Effects of Growth Factors on Proliferation and Differentiation of Human Mammary Stem/Progenitor Cells

Abstract

Stem/progenitor cells (SPC) are the multi-potent progenitors that give rise to all mammary gland lineages. With cancer biology beginning to focus on tumorigenic stem cells as being the origin of clinically relevant metastatic cells, isolation of normal mammary SPCs would prove essential for comparison studies between the healthy and tumorigenic cell populations. However, the isolation and purification of SPCs has remained difficult, as no assay has been developed which demonstrates the two key stem cell properties: ability to differentiate into multi-lineage progeny and to self-renew. In an effort to cultivate a suspension culture enriched in SPCs an assay was developed in which human mammary epithelial cells (HMEC) were grown in low-adherence conditions in the form of mammospheres. These mammospheres were found to generate more multilineage colonies when placed in differentiating conditions, signaling the presence of SPCs. Additionally, this culture system was used to investigate the effects of various growth factors implicated in mammary gland morphogenesis on cell fate determination. We showed that LIF, SCF, EGF, and bFGF support proliferation of HMEC in an undifferentiated state in suspension culture, whereas PE and BMP4 favor differentiation along ductal epithelial and myoepithelial lineages respectively. Direct study of SPCs would not only be useful in studying mammary gland development, but also provide a means of comparison, through analysis of gene and surface protein expression, between healthy and tumorigenic tissues. This comparison could facilitate the identification of specific pathways important for growth and survival of tumors and permit the development of new therapeutic strategies targeting these pathways.