Growth control in cultured 3T3 fibroblasts: neutralization and identification of a growth-inhibitory factor by a monoclonal antibody

Functions of fatty acid binding proteins

Cytosolic fatty acid binding proteins (FABP) belong to a gene family of which eight members have been conclusively identified. These 14-15 kDa proteins are abundantly expressed in a highly tissue-specific manner. Although the functions of the cytosolic FABP are not clearly established, they appear to enhance the transfer of long-chain fatty acids between artificial and native lipid membranes, and also to have a stimulatory effect on a number of enzymes of fatty acid metabolism in vitro. These findings, as well as the tissue expression, ligand binding properties, ontogeny and regulation of these proteins provide a considerable body of indirect evidence supporting a broad role for the FABP in the intracellular transport and metabolism of long-chain fatty acids. The available data also support the existence of structure- and tissue-specific specialization of function among different members of the FABP gene family. Moreover, FABP may also have a possible role in the modulation of cell growth and proliferation, possibly by virtue of their affinity for ligands such as prostaglandins, leukotrienes and fatty acids, which are known to influence cell growth activity. FABP structurally unrelated to the cytosolic gene family have also been identified in the plasma membranes of several tissues (FABPpm). These proteins have not been fully characterized to date, but strong evidence suggest that they function in the transport of long-chain fatty acids across the plasma membrane.

Culture conditions for arresting and stimulating the proliferation of a rainbow trout fibroblast cell line, RTG-2

Conditions for arresting and stimulating the proliferation of the rainbow trout fibroblast cell line RTG-2 have been examined and the time course of events after stimulation determined. Quiescent populations were achieved in two ways. Cultures grown to confluency without a medium change for at least 7 d had fewer than 5% of the cells in S phase and few mitotic figures. Cultures deprived of serum, which could be done for up to 3 d without a loss in cell number, also achieved quiescence. After 3 d without serum, less than 1% of cells were in S phase and mitotic figures were infrequent. Addition to these cultures of fresh serum-containing medium brought about the synchronous entry of cells into S phase and mitosis. For cultures in which either the medium had been changed after 7 d without a change or serum-containing medium had been added after 3 d of serum deprivation, DNA synthesis increased after a lag period of 20 to 24 h, was pronounced between 30 and 45 h, and then declined. This was followed by a peak in the mitotic index. These protocols for arresting and subsequently stimulating RTG-2 proliferation should allow the G1-S transition to be studied in a representative of teleosts.

Absence of three secreted proteins and presence of a 57-kDa protein related to irreversible arrest of cell growth

Two subsets of proteins from the conditioned medium of normal human diploid fibroblasts were detected by NaDodSO4/polyacrylamide gel electrophoresis analysis; the presence of these subset is related to the nonproliferative state of in vitro aged cells. One subset consists of three proteins (molecular mass from 80 to 87 kDa) secreted by replicating cells in sparse cultures, as well as by quiescent cells in confluent or serum-starved cultures of young fibroblasts. These proteins disappear from the medium when cultures reach the state of senescence. The other subset consists of a single protein of 57 kDa, detected only in conditioned medium of senescent fibroblast cultures. The results suggest that when human fibroblasts reach senescence, they secrete the 57-kDa protein and concomitantly stop secreting the other three proteins normally found in the culture medium of young fibroblasts. Therefore, the alternative secretion of these protein subsets could specifically signify irreversible arrest of cell growth during in vitro cellular aging.

Development of in vitro immunization in murine and human hybridoma technology

The development of in vitro immunization in murine and human hybridoma technology is reviewed. Special consideration is given to: technical aspects of in vitro immunization of mouse B cells; quality of antibodies produced by in vitro immunization; advantages of murine in vitro immunization; technical aspects of in vitro immunization of human B cells; and the advantages of human in vitro immunization and human monoclonal antibodies. Future developments likely to be made in this area of immunotechnology are outlined.

