Starving cancer: the molecular mechanisms of nutrient transporter loss induced by anti-neoplastic sphingolipids
Sphingolipids are found in all kingdoms of life. In mammals, sphingolipids regulate multiple cellular processes. Ceramide is a sphingolipid of interest to cancer biologists due to its regulation of cell cycle, membrane morphology, cell migration, differentiation, and cell death. Disruptions in ceramide metabolism are associated with disease. Reduced ceramide in cancer cells leads to increased malignancy and metastasis. Conversely, increasing cellular ceramide leads to differentiation, proliferative arrest, and cell death. Our lab has developed synthetic sphingolipids that effectively and selectively induce cancer cell death in vivo. We have recently described one such synthetic sphingolipid, SH-BC-893 (893), that is selective and effective in murine cancer models without notable toxicity to the animal.
Our publications show that the selectivity of sphingolipids for cancer cells is due to oncogene-driven constitutive anabolism of cancer cells. Highly anabolic TSC2 knockout cells are resistant to sphingolipid-induced death but are sensitized by expression of oncogenic KRAS.
893 inhibits multiple, parallel cellular nutrient acquisition mechanisms. One mechanism is by decreasing surface nutrient transporter expression, an activity shared with the endogenous sphingolipid ceramide. However, 893 further limits nutrient acquisition by reducing lysosomal fusion to prevent access to intracellular metabolites from LDL, autophagosomes, and macropinosomes. These phenotypes were not observed with ceramide.
The molecular mechanism through which ceramide and synthetic sphingolipids induce loss of surface nutrient transporters is not yet known. The work presented here suggests that recycling of nutrient transporter proteins is inhibited. We show that actin remodeling, ARF6 inactivation, PKC and Rac1 activation all appear to contribute to sphingolipid-induced nutrient transporter loss. ARF6 inactivation may be downstream of sphingolipid-induced PP2A activation similar to other effects. Preliminary data suggests ARF6 inactivation may result from PP2A-mediated de-phosphorylation of the ARF6 guanine nucleotide exchange factor Grp1, which activates ARF6. Both endogenous sphingolipids and 893 inactivate ARF6. Interestingly, ARF6 activation is associated with tumor progression, grade, and metastasis in multiple cancer classes. We found that combining sphingolipid with ARF6 inhibitors synergistically and selectively kill cancer cells. These findings clarify the mechanism of sphingolipid-induced cell death and identify inactivation of ARF6 playing a role in nutrient transporter loss.