Muscle mechano growth factor is preferentially induced by growth hormone in growth hormone-deficient lit/lit mice

Abstract

Two muscle insulin-like growth factor-I (IGF-I) mRNA splice variants (IGF-IEa and IGF-IEb) have been identified in rodents. IGF-IEb, also called mechano growth factor (MGF) has been found to be upregulated by exercise or muscle damage. Growth hormone (GH) is the principal regulator of IGF-I expression in several tissues including skeletal muscle. Therefore, we investigated the effect of chronic GH excess or disruption of GH receptor (GHR) signalling, and the acute effect of GH administration on expression of muscle IGF-I isoforms using transgenic mice that express bovine GH (bGH), GHR gene-disrupted (GHR-/-) mice and GH-deficient lit/lit mice before and after exogenous GH administration. MGF mRNA in skeletal muscle was increased in bGH mice whereas it was decreased in GHR-/- mice compared with control animals. Exogenous GH administration to dwarf lit/lit mice significantly increased muscle MGF but not IGF-IEa mRNA 4 h after treatment. Twelve hours after GH treatment, both MGF and IGF-IEa mRNAs in muscle were increased compared with vehicle-treated lit/lit mice. In contrast in GH-sufficient lit/+ mice, both MGF and IGF-IEa mRNAs were increased 4 h after and returned to the basal level 12 h after GH treatment. Hepatic IGF-I isoforms were regulated in parallel by GH. Thus, our results demonstrated that: (1) MGF mRNA in skeletal muscle is expressed in parallel with GH action; (2) MGF mRNA in muscle is produced preferentially in the situation of GH deficiency in contrast to the pattern in the GH-sufficient state; and (3) the induction of IGF-I isoforms by GH is tissue-specific.

Figure 1. Detection of IGF-IEa and MGF mRNA in skeletal muscle

A, alternative splicing process producing murine IGF-I isoforms and the position of primers used in this study. The grey box indicates the coding region of mature IGF-I protein. The black box indicates the 52-base pair nucleotides that are inserted in IGF-IEb protein. B, detection of both IGF-IEa and IGF-IEb (MGF) in skeletal muscle of wild-type mice (lane: 1–3 and 7–9) and GHR−/− mice (lane: 4–6 and 10–12). Amplified products by RT-PCR were separated on 2% agarose gel and visualized with ethidium bromide staining. C, sequence analysis of the band shown in B confirmed that the amplified products were IGF-IEa and IGF-IEb (MGF), respectively.

Figure 2. Expression of IGF-IEa and MGF mRNA in skeletal muscle

Expression of IGF-IEa (A) and MGF (B) mRNA in skeletal muscle in bGH, GHR−/− and lit/lit mice. Non-transgenic or non-knockout littermates were used as controls for bGH or GHR−/−, respectively. lit/+ mice were used as controls for lit/lit mice; n = 6 in each group. *P < 0.05; **P < 0.01.

Figure 3. Expression of IGF-IEa and MGF mRNA in skeletal muscle

Expression of IGF-IEa (A and C) and MGF (B and D) mRNA in skeletal muscle of lit/+ mice or lit/lit mice 4 h (A and B) or 12 h (C and D) after GH- or vehicle-treatment; n = 5 in each group. **P < 0.01; NS, not significant.

Figure 4. Expression of IGF-IEa and MGF mRNA in liver

Expression of IGF-IEa (A and C) and MGF (B and D) mRNA in liver of lit/+ mice or lit/lit mice 4 h (A and B) or 12 h (C and D) after GH- or vehicle-treatment; n = 5 in each group. **P < 0.01; ***P < 0.001; NS, not significant.