Keratin 15 Expression in Stratified Epithelia: Downregulation in Activated Keratinocytes

Abstract

Keratin 15 (K15) is a type I keratin without a defined type II partner whose 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 the 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 outer root sheath. By comparison, K14 expression was found throughout the outer root sheath of hair follicles; however, when both K14 alleles were naturally ablated, the expression of K15 was also observed throughout the outer root 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 epidermis suggested an upregulation of the K15 gene in the absence of the K14 protein. In organotypical cultures where differentiating keratinocytes expressed markers of activated phenotype, i.e., K6 and K16, expression of K15 was undetectable. The expression of K15 mRNA and protein was also downregulated in two hyperproliferating situations, psoriasis and hypertrophic scars. Because keratinocytes in psoriasis and hypertrophic scars are activated, we conclude that K15 expression is not compatible with keratinocyte activation and the K15 gene is downregulated to maintain the activated phenotype.

K15 Expression Implies Lateral Differentiation within Stratified Epithelial Basal Cells

“…The first is an anti-C-terminal peptide polyclonal antibody to K15, IMP15, which enabled double immunofluorescence to be carried out, and the second is a monoclonal antibody, LC18N, which arose fortuitously. These antibodies were used in addition to monoclonal antibody LHK15 (Waseem et al, 1999) to give greater confidence in interpreting immunohistochemistry results. In immunoblots of protein extracted from human epidermis, IMP15 recognizes a band at 50 kDa compared with a 48 kDa band in mouse extracts (Fig.…”

“…K15 has been reported to be expressed throughout the adult human esophagus based on in situ hybridization (Leube et al, 1988), but more recently it was shown with antibody LHK15 that K15 protein is also confined to the basal layer in human esophagus (Waseem et al, 1999).…”

“…Other keratin antibodies were LE41 (K8) and LE61 (K18) (Lane, 1982), LL002 (K14) (Purkis et al, 1990), LL025 (K16) (Wilson et al, 1994), LHK6 (K6) and LHK15 (K15) (Waseem et al, 1999), LP2K (K19) (Stasiak et al, 1989) RCK102 (K5 &K8) (Broers et al, 1986), and LH2 (K10) (Leigh et al, 1993). Additional monoclonal antibodies used were E-cadherin (Transduction Laboratories, Lexington, Kentucky) diluted 1:50, 11-5F (anti-desmoplakin) (Parrish et al, 1987) diluted 1:100; and MM-1 (anti-Ki67) (Novacastra, Newcastle upon Tyne, United Kingdom).…”

“…It has now become clear that significant amounts of K15 are present in the basal layer of several stratified epithelial tissues (Lloyd et al, 1995;Waseem et al, 1999). It has even been suggested that K15 may be a marker for stem cells in the hair follicles (Lyle et al, 1998).…”

Keratin down-regulation in vimentin-positive cancer cells is reversible by vimentin RNA interference, which inhibits growth and motility

“…Squamous carcinoma cells expressing low levels of K19 were reported to have a higher capacity to invade in vitro, which was decreased by ectopic expression of this IF protein (9). In addition, down-regulation of K15 has been reported in hyperproliferative conditions such as psoriasis and hypertrophic scarring (10), whereas K14 down-regulation was documented in a model of cervical carcinoma (11). However, the relationship between keratin expression and tumor progression remains unclear, as a recent report indicated that K14 overexpression correlated with high-risk squamous cell carcinoma (SCC; ref.…”