HSP25 Is Involved in Two Steps of the Differentiation of PAM212 Keratinocytes

HSP25 is a member of the small heat shock protein family. This 25-kDa protein exhibits a highly specific distribution during mouse embryonic development. Although multiple functions have been proposed for HSP25, the role it plays during differentiation is still unknown. High levels of HSP25 can be detected in embryonic and adult skin. During epidermis differentiation, the concentration of HSP25 increases with the distance of keratinocytes from the basal layer, in parallel with the extent of keratinization. We used an ex vivo cellular system, PAM212 cells, to analyze quantitatively and qualitatively the dynamics of HSP25 production and phosphorylation during the differentiation of keratinocytes. Our observations suggest that HSP25 is involved in two steps of PAM212 keratinocyte differentiation. Shortly after the induction of differentiation, a transient hyperphosphorylation of HSP25 seems to be essential for the expression of differentiation markers. Later, the chaperone-active form of HSP25 is organized progressively into characteristic aggregates involved in the dynamics of keratin filament networks.