Ganglioside G_D2 in small cell lung cancer cell lines: Enhancement of cell proliferation and mediation of apoptosis

Expression levels of gangliosides and glycosyltransferase genes responsible for their syntheses in human lung cancer cell lines and a normal bronchial epithelial cell line were analyzed. Both non-small cell lung cancers and small cell lung cancers (SCLCs) mainly expressed G_M2 and G_M1, whereas only SCLCs expressed b-series gangliosides, such as G_D2, G_D1b, and G_T1b. Accordingly, many SCLC cell lines showed up-regulation of the G_D3 synthase gene. Consequently, we introduced G_D3 synthase cDNA into a SCLC line with low expression of b-series gangliosides and analyzed the effects of newly expressed gangliosides on tumor phenotypes. The transfectant cells expressing high levels of G_D2 and G_D3 exhibited markedly increased growth rates and strongly enhanced invasion activities. Addition of anti-G_D2 monoclonal antibodies into the culture medium of these cells resulted in the marked growth suppression of G_D2-expressing cell lines with reduced activation levels of mitogen-activated protein kinases but not of nonexpressants, suggesting that G_D2 plays important roles in cell proliferation. Moreover, G_D2-expressing cells treated with anti-G_D2 antibodies showed features of apoptotic cell death at 30 min after addition of antibodies, i.e., shrinkage of cytoplasm, binding of Annexin V, and staining with propidium iodide, followed by DNA fragmentation. This G_D2-mediated apoptosis was associated with caspase-3 activation and partly inhibited by a caspase inhibitor, z-Val-Ala-Asp-fluoromethyl ketone. The finding that anti-G_D2 antibodies suppressed the cell growth and induced apoptosis of SCLC cells strongly suggested the usefulness of G_D2 as a target for the therapy of disastrous cancer, although the precise mechanisms for apoptosis remain to be clarified.