ICAM-1 is a cell surface glycoprotein that serves as the major ligand for LFA-1, an anchoring receptor on all T cells whose interaction with ligand is critical for proper T-cell function. The regulation of ICAM-1 expression is critically important in both normal inflammatory and pathologic processes. We have established the kinetics and molecular regulatory level for cytokine-induced de novo ICAM-1 expression by human keratinocytes (HK) in culture. We have established an in vitro leukocyte adhesion assay for resting and blast T cells to cultured HK and have demonstrated that the marked increase in adhesion of T cells to IFN-gamma-treated HK is primarily due to the induction of HK ICAM-1 expression. We have further explored the regulatory mechanisms of ICAM-1 gene expression and shown that ICAM-1 expression in HK is induced by short-term exposure to cycloheximide, implicating the existence of a transacting protein with a short half-life that is involved in the constitutive repression of ICAM-1 in HK. We have shown that the kinetics for IFN-gamma-induced expression of HLA-DRbeta by HK (DR) is different from those for ICAM-1. Additionally, TGF-beta enhances the induced expression of ICAM-1 and suppresses the induced expression of DR. We have isolated a human genomic clone containing the 5' flanking transcriptional regulatory regions of ICAM-1, which includes consensus sequences for promoter and enhancer elements, and have established the transcription start site by primer extension and S1 nuclease protection assays. A 282 bp fragment including the cap site at its 3' end is a potent functional promoter and can drive the expression of a heterologous reporter gene to a degree comparable to the SV40 promoter and enhancer. We are currently analyzing additional 5' flanking and intronal regions for repressor and enhancer functions as well as for consensus and functional elements for cytokine induced transcriptional activation.
