Differentiation-dependent alternative splicing and expression of the extracellular matrix protein 1 gene in human keratinocytes.

The human extracellular matrix protein 1 (Ecm1) gene is located at chromosome band 1q21 close to the epidermal differentiation complex and is transcribed in two discrete mRNAs: a full length Ecm1a and a shorter, alternatively spliced, Ecm1b transcript, the expression of which is restricted to tonsils and skin. The chromosomal localization and the Ecm1b expression in skin prompted us to investigate the role of Ecm1 in keratinocyte differentiation. In this study, we provide evidence for the existence of a relationship between keratinocyte differentiation and expression of the Ecm1b transcript. Cultures of subconfluent undifferentiated normal human keratinocytes express only Ecm1a. Upon reaching confluence, the cells start to differentiate, as measured by keratin K10 mRNA expression. Concomitantly Ecm1b mRNA expression is induced, although expression of Ecm1a mRNA remains unchanged. In addition, treatment of undifferentiated normal human keratinocyte cells with 12-O-tetradecanoyl-phorbol-13-acetate strongly induces the expression of Ecm1b mRNA. Expression of Ecm1b can also be induced by coculturing normal human keratinocytes with lethally irradiated feeder cells and by a diffusible factor secreted by stromal cells. In adult human skin, Ecm1a mRNA is expressed throughout the epidermis with the strongest expression in the basal and first suprabasal cell layers, whereas expression of Ecm1b mRNA is predominantly found in spinous and granular cell layers. Immunohistochemically, Ecm1a expression is almost completely restricted to the basal cell layer, whereas Ecm1b is detected in the suprabasal layers. These results are strongly suggestive of a role for Ecm1b in terminal keratinocyte differentiation, which is also supported by the localization of the Ecm1 gene at 1q21. Refinement of its genomic localization, however, placed Ecm1 centromeric of the epidermal differentiation complex.