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The cAMP-HMGA1-RBP4 system: a novel biochemical pathway for modulating glucose homeostasis.

Chiefari E, Paonessa F, Iiritano S, Le Pera I, Palmieri D, Brunetti G, Lupo A, Colantuoni V, Foti D, Gulletta E, De Sarro G, Fusco A, Brunetti A - BMC Biol. (2009)

Bottom Line: These results indicate that HMGA1 is an important modulator of RBP4 gene expression in vivo.Further, they provide evidence for the identification of a novel biochemical pathway involving the cAMP-HMGA1-RBP4 system, whose activation may play a role in glucose homeostasis in both rodents and humans.Elucidating these mechanisms has importance for both fundamental biology and therapeutic implications.

View Article: PubMed Central - HTML - PubMed

Affiliation: Dipartimento di Medicina Sperimentale e Clinica G. Salvatore, Catanzaro, Italy. echiefari@libero.it

ABSTRACT

Background: We previously showed that mice lacking the high mobility group A1 gene (Hmga1-knockout mice) developed a type 2-like diabetic phenotype, in which cell-surface insulin receptors were dramatically reduced (below 10% of those in the controls) in the major targets of insulin action, and glucose intolerance was associated with increased peripheral insulin sensitivity. This particular phenotype supports the existence of compensatory mechanisms of insulin resistance that promote glucose uptake and disposal in peripheral tissues by either insulin-dependent or insulin-independent mechanisms. We explored the role of these mechanisms in the regulation of glucose homeostasis by studying the Hmga1-knockout mouse model. Also, the hypothesis that increased insulin sensitivity in Hmga1-deficient mice could be related to the deficit of an insulin resistance factor is discussed.

Results: We first show that HMGA1 is needed for basal and cAMP-induced retinol-binding protein 4 (RBP4) gene and protein expression in living cells of both human and mouse origin. Then, by employing the Hmga1-knockout mouse model, we provide evidence for the identification of a novel biochemical pathway involving HMGA1 and the RBP4, whose activation by the cAMP-signaling pathway may play an essential role for maintaining glucose metabolism homeostasis in vivo, in certain adverse metabolic conditions in which insulin action is precluded. In comparative studies of normal and mutant mice, glucagon administration caused a considerable upregulation of HMGA1 and RBP4 expression both at the mRNA and protein level in wild-type animals. Conversely, in Hmga1-knockout mice, basal and glucagon-mediated expression of RBP4 was severely attenuated and correlated inversely with increased Glut4 mRNA and protein abundance in skeletal muscle and fat, in which the activation state of the protein kinase Akt, an important downstream mediator of the metabolic effects of insulin on Glut4 translocation and carbohydrate metabolism, was simultaneously increased.

Conclusion: These results indicate that HMGA1 is an important modulator of RBP4 gene expression in vivo. Further, they provide evidence for the identification of a novel biochemical pathway involving the cAMP-HMGA1-RBP4 system, whose activation may play a role in glucose homeostasis in both rodents and humans. Elucidating these mechanisms has importance for both fundamental biology and therapeutic implications.

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Comparison of RBP4 mRNA levels in glucagon-injected wild-type and Hmga1-deficient mice, and liver RBP4 expression in wild-type mice during fasting and fed. Total RNA was isolated from liver and fat of 3-h-fasted mice, before (time 0) and after 9 h of intraperitoneal injection of glucagon, and RBP4 mRNA was measured by qRT-PCR and normalized to RPS9 mRNA abundance. Results are the mean values ± s.e.m. from 6–8 animals per group. Black bars, Hmga1+/+, n = 8; gray bars, Hmga1+/-, n = 6; white bars, Hmga1-/-, n = 6. *P < 0.05 versus each control (time 0). Western blots for HMGA1 protein expression are shown in liver and fat from all three genotypes (top). The levels of RBP4 mRNA and protein (shown at the bottom of the figure) were measured in liver of fed and 6-h-fasted wild-type mice (6 animals per group), using qRT-PCR and Western blot (WB), respectively. *P < 0.05 versus fed mice. Inset, cAMP was measured in liver from control and Hmga1-deficient mice, in both basal conditions and 3 h after the intraperitoneal injection of glucagon (1 mg/kg body weight), as described in the Methods section. The data are mean ± s.e.m. for 4–6 animals per group.
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Figure 6: Comparison of RBP4 mRNA levels in glucagon-injected wild-type and Hmga1-deficient mice, and liver RBP4 expression in wild-type mice during fasting and fed. Total RNA was isolated from liver and fat of 3-h-fasted mice, before (time 0) and after 9 h of intraperitoneal injection of glucagon, and RBP4 mRNA was measured by qRT-PCR and normalized to RPS9 mRNA abundance. Results are the mean values ± s.e.m. from 6–8 animals per group. Black bars, Hmga1+/+, n = 8; gray bars, Hmga1+/-, n = 6; white bars, Hmga1-/-, n = 6. *P < 0.05 versus each control (time 0). Western blots for HMGA1 protein expression are shown in liver and fat from all three genotypes (top). The levels of RBP4 mRNA and protein (shown at the bottom of the figure) were measured in liver of fed and 6-h-fasted wild-type mice (6 animals per group), using qRT-PCR and Western blot (WB), respectively. *P < 0.05 versus fed mice. Inset, cAMP was measured in liver from control and Hmga1-deficient mice, in both basal conditions and 3 h after the intraperitoneal injection of glucagon (1 mg/kg body weight), as described in the Methods section. The data are mean ± s.e.m. for 4–6 animals per group.

