Transforming growth factors released from Kirsten sarcoma virus transformed cells do not compete for epidermal growth factor membrane receptors

Ectopic peptides released by a human melanoma cell line that modulate the transformed phenotype

Cells derived from a human melanoma strain by low serum selection in monolayer were found to be capable of growth in semi-solid medium, forming colonies ranging from tight, to loose, or dispersed. These phenotypes were found to be stable on cloning and retesting. Examination of the serum-free media conditioned by these clones indicates that the clones release activities capable of inducing agar colonies, in an indicator cell line (NRK, 49F), that express phenotypes similar to those of the melanoma clones used to produce the serum-free conditioned media (SF-CM). These SF-CM contained a transforming growth factor (TGF) beta (apparent Mr 14 700) and a single, apparently high Mr TGF-alpha species. Co-eluting with the TGF-alpha at an Mr of approximately 22 500 was a previously undescribed activity capable of modulating the phenotype of the NRK agar colonies induced by the combination of TFGs alpha and beta. This new activity can modulate the usual tight colony to express a loose or dispersed phenotype characteristic of the producer cells. The possible role these ectopic peptides play in the expression of the transformed phenotype by the tumour cells producing them, and the possible correlation between the 22 500 Mr EGF-like peptide (TGF-alpha) released by this particular tumour line and high Mr EGF-like peptides found in the urine of cancer patients, are both discussed.

Purification of melanoma growth stimulatory activity

The Hs0294 human malignant melanoma cell line produces a monolayer mitogen that stimulates the serum free growth of low-density cultures of Hs0294 cells. This report describes the purification of that mitogen, termed MGSA for melanoma growth stimulatory activity, from serum-free conditioned medium from the Hs0294 cultures. MGSA has been purified from acetic acid extracts of lyophilized conditioned medium by gel filtration, reverse-phase high-pressure liquid chromatography (RP-HPLC), and preparative electrophoresis, resulting in a greater than 400,000-fold purification. MGSA bioactivity resides in acid- and heat-stable polypeptides of high and low molecular weight (24-28 kd and less than 14-16 kd). However, the majority of the activity is reproducibly associated with the approximately 16-kd moiety eluting from RP-HPLC at approximately 35% acetonitrile. Reduction with dithiothreitol or B-mercaptoethanol results in a loss of biological activity but does not convert the 24-28-kd moieties to the less than 14-16-kd forms of MGSA. 125I-MGSA that has been purified by preparative electrophoresis (16 kd) specifically binds to Hs0294 melanoma cells and retains 100% of the growth-stimulatory activity. The 16-kd MGSA stimulates the proliferation of Hs0294 cells at concentrations of 0.3-30 pM. The electrophoretic mobility of MGSA is also unaltered by the preparative electrophoresis procedure, further demonstrating that this procedure does not alter the biochemical integrity of the growth factor. Purified MGSA does not enable anchorage-independent growth of normal rat kidney (NRK) cells and is therefore different from the previously described transforming growth factors. The amino acid composition of MGSA differs from that of other previously described growth factors. These data demonstrate that MGSA represents a separate class of growth factors with biological and biochemical properties different from other growth factors.

Neuroblastoma cells express c-sis and produce a transforming growth factor antigenically related to the platelet-derived growth factor

Mouse neuroblastoma Neuro-2A cells produce transforming growth factors during exponential growth in a defined hormone-free medium, which, on Bio-Gel columns in 1 M HAc, elute at a molecular size of 15 to 20 kilodaltons (kDa). These neuroblastoma-derived transforming growth factors have strong mitogenic activity, but they do not compete with epidermal growth factor for receptor binding (E. J. J. van Zoelen, D. R. Twardzik, T. M. J. van Oostwaard, P. T. van der Saag, S. W. de Laat, and G. J. Todaro, Proc. Natl. Acad. Sci. U.S.A. 81:4085-4089, 1984). In this study approximately 80% of the mitogenic activity was immunoprecipitated by antibodies raised against platelet-derived growth factor (PDGF). Immunoblotting indicated a true molecular size of 32 kDa for this PDGF-like growth factor. Analysis of poly(A)+ RNA from Neuro-2A cells demonstrated the expression of the c-sis oncogene in this cell line, whereas in vitro translation of the RNA yielded a 20-kDa protein recognized by anti-PDGF antibodies. Separation by reverse-phase high-pressure liquid chromatography demonstrated the presence of two distinct mitogenic activities in neuroblastoma-derived transforming growth factor preparations, one of which is antigenically related to PDGF. Both activities had the ability to induce anchorage-independent growth in normal rat kidney cells, both in the presence and in the absence of epidermal growth factor. It is concluded that Neuro-2A cells express c-sis with concomitant production and secretion of a PDGF-like growth factor, which plays a role in the induction of phenotypic transformation on normal rat kidney cells.

Lowering extracellular Na+ concentration releases autocrine growth factors from renal epithelial cells

Sodium influx is an important early signal during the onset of mitogenesis in many types of cells. From this observation, one would predict that a decrease in extracellular Na+ concentration might retard cell proliferation. We tested this prediction by exposing sets of cultures of monkey kidney epithelial cells (BSC-1 line) to medium with progressively reduced concentrations of Na+, and we measured the effect on cell multiplication. Unexpectedly, a reduction of the Na+ concentration from 155 mM (control) to 130 mM stimulated proliferation of epithelial cells but not of fibroblasts. Exposure of BSC-1 cells to low Na+ medium for 5 min was sufficient to commit them to accelerated growth. Further study revealed that the cells released two growth factors during this period: anionic proteins with apparent molecular weights of 6200 and 9000 whose properties differ from those of other known growth factors. Thus, a reduction in extracellular Na+ concentration apparently signaled the rapid release of autocrine growth factors that stimulate renal epithelial cell multiplication.

