THE PROXIMAL PROMOTER OF THE MOUSE LORICRIN GENE CONTAINS A FUNCTIONAL AP-1 ELEMENT AND DIRECTS KERATINOCYTE-SPECIFIC BUT NOT DIFFERENTIATION-SPECIFIC EXPRESSION

Loricrin gene expression is limited to terminally differentiating kera tinocytes of stratified squamous epithelia. To define the regulatory e lements that mediate the expression of the loricrin gene, we replaced the loricrin coding sequences from a 6.5-kilobase genomic fragment wit h the chloramphenicol acetyltransferase gene and transfected this cons truct into cultured mouse keratinocytes. High expression levels were o bserved in both undifferentiated as well as differentiating cells. Tra nsgenic mice bearing a similar construct, but with beta-galactosidase as the reporter gene, corroborated these in vitro findings and showed tissue and cell type-specific, but not differentiation-specific expres sion. Deletion analysis of the promoter region determined that sequenc es up to -60 base pairs from the start of transcription could be remov ed without significant loss of promoter activity. Within these proxima l 60 base pairs is an evolutionarily conserved AP-1 element that is re cognized by both purified c-Jun and AP-1 factors from keratinocytes in vitro. Mutation of this AP-1 site abolished the activity of the loric rin promoter. These studies show that elements directing expression of the loricrin gene to the stratified squamous epithelia are contained within a 6.5-kilobase genomic fragment, and those elements required to restrict expression to differentiated keratinocytes lie outside this region.