Insulin-like Growth Factor-I Induces Renal Cell Hypertrophy via a Calcineurin-dependent Mechanism

Insulin-like growth factor-I (IGF-I) may play an important role in the development of renal hypertrophy. In this study we determined the effect of IGF-I on cultured mesangial cells (MCs) and examined activation of key signaling pathways. IGF-I induced hypertrophy as determined by an increase in cell size and an increase in protein to DNA ratio and increased accumulation of extracellular matrix (ECM) proteins. IGF-I also activated both Erk1/Erk2 MAPK and phosphatidylinositol 3-kinase (PI3K) in MCs. Inhibition of either MAPK or PI3K, however, had no effect on IGF-I-induced hypertrophy or ECM production. Next, we examined the effect of IGF-I on activation of the calcium-dependent phosphatase calcineurin. IGF-I treatment stimulated calcineurin activity and increased the protein levels of calcineurin and the calcineurin binding protein, calmodulin. Cyclosporin A, an inhibitor of calcineurin, blocked both IGF-I-mediated hypertrophy and up-regulation of ECM. In addition, calcineurin resulted in sustained Akt activation, indicating possible cross-talk with other signaling pathways. Finally, IGF-I treatment resulted in the calcineurindependent nuclear localization of NFATc1. Therefore, IGF-I induces hypertrophy and increases ECM accumulation in MCs. IGF-I-mediated hypertrophy is associated with activation of Erk1/Erk2 MAPK and PI3K but does not require either of these pathways. Instead, IGF-I mediates hypertrophy via a calcineurin-dependent pathway.