Transforming growth factor activity of bovine brain-derived growth factor

Mechanisms of cell transformation in the Syrian hamster embryo (SHE) cell transformation system

The Syrian hamster embryo (SHE) cell transformation system has been used for investigational studies of basic mechanisms of neoplastic transformation, as well as determining the carcinogenic potential of chemical, physical, and biological agents. Many of these investigations utilize an intermediate step in the SHE cell neoplastic transformation process, known as morphological transformation, as an indicator that the cells have acquired an increased potential to progress to malignancy. While the nature of the morphologically transformed phenotype is not completely understood, it is believed to result from a block in the cellular differentiation of stem cells present within the SHE cell population. In terms of determination of the transforming potential of biological/chemical/physical agents, more than 500 agents have been tested in the SHE cell transformation assay with an 80-90% correlation between MT and carcinogenic potential. As such, the SHE cell transformation assay has utility as a test to provide short-term information on the carcinogenic potential of chemicals. One class of agents of current interest with regard to SHE cell transformation assay utilization consists of growth and differentiation factors (GDFs). Analysis of the SHE cell transformation potential of the GDFs, epidermal growth factor (EGF), fibroblast growth factor 4 (FGF-4), platelet-derived growth factor AA (PDGF AA), PDGF AB, PDGF BB, and the antimitogenic GDF, transforming growth factor beta one (TGF-beta1), was performed. All GDFs, with the exception of TGF-beta1, induced SHE cell transformation. However, an interesting difference between the GDFs was observed--PDGF A/B and PDGF B/B, but not PDGF A/A, EGF, or FGF-4, induced transformation after both a transient 1-day exposure and a continuous 7-day exposure, while continuous 7-day exposure was required for transformation by PDGF A/A, EGF, and FGF-4. Interestingly, both transient 1-day and continuous 7-day TGF-beta1 exposure resulted in suppression of transformation induced by a variety of transforming agents including growth factors, Ames assay-positive carcinogens, Ames assay-negative carcinogens, and spontaneous transformation. Interestingly TGF-beta1 was not able to suppress transformation by the tumor promoter, TPA. Together, these data demonstrate the utility of the Syrian hamster embryo cell transformation system for analyzing the transforming potential of GDFs and for characterizing differences in transforming mechanisms between different GDFs.

Requirement of heparan sulfate for bFGF-mediated fibroblast growth and myoblast differentiation

Basic fibroblast growth factor (bFGF) binds to heparan sulfate proteoglycans at the cell surface and to receptors with tyrosine kinase activity. Prevention of binding between cell surface heparan sulfate and bFGF (i) substantially reduces binding of fibroblast growth factor to its cell-surface receptors, (ii) blocks the ability of bFGF to support the growth of Swiss 3T3 fibroblasts, and (iii) induces terminal differentiation of MM14 skeletal muscle cells, which is normally repressed by fibroblast growth factor. These results indicate that cell surface heparan sulfate is directly involved in bFGF cell signaling.

Structural analysis of a mature hst-1 protein with transforming growth factor activity

A recombinant hst-1 protein produced in silkworm cells by a recombinant baculovirus, previously shown to be a potent mitogen for NIH3T3 cells and human endothelial cells, also stimulated anchorage-independent growth of NRK-49F cells. Amino acid sequence analysis revealed that the amino-terminal sequence with 58 amino acids was cleaved off in silkworm cells. These results indicated that the mature hst-1 protein consisting of 148 amino acids had transforming growth factor activity.

Isolation and partial purification of growth factors with TGF-like activity from human malignant gliomas

The effect of concentrated conditioned medium from each of eight human malignant glioma cell lines on the growth of indicator cells (normal rat kidney fibroblasts (NRK), clone 14) was determined in monolayer and in soft agar assay systems. The conditioned medium from all cell lines was mitogenic in the monolayer assay, but only SF-210, U-343 MG-A, and U-251 MG produced soluble factors that caused NRK cells to grow in soft agar. The soluble growth-promoting factors from these three cell lines were acid- and heat-stable (60 degrees C for 30 minutes) but were inactivated by trypsin (100 microns/ml) and dithiothreitol (50 microM). The growth factors from SF-210 and U-343 MG-A were further purified by molecular-sieve chromatography. The partially purified growth factor from U-343 MG-A retained transforming growth factor (TGF)-like activity, had a molecular weight of 9 kD, was potentiated by TGF-beta in the soft agar assay, competed effectively with 125I-epidermal growth factor (EGF) radiolabeled for the EGF receptor on A 431 epidermoid carcinoma cells, and was completely inhibited by monoclonal antibodies to TGF-alpha. The partially purified growth factor from SF-210 had a molecular weight of 17 kD, was not inhibited by monoclonal antibodies to platelet-derived growth factor (PDGF) or TGF-alpha, and did not bind to a heparin-Sepharose column. These results imply that U-343 MG-A secretes a growth factor with TGF-alpha-like activity, and SF-210 secretes a TGF with neither TGF-alpha nor TGF-beta activity.

