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 chromoso me band 1q21 close to the epidermal differentiation complex and is transcri bed in two discrete mRNAs: a full length Ecm1a and a shorter, alternatively spliced, Ecm1b transcript, the expression of which is restricted to tonsil s and skin. The chromosomal localization and the Ecm1b expression in skin p rompted us to investigate the role of Ecm1 in keratinocyte differentiation. In this study, we provide evidence for the existence of a relationship bet ween keratinocyte differentiation and expression of the Ecm1b transcript. C ultures 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 expressi on is induced, although expression of Ecm1a mRNA remains unchanged. In addi tion, treatment of undifferentiated normal human keratinocyte cells with 12 -O-tetradecanoyl-phorbol-13-acetate strongly induces the expression of Ecm1 b mRNA. Expression of Ecm1b can also be induced by coculturing normal human keratinocytes with lethally irradiated feeder cells and by a diffusible fa ctor secreted by stromal cells. In adult human skin, Ecm1a mRNA is expresse d throughout the epidermis with the strongest expression in the basal and f irst suprabasal cell layers, whereas expression of Ecm1b mRNA is predominan tly found in spinous and granular cell layers. Immunohistochemically, Ecm1a expression is almost completely restricted to the basal cell layer, wherea s Ecm1b is detected in the suprabasal layers. These results are strongly su ggestive of a role for Ecm1b in terminal keratinocyte differentiation, whic h is also supported by the localization of the Ecm1 gene at 1q21. Refinemen t of its genomic localization, however, placed Ecm1 centromeric of the epid ermal differentiation complex.