The use of biological assays for detection of polypeptide growth factors

Electrochemical study of insulin at the polarized liquid-liquid interface

This paper reports on the electrochemical behavior of bovine insulin at the interface between two immiscible electrolyte solutions (ITIES). The voltammetric ion-transfer response obtained in the presence of insulin was dependent on the aqueous phase pH conditions and on the nature of the organic phase electrolyte employed in experiments. Optimal detection was obtained at acidic pH below the isoelectric point of insulin where it was positively charged. A shift in transfer potentials to lower potential values was observed with decreasing hydrophobicity of the anion of the organic phase electrolyte. No ion-transfer response was observed at pH values of the aqueous phase above the isoelectric point, where insulin was negatively charged. These results suggest that the voltammetric response is due to ion-pairing interactions at the ITIES between positively charged insulin and the hydrophobic anion of the organic phase electrolyte, together with adsorption of the ion-pair at the interface. The voltammetric response was obtained for insulin at concentrations down to 1 muM. These results show that electrochemistry is useful in studying the behavior of this important protein molecule at the polarized water-1,2-DCE interface and provides an alternative detection mode for bioanalytical applications.

Ligand depletion negatively controls the mitogenic activity of epidermal growth factor

EGF activates the ErbB1 receptor, but there appears only a limited correlation between its receptor binding affinity and mitogenic activity. This is indicated by our present observation that in cells with high ErbB1 expression, including SUM102 breast tumor cells, low affinity EGF/Notch chimeras have similarly high mitogenic activity as EGF, in spite of the fact that EGF is superior in inducing receptor tyrosine phosphorylation and p42/p44 MAP-kinase activity. However, as a result of receptor-mediated internalisation high-affinity ligands such as EGF are depleted much more rapidly from the extracellular medium than low-affinity EGF/Notch chimeras. As a consequence, the mitogenic activity of EGF on ErbB1 overexpressing cells is limited by substantial degradation of internalised ligand in the period before cells enter S-phase, a phenomenon that is not observed for low affinity mutant ligands. The mitogenic activity of EGF on ErbB1 overexpressing cells does therefore not only depend on the applied concentration but also on the total amount of ligand added, and is strongly underestimated when tested in a limited assay volume. No such dependence on the incubation volume was observed for EGF activity on cells with low ErbB1 expression levels and on cells for which EGF is growth inhibitory.

Transforming growth factor beta1 decreases cholesterol supply to mitochondria via repression of steroidogenic acute regulatory protein expression

Transforming growth factor-betas (TGF-betas) constitute a family of dimeric proteins that affect growth and differentiation of many cell types. TGF-beta1 has also been proposed to be an autocrine regulator of adrenocortical steroidogenesis, acting mainly by decreasing the expression of cytochrome P450c17. Here, we demonstrate that TGF-beta1 has a second target in bovine adrenocortical cells, namely the steroidogenic acute regulatory protein (StAR). Indeed, supplying cells with steroid precursors revealed that TGF-beta1 inhibited two steps in the steroid synthesis pathway, one prior to pregnenolone production and another corresponding to P450c17. More specifically, TGF-beta1 inhibited pregnenolone production but neither the conversion of 25-hydroxycholesterol to pregnenolone nor P450scc activity. Thus, TGF-beta1 must decrease the cholesterol supply to P450scc. We therefore examined the effect of TGF-beta1 on the expression of StAR, a mitochondrial protein implicated in intramitochondrial cholesterol transport. TGF-beta1 decreased the steady state level of StAR mRNA in a time- and concentration-dependent manner. This inhibition occurs at the level of StAR transcription and depends on RNA and protein synthesis. It is likely that the TGF-beta1-induced decrease of StAR expression that we report here may be expanded to other steroidogenic cells in which a decrease of cholesterol accessibility to P450scc by TGF-beta1 has been hypothesized.

