EPIDERMAL GROWTH FACTOR IN MAN AND MOUSE: PHYSIOLOGICAL, CLINICAL AND EXPERIMENTAL STUDIES

Abstract

Epidermal growth factor (EGF), which was first isolated from the mouse submaxillary gland and later from human urine belongs to a new class of polypeptides collectively known as growth factors. These growth factors regulate cell growth, tissue differentiatIon and maturation. EGF has interesting interactions with the endocrine system particularly in the thyroid gland, where it stimulates proliferation and inhibits differentiated thyrocyte function in vitro. The synthesis of EGF in mouse submaxillary gland is, in turn, regulated by hormones such as thyroxine and testosterone. Ever since its discovery, attempts have been made to assign a role to EGF in normal physiology or in the causation of disease. The results have so far proved inconclusive. The aim of the present study was to investigate the significance of EGF in man and mouse by using radioligand binding assays to measure EGF levels in a number of physiological, clinical and experimental situations. Specific and sensitive radio immunoassays and radioreceptor assays were developed for both human and mouse EGF. Methodologies for the extraction of EGF from human and mouse tissues were validated. The human (h) EGF radioimmunoassay was optimised for use in saliva, urine and thyroid tissue homogenates and applied to populations of healthy subjects and patients with various diseases. The concentration of (m) EGF in homogenates from mouse thyroid and other tissues was measured by the homologous mEGF radioimmunoassay. The biological activities of both hEGF and mEGF detected by the radioimmunoassays were assessed in separate radio receptor assays and their molecular integrity, by gel chromatography. The main findings in this work were as follows: hEGF was found to be normally present in saliva of healthy subjects from the different racial groups without significant racial variations. The mean salivary hEGF concentration (pmol) was 368.55 ± 30.00 in Pakistanis, 457.25 ± 54.21 in Black Africans, 414.00 ± 31.20 in Chinese and 450.00 ± 58.22 in white Britons, p > 0.05. The mean salivary hF concentration in 27 pregnant famales and 23 healthy age - matched controls were similar (713.19 ± 56.10 vs 665.22±51.66 pmol/1); there was no correlation between salivary hEGF and gestational age among the former. Significant increases in salivary hEGF levels were observed In patients with goitrous thyrotoxicosis and in those with terminal renal failure. Declining urinary excretion of hEGF was observed in diabetic patients with worsening proteinuria. hEGF was found measurable in every human thyroid gland examined and increased concentrations were found in glands from patients with uncorrected thyrotoxicosis. Studies using immunochemistry localized hEGF to the cytoplasm of the follicular cells in frozen human thyroid sections. Studies in the mouse demonstrated for the first time the presence of mEGF within murine thyroids. The mean mEGF concentration in thyroid homogenates was 26.10 ±6.00 ng/mg protein (range 5.50 - 68.00 ng/mg). This was equivalent to 1.5 ng/mg wet weight and was of the order of 20 to 150 times the mEGF concentration previously reported in mouse testes, brain or pituitary. Extracts from the mouse submaxillary gland expectedly contained abundant mEGF (600 - 1220ng/mg wet weight) but there was no correlation between thyroid and submaxillary mEGF content; Sialoadenectomy augmented thyroid mEGF content. Exposure of mice to various hormonal and metabolic manipulations indicated that mouse thyroid and submaxillary mEGF was responsive to thyroid hormone status, androgen status and to changes in dietary iodine. Further studies in newborn mice revealed that their thyroids had begun to t elaborate appreciable amounts of mEGF while the growth factor remained undetectable in their submaxillary glands. In conclusions these mouse indicate that EGF is a normal constituent of human saliva, human urine and of both human and murine thyroid tissues with differinig levels relating to different disease states such as thyrotoxicosis, diabetic nephropathy and end - stage renal failure. Furthermore, evidence has been presented to implicate the thyroid gland as a probable site of EGF synthesis in both man and the mouse.