The Role of the Golgi Complex in Sulfate Metabolism

The formation of large aggregates by ionic interactions between acidic glucosaminoglycans and cationic secretory proteins has been proposed as one of the critical steps in the concentration process in the condensing vacuoles of secretory cells. In this paper, this hypothesis was tested by studies on the interactions between bovine chymotrypsinogen A and chondroitin sulfate as a simplified model. Small amounts of chondroitin sulfate were found able to induce chymotrypsinogen precipitation. Like zymogen granules, the resulting aggregates were moderately sensitive to ionic strength and insensitive to osmolality. Moreover, their pH dependence was similar to that of isolated zymogen granules. When sulfated glucosaminoglycans isolated from the zymogen granules of the guinea pig pancreas were used instead of chondroitin sulfate, the same kind of interactions with chymotrypsinogen were obtained. Our data support the hypothesis that the strong ionic inteFactions between those sulfated glucosaminoglycans and cationic proteins could be responsible for the concentration process.KEY WORDS chymotrypsinogen A 9 chondroitin sulfate glucosaminoglycans molecular aggregation zymogen granule condensation turbidity A sulfated macromolecular fraction has recently been isolated from the zymogen granules of the guinea pig pancreas (17, 18). Preliminary results indicate that this fraction is composed of acidic glucosaminoglycans, principally heparan sulfate and chondroitin sulfate (18). Such compounds have been implicated in the process by which pancreatic acinar cells concentrate their secretory proteins in the condensing vacuoles of the Golgi complex (23,16,9). The formation of large aggregates by ionic interactions between those polyanions and cationic secretory proteins could reduce the osmotic activity within the vacuoles and lead to concentration of their content by water loss. In this paper, this hypothesis was tested by studies on the interactions between bovine chymotrypsinogen A (ChTg) and chondroitin sulfate (Ch-SO4) as a simplified model. The results indicate that the ionic interactions of these compounds are strong enough to induce coprecipitation. Sulfated polyanions isolated from the zymogen granules of the guinea pig pancreas were found to interact with bovine ChTg in a manner comparable to that of Ch-SO4. MATERIALS AND METHODSBovine chymotrypsinogen A (Worthington Biochemical Corp., Freehold, N.J.) dissolved (1 mg/ml) in 2 mM Tris maleate buffer, pH 6, was mixed with solutions of J. CELL BIOLOGY 9 The Rockefeller University Press -0021-9525/78/0901-095151.00 951 on

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Ionic interactions between bovine chymotrypsinogen A and chondroitin sulfate A.B.C.. A possible model for molecular aggregation in zymogen granules.

Reggio¹,

Dagorn²

1978

“…Since carbohydrate chains of GAGs are largely assembled within the Golgi apparatus by multiple enzymes (31), a general defect in processing should affect GAG synthesis. Moreover, the apparent localization of sulfation to the trans-Gol~ apparatus (7,44) would serve to focus on later steps of the processing sequence.…”

mentioning

confidence: 99%

Glycosaminoglycan synthesis is depressed during mitosis and elevated during early G1.

Preston¹,

Regula²,

Sager³

et al. 1985

[35S]Sulfate incorporation was measured in populations of Chinese hamster ovary cells enriched for mitotics, early G1 cells, and interphase monolayers or suspensions. Incorporation was determined by biochemical analysis of extracts and quantitative autoradiography of thick sections. 90% of [35S]sulfate was incorporated into glycosaminoglycan (GAG). Incorporation was depressed fourfold in mitotics and stimulated by from two-to three-fold in early G1 cells relative to mixed interphase cells. GAG synthesis was maintained into late G2. Thus, the rate of GAG biosynthesis was correlated temporally with the detachment and reattachment of cells to substrate.Inhibitors of protein synthesis brought about the rapid arrest of GAG biosynthesis. However, xylosides, which bypass the requirement for core protein, did not bring oligosaccharide sulfation in mitotics to interphase levels. These observations indicate an inhibition of Golgi processing and are consistent with a generalized defect of membrane vesicle-mediated transport during mitosis.In their pioneering work on glycosaminoglycan (GAG) ~ synthesis by cultured cells, Kraemer and Tobey (14) showed that a significant fraction of trypsin-releasable (i.e., surface) GAGs was shed during mitosis. Although the Chinese hamster ovary (CHO) cells analyzed were grown in suspension, viewed from the perspective of recent studies of cell adhesion (e.g., reference 32), it seemed likely that the shedding of GAGs was related to the familiar ease with which mitotic cells are detached from monolayers. We hypothesized that any effect of shedding would be reinforced by a coordinate reduction in GAG synthesis that would delay replacement of surface GAGs, and that a rapid resumption of GAG synthesis in early G i would be required for cell reattachment.A parallel motivation for our studies arose from the accumulating evidence that membrane vesicle-mediated transport is depressed during mitosis. This is shown clearly in work from our laboratory demonstrating depression of endocytosis ( 1, 2) and transferrin recycling (35) and from that of Warren and colleagues showing a decrease in surface transferrin receptors (41), and in depression of histamine secretion (9). These studies focus primarily on fusion and budding events at the plasma membrane. In addition, the insertion of the G t Abbreviations used in this paper. CHO, Chinese hamster ovary; GAG, glycosaminoglycan.protein of vesicular stomatitis virus is also depressed during mitosis (42), which suggests a defect in its processing through the Golgi apparatus. Since carbohydrate chains of GAGs are largely assembled within the Golgi apparatus by multiple enzymes (31), a general defect in processing should affect GAG synthesis. Moreover, the apparent localization of sulfation to the trans-Gol~ apparatus (7, 44) would serve to focus on later steps of the processing sequence.We describe here the rates of labeled sulfate incorporation into CHO cells during mitosis, in their subsequent movement into G1, and in late G2. We followed incorporat...