Fetal and postnatal sera differentially modulate human dermal fibroblast phenotypic and functional features in vitro

Fetal wounds heal without scar formation, fibrosis, or contracture. Compared with adult wounds, they are characterized by major differences in the extracellular matrix and the absence of myofibroblastic cells. The reasons for these differences are not well known and determination of factors affecting the absence of scarring in the fetus may lead to strategies for controlling adult pathological scarring. In the present study, we have assessed the effects of serum on the behavior of normal human dermal fibroblasts. Using an in vitro approach, we investigated the effects of fetal and adult serum on cell properties such as growth rate, collagen synthesis, gelatinase activities, and differentiation to myofibroblasts using biochemical, morphological, and ultrastructural parameters. We studied the induction of alpha-smooth muscle (alpha-SM) actin in fibroblasts, and its correlation with increased collagen gel contraction by the cells. Our results showed that, compared with FBS (fetal bovine serum), postnatal calf serum (PCS) decreased mitogenic activity and collagenase synthesis but not collagen synthesis. Furthermore, cells cultured with PCS differentiated to myofibroblasts with an increase in cell diameter, number of stress fibers, alpha-SM actin expression, and collagen gel contraction. To characterize the molecules involved in this differentiation process, the amount of transforming growth factor beta (TGFbeta) in FBS and PCS was determined and the effect of neutralizing anti-TGFbeta antibody was evaluated. It was determined that FBS contained more TGFbeta than PCS, but that essentially all the TGFbeta was latent in both sera. However, results obtained with anti-TGFbeta antibody show that active TGFbeta is present when human dermal fibroblasts are cultured with medium containing PCS. These results suggest that, in the presence of PCS but not FBS, the cells either produce active TGFbeta or an enzyme that is able to activate latent serum TGFbeta. Alternatively, sera may contain two different forms of latent TGFbeta, the PCS form being activated by the dermal fibroblast cells. A similar mechanism may be involved, at least in part, in skin wound healing and may underlie the appearance of myofibroblasts in postnatal wounds.

Lyophilized keratinocyte cell lysates contain multiple mitogenic activities and stimulate closure of meshed skin autograft-covered burn wounds with efficiency similar to that of fresh allogeneic keratinocyte cultures

For several years, grafting with allogeneic keratinocyte cultures has been used successfully as a wound-healing therapy both by us and by many other groups. Since their postgrafting survival time is limited, the effect of these cultures is generally explained by the production of wound repair-stimulating factors that promote proliferation and migration of resident cells. In this study we show that lysates of cultured keratinocytes contain mitogenic activity for keratinocytes, endothelial cells, and fibroblasts. In addition, the lysates inhibit the contraction of collagen gels by human skin fibroblasts. On the basis of these observations and of in vivo data obtained by ourselves and others, we have evaluated the effect of total keratinocyte lysates on the healing of meshed skin autograft-covered burn wounds. Twenty burn wounds were tangentially excised and autografted with one to three meshed conventional skin transplants. An area treated with a gel containing lysated keratinocyte cultures was compared with an area treated with placebo-gel in terms of epithelialization on day 5. In six patients an additional fresh keratinocyte alloculture was applied as a positive control. Results indicate that the newly formed epithelium (difference between percentage of epithelialization on day 5 and on day 0) was 31.1 percent in the treated area compared with 16.5 percent in the placebo area. This result is comparable with the value obtained by treatment with fresh keratinocyte allocultures, namely, 33.8 percent. These figures show a twofold stimulation of epithelialization.

Heparin-like polymers derived from dextran enhance colonic anastomosis resistance to leakage

A new tissue repair agent, RGTA11, is described for its ability to enhance colonic anastomosis repair and resistance to leakage. RGTA11 is a dextran derivative containing 110% carboxymethyl groups, 2.6% carboxymethyl benzylamide groups, and 36.6% carboxymethyl benzylamide sulfonate groups. RGTA11 was deemed efficient to protect the heparin-binding growth factors FGF2 against trypsin digestion. By this property RGTA11 mimicked heparin or heparan sulfate. We have also found that RGTA11 protected TGF beta 1 against trypsin digestion while heparin did not. RGTA11 was then tested in an in vivo wound-healing model of colonic anastomosis. Our results indicate that after 48 h, RGTA11- or RGTA11/FGF-2-treated animals presented a resistance of the anastomosis to leakage which was increased twofold (p < 0.05) over untreated controls. After 96 h and until day 7 there was no more difference with control animals. Our results suggest that RGTA11 presents potential clinical interest by preventing earlier leakage of colonic anastomosis.

