Binding and processing of epidermal growth factor in Panc-I human pancreatic carcinoma cells

The binding of ¹²⁵I-labeled epidermal growth factor (EGF) was studied in Panc-I human pancreatic carcinoma cells. At 37°C, binding was rapid and associated with marked endocytosis of the ligand. Bound EGF was sequentially converted to a number of more acidic species as follows: pI 4.55 to pI 4.2, to pI 4.35, to pI 4.0. EGF internalization and processing were blocked at 4°C. EGF did not alter cell growth when Panc-I cells were incubated in the presence of 2 to 10% serum. In contrast, when the serum concentration was lowered to 0.1%, EGF significantly enhanced cell replication after 6 days of culture.     

Interactions between the receptors for platelet-derived growth factor and epidermal growth factor

Preincubation of Swiss 3T3 cells or human fibroblasts with purified platelet-derived growth factor (PDGF) at 4 degrees C or 37 degrees C rapidly inhibits subsequent binding of 125I-epidermal growth factor (125I-EGF). The effect does not result from competition by PDGF for binding to the EGF receptor since (a) very low concentrations of PDGF are effective, (b) cells with EGF receptors but no PDGF receptors are not affected, and (c) the inhibition persists even if the bound PDGF is eluted before incubating the cells with 125I-EGF. PDGF does not affect 125I-insulin binding nor does EGF affect 125I-PDGF binding under these conditions. Endothelial cell-derived growth factor also competes for binding to PDGF receptors and inhibits 125I-EGF binding. The inhibition demonstrated by PDGF seems to result from an increase in the Kd for 125I-EGF binding with no change in the number of EGF receptors.     

Characterization of insulin-like growth factor binding proteins by two-dimensional polyacrylamide gel electrophoresis and ligand blot analysis.

Insulin-like growth factor binding proteins (IGFBPs) in pregnant baboon serum and tissue culture media obtained following explant culture of uteri from pregnant baboons were characterized by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (2D SDS-PAGE) followed by Western ligand blot analysis using 125I-labeled IGF-I. IGFBP-1 (Mr 30,000; pI 4-4.2), IGFBP-2 (Mr 34,000, pI 5.7-6.2), IGFBP-3 (doublet Mr 42-48,000; pI 6.2-6.8), and IGFBP-4 (Mr 24,000; pI 5.7-6.0) were clearly separated from one another. The authenticity of IGFBP-1, -2, and -3 was verified by immunoprecipitation using polyclonal antibodies followed by ligand blotting. Specificity of 125I-labeled IGF-I binding to IGFBPs was also determined by competitive binding studies using unlabeled IGF-I and -II. This technique allows for the identification of IGFBPs in complex biological fluids on the basis of their characteristic Mr and pI with or without the availability of specific antibodies and can be done rapidly using the mini 2D SDS-PAGE systems.     

Sequential processing of epidermal growth factor in early and late endosomes of rat liver.

We have used isolated perfused rat livers to examine the intracellular processing of 125I-epidermal growth factor (EGF) and to determine where in the endocytic pathway the hydrolases which degrade EGF are acting. Following uptake of 125I-EGF at 37 or 16 degrees C, subcellular fractions enriched in endosomes and lysosomes were isolated, and their 125I-EGF content was examined by reverse-phase high performance liquid chromatography. Three forms of EGF processed at their carboxyl termini are generated in endosomes. At 37 degrees C, EGF is first processed in early endosomes by a carboxypeptidase B-like protease and is further processed in late endosomes by a trypsin-like protease and then a carboxypeptidase B-like protease. At 16 degrees C, entry of EGF into late endosomes is slowed, and only the first processed form is generated over 60 min. Longer perfusions (180 min) at 16 degrees C result in some processing (7%) by proteases found in late endosomes. EGF-horseradish peroxidase cytochemistry confirmed that the additional processing detected at 180 min correlated with movement of EGF from tubulovesicular to multivesicular endosomes. These results, combined with in vitro incubations of EGF in isolated endosomal and lysosomal fractions, suggest that different proteases are active at selective points in the endocytic pathway and that the full complement of proteases needed for complete degradation of EGF is active only in lysosomes.     

