Endocrine Abstracts

IGFBP-5 is an important member of the IGF axis in the rodent mammary gland and can function by sequestering IGF-I (insulin-like growth factor-I) - a polypeptide growth factor involved in the growth and differentiation of the mammary gland. Removal of free IGF-I results in inhibition of IGF-I signalling through the cell surface Type-I IGF receptor, and since IGFBP-5 is highly up regulated during mammary gland involution in vivo, it is thought to have a pro-apoptotic role during mammary epithelial cell death. To examine in more detail the regulation of IGFBP-5 in mammary epithelial cells during postnatal development, a primary cell culture model has been established. Isolated mammary organoids, from pregnant mice, form three-dimensional structures called mammospheres when plated onto extracellular matrix. When treated with lactogenic hormones mammospheres will differentiate to become fully functional alveoli. We have found that IGFBP-5 is up regulated during differentiation of mammospheres. Quantitative RT-PCR and Western blotting analyses have shown that IGFBP-5 is up regulated at the level of mRNA and protein expression and this confirms our earlier findings using the mouse mammary epithelial cell line HC11. Mammary organoids which are plated onto a dilute extracellular matrix form epithelial monolayers and IGFBP-5 is also up regulated in monolayer cultures following addition of lactogenic hormones. However, less intact IGFBP-5 is present due to extensive proteolysis in monolayer cultures. Experiments are currently underway to examine the IGFBP-5 profile during cell death in mammosphere cultures. Induction of apoptosis using prolactin withdrawal and/or TGF-beta 3 and IGFBP-5 addition with subsequent activation of caspase- 3 will confirm whether the cells have undergone apoptosis. Utilising primary cell cultures derived from the mammary gland permits IGFBP-5 profiling in a model which more closely resembles the in vivo phenotype than mammary epithelial cell lines. Our work suggests that IGFBP-5 may have dual functionality in the mouse mammary gland during differentiation and involution of this tissue.