Further studies on the enhancing factor and its possible mechanism of action

In this study the nature of binding of enhancing factor (EF) and its mode of action are examined. EF binds to A431 cells through its own receptor, which is distinct from the receptor for epidermal growth factor (EGF). EF binds to the cell membrane and in turn provides a binding site for EGF. Data analyzed from Scatchard plots show that prior treatment of formalin-fixed A431 cells with EF for 30 minutes results in an increase in the number of binding sites for 125I-EGF. 3H-Thymidine incorporation studies, using the EGF-receptorless cell line NR-6, indicate that neither EF nor EGF alone stimulates the cells to synthesise DNA, but when both are added together the cells show 3H-thymidine incorporation. The role of EF may be to trap EGF and make it available to the cells through its own receptors even in the absence of EGF receptors. EF also induces anchorage-independent growth of normal fibroblasts in soft agar only in the presence of EGF.

Neuroblastoma cells express c-sis and produce a transforming growth factor antigenically related to the platelet-derived growth factor

Mouse neuroblastoma Neuro-2A cells produce transforming growth factors during exponential growth in a defined hormone-free medium, which, on Bio-Gel columns in 1 M HAc, elute at a molecular size of 15 to 20 kilodaltons (kDa). These neuroblastoma-derived transforming growth factors have strong mitogenic activity, but they do not compete with epidermal growth factor for receptor binding (E. J. J. van Zoelen, D. R. Twardzik, T. M. J. van Oostwaard, P. T. van der Saag, S. W. de Laat, and G. J. Todaro, Proc. Natl. Acad. Sci. U.S.A. 81:4085-4089, 1984). In this study approximately 80% of the mitogenic activity was immunoprecipitated by antibodies raised against platelet-derived growth factor (PDGF). Immunoblotting indicated a true molecular size of 32 kDa for this PDGF-like growth factor. Analysis of poly(A)+ RNA from Neuro-2A cells demonstrated the expression of the c-sis oncogene in this cell line, whereas in vitro translation of the RNA yielded a 20-kDa protein recognized by anti-PDGF antibodies. Separation by reverse-phase high-pressure liquid chromatography demonstrated the presence of two distinct mitogenic activities in neuroblastoma-derived transforming growth factor preparations, one of which is antigenically related to PDGF. Both activities had the ability to induce anchorage-independent growth in normal rat kidney cells, both in the presence and in the absence of epidermal growth factor. It is concluded that Neuro-2A cells express c-sis with concomitant production and secretion of a PDGF-like growth factor, which plays a role in the induction of phenotypic transformation on normal rat kidney cells.

Further study of beta-TGFs released by virally transformed and non-transformed cells

Chicken embryo fibroblasts sensitized by ts RSV respond to TGFs present in the media of non-transformed FR3T3 and NRK-4 rat cells and of the same cells transformed by KiMSV or RSV. They also respond to TGFs present in the media of BHK hamster cells transformed by MoMSV, PyV or RSV. Two other indicator rat cell lines, untransformed NRK-4 and FR3T3, sensitized by ts KiMSV, respond to the same TGF-containing media, and this response is increased by exogenous EGF. Normal FR3T3 cells failed to respond to any of the media. The most sensitive target cells were the ts KiMSV-FR3T3 cells at the restrictive temperature (39.5 degrees C). All the media tested on NRK-49F target cells required EGF for their TGF activity which was essentially dependent on prior activation by acidification. These data show that the above media from non-transformed or transformed cells contain beta-TGFs, with no detectable accompanying alpha-TGF activity. The release of and the response to these TGFs are not interdependent. A function of ts mutant src and k-ras viral oncogenes, still expressed at the restrictive temperature, can sensitize non-responsive cells, without there being any specificity towards the TGF producer cells.

Normal embryo fibroblasts release transforming growth factors in a latent form

Normal chicken, mouse, and human embryo fibroblasts release into their culture media transforming growth factors (TGFs) in a latent form. Their soft agar colony-forming activity on two widely used target cells, rat NRK-49F and mouse AKR-2B, is essentially revealed only after prior acidification of cell-conditioned media. These TGFs are EGF-dependent when assayed on NRK-49F cells and EGF-independent on AKR-2B cells. The TGF activity from the chicken source is released in three (apparent) molecular weight forms of 500 kd, 125 kd, and 20 kd.

Human transforming growth factor-alpha: precursor structure and expression in E. coli

Transforming growth factor-alpha (TGF-alpha) is secreted by many human tumors and can induce the reversible transformation of nontransformed cell lines. Using long synthetic deoxyoligonucleotides as hybridization probes we isolated an exon coding for a portion of TGF-alpha from a human genomic DNA library. Utilizing this exon as a probe, a cell line derived from a human renal cell carcinoma was identified as a source of TGF-alpha mRNA. A cloned TGF-alpha cDNA was isolated from a cDNA library prepared using RNA from this cell line, and was found to encode a precursor polypeptide of 160 amino acids. The 50 amino acid mature TGF-alpha produced by expression of the appropriate coding sequence in E. coli binds to the epidermal growth factor receptor and induces the anchorage independence of normal mammalian cells in culture.