Fetal growth restriction and the programming of heart growth and cardiac insulin-like growth factor 2 expression in the lamb

Abstract

Reduced growth in fetal life together with accelerated growth in childhood, results in a ~50% greater risk of coronary heart disease in adult life. It is unclear why changes in patterns of body and heart growth in early life can lead to an increased risk of cardiovascular disease in adulthood. We aimed to investigate the role of the insulin-like growth factors in heart growth in the growth-restricted fetus and lamb. Hearts were collected from control and placentally restricted (PR) fetuses at 137-144 days gestation and from average (ABW) and low (LBW) birth weight lambs at 21 days of age. We quantified cardiac mRNA expression of IGF-1, IGF-2 and their receptors, IGF-1R and IGF-2R, using real-time RT-PCR and protein expression of IGF-1R and IGF-2R using Western blotting. Combined bisulphite restriction analysis was used to assess DNA methylation in the differentially methylated region (DMR) of the IGF-2/H19 locus and of the IGF-2R gene. In PR fetal sheep, IGF-2, IGF-1R and IGF-2R mRNA expression was increased in the heart compared to controls. LBW lambs had a greater left ventricle weight relative to body weight as well as increased IGF-2 and IGF-2R mRNA expression in the heart, when compared to ABW lambs. No changes in the percentage of methylation of the DMRs of IGF-2/H19 or IGF-2R were found between PR and LBW when compared to their respective controls. In conclusion, a programmed increased in cardiac gene expression of IGF-2 and IGF-2R may represent an adaptive response to reduced substrate supply (e.g. glucose and/or oxygen) in order to maintain heart growth and may be the underlying cause for increased ventricular hypertrophy and the associated susceptibility of cardiomyocytes to ischaemic damage later in life.

Figure 1. Cardiac IGF-2 mRNA expression in control and PR fetuses

Cardiac IGF-2 mRNA expression normalized to RpP0 of PR and control fetuses (A). Relationship between cardiac IGF-2 mRNA expression and both the per cent mononucleated cardiomyocyte (B) and left ventricle relative binucleated cardiomyocyte length (C). Values are mean ± SEM. *Significantly different from control fetuses (P < 0.05). Control, open bars/circles; PR, filled bars/circles.

Figure 2. Cardiac IGF-1R mRNA and protein expression in control and PR fetuses

Cardiac IGF-1R mRNA expression normalized to RpP0 (A) and IGF-1R protein abundance (B) in PR and control fetuses. Relationship between IGF-1R protein abundance and both the per cent mononucleated cardiomyocyte (C) and left ventricle relative binucleated cardiomyocyte length (D). Western blot of cardiac IGF-1R protein in control and PR fetuses (E). Values are mean ± SEM. *Significantly different from control fetuses (P < 0.05). Control, open bars/circles; PR, filled bars/circles.

Figure 3. Cardiac IGF-2R mRNA and protein expression in control and PR fetuses

Cardiac IGF-2R mRNA expression normalized to RpP0 (A) and IGF-2R protein abundance (B) in PR and control fetuses. Relationship of cardiac IGF-2R mRNA expression to the relative length of binucleated cardiomyocytes (C). Western blot of cardiac IGF-2R protein in control and PR fetuses (D). Values are mean ± SEM. *Significantly different from control fetuses (P < 0.05). Control, open bars/circles; PR, filled bars/circles.

Figure 4. Cardiac IGF-2 mRNA expression in ABW and LBW lambs

A, cardiac IGF-2 mRNA expression normalized to RpP0 in LBW and ABW lambs. B, relationship between cardiac IGF-2 mRNA expression and the relative weight of the left ventricle. Values are mean ± SEM. *Significantly different from ABW lambs (P < 0.05). ABW, open bars/circles; LBW, filled bars/circles.

Figure 5. Cardiac IGF-2R mRNA and protein expression in ABW and LBW lambs

Cardiac IGF-2R mRNA expression normalized to RpP0 (A) and IGF-2R protein abundance (B) in LBW and ABW lambs. Relationship between cardiac IGF-2R protein abundance and relative left ventricle weight in ABW (C) and LBW lambs (D). Western blot of cardiac IGF-2R protein in ABW and LBW lambs (E). Values are mean ± SEM. *Significantly different from ABW lambs (P < 0.05). ABW, open bars/circles; LBW, filled bars/circles.