Identification of a functional determinant of differentiation-dependent expression in the involucrin gene

Involucrin is an integral component of the cornified envelope which is a characteristic feature of the differentiated keratinocyte. Involucrin expression is tightly linked to the onset of differentiation and first expressed in the immediate suprabasal layers of the epidermis. We have identified a transcriptional response element within the distal 5'-flanking region of the involucrin gene which contributes to differentiation-dependent expression. Deletion of this site impairs differentiation-dependent promoter activity in transient transfection analysis, and conversely, this region imparts differentiation-dependent expression to a heterologous promoter. The identified site bears sequence similarity to several AP2-like response elements identified in keratinocyte-specific genes and binds a protein complex (keratinocyte differentiation factor, KDF-1) which is distinct from AP2 by several criteria. The migration of KDF-1 is distinct from AP2 in electrophoretic mobility shift assays, KDF-1 is antigenically unrelated to AP2 since AP2 specific antibodies do not supershift the KDF-1-DNA complex and KDF-1 is poorly competed by oligonucleotides representing consensus AP2 recognition sequences. In addition, the KDF-1 complex is not detected in nuclear extracts derived from human dermal fibroblasts or an enriched population of basal keratinocytes. These findings provide insights to the underlying basis of differentiation-dependent expression of a keratinocyte specific gene.