Hsieh, T.; Rosario, C.; Duan, C.*: INSULIN-LIKE GROWTH FACTOR (IGF) BINDING PROTEIN-5 STIMULATES CELL MOTILITY THROUGH AN IGF-INDEPENDENT MECHANISM THAT MAY INVOLVE IN ITS NUCLEAR TRANSLOCATION
Porcine vascular smooth muscle cells (VSMCs) secrete insulin-like growth factor binding protein-5 (IGFBP-5). In this report, we show that IGFBP-5 regulates VSMC motility through an IGF-dependent as well as an IGF-I-independent mechanism. The migration-promoting effects of IGFBP-5 and/or IGF-I were analyzed by a trans-well migration assay (directed migration) and a gold particle cell motility assay (random motility). IGF-I is a strong regulator of directed migration in VSMCs. When incubated with IGF-I at equal molar concentration, IGFBP-5 caused a 3.6-fold increase in directed migration towards IGF-I. IGFBP-5 alone caused a significant increase in basal migration and random motility. While addition of an IGF neutralizing antibody abolished the chemotactic effect of IGF-I, it did not affect the IGFBP-5-induced changes, indicating such change is not due to the interaction between IGFBP-5 and endogenous IGF-I. To investigate the IGF-I-independent mechanism(s) that IGFBP-5 utilizes to alter VSMC motility, the nuclear localization of IGFBP-5 was examined by immunocytochemistry. IGFBP-5 was easily detectable in the nuclei of cultured VSMCs. When added to cultured VSMCs, Cy3-tagged IGFBP-5 was translocated into nuclei. In addition to a 1.7 fold increase in IGF-I-induced migration, targeted expressing of IGFBP-5 in VSMC nuclei resulted in a 6-fold increase in the basal migration and motility in the absence of IGF-I. These findings suggest that IGFBP-5 plays an important role in regulating VSMC motility through a ligand-independent mechanism that may involve in nuclear translocation of this unique IGFBP (Supported by NIH R01HL60679 and NSF Grant 978911).