An actin-binding protein Girdin regulates the motility of breast cancer cells.

Girdin (girders of actin filaments) is a novel actin-binding Akt substrate that plays an important role in actin organization and Akt-dependent cell motility in fibroblasts. Here, we find that Girdin is expressed in a variety of cancer cell lines, including the breast cancer cell line MDA-MB-231, and is phosphorylated by the stimulation of insulin-like growth factor (IGF-I). In vitro migration and invasion assays showed that Girdin is required for the IGF-I-dependent cell movement of MDA-MB-231 cells. Short hairpin interfering RNA directed against Girdin markedly inhibited the metastasis of s.c. transplanted MDA-MB-231 cells in nude mice. In addition, Girdin is highly expressed in a variety of human malignant tissues, including breast, colon, lung, and uterine cervical carcinomas. These findings highlight the important role of Girdin in tumor progression in which the Akt signaling pathway is aberrantly activated.

Dok-4 regulates GDNF-dependent neurite outgrowth through downstream activation of Rap1 and mitogen-activated protein kinase.

During development of the central and peripheral nervous systems, neurite extension mediated via glial-cell-line-derived neurotrophic factor (GDNF) and its receptor RET is critical for neuronal differentiation. In the present study, we investigated the role of the RET substrate Dok-4 in neurite outgrowth induced by the GDNF/RET signaling pathway. In TGW neuroblastoma cells, which endogenously express both RET and Dok-4, depletion of Dok-4 through treatment with small interfering RNA resulted in a marked decrease in GDNF-stimulated neurite outgrowth. By contrast, exogenous expression of wild-type Dok-4 induced sustained p44/42 mitogen-activated protein kinase (ERK1/2) activation and enhanced neurite outgrowth. Expression of Dok-4 mutants in which the tyrosine residues at codons 187, 220 and 270, conserved between Dok-4, -5, and -6, were each replaced with a phenylalanine inhibited sustained ERK1/2 activation and neurite outgrowth. We also found that Dok-4 induced a significant activation of the small G protein Rap1 and that expression of a dominant active Rap1 mutant restored neurite outgrowth in Dok-4-depleted cells. By contrast, expression of a dominant negative Rap1 mutant impaired GDNF-stimulated neurite outgrowth from TGW cells. Finally, we found that neurite formation in cultured rat hippocampal neurons was enhanced by the expression of Dok-4. Together, our results suggest that Dok-4, through activation of the Rap1-ERK1/2 pathway, regulates GDNF-mediated neurite outgrowth during neuronal development.