Abstract
We have previously demonstrated that insulin‐like growth factor binding protein‐5 (IGFBP‐5) is upregulated following treatment of the mouse mammary epithelial cell line HC11 with lactogenic hormones (dexamethasone, insulin, and prolactin—DIP). In addition, we have also shown that IGFBP‐5 is upregulated in mammary epithelial cells in vivo during involution of the rodent mammary gland. We have, therefore, postulated that there may be a dual regulation of IGFBP‐5 expression during the temporally separated processes of differentiation and apoptosis of mammary epithelial cells. To test this hypothesis further, we have used a phenotypically differentiated model, which comprises primary cultures of mouse mammary epithelial cells grown on a layer of EHS (Engelbreth–Holm–Swarm) extracellular matrix. We show that lactogenic hormone treatment (hydrocortisone, insulin, and prolactin—HIP) of these cultures induces the upregulation of IGFBP‐5 thus replicating the results obtained with the HC11 cell line. In addition, following the induction of apoptosis in primary cultures of mammary epithelial cells by treatment with TGFβ‐3, IGFBP‐5 expression is also upregulated. In parallel with this upregulation of IGFBP‐5, there is also an increase in the levels of cleaved caspase‐3, a well‐characterized marker of cellular apoptosis. These findings confirm previous in vivo work demonstrating an increase in IGFBP‐5 expression during involution of the mouse mammary gland. When HC11 cells are cultured under serum‐free conditions (a well‐characterized apoptotic insult in cell culture), there is also an increase in cleaved caspase‐3 levels. Unexpectedly, in the presence of TGFβ‐3, caspase‐3 levels are attenuated. In the presence of DIP, caspase‐3 levels are also decreased in HC11 cells. As described previously, TGFβ‐3 inhibits β‐casein synthesis in HC11 cells. In the HC11 cell line (in contrast to primary cultures of mammary epithelial cells), there is no evidence for TGFβ‐3 induction of IGFBP‐5 under either serum‐free or DIP‐supplemented conditions. We believe our data with primary cultures of mammary epithelial cells support the hypothesis of dual regulation of IGFBP‐5 expression during both differentiation and apoptosis in the mammary gland and emphasizes the importance of using appropriate cell culture models to investigate such phenomena in this tissue. We discuss the possible implications of our observations in relation to the physiological processes of pregnancy, lactation, and involution in the mammary gland and the associated changes in mammary epithelial cell function. J. Cell. Physiol. 207: 471–479, 2006. © 2006 Wiley‐Liss, Inc.