective inhibition of hepatic FoxO6 activity in diabetic mice.

Conclusions: Our data uncover a FoxO6-dependent pathway by which the liver orchestrates insulin regulation of gluconeogenesis, providing the proof-of-concept that selective FoxO6 inhibition is beneficial for curbing excessive hepatic glucose production and improving glycemic control in diabetes.

Show MeSH
Related in: MedlinePlus