An acidic fibroblast growth factor stimulates DNA synthesis, inhibits collagen and alkaline phosphatase synthesis and induces resorption in bone

We have examined the effects of an acidic fibroblast growth factor (aFGF) on bone cell growth and differentiation and on osteoclastic resorption using rat fetus organ cultured calvariae and long bones, respectively. Low concentrations of this aFGF stimulated DNA synthesis (1.28 ng/ml) and inhibited collagen formation (1.28 ng/ml) and alkaline phosphatase activity (0.64 ng/ml) in the isolated calvariae. The inhibition of collagen synthesis was independent of the aFGF's mitogenic effect, and was evident in periosteum-containing and periosteum-free bones, pointing to osteoblasts as the aFGF-responsive cells. Our preparation of aFGF enhanced resorption in the long bones by a calcitonin-inhibitable mechanism. PGE2 release accompanied and indomethacin prevented the enhancement of resorption. By contrast, indomethacin did not block the stimulation of DNA and collagen synthesis caused by our aFGF. These results indicate that our aFGF exerts a PGE2-independent effect on DNA synthesis and collagen synthesis and a PGE2-dependent effect on resorption in bone tissue in vitro.

Native and a synthetic analogue of the malignancy-associated parathyroid hormone-like protein have in vitro transforming growth factor-like properties

A human parathyroid-like protein (PLP) has recently been isolated and cloned from human tumors associated with the paraneoplastic syndrome, humoral hypercalcemia of malignancy. PLP shares NH2-terminal amino acid sequence similarity with PTH but has a unique primary structure thereafter. Studies reported to date have indicated that both native and synthetic amino-terminal PLP polypeptides display actions in vivo and in vitro that are similar to those of PTH. We report here that purified native PLP and synthetic 36Tyr(1-36)amide human PLP induce epidermal growth factor-dependent transformation of NRK 49F cells in soft agar. Further, the synthetic peptide induces a significant increase in the biosynthesis of fibronectin by human dermal fibroblasts. (1-34)PTH does not display either of these biological activities. These data indicate that there are qualitative differences between PTH and the recently identified PLP. The latter hormone appears to possess transforming growth factor-like properties that may be relevant to its physiological actions.

Clinical applications of heparin-binding growth factors

The family of HBGFs represents one of the most important families of mediators yet described, capable of inducing mesenchymal cell proliferation and differentiation, tissue regeneration, morphogenesis, and neovascularization, and it is clear their clinical potential is enormous. While some obvious applications of HBGFs, such as in wound healing and seeding of vascular prostheses, are already being examined in detail, the realization of their full clinical potential will require the co-ordinated efforts of many laboratories in a wide spectrum of fields. A better understanding is needed of the pathophysiological roles of HBGFs in vivo. For example, if abnormal expression of HBGFs is the cause of certain pathologies characterized by abnormal vascularization, the clinical potential of HBGF antagonists as inhibitors of angiogenesis will be considerable. A better understanding is also needed of the relationship between HBGF structure and function, susceptibility to proteolysis, in-vivo stability, and synergism with other biological response modifiers. In addition, many clinical applications will be limited by our ability to target HBGFs to selected sites in the body, while others will be limited by undesirable side-effects. Indeed, the minimization of such side-effects may rapidly become a central issue in the in-vivo use of HBGFs. For example, the presence of HBGFs in ocular tissues, their role in phototransduction, their ability to induce neovascularization, and the clear link between abnormal ocular neovascularization and blindness, suggest that the eye may be an organ particularly sensitive to local changes in HBGF levels. Finally, HBGFs will almost certainly have extremely potent immunoregulatory effects. Nevertheless, the application of HBGFs in a variety of clinical situations should lead to many innovative therapeutic advances.