Effects of dexamethasone on the expression of transforming growth factor-beta in mouse embryonic palatal mesenchymal cells

The central role of TGF-beta in the development of the embryonic palate has been well characterized. TGF-beta inhibits mesenchymal cell proliferation, induces medial edge epithelial cell differentiation, and modulates the expression of extracellular matrix proteins as well as the proteases that act upon them. Mechanisms by which TGF-beta expression itself is regulated are less well understood. Glucocorticoids are recognized in several cellular systems as able to regulate the expression of TGF-beta. This study was therefore designed to examine whether glucocorticoids affect the expression of TGF-beta isoforms in embryonic palatal cells. Based on flow cytometric analysis and viability determination, confluent primary cultures of mouse embryonic palate mesenchymal (MEPM) cells exposed to up to 10(-6) M dexamethasone (dex) exhibited no signs of cytotoxicity after 24 hours of exposure. Northern blot analyses revealed that dexamethasone reduced steady-state mRNA levels of TGF-beta 3 in a dose-dependent manner as early as 4 hours after treatment but had little effect on TGF-beta 1 and TGF-beta 2 expression up to 24 hours of dex exposure. Dex also reduced the synthesis of both latent and mature forms of TGF-beta protein by approximately four-fold as determined by the mink lung epithelial cell growth inhibition bioassay. Assessment of the ratio of mature to latent protein found in conditioned medium of control compared to dex-treated cultures indicated that dexamethasone may reduce the activation of latent TGF-beta to mature biologically active TGF-beta. Dexamethasone inhibited the proliferation of MEPM cells despite the down-regulation of TGF-beta suggesting that dex-induced growth inhibition of MEPM cells is not mediated by TGF-beta. These data suggest that dex modulates TGF-beta signaling pathways directly by down-regulating TGF-beta expression and possibly indirectly by altering the availability of mature TGF-beta necessary to exert its biological effects in the developing palate.

Mutations at positions 11 and 60 of insulin-like growth factor 1 reveal differences between its interactions with the type I insulin-like-growth-factor receptor and the insulin receptor

Insulin-like growth factor 1 (IGF-1) and three analogues ([V11I]IGF-1, [V11T]IGF-1, and [Y60F]IGF-1), constructed by site-directed mutagenesis, were expressed as fusion proteins and secreted into the periplasmic space of Escherichia coli. Purified IGF were obtained following IgG Sepharose affinity and cation-exchange chromatographies of the products of hydroxylamine cleavage of fusion proteins. The properties of the mutants were assessed using (a) quantification of affinities for the human insulin receptor overexpressed on NIH 3T3 cells, (b) quantification of affinities for the type I IGF receptor via competition for binding to a monolayer of MDA-MB-231 cells, (c) promotion of the in vitro mitogenesis of growth-arrested MCF-7 cells in the presence of 17-beta-oestradiol, and (d) a competition assay for binding to IGF-binding proteins secreted by MCF-7 cells. The mutants exhibited decreases in affinity for the insulin receptor, relative to IGF-1, of 2.6-, 3.8- and, 8.8-fold for [Y60F]IGF-1, [V11I]IGF-1, and [V11T]IGF-1, respectively. IGF-1, [V11I]IGF-1, and [Y60F]IGF-1 were of equal potency in the growth assay and in affinity for the type I IGF receptor. [V11T]IGF-1 exhibited a three fold loss of potency in the type I IGF receptor-binding and growth assays. The mutants did not differ significantly from IGF-1 in their affinities for the IGF-binding proteins. The full-activity of [Y60F]IGF-1 at the type I IGF receptor, in contrast to the weakened receptor affinity of IGF-1 with a Leu substitution at this position, indicates a requirement for an aromatic ring, rather than a hydroxyl group, in the interaction of IGF-1 with the type I IGF receptor. The decrease in affinity for the insulin receptor of all the mutants indicates that, as in insulin, the residues Val11 and Tyr60 are important for the interaction of IGF-1 with the insulin receptor. The unchanged or minor changes in the affinities of the mutants for the type I IGF receptor contrast with the more deleterious effects of the mutations on insulin receptor binding and with the properties of analogues of insulin mutated at equivalent sites: 3-fold and 5-10-fold reductions in biological activity for [VB12I]insulin and [YA19F]insulin, respectively. Thus, the results obtained using the mutants indicate important differences between the IGF-1/type I IGF receptor and insulin/insulin receptor interactions.

