S100 proteins as calcium signaling mediators and transglutaminase substrates in keratinocyte biology

Calcium is an important regulator of keratinocyte differentiation. A key mediator of calcium signaling in the skin is the S100 protein family. S100 proteins are important for epidermal differentiation since selected members are expressed in the epidermis and the corresponding genes are clustered on the chromosomal band 1q21, in the region known as the “epidermal differentiation complex”. In addition, deregulated expression of some S100 proteins is associated with severe epidermal diseases. Thus, S100A7 is overexpressed in psoriasis.; Despite the potential importance of these proteins in keratinocyte biology, studies aiming to elucidate S100 proteins role in epidermis are very limited. The overall goal of the present thesis is to understand the role(s) of S100 proteins in human keratinocytes.; To investigate the role of S100A7 in keratinocytes, we developed an adenoviral expression system for S100A7 and its potential target, the epidermal fatty acid binding protein (EFABP). Our data indicates that EFABP is part of the same functional complex with S100A7. An increase in the intracellular calcium concentration determines the relocation of both proteins at the cell periphery in structures that share properties with focal adhesions. This relocation correlates with S100A7 localization in human epidermis: cytosolic localization in basal, non-differentating keratinocytes vs. cell periphery localization in more differentiating keratinocytes of the suprabasal layers.; Previous studies demonstrated that S100A10 and S100A11 are incorporated in the cornified envelope structures as the keratinocytes differentiate. Since we showed that S100A7 redistributes to the cell periphery with keratinocyte differentiation, we examined whether this protein as well as other S100 proteins can function as transglutaminase substrates. We determined that the residues covalently modified by transglutaminases are in the solvent exposed areas of the proteins, within the same regions that are required for S100 proteins binding to their targets. This might be a mechanism that inactivates S100 protein function with the ultimate result of terminating the calcium signaling. In contrast, we show that S100A7 is unable to form crosslinked homomers in vitro, although it has transglutaminase-reactive glutamine and lysine residues. Moreover, S100A7 forms multimers in psoriatic epidermis, suggesting that this protein might become crosslinked in vivo to other non-S100 proteins.