Growth inhibition and compensation in response to neonatal hypoxia in rats

Abstract

Background: Hypoxia (Hx) is an important disease mechanism in prematurity, childhood asthma, and obesity. In children, Hx results in chronic inflammation.

Methods: We investigated the effects of Hx (12% O2) during postnatal days 2-20 in rats. Control groups were normoxic control (Nc), and normoxic growth restricted (Gr) (14-pup litters).

Results: The Hx-exposed and Gr rats had similar decreases in growth. Hx increased plasma tumor necrosis factor-α (TNF-α) and interleukin 6 (IL-6) levels and decreased insulin-like growth factor 1 (IGF-I) and vascular endothelial growth factor (VEGF) levels. Hx resulted in hypertrophy of the right ventricle (RV) but disproportionate decrements in limb skeletal muscle (SM) growth. miR-206 was depressed in the hypertrophied RV of Hx rats but was increased in growth-retarded SM. Hx resulted in decreased RV messenger RNA (mRNA) level for myostatin but had no effect on SM myostatin. The mRNA for Hx-sensitive factors such as hypoxia inducible factor-1α (HIF-1α) was depressed in the RV of Hx rats, suggesting negative feedback.

Conclusion: The results indicate that Hx induces a proinflammatory state that depresses growth-regulating mechanisms and that tissues critical for survival, such as the heart, can escape from this general regulatory program to sustain life. This study identifies accessible biomarkers for evaluating the impact of interventions designed to mitigate the long-term deleterious consequences of Hx that all too often occur in babies born prematurely.

Figure 1. Effects of Hypoxia and Litter Size on Body and Muscle Mass

a) Both large litter size in normoxia (Gr – Grey Column) and exposure to hypoxia (Hx – Hatched Column) (litter size 4) resulted in depressed body mass relative to pups maintained in normoxia (Nc - White Column) (litter size 4) at 21 days postpartum in rats. b) The relative mass of the leg muscle Medial Gastrocnemius and the concentration of RNA in that muscle were depressed at 21 days post partum in the Hx neonates. The left three columns are data from female rats, the right 3 columns are data from the male rats. N ≥ 7 *, P<0.05 vs. Nc; §, P<0.05 vs. Gr.

Figure 2. Effects of Hypoxia and Litter Size on the Right Ventricle

The right ventricular a) mass, b) DNA content, c) myofibrillar protein content and d) β-myosin heavy chain percent were dramatically altered to support the increased demands of the hypoxic treatment at 21 days post partum. N ≥ 7. *, P<0.05 vs. Nc; §, P<0.05 vs. Gr.

Figure 3. Effects of Hypoxia and Litter Size on Circulating Growth Mediators

At 21 days post partum, plasma concentrations of a) IGF-I were depressed in both the Gr and Hx neonates. Plasma concentrations of b) VEGF and c) GH were depressed only in the Hx groups. n ≥ 7, *, P<0.05 vs. Nc; §, P<0.05 vs. Gr.

Figure 4. Effects of Hypoxia and Litter Size on Hypoxia Sensitive mRNA in the Right Ventricle

The Abundance of mRNA for a) HIF1α, b) HYOU1, c) VEGF and d) PDGFRβ was altered in the in the right ventricle at 21 days post partum. An exception to this was seen for VEGF in the Hx male neonates. n ≥ 7, *, P<0.05 vs. Nc; §, P<0.05 vs. Gr. AU; arbitrary units.

Figure 5. Effects of Hypoxia and Litter Size on Growth Related mRNA in the Right Ventricle and Skeletal Muscle

The abundance of mRNA for a) Myostatin and b) IGFBP5 was significantly depressed in the right ventricles of the Hx neonates at 21 days post partum. The abundance of mRNA for c) Nos3 and d) IGFBP4 was depressed in the MG muscle. n ≥ 7, *, P<0.05 vs. Nc; §, P<0.05 vs. Gr.

Figure 6. Effects of Hypoxia and Litter Size on microRNA Expression in Heart and Skeletal Muscle

At 21 days post partum, the abundance of a) miR1 was depressed in the RV of male neonates. At this time point, the abundance of b) miR206 was depressed in the RV of both genders. In MG muscle miR1 (c) was depressed in both the Gr and Hx female neonates. In contrast, the miR206 (d) was increased in both genders of Hx neonates. n ≥ 7, *, P<0.05 vs. Nc; §, P<0.05 vs. Gr. AU: arbitrary units.