Binding of human epidermal growth factor to tissue homogenates of the rat

Tissue distribution and plasma clearance of heparin-binding EGF-like growth factor (HB-EGF) in adult and newborn rats

Heparin-binding EGF-like growth factor (HB-EGF), a member of the epidermal growth factor (EGF) family, can protect intestinal epithelial cells from various forms of injury in vitro and attenuate intestinal ischemia/reperfusion damage in vivo. With the goal of eventual clinical use of HB-EGF to protect the intestines from injury in neonates, children, and adults, the pharmacokinetics and biodistribution of 125I-labeled HB-EGF were investigated. After intravenous bolus, HB-EGF had a distribution half-life of 0.8 min and an elimination half-life of 26.67 min. After gastric administration, the bioavailability was 7.8%, with a 2.38 h half-life in the absorption phase and an 11.13 h half-life in the elimination phase. After intravenous dosing, most radioactivity was found in the plasma, liver, kidneys, bile, and urine, whereas it was mainly distributed in the gastrointestinal tract after intragastric administration. The degradation of 125I-HB-EGF in plasma from newborn rats was lower than that in adult rats after gastric administration. This supports the feasibility of enteral administration of HB-EGF in the treatment of gastrointestinal diseases, including newborns afflicted with necrotizing enterocolitis.

Perturbation of epidermal growth factor clearance after radioiodination and its implications

The clearance of human epidermal growth factor (hEGF1-53) has been thought to be mediated mainly by a high-capacity receptor system, yet relatively low in vivo clearance rates (<10 mL/min/kg) and long terminal elimination half-lives (>120 min) have been observed in rats receiving the peptide that was iodinated by the oxidative chloramine-T (CT) method. We investigated if a mild, less oxidative iodination by the lactoperoxidase (Enzymobeads, EB) method, which is known to yield an iodinated peptide with receptor-binding equivalence, could produce a labeled peptide that behaves pharmacokinetically similar to the native material. For comparison, a parallel study was also conducted with EB-125I-hEGF1-48, which in its native form has a much reduced receptor binding activity due to the loss of the C-terminal pentapeptide. Plasma radioactivity concentrations were determined by trichloroacetic acid (TCA) precipitation and immunoprecipitation. Rats cleared unlabeled hEGF1-53 and hEGF1-48 markedly faster (CL(tot) > 120 mL/min/kg) than their radiolabeled counterparts. Approximately 96% of the hEGF1-53 dose was cleared during the initial phase (0-4 min), as opposed to only 5-14% for the iodinated peptide. Similar change was also observed for EB-125I-hEGF1-48 and CT-125I-hEGF1-53. The pharmacokinetic behavior of EB-125I-hEGF1-53 was, in fact, comparable to that of CT-125I-hEGF1-53. These observations indicate that receptor-binding equivalence does not have direct relationship with in vivo EGF clearance. Both iodination methods (oxidative CT and less oxidative EB) might have perturbed one or more steps in the cascade of ligand-receptor internalization and intracellular procession, which in turn modified the disposition of the peptides. In addition, the two independent precipitation techniques for the same peptide generated different kinetic outcomes. The overall experimental results suggest that it is unacceptable to use an iodinated form to characterize the disposition of peptides/proteins like EGF with a specific receptor system mediating its clearance.

Dose-dependent plasma clearance of human epidermal growth factor in rats

Exogenously administered human epidermal growth factor (hEGF) shows a marked dose-dependent plasma disposition in rats. In the present study, total plasma clearance (CLtotal) of hEGF was examined from the viewpoint of hepatic blood flow and accessible EGF receptors in a dosing range of hEGF from 30 to 1000 micrograms/kg. In rats in which down-regulation of EGF receptors caused a reduction in their number (50% below the normal level) (group 1), the CLtotal of hEGF was decreased only at a medium dose (100 micrograms/kg) compared to those in normal rats. In rats in which hepatic intoxication with carbon tetrachloride caused 50% reduction of both EGF receptor number and hepatic blood flow (group 2), CLtotal was decreased at low and medium hEGF doses. The decrease at a low hEGF dose was proportional to the decrease in the hepatic blood flow. In rats with reduced hepatic blood flow caused by hypothermia (group 3), CLtotal was decreased at all hEGF doses examined, and the decrease at a low hEGF dose (50 micrograms/kg) was proportional to the hepatic blood flow. The decreases of CLtotal at a medium hEGF dose in groups 1, 2 and 3 were well accounted for by the decrease of hepatic blood flow and/or EGF receptor number in a well-stirred model.(ABSTRACT TRUNCATED AT 250 WORDS)

