Semaphorin 3B (SEMA3B) induced apoptosis in cancer cells is associated with inhibition of the protein kinase B (PKB)/AKT pathway

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Semaphorin 3B (SEMA3B) is a secreted member of the semaphorin family that is characterized by the presence of a highly conserved semaphorin domain. These proteins have been implicated in axon guidance by sending repulsive cues to growing axons in species from flies to vertebrate. Recently, ourselves and others have described tumor suppressor activity of SEMA3B in non-small cell lung cancer (NSCLC) and ovarian cancer cells (Tomizawa et al, PNAS 98: 13954, 2001; Tse et al, Cancer Res. 62: 542, 2002). We have also shown the antagonistic relationship between VEGF₁₆₅ and SEMA3B, which coincided with other semaphorin family members. We now investigate the signaling pathway(s) used by SEMA3B to inhibit cell growth or induce apoptosis in lung and breast cancer cells. Protein kinase B (PKB)/AKT promotes cell survival signals through the phosphoinositide 3-kinase (PI3K) pathway, leading to inactivation of a series of pro-apoptotic proteins. We found that transfection of various lung cancer cell lines with the plasmid encoding SEMA3B severely restricts proliferation and clonogenic potential of these cells. The conditioned medium derived from COS7 cells transiently expressing SEMA3B (exogenous SEMA3B) also inhibited lung cancer growth in vitro. Furthermore, we observed that exogenous treatment with SEMA3B inhibited or decreased AKT phosphorylation in serine 473 leading to decreased AKT activity. We also observed SEMA3B induced changes in phosphorylation of other proteins important in apoptotic pathways including BAD, GSK-3beta, FKHR, and PTEN. These results lead to the hypothesis that apoptotic effects of SEMA3B on tumor cells might be mediated in part via blockade of the AKT pathway.