Transforming growth factor-alpha. A model for membrane-anchored growth factors

Decreased expression of transforming growth factor alpha during differentiation of human pancreatic cancer cells

The relationship between cell differentiation and transforming growth factor alpha (TGF-alpha) expression in human pancreatic cancer cells was analyzed in Capan 1 cells. These cells differentiate either spontaneously or after butyrate treatment. During differentiation (spontaneous or butyrate induced), TGF-alpha messenger RNA (mRNA) levels decreased, whereas the TGF-beta 1 mRNA levels remained unchanged. TGF-alpha was present in cells as proTGF-alpha, which decreased after butyrate treatment. Secretion of TGF-alpha was not found. Under the two conditions of differentiation, the membrane-bound protein kinase C activity was also reduced. Conversely, long-term phorbol ester treatment increased both membrane-bound protein kinase C activity (260%) and TGF-alpha mRNA level (500%), a not significant increase of TGF-beta 1 mRNA was observed. However, phorbol 12-myristate-13-acetate did not induce TGF-alpha synthesis or secretion. These data suggest that expression of TGF-alpha can be reduced in cancer cells; they also suggest the existence of a relationship between TGF-alpha expression and cell differentiation. In addition, the protein kinase C-induced TGF-alpha mRNA level was not followed by the increase of TGF-alpha biosynthesis, suggesting a translational control. Finally, the expression of TGF-alpha and -beta 1 messengers appears to be differently regulated.

Secretion and localized transcription suggest a role in positional signaling for products of the segmentation gene hedgehog

The segment polarity genes engrailed and wingless are expressed in neighboring stripes of cells on opposite sides of the Drosophila parasegment boundary. Each gene is mutually required for maintenance of the other's expression; continued expression of both also requires several other segment polarity genes. We show here that one such gene, hedgehog, encodes a protein targeted to the secretory pathway and is expressed coincidently with engrailed in embryos and in imaginal discs; maintenance of the hedgehog expression pattern is itself dependent upon other segment polarity genes including engrailed and wingless. Expression of hedgehog thus functions in, and is sensitive to, positional signaling. These properties are consistent with the non-cell autonomous requirement for hedgehog in cuticular patterning and in maintenance of wingless expression.

The junction between cytokines and cell adhesion

Several aspects of the interactions between growth factors and cell adhesion are described. Recent advances in the field come from the identification of molecules resembling growth factors or growth factor receptors, which bear cell adhesion motifs as well as molecules participating in both cell growth control and adhesion.

Transforming growth factor-alpha gene expression in the hypothalamus is developmentally regulated and linked to sexual maturation

Hypothalamic injury causes female sexual precocity by activating luteinizing hormone-releasing hormone (LHRH) neurons, which control sexual development. Transforming growth factor-alpha (TGF-alpha) has been implicated in this process, but its involvement in normal sexual maturation is unknown. The present study addresses this issue. TGF-alpha mRNA and protein were found mostly in astroglia, in regions of the hypothalamus concerned with LHRH control. Hypothalamic TGF-alpha mRNA levels increased at times when secretion of pituitary gonadotropins--an LHRH-dependent event--was elevated, particularly at the time of puberty. Gonadal steroids involved in the control of LHRH secretion increased TGF-alpha mRNA levels. Blockade of TGF-alpha action in the median eminence, a site of glial-LHRH nerve terminal association, delayed puberty. These results suggest that TGF-alpha of glial origin is a component of the developmental program by which the brain controls mammalian sexual maturation.

The gene lin-3 encodes an inductive signal for vulval development in C. elegans

The lin-3 gene is necessary for induction of the Caenorhabditis elegans vulva by the anchor cell. It encodes a molecule similar to epidermal growth factor and to transforming growth factor-alpha and acts through the epidermal growth factor receptor homologue let-23. Expression of lin-3 in the anchor cell stimulates vulval induction; lin-3 may encode the vulval inducing signal.

In vitro secretion of transforming growth factor alpha (TGF-alpha): a comparison of the A431 cell line with three human oesophageal squamous cell carcinoma lines

Transforming growth factor alpha (TGF-alpha) is a single chain polypeptide which exists in a variety of forms differing in molecular weight. These forms are variously present in normal and neoplastic cells. Of particular interest are TGF-alpha's well-known mitogenic properties. The transition from a normal to a neoplastic cellular state results from signalling defects that may depend upon, inter alia, abnormal levels of expression and secretion of TGF-alpha. It is known that the secretion of TGF-alpha may be enhanced appreciably by agents such as phorbol 12-myristate 13-acetate (PMA), serum factors and epidermal growth factor (EGF). Here, we compare the efficacy of these three agents in the elevation of TGF-alpha secretion in the well studied A431 cell line with their previously undocumented efficacy in certain interesting, but little known, human oesophageal squamous cell carcinoma (SCC) lines.

UV induction of transforming growth factor alpha in melanoma cell lines is a posttranslational event

Low, mitogenic fluences of UVC (3.7-5.6 Jm-2) have previously been shown to cause increases of radioimmunoassayable transforming growth factor alpha (TGF alpha) in the medium and cells of cultures of melanocytes, melanoma lines, and HeLa cells (Ellem, K.A.O., Cullinan, M., Baumann, K.C., Dunstan, A.: Carcinogenesis 9:797-801, 1988). Here the cellular mechanism of this increase is explored by Northern blotting to detect any changes in TGF alpha mRNA levels, and the use of inhibitors of macromolecular synthesis to attempt to block the increase in TGF alpha protein. We were unable to detect any increase in TGF alpha mRNA levels attributable to UVC between 2 and 24 hours after irradiation. Inhibition of DNA synthesis (arabinosylcytosine, 10 microM), RNA synthesis (actinomycin D, 3 micrograms/ml; DRB 93 microM), or protein synthesis (cycloheximide, 10 micrograms/ml) failed to prevent the UVC induced increase in TGF alpha. We conclude that the UVC induction of TGF alpha is by a posttranslational mechanism. There was considerable discordance between the amount of TGF alpha protein and its mRNA in cultures of 15 different melanoma cell lines, which again emphasized that posttranscriptional mechanisms modulate the release of immunodetectable TGF alpha. We also found that the inhibitors themselves were capable of inducing an increase in TGF alpha in MM229 cultures. This suggests that the inhibitors and UV may effect the increase by a common mechanism, perhaps the activation of cell surface proteases as suggested for other stimuli (e.g., Pandiella, A., and Massagué, J.: Proc. Natl. Acad. Sci., USA 88:1726-1730, 1991) and that the response may be part of a global response to perturbation of DNA synthesis.

Neu differentiation factor: a transmembrane glycoprotein containing an EGF domain and an immunoglobulin homology unit

We recently reported that a 44 kd glycoprotein secreted by transformed fibroblasts stimulates tyrosine phosphorylation of the product of the neu proto-oncogene and induces differentiation of mammary tumor cells to milk-producing, growth-arrested cells. A partial amino acid sequence of the protein, termed Neu differentiation factor (NDF), enabled cloning of the corresponding complementary DNA. The deduced structure of the precursor of NDF indicated that it is a transmembrane protein whose extracellular portion contains an EGF-like domain that probably functions as a receptor recognition site. In addition, the ectodomain contains one immunoglobulin homology unit. Despite the lack of a recognizable hydrophobic signal peptide at the N-terminus, a recombinant NDF, like the natural molecule, is released into the medium of transfected COS-7 cells in a biologically active form. Northern blot analysis indicated the existence of several NDF transcripts, the major ones being 1.8, 2.6, and 6.7 kb in size. Transformation by the ras oncogene dramatically elevated the expression of NDF in fibroblasts.

Expression cloning of a diphtheria toxin receptor: identity with a heparin-binding EGF-like growth factor precursor

A monkey cDNA (pDTS) encoding a diphtheria toxin (DT) sensitivity determinant was isolated by expression cloning in mouse L-M cells. Mouse cells are naturally resistant to DT, because they lack functional cell surface receptors for the toxin. Unlike wild-type L-M cells, pDTS-transfected mouse cells are extremely toxin sensitive and specifically bind radioiodinated DT. Intoxication of the transfected cells requires receptor-mediated endocytosis of the bound toxin. The cDNA is predicted to encode an integral membrane protein that is identical to the precursor of a heparin-binding EGF-like growth factor. The DT sensitivity protein is thus a growth factor precursor that DT exploits as a receptor.

Binding of epidermal growth factor to receptors in preparations of enriched porcine parietal cells and inhibition of aminopyrine uptake

Preparations of isolated porcine gastric cells, enriched in parietal cells, were used to study binding of epidermal growth factor (EGF) to receptors and subsequent inhibition of [14C]aminopyrine uptake. EGF in concentrations from 10(-10) to 10(-7) M inhibited aminopyrine uptake stimulated by 10(-5) M histamine with an IC50 of 3 x 10(-10) M. [125]EGF bound in a saturable and specific manner to sites on cells in preparations containing 40-90% parietal cells. Mean apparent dissociation constant for the sites was 1.6 x 10(-9) M, with an average number of approximately 20,000 sites per cell. Endocytosis of ligand by parietal cells was limited, amounting to 10-20% of bound EGF after 1 h of incubation at 37 degrees C. Occupation of a fraction of the receptors caused a maximal reduction by 40% of aminopyrine uptake in histamine-stimulated cells, suggesting the occurrence of spare receptors. The results indicate the existence of specific receptors for EGF on porcine parietal cells exerting a regulatory influence on acid secretion.

Differentiation inhibiting activity (DIA/LIF) and mouse development

Analysis of the differentiation in culture of murine embryonic stem (ES) cells has resulted in the identification and characterization of the regulatory factor differentiation inhibiting activity (DIA). DIA specifically suppresses differentiation of the pluripotential ES cells without compromise of their developmental potential. DIA is identical to the pleiotropic cytokine leukaemia inhibitory factor (LIF) which has a broad range of biological activities in vitro and in vivo. It is produced in both diffusible and matrix-localised forms whose expression is differentially regulated. The compartmentalization of DIA/LIF and the modulation of its expression during stem cell differentiation and by other cytokines may be significant elements in the control of early embryo development. These features may also indicate general principles of the regulatory networks which govern stem cell renewal and differentiation in later development.

Androgen-responsive expression and mitogenic activity of schwannoma-derived growth factor on an androgen-dependent Shionogi mouse mammary carcinoma cell line

We report here, using the androgen-dependent SC2G cell line derived from Shionogi mouse mammary carcinoma SC115, that cDNA encoding mouse homologue of the rat schwannoma-derived growth factor (SDGF) was isolated and its protein product was identified to be an autocrine growth factor which is expressed in response to androgen in this hormone-dependent cell line. The androgen-independent SC1G cells derived from SC115 were shown to express SDGF even in the absence of androgen, while Northern analysis probed with mouse mammary tumor virus (MMTV) DNA showed that the androgen inducible transcriptional machinery remains functional in this hormone independent cell line.

Anthralin decreases keratinocyte TGF-alpha expression and EGF-receptor binding in vitro

Anthralin is an effective topical treatment for active psoriasis; however, its mechanism of action is unknown. Both TGF-alpha and its receptor, the EGF receptor, are overexpressed in active psoriatic plaques and might, therefore, play a role in psoriatic epidermal hyperplasia. In order to assess whether anthralin might act via alteration of this growth factor pathway, we examined the in vitro effects of pharmacologic concentrations of anthralin on cultured normal human keratinocytes. Keratinocyte proliferation was inhibited by 98% at an anthralin concentration of 10 ng/ml. In contrast, lymphocyte proliferation was inhibited by only 50% at an anthralin concentration of 10 micrograms/ml. Anthralin treatment did not induce cell-cycle-specific growth arrest as assessed by flow-cytometric analysis of acridine-orange-stained keratinocytes. Northern analysis of anthralin-treated keratinocytes demonstrated a marked decrease in TGF-alpha mRNA expression. Anthralin-treated keratinocytes showed decreased binding of 125I-EGF and 125I-IGF-I to their respective receptors, but EGF receptor binding was inhibited to a greater extent. Anthralin decreased ligand-binding affinity and cell-surface numbers of EGF receptors as assessed by Scatchard analysis of 125I-EGF binding to anthralin-treated keratinocytes. These results indicate that anthralin alters components of the EGF receptor pathway in cultured keratinocytes and that these effects might contribute to the clinical efficacy of anthralin in the treatment of active psoriasis.

Intercellular communication and cell-cell adhesion

In developmental biology, binary cell-cell interactions often determine the fate of one or both cell partners. The two cells must adhere to one another to allow chemical signals to be transmitted in one or both directions across the regions of cell-cell contact. The molecular mechanisms of cell-cell adhesion and intercellular communication, even if they are mediated by different cell surface components, may be functionally integrated in several different ways. Studies of helper T cells with antigen-presenting B cells in culture have illuminated such binary interactions. The possible application of similar mechanisms to other binary developmental systems is briefly explored.

Regulation of transforming growth factor gene expression in human endometrial adenocarcinoma cells

We have examined the effects of medroxyprogesterone acetate (MPA) and 4-hydroxytamoxifen (OH-TAM) on the cell proliferation and the expression of TGF-alpha and TGF-beta genes in Ishikawa cells and HEC-50 human endometrial adenocarcinoma cells. The effects of exogenous TGF-alpha, TGF-beta and anti-EGF receptor monoclonal antibody on cell proliferation were also determined. Antisense oligonucleotides were used to determine the effects of endogenous expression of TGF-alpha and TGF-beta. In both cell lines, MPA resulted in a time and dose-dependent inhibition of cell proliferation whereas OH-TAM had no effect on HEC-50 cell proliferation. The relative abundance of TGF-alpha mRNA was significantly reduced by MPA in Ishikawa cells but not in HEC-50 cells. In Ishikawa cells, a reduction in TGF-alpha mRNA abundance was observed with OH-TAM under conditions where both inhibition and stimulation of cell proliferation were demonstrated. Anti-EGF receptor monoclonal antibody inhibited Ishikawa cell growth but had little effect on HEC-50 cell proliferation. Exogenous TGF-alpha stimulated proliferation of both cell lines whereas exogenous TGF-beta inhibited proliferation of Ishikawa cells but stimulated proliferation of HEC-50 cells. Antisense oligonucleotides to TGF-beta inhibited proliferation of HEC-50 cells. From these data we conclude that the antiproliferative effects of progestins and OH-TAM on endometrial cancer cells appear to be mediated by different mechanisms.

Endothelial cell interactions with granulocytes: tethering and signaling molecules

The adhesion of granulocytes to endothelial cells requires regulated expression of molecules on both the endothelial cell and the granulocyte. These pro-adhesive molecules have diverse structures and mechanisms of expression, and act either to tether the two cells together or as signals that induce activation-dependent adhesion events. Combinations of tethering and signaling molecules regulate endothelial-cell-granulocyte interactions at the endothelial surface.

Increases in pericellular proteolysis at developing neuromuscular junctions in culture

To determine whether localized changes in pericellular proteolysis contribute to synapse formation, we examined the degradative actions of developing Xenopus laevis nerve and muscle cells on films of extracellular matrix proteins adsorbed to the glass surface of a tissue culture chamber. Skeletal myocytes, growing neurites, and fibroblasts all removed fluorescent fibronectin and laminin from the culture substratum at regions of close cell-surface contact. In addition, however, motor neurites also displayed a particularly enhanced rate of gelatin elimination at developing neuromuscular junctions. It has already been shown (a) that there is a similar remodeling of organized muscle basal lamina proteoglycan accumulations along the path of nerve-muscle contact and (b) that this is the earliest detectable biochemical change specific to developing neuromuscular junctions. Our observations thus suggest that the establishment of motoneuron-muscle contact leads to a further activation of pericellular proteinases along both the pre- and the postsynaptic surfaces of the developing junction. We therefore consider whether site-specific proteinase-activation cascades could contribute to the inductive signals that direct synaptic differentiation.

Lipofectin enhances cellular uptake of antisense DNA while inhibiting tumor cell growth

A natural DNA oligomer (15-mer) was synthesized with a sequence complementary to the translation initiation codon region of the human TGF-alpha mRNA and mixed with Lipofectin to form unilamellar complexes. It was found that tumor cell growth was inhibited when HCT116 cells were treated with Lipofectin-DNA oligomer complexes or with Lipofectin alone. Uptake of 32P-labeled 15-mers into colon tumor cells was compared in the presence and absence of Lipofectin. The amount of labeled oligomer found in cells that received optimal ratios of Lipofectin to DNA was 4- to 10-fold higher than the amount found in cells that received 32P-labeled DNA alone. Although Lipofectin-antisense DNA oligomer treatment of HCT116 cells caused a dose-dependent inhibition of cell growth, there was a subsequent rise in target mRNA product. Because the mechanism of growth inhibition could not involve an inhibition of TGF-alpha expression, it was concluded that Lipofectin probably exerts a nonspecific, detergent-like effect upon the cell membrane, producing an enhancement of TGF-alpha processing and release.

Purification of recombinant human tumor necrosis factor precursor from Escherichia coli

To study its biological functions, tumor necrosis factor precursor (proTNF) with a molecular size of 26-KDa was obtained as a recombinant protein from Escherichia coli. The recombinant proTNF was successfully accumulated in the insoluble form, corresponding to about 10-15% of total E. coli proteins. Solubilization, gel filtration and anion exchange chromatography were performed under denatured conditions followed by dialysis in phosphate-buffered saline. These processes removed most of the contaminating bacterial proteins, yielding proTNF with a purity of about 70-80%. This recombinant proTNF is expected to be useful for functional studies on activated macrophages with membrane integrated proTNF.

Transmembrane form of the kit ligand growth factor is determined by alternative splicing and is missing in the Sld mutant

The ligand (KL) for the c-kit receptor is a growth factor encoded at the mouse steel (Sl) locus. KL exists in both cell surface and soluble forms, though little is known of the regulation and functional significance of these forms. We show here that tissue-specific alternative splicing gives two types of KL mRNA. Both encode a transmembrane domain, but in transfected cells one produced the soluble form of KL at relatively high levels, whereas the other preferentially gave the cell surface form. Cell surface KL not only stimulated proliferation, but also mediated cell-cell adhesion. The SId allele, which impairs development of hematopoietic cells, melanocytes, and germ cells, has a deletion in the KL gene removing the transmembrane and intracellular domains. Expression of a corresponding cDNA gave a soluble protein that stimulated cellular proliferation but was not associated with the cell surface. These results provide evidence that cell surface KL has a critical role in the intact organism.