Involvement of β1-Integrin Up-regulation in Basic Fibroblast Growth Factor- and Epidermal Growth Factor-induced Proliferation of Mouse Neuroepithelial Cells

In neural stem cells, basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) promote cell proliferation and self-renewal. In the bFGF-and EGF-responsive neural stem cells, beta 1-integrin also plays important roles in crucial cellular processes, including proliferation, migration, and apoptosis. The cross-talk of the signaling pathways mediated by these growth factors and beta 1-integrin, however, has not been fully elucidated. Here we report a novel molecular mechanism through which bFGF or EGF promotes the proliferation of mouse neuroepithelial cells (NECs). In the NECs, total beta 1-integrin expression levels and proliferation were dose-dependently increased by bFGF but not by EGF. EGF rather than bFGF strongly induced the increase of beta 1-integrin localization on the NEC surface. bFGF-and EGF-induced beta 1-integrin up-regulation and proliferation were inhibited after treatment with a mitogen-activated protein kinase kinase inhibitor, U0126, which indicates the dependence on the mitogen-activated protein kinase pathway. Involvement of beta 1-integrin in bFGF-and EGF-induced proliferation was confirmed by the finding that NEC proliferation and adhesion to fibronectin-coated dishes were inhibited by knockdown of beta 1-integrin using small interfering RNA. On the other hand, apoptosis was induced in NECs treated with RGD peptide, a small beta 1-integrin inhibitor peptide with the Arg-Gly-Asp motif, but it was independent of beta 1-integrin expression levels. Those results suggest that regulation of beta 1-integrin expression/localization is involved in cellular processes, such as proliferation, induced by bFGF and EGF in NECs. The mechanism underlying the proliferation through beta 1-integrin would not be expected to be completely identical, however, for bFGF and EGF.