FHL3 negatively regulates human high-affinity IgE receptor β-chain gene expression by acting as a transcriptional co-repressor of MZF-1

The high-affinity IgE receptor FcepsilonRI plays a key role in triggering allergic reactions. We recently reported that human FcepsilonRI beta-chain gene expression was down-regulated by a transcription factor, MZF-1, through an element in the fourth intron. In the present study, we found that this transcriptional repression by MZF-1 required FHL3 (four and a half LIM domain protein 3) as a cofactor. Yeast two-hybrid and immunoprecipitation assays demonstrated that FHL3 bound MZF-1 in vitro and in vivo. Overexpression of FHL3 in KU812 cells suppressed the beta-chain promoter activity through the element in the fourth intron in an MZF-1-dependent manner. Furthermore, results from pull-down assays and gel-filtration chromatography employing nuclear extracts indicated that MZF-1 and FHL3 formed a complex of high molecular mass with some additional proteins in the nucleus. Granulocyte-macrophage colony-stimulating factor, which was reported to decrease Fepsilon6RI expression, induced the accumulation of FHL3 in the nucleus, in accordance with the repressive role of FHL3 in beta-chain gene expression.