Transforming growth factor beta secretion from primary breast cancer fibroblasts

Transforming growth factor beta (TGF-beta) is a hormonally regulated growth inhibitor with autocrine and/or paracrine functions in human breast cancer. In vivo, enhanced immunohistochemical staining of extracellular TGF-beta 1 has been detected around stromal fibroblasts in response to the antiestrogen treatment. We have investigated the effects of tamoxifen on the production of TGF-beta by primary human breast fibroblast cultures in serum-free medium. Highly variable levels of mainly latent TGF-beta 1 were detected in conditioned media from both tumor and normal tissue derived fibroblasts. Hydroxy-tamoxifen was shown to increase latent TGF-beta 1 secretion in three of the eight tumor tissue-derived fibroblast cultures. Such effect of hydroxy-tamoxifen was not observed in fibroblast cultures established from normal adjacent breast tissue.

Transforming growth factor-beta: possible roles in Dupuytren's contracture

Transforming growth factor-beta (TGF-beta) is a multifunctional polypeptide that stimulates extracellular matrix deposition and fibroblast proliferation. Because both these features characterize Dupuytren's contracture, we investigated a possible role for TGF-beta in the etiology of this disorder. We studied receptor expression for TGF-beta, effects of TGF-beta 1 on DNA-synthesis, and in vitro production of TGF-beta 1 and TGF-beta 2 in both normal and Dupuytren-derived fibroblasts. We also studied the effects of epidermal growth factor (EGF) on growth of the different cell types. TGF-beta receptor profiles were different between the two cell types, as were TGF-beta 1 and EGF-induced stimulation of cell growth. Both cell types secreted both active and latent TGF-beta. Our results suggest that growth factors such as TGF-beta and EGF may play a role in Dupuytren's contracture.

Characterization of a rat C6 glioma-secreted follistatin-related protein (FRP). Cloning and sequence of the human homologue

A protein was isolated from rat C6 glioma-conditioned medium and was biochemically characterized. The heparin-binding protein has a native molecular mass of 55-75,000 Da, a molecular mass of 40-48,000 Da under denaturing conditions, and a pI of 5.0-6.0. Based on the determined partial amino acid sequences, the full lenght cDNA encoding the rat and human proteins were cloned. The cDNA sequences identified the isolated rat and human protein as the homologue of a recently reported mouse osteoblast-transforming-growth-factor-beta 1-inducible protein, encoded by the TSC-36 gene [Shibanuma, M., Mashimo, J., Mita, A., Kuroki, T. & Nose, K. (1993) Eur. J. Biochem. 217, 13-19]. Analysis of the human, rat and mouse amino acid sequences indicates that these proteins are highly conserved (> 92% sequence identity). Sequence similarities with follistatin and the follistatin-like domain of agrin are revealed. The relationship with follistatin and agrin points to possible common functions for the cloned follistatin-related proteins (FRP). The protein has no effect on the inhibitory action of transforming growth factor-beta 1, on CCl-64 cell growth.

