The gene for human involucrin (hINV) is selectively expressed in stratifying epithelial cells lining external body surfaces. Previously, we characterized the hINV promoter 5´ distal regulatory region (DRR) located between nt -2473 and -2088 upstream of the transcription start site. This region is required for optimal hINV gene expression. The DRR contains weak and strong activator elements. The strong activator comprises AP1- and Sp1-binding sites that combine to drive high-level promoter expression in human keratinocytes. Here we show that the hINV promoter is expressed in a cell-specific manner in vitro and that the DRR contains elements that are partly responsible for cell-type-specific expression of hINV. hINV promoter activity is barely detectable in 3T3 fibroblasts or HEK-293 human embryonic kidney cells. Reporter plasmids containing the full-length promoter or the isolated DRR can, however, be activated in 3T3 and HEK-293 cells by co-transfection with a plasmid encoding the transcription factor Sp1. Consistently with the lower hINV promoter activity, immunoblotting studies indicate that Sp1 protein levels are lower in 3T3 and HEK-293 cells than in human epidermal keratinocytes. Increased Sp1 protein in transfected 3T3 cells and HEK-293 cells correlates with increased promoter activity. In addition, Sp1 transfection activates the expression of the endogenous gene for hINV in HEK-293 cells. These studies suggest that Sp1 might have a role in cell-specific expression of hINV.

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