Regulation of IGF-I, IGFBP-4 and IGFBP-5 gene expression by loading in mouse skeletal muscle.

Gene expression of IGF-I, IGFBP-4 and IGFBP-5 was studied in hindhimb skeletal muscle of mice, which were either overloaded or unloaded for 8 days. Overloading induced a 15% hypertrophy in soleus muscle associated with a 60% increase of IGF-I transcript levels and a doubling of IGFBP-4 mRNA levels. IGFBP-5 mRNA levels were decreased to one third of the control value. Changes in IGFBPs mRNA always preceded changes in IGF-I gene expression. Unloading by hindlimb suspension resulted in atrophy of soleus muscle (20%) and phenotype change towards the fast type associated with a transient decrease of IGF-I mRNA (30%) and a sustained increase (x2) of IGFBP-5 transcript. These alterations in IGFBPs expression, in unloaded or overloaded soleus, suggest that they may play a role in skeletal muscle adaptation to changes in loading.