Biosynthesis of the epidermal growth factor receptor in human epidermoid carcinoma-derived A431 cells

Using human-specific antibody reagents, we have examined the biosynthesis of the epidermal growth factor receptor in human epidermoid carcinoma-derived A431 cells. Four Mr species (Mr = 70,000, 95,000, 135,000, and 145,000) are detected when cells are subjected to a brief pulse of L-[35S]methionine; an Mr = 165,000 species is detected after 45-60 min of exposure of cells to radiolabel. In pulse-chase experiments, the four lower Mr species appear to bear a precursor relation to the Mr = 165,000 protein. The molecule acquires N-linked oligosaccharide cotranslationally, and two of the species (Mr = 95,000 and 145,000) are susceptible to digestion with endo-beta-N-acetylglucosaminidase H. The Mr = 145,000 and Mr = 165,000 proteins, which become labeled with 125I-epidermal growth factor after treatment of intact cells with a bifunctional cross-linking reagent, are phosphorylated at serine and threonine on identical tryptic peptides.     

Acute Regulation of the Epidermal Growth Factor Receptor in Response to Nerve Growth Factor

PC12 cells possess specific receptors for both nerve growth factor and epidermal growth factor, and by an unknown mechanism, nerve growth factor is able to attenuate the propagation of a mitogenic response to epidermal growth factor. The differentiation response of PC12 cells to nerve growth factor, therefore, predominates over the proliferative response to epidermal growth factor. We have observed that the addition of nerve growth factor to PC12 cells rapidly produces a decrease in surface 125I-epidermal growth factor binding capacity. Unlike previously described nerve growth factor effects on 125I-epidermal growth factor binding capacity, which required several days of nerve growth factor exposure, the decreases we report occur within minutes of nerve growth factor addition: A 50% decrease in 125I-epidermal growth factor binding capacity is evident at 10 min. This rapid nerve growth factor response is concentration dependent; inhibition of 125I-epidermal growth factor binding is detectable at nerve growth factor levels as low as 0.2 ng/ml and is maximal at approximately 50 ng/ml, consistent with known ranges of biological activity. No demonstrable differences in the rate of epidermal growth factor receptor synthesis or degradation were observed in cells acutely exposed to nerve growth factor. Scatchard analysis revealed that acute nerve growth factor treatment decreased the number of both high- and low-affinity 125I-epidermal growth factor binding sites, while the receptor affinity remained unchanged. We have also investigated the involvement of various potential intracellular mediators of nerve growth factor action and of known intracellular modulatory systems of the epidermal growth factor receptor for their capacity to participate in this nerve growth factor activity.     

Insulin-like growth factor characteristics of an acidic non-suppressible insulin-like activity.

The biological activities of an acidic form of non-suppressible insulin-like activity (ILA pI 4.8) have been studied. ILA pI 4.8 was isolated from Cohn fraction IV-1 of human serum by pH 5.5 ion-exchange chromatography on SP-Sephadex. Carrier-bound ILA was eluted at pH 9.7 and then sequentially gel chromatographed in 1% formic acid on Sephadex G-75 and Bio-Gel P-30. The low-Mr (7000) active material was subjected to flat bed isoelectric focusing. Overall recovery was 87 munit of insulin equivalents/100 g of Cohn fraction IV-1, with a specific activity in the range 4-10 munit/mg of protein, representing a purity of 1-6%. This material has been tested in a variety of insulin-like growth factor (IGF)/somatomedin assay systems. It stimulated, in a dose-related manner, [14C]glucose conversion into lipid by isolated rat adipocytes, 35SO4(2-) incorporation into weanling rat costal cartilage and [3H]thymidine incorporation into DNA of cultured human fibroblasts. Like IGF-I and -II, ILA pI 4.8 was able to inhibit degradation of 125I-insulin by crude homogenates of rat liver. In addition, the biological activity of ILA pI 4.8 was completely suppressible by a recently described inhibitor of IGF-I and IGF-II. ILA pI 4.8 was able to compete, in a parallel manner, with 125I-IGF-I and 125I-IGF-II and, at higher doses, with 125I-insulin in a placental radioreceptor assay. No cross-reactivity was seen in a radioimmunoassay for IGF-I and -II C-peptides, but at higher concentrations parallel displacement was observed in a somatomedin C/IGF-I radioimmunoassay using two different antisera. These data indicate that ILA pI 4.8 does possess many of the biological activities previously reported for the IGFs. Since ILA pI 4.8 does occur naturally in serum, it would appear reasonable to tentatively include it as one of the IGF/somatomedin family.     