Epidermal growth factor receptor in normal ovaries and benign ovarian tumours

Epidermal growth factor receptor (EGFR) was assayed in 52 women who had normal ovaries removed at hysterectomy and in 30 women with benign ovarian tumours. The histology of each ovary was recorded. A single point screen was performed on all samples and in positive cases a full Scatchard analysis. EGFR was present in 8 of 52 normal ovaries (15.4%) and 3 contained the high-affinity component while 5 had the low affinity component. In the benign ovarian tumour group 4 of 30 tumours (13.3%) had receptor present, one was high affinity and 3 were low affinity in type. We can conclude that EGFR is detectable only at low frequency in normal and benign ovarian tumours.

Comparison of the methods for determining cell-surface and intracellular receptors for epidermal growth factor in the rat liver

We compared methods for determining the distribution of epidermal growth factor (EGF) receptors between the cell surface and the cell interior in the rat liver. Incubation of isolated hepatocytes with 100 nM EGF for 20 min at 37 degrees C remarkably decreased the cell-surface EGF receptor density (internalization of receptors). The detergent Brij 35 was previously reported to permit assay of the intracellular latent EGF receptors in liver homogenates, but in the present investigation, Brij 35 lowered the affinity of EGF for the receptor depending on the detergent concentration, and the appearance of latent receptors was not observed. In contrast, permeabilization of the cells with digitonin, followed by an acid-washing procedure, increased the EGF binding capacity to close to the control level. Hence, the EGF receptors, internalized together with EGF molecules, were not degraded for at least 20 min, and the digitonin method is suitable for quantifying the intracellular EGF receptors. The binding capacities of the digitonin-treated and untreated control cells showed no difference upon digitonin treatment, suggesting that the bulk of EGF receptors exists on the cell surface. Further, cell-surface EGF receptor density was determined after the i.v. administration of EGF (300 micrograms/kg) to rats. Isolated hepatocytes prepared 30 min after the administration of EGF showed little binding for EGF on the cell surface, while the cell-surface EGF receptor density recovered to close to control values in cells prepared after 3 hr.

Kinetic analysis of receptor-mediated endocytosis of epidermal growth factor by isolated rat hepatocytes

The interaction of epidermal growth factor (EGF) with cell surface receptors and their subsequent endocytosis in isolated rat hepatocytes were analyzed by measuring changes in the concentrations of cell surface-bound, internalized, and degraded EGF. The kinetic model proposed by Wiley and Cunningham (Cell 25: 433-440, 1981) and Gex-Fabry and Delisi [Am. J. Physiol. 247 (Regulatory Integrative Comp. Physiol. 16): R768-R779, 1984] was basically utilized for the model analysis. The following kinetic parameters were obtained: association and dissociation rate constants for EGF-receptor interaction, internalization rate constant for EGF-receptor complex (kappa e), internalization rate constant for free receptor (kappa t), sequestration rate constant (kappa s) of the complex from shallow (exchangeable) to deep (nonexchangeable) membraneous compartment, intracellular degradation rate constant and initial cell-surface receptor density. The kappa s value, which was obtained by analyzing the time profiles of EGF association with cells, was approximately 5-10 times larger than the kappa e value determined by directly measuring internalized EGF with the acid-washing technique. This suggests the necessary presence of deep (nonexchanging) compartment of the complex in the plasma membrane. The calculated kappa e value is at least several times larger than the kappa t value, yielding the kinetic basis for the occurrence of receptor downregulation induced by excess EGF. We conclude that, in the overall receptor-mediated processing of EGF after bound to the cell surface receptors, the dissociation process is rapid [half-time (t1/2) less than 1 min], the degradation process is much slower (t1/2 approximately equal to 3 h), and the receptor internalization process is intermediate (t1/2 approximately equal to 6-7 min). In addition, two pools for EGF-receptor complex in the plasma membrane seem to be present, although their identification cannot be made.