Generation and characterization of mouse microglial cell lines

A murine cell line (MMGT1) has been established after transfection of primary microglial cell cultures with a v-myc-containing plasmid. This cell line was comparable with primary microglial cells with respect to morphology, presence of acetylated low density lipoprotein receptor, non-specific esterase, CD63, major histocompatibility complex antigens and CD11, and binding for Ricinus communis agglutinin. Primary microglia as well as MMGT1 cells were negative for glial fibrillary acidic protein. Different MMGT1 strains were obtained after subcloning, two of which resembled histiocytes (F4/80 and BM-8). These cell strains, MMGT12 and 16, were able to opsonize latex beads, and could be induced by endotoxins (LPS) to secrete TNF-alpha, IL-1, IL-6, TGF-beta, and EGF. The other subclones had intermediate (MCA519, ER-MP20) or mixed macrophage characteristics and did not react to endotoxin by an increase in TNF-alpha, IL-1, and TGF-beta. Our newly established murine microglial lines may prove to be useful models to study inflammation and repair in the brain.

High titre anticytokine antibodies obtained by intralymphnode immunization with low amounts of antigen

Intralymphnode immunization was performed on rabbits to obtain anticytokine antibodies using low or very low amounts of the following purified cytokines: CSF-1 (or M-CSF: 10, 2 or 0.2 microgram/injection), GM-CSF (10 micrograms/injection), IL-2 (10 micrograms/injection) and HILDA/LIF (10 micrograms for the first injection and 5 micrograms/injection for boosts). This technique is easily performed by dissection of the popliteal lymphnode. Specific high titre antibodies were obtained after the first or second boost for antigen doses between 10 (for all cytokines tested) and 0.2 microgram (for CSF-1) per injection. In most cases, these antibodies could be used for immunoprecipitation, competition assays, dot immunoblotting, neutralization of biological activity and receptor binding inhibition. Some applications show that these tools are useful for cytokine research projects. For newly identified cytokines available in limited amounts, this method of obtaining specific polyclonal antibodies is an interesting alternative to the expensive, time-consuming and technically more demanding monoclonal antibody method.

Secretion of transforming growth factor-beta isoforms by embryonic stem cells: isoform and latency are dependent on direction of differentiation

Murine embryonic stem (ES) cells are maintained in an undifferentiated state when cultured in medium conditioned by Buffalo rat liver (BRL) cells. BRL conditioned medium (CM) contains a differentiation inhibitory activity (DIA) that is synonymous with leukemia inhibitory factor (LIF). ES cells in monolayer culture can be induced to differentiate by addition of all-trans retinoic acid (RA) to the BRL CM, when they mainly form cells resembling parietal endoderm, or by culture in medium not conditioned by BRL cells. ES cells thus deprived of LIF/DIA differentiate spontaneously to a cell type that expresses Brachyury (T), a marker of early mesoderm. Northern blot analyses have shown previously that transcripts for transforming growth factor beta 1 (TGF-beta 1) are detected in undifferentiated cells while transcripts for TGF-beta 2 and TGF-beta 3 only become detectable after differentiation. We have now determined levels of TGF-beta protein in CM and in the extracellular matrix (ECM) and have used neutralizing antibodies specific for TGF-beta 1 and TGF-beta 2 that do not react with recombinant human TGF-beta 3 to determine the isoform secreted. Using the growth inhibition of mink lung CCL64 cells as a bioassay for TGF-beta activity, we demonstrate that undifferentiated ES cells secrete latent TGF-beta 1 into the medium but no activity is found in their ECM. Cells induced to differentiate with RA contain TGF-beta 2 in both active and latent forms in their CM. Likewise their ECM contains TGF-beta 2 as the sole isoform. ES cells deprived of LIF/DIA secrete both TGF-beta 1 and TGF-beta 2 isoforms in their CM but TGF-beta-like activity remains after addition of neutralizing antibodies for TGF-beta 1 and TGF-beta 2. This active TGF beta is the major component of the TGF-beta activity in this CM. By contrast, ECM from LIF/DIA deprived cells contains only the TGF-beta 1 and beta 2 isoforms. The remaining activity in CM correlates with high expression of TGF-beta 3 by Northern blot analysis in these cells. We speculate that TGF-beta 3 is secreted by these cells and may be activated more efficiently and/or in a different manner to TGF-beta 1 and TGF-beta 2, since it is present in CM only in its active form.

