Notch protein expression changes in human skin during ageing, keratinocyte differentiation and UVB-exposure

Epidermal integrity is guaranteed by the presence of keratinocyte stem cells (KSCs) and the correct balance between cell proliferation, differentiation and apoptosis. The microenviroment or “niche” where KSCs reside has a key role in the regulation of these processes and comprise all the factors expressed or released by skin cells. During ageing, epidermal niche changes determine the behavior of KSCs and their progeny (or transit amplifying (TA) cells). Notch proteins (Notch-1, -2, -3, -4) comprise a family of surface receptors which are implicated in maintaining epidermal homeostasis and, for this reason, they are fundamental for the comprehension of the mechanism within epidermal niche in normal condition and during photoageing. First, we have collected skin samples from different age donors: young (Y-under 20 years), adult (A-between 20 and 50 years) and old (O-over 50 years). We have shown that CK15 and CK10 expression don’t change with age, while there is a reduction in Ki67-positive cells and a increasing in the number of epidermal layers expressing involucrin. In culture, keratinocytes present a reduction in proliferation and in CFE in direct ratio to age. In particular, we have shown that TA cells are eventually the most involved. In skin biopsies, Notch-1 expression shown a reduction with age, while Notch-2 presents a different localization. In keratinocyte cell culture, Notch-1 decrease with age, while Notch-2 expression seems to be higher Y cells and constant in A and O cells. Notch-1 and Notch-2 are mainly expressed by TA cells and both proteins present a cytoplasmic localization in KSCs and TA cells, except for Notch-1 which is also present in the nucleus of TA cells. Moreover, both proteins are mainly expressed and activated after stimulus with calcium. Notch-1 activation decrease in confluent and more differentiated keratinocytes from different age, while its expression present a reduction in subconfluent cells and a increase in confluent cells in direct ratio to age. Notch-2 is mainly expressed and activated with confluence in all age class. Furthermore, Notch-1 is strongly activated after stimulus with UVB75 in Y keratinocyte. In addition, it is re-activated in O keratinocytes after stimulus with UVB5 and it is not expressed in more aged keratinocytes with respect to Y ones. Notch-2 is mainly expressed after stimulus with UVB5. This data confirm the role of Notch proteins within epidermal niche and their possible involvement in the mechanisms of photoaging.