P28. Primary mammary epithelial cells from the lactating epithelium have distinct growth and differentiation capacity

Abstract

The mammary gland is a unique organ in its ability to undergo repetitive cycles of growth, differentiation, function and involution in the mature adult. These processes depend upon a pool of committed multipotent cells that undergo asymmetric division to give rise to more differentiated progeny that proliferate at the onset of gestation. It is normally not possible to access these cells from lactating women to generate primary mammary epithelial cultures for research as surgical intervention is rare during lactation. We have identified cells expressing markers for multipotent mammary epithelial cells (CD49f, CD24 and p63) in human breastmilk samples. These primary mammary epithelial cells (PMEC) can be expanded and manipulated in epithelial-specific monolayer culture to produce differentiated cells expressing luminal (CK18) and myoepithelial (CK14) markers. Using the immortalised non-tumorigenic mammary line MCF10A and tumorigenic mammary line MDA231 as a comparison, we show that PMEC have a distinctive response to extra-cellular matrix (ECM) and growth and functional regulators. In monolayer culture PMEC bypass replicative senescence at day 8 in response to ethanolamine and hormones prolactin, placental lactogen and RANKL, while immortal lines show no response in their rapid growth pattern. In three-dimensional ECM culture PMEC and MCF10A produce alveolus-like structures that maintain a basal layer of CD49f-positive cells and a luminal layer of CK18-positive cells, while MDA231 form disorganized invasive colonies. Uniquely, PMEC also produce hallmark milk proteins beta-casein and alpha-lactalbumin in response to prolactin stimulation in the luminal secretory layer. These data imply that the cell population expanded during lactation is receptive to distinct stimuli that results in differentiation in ways not observed in cells taken from the non-lactating gland. We demonstrate that human milk is a non-invasive source of these cells, providing a tool for further research into the specific pathways that regulate differentiation. Using such models may be useful to further understand mechanisms that underlie mammary gland pathologies.