Inhibition of synthesis of rat parotid secretory proteoglycan in a gland slice system

Parotid secretory granules: crossroads of secretory pathways and protein storage

Saliva plays an important role in digestion, host defense, and lubrication. The parotid gland contributes a variety of secretory proteins-including amylase, proline-rich proteins, and parotid secretory protein (PSP)-to these functions. The regulated secretion of salivary proteins ensures the availability of the correct mix of salivary proteins when needed. In addition, the major salivary glands are targets for gene therapy protocols aimed at targeting therapeutic proteins either to the oral cavity or to circulation. To be successful, such protocols must be based on a solid understanding of protein trafficking in salivary gland cells. In this paper, model systems available to study the secretion of salivary proteins are reviewed. Parotid secretory proteins are stored in large dense-core secretory granules that undergo stimulated secretion in response to extracellular stimulation. Secretory proteins that are not stored in large secretory granules are secreted by either the minor regulated secretory pathway, constitutive secretory pathways (apical or basolateral), or the constitutive-like secretory pathway. It is proposed that the maturing secretory granules act as a distribution center for secretory proteins in salivary acinar cells. Protein distribution or sorting is thought to involve their selective retention during secretory granule maturation. Unlike regulated secretory proteins in other cell types, salivary proteins do not exhibit calcium-induced aggregation. Instead, sulfated proteoglycans play a role in the storage of secretory proteins in parotid acinar cells. This work suggests that unique sorting and retention mechanisms are responsible for the distribution of secretory proteins to different secretory pathways from the maturing secretory granules in parotid acinar cells.

A sulfated proteoglycan is necessary for storage of exocrine secretory proteins in the rat parotid gland

Sulfated proteoglycans have been proposed to play a role in the sorting and storage of secretory proteins in exocrine secretory granules. Rat parotid acinar cells expressed a 40- to 60-kDa proteoglycan that was stored in secretory granules. Treatment of the tissue with the proteoglycan synthesis inhibitor paranitrophenyl xyloside resulted in the complete abrogation of the sulfated proteoglycan. Pulse-chase experiments in the presence of the xyloside analog showed a significant reduction in the stimulated secretion and granule storage of the newly synthesized regulated secretory proteins amylase and parotid secretory protein. Inhibition of proteoglycan sulfation by chlorate did not affect the sorting of these proteins. The effect of proteoglycan synthesis inhibition on protein sorting was completely reversed upon treatment with a weak acid. These results suggest that the sulfated proteoglycan is necessary for sorting and storage of regulated secretory proteins in the exocrine parotid gland. Preliminary evidence suggests that the mechanism involves the modulation of granule pH by the proteoglycan rather than a direct interaction with other granule components.

High-performance liquid chromatography analysis of chondroitin sulphate isomers in human whole saliva in a variety of clinical conditions

OBJECTIVES

Tests have been carried out to assess the level of unsaturated disaccharide isomers obtained from chondroitin sulphate in whole saliva, which contains chondroitin sulphate derived from gingival crevicular fluid (GCF).

MATERIALS AND METHODS

Whole saliva was collected from periodontally diseased subjects (PDS), clinically healthy subjects (CHS) and edentulous subjects (ES). Glycosaminoglycans (GAG) were liberated by digestion with Pronase E, and precipitated with cetylpyridinium chloride and ethanol. The unsaturated disaccharides obtained by chondroitinase ACII digestion of the liberated GAG were analysed by high-performance liquid chromatography. The unsaturated disaccharides included delta Di-0S, delta Di-6S and delta Di-4S.

RESULTS AND CONCLUSIONS

Analysis of data indicated that delta Di-0S, delta Di-6S and delta Di-4S were found in all PDS samples. The amount (ng ml-1 collected whole saliva) of delta Di-0S, delta Di-6S and delta Di-4S (P < 0.01) indicated significant differences between CHS and PDS whole saliva samples. The quantities of delta Di-0S and delta Di-4S (P < 0.01) indicated significant differences between PDS and ES whole saliva. The amount of delta Di-0S (P < .05) and delta Di-6S (P < 0.01) also indicated significant differences between CHS and ES whole saliva. These results indicate that chondroitin sulphate in PDS and CHS whole saliva is representative of that previously reported in gingival crevicular fluid and so provides a useful and alternative means of assessing the role of GAG as indicators of periodontal disease.

Alpha- and beta-xylosides modulate the synthesis of fibronectin and thrombospondin-1 by endothelial cells

We have previously shown that both p-nitrophenyl-alpha-D-xylopyranoside (alpha-xyloside) and p-nitrophenyl-beta-D-xylopyranoside (beta-xyloside) inhibit endothelial morphogenesis in vitro. In order to determine the mechanism for this inhibition, we have now investigated the effects of these compounds on the synthesis of proteoglycans and proteins by bovine aortic endothelial cells. Consistent with their well-recognised modes of action, beta-xyloside, but not alpha-xyloside, enhanced the secretion of free glycosaminoglycans into the medium. Furthermore, although both xylosides inhibited proteoglycan deposition into the cell layer/matrix, only beta-xyloside altered the nature of the proteoglycans synthesised by the cells. Both alpha- and beta-xylosides markedly inhibited total protein synthesis by endothelial cells in the absence of any effect on cell growth. This inhibition was time- and dose-dependent and was not due to the enzymatic release of p-nitrophenol by the cells. The synthesis of fibronectin and thrombospondin-1 were specifically and differentially modulated by both alpha- and beta-xylosides. That is, xylosides markedly reduced fibronectin levels relative to other proteins in both the medium and the cell layer/matrix. In contrast, the relative levels of thrombospondin-1 were increased in the xyloside-treated cultures both in terms of mRNA and protein. These studies demonstrate novel effects of xylosides on protein synthesis. Furthermore, they suggest that the inhibition of endothelial morphogenesis by xylosides may be due to the actions of these compounds on the synthesis of specific proteins.

Identification and characterization of a Candida albicans-binding proteoglycan secreted from rat submandibular salivary glands

A previously identified Candida albicans-binding glycoprotein secreted from rat submandibular glands (RSMG) has been further purified from an aqueous RSMG extract by ion-exchange chromatography and gel filtration. Biochemical analysis of the glycoprotein revealed high levels of uronic acid and sulfate, suggesting that it was a proteoglycan. Its amino acid and carbohydrate compositions were similar to those observed for other proteoglycans and differed significantly from those of RSMG mucin, the major secretory glycoprotein of RSMG. In addition, the apparent molecular weight of the glycoprotein was reduced following treatment with either chondroitinase ABC or heparitinase, demonstrating the presence of chondroitin sulfate and heparan sulfate. On the basis of its structure and anatomical source, the glycoprotein is referred to as submandibular gland secreted proteoglycan 1 (SGSP1). SGSP1 also binds monoclonal antibody 1F9, which recognizes the human blood group A carbohydrate epitope found on RSMG mucin. Hence, SGSP1 appears to be a hybrid molecule with carbohydrate structures found in both proteoglycans and RSMG mucin. Enzymatic digestion of SGSP1, followed by its interaction with a radiolabelled C. albicans strain in a filter-binding assay, demonstrated that binding to this strain appears to be mediated primarily via the heparan sulfate side chains of SGSP1 and not via the blood group A oligosaccharide.