Expression of epidermal growth factor-related proteins in the aged adult mouse mammary gland and their relationship to tumorigenesis.

Abstract

Mammary glands from female BALB/c mice of different ages and parity were screened for production of three epidermal growth factor (EGF) related transforming growth factors and their corresponding mRNAs. Glands were obtained from 2-26-month-old nulliparous, 4-26-month-old parous, and 2-8-month-old midpregnant mice. Reverse-transcribed polymerase chain reaction (RT-PCR) was used to screen for mRNA from the transforming growth factor alpha (TGF alpha), cripto-1 (CR-1), and amphiregulin (AR) genes in extracts of whole mammary glands. TGF alpha, CR-1, and AR transcripts were detected in all of the mammary glands assayed. In situ hybridization was then used to localize these mRNAs among various cell types in sections of glands. TGF alpha mRNA levels were low in the mammary epithelium from young nulliparous mice, high in the stroma of midpregnant mammary glands, and highest in luminal epithelium of the aged glands. AR mRNA levels were high and remained unchanged in all developmental stages. CR-1 mRNA level increased with age and was detected primarily in epithelium, with some scattered expression in adjacent stroma. Finally, TGF alpha, CR-1, and AR proteins were immunolocalized in histological sections of mammary glands from the various developmental stages. TGF alpha was detected sporadically in midpregnant mice, with more conspicuous reactivity seen in 18-26-month-old mice (38% of mice). CR-1 immunoreactivity was detected in 100% of the 18-26-month-old glands but not in any other age groups. Strong AR immunoreactivity was observed in in all glands, including 100% of the 18-26-month-old glands. Staining for all three of these growth factors was observed primarily in the epithelium, with some reactivity detected in the periductal fibroblasts. No significant difference was discerned between glands from nulliparous and parous animals. We also found intense CR-1 and AR mRNA expression and strong immunoreactivity in seven different carcinogen-induced and eight spontaneous mammary tumors. Our results demonstrate that these growth factors accumulate in significant amounts in the old gland of both nulliparous and parous mice. The observations suggest that these growth factors are positioned to contribute to abnormal development in the older mammary gland, predisposing them to tumorigenesis.