| Insulin-like growth factor I (IGF-I) is a potent stimulator of survival and mitogenesis in human neuroblastoma (NBL) cells, and contributes to the tumorigenic properties of NBL cancer. IGF-I is highly expressed in bone, the most common site of NBL metastasis, and may also regulate the motility of NBL cells, increasing their potential for metastasis. We find that IGF-I causes dynamic changes in the morphology of SH-SY5Y human NBL cells that lead to lamellipodium extension, the membrane protrusion found at the leading edge of motile cells. IGF-I increases the motility of SH-SY5Y cells, depending largely upon phosphatidylinositol 3-kinase (PI-3K) second messenger signaling. IGF-I activates rac, a known stimulator of actin polymerization that underlies lamellipodium extension, and its targets, the p21-activated kinases (PAKs) 1 and 2. IGF-1 increases cofilin phosphorylation downstream of PI-3K, rac, and LIM kinase. These findings suggest that regulation of cofilin phosphorylation is part of the mechanism of IGF-I mediated lamellipodium extension. While expression of LIMK and cofilin mutants has little effect on the short term elaboration of lamellipodium in SH-SY5Y cells stimulated with IGF-I, overexpression of LIMK or wild type cofilin is inhibitory to IGF-I mediated motility. Expression of dominant negative LIMK or constitutively active cofilin increases motility in serum-starved SH-SY5Y cells. Thus, cofilin activity is required for IGF-I mediated motility, but not for short-term lamellipodium extension. IGF-I stimulated inactivation of cofilin must be spatially regulated within the cell or coordinated with re-activation by phosphatases to contribute to the process of actin dynamics within the lamellipodium. |
