Control of growth hormone receptor and insulin-like growth factor-I expression by cortisol in ovine fetal skeletal muscle

Abstract

Insulin-like growth factor (IGF)-I has an important role in myogenesis but its developmental regulation in skeletal muscle before birth remains unknown. In other tissues, cortisol modulates IGF gene expression and is responsible for many of the prepartum maturational changes essential for neonatal survival. Hence, using RNase protection assays and ovine riboprobes, expression of the IGF-I and growth hormone receptor (GHR) genes was examined in ovine skeletal muscle during late gestation and after experimental manipulation of fetal plasma cortisol levels by fetal adrenalectomy and exogenous cortisol infusion. Muscle IGF-I, but not GHR, mRNA abundance decreased with increasing gestational age in parallel with the prepartum rise in plasma cortisol. Abolition of this cortisol surge by fetal adrenalectomy prevented the prepartum fall in muscle IGF-I mRNA abundance. Conversely, raising cortisol levels by exogenous infusion earlier in gestation prematurely lowered muscle IGF-I mRNA abundance but had no effect on GHR mRNA. When all data were combined, plasma cortisol and muscle IGF-I mRNA abundance were inversely correlated in individual fetuses. Cortisol is, therefore, a developmental regulator of IGF-I gene expression and is responsible for suppressing expression of this gene in ovine skeletal muscle near term. These observations have important implications for muscle development both before and after birth, particularly during conditions which alter intrauterine cortisol exposure.

Figure 1. Ontogeny of GHR and IGF-I gene expression in ovine fetal skeletal muscle

Autoradiograms of RNase protection assay using the GHR and IGF-I riboprobes with 50 mg of total RNA from skeletal muscle of groups of intact fetuses aged 100-145 days of gestation. For IGF-I mRNA, protected probes gave bands at 132 nucleotides (132 n) for Class 1 transcripts and at 147 nucleotides (147 n) for Class 2 transcripts.

Figure 2. Changes in plasma cortisol concentration and Class 1 IGF-I mRNA abundance with gestational age

Mean (± s.e.m.) values of plasma cortisol concentration (A) and Class 1 IGF-I mRNA abundance (B) with respect to gestational age in skeletal muscle from intact (•) and AX (▵) sheep fetuses. Values with different letters are significantly different from each other (ANOVA, P < 0.05). † Significantly different from values seen in intact fetuses at the same gestational age (P < 0.01).

Figure 3. Effect of adrenalectomy on IGF-I gene expression in ovine fetal skeletal muscle at term

Autoradiograms of Class 1 and Class 2 IGF-I mRNA in skeletal muscle from AX and intact sheep fetuses at 143-145 days of gestation.

Figure 4. Effect of cortisol infusion on IGF-I gene expression in ovine fetal skeletal muscle

Autoradiograms of Class 1 and Class 2 IGF-I mRNA transcripts in skeletal muscle from intact and AX fetuses infused with saline or cortisol for 5 days before delivery at either 110-114 days or 127-131 days. Details of the dose of cortisol are given in the text.

Figure 5. The relationship between the plasma cortisol concentration and skeletal muscle Class 1 IGF mRNA abundance in individual sheep fetuses

•,intact control fetuses,unoperated and saline infusion;, cortisol infused intact fetuses; ▴, adrenalectomized fetuses; , cortisol infused adrenalectomized fetuses. All animals, log₁₀y = 1.479-[0.688 log₁₀x], n = 37, r = −0.761, P < 0.001; control animals alone, log₁₀y = 1.580 − [0.760 log₁₀x], n = 19, r = −0.805, P < 0.001).