Phenotypic expression of IGF binding protein transcripts in muscle, in vitro and in vivo

The actions of the insulin-like growth factors (IGF-I and ICF-II) which are essential components of skeletal muscle growth are mediated via their rece ptors and modulated by six binding proteins (IGFBPs). We studied IGFBP tran scripts in C2C12 cell cultures and in adult control and denervated gastrocn emius muscle. IGFBP-2, -4, -5, and -6 were detected in C2C12 cells. IGFBP-6 mRNA levels remained unchanged, IGFBP-2 levels decreased and IGFBP-4 and - 5 increased over 1, 5, and 9 days after serum reduction. In a range of adul t muscles studied, IGFBP-4 mRNA levels were similar, IGFBP-5 was present at different levels in slow and fast muscles and IGFBP-6 had the lowest expre ssion in the tibialis anterior. Denervation resulted in dramatic up-regulat ion of IGFBP-4 and -5 transcripts but there was no change in IGFBP-6. These results suggest that either lack of neural input and/or mechanical loading , both of which contribute to muscle atrophy, affect IGFBP expression. (C) 2000 Academic Press.