To investigate the cellular functions of sulfated
glycosphingolipids, we introduced the
cerebroside sulfotransferase (CST) gene into J5 cells, a subclone of 3LL
Lewis lung carcinoma cells. The J5 cells lack
acidic glycosphingolipids but accumulate their common biosynthetic precursor,
lactosylceramide. We established the stable CST transfectants, J5/CST-1 and J5/CST-2 clones, highly expressing sulfated
lactosylceramide (SM3). Both clones exhibited more spherical morphology in comparison to mock transfectant, and their adhesiveness to
fibronectin and
laminin was significantly lower. The loss of cell-substratum interactions in these SM3-expressing cells could be attributed to decreased expression of
integrins (alpha(5), alpha(6), and beta(1)) on the cell surface and their whole cellular levels. However, the levels of H-2K(b) and H-2D(b)
antigens remained unchanged.
Reverse transcriptase-polymerase chain reaction and Northern blot analyses for these
integrins exhibited significant decrease of beta(1) gene expression in J5/CST-1 and 2, but there was no change in the levels of alpha(5) and alpha(6) transcripts. Deglycosylation by
endoglycosidase H treatment clearly demonstrated that the precursor form of beta(1)
integrin, possessing high
mannose oligosaccharide chains, was preferentially decreased in the CST transfectants. These results demonstrate that endogenous SM3 negatively regulates beta(1)
integrin expression at the transcriptional level, and the decrease of alpha
integrin proteins in the CST transfectants was due to the post-transcriptional modification. We suggest the putative importance of the intracellular pre-beta(1)
integrin pool for normal
integrin maturation and subsequent function. Although the rates of cell proliferation in vitro for mock and CST transfectants were similar, tumorigenicity of J5/CST-1 and -2 cells inoculated into syngeneic C57/BL6 mice was greatly decreased or even absent. This was probably due to global loss of the efficient cell-matrix interactions, which are essential for the development of malignant
tumors in vivo. Thus, we showed the evidence that cellular SM3 negatively regulates the cell-substratum interaction, resulting in the loss of tumorigenicity.