Insulin-like growth factor I receptor signaling is required for exercise-induced cardiac hypertrophy - PubMed (original) (raw)

Insulin-like growth factor I receptor signaling is required for exercise-induced cardiac hypertrophy

Jaetaek Kim et al. Mol Endocrinol. 2008 Nov.

Abstract

The receptors for IGF-I (IGF-IR) and insulin (IR) have been implicated in physiological cardiac growth, but it is unknown whether IGF-IR or IR signaling are critically required. We generated mice with cardiomyocyte-specific knockout of IGF-IR (CIGF1RKO) and compared them with cardiomyocyte-specific insulin receptor knockout (CIRKO) mice in response to 5 wk exercise swim training. Cardiac development was normal in CIGF1RKO mice, but the hypertrophic response to exercise was prevented. In contrast, despite reduced baseline heart size, the hypertrophic response of CIRKO hearts to exercise was preserved. Exercise increased IGF-IR content in control and CIRKO hearts. Akt phosphorylation increased in exercise-trained control and CIRKO hearts and, surprisingly, in CIGF1RKO hearts as well. In exercise-trained control and CIRKO mice, expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) and glycogen content were both increased but were unchanged in trained CIGF1RKO mice. Activation of AMP-activated protein kinase (AMPK) and its downstream target eukaryotic elongation factor-2 was increased in exercise-trained CIGF1RKO but not in CIRKO or control hearts. In cultured neonatal rat cardiomyocytes, activation of AMPK with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) prevented IGF-I/insulin-induced cardiomyocyte hypertrophy. These studies identify an essential role for IGF-IR in mediating physiological cardiomyocyte hypertrophy. IGF-IR deficiency promotes energetic stress in response to exercise, thereby activating AMPK, which leads to phosphorylation of eukaryotic elongation factor-2. These signaling events antagonize Akt signaling, which although necessary for mediating physiological cardiac hypertrophy, is insufficient to promote cardiac hypertrophy in the absence of myocardial IGF-I signaling.

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Figures

Figure 1

Cardiomyocyte Deletion of IGF-IR Western blot analysis of IGF-IR protein (molecular mass 95 kDa) obtained from lysates of isolated cardiomyocytes (40 μg), liver (60 μg), and skeletal muscle (60 μg) from WT and CIGF1RKO mice. GAPDH is the loading control.

Figure 2

Impact of IGF-IR Deletion on Cardiac Growth A, HW/BW ratio in male WT and CIGF1RKO mice at postnatal d 10 and 3 or 8 wk of age, respectively. Numbers of mice in each group are indicated on the bars. B, Representative phase contrast images of isolated cardiomyocytes from hearts of 8-wk-old male WT and CIGF1RKO mice. Magnification, ×10. Myocyte area and circumference (at least 100 cells in each of three mice) are shown in the table. Data are mean ±

sem

Figure 3

Attenuated Cardiac Hypertrophy in Swim-Trained CIGF1RKO Mice A, VW/TL ratio in exercise-trained (Sw) and sedentary (Sed) WT and CIGF1RKO mice. Numbers of ventricles are indicated on the bars. B, Immunostaining of ventricular sections with FITC-conjugated wheat germ agglutinin (magnification, ×40) and quantification of cross-sectional area from at least 100 myocytes per ventricle in randomly selected fields of sections from WT and CIGF1RKO hearts. Numbers of sections are indicated on the bars. C, VW/TL in exercise-trained (Sw) and sedentary (Sed) WT and CIRKO mice. Numbers of ventricles are indicated on the bars. D, Immunostaining of ventricular sections with FITC-conjugated wheat germ agglutinin (magnification, ×40) and quantitation of cross-sectional area in WT and CIRKO mice. Numbers of sections are indicated on the bars. NS, Not significant.