“…The absence of secretory granules in GDF cells, therefore, may be due to substantially lower levels of synthesis of the secretory proteins and/or to a defect in the yet to be elucidated concentration mechanism(s) in the Golgi or the post-Golgi vesicles (13,18,27). For example, differences in the synthesis of sulfate-containing macromolecules (33,34,46) in the Golgi elements of the GDF and GEF neoplastic ceils might play a significant role.…”

Section: Discussionmentioning

confidence: 99%

Intracellular transport and storage of secretory proteins in relation to cytodifferentiation in neoplastic pancreatic acinar cells.

Becich¹,

Bendayan²,

Reddy³

1983

The pancreatic acinar carcinoma established in rat by Reddy and Rao (1977, Science 198:78-80) demonstrates heterogeneity of cytodifferentiation ranging from cells containing abundant well-developed secretory granules to those with virtually none. We examined the synthesis intracellular transport and storage of secretory proteins in secretory granule-enriched (GEF) and secretory granule-deficient (GDF) subpopulations of neoplastic acinar cells separable by Percoll gradient centrifugation, to determine the secretory process in cells with distinctly different cytodifferentiation. The cells pulse-labeled with [3H]leucine for 3 min and chase incubated for up to 4 h were analyzed by quantitative electron microscope autoradiography. In GEF neoplastic cells, the results of grain counts and relative grain density estimates establish that the label moves successively from rough endoplasmic reticulum (RER) ~ the Golgi apparatus ~ post-Golgi vesicles (vacuoles or immature granules) --~ mature secretory granules, in a manner reminiscent of the secretory process in normal pancreatic acinar cells. The presence of -40% of the label in association with secretory granules at 4 h postpulse indicates that GEF neoplastic cells retain (acquire) the essential regulatory controls of the secretory process. In GDF neoplastic acinar cells the drainage of label from RER is slower, but the peak label of -20% in the Golgi apparatus is reached relatively rapidly (10 min postpulse). The movement of label from the Golgi to the post-Golgi vesicles is evident; further delineation of the secretory process in GDF neoplastic cells, however, was not possible due to lack of secretory granule differentiation. The movement of label from RER --* the Golgi apparatus --> the post-Golgi vesicles suggests that GDF neoplastic cells also synthesize secretory proteins, but to a lesser extent than the GEF cells. The reason(s) for the inability of GDF cells to concentrate and store exportable proteins remain to be elucidated.IntraceUular aspects of the process of protein secretion have been unraveled in the adult pancreatic exocrine acinar cell by the classical autoradiographic studies of Caro and Palade (4) and . These studies have identified six successive steps in the secretory process namely, synthesis, segregation, intraceUular transport via the Golgi complex, concentration, intracellular storage of discharge (for detailed discussion of this process see references 18 and 27). The existence of such a secretory process in a variety of highly differentiated eucaryotic cell types is now well documented (5,6,8,15,18,27,47). Information regarding the acquisition of various steps of the secretory process during cytodifferentiation is, however, lacking. The studies with developing embryonic rat pancreas have provided a greater understanding of the sequential events involved in the histogenesis, the differentiation of the secretory apparatus, and the regulation of expression of pancreas specific genes in this organ (24,28,42). Further delineation of intrac...