Acidic structural proteins of connective tissue. II. Evidence for disulfide cross-links

A noncollagenous matrix for attachment of rat hepatocytes in culture

Adult rat hepatocytes efficiently attach to intact connective tissue fibers prepared from normal rat liver; this material has been given the name of "biomatrix". The cells remain alive and differentiated for at least 4 months in culture. The liver biomatrix contains both collagen and noncollagenous glycoproteins. Heretofore, the functions of the different components of the biomatrix in facilitating cell adhesion and promoting survival and differentiation of rat hepatocytes have not been investigated. We now report on an aggregate of glycoproteins which were with dilute acetic acid from liver biomatrix and which facilitate hepatocyte adhesion to plastic dishes and collagen. One protein in the extract, with a molecular weight larger than 300,000, was selectively immunoprecipitated with an antibody prepared against the extract and immunoadsorbed to glutaraldehyde-cross-linked rat serum. The antibody did not cross-react with cold-insoluble globulin and selectively inhibited cell attachment. The protein of the molecular weight of 300,000 appears to be specific for liver, since similar extracts prepared from rat lung, kidney, and heart biomatrices did not cross-react with the antibody. Furthermore, the latter extracts did not facilitate cell attachment above values obtained with plain culture dishes. The results suggest that glycoproteins in the biomatrix, which are antigenically distinct from fibronectin, can mediate attachment of rat hepatocytes to plastic dishes and collagen.

Structural glycoprotein, fact or artefact

It has been repeatedly claimed that structural glycoprotein or SGP is a major component of many connective tissues, particularly aorta. SGP is usually defined as material extractable from tissue with chaotropic agents such as urea and having an amino acid composition matching that of other, similar extracts. It has been claimed that SGP comprises 10-60% of the dry weight of aortic tissue. This paper describes a study in which aorta was exhaustively extracted with agents previously claimed to extract SGP. All the extracts yielded mixtures of proteins which, upon separation by polyacrylamide gel electrophoresis, were found to consist largely of actin, collagen, myosin and other known proteins. No significant quantity of any one unidentified protein was found which could have been SGP. This study casts grave doubt upon the existence of SGP as a major component of aorta.

Cold-insoluble globulin (fibronectin) in connective tissues of adult human lung and in trophoblast basement membrane

Cold-insoluble globulin (CIG), which is immunochemically indistinguishable from the fibroblast surface protein known as large external transformation-sensitive glycoprotein and fibronectin, was detected immunologically in connective tissue fractions from adult human lung. The fractions tested were (a) intact parenchyma, (b) acidic structural glycoproteins (ASG) extracted from lung parenchyma with 0.3 M acetic acid, and (c) isolated alveolar basement membrane (ABM). For comparison with ABM, preparations of human glomerular basement membrane and human trophoblast basement membrane (TBM) were tested. CIG was not detected in glomerular basement membrane but was present in large amounts in TBM. The CIG antigen could be solubilized from the parenchyma and from ABM by collagenase digestion which indicates that CIG occurs in lung connective tissue in association with collagen. Fibrinogen antigenic determinants were present in the ASG fraction, but the question of whether CIG and fibrin(ogen) are associated in lung connective tissue requires further study. When CIG was quantified by electroimmunoassay, intact lung parenchyma contained approximately equal to 0.4% CIG, ASG contained 3-4.5% CIG, ABM contained 0.1-0.9% CIG and TBM contained 1.5%-7.2% Cg. the evidence suggests that CIG is a chemical constituent of lung connective tissue matrix where it may influence the function of alveoli.