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ABSTRACT
125I-Labelled mouse epidermal growth factor (125I-EGF) was transferred intact and undegraded from circulating blood into milk in conscious lactating goats. Greater than 90% of the total radioactivity present in milk from the infused gland was in the aqueous phase and more than 72% was acidprecipitable. This radiolabelled material co-eluted with authentic EGF through gel filtration and was immunoprecipitable by a specific rabbit anti-mouse EGF immunoglobulin. Mammary uptake of125I-EGF infused into mammary arterial blood (close-arterial infusion) for 1 h varied from 20 to 83% at different stages of the reproductive cycle. Only 0·5–2·9% of the infused 125I-EGF was transferred into milk during the first 3 h after the start of the infusion, which represents 0·7–6·3% of mammary uptake of EGF.
The kinetics of transfer of 125I-EGF were followed in two lactating goats. Radioactivity reached peak levels in milk about 120 min after the start of a 1 h close-arterial infusion into the mammary gland, with an initial lag of about 30 min when little transfer occurred. Transfer was slower in two non-lactating goats with maximal levels of activity in milk being reached after about 180 min. The results are consistent with a transcellular transfer, whereby the factor is bound to receptors on the baso-lateral membrane, internalized by epithelial cells and subsequently secreted across the apical membrane into the alveolar lumen. The low level of degraded labelled EGF in milk (and mammary vein blood) suggests a modification of the normal pathway of EGF degradation such that the delivery of internalized factor to lysosomes is avoided.
J. Endocr. (1986) 109, 325–332
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ABSTRACT
The polypeptide mitogen, epidermal growth factor (EGF), was originally isolated from mouse submaxillary gland (SMG). In mice, these glands are sexually dimorphic; the SMG of male animals typically contains up to 400 pmol EGF/mg protein whereas EGF concentrations in the SMG of female mice are only 5–20 pmol/mg protein. When mice were castrated at 8 weeks of age, EGF mRNA levels in the SMG fell rapidly to the low levels observed in control female animals. Although the concentration of EGF in the SMG, measured by radioimmunoassay, also fell to very low levels (13·8 ± 0·7 (s.e.m.) pmol/mg protein) in the castrated mice, the rate of decrease was considerably slower than that of the mRNA. In contrast to the loss of EGF and EGF mRNA from mice following castration, ovariectomy led to a rise in EGF mRNA levels in SMG, with a maximal increase (approximately 100-fold) 4–6 weeks after ovariectomy. Concentrations of EGF in the SMG also rose markedly in the ovariectomized animals, from a control value of 5·4 ± 0·8 to 34·7 ± 7·9 pmol/mg protein at 6 weeks. In mice, the kidney displays the second highest level of EGF gene expression. However, in contrast to the effects on SMG, kidney EGF mRNA was not affected by either castration or ovariectomy. These results provide further evidence for the tissue-specific control of EGF gene expression in response to steroid hormones.
Journal of Endocrinology (1989) 121, 501–506
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ABSTRACT
The concentration of epidermal growth factor (EGF) in human mammary secretions was about 50 nmol/l for several weeks prepartum. It then fell to about 13 nmol/l within 4 days after parturition, in parallel with the decrease in protein concentration which is associated with the onset of lactation. In contrast, the concentration of EGF in urine samples from the same donors remained constant throughout this period. All the immunoreactive EGF in mammary secretions competed at the EGF receptors on Swiss mouse 3T3 fibroblasts. The stimulation of these cells by samples of mammary secretions was, however, much greater than that induced by EGF alone, indicating the presence of other factors which synergize with EGF. Gel filtration of mammary secretions revealed two major peaks of mitogenic activity, corresponding to EGF and a factor of higher molecular weight. The latter could synergize with added EGF, insulin or bombesin, and thus falls into a different functional class from any of these factors.
J. Endocr. (1987) 112, 151–159
