IGF-2R-Gαq signaling and cardiac hypertrophy in the low-birth-weight lamb

Abstract

The cardiac insulin-like growth factor 2 receptor (IGF-2R) can induce cardiomyocyte hypertrophy in a heterotrimeric G protein receptor-coupled manner involving αq (Gαq) or αs (Gαs). We have previously shown increased left ventricular weight and cardiac IGF-2 and IGF-2R gene expression in low-birth-weight (LBW) compared with average-birth-weight (ABW) lambs. Here, we have investigated the cardiac expression of IGF-2 gene variants, the degree of histone acetylation, and the abundance of proteins in the IGF-2R downstream signaling pathway in ABW and LBW lambs. Samples from the left ventricle of ABW and LBW lambs were collected at 21 days of age. There was increased phospho-CaMKII protein with decreased HDAC 4 abundance in the LBW compared with ABW lambs. There was increased GATA 4 and decreased phospho-troponin I abundance in LBW compared with ABW lambs, which are markers of pathological cardiac hypertrophy and impaired or reduced contractility, respectively. There was increased histone acetylation of H3K9 at IGF-2R promoter and IGF-2R intron 2 differentially methylated region in the LBW lamb. In conclusion, histone acetylation of IGF-2R may lead to increased IGF-2R mRNA expression and subsequently mediate Gαq signaling early in life via CaMKII, resulting in an increased risk of left ventricular hypertrophy and cardiovascular disease in adult life.

Keywords: histone acetylation; insulin-like growth factor 2 receptor; low birth weight.

Fig. 1.

Low birth weight decreased histone acetylation at IGF-2 promoter 2. Acetylation of histone H3K9 and H4K8 at IGF-2 promoter 1 (A and B) in low birth weight (LBW) compared with average birth weight (ABW) lambs. Acetylation of histone H3K9 and H4K8 at IGF-2 promoter 2 (C and D) in LBW compared with ABW lambs. Acetylation of histone H3K9 and H4K8 at IGF-2 promoter 4 (E and F). Sample size for each group is indicated in the bar. *Significantly different from ABW lambs (P < 0.05). ABW, average birth weight (white bar); LBW, low birth weight (black bar); MNE, mean normalized expression.

Fig. 2.

Low birth weight activates a CaMKII and HDAC 4 pathway. Cardiac CaMKII (A), phospho-CaMKII (B), HDAC 4 (C) and phospho-HDAC 4 (D) protein in LBW compared with ABW lambs. Images of the Western blots for cardiac CaMKII, phospho-CaMKII, HDAC 4, and phospho-HDAC 4 abundance in ABW and LBW lambs (E). Sample size for each group is indicated in the bar. *Significantly different from ABW lambs (P < 0.05). ABW or A, average birth weight (white bar); LBW or L, low birth weight (black bar); B, blank; U, unanalyzable; au, arbitrary units.

Fig. 3.

Low birth weight affects cardiac pathological hypertrophy and contractility markers. Cardiac GATA 4 (A), ANP (B), phospho-troponin I (C), and SERCA²⁺ ATPase (D) in LBW compared with ABW lambs. Images of the Western blots for cardiac GATA 4, ANP, phospho-troponin I, and SERCA²⁺ ATPase abundance in ABW and LBW lambs (E). Sample size for each group is indicated in the bar. *Significantly different from ABW lambs (P < 0.05). ABW or A, average birth weight (white bar); LBW or L, low birth weight (black bar); B, blank; U, unanalyzable; au, arbitrary units.

Fig. 4.

Low birth weight increased histone acetylation at IGF-2R promoter. Acetylation of histone H3K9 (A) and H4K8 (B) of IGF-2R promoter (A) in LBW compared with ABW lambs. Sample size for each group is indicated in the bar. *Significantly different from ABW lambs (P < 0.05). ABW, average birth weight (white bar); LBW, low birth weight (black bar).

Fig. 5.

Low birth weight increased histone acetylation at IGF-2R intron 2 DMR. Acetylation of histone H3K9 (A) and H4K8 (B) of IGF-2R intron 2 DMR in LBW compared with ABW lambs. Sample size for each group is indicated in the bar. *Significantly different from ABW lambs (P < 0.05). ABW, average birth weight (white bar); LBW, low birth weight (black bar).

Fig. 6.

Activation of cardiac IGF-2R increases phospho-CaMKII and GATA 4 with decreased of HDAC 4 protein abundance but does not increase any of the pathological cardiac hypertrophy markers in the LBW lambs compared with ABW lambs at 21 days after birth. ↑, increased protein expression compared with ABW lambs; ↓, decreased protein expression compared with ABW lambs; ↔, no changes in protein expression compared with ABW lambs; P, phospho-protein.