Heparan Sulfate Proteoglycans in Rat Ovarian Granulosa

Rat ovarian granulosa cells synthesize two distinct species of plasma membrane-intercalated heparan sulfate (HS) proteoglycans; glycosylphosphatidylinositol (GP1)-anchored and core protein-intercalated HS proteoglycans. Both species of HS proteoglycans are primarily localized on the plasma membrane. Cell surface localization of GPI-anchored and protein-intercalated HS proteoglycans can be determined by their accessibility to exogenously added phosphatidylinositol-specific phospholipase C (PI-PLC) and trypsin, respectively. Kinetic parameters for the processes involving their transfer from the Golgi to the cell surface, endocytosis and secretion, and the modes of intracellular degradation were determined by metabolic labeling experiments using [S5S]sulfate and various chase protocols in combination with the use of PI-PLC and trypsin in rat ovarian granulosa cells. The experiments demonstrated that (i) both HS proteoglycan species are transferred from the Golgi to the cell surface with an average transit time of -12 min. (ii) GPI-anchored HS proteoglycans are endocytosed with a t H -3 h, without being shed into the medium, and they are rapidly degraded, tLh -25 min, without generating recognizable degradation intermediates. (iii) Protein-intercalated HS proteoglycans are partly (-30%) shed from the cell surface into the medium and the remaining -70% are endocytosed with a tIA -4 h. After endocytosis, they undergo a slow (tLh -4 h) stepwise degradation generating distinct HS oligosaccharides as degradation intermediates. These results indicate that the GPI-anchored and the protein-intercalated HS proteoglycans have distinct secretory, endocytotic, and intracellular degradation pathways probably due to the differences in their anchor structures.