Differential expression of the $_{Hs}$Kin17 protein during differentiation of in vitro reconstructed human skin

In eukaryotic cells, various proteins homolo-gous to the $E.\ coli$ RecA protein are involved in the elimination of DNA damage. These proteins contribute to the repair of double-strand breaks and to genetic re-combination. The mouse Kin17 protein is recognised by antibodies directed against the RecA protein. Kin17 has a zinc-finger domain allowing binding to curved DNA stretching over illegitimate recombination junctions. In the present study, we identified the human counterpart of the mouse Kin17 protein (named $_{Hs}$Kin17) in skin cells. We employed an in vitro reconstructed skin model composed of an epidermal sheath lying on a dermal matrix with human fibroblasts embedded in rat collagen type I. The maturation programme (proliferation versus differentiation) of kera-tinocytes was highly dependent on stromal cells. Im-munohistochemical staining of frozen sections obtained from skin specimens was monitored by an interactive laser cytometer. In this way we analysed protein levels in both dermal and epidermal compartments. After having characterised the epithelium, we focused our attention on $_{Hs}$Kin17 expression. We detected $_{Hs}$Kin17 in human keratinocytes. $_{Hs}$Kin17 protein levels increased in proliferating epithelial keratinocytes after 7 days of culture. After 2 weeks of culture, epidermal sheaths acquired most of the differentiated features of mature epithelium. At this time, $_{Hs}$Kin17 protein dropped below measurable levels in the stratum cor-neum, and diminished in nucleated cells. This study showed that $_{Hs}$Kin17 is expressed in human reconstructed epithelium under conditions of hyperprolifer-ation.