Phenotypic transformation of normal rat kidney cells by transforming growth factor beta is not paralleled by enhanced production of a platelet-derived growth factor

Phenotypic transformation of normal rat kidney (NRK) cells requires the concerted action of multiple polypeptide growth factors. Serum-deprived NRK cells cultured in the presence of epidermal growth factor (EGF) become density-inhibited at confluence, but they can be restimulated by a number of defined polypeptide growth factors, resulting in phenotypic cellular transformation. Kinetic data show that restimulation by transforming growth factor beta (TGF-beta) and retinoic acid is delayed when compared to induction by platelet-derived growth factor (PDGF), indicating that both TGF beta and retinoic acid may exert their growth-stimulating action by an indirect mechanism. Northern blot analysis shows that NRK cells express the genes for various polypeptide growth factors, including TGF beta 1, PDGF A-chain and basic fibroblast growth factor, but that the levels of expression are not affected by TGF beta or retinoic acid treatment. NRK cells also secrete low amounts of a PDGF-like growth factor into their extracellular medium, but the levels of secretion are insufficient to induce mitogenic stimulation and are unaffected by agents inducing phenotypic transformation. In combination with studies on the effects of anti-PDGF antibodies, it is concluded that phenotypic transformation of NRK cells by TGF beta and retinoic acid is not the result of enhanced production of a PDGF-like growth factor.

Purification process monitoring in monoclonal antibody preparation: contamination with viruses, DNA and peptide growth factors

Administration in vivo of monoclonal antibodies to humans is challenged by considerations regarding their safety. Contamination with viruses, potentially oncogenic nucleic acids and biologically active components like growth factors and hormones forms a serious point of concern in this respect. We have investigated the potential risk of viral contamination by measuring the reduction of 12 different viruses (after spiking) in the standard downstream purification process of ascitic fluid. Depending on the type of virus added and the purification step employed, the reduction of infectious virus particles varies considerably. The overall reduction ranges from about 10(3), observed for a member of the family of Papovaviridae, to more than 10(12) for members of the families of Herpesviridae and Orthomyxoviridae. Using hybridization analysis with a mouse (genomic) DNA probe, we show that the amount of residual DNA in ascitic fluids may also vary considerably, ranging from 75 ng/ml to 1 microgram/ml. In crude preparations produced in cell culture, much lower DNA concentrations are found (0.3 ng/ml). When standard downstream purification procedures are applied to ascitic fluid, a significant reduction of residual DNA levels is observed in the purified monoclonal antibody preparations and in intermediate fractions. The overall reduction factors vary from about 10(3) to 10(4), which is also confirmed by spiking experiments with either purified DNA or crude chromatin-like DNA. Using in-vitro cellular assays, we further show that peptide growth factors like PDGF and TGF beta are present in considerable amounts in ascitic fluids. The observed biological activities, however, are completely eliminated during the purification steps applied.

Analysis of receptor binding displacement curves by a nonhomologous ligand, on the basis of an equivalent competition principle

An exact method for the analysis of receptor-ligand binding data when labeled bound ligand is displaced by a nonhomologous ligand with a different dissociation constant is described. The present method, which is based on an equivalent competition principle for the homologous and the nonhomologous ligand, converts displacement curves into a linear form and is also applicable to situations in which free concentrations of ligand are significantly smaller than the added concentrations as a result of ligand binding. It is shown that the dissociation constant of the nonhomologous ligand is given directly by the concentration of this nonhomologous ligand added and the free concentration of unlabeled homologous ligand required to give the same level of displacement of labeled bound ligand. On the basis of these displacement characteristics, all binding parameters for receptor interaction of the nonhomologous ligand can be obtained and expressed, for example, in a Scatchard plot. The present method, which is referred to as the equivalent competition method, is also evaluated in this study with respect to the effects of nonspecific ligand binding and the presence of multiple receptor classes.