Glycolipid biosynthesis: biosynthesis of mannose- and fucose-containing glycolipids by HeLa cells

Distribution of glycosyltransferases among Golgi apparatus subfractions from liver and hepatomas of the rat

Glycosyltransferase activities of highly purified fractions of Golgi apparatus, plasma membrane and endoplasmic reticulum, all from the same homogenates, were analyzed and compared. Additionally, Golgi apparatus were unstacked and the individual cisternae separated into fractions enriched in cis, median and trans elements using the technique of preparative free-flow electrophoresis. Golgi apparatus from both liver and hepatomas were enriched in all glycosyltransferases compared to endoplasmic reticulum and plasma membranes. However, Golgi apparatus from hepatomas showed both elevated fucosyltransferase and galactosyltransferase activities but reduced sialyltransferase and dipeptidyl peptidase IV (DPP IV) activities compared to liver. Activity of N-acetylglucosaminyltransferase was approximately the same in both liver and hepatoma Golgi apparatus. With normal liver, sialyl- and galactosyltransferase activities and DPP IV showed a marked cis-to-trans gradient of activity. Fucosyltransferase was concentrated in two regions of the electrophoretic separations, one corresponding to cis cisternae and one corresponding to trans cisternae. N-Acetylglucosaminyltransferase activity was more widely distributed but the endogenous acceptor activity was predominantly cis. With hepatoma Golgi apparatus, the pattern for DPP IV was similar to that for liver but those of sialyl- and galactosyltransferases differed markedly from liver. Instead of activity increasing cis to trans, the activities for sialyl- and galactosyltransferases decreased. For fucosyltransferases, activity dependent on exogenous acceptor was medial whereas with endogenous acceptor, two activity peaks, cis and trans, still were observed. For N-acetylglucosaminyltransferase the pattern for hepatoma was similar to that for liver. The results indicate alterations in the distribution of glycosyltransferase activities within the Golgi apparatus in hepatotumorigenesis that may reflect altered cell surface glycosylation patterns.

Role of the Golgi apparatus in cellular pathology

The Golgi apparatus response to pathological disorders is predominantly as an intermediary component of membrane biogenesis where it is involved in processing, sorting and secretion of materials via secretory granules, and in the formation of lysosomes. A common initial response of the Golgi apparatus to any stress is an alteration or cessation of secretory activity. In the transformed cell, the Golgi apparatus is altered both morphologically and biochemically, suggesting a shift from a secretory to a membrane-generating mode of functioning. However, since fewer or less well-developed Golgi apparatus are frequently found in transformed cells, analytical methods of membrane isolation developed for normal tissues may not always yield equivalent results when applied to tumors. Cell surface alterations characteristic of malignant cells may result from modifications occurring at the level of the Golgi apparatus. Some lysosomal dysfunctions may result from underglycosylation of acid hydrolases by the Golgi apparatus. The use of cell-free systems between endoplasmic reticulum and Golgi apparatus or within Golgi apparatus cisterane is providing a new approach to the elucidation of the role of the Golgi apparatus in normal as well as pathological states.

Synthesis of glycoprotein, glycolipid, protein, and lipid in synchronized L5178Y cells

Synthesis of four macromolecular classes found in membranes-glycoprotein, glycolipid, protein, and lipid-was measured as a function of time of the cell cycle in synchronized L5178Y cells. Incorporation of leucine, choline, fucose, glucosamine, or thymidine into the cells, protein, nucleic acid, or lipid was measured by pulse-labeling for (1/2) hr at (1/2) hr intervals after release from the mitotic block. The amount of protein, lipid, glycoprotein, or glycolipid released or secreted into the medium by the L5178Y cells was also measured as a function of time of the cell cycle. Cellular protein was found to be synthesized throughout the cell cycle, with the highest synthesis occurring in the S period; synthesis was depressed in the M period. Cellular glycoprotein was synthesized at approximately the same times as protein, except that the rates of glycoprotein synthesis in the S period relative to other periods were much greater than for protein. Secreted protein was synthesized throughout the cell cycle without any general pattern, except that secretion was elevated in the late S and G(2) periods. Secreted glycoprotein was similar to secreted protein. Cellular lipid and cellular glycolipid were synthesized almost exclusively in the G(2) and M periods; there was no synthesis in the G(1) and S periods. Release or secretion of glycolipid and lipid also occurred in the G(2) and M periods.