Inhibition of tumor progression by inhibition of ganglioside biosynthesis: Models and mechanisms

Gangliosides are complex sphingolipids containing polar head groups, located primarily in cell membranes. They have long been recognized as sites of biological recognition and are involved in complex signal transduction processes, cell proliferation, differentiation, adhesion and apoptosis. Thus, gangliosides are potentially influential in tumor development and progression. Our laboratory and this study have established that gangliosides have the potential to influence tumor growth, and that experimental manipulation of ganglioside synthesis is able to slow down tumor progression and delay tumor onset. We found that treatment with the iminosugar OGT2378, an inhibitor of ganglioside synthesis at the level of glucosylceramide synthase, significantly impedes neuroectodermal tumor growth in an orthotopic, syngeneic MEB4 melanoma tumor model and, in exploratory studies, in an orthotopic, xenogeneic SKNSH neuroblastoma tumor model. In addition, in an orthotopic, xenogeneic LAN-1 neuroblastoma tumor model, treatment decreased the propensity of the tumor to metastasize.Morphology, growth characteristics, or apoptosis of these cells were not altered in response to OGT2378 treatment in vitro. Histopathologically, tumor tissue from ganglioside depleted MEB4 melanoma tumors appeared more highly differentiated and showed a strong reduction of PCNA expression indicating decreased mitotic activity. To further investigate the mechanism involved, ganglioside depleted neuroectodermal tumor cells were cultured in normoxic and hypoxic conditions. Ganglioside depleted neuroectodermal tumor cells exhibited a significant increase in apoptosis when cultured in severe hypoxia, an effect that was partly reversed by both the addition of exogenous gangliosides and inhibition of the de novo ceramide synthesis. Ganglioside depletion had no effect on the activation of either HIF-1alpha, protein kinase B/Akt or mTOR caused by hypoxia, while VEGF secretion was moderately augmented.Our results establish that ganglioside inhibition significantly inhibits tumor growth. We have implicated cell proliferation, differentiation, cell signaling, apoptosis, and host immune responses as processes all altered by interference with tumor ganglioside metabolism. Both tumor cell membrane gangliosides as well as gangliosides shed in the tumor microenvironment play important roles in tumor development, suppressing host immune function and enhancing microenvironmental stromal cells (fibroblasts, endothelial cells) to provide favorable conditions for tumor growth, whereas ganglioside depletion impedes the tumor cell's stress response.