Figure 4

IGF-IR Expression A, IGF-IR expression in the hearts of sedentary (Sed) and swim-trained (Sw) WT, CIGF1RKO, and CIRKO mice. Upper panels are representative blots, and lower panel is densitometry of results from five to seven hearts per group. *, P < 0.05; **, P < 0.01; NS, not significant. a, P < 0.05 vs. WT Sed (by Student’s t test). B, mRNA levels expressed as fold change compared with sedentary WT. n = 7–8 per group. *, P < 0.05; **, P < 0.01 vs. WT Sed; †, P < 0.001 vs. WT swim; ¶, P < 0.001 vs. CIRKO Sed.

Figure 5

Akt-Mediated Signaling Total and phosphorylated Akt (A) and total and phosphorylated GSK3β (B) in sedentary (Sed) and swim-trained (Sw) CIGF1RKO and CIRKO and their respective controls. Upper panels are representative blots, and lower panel is densitometry of results from five to six hearts per group. *, P < 0.05; **, P < 0.01; †, P < 0.001.

Figure 6

AMPK Signaling and Glycogen Content A, Total and phosphorylated AMPK and ACC. Upper panels are representative blots, and lower panel is densitometry of results from three to four hearts per group expressed as fold change relative to sedentary WT. **, P < 0.01 vs. CIGF1RKO Sed, and P < 0.05 vs. WT swim for p-AMPK; *, P < 0.01 vs. CIGF1RKO Sed; †, P < 0.05 vs. WT Sed for p-ACC. B, Total and phosphorylated eEF2. Upper panel shows a representative blot, and the lower panel is densitometry of results from five to seven hearts per group. *, P < 0.05. C, Myocardial glycogen content in sedentary or swim-trained WT or CIGF1RKO mice. Numbers of ventricles are indicated on the bars.

Figure 7

PGC-1α Expression A, PGC-1α mRNA in sedentary or swim-trained mice, expressed as fold change relative to sedentary WT. *, P < 0.05 vs. WT swim; **, P < 0.001 vs. WT Sed; ¶, P < 0.01 vs. WT Sed; †, P < 0.001 vs. CIRKO Sed. or vs. WT swim. B and C, Total and phosphorylated FOXO3 and FOXO1 (B) and total and phosphorylated CREB (C) in sedentary (Sed) and swim-trained (Sw) CIGF1RKO and their respective controls. *, P < 0.05 vs. Sed.

Figure 8

AICAR Inhibits IGF-I/Insulin-Mediated Cardiomyocyte Hypertrophy A, Upper panel shows representative fluorescence micrographs of neonatal rat ventricular cardiomyocytes treated with or without 100 n

insulin plus 10 n

IGF-I for 72 h in the presence of absence of 1 m

AICAR and immunostained with α-actinin (magnification, ×20), and lower panel shows image quantification (mean ±

sem

) of data from four independent experiments where at least 100 cells were measured per condition. B, Representative immunoblots showing total and phosphorylated AMPK and eEF2, respectively. Densitometry is shown below each blot. Significant differences are indicated on each figure.

Figure 9

Proposed Model for the Signaling Mechanisms Responsible for the Blunted Hypertrophic Response in Mouse Hearts that Lack IGF-IR (CIGF1RKO) In WT and CIGF1RKO hearts, exercise training leads to increased phosphorylation of Akt (p-Akt), which promotes cardiac hypertrophy by phosphorylating and inhibiting GSK3β and by activating mTOR and S6 kinase (S6K). In CIGF1RKO mice, there is increased activation of FOXO3 by mechanisms that might be Akt dependent or Akt independent. FOXO3 is a positive regulator of PGC-1α expression. Nuclear exclusion of FOXO3 by phosphorylation reduces the FOXO3-dependent activation of the PGC-1α gene. Reduced PGC-1α expression limits the mitochondrial adaptations to exercise training, thereby promoting energetic stress, which leads to activation of AMPK. AMPK activation will increase the phosphorylation of eEF2, which inhibits protein synthesis and antagonizes pro-hypertrophic signaling pathways. Increased AMPK phosphorylation might also antagonize MAPK signaling which is also pro-hypertrophic.

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Insulin-like growth factor I receptor signaling is required for exercise-induced cardiac hypertrophy - PubMed (original) (raw)