This project concerns the significance and biochemical mechanisms of progressive losses of sialoglycosphingolipids (gangliosides) from the mammalian cell surface during tumorigenesis. In rodents in which hepatomas were induced by 2-acetylaminofluorene, glycolipid simplifications are late events of the tumorigenic progression, the end result of an enzymatic cascade, and correlated with metastatic potential. Higher gangliosides, including those lost during tumorigenesis, may bind fibronectins and other cell adhesion proteins, although new results suggest an additional ability of a fibronectin-cell surface receptor interaction involved in growth control not mediated by gangliosides. Yet ganglioside simplifications correlate with the reduced ability of malignant tumors to bind fibronectins and help explain how losses of gangliosides might facilitate metastasis through reduced cell adhesion. Relationships among ganglioside simplification, loss of fibronectin receptors, failure to bind fibronectins and other adhesion proteins, and metastatic potential have been analyzed and correlated by a combination of techniques and approaches. The critical question of a function for gangliosides in situ as surface receptors for fibronectins and other adhesion proteins was approached from near-neighbor analyses. A novel, photolabile, heterobifunctional cross-linking procedure was developed that is specific for galactose-containing glycoconjugates and has established the proximity of disialogangliosides and fibronectin at the cell surface. Mechanisms whereby gangliosides are transported from sites of synthesis at the Golgi apparatus to sites of insertion at the plasma membrane and certain internal membranes were also investigated along with the kinetics of ganglioside biosynthesis and transport. New information concerns the basic mechanisms leading to ganglioside simplification through studies of the nature of the enzymatic changes involved. Also under investigation is the apparent buildup of higher ganglioside precursors and products of the monosialoganglioside pathway that precedes or accompanies the depletion of the more complex higher gangliosides. (A)