Mentions: Based on our observations in intact cultured cells, indicating a role for the cAMP signaling pathway in HMGA1 and RBP4 gene expression, cAMP-inducible transcriptional activation of the Hmga1 and RBP4 genes was investigated in vivo, in whole animals, by systemic administration of the intracellular cAMP-elevating hormone glucagon. Under these conditions, glucagon-stimulated cAMP responses in terms of both Hmga1 and RBP4 mRNA expression were first analyzed in wild-type control mice. Consistent with our data in Hepa1 cells, Hmga1 and RBP4 mRNA levels significantly increased in liver and fat of normal mice after intraperitoneal injection of glucagon (Figure 5). Time course analyses revealed that the induction and accumulation of Hmga1 mRNA preceded the expression of RBP4 mRNA in both tissues. In liver, RBP4 mRNA appeared after that for Hmga1, peaked after 6 h following glucagon injection, and then remained at a plateau (Figure 5). In fat, RBP4 mRNA appeared at 1 h after Hmga1 mRNA, peaked after 6 h of glucagon stimulation, and decreased smoothly thereafter (Figure 5). Increased levels of Hmga1 and RBP4 mRNAs were paralleled by the increase of Hmga1 and RBP4 protein expression, as measured by Western blot analysis of proteins from liver and fat of glucagon-injected animals (Figure 5). Interestingly, when similar experiments were carried out in glucagon injected Hmga1-deficient mice, tissue expression of RBP4 mRNA was severely attenuated in liver and fat from heterozygous (Hmga1+/-) and Hmga1- (Hmga1-/-) animals (Figure 6), thereby indicating that HMGA1 is indeed required for maximal induction of the RBP4 gene in vivo, in the whole organism, and that the glucagon/adenylate cyclase system regulates both HMGA1 and RBP4 gene and protein expression. Consistent with this conclusion, liver RBP4 mRNA and protein expression levels were lower in fed wild-type mice, becoming higher during fasting, when circulating glucagon increases (Figure 6).


The cAMP-HMGA1-RBP4 system: a novel biochemical pathway for modulating glucose homeostasis.

Chiefari E, Paonessa F, Iiritano S, Le Pera I, Palmieri D, Brunetti G, Lupo A, Colantuoni V, Foti D, Gulletta E, De Sarro G, Fusco A, Brunetti A - BMC Biol. (2009)

