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

In response to interferon gamma (IFN gamma), 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, pI gamma RE, has been shown to confer IFN gamma-dependent transcription enhancement. By electrophoretic mobility shift assays (EMSA), pI gamma RE forms a distinct complex with proteins from IFN gamma-treated human keratinocytes, termed gamma response factor (GRF). Binding of GRF is tyrosine phosphorylation-dependent, and mutations of pI gamma RE 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 Stat1 alpha-like protein; however, non-ICAM-1 IFN gamma-responsive elements (REs) known to bind Stat1 alpha homodimers fail to compete for GRF binding in EMSA, and pI gamma RE does not cross compete with these REs that complex with homodimeric stat1 alpha. The pI gamma RE GRF complex also displays a distinctly different electrophoretic mobility compared to that of IFN gamma REs complexed to homodimeric Stat1 alpha. These findings indicate that a distinct complex containing a Stat1 alpha-like protein mediates IFN gamma-induced ICAM-1 gene transcription and identifies a subset of IFN gamma-responsive genes that appear to be regulated by this complex.