Attenuated processing of epidermal growth factor in the face of marked degradation of transforming growth factor-alpha

T3M4 human pancreatic carcinoma cells avidly bound and internalized 125I-labeled epidermal growth factor (EGF) but did not readily degrade the ligand. Pulse-chase experiments in which the cell-bound radioactivity was allowed to dissociate into the incubation medium in the presence of unlabeled EGF indicated that the majority of the released 125I-EGF consisted of intact EGF and a slightly processed species that readily bound to the cell. Omission of unlabeled EGF during the chase period markedly decreased the amount of radioactivity in the incubation medium, mainly as a result of the rebinding of EGF to the cells. In contrast, T3M4 cells readily degraded 125I-labeled transforming growth factor-alpha (TGF-alpha), and the released radiolabeled products did not rebind to the cells. Both ligands were released from T3M4 cells under acidic conditions, complete dissociation occurring at a pH of 4.5 for EGF, and a pH of 6.5 for TGF-alpha. A 431 human epidermoid carcinoma cells and ASPC-1 human pancreatic carcinoma cells also failed to extensively degrade 125I-EGF, whereas Rat-1 fibroblasts markedly degraded the growth factor. As in the case of T3M4 cells, ASPC-1 cells extensively degraded 125I-TGF-alpha. Degradation of either ligand was blocked by the lysosomotropic compound methylamine in all the tested cell lines. Immunoprecipitation of the EGF receptor with specific polyclonal antibodies and Western blot analysis revealed the anticipated 170-kDa protein in T3M4 cells. Both EGF and TGF-alpha enhanced EGF receptor degradation, but TGF-alpha was less effective than EGF. These findings indicate that in certain cell types EGF and TGF-alpha may be differentially processed.     

Binding, internalization and intracellular processing of 125I-epidermal growth factor purified by isoelectric focusing

When epidermal growth factor (EGF) which had been extensively purified by HPLC was subjected to iodination with sodium ¹²⁵iodide, 5 major species of differing isoelectric points were produced. Some of these species bound to rat fibroblasts with different affinities but were internalized with equal efficiency. Examination of the internalized ¹²⁵I-labelled molecules revealed processing of all the ¹²⁵I-EGF species to macromolecules with more acidic isoelectric points. The ¹²⁵I-EGF species with a pI of 4.5 corresponded in electrofocusing behavior with intact non-iodinated EGF. Other EGF species probably represented molecules which were covalently modified as a result of the iodination procedure.     

Affinity labeling of a nerve growth factor receptor component on rat pheochromocytoma (PC12) cells

