Custom-made chemical glycosylation of a carrier yields a neoglycoconjugate. Such a synthetic probe exposes a carbohydrate part that can display ligand properties for appropriate sugar receptors like lectins on cells or in tissue sections, unless their binding sites are harmed by the processing of the specimen or blocked by high-affinity endogenous ligands. The use of labelled neoglycoconjugates in histology and pathology, termed reverse lectin histochemistry, is complementary to the application of the isolated and labelled tissue lectin(s). Concomitant experiments with both classes of marker allow us to gain histochemical information on the capacity for distinct glycobiological interactions in situ. The results have a potential relevance to enhance our understanding of the participation of this type of molecular interplay in physiologically important recognitive processes.
TGF-β1 is the prototype of the so called TGF-β growth factor family. The family consists of an increasing number of different polypeptide modulators of cell growth, differentiation and morphogenesis. Numerous ways to regulate the expression of the TGF-β genes have been identified. Their expression can be affected by autocrine and paracrine mechanisms, and also by some other growth factors. TGF-β:s are also subject to regulation by retinoids, steroid hormones and vitamin D. Active TGF-β regulates transcription, and there is an increasing list of genes that are affected by TGF-β. A characteristic feature in the biology of TGF-β:s is that they are usually secreted from cells in latent forms. TGF-β was originally isolated from the conditioned medium of sarcoma virus transformed cells. The purification involved acidification, which, on the basis of current understanding, activates latent forms. Platelets and placenta are rich sources of TGF-β. We have recently found that TGF-β is not just secreted from cultured cells but is deposited in a latent form to the pericellular space, namely to the extracellular matrix. The latent complex consists of the small latent complex (TGF-β and its propeptide) and a high molecular weight protease resistant binding protein, LTBP (latent TGF-β binding protein). Protease-mediated release from the matrix is supposedly an initial step in the activation of the molecule, which possibly occurs in a coordinate way at the cell surface. The association of latent TGF-β complexes with the pericellular matrices and their release by proteinases implicates that these events participate in a number of biological events where enhanced or focal proteolysis takes place, including cell invasion, tissue remodeling and wound healing.
The class of mammalian biologically active polypeptides called growth factors influence the proliferation, differentiation, motility, maintenance and apoptosis of target cells. Growth factors such as epidermal growth factor(EGF) and fibroblast growth factor (FGF) elicit responses in cells by interacting with the extracellular domain of their receptors. This binding results in the activation of the intrinsic tyrosine kinase activity of the receptor and signaling. It has become apparent that accessory receptors, namely heparan sulfate (HS) proteoglycan, are required for the action of some growth factors. The mechanism of action of the FGFs serves as the prototype for this phenomenon. Amphiregulin and heparin-binding EGF-like growth factor (HB-EGF), two ligands which function via the activation of the EGF receptor have been shown to require extracellular HS proteoglycan for bioactivity. The heparin-binding regions of amphiregulin and HB-EGF have been localized to a basic -20 amino acid segment which lies just adjacent to the EGF-like domain of these mitogens. Based upon experimental data, a mechanism is proposed for amphiregulin action in which the HS proteoglycan is an integral membrane protein which either presents amphiregulin to the receptor or stabilizes the bivalent binding of one molecule of amphiregulin to the activated receptor dimer.
Studies of the initiation of the polysaccharide chains in proteoglycans containing heparins, heparan sulfates, or chondroitin sulfates-all linked to the core proteins of the respective proteoglycans by a xylose→serine linkage-began about 30 years ago. As had been intended, these investigations led to the demonstration of transfer of xylose from UDP-xylose to serine hydroxyl groups in an acceptor of protein nature, which was presumably the core protein of the proteoglycan under study. This reaction is catalyzed by the enzyme proteoglycan core protein xylosyltransferase. However, they also resulted in the discovery, in extracts of a mouse mastocytoma and hen oviduct, of a second xylosyltransferase which gave a product of a different kind. The property that set this product apart was its resistance to alkali treatment under conditions that cleave the xylose→serine linkage in the proteoglycans. We have found an analogous xylosyltransferase in kidney and muscle, and extensive characterization of the alkali-stable product formed by a soluble enzyme preparation from rat kidney has led to the conclusion that the enzyme (and the acceptor!) are identical to glycogenin. This substance is the core protein of glycogen proteoglycan and is also a self-glycosylating enzyme that normally uses UDP-glucose as the glycosyl donor and catalyzes early steps in glycogen biosynthesis. In this review, we summarize our current knowledge of the polysaccharide chain initiating enzymes glycogenin and proteoglycan core protein xylosyltransferase and how the discovery that both enzymes can use the common substrate UDP-xylose has led to new insights into glycogen and proteoglycan biosynthesis and its regulation.
P-selectin (CD62P) belongs to the selectin family of cell adhesion molecules, which includes E-selectin (CD62E) and L-selectin (CD62L). It contains a NH2-terminal domain of -120 amino acid residues homologous to Ca2+-dependent animal lectins and functions as an inducible receptor on activated platelets and endothelial cells for most leukocytes. The high-affinity human leukocyte ligand for P-selectin is a dimeric transmembrane sialomucin, called PSGL-1 (P-selectin Glycoprotein Ligand-1). As this molecule also serves as a ligand for E-selectin, we refer to it as P/ESGL-1 (P- and E-selectin Glycoprotein Ligand-1). There is an increasing body of evidence from work both in vitro and in vivo to support an important physiological and pathological role for P-selectin in mediating adhesion of leukocytes to endothelial cells and platelets in inflammation.