Insulin-like growth factor I (IGF-I) binding protein-5 (IGFBP-5) is produced by mesangial cells (MCs) and likely functions to modulate glomerular IGF-I activity. Although IGFBP-5 may be inhibitory for IGF-stimulated MC activity, preliminary studies suggested that IGFBP-5 acts directly on MCs. To investigate this further, we evaluated the effects of IGFBP-5 on rat MC migration. We found that the carboxy-truncated fragment, IGFBP-5-(1–169), inhibited IGF-I-stimulated migration, but intact IGFBP-5 simulated migration when IGF-I was not present. Demonstration that125I-labeled IGFBP-5 directly binds to MCs further supports an independent role for IGFBP-5. Because heparin inhibited MC binding of125I-IGFBP-5, we tested the heparin binding peptide, IGFBP-5-(201–218), for stimulatory activity. IGFBP-5-(201–218) stimulated MC migration, and this effect was inhibited by heparin. Because the disintegrin, kistrin, blocked IGF-I-induced migration but not migration induced by IGFBP-5-(201–218), the migratory induction mechanism for the two peptides is different. These data indicate that separate, specific regions of IGFBP-5 are responsible for interactive effects with IGF-I as well as direct effects on MC activity.

These data demonstrate that IGFBP-5201-218 preferentially activates Cdc42 and induces the formation of long filopodia with unique substratum attachments that produce a novel mode of locomotion.

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