Transforming growth factor alpha-Pseudomonas exotoxin fusion protein prolongs survival of nude mice bearing tumor xenografts

Transforming growth factor alpha (TGF alpha)-Pseudomonas exotoxin 40 (PE40) is a chimeric protein consisting of an N-terminal TGF alpha domain fused to a C-terminal 40-kDa segment of the Pseudomonas exotoxin A protein. TGF alpha-PE40 exhibits the receptor-binding activity of TGF alpha and the cell-killing activity of PE40. These properties make TGF alpha-PE40 an effective cytotoxic agent for cells that possess epidermal growth factor receptors (EGFR). However, the utility of this protein as an anticancer agent has been unclear because many normal tissues express EGFR and may be damaged by exposure to TGF alpha-PE40. To address this issue, we injected nude mice with a lethal inoculum of either A431 or HT29 human tumor cells that possess EGFR or with Chinese hamster ovary (CHO) tumor cells that lack EGFR. Animals were treated with a derivative of TGF alpha-PE40 in which the cysteine residues are replaced by alanine, termed "TGF alpha-PE40 delta cys," or with saline once a day for 5 days. Mice bearing EGFR+ tumor cells lived significantly (P less than 0.001) longer when treated with TGF alpha-PE40 delta cys compared with saline-treated controls (median survival: A431 cells, 51.5 vs. 25.5 days; HT29 cells, 101 vs. 47.5 days). TGF alpha-PE40 delta cys did not prolong the survival of mice bearing tumor cells that lack EGFR (median survival: CHO cells, 15.5 vs. 19.5 days). The only toxicity to normal tissues was mild periportal hepatic necrosis. These studies indicate that a therapeutic window exists in vivo for the use of some growth factor-toxin fusion proteins as anticancer agents.

Kinetic analysis of the downregulation of epidermal growth factor receptors in rats in vivo

We previously clarified the specific binding sites for epidermal growth factor (EGF) in several organs in rats based on in vivo kinetic analysis (D. C. Kim, Y. Sugiyama, H. Sato, T. Fuwa, T. Iga, and M. Hanano. J. Pharm. Sci. 77: 200-207, 1988). In the present study, we have determined the extent of the receptor downregulation and the recovery rate of the available receptors for EGF in several organs in vivo. At the specified times (30 min-24 h) after intravenous administration of excess unlabeled EGF (300 micrograms/kg), the early-phase (less than 3 min) uptake clearances (k1) of the tracer amount of 125I-EGF, which are proportional to the cell-surface available receptor densities, were determined in the liver, kidney, duodenum, jejunum, ileum, stomach, and spleen. As the result, the k1 value in each organ at 30 min after intravenous administration of unlabeled EGF was lowered close to the receptor-independent clearance value, indicating that the cell-surface receptors were almost completely downregulated, and thereafter, the k1 value showed gradual recovery to the control level. Furthermore, the recovery half-lives showed interorgan differences, namely the half-life (20 min) in the liver was much shorter than those (2-4.5 h) in other organs. These results were considered to reflect the processes of the recycling of internalized EGF receptors to the cell-surface or recruitment of new receptors. It was concluded that the recovery rate of the downregulated receptors in the liver, which is most responsible for the plasma clearance of EGF, is much faster than those in other organs.