Clonal PC12 rat pheochromocytoma cells were sequentially incubated with 125I-labeled nerve growth factor and the photoreactive bifunctional agent hydroxysuccinimidyl-p-azidobenzoate. This treatment effected the crosslinking of 125I nerve growth factor to a PC12 cell component that exhibits an apparent Mr = 148 000-158 000, and consists of a single polypeptide chain with internal disulfide bonds. The amount of label associated with this Mr = 148 000-158 000 species was proportional to the degree of occupancy of nerve growth factor receptors by 125I-labeled nerve growth factor. Affinity labeling of this species was inhibited by the presence of 0.2 microM unlabeled nerve growth factor during incubation of PC12 cells with 125I nerve growth factor. In membranes prepared from PC12 cells hydroxysuccinimidyl-p-azidobenzoate effected the crosslinking of 125I-labeled nerve growth factor to an Mr = 120 000-130 000 species but not to the Mr = 148 000-158 000 component observed in intact cells. The kinetics of 125I nerve growth factor affinity labeling of the Mr = 148 000-158 000 species closely paralleled the time-course of 125I nerve growth factor association to two kinetically distinct forms of nerve growth factor receptors in PC12 cells. The data indicate that the Mr = 148 000-158 000 species affinity-labeled by 125I nerve growth factor is the native form of a component associated with kinetically different nerve growth factor receptors in PC12 cells.     

Protamine enhances epidermal growth factor (EGF)-stimulated mitogenesis by increasing cell surface EGF receptor number. Implications for existence of cryptic EGF receptors

Treatment of Swiss mouse 3T3 cells and human epidermoid carcinoma A431 cells with protamine at 37 degrees C increased the 125I-epidermal growth factor (EGF) binding activity at 4 degrees C. The effect of protamine on the increase of 125I-EGF binding activity appeared to be time, temperature, and dose dependent. This up-modulation of 125I-EGF binding by protamine correlated with protamine enhancement of EGF-stimulated mitogenesis, with respect to the magnitude of the effect and the dose response curves. Scatchard plot analyses indicated that protamine induced an increase in numbers of both high and low affinity EGF receptors without affecting their affinities. Protamine also increased functionally active EGF receptors in plasma membranes and solubilized membranes. This was evidenced by Scatchard plot analyses and by a protamine-induced increase of 125I-EGF-EGF receptor complex and an increase in EGF-stimulated phosphorylation of the EGF receptor. Combined with column chromatography of the solubilized EGF receptor on protamine-agarose gel, these results suggest that protamine may increase the EGF receptor number by directly activating cryptic EGF receptors in the plasma membrane.     

Partial purification and characterization of a growth factor present in goat's colostrum. Similarities with platelet-derived growth factor.

A factor in goat's colostrum which stimulates DNA synthesis and cell proliferation in Swiss 3T3 fibroblasts has been purified approx. 350-fold by a sequence of acid precipitation, cation-exchange chromatography and gel filtration. The growth factor is a highly basic, heat stable (100 degrees C for 5 min) polypeptide with Mr approx. 35000. The polypeptide resists denaturation by guanidinium chloride or urea but is totally inactivated by treatment with reducing agents. The factor, which we have termed colostric basic growth factor ( CBGF ), inhibits the binding of 125I-labelled epidermal growth factor (125I-EGF) to Swiss 3T3 fibroblasts but does not inhibit 125I-EGF binding to epidermoid A431 cells. CBGF interacts synergistically with plasma in stimulating DNA synthesis in quiescent Swiss 3T3 cells. The chemical and biological properties of CBGF are thus very similar to the properties reported for the human platelet-derived growth factor. Although high concentrations of CBGF are present in the colostrum of goats, cows, and sheep, the milk of these species contains little or no factor. The origin and possible functions of CBGF are unknown.     

Regulation of transferrin receptor expression at the cell surface by insulin-like growth factors, epidermal growth factor and platelet-derived growth factor.

Addition of platelet-derived growth factor (PDGF), recombinant insulin-like growth factor I (rIGF-I) or epidermal growth factor (EGF) to BALB/c 3T3 fibroblasts causes a marked increase in the binding of [125I]diferric transferrin to cell surface receptors. This effect is very rapid and is complete within 5 min. The effect of EGF is transient, with [125I]diferric transferrin binding returning to control values within 25 min. In contrast, PDGF and rIGF-I cause a prolonged stimulation of [125I]diferric transferrin binding that could be observed for up to 2 h. The increase in the binding of [125I]diferric transferrin caused by growth factors was investigated by analysis of the binding isotherm. Epidermal growth factor, PDGF and rIGF-I were found to increase the cell surface expression of transferrin receptors rather than to alter the affinity of the transferrin receptors. This result was confirmed in human fibroblasts by the demonstration that EGF, PDGF and rIGF-I could stimulate the binding of a monoclonal antibody directed against the transferrin receptor (OKT9) to the cell surface. Furthermore, PDGF and rIGF-I stimulated the sustained uptake of [59Fe]diferric transferrin by BALB/c 3T3 fibroblasts, while EGF transiently increased uptake. Thus the effect of these growth factors to increase the cell surface expression of the transferrin receptor appears to have an important physiological consequence.     

