Notch-1 and Notch-2 modulate keratinocyte stem cell viability and differentiation during skin ageing and UVB exposure

Notch are a family of surface receptors implicated in maintaining epidermal homeostasis. In the epidermis, CK15 and CK10 expression does not change with age, while there is a reduction in Ki67-positive cells and an increased involucrin expression. In culture, keratinocytes display reduced proliferation and a lower colony forming efficiency, as a function of age. Transit amplifying cells appear more affected than stem keratinocytes by ageing. In epidermal sections, Notch-1 expression shows a reduction with age, while Notch-2 is located in the upper layers in Y (under 20-years), in all layers in A (between 20 and 60-years), and predominantly in the basal layer in O (over 60-years). In cultures, Notch-1 activation decreases with age, while Notch-2 seems to be more activated in Y cells than in A and O cells. Notch-1 and Notch-2 are mainly expressed in the cytoplasm of TA cells, Notch-1 being present also in the nucleus. Notch protein inhibition reduces keratinocyte stem cell viability, possibly through survivin downregulation. Inhibiting Notch-1 also induces G1 arrest in keratinocytes at all ages. Notch proteins are activated by calcium, while Notch-1 activation decreases in more differentiated keratinocytes. On the other hand, Notch-2 is activated at cell confluence, in all age groups. Furthermore, Notch-1 is up-regulated upon UVB irradiation (75 mJ/cm2) in Y keratinocytes and is re-activated in O keratinocytes after UVB (5 mJ/cm2). Taken together, these data confirm the role of Notch proteins within epidermal niche and their possible involvement in the mechanisms of photoaging.