Comparison of RBP4 mRNA levels in glucagon-injected wild-type and Hmga1-deficient mice, and liver RBP4 expression in wild-type mice during fasting and fed. Total RNA was isolated from liver and fat of 3-h-fasted mice, before (time 0) and after 9 h of intraperitoneal injection of glucagon, and RBP4 mRNA was measured by qRT-PCR and normalized to RPS9 mRNA abundance. Results are the mean values ± s.e.m. from 6–8 animals per group. Black bars, Hmga1+/+, n = 8; gray bars, Hmga1+/-, n = 6; white bars, Hmga1-/-, n = 6. *P < 0.05 versus each control (time 0). Western blots for HMGA1 protein expression are shown in liver and fat from all three genotypes (top). The levels of RBP4 mRNA and protein (shown at the bottom of the figure) were measured in liver of fed and 6-h-fasted wild-type mice (6 animals per group), using qRT-PCR and Western blot (WB), respectively. *P < 0.05 versus fed mice. Inset, cAMP was measured in liver from control and Hmga1-deficient mice, in both basal conditions and 3 h after the intraperitoneal injection of glucagon (1 mg/kg body weight), as described in the Methods section. The data are mean ± s.e.m. for 4–6 animals per group.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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Figure 6: Comparison of RBP4 mRNA levels in glucagon-injected wild-type and Hmga1-deficient mice, and liver RBP4 expression in wild-type mice during fasting and fed. Total RNA was isolated from liver and fat of 3-h-fasted mice, before (time 0) and after 9 h of intraperitoneal injection of glucagon, and RBP4 mRNA was measured by qRT-PCR and normalized to RPS9 mRNA abundance. Results are the mean values ± s.e.m. from 6–8 animals per group. Black bars, Hmga1+/+, n = 8; gray bars, Hmga1+/-, n = 6; white bars, Hmga1-/-, n = 6. *P < 0.05 versus each control (time 0). Western blots for HMGA1 protein expression are shown in liver and fat from all three genotypes (top). The levels of RBP4 mRNA and protein (shown at the bottom of the figure) were measured in liver of fed and 6-h-fasted wild-type mice (6 animals per group), using qRT-PCR and Western blot (WB), respectively. *P < 0.05 versus fed mice. Inset, cAMP was measured in liver from control and Hmga1-deficient mice, in both basal conditions and 3 h after the intraperitoneal injection of glucagon (1 mg/kg body weight), as described in the Methods section. The data are mean ± s.e.m. for 4–6 animals per group.
Mentions: Based on our observations in intact cultured cells, indicating a role for the cAMP signaling pathway in HMGA1 and RBP4 gene expression, cAMP-inducible transcriptional activation of the Hmga1 and RBP4 genes was investigated in vivo, in whole animals, by systemic administration of the intracellular cAMP-elevating hormone glucagon. Under these conditions, glucagon-stimulated cAMP responses in terms of both Hmga1 and RBP4 mRNA expression were first analyzed in wild-type control mice. Consistent with our data in Hepa1 cells, Hmga1 and RBP4 mRNA levels significantly increased in liver and fat of normal mice after intraperitoneal injection of glucagon (Figure 5). Time course analyses revealed that the induction and accumulation of Hmga1 mRNA preceded the expression of RBP4 mRNA in both tissues. In liver, RBP4 mRNA appeared after that for Hmga1, peaked after 6 h following glucagon injection, and then remained at a plateau (Figure 5). In fat, RBP4 mRNA appeared at 1 h after Hmga1 mRNA, peaked after 6 h of glucagon stimulation, and decreased smoothly thereafter (Figure 5). Increased levels of Hmga1 and RBP4 mRNAs were paralleled by the increase of Hmga1 and RBP4 protein expression, as measured by Western blot analysis of proteins from liver and fat of glucagon-injected animals (Figure 5). Interestingly, when similar experiments were carried out in glucagon injected Hmga1-deficient mice, tissue expression of RBP4 mRNA was severely attenuated in liver and fat from heterozygous (Hmga1+/-) and Hmga1- (Hmga1-/-) animals (Figure 6), thereby indicating that HMGA1 is indeed required for maximal induction of the RBP4 gene in vivo, in the whole organism, and that the glucagon/adenylate cyclase system regulates both HMGA1 and RBP4 gene and protein expression. Consistent with this conclusion, liver RBP4 mRNA and protein expression levels were lower in fed wild-type mice, becoming higher during fasting, when circulating glucagon increases (Figure 6).

Bottom Line: These results indicate that HMGA1 is an important modulator of RBP4 gene expression in vivo.Further, they provide evidence for the identification of a novel biochemical pathway involving the cAMP-HMGA1-RBP4 system, whose activation may play a role in glucose homeostasis in both rodents and humans.Elucidating these mechanisms has importance for both fundamental biology and therapeutic implications.

View Article: PubMed Central - HTML - PubMed

Affiliation: Dipartimento di Medicina Sperimentale e Clinica G. Salvatore, Catanzaro, Italy. echiefari@libero.it

ABSTRACT

Background: We previously showed that mice lacking the high mobility group A1 gene (Hmga1-knockout mice) developed a type 2-like diabetic phenotype, in which cell-surface insulin receptors were dramatically reduced (below 10% of those in the controls) in the major targets of insulin action, and glucose intolerance was associated with increased peripheral insulin sensitivity. This particular phenotype supports the existence of compensatory mechanisms of insulin resistance that promote glucose uptake and disposal in peripheral tissues by either insulin-dependent or insulin-independent mechanisms. We explored the role of these mechanisms in the regulation of glucose homeostasis by studying the Hmga1-knockout mouse model. Also, the hypothesis that increased insulin sensitivity in Hmga1-deficient mice could be related to the deficit of an insulin resistance factor is discussed.

Results: We first show that HMGA1 is needed for basal and cAMP-induced retinol-binding protein 4 (RBP4) gene and protein expression in living cells of both human and mouse origin. Then, by employing the Hmga1-knockout mouse model, we provide evidence for the identification of a novel biochemical pathway involving HMGA1 and the RBP4, whose activation by the cAMP-signaling pathway may play an essential role for maintaining glucose metabolism homeostasis in vivo, in certain adverse metabolic conditions in which insulin action is precluded. In comparative studies of normal and mutant mice, glucagon administration caused a considerable upregulation of HMGA1 and RBP4 expression both at the mRNA and protein level in wild-type animals. Conversely, in Hmga1-knockout mice, basal and glucagon-mediated expression of RBP4 was severely attenuated and correlated inversely with increased Glut4 mRNA and protein abundance in skeletal muscle and fat, in which the activation state of the protein kinase Akt, an important downstream mediator of the metabolic effects of insulin on Glut4 translocation and carbohydrate metabolism, was simultaneously increased.

Conclusion: These results indicate that HMGA1 is an important modulator of RBP4 gene expression in vivo. Further, they provide evidence for the identification of a novel biochemical pathway involving the cAMP-HMGA1-RBP4 system, whose activation may play a role in glucose homeostasis in both rodents and humans. Elucidating these mechanisms has importance for both fundamental biology and therapeutic implications.

Show MeSH
Related in: MedlinePlus