A unique family of endothelial cell polypeptide mitogens: the antigenic and receptor cross-reactivity of bovine endothelial cell growth factor, brain-derived acidic fibroblast growth factor, and eye-derived growth factor-II

Bovine brain, hypothalamus, pituitary, and retina contain potent anionic polypeptide mitogens for endothelial cells. Immunological assays using murine monoclonal antibodies against bovine endothelial cell growth factor (ECGF) and radioreceptor assays using [125I]ECGF were performed to determine the cross-reactivity of ECGF with bovine acidic pI brain-derived fibroblast growth factor (acidic FGF) and bovine eye-derived growth factor-II [EDGF-II). We observed that acidic FGF and EDGF-II are recognized by anti-ECGF monoclonal antibodies and compete with [125I] ECGF for receptor occupancy. Furthermore, the biological activity of ECGF, acidic FGF, and EDGF-II is potentiated by the glycosaminoglycan, heparin. These results argue that ECGF, acidic FGF, and EDGF-II belong to a common family of polypeptide growth factors.     

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.     

Association of bovine brain-derived growth factor receptor with protein tyrosine kinase activity

Bovine brain-derived growth factor (BDGF) is very similar to endothelial cell growth factor and brain-derived acidic fibroblast growth factor in terms of pI (5.7) and molecular weight (approximately 17,000). BDGF has a wide spectrum of cell specificity, including bovine aorta endothelial cells and Swiss mouse 3T3 cells. BDGF stimulates the phosphorylation of a 135-kDa protein in plasma membranes of 3T3 cells. The optimal concentration for stimulation of phosphorylation is close to the Kd of 125I-BDGF binding to receptor, suggesting that the BDGF-stimulated 32P-labeled 135-kDa protein may be the BDGF receptor. The alkaline stability of this 32P-labeled 135-kDa phosphoprotein and phosphoamino acid analysis of the acid hydrolysates indicate that the phosphorylation occurs at tyrosine residues. The molecular size of BDGF receptor is estimated as approximately 135 kDa by cross-linking 125I-BDGF to its receptor in 3T3 cells, using a bifunctional reagent, ethylene glycolbis(succinimidylsuccinate). Both BDGF-stimulated phosphorylation and 125I-BDGF binding to receptor can be inhibited by protamine. These results suggest that the BDGF receptor is a 135-kDa protein which is associated with a protein tyrosine kinase activity.     

Platelet-derived growth factor. III. Identification of a platelet-derived growth factor receptor by affinity labeling

Two homobifunctional cross-linking reagents have been used to cross-link 125I-platelet-derived growth factor (PDGF) to a cell surface component with an approximate Mr = 164,000 that has many characteristics of a specific PDGF receptor. Excess unlabeled PDGF competed for labeling of this component, while high concentrations of fibroblast growth factor, insulin, epidermal growth factor, low density lipoprotein or acetylated low density lipoprotein had no effect. Preincubation of cells with 125I-PDGF at 37 degrees C reduced specific 125I-PDGF binding (down regulation) and produced a parallel decrease in the amount of the 164,000-dalton receptor available for labeling. The 164,000-dalton component contains at least some protein that is accessible to trypsin in the extracellular medium. A complex of comparable Mr is seen on all PDGF-responsive cell types examined, but not on a nonresponsive cell type. 125I-PDGF does not become covalently cross-linked to this component in the absence of a cross-linking reagent.     

