A purified structural glycoprotein determines a strong inhibition of protein synthesis in fibroblast cultures

Fifty years of structural glycoproteins

During decades preceding and following the last war, a favourite subject of biochemists was to study glycoproteins. One class of these substances, found in connective tissues were characterised as polysaccharides, most of them found to be linked to proteins, designated later as glycosaminoglycans and proteoglycans. Another family of glycoconjugates represented epithelial mucins as found in the gastro-intestinal and respiratory tracts and conduits. A third family of glycoconjugates is represented by circulating glycoproteins isolated from the blood plasma, mostly studied by medical biochemists in relation to pathological conditions comprising those increasing during the inflammatory reaction: acute phase glycoproteins. Their study suggested that they might be derived from connective tissues. Although inflammatory glycoproteins derive mostly from the liver, the possibility of connective tissue origin of glycoproteins remained open. Using cornea, an avascular tissue, we could show that connective tissues also synthesize glycoproteins. We proposed to designate them "structural glycoproteins" (SGP-s) to distinguish them from circulating, blood-born glycoproteins coming from the liver. They play locally "structural" roles in connective tissues where they are synthesized. Soon after fibronectin was identified and shown to mediate cell-matrix interactions. A large family of glycoproteins were then isolated from a variety of sources, cells, tissues others than liver, confirming our original hypothesis. The first experiments on these glycoproteins were published from 1961/1962 giving the opportunity to recapitulate this biochemical adventure 50 years later, together with the celebration of the foundation of the first connective tissue society in Europe, as described in the first article in this issue.

Nonisotopic evaluation of collagen in fibroblasts cultures

A method for the evaluation of collagen concentrations in the medium of fibroblasts in culture was developed. Collagen was precipitated with other proteins by addition of ethanol and hydrolyzed by 6 M HCl. The primary amino acids of the hydrolyzate were reacted with o-phthalaldehyde (OPA) and secondary amino acids (Pro, Hyp) were derivatized with 9-fluorenylmethyl-chloroformate (FMOC-Cl). The mixture was separated by isocratic HPLC on a reverse-phase column. FMOC-derivatives were detected by fluorometry, whereas OPA-derivatives were not. This method is suitable for the monitoring of collagen metabolism in fibroblast cultures exposed to various effectors.

Effects of isaxonine phosphate and analogs on fibroblast metabolism in culture

Human skin fibroblasts in confluent cultures were incubated for 24 h in the presence of isaxonine phosphate (Nerfactor) and several related factors. The incorporation of 14C-proline into secreted proteins and the release of collagen into the medium were inhibited. When the cells were incubated for an additional period of 24 h after thorough washing, protein and collagen syntheses were found to be identical to those of controls, demonstrating that the inhibition of protein synthesis was independent of any toxic effect. When cells were incubated in the presence of both isaxonine and colchicine, the secretion of collagen was more inhibited than by colchicine alone, and proteins accumulated in the cells.

A study of collagen metabolism in cell cultures by fluorometric determination of proline and hydroxyproline

A technique of derivatization of proline (Pro) and 4-hydroxyproline (Hyp) by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole permitted the measurement of Pro and Hyp radioactivities, concentrations, and specific activities in the main fractions separated from cultures of fibroblast cells (extracellular collagen and non-collagen proteins, intracellular free Pro and Hyp, Pro- and Hyp-containing peptides, procollagen, and non-collagen proteins). The evaluation of collagen in the medium was obtained from as few as 10(4) cells. The method might advantageously replace [14C] Pro or [3H] Pro incorporation studies. It permits measurement of the size of the Pro pool and the amount of peptides formed by intracellular catabolism of collagen. It demonstrates that the time necessary for a full equilibration of intracellular Pro and intracellular collagen is longer than is generally believed. It avoids the uncertainties of protein labelling, which may vary with uncontrolled variations of the intracellular Pro specific activity.

Inhibition of collagen production in scleroderma fibroblast cultures by a connective tissue glycoprotein extracted from normal dermis

It was shown in a previous paper that a connective tissue glycoprotein (CTGP) extracted from normal rabbit dermis was able to inhibit total protein and collagen syntheses by normal dermis fibroblast cultures. In the present study, the effects of CTGP on scleroderma fibroblasts were investigated. [14C]Proline incorporation into total proteins of the supernatant was not significantly different from that found in controls. By contrast, the amount of collagen, expressed as percentage of total secreted protein, was far higher in scleroderma cultures than in normal ones (14.4% +/- 6.0% vs 4.6% +/- 0.9%). Addition of CTGP to the medium induced a concentration-dependent inhibition of [14C]proline incorporation into proteins from both control and scleroderma cells. In control cultures, no significant decrease of the percentage of collagen was observed, but over 60 micrograms/ml, both cytotoxic effects and inhibition of protein synthesis occurred. In scleroderma cultures, the inhibition was twice as effective on collagen as on noncollagen protein synthesis. The inhibition of collagen secretion was not related either to changes in collagen hydroxylation or to the intracellular catabolism of newly synthesized procollagen.

Inhibition of fibroblastic cell division by a fraction of structural glycoproteins extracted from rabbit dermis

A fraction of structural glycoproteins (SGP) extracted from rabbit dermis exerted a concentration-dependent inhibition on the multiplication of normal dermal fibroblasts and of mouse sarcoma fibroblastic cells CCRF S-180 II. The inhibition was fully reversible when the SGP was eliminated from the culture medium.

Separation and evaluation of the cis and trans isomers of hydroxyprolines: effect of hydrolysis on the epimerization

A procedure has been developed which can detect the hydroxyproline isomers trans-4-hydroxyproline (Hyp), trans-3-hydroxyproline, cis-4-hydroxyproline, and cis-3-hydroxyproline present in hydrolysates of collagens. The method involves hydrolyzing collagen, and reacting the primary amino acids with o-phthaladehyde (OPA) and the hydroxyprolines and proline with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) which combines specifically with secondary amino acids. The proline and hydroxyprolines are then separated by thin-layer chromatography and quantified by using a scanning spectrofluorometer. The method was used to show that both trans-4-L-hydroxyproline and trans-3-L-hydroxyproline were epimerized as a function of hydrolysis time to the cis isomers. An appreciable amount of trans-3-Hyp was degraded. Hydrolysis with 6 N HCl in the presence of 6% trichloroacetic acid gave greater epimerization than the 6 N HCl alone. Alkaline hydrolysis in 0.2 M Ba(OH)2 caused more epimerization of trans-4-Hyp and trans-3-Hyp compared with acid hydrolysis but less degradation, so that alkaline hydrolysis is proposed for the evaluation of trans-3-Hyp, provided that the total of the cis and trans isomers be considered in this case.