Interferon γ-dependent induction of human intercellular adhesion molecule-1 gene expression involves activation of a distinct STAT protein complex

Abstract

In response to interferon γ (IFNγ), intercellular adhesion molecule-1 (ICAM-1) is expressed on human keratinocytes, a cell type that is critically involved in cutaneous inflammation. An ICAM-1 5' regulatory region palindromic response element, pIγRE, has been shown to confer IFNγ- dependent transcription enhancement. By electrophoretic mobility shift assays (EMSA), pIγRE forms a distinct complex with proteins from IFNγ-treated human keratinocytes, termed γ response factor (GRF). Binding of GRF is tyrosine phosphorylation-dependent, and mutations of pIγRE that disrupt the palindromic sequence or alter its spatial relationship abrogate GRF binding. Supershift EMSAs using antibodies to characterized STAT proteins suggest that GRF contains a Stat1α-like protein; however, non-ICAM-I IFNγ-responsive elements (REs) known to bind Stat1α homodimers fail to compete for GRF binding in EMSA, and pIγRE does not cross-compete with these REs that complex with homodimeric stat1α. The pIγRE-GRF complex also displays a distinctly different electrophoretic mobility compared to that of IFNγREs complexed to homodimeric Stat1α. These findings indicate that a distinct complex containing a Stat1α-like protein mediates IFNγ-induced ICAM-1 gene transcription and identifies a subset of IFNγ-responsive genes that appear to be regulated by this complex.