Epithelial-mesenchymal transition (EMT) is a differentiation switch that allows epithelial cells to migrate and invade through their local tissue environment to circulation system during metastasis. Both of cancer and mesenchymal cells in the tumor microenvironment can secrete a vast diversity of signaling proteins to act on EMT. It has been reported that some secreted glycoproteins acting on EMT can modulate cancer cell migration and invasion through the TGF-\#946; receptor signaling pathway. SCUBE3 (signal peptide-CUB-EGF-like domain-containing protein 3), one of the evolutionarily-conserved SCUBE gene family, encodes a secreted plasma membrane-associated glycoprotein which contains a N-terminal signal peptide sequence, nine copies of EGF-like repeats, a N-linked glycosylated spacer region and a C-terminal CUB (complement proteins C1r/C1s, Uegf, and Bmp1) domain. In this study, we found that the SCUBE3 mRNA signature correlates positively with invasion ability in lung cancer cell lines, as well as the clinical outcome of NSCLC (non-small cell lung cancer) patients. Knockdown of SCUBE3 expression in highly invasive lung cancer cell lines, CL1-5 and Hop62, could inhibit the abilities of cell migration and invasion, but not cell adhesion or proliferation. By using protein arrays and co-immunoprecipitation assays, we found that SCUBE3 interacts with TGFBR2 via the C-terminal CUB domain. The phosphorylation of Smad2/3, the downstream targets of TGF-\#946; receptors, could be induced when cells were treated with SCUBE3. Conversely, SCUBE3-induced Smad2/3 phosphorylation could be blocked by a TGF-\#946; receptor inhibitor, SB431542. Purified SCUBE3 CUB domain could also enhance the PAI-1 (a downstream of Smad2/3) luciferase reporter activity. Furthermore, the treatments of A549 and EKVX lung cancer cells, which lack endogenous SCUBE3 expression, with the purified SCUBE3 CUB domain could upregulate the expression of the EMT regulators, vimentin and Slug, and promote cell migration. In conclusion, our findings suggest that SCUBE3 can trigger EMT through inducing the TGF-\#946; receptor signaling pathway and may function as a novel invasion promoter in lung cancer.
Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3891.
100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO