Heparin-like molecules on the cell surface potentiate binding of diphtheria toxin to the diphtheria toxin receptor/membrane-anchored heparin-binding epidermal growth factor-like growth factor

Diphtheria toxin receptor (DTR), which is identical to the membrane-anchored form of heparin-binding EGF-like growth factor (proHB-EGF), has a high affinity for heparin. We studied the effect of heparin-like molecules on the binding of diphtheria toxin (DT) to DTR/proHB-EGF. Mutant Chinese hamster ovary (CHO) cells deficient in heparan sulfate (HS) proteoglycans were about 15 times less sensitive to DT than wild type CHO-K1 cells. When free heparan sulfate or heparin was added to the culture medium, DT sensitivity of the mutant cells was fully restored. Studies of binding of 125I-labeled DT to HS-deficient CHO cells transfected with human DTR/proHB-EGF cDNA indicated that the increased sensitivity to DT after addition of heparin is due to increased binding of DT to cells. Vero cells display a relatively large amount of heparan sulfate residues compared to CHO-K1 cells or L cells. Enhancement of DT binding by the addition of heparin was also observed with CHO-K1 cells and L cells that had been transfected with human DTR/proHB-EGF cDNA, but the degree of enhancement was less than that observed with the HS-deficient CHO cells. Addition of heparin did not affect DT binding or DT sensitivity of Vero cells. Heparin-dependent binding was observed when intact Vero cells were treated with heparitinase or when the cell membrane was solubilized with a neutral detergent. Scatchard plot analysis for the binding of DT to a recombinant HB-EGF in vitro and to L cells expressing human DTR/proHB-EGF revealed that heparin increases the affinity of DTR/proHB-EGF for DT but does not change the number of binding sites. Although DRAP27/CD9 is known to enhance DT binding to DTR/proHB-EGF, the results indicate that heparin and DRAP27/CD9 increase DT binding by independent mechanisms. Thus, heparin-like molecules, probably in the form of heparan sulfate proteoglycan on the cell surface, are a third factor required for maximal DT binding activity of cells.

Peripheral blood T lymphocytes and lymphocytes infiltrating human cancers express vascular endothelial growth factor: a potential role for T cells in angiogenesis

CD3+ peripheral blood T lymphocytes were evaluated for expression of vascular endothelial growth factor (VEGF), an endothelial cell mitogen and potent angiogenic factor. VEGF mRNA expression was confirmed in CD3+ cells and Jurkat cells, a human T-cell line, by reverse transcription-PCR and in CD4+ and CD8+ T cell subtypes by Northern blot hybridization. Steady-state levels of VEGF mRNA were inducible in CD3+ T cells by hypoxia, a known inducer of VEGF mRNA accumulation. Secreted VEGF was detected in CD4+ and CD8+ T cell- and Jurkat cell-conditioned medium, indicating that T lymphocytes are capable of exporting bioactive concentrations of VEGF into the extracellular space. Human prostate and bladder cancers (prostatic adenocarcinoma and transitional cell carcinomas) were evaluated for VEGF mRNA expression by in situ hybridization. Tumor-infiltrating lymphocytes (TIL), identifiable immunocytochemically as T cells, along with tumor cells in these cancers, expressed VEGF mRNA. TIL in bladder cancers could be labeled with a specific anti-VEGF mAb, indicating that TIL are likely to be able to secrete VEGF protein in situ at bioactive concentrations. The finding that peripheral T cells and TIL in human tumors synthesize a factor known to be a specific mediator of neovascularization suggests a role for T lymphocytes as cellular effectors of angiogenesis.

Isolation and characterization of heparin-binding growth factors in human leiomyomas and normal myometrium

Uterine leiomyomas (fibroids) are benign, smooth muscle cell (SMC) tumors of the myometrium containing abundant extracellular matrix (ECM). Heparin-binding growth factors present in leiomyoma and normal myometrial fresh tissue were isolated using heparin-affinity fast protein liquid chromatography. Purification of these growth factors was monitored by the stimulation of [3H]thymidine incorporation into BALBc-3T3 cells and myometrial SMC. Western blot analysis confirmed that two consistent peaks of growth factor activity (eluting at 0.5 M NaCl and 1.7 M NaCl) were platelet-derived growth factor (PDGF), 31 kDa, and basic fibroblast growth factor (bFGF), 18 kDa, respectively. Northern blot analysis of leiomyoma and myometrial tissue revealed three RNA transcripts (2.8, 2.3, and 1.9 kb) for PDGF-A chain, one RNA transcript (4.0 kb) for PDGF-B chain, and two RNA transcripts (3.7 and 3.5 kb) for bFGF. RNase protection assay showed elevated expression of the bFGF mRNA transcript in leiomyomas in 3 out of 5 patients. Immunoperoxidase staining of paraffin-embedded tissue showed that PDGF was predominantly intracellular in both vascular and myometrial SMC. Basic FGF, by contrast, was found primarily bound to the ECM of myometrium and fibroids. Leiomyomas showed much stronger staining for bFGF due to the large areas of ECM in these tumors. A third mitogenic peak eluting at 1.1 M NaCl was also seen in both myometrial and leiomyoma tissue. This peak was not definitively identified by Western blotting. However, Northern analysis for heparin binding-epidermal growth factor (HBEGF), which also elutes at 1.1 M NaCl, detected one RNA transcript for HBEGF (2.5 kb) in normal myometrium but little or no expression in the corresponding leiomyoma tissue. Immunoperoxidase staining showed that HBEGF was a cell-membrane-associated protein in both normal myometrial and leiomyoma SMC with more intense staining in normal myometrium. These results show that both leiomyomas and myometrium synthesize a number of heparin-binding growth factors. The enhanced growth of leiomyomas may be due, in part, to the presence of large quantities of bFGF that are stored in the ECM of these tumors. In addition, the level of HBEGF mRNA declines during the transformation of myometrial SMC into leiomyomas.

Induction of heparin-binding EGF-like growth factor expression during myogenesis. Activation of the gene by MyoD and localization of the transmembrane form of the protein on the myotube surface

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) gene expression and protein localization were analyzed during the process of myogenic differentiation. The mouse HB-EGF gene was isolated, and a 1.8-kilobase genomic fragment flanking the 5' end of the cDNA was cloned. This fragment contains two sequences which match the consensus CANNTG sequence for E-boxes, binding sites for the MyoD family of DNA-binding transcription factors that regulate myogenesis. Accordingly, HB-EGF synthesis was analyzed in 10T1/2 cells and C2C12 cells which are used commonly for the study of myogenesis. HB-EGF gene expression was upregulated in both cell types during myogenesis. In 10T1/2 cells, direct activation of HB-EGF gene expression by MyoD was shown in that: i) transient transfection of these cells with a plasmid expressing MyoD resulted in a 10-20-fold increase in endogenous HB-EGF mRNA levels; ii) co-transfection of MyoD and an HB-EGF promoter-reporter plasmid resulted in a 5-10-fold increase in reporter activity, an increase that was abrogated by deletion of a putative HB-EGF proximal E-box sequence; and iii) incubation of MyoD protein with a 25-base pair double-stranded oligonucleotide corresponding to the HB-EGF proximal E-box sequence resulted in retarded electrophoretic mobility of the oligonucleotide. In C2C12 cells, differentiation of myoblasts into myotubes resulted in a 40-50-fold increase in HB-EGF promoter activity. In addition, immunostaining and laser confocal microscopy detected HB-EGF protein in C2C12 myotubes but not in myoblasts. The HB-EGF produced was in its transmembrane form and localized to the myotube surface. Taken together, it was concluded that during skeletal muscle cell differentiation, MyoD plays a direct role in activating HB-EGF gene expression and that HB-EGF protein is expressed preferentially in myotubes and in its membrane-anchored form.

Phorbol ester induces the rapid processing of cell surface heparin-binding EGF-like growth factor: conversion from juxtacrine to paracrine growth factor activity

Vero cell heparin-binding epidermal growth factor-like growth factor (HB-EGF) is synthesized as a 20- to 30-kDa membrane-anchored HB-EGF precursor (proHB-EGF). Localization and processing of proHB-EGF, both constitutive and 12-O-tetradecanoylphorbol 13-acetate (TPA)-inducible, was examined in Vero cells overexpressing recombinant HB-EGF (Vero H cells). Flow cytometry and fluorescence immunostaining demonstrated that Vero cell proHB-EGF is cell surface-associated and localized at the interface of cell to cell contact. Cell surface biotinylation and immunoprecipitation detected a 20- to 30-kDa heterogeneous proHB-EGF species. Vero H cell surface proHB-EGF turned over constitutively with a half-life of 1.5 h. Some of the 20- to 30-kDa cell surface-associated proHB-EGF was processed and a 14-kDa species of bioactive HB-EGF was released slowly, but most of the proHB-EGF was internalized, displaying a diffuse immunofluorescent staining pattern and accumulation of proHB-EGF in endosomes. Addition of TPA induced a rapid processing of proHB-EGF at a Pro148-Val149 site with a half-life of 7min. The TPA effect was abrogated by the protein kinase C inhibitors, staurosporine and H7. Kinetic analysis showed that loss of cell surface proHB-EGF is maximal at 30 min after addition of TPA and that proHB-EGF is resynthesized and the initial cell surface levels are regained within 12-24 h. Loss of cell surface proHB-EGF was concomitant with appearance of 14- and 19-kDa soluble HB-EGF species in conditioned medium. Vero H cell-associated proHB-EGF is a juxtacrine growth factor for EP170.7 cells in coculture. Processing of proHB-EGF resulted in loss of juxtacrine activity and a simultaneous increase in soluble HB-EGF paracrine mitogenic activity. It was concluded that processing regulates HB-EGF bioactivity by converting it from a cell-surface juxtacrine growth factor to a processed, released soluble paracrine growth factor.

T lymphocytes that infiltrate tumors and atherosclerotic plaques produce heparin-binding epidermal growth factor-like growth factor and basic fibroblast growth factor: a potential pathologic role

Despite significant infiltration into tumors and atherosclerotic plaques, the role of T lymphocytes in these pathological conditions is still unclear. We have demonstrated that tumor-infiltrating lymphocytes (TILs) and plaque-infiltrating lymphocytes (PILs) produce heparin-binding epidermal growth factor-like growth factor (HB-EGF) and basic fibroblast growth factor (bFGF) in vitro under nonspecific conditions and in vivo in tumors by immunohistochemical staining. HB-EGF and bFGF derived from TILs and PILs directly stimulated tumor cells and vascular smooth muscle cells (SMCs) in vitro, respectively, while bFGF displayed angiogenic properties. Therefore, T cells may play a critical role in the SMC hyperplasia of atherosclerosis and support tumor progression by direct stimulation and angiogenesis.

Mouse chromosomal location of three EGF receptor ligands: amphiregulin (Areg), betacellulin (Btc), and heparin-binding EGF (Hegfl)

The products of five distinct loci, Egf, Tgfa, Areg, Btc, and Hegfl act as ligands for the epidermal growth factor receptor. Egf and Tgfa have previously been mapped to mouse chromosomes 3 and 6, respectively, but the mouse chromosomal locations of Areg, Btc, and Hegfl have not been reported. Here, we show that Areg and Btc are tightly linked on mouse chromosome 5, while Hegfl maps to mouse chromosome 18. We also provide evidence that a putative sixth family member, Sdgf, is in fact a species variant of Areg. These results confirm and extend known relationships between mouse and human chromosomes, and they also provide new information regarding the evolution of this important gene family.

Carboxyl-terminal truncation of leucine76 converts heparin-binding EGF-like growth factor from a heparin-enhancible to a heparin-suppressible growth factor

Previous studies have indicated that heparin differentially regulates heparin-binding EGF-like growth factor (HB-EGF) and amphiregulin (AR) mitogenic activity. To further explore this phenomenon, these mitogens were compared under identical cell culture conditions in two different assays. The results of our present investigation demonstrated that AR-mediated mitogenic activity in the murine AKR-2B fibroblast-like cell line was inhibited by heparin, while HB-EGF activity was enhanced. However, the absolute effect of heparin appeared to be cell type specific since HB-EGF mitogenic activity was not dramatically affected by coincubation with heparin when tested on human dermal fibroblasts. Several studies have indicated that mutation of a conserved leucine in the carboxyl-terminal region of both EGF and transforming growth factor-alpha results in decreased affinity for EGF receptors. Since this leucine is present in the analogous position of HB-EGF, but absent in AR, we examined the effect of deleting this residue by carboxyl-terminal truncation of HB-EGF. Analysis of recombinant forms of HB-EGF demonstrated that HB-EGF can be converted to a heparin-inhibited growth factor if the putative mature form of the protein is truncated by two residues (leucine76 and proline77) at the carboxyl terminus. Further analysis demonstrated that only leucine76 appears to be required for heparin-dependent enhancement of HB-EGF-mediated mitogenic activity, indicating that this amino acid may play a pivotal role in controlling the response of HB-EGF to heparin or related glycosaminoglycan sulfates. Our results also suggest that expression of different HB-EGF forms in vivo could result in the production of HB-EGFs with divergent responses to sulfated glycosaminoglycans and proteoglycans.

Cells transfected with the basic fibroblast growth factor gene fused to a signal sequence are invasive in vitro and in situ in the brain

Invasiveness is a critical event in the development of malignancy in brain tumors. A potential molecular mediator is basic fibroblast growth factor (bFGF). NIH-3T3 cells transfected with the bFGF gene fused with a signal peptide sequence (signal peptide bFGF) acquire an invasive phenotype as measured by in vitro assays of invasion including: 1) the formation of branching networks on Matrigel; 2) invasiveness in a chemoinvasion assay; 3) migration in a cell spreading assay; 4) detection of an Mr 92,000 gelatinase; and 5) local invasion into the surrounding neuropil after injection in the athymic mouse brain. By contrast, cells transfected with only the native bFGF gene (wild-type bFGF): 1) formed discrete cell clusters on Matrigel; 2) were less invasive and migratory in vitro; 3) released minimal Mr 92,000 collagenase; and 4) in vivo formed a pseudocapsule that separated the tumor cells from the neuropil. Quantitation of bFGF in the conditioned serum-free medium of the cell lines by enzyme-linked immunosorbent assay demonstrated that the signal peptide-bFGF cell clone secreted bFGF. These findings suggest a role for bFGF-mediated pathways and collagenase as molecular determinants of invasiveness in the brain.

The membrane protein CD9/DRAP 27 potentiates the juxtacrine growth factor activity of the membrane-anchored heparin-binding EGF-like growth factor

The membrane-anchored heparin-binding EGF-like growth factor precursor (proHB-EGF)/diphtheria toxin receptor (DTR) belongs to a class of transmembrane growth factors and physically associates with CD9/DRAP27 which is also a transmembrane protein. To evaluate the biological activities of proHB-EGF/DTR as a juxtacrine growth factor and the biological significance of its association with CD9/DRAP27, the mitogenic activity of proHB-EGF/DTR was analyzed using stable transfectants of mouse L cells expressing both human proHB-EGF/DTR and monkey CD9/DRAP27, or either one alone. Juxtacrine activity was assayed by measuring the ability of cells in co-culture to stimulate DNA synthesis in an EGF receptor ligand dependent cell line, EP170.7. LH-2 cells expressing human proHB-EGF/DTR stimulated EP170.7 cell growth moderately. However, LCH-1 cells, a stable co-transfectant expressing both human proHB-EGF/DTR and monkey CD9/DRAP27 cDNAs, dramatically unregulated the juxtacrine growth factor activity of proHB-EGF/DTR approximately 25 times over that of LH-2 cells even though both cell types expressed similar levels of proHB-EGF/DTR on the cell surface. Anti-CD9/DRAP27 antibodies which were not able to neutralize the mitogenic activity of soluble HB-EGF suppressed LCH-1 cell juxtacrine growth activity to the same extent as did anti-HB-EGF neutralizing antibodies and CRM 197, specific inhibitors of human HG-EGF. These findings suggest that optimal expression of the juxtacrine growth activity of proHB-EGF/DTR requires co-expression of CD9/DRAP27. These studies also indicate that growth factor potentiation effects which have been observed previously for soluble growth factors also occurs at the level of cell surface associated growth factors.

Diphtheria toxin binds to the epidermal growth factor (EGF)-like domain of human heparin-binding EGF-like growth factor/diphtheria toxin receptor and inhibits specifically its mitogenic activity

The membrane anchored form of human heparin-binding epidermal growth factor-like growth factor (HB-EGF) acts as the diphtheria toxin (DT) receptor. Transfection of human HB-EGF cDNA into mouse LC cells, L cells stably expressing DRAP27, conferred sensitivity to DT, but transfection of mouse HB-EGF cDNA did not. To define the essential regions of HB-EGF that serve as the functional DT receptor, we examined the sensitivity to DT and DT binding of cells expressing several human/mouse HB-EGF chimeras. It was found that DT binds to the EGF-like domain of the human HB-EGF. However, mouse HB-EGF does not serve as a functional DT receptor due to non-conserved amino acid substitutions in this domain. In addition, CRM197, a non-toxic mutant of DT, inhibited strongly the mitogenic activity of the secreted form of human HB-EGF, but not of mouse HB-EGF and other EGF receptor-binding growth factors. These results confirmed further that DT interacts with the EGF-like domain of HB-EGF and that this interaction is specific for human HB-EGF.

The role of growth factors in vascular cell development and differentiation

The control of vascular growth and differentiation is a complex system of activity and interaction between positive and negative modulators of these processes. A number of important stimulators and inhibitors of both smooth muscle cells and endothelial cells have now been purified and biochemically characterized. Imbalances in the activity of these factors can result in serious pathologies. In this chapter, we briefly discuss the biology of blood vessel development and growth, review the current literature which describes these stimulators and inhibitors, and discuss current therapeutic strategies designed around these growth modulators.

Purification and characterization of transmembrane forms of heparin-binding EGF-like growth factor

Heparin-binding epidermal growth factor-like growth factor (HB-EGF), whose cDNA has a predicted 208-codon open reading frame, is synthesized as a membrane-spanning precursor that is processed to release mature mitogenic proteins of approximately 73-87 amino acids in length. Previous work has focused on the structural and biological properties of secreted HB-EGF. In this study, human recombinant transmembrane HB-EGF, produced by expression of HB-EGF1-208 cDNA in a baculovirus system, has been isolated, purified, and characterized structurally and biologically. Two isoforms of transmembrane HB-EGF (HB-EGFTM) were purified from membrane fractions of infected insect cells by a combination of heparin affinity chromatography and reversed-phase high performance liquid chromatography. The isoform designated as HB-EGFTM-1, a 21.5-kDa protein, yielded no N-terminal sequence, suggesting that it is N-terminally blocked. However, HB-EGFTM-II, a 24-kDa protein, was N-terminally sequenced and found to be initiated at Asp63 in the 208-amino acid residue primary translation product. This N terminus is the same as that determined for a 18-kDa isoform of secreted HB-EGF purified from the conditioned medium of insect cells expressing HB-EGF1-149 cDNA and is also identical to the N terminus of the longest form of secreted HB-EGF initially purified from human macrophage-like U-937 cell conditioned medium. HB-EGFTM-II cross-reacted on a Western blot with an antibody directed against the 16 C-terminal amino acids of the cytoplasmic tail of HB-EGF, indicating that it contains a putative transmembrane domain. HB-EGFTM-II was bioactive and stimulated the proliferation of BALB/c 3T3 cells and smooth muscle cells and the motility of smooth muscle cells, albeit with approximately 10-25% of the specific activity of secreted HB-EGF isoforms. We concluded that transmembrane HB-EGF is bioactive when isolated, consistent with the possibility of its functioning as a juxtacrine growth factor when still tethered to the cell.

Syndecans, cell surface heparan sulfate proteoglycans, are induced by a proline-rich antimicrobial peptide from wounds

Cell surface heparan sulfate proteoglycans, such as the syndecans, are required for cellular responses to heparin-binding growth factors and extracellular matrix components. Expression of syndecan-1 and -4 is induced in mesenchymal cells during wound repair in the mouse, consistent with a role for syndecans in regulating cell proliferation and migration in response to these effectors. Here we show that wound fluid contains inductive activity that mimics the in vivo induction in time of appearance, specificity for mesenchymal cells, and selectivity for syndecan-1 and -4. We have purified and synthesized a 4.8-kDa proline-rich protein from wound fluid that reproduces this induction of syndecan-1 and -4 in cultured cells. This peptide, identical to the antibacterial peptide PR-39, is released into the wound by the cellular infiltrate and induces syndecan expression at the same peptide concentrations that lyse bacteria. These results indicate that wounds contain a multifunctional protein that induces mammalian cells to express cell surface heparan sulfate proteoglycans as part of the wound repair process and that kills bacteria as part of a nonimmune defense mechanism.

Biosynthesis and processing by phorbol ester of the cells surface-associated precursor form of heparin-binding EGF-like growth factor

Human MDA MB 231 cells were found to synthesize mostly the cell surface-associated precursor form of heparin-binding EGF-like growth factor (HB-EGF), a 27-kDa protein. Evidence for this form of HB-EGF included increased fluorescence intensity when cells were analyzed by flow cytometry using anti-HB-EGF antibodies, lack of HB-EGF in conditioned medium, and sensitivity to diphtheria toxin, for which HB-EGF is the receptor. Phorbol ester treatment of cells resulted, within 30 minutes, in loss of cell surface 27 kDA HB-EGF, lack of interaction with anti-HB-EGF antibodies, accumulation of active 21 kDa HB-EGF in conditioned medium, and the acquisition of diphtheria toxin resistance. It was concluded that cell surface-associated HB-EGF is the precursor of a bioactive growth factor, is biologically active as the receptor for diphtheria toxin, and is susceptible to rapid processing.

Differential regulation of heparin-binding epidermal growth factor-like growth factor in the adult ovariectomized mouse uterus by progesterone and estrogen

Expression of heparin-binding epidermal growth factor-like growth factor (HB-EGF) was studied in the adult ovariectomized mouse uterus in response to progesterone (P4) and/or 17 beta-estradiol (E2) using Northern blotting, in situ hybridization, and immunohistochemistry. A 2.5-kilobase transcript of HB-EGF messenger RNA (mRNA) was detected in total uterine RNA samples. Although low levels of this mRNA were detected in uterine samples of oil-treated ovariectomized mice (control), an injection of E2 promptly up-regulated the levels. The mRNA levels peaked at 2 h and returned to basal levels after 12 h. Injection of P4 alone did not influence the basal levels; however, coinjection of E2 with P4 caused a rapid, but transient, up-regulation of the mRNA. The levels peaked between 2-4 h and declined 6 h after the hormone injections. Coinjection of E2 with P4 after 1 day of P4 priming also resulted in peak levels of HB-EGF mRNA at 2 h; however, the levels were not sustained thereafter. Because P4 and E2 differentially regulate heterogeneous uterine cell types, in situ hybridization was performed to determine cell-specific expression of HB-EGF mRNA in the ovariectomized uterus before and after steroid treatments. In the oil-treated uterine sections, very low levels of autoradiographic signals were observed in the luminal epithelium. In contrast, an injection of E2 resulted in a marked accumulation of HB-EGF mRNA primarily in uterine epithelial cells within 2 h. Although specific hybridization signals could not be detected in any uterine cell types after P4 treatment, combined treatment with P4 and E2 resulted in an accumulation of HB-EGF mRNA in stromal cells. To determine whether uterine HB-EGF mRNA was translated, cellular distribution of HB-EGF protein was investigated by immunohistochemistry. In oil-treated uterine sections, an overall weak immunostaining was noted, whereas no staining could be detected in uterine sections after P4 treatment. In contrast, positive immunostaining was noted in epithelial cells after E2 treatment. Coinjection of P4 with E2 caused immunostaining in the stroma. These results are consistent with those of in situ hybridization. The present investigation establishes that in the adult ovariectomized mouse uterus, E2 regulates HB-EGF expression in the epithelium, whereas expression of HB-EGF in the stroma is regulated by P4 and E2.

The heparin-binding domain of heparin-binding EGF-like growth factor can target Pseudomonas exotoxin to kill cells exclusively through heparan sulfate proteoglycans

Heparin-binding EGF-like growth factor (HB-EGF) is a smooth muscle cell mitogen composed of both EGF receptor and heparin-binding domains. To better understand the function of its domains, intact HB-EGF or its heparin-binding (HB) domain (amino acids 1-45) were fused to a mutant Pseudomonas exotoxin with an inactivated cell-binding domain. The resulting chimeric toxins, HB-EGF-PE* and HB-PE*, were tested on tumor cells, proliferating smooth muscle cells and a mutant Chinese hamster ovary cell line deficient in heparan sulfate proteoglycans (HSPGs). Two targets were found for HB-EGF-PE*. Cells were killed mainly through EGF receptors, but the HB domain was responsible for killing via HSPGs. HB-PE* did not bind to the EGF receptor and thus was cytotoxic by interacting exclusively with HSPGs. We conclude that the HB domain of HB-EGF is able to mediate internalization through HSPGs, without requiring the EGF receptor.

Heparin-binding EGF-like growth factor, which acts as the diphtheria toxin receptor, forms a complex with membrane protein DRAP27/CD9, which up-regulates functional receptors and diphtheria toxin sensitivity

DRAP27, the monkey homolog of human CD9 antigen (DRAP27/CD9) and diphtheria toxin receptor (DTR) were expressed in mouse L cells. L cells transfected transiently with both DRAP27/CD9 and DTR cDNA bound approximately 10 times more diphtheria toxin (DT) than cells transfected with DTR alone. Stable L cell transfectants expressing both DTR and DRAP27/CD9 (LCH-1 cells) had 15 times more cell surface DT-binding sites and were 20 times more sensitive to DT than were stable L cell transfectants expressing DTR alone (LH-1 cells). Increased DT-binding and DT sensitivity were not due to increased DTR transcription or increased cell surface DTR protein. Co-immunoprecipitation of DRAP27/CD9 with DTR and chemical cross-linking suggest a tight association of these membrane-bound proteins. In addition, the identity of DTR and a growth factor (HB-EGF) was established. Immobilized DT specifically adsorbed HB-EGF precursor solubilized from transfected L cells and [125I]DT bound to immobilized recombinant HB-EGF. We conclude that DRAP27/CD9 associates tightly with DTR/HB-EGF and up-regulates the number of functional DTRs and DT sensitivity, and that HB-EGF is identical to DTR.

Heparin-binding EGF-like growth factor gene is induced in the mouse uterus temporally by the blastocyst solely at the site of its apposition: a possible ligand for interaction with blastocyst EGF-receptor in implantation

Heparin-binding EGF-like growth factor (HB-EGF) is a newly discovered member of the EGF family of growth factors. HB-EGF can bind to two loci on cell surfaces, heparan sulphate proteoglycans and EGF-receptor (EGF-R), and either one or both of these interactions could play a role in cell-cell interactions. In the rodent, increased endometrial vascular permeability at the site of blastocyst apposition is considered to be an earliest discernible prerequisite event in the process of implantation and this event coincides with the initial attachment reaction between the blastocyst trophectoderm and uterine luminal epithelium. This investigation demonstrates that the HB-EGF gene is expressed in the mouse uterine luminal epithelium surrounding the blastocyst 6–7 hours before the attachment reaction that occurs at 2200–2300 hours on day 4 of pregnancy. It was further demonstrated that this gene is not expressed in the luminal epithelium at the site of the blastocyst apposition during the progesterone-maintained delayed implantation, but is readily induced in the luminal epithelium surrounding an activated blastocyst after termination of the delay by an estrogen injection. In vitro studies showed that HB-EGF induced blastocyst EGF-R autophosphorylation, and promoted blastocyst growth, zona-hatching and trophoblast outgrowth. These results suggest possible interactions between the uterine HB-EGF and blastocyst EGF-R very early in the process of implantation, earlier than any other embryo-uterine interactions defined to date at the molecular level.

T lymphocytes synthesize and export heparin-binding epidermal growth factor-like growth factor and basic fibroblast growth factor, mitogens for vascular cells and fibroblasts: differential production and release by CD4+ and CD8+ T cells

T lymphocytes infiltrate wounds, tumors, and atherosclerotic plaques, pathophysiological processes characterized by the migration and proliferation of vascular cells and fibroblasts. Although T lymphocytes are known to produce cytokines for inflammatory cells, it has not been demonstrated that they synthesize growth factors that are mitogenic for vascular cells and fibroblasts. We demonstrate that cultured T lymphocytes isolated from normal human peripheral blood synthesize and export two well-characterized growth factors, heparin-binding epidermal growth factor-like growth factor (HB-EGF) and basic fibroblast growth factor (bFGF). This conclusion is based on mRNA expression analysis, heparin-affinity chromatography profiles, target-cell specificity, and functional inhibition by specific neutralizing antibodies. Atypically, a substantial amount of T-cell-derived bFGF-like activity appears to be constitutively released into conditioned medium, almost as much as is associated with T-cell lysates. bFGF is synthesized and exported by purified CD4+ and CD8+ T cells, whereas HB-EGF is synthesized and exported primarily by CD4+ T cells. The T-cell-derived HB-EGF and bFGF activities are potent mitogens for fibroblasts and smooth muscle cells, and the bFGF-like activity is also mitogenic for endothelial cells. These results suggest that T lymphocytes may play key roles in mediating smooth muscle hyperplasia associated with atherosclerosis and in angiogenesis associated with wound healing and tumor growth by acting locally to deliver vascular-cell growth factors to tissues.

Lysophosphatidylcholine upregulates the level of heparin-binding epidermal growth factor-like growth factor mRNA in human monocytes

Lysophosphatidylcholine is increased in the plasma of hypercholesterolemic patients, is a component of oxidatively modified low-density lipoprotein, and, as such, may play an important role in atherosclerosis. Here we demonstrate that in human monocytes, lysophosphatidylcholine increases the level of mRNA encoding the heparin-binding epidermal growth factor-like growth factor (HB-EGF), a potent smooth muscle mitogen. Lysophosphatidylcholine treatment also enhances the release of heparin-binding mitogenic activity by these cells in culture. The anti-inflammatory glucocorticoid dexamethasone inhibits the upregulation of HB-EGF mRNA induced by either lysophosphatidylcholine or bacterial lipopolysaccharide in cultured monocytes. However, the responses induced by lysophosphatidylcholine and by lipopolysaccharide differ in their kinetics. In addition, the response to lysophosphatidylcholine is resistant to the action of cycloheximide, whereas the response to lipopolysaccharide is not, suggesting that the activation mechanisms induced by these two stimuli are different. Since a nuclear run-on assay showed no effect of lysophosphatidylcholine on the transcription of the HB-EGF gene, we speculate that lysophosphatidylcholine may increase the level of HB-EGF mRNA by altering the processing or degradation of primary or mature transcripts. Lysophosphatidylcholine enhancement of monocyte production of HB-EGF may represent an important result of the interactions among oxidized low-density lipoprotein and monocyte-derived macrophages and may play a role in initiation of smooth muscle proliferation in atherogenesis.