Tumor-promoting phorbol diesters mediate phosphorylation of the epidermal growth factor receptor

4 beta-Phorbol 12 beta-myristate 13 alpha-acetate (PMA) markedly inhibited the binding of low concentrations (less than 10(-9 m) of 125I-epidermal growth factor (EGF) to A431 human epidermoid carcinoma cells. However, very little change in the binding of 125-I-EGF at high concentrations (greater than 10(-8) M) was observed in response to PMA. Affinity labeling of the 170,000-dalton EGF receptor with 125I-EGF and disuccinimidyl suberate was also decreased by the tumor promoter at low, but not high, concentrations of 125I-EGF. In order to examine this action of PMA on the EGF receptor, the receptor phosphorylation state was evaluated in A431 cells that had been incubated with [32P]phosphate for 3 h prior to the addition of PMA. The 32P content of the EGF receptor purified with EGF-Sepharose was increased by 38% compared with the same amount of receptor isolated from control cells. The increase in EGF receptor phosphorylation was dose-dependent with a half-maximal effect between 0.1 and 1 nM PMA and was specific for tumor promoting analogues of phorbol diesters. Phosphoamino acid analysis indicated that the increase in the 32P content of the EGF receptor was mainly due to phosphoserine. These results demonstrate that the EGF receptor is a target for PMA action and suggest that the mechanism of PMA action on the response of cells to epidermal growth factor may be mediated in part by phosphorylation of the EGF receptor.     

Ligand-induced desensitization of 125I-epidermal growth factor internalization

The internalization of 125I-epidermal growth factor (EGF) by A431 cells was investigated. Control cells were able to internalize over 80% of receptor-bound 125I-EGF. By contrast, cells treated with EGF before incubation with 125I-EGF internalized only 50% of the surface-bound radioligand. The ligand-induced decrease in 125I-EGF internalization showed a dose response to EGF with half-maximal effect occurring at 3 nM. The alteration in the extent of 125I-EGF internalization did not require extended treatment with high concentrations of the hormone. When the internalization of picomolar versus nanomolar concentrations of EGF were compared, the lower concentrations of 125I-EGF were more completely internalized than the higher concentrations of radioligand. These data are consistent with the hypothesis that occupation of the EGF receptor by hormone rapidly leads to the activation of cellular processes which effectively desensitize the system to further ligand-induced internalization. The decrease in the extent of ligand internalization occurred in cells in which the protein kinase C (Ca2+/phospholipid-dependent enzyme) activity had been down-regulated by prolonged treatment with 12-O-tetradecanoyl-phorbol-13-acetate implying that the desensitization process is independent of protein kinase C. However, the effects of EGF on the extent of hormone internalization could be mimicked by the addition of A23187 and could be prevented by pretreatment of the cells with calmodulin antagonists suggesting the possibility that Ca2+-calmodulin is involved in the regulation of EGF receptor internalization in A431 cells.     

Characterization of the epidermal growth factor receptor in preimplantation pig conceptuses.

Embryos recovered from sows on Days 9-13 of pregnancy (Day 0 = first day of estrus) exhibited saturable and time-dependent specific binding of 125I-epidermal growth factor (EGF). The specific binding (pg/mg protein) was greater (P less than 0.001) for Day 13 elongated conceptuses than for conceptuses of earlier stages. Scatchard analyses showed two classes of binding sites (Kd = 7.0 +/- 2.6 x 10(-11) M, Bmax = 6.2 +/- 1.4 fmol/mg protein and Kd = 3.4 +/- 0.2 x 10(-8) M, Bmax = 420 +/- 80 fmol/mg protein). The EGF receptor in Day 13 conceptus membranes is a 170-kDa protein and was phosphorylated in the presence of EGF and adenosine triphosphate. EGF stimulated protein tyrosine kinase activity about 1.6-fold over basal levels. The results show that the preimplantation pig conceptus possesses EGF-binding sites with the properties of functional EGF-receptors.