Cell surface proteoglycans (PGs) play key roles in several aspects of cell behavior, including modulating responses to growth and differentiation factors and modulating cell adhesion and migration. Cell surface PGs are similar to their ECM counterparts, in the sense that they consist of core proteins that contain one to several glycosaminoglycans. One of the most striking features of cell surface PGs as a group is their structural diversity, both from the standpoint of the glycosaminoglycans and from the standpoints of the different core proteins that can be expressed as PGs. This purpose of this brief review is to summarize some of the evidence implicating an important role for cell surface PG in mediating cell adhesion to components of the ECM. The approach is to briefly review some of what is known regarding the structure and function of cell surface PGs, and to discuss this information with respect to integrin mediated cell adhesion, using fibronectin as a prototype ECM component. Collectively, much of the data suggest that cell surface PGs modulate cell adhesion to the ECM by acting in conjunction with other cell adhesion receptors such as integrins. Since fibronectin has multiple closely spaced cellular recognition sites that can bind cell surface PGs and integrin, a working model for cell adhesion to fibronectin is presented in which these sites act in conjunction with one another to cluster cell surface PGs and integrins on the cell surface, creating the necessary signals to promote adhesion. This working model can serve as a conceptual base from which to partially explain mechanism(s) by which fibronectin and other ECM components could transmit information into cells, and it has direct implications in helping to explain cell type-specific behavior associated with many pathological states, including tumor cell invasion and metastasis.
In recent years β1-6-N-acetylglucosaminyltransferase activities have been described in numerous mammalian tissues and cell types. From these studies it is clear that alterations in the expression level of these enzymes and their corresponding carbohydrate structures on the cell surface are associated with development, differentiation and oncogenesis. However, the molecular basis for these changes as well as their functional significance remain at present unknown. Recent advances in the molecular cloning and expression of β1, 6-N-acetylglucosaminyltransferase cDNAs will provide exciting new approaches to study their differential expression patterns at the molecular level and to correlate the expression of the corresponding carbohydrates with functionS.
Proteoglycans including heparin, heparan sulfate, chondroitin sulfate, dermatan sulfate, keratan sulfate and so on which exhibit many kinds of biological functions, have become good targets for the synthetic carbohydrate chemistry. Described in this minireview are the syntheses of proteoglycans, featuring the modern technologies in oligosaccharide synthesis.
Tn-syndrome (also referred to as Permanent mixed-field polyagglutinability) is due to an acquired defect of N-acetylgalactosaminide ß1, 3galactosyltransferase activity (ß1, 3Gal-T, E. C. 2.4. 1.122) in all blood cells. The deficiency encompasses a constant number of cells in an affected individual; the proportion of deficient cells, however, shows considerable variation among individuals. The Tn-syndrome is characterized by the co-expression of the Tn- and sialosyl-Tn-antigens whose molecular nature are GalNAcα(1-R) or Neu5Acα(2-6)GalNAcα(1-R), respectively, linked O-glyco-sidically to thr/ser on plasma membrane glycoproteins. The molecular cause is a repressed activity of ß1, 3Gal-T in affected cells as shown by its reversibility after treating them with 5-azacytidine. Clinically, the idiopathic form of the Tn-syndrome is correlated with hemolytic anemia and thrombopenia of variable clinical severity. Expression of Tn- and sialosyl-Tn is also found as an epiphenomenon in a variety of neoplastic disorders for which multiple causes for altered glycosylation may be responsible. These conditions are not reviewed here. The Tn-syndrome recalls in many respects paroxysmal nocturnal hemoglobinuria (PNH). However, 5-azacytidine treatment did not reverse absence of CD48 expression in a case with idiopathic PNH.
The anti-A+N+Tn blood type specific lectin from Moluccella laevis exhibits several unusual features, from both a molecular and functional point of view. It is a glycoprotein (ca. 10% neutral sugar), which is extremely stable to denaturation and has an unusual quaternary structure. In contrast to other lectins, which are oligomers of identical (or nearly identical) subunits, the M. laevis lectin consists of three kinds of subunit: one of 67kDa, made up of two polypeptides of 46 and 28kDa held together by S-S bonds and two non-covalently linked subunits of 42 and 26kDa. The latter subunit has been isolated and shown to possess both anti-A and anti-N activity. The lectin has a very high affinity for methyl α-N-acetyl-galactosaminide (about 2, 000 times higher than for galactose and 200 times higher than for methyl α-galactoside) and also binds strongly to NeuAcα2-6GalNAc. This accounts for its blood type A and Tn specificity. The N specificity of the lectin may be ascribed to a small number of unsubstituted (and/or NeuAcα2-6 substituted) O-linked α-GalNAc residues, postulated to be present in glycophorin A of blood type NN but not in glycophorin A of blood type MM. Since the expression of the Tn and sialosyl-Tn determinants is associated with different types of tumor (e. g. breast and colon), the lectin may be useful in cancer research and diagnosis.
Recent studies have begun to delineate a basic framework on which the pathways of intracellular cholesterol trafficking can be understood. Our studies, and those of others, on the defect in intracellular cholesterol transport in Niemann-Pick type C disease have aided in defining some of the routes by which cholesterol moves between various subcellular organelles and the plasma membrane. Here, we describe these pathways and review the findings that led to their formulation.