Endocrine Abstracts

The insulin-like growth factor binding proteins (IGFBPs-1 to -6) complex IGF-I and -II and were originally assumed to limit IGF access to the main IGF signalling receptor (IGFR) and decrease IGF activity. IGFBP-5 is the most conserved of the IGFBPs. Examination of its primary sequence reveals that, in addition to the key IGF binding domain, additional functional motifs are present that are not related to its IGF binding function.

IGFBP-5 is upregulated during differentiation of several key lineages. We initially investigated the hypothesis that IGFBP-5 has an IGF-independent role in myogenesis. Using a non-IGF binding mutant form of IGFBP-5, we partitioned its function into IGF-dependent inhibition of myoblast differentiation, and IGF-independent cell survival (Cobb et al. 2004 J. Cell. Sci 117:1737). As IGFBP-5 is also induced in some cancers, we suggest that IGFBP-5 has an important role in the determination of cell fate. The aim of the current study was to extend IGF-independent actions of IGFBP-5 and examine i, direct effects on differentiation and ii, intracellular signalling pathways.

C2 myoblasts were transfected (pBabe retrovirus) with either wild-type IGFBP-5 (wtBP5) or non-IGF binding IGFBP-5 (mutBP5) and induced to differentiate by serum withdrawal. WtBP5 inhibited, but surprisingly mutBP5 accelerated, myogenesis, assessed by cell morphology, myosin heavy chain (MHC) immunocytochemistry and MHC protein levels (P<0.01). IGFR phosphorylation was decreased by wtBP5 (P<0.01) and unchanged by mutBP5. Phosphorylation of the key IGFR target, Akt, was decreased by wtBP5 but unexpectedly increased two-fold by mutBP5 (P<0.01); mutBP5 also increased the activity of Akt downstream targets (P-Bad S136, p21). Two other key myogenic protiens, p38MAPK and RhoA, as well as the anti-apoptotic Bcl-XL, were activated by mutBP5.

In summary, IGFBP-5 may activate key signalling pathways that regulate apoptosis and differentiation, independent of IGFR activation and via unidentified upstream mechanisms which could regulate cell fate in vivo.