Based on gene expression patterns, breast cancers can be divided into subtypes 
that closely resemble various developmental stages of normal mammary epithelial 
cells (MECs). Thus, understanding molecular mechanisms of MEC development is 
expected to provide critical insights into initiation and progression of breast 
cancer. Epidermal growth factor receptor (EGFR) and its ligands play essential 
roles in normal and pathological mammary gland. Signals through EGFR is required 
for normal mammary gland development. Ligands for EGFR are over-expressed in a 
significant proportion of breast cancers, and elevated expression of EGFR is 
associated with poorer clinical outcome. In the present study, we examined the 
effect of signals through EGFR on MEC differentiation using the human telomerase 
reverse transcriptase (hTERT)-immortalized human stem/progenitor MECs which 
express cytokeratin 5 but lack cytokeratin 19 (K5(+)K19(-) hMECs). As reported 
previously, these cells can be induced to differentiate into luminal and 
myoepithelial cells under appropriate culture conditions. K5(+)K19(-) hMECs 
acquired distinct cell fates in response to EGFR ligands epidermal growth factor 
(EGF), amphiregulin (AREG) and transforming growth factor alpha (TGFα) in 
differentiation-promoting MEGM medium. Specifically, presence of EGF during in 
vitro differentiation supported development into both luminal and myoepithelial 
lineages, whereas cells differentiated only towards luminal lineage when EGF was 
replaced with AREG. In contrast, substitution with TGFα led to differentiation 
only into myoepithelial lineage. Chemical inhibition of the MEK-Erk pathway, but 
not the phosphatidylinositol 3-kinase (PI3K)-AKT pathway, interfered with 
K5(+)K19(-) hMEC differentiation. The present data validate the utility of the 
K5(+)K19(-) hMEC cells for modeling key features of human MEC differentiation. 
This system should be useful in studying molecular/biochemical mechanisms of 
human MEC differentiation.