The epidermis is a stratified epithelium with a stem cell subpopulation in the basal layer that constantly replicates and periodically detaches from the base, undergoing a differentiation process that involves various developmen- tal signals and regulatory pathways. During the last 10 years, a number of studies tried to elucidate the intricate scenario that maintains the epithelial shield during the entire life span. In our study, we investigated the role of Numb in the skin compartment and, in particular, its involvement in stem cell maintenance. Numb expression in the skin compartment was assessed by immunofluorescence and immunohistochemistry analysis. We evaluated Numb expression in primary epithelial cells at various differentiative stages. Moreover, we overexpressed Numb in the isolated population enriched for undifferentiated progenitors to establish its involvement in in vitro differ- entiation. We demonstrated that Numb in high-proliferating epithelial undifferentiated progenitors contributes to the maintenance of an undifferentiated state. This regulation involves the E3 ligases Itch binding. Moreover, the analysis of a cohort of cutaneous carcinomas showed that Numb is highly expressed in squamous cell carcinoma (SCC), where we observed a direct correlation between the expression of Numb and Ki-67. Our data indicate for the first time that Numb is involved in the maintenance of the undifferentiated proliferating stem cell pool in the epithelial basal layer and its expression could become a new marker in skin cancer.
- Notch INTRODUCTION The epidermis is formed by several layers of cells, and it is maintained by a mitotically active basal stratum enriched in stem cells. During their life span, epithelial stem cells, in the basal layer, undergo symmetric and asymmetric divi- sion cycles, generating daughter stem cells and a dividing progenitor population, known as transiently amplifying (TA) cells. Epithelial stem cells were originally identified and isolated on the basis of their adhesive characteristics (26). In particular, the basal keratinocytes that rapidly adhere (ADH+) to collagen-treated dishes are enriched in stem cells, as indicated by their colony-forming efficiency (CFE), and have a high expression of b1 integrin, which allows a rapid attachment to collagen IV-treated plates. In contrast, basal cells, expressing a lower amount of b1 integrin, slowly adhere (ADH−) to the collagen IV and are committed to terminal differentiation after few divisions.
- Stem cell