Contact inhibition of growth of human diploid fibroblasts by immobilized plasma membrane glycoproteins

The human embryonic fibroblasts used in this study show pronounced inhibition of growth when reaching a critical cell density. High cell density and growth inhibition has previously been mimicked by the addition of glutaraldehyde-fixed cells or of isolated plasma membranes to sparsely seeded proliferating fibroblasts (Wieser, R. J., R. Heck, and F. Oesch, 1985, Exp. Cell Res., 158:493-499). In this report, we describe the successful solubilization of the growth-inhibiting glycoproteins and their covalent coupling to silicabeads (10 microns), which had been derivatized with 3-isothiocyanatopropyltriethoxysilane. The beads, bearing the plasma membrane proteins, were added to sparsely seeded, actively proliferating fibroblasts, and growth was measured by the determination of cell number or of incorporation of [3H]thymidine into DNA. The growth was inhibited in a concentration-dependent manner, whereby 50% inhibition was achieved with 0.3 micrograms of immobilized protein added to 5 X 10(3) cells. Terminal galactose residues of plasma membrane glycoproteins with N-glycosydically bound carbohydrates were responsible for the inhibition of growth. Dense cultures of human fibroblasts are characterized by an accelerated synthesis of procollagen type III. We have found that this cellular response can also be induced by the addition of immobilized plasma membrane glycoproteins to sparsely seeded cells. These observations support the conclusion that the addition of immobilized plasma membrane glycoproteins to sparsely seeded fibroblasts mimics the situation occurring at high cell density. These results show that cell-cell contacts via plasma membrane glycoproteins carrying terminal galactose residues are important for the regulation of the proliferation of cultured human fibroblasts and presumably of the accelerated synthesis of collagen type III.

Growth control in cultured 3T3 fibroblasts. V. Purification of an Mr 13,000 polypeptide responsible for growth inhibitory activity

A growth regulatory factor, which reversibly inhibits DNA synthesis and proliferation of fibroblasts, has been isolated from medium conditioned by exposure to density-inhibited mouse 3T3 cells. This factor, termed FGR-s (13K), yielded a single polypeptide (Mr 13,000) when analyzed by SDS PAGE under both reducing and nonreducing conditions. The dose-response curve of growth inhibition by FGR-s (13K) showed that 50% inhibition of 3T3 cell proliferation was achieved at a concentration of approximately 3 ng/ml, corresponding to approximately 0.23 nM. The activity of FGR-s (13K) was depleted by passing the material over an affinity column containing the monoclonal antibody 2A4; this monoclonal antibody had been previously characterized to bind to the Mr 13,000 polypeptide. These results indicate that we have purified a growth regulatory factor that acts to inhibit the proliferation of cells in an autocrine pathway.

Loss of reciprocal modulations of growth and liver function of hepatoma cells in culture by contact with cells or cell membranes

In mature rat hepatocytes in primary culture, many metabolic functions and cell growth are controlled reciprocally by cell density, and this reciprocal regulation is mediated by a cell-surface modulator through cell-cell contact. Cultured RY-121B cells from Reuber hepatoma and MH1C1 cells from Morris hepatoma, which retain some liver-specific functions, did not show any cell-density dependency of either cell growth or hepatocyte-specific functions, such as induction of tyrosine aminotransferase by dexamethasone. However, when RY-121B cells were cocultured with a low density of rat hepatocytes as monolayers in direct contact, they exerted contact-dependent control of DNA synthesis and of differentiated function of the hepatocytes. Furthermore, plasma membranes from various tumor cells including these hepatoma cells had strong modulator activity on primary cultures of normal rat hepatocytes, and their activity mimicked the reciprocal effects of cell density on DNA synthesis and induction of tyrosine aminotransferase. On the contrary, addition of plasma membranes from normal adult rat liver to sparse cultures of RY-121B or MH1C1 cells did not cause any inhibition of active DNA synthesis or enhancement of induction of tyrosine aminotransferase in these cells. These results suggest that hepatoma cells have lost cell density-dependent regulations of many cellular activities and cell growth because they have lost the ability to respond to the cell surface modulator, although they retain modulator activity on their plasma membranes.