Decrease in the number of receptors for epidermal growth factor in the liver of D-galactosamine-intoxicated rats

Hepatic transport of epidermal growth factor (EGF) was studied in D-galactosamine-intoxicated rats by the multiple-indicator dilution (MID) method. The extraction ratio of 125I-labeled EGF in the intoxicated rats, obtained from a model-independent analysis of the dilution curves, decreased to 45% of the control values. A distributed two-compartment model was fitted to the dilution data by nonlinear least-squares regression, and the kinetic parameters, kon.PT (product of on-rate constant and receptor density), koff (off-rate constant) and ks (sequestration rate constant) were determined. The values of kon.PT and ks in the intoxicated rats decreased to approximately one-half and one-third of those in the control rats respectively. Similar decreases in the kon.PT and ks values in the intoxicated rats were also observed for the transport of 125I-labeled insulin, a positive control, into the liver. The 125I-labeled EGF binding experiment at equilibrium using liver homogenates revealed that the intoxication reduced the receptor density (PT) to one-third of the control values, whereas the equilibrium dissociation constant (kd) did not change significantly. The activities of Na+,K+-ATPase, cytochrome P-450 and glutathione S-transferase decreased in the intoxicated rats to 70-80% of the control values. The number of nuclei per unit area of tissue slices was also reduced to 70% of the control. Thus, the extent to which the enzyme activities and the number of nuclei decreased in the intoxicated liver was smaller than that of the number of EGF receptors. It is concluded that the reduction of EGF receptors cannot be explained by the "intact hepatocyte hypothesis" but rather by the functional change of hepatocytes induced by the administration of D-galactosamine.

Epidermal growth factor as a regulatory hormone maintaining a low pH microclimate in the rat small intestine

This study was designed to determine the effect of epidermal growth factor (EGF) in the lumen on the pH of the intestinal surface in the rat jejunum, which is referred to as "microclimate-pH". In the control experiment, a significant pH gradient was observed between the mucosal surface (approximately pH 6.8) and the bulk phase (approximately pH 7.3). The microclimate-pH was decreased by 0.2-0.6 pH units after addition of higher concentrations of EGF (3-100 nM) to the lumen. The microclimate-pH thus decreased recovers to the control value by replacing EGF with TES buffer, suggesting that the EGF effect is reversible. Considering that the Na+-H+ exchanger exists on the luminal membrane of the intestinal cells, the decrease in the microclimate-pH which was induced by EGF added to the luminal side may be due to the activation of Na+-H+ exchanger.

Receptor-mediated transport of peptide hormones and its importance in the overall hormone disposition in the body

A remarkable feature of the pharmacokinetics of polypeptide hormones is the contribution of specific binding sites (receptors) to the polypeptide hormone distribution and clearance in the body. The concept of "transport receptor" is now well established, and receptor-mediated endocytosis (RME) is recognized as a general mechanism in the uptake of biologically important peptide hormones. This article focuses on the kinetic analysis of the RME of polypeptides, based mainly upon the observations of the kinetics of epidermal growth factor in the liver. The following points are emphasized: (1) How can we determine the existence and the kinetic constants of polypeptide RME in vivo and in the perfused liver system? A liver perfusion method, the single-pass multiple-indicator dilution technique, has been shown to be suitable for analyzing the dynamics of interaction of peptide hormones with their cell surface receptors. (2) What is the importance of down-regulation of transport receptors to the overall kinetics of polypeptides in vivo? Time profiles of polypeptide plasma concentrations and their surface receptors in the liver after iv administration of epidermal growth factor were simulated with a physiologic pharmacokinetic model that includes kinetic constants representing the interaction of polypeptides and their receptors.

Kinetic analysis of the elimination process of human epidermal growth factor (hEGF) in rats

Pharmacokinetic study of human epidermal growth factor (hEGF) in rats was performed in vivo. The hepatic extraction ratio (EH) of [125I]hEGF, determined from the difference between the artery and the hepatic vein plasma concentrations at steady state, was 0.19. The hepatic clearance (CLH:7.56 ml/min/kg body wt), calculated by multiplying EH by the hepatic plasma flow rate (QP,H), was approximately 70% of the total body clearance (CLtot: 10.8 ml/min/kg body wt), which was determined from the steady-state arterial plasma concentration and the infusion rate. These results indicated that the liver is the main organ responsible for the removal of [125I]hEGF from the systemic circulation in rats. The renal extraction ratio (ER) of [125I]hEGF was half of that of [14C]inulin; this may have resulted from the plasma protein binding of [125I]hEGF, which was approximately 50% as determined by the charcoal adsorption method and the equilibrium gel-filtration method. The renal clearance (CLR:2.65 ml/min/kg body wt), calculated by multiplying ER by the renal plasma flow rate (QPR), was approximately 17% of the CLtot (15.6 ml/min/kg body wt), indicating a minor contribution of CLR to CLtot compared with that of CLH to CLtot. The CLR of [125I]hEGF calculated from the urinary excretion data was one-tenth of that calculated from the plasma concentration difference between the femoral artery and the renal vein at steady state. These results suggest that the bulk of [125I]hEGF cleared from the plasma by the kidney may have been metabolized further in the renal tubules before appearing in the urine.

