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 s mall cell lung cancers (SCLCs) mainly expressed G(M2) and G(M1), whereas on ly 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) synth ase gene. Consequently, we introduced G(D3) synthase cDNA into a SCLC line with low expression of b-series gangliosides and analyzed the effects of ne wly expressed gangliosides on tumor phenotypes. The transfectant cells expr essing high levels of G(D2) and G(D3) exhibited markedly increased growth r ates and strongly enhanced invasion activities. Addition of anti-G(D2) mono clonal antibodies into the culture medium of these cells resulted in the ma rked growth suppression of G(D2)-expressing cell lines with reduced activat ion 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., shri nkage of cytoplasm, binding of Annexin V, and staining with propidium iodid e, followed by DNA fragmentation. This G(D2)-mediated apoptosis was associa ted with caspase-3 activation and partly inhibited by a caspase inhibitor, z-Val-Ala-Asp-fluoromethyl ketone. The finding that anti-G(D2) antibodies s uppressed the cell growth and induced apoptosis of SCLC cells strongly sugg ested the usefulness of G(D2) as a target for the therapy of disastrous can cer, although the precise mechanisms for apoptosis remain to be clarified.