Gangliosides expressed by the renal cell carcinoma cell line SK-RC-45 are involved in tumor-induced apoptosis of T cells

It is now understood that the genetic plasticity of cancer cells can lead to alterations that confer selective growth advantages to the tumor, some of which play a role in immune escape. A number of mutations veiling tumor cells from host immune defenses have been well characterized but more recent studies suggest that a variety of tumors can also express products that are actually toxic for the immune effectors. A component of this tumor-induced T-cell death has been attributed to receptor-mediated apoptosis. Some tumors, however, synthesize soluble factors that mediate similar effects. In this regard, we previously showed that supernatants from explanted renal cell carcinoma (RCC) tumors sensitized normal T cells to activation induced cell death, and the responsible products had the features of gangliosides. We have also shown that renal tumor lines, including SK-RC-45, induce apoptosis of both Jurkat cells and normal T lymphocytes. Here, we used the ganglioside synthesis inhibitor PPPP to define the role of gangliosides in RCC cell line (SK-RC-45)-mediated T cell and Jurkat cell apoptosis and to elucidate the proapoptotic molecular events by which the glycosphingolipids produce their effects. The ganglioside-synthesizing SK-RC-45 line stimulated the TUNEL (terminal deoxynucleotidyl transferase-mediated nick end labeling) positivity of cocultured T cells by a mechanism that involved decreasing lymphocyte expression levels of Bcl-2 and Bcl-(XL), inducing cytochrome c release from their mitochondria and activating caspases 9 and 3. These proapoptotic events were partially or completely abrogated when tumor cells were pretreated with PPPP for 5 days before the SK-RC-45/T lymphocyte coincubation, a regimen that reduced tumor-associated ganglioside levels by 70-80%. Our results suggest that gangliosides may be key mediators of RCC-induced T-cell apoptosis and imply that they contribute to the T-cell dysfunction in the tumor microenvironment.