Dynamic determination of kinetic parameters for the interaction between polypeptide hormones and cell-surface receptors in the perfused rat liver by the multiple-indicator dilution method

Hepatic elimination of epidermal growth factor (EGF) via receptor-mediated endocytosis was studied by a multiple-indicator dilution method in the isolated perfused rat liver, in which cell polarity and spatial organization are maintained. In this method EGF was given with inulin, an extracellular reference, as a bolus into the portal vein, and dilution curves of both compounds in the hepatic vein effluent were analyzed. Analysis of the dilution curve for EGF, compared with that for somatostatin, which showed no specific binding to isolated liver plasma membranes, resulted as follows: (i) both extraction ratio and distribution volume of 125I-labeled EGF decreased as the injected amount of unlabeled EGF increased; (ii) the ratio plot [ln (inulin/EGF) versus time] of the dilution curve for EGF exhibited an upward straight line initially for a short period of time (approximately equal to 10 sec), whereas the ratio plot [ln (inulin/somatostatin) versus time] of somatostatin gradually decreased. The multiple-indicator dilution method was used for other peptides also. Insulin and glucagon, known to have hepatocyte receptors, behaved similarly to EGF in shape of their ratio plots. Thus, analysis of dilution curves can reveal whether or not the cell surface has receptors for certain peptides. In addition, the dilution curves for EGF at various doses (tracer approximately equal to 30 micrograms) were analyzed simultaneously based on a kinetic model incorporating the perfusion rate, the association rate constant of EGF to surface receptors (kappa on), the dissociation rate constant of EGF from the EGF-receptor complex (kappa off), and the sequestration rate constant of the complex. The kinetic parameters [the dissociation constant (Kd = kappa off/kappa on) and the number of surface receptors] calculated by this analysis were comparable with reported values obtained by in vitro direct binding measurements at equilibrium using liver homogenates. We conclude that the multiple-indicator dilution method is a good tool for analyzing the dynamics of peptide hormones--cell-surface receptor interaction under a condition in which spatial architecture of the liver is maintained.

Kinetic analysis of in vivo receptor-dependent binding of human epidermal growth factor by rat tissues

Kinetic analysis of the tissue distribution of human epidermal growth factor (hEGF) in rats was performed in vivo. The plasma disappearance half-life of [125I]hEGF was prolonged by coadministration of unlabeled hEGF, indicating saturation of the mechanism for hEGF removal from the systemic circulation. To analyze the contribution of each tissue to the uptake of hEGF, the amount of [125I]hEGF taken up by each tissue was determined after coadministration of various amounts of unlabeled hEGF. Kinetic analysis of the data yielded the following results. (1) Among the tissues examined, the distribution of [125I]hEGF to the liver, kidney, small intestine, stomach, and spleen was much greater than that accounted for by the distribution to the extracellular space of each tissue. (2) The binding (or uptake) of hEGF by these tissues showed remarkable saturation, which may represent the receptor-dependent binding (or uptake) mechanism. (3) The apparent binding (or uptake) clearance per gram of tissue at the low dose (in the range of first-order kinetics), defined with regard to the arterial plasma concentration, was greatest in the kidney, followed by the liver and small intestine. The larger binding (or uptake) clearance of the kidney compared with that of the liver can be attributed to the higher plasma flow rate (per gram of tissue) in the kidney. However, the intrinsic ability to take up hEGF was much greater in the liver than that in the kidney. The hepatic binding (or uptake) of hEGF at the low dose was almost limited by the hepatic plasma flow rate.(ABSTRACT TRUNCATED AT 250 WORDS)