Monoclonal antibodies directed to two different domains of human plasma fibronectin: their specificities

Probing molecular polymorphism of fibronectins with antibodies directed to the alternatively spliced peptide segments

Molecular heterogeneity of fibronectins (FNs) isolated from plasma, cultured fibroblasts, and placenta was studied with site-specific antibodies recognizing alternatively spliced peptide segments, termed ED-A and IIICS/delta 2. The antibodies were raised in rabbits by immunization with synthetic peptides. Neither the ED-A nor the IIICS/delta 2 extra peptide segment was present in the major subunits of plasma FN, although a minor subunit contained the latter extra segment. Cellular FN consisted of at least four subunits differing in size of the fragments generated by cleavage of the C-terminal region with cathepsin D. These fragments were distinct from each other in the reactivity with anti-ED-A and anti-IIICS/delta 2 antibodies, suggesting that all combinations of the presence or absence of the extra segments were produced by cultured fibroblasts. Placental FN was more heterogeneous than plasma and cellular FNs, consisting of five, or probably more, subunits. Among these, the two smaller subunits appeared to be closely similar to the major subunits of plasma FN, whereas the other subunits were more related to those of cellular FN in the size of cathepsin D cleaved C-terminal fragments and in the reactivity with anti-peptide antibodies. These results, taken together, indicate that the FNs produced by different tissues or cell types are distinct from each other in the number and types of subunits, which are partly, if not all, defined by alternative splicing at the ED-A and IIICS regions.

The oncofetal domain of fibronectin defined by monoclonal antibody FDC-6: its presence in fibronectins from fetal and tumor tissues and its absence in those from normal adult tissues and plasma

An IgG1 monoclonal antibody, FDC-6, was established, which defines a unique fibronectin (FN) domain, located between the "Hep-2" and the "Fib-2" domains, in the COOH-terminal region of FNs isolated from hepatoma, sarcoma, and fetal fibroblasts. A systematic study with this antibody indicates the presence of two classes of human FNs. (i) FN from fetal connective tissue, placenta, amniotic fluid, hepatoma, and colon carcinoma as well as cell lines from fetal tissues (WI-38), hepatomas (HuH-6 and HuH-7), and sarcoma (VA13) was characterized by the presence of the FDC-6-defined domain and by a high molecular weight (subunit Mr, 310,000-335,000). (ii) In contrast, FN from normal adult tissues and plasma was characterized by a lower molecular weight (subunit Mr, 285,000-295,000) and lack of reactivity with FDC-6 and is therefore devoid of the FDC-6-defined domain. The FDC-6-defined domain is therefore called the "oncofetal" domain, and FN containing this domain is hereby called "oncofetal FN" (onf FN). The onf FN is similar to the previously known "cellular-form" FN. FN from normal adult tissues and plasma, lacking the oncofetal domain, is hereby called "normal FN" (nor FN). The nor FN is similar to the previously known "plasma-form" FN. Development of FN from fetal to adult form is associated with loss of the oncofetal domain defined by the FDC-6 antibody, and oncogenic transformation is associated with activation in synthesis of the oncofetal domain defined by the FDC-6 antibody.

The role of intermolecular disulfide bonding in deposition of GP140 in the extracellular matrix

Human WI-38 fibroblasts in cultures synthesized at least three molecular forms of the major, extracellular matrix glycoprotein (GP), GP140: (a) cytoplasmic GP140 (1.2 ng of GP140/micrograms of cell protein) was detergent-soluble, underglycosylated, and possessed detectable levels of intermolecular disulfide bonding; (b) matrix GP140 (3.6 ng of GP140/micrograms of cell protein) was detergent-insoluble, more highly glycosylated and polymerized by intermolecular disulfide bonding, and co-distributed in the extracellular matrix with fibronectin; and (c) released GP140 (2 ng of GP140/micrograms of cell protein per 24 h) was recovered in the conditioned culture media and lacked intermolecular disulfide bonding. Cytoplasmic GP140 was the immediate biosynthetic precursor of the matrix form of GP140. In addition, various human adult and fetal tissues contained a form of GP140 that resembled the fibroblast matrix GP140 in the degree of intermolecular disulfide bonding, relative molecular mass, and immunological reactivity. Analysis of the sequence of events in assembly of GP140 and fibronectin in the extracellular matrix detected the following: (a) fibronectin was first to appear in the extracellular matrix; (b) GP140 accumulated in the cytoplasm, then deposited in the extracellular matrix and co-aligned with the established fibronectin; and (c) maturation of the extracellular matrix proceeded by continued intermolecular disulfide bonding. To evaluate possible roles for intermolecular disulfide bonding in cell interactions, a unique assay system was utilized based on the ability of labeled cells to incorporate radioactive matrix components into a biotinylated exogenous matrix. Precipitation of the biotinylated matrix from extracts of the cultures using avidin indicated: (a) disulfide bonding of radioactive GP140 and fibronectin into the exogenous biotinylated matrix required cell contact with the matrix. The newly deposited GP140 and fibronectin derived from the cells and not from GP140 and fibronectin present in the conditioned culture media. (b) Pro-alpha 1 and Pro-alpha 2 procollagens, present in the culture media, bound to the exogenous matrix in a noncovalent manner and were independent of cell contact. (c) SV40 transformed cells (WI-38 VA13) synthesized released form GP140 but did not deposit GP140 into the biotinylated matrix.

Monoclonal antibodies used as probes for the structural organization of the central region of fibronectin

Two monoclonal mouse antibodies against human plasma fibronectin were compared in their reactivity for proteolytic fragments of the antigen by enzyme immunoassay and immunoblotting. These antibodies were shown to react with two different structures within a short segment (about 30 kDa) located about one-third away from the C-terminus of the fibronectin chains.