Abstract P5-03-04: The meaning and significance of p63+/K5/14+ somatic stem cells of the normal breast salivary gland epithelium
Authors: Boecker W , Stenman G , Loening T , Andersson MK , Berg T , Buchwalow I 
Published: August 2014

Abstract
Background: Somatic breast stem cells are the focus of intensive efforts worldwide not only at elucidating their nature and properties but also at increasing our understanding of proliferative diseases of breast. Despite the many important advances on stem cells in other organs, substantial uncertainties regarding the identity of somatic breast stem cells still hinder further advances and a conceptual approach in the field of physiological epithelial regeneration and corresponding tumors.
Material and Methods: For that reason, we performed an in situ Triple Immunofluorescence Lineage/Differentiation Tracing (isTILT) and qRT-PCR study of formalin-fixed breast tissues. We used basal (K5/K14), glandular (K7/K8/18), and epidermal-specific squamous (K10) keratins, the stem cell marker p63, and smooth muscle actin (SMA; myoepithelial marker) with the aim to trace the two cell lineages and define their cellular hierarchy. We investigated 5 cases of the breast and 5 cases of the salivary gland tissues.
Results: First of all, the breast gland contains p63 and K5/14 co-expressing progenitor cells observed in small clusters of 2-3 cells at the interface between myoepithelial and luminal layer of breast ducts. Interestingly, p63 is not preserved in luminal cells, rather the glandular differentiation involves down-regulation of p63 with the emergence of K5/14-positive glandular progenitors and furthermore sequential modulation of keratins with a shift from basal keratins K5/14 to glandular keratins K8/18. In contrast, p63 was preserved and co-expressed with both K5/14 and myoepithelial markers such as SMA in the myoepithelial lineage. In contrast p63+/K5/14+ cells could not be found in terminal ducts-lobular units, rather these structures are characterized by K5/14+/K8/18+ intermediary and K8/18+ glandular cells. Interestingsly, the same cellular make up and principles were observed in striated and excretory ducts of salivary glands. In contrast, intercalated ducts and acini contained only K7+ glandular and p63+/SMA+/K5/14+ myoepithelial cells, but lacked K5/14+/K7+ intermediare cells.
Conclusion: Here, we localized for the first time a group of cells with this unique p63+/K5/14+ phenotype in the normal breast duct epithelium. Using isTILT experiments and qRNA analyses with p63 as a stem cell marker, we demonstrated these p63+/K5/14+ physiological progenitors residing at the interface of myoepithelial and luminal cells of the breast ducts. From our isTILT-findings, we suggest that in the glandular lineage of breast ducts, p63 is down-regulated with a sequential shift in keratin expression from basal keratins K5/14 to glandular keratins K7 and/or K8/18 as the cells mature. Interestingly terminal ducts-lolular units are devoid of p63+/K5/14+ progenitor cells. In salivary glands, these p63+/K5/14+ progenitors are found to sit in a basal position adjacent to the basement membrane of striated and excretory ducts. We conclude that p63+/K5/14+ progenitors and the glandular/myoepithelial lineage differentiation are tightly regulated and occur in defined contexts of tissue physiology.
Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-03-04.