Keratin 15 expression in stratified epithelia: Downregulation in activatedkeratinocytes

Keratin 15 (K15) is a type I keratin without a defined type II partner whos e expression in epidermal diseases has not been investigated. In this study we have used LHK15, a monoclonal antibody raised against the last 17 amino acids of the K15 polypeptide, to show that K15 is expressed primarily in t he basal keratinocytes of stratified tissues, including the fetal epidermis and fetal nail. Although K15 in normal hair follicles was virtually absent from hair bulbs, it was expressed by a subset of keratinocytes in the oute r root sheath. By comparison, K14 expression was found throughout the outer root sheath of hair follicles; however, when both K14 alleles were natural ly ablated, the expression of K15 was also observed throughout the outer ro ot sheath of the follicles. Expression of K15 mRNA was assessed by in situ hybridization and corroborated the data from immunostaining. An increase in K15 mRNA and protein expression in hair follicles from the K14 ablated epi dermis suggested an upregulation of the K15 gene in the absence of the K14 protein. In organotypical cultures where differentiating keratinocytes expr essed markers of activated phenotype, i.e., K6 and K16, expression of K15 w as undetectable. The expression of K15 mRNA and protein was also downregula ted in two hyperproliferating situations, psoriasis and hypertrophic scars. Because keratinocytes in psoriasis and hypertrophic scars are activated, w e conclude that K15 expression is not compatible with keratinocyte activati on and the K15 gene is downregulated to maintain the activated phenotype.