Heparin-binding EGF-like growth factor synthesis by smooth muscle cells

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) has been previously demonstrated to be a potent mitogen for smooth muscle cells. Evidence is now presented that these cells synthesize HB-EGF as well. Cultured fetal human vascular smooth muscle cells express 2.5-kb HB-EGF mRNA. These cells also release an HB-EGF-like activity that (i) stimulates smooth muscle cell and BALB/c 3T3 cell but not endothelial cell proliferation; (ii) binds to TSK heparin affinity columns and is eluted with 0.9-1.2 M NaCl, and (iii) triggers phosphorylation of a protein with the same molecular weight as the 170-kD EGF receptor. In addition, 125I-HB-EGF can be cross-linked to the EGF receptor on fetal human vascular smooth muscle cells. These results suggest that smooth muscle cells can both synthesize and respond to HB-EGF, and that HB-EGF may therefore be involved in autocrine regulation of these cells.

Regulation of basic fibroblast growth factor mRNA expression in rat C6 glioma cells

Multiple basic fibroblast growth factor (bFGF) mRNAs are transcribed in rat brain at 6.0, 3.7, 2.5, 1.8, 1.6, 1.4, and 1.0 kb. These seven transcripts are also seen in Rat-1 fibroblasts and ras-transformed Rat-1 fibroblasts in culture. However, only a single bFGF transcript at 6.0 kb is detectable in the rat astrocytoma cell line, C6, and this mRNA is identical to that seen in a primary culture of cortical astrocytes. C6 glioma cells also transcribe message for FGF receptor 1 (FR1), suggesting possible autocrine growth by these cells. Growth factor activity in a C6 cell lysate was characterized by heparin affinity chromatography and Western blot analysis using an anti-bFGF antibody. Proteins of 18, 21.5, and 22 kDa were detected in C6 cells, indicating that the 6.0-kb mRNA is translated into the three characteristic bFGF proteins. Rat-1 fibroblasts also synthesize bFGF proteins of identical molecular weight. The small transcripts detected in brain probably represent bFGF or FGF-related mRNAs in cell types other than glia, such as fibroblasts, endothelial cells, or neurons. In cultured C6 cells, bFGF protein levels are highest in confluent, quiescent cells, whereas mRNA levels are low. Addition of serum, phorbol ester, or cycloheximide to both C6 cells and fibroblasts induces the level bFGF mRNA transcripts 10-fold after 1-4 h. This rapid induction after cell activation indicates that bFGF is an early response gene. Therefore, even though there are abundant intracellular stores of the factor, the transcriptional activation seen after mitogenic activation of cells implies that de novo bFGF mRNA synthesis is an important part of the mitogenic response.

Glucocorticoid inhibits thrombin-induced expression of platelet-derived growth factor A-chain and heparin-binding epidermal growth factor-like growth factor in human aortic smooth muscle cells

Proliferation of smooth muscle cells (SMCs) in atherosclerosis may be modulated by several growth regulatory molecules. At least two mitogens for SMCs, platelet-derived growth factor (PDGF) A-chain and heparin-binding epidermal growth factor-like growth factor (HB-EGF), can be produced by SMCs themselves and may stimulate smooth muscle proliferation in an autocrine or paracrine fashion. We examined the effects of thrombin, which may be generated at the site of vascular injury during atherogenesis, and the potent anti-inflammatory glucocorticoid, dexamethasone (DEX), on the expression of the genes encoding these two growth factors. Since both PDGF A-chain and HB-EGF have affinity for heparin, we also examined the effect of thrombin and DEX on the release of heparin binding mitogenic activity from SMCs. Treatment of SMCs with thrombin resulted in increases both in the level of the PDGF-A and HB-EGF transcripts in the cells, as well as in released heparin-binding growth factor activity. DEX inhibits the thrombin-stimulated release of mitogenic activity in a dose-dependent manner. An enzyme-linked immunoadsorbent assay showed that DEX inhibits both constitutive and thrombin-stimulated release of PDGF-AA. DEX also decreases both constitutive and thrombin-stimulated mRNA levels for PDGF A-chain and HB-EGF and destabilizes the transcripts for both growth factors. A nuclear run-on assay revealed that DEX acts, in addition, to inhibit constitutive and thrombin-stimulated transcription of the PDGF A-chain and HB-EGF genes. Thus, these findings indicate that expression of PDGF A-chain and HB-EGF may be regulated by thrombin and glucocorticoid at the transcription level. Our results are consistent with the involvement of thrombin-induced growth factor expression in neointimal SMC proliferation and suggest the possibility that intimal proliferation may be attenuated by glucocorticoids.