Regulation of Major Histocompatibility (MHC) Class II Human Leukocyte Antigen-DRα Gene Expression in Thyrocytes by Single Strand Binding Protein-1, a Transcription Factor That Also Regulates Thyrotropin Receptor and MHC Class I Gene Expression

The single strand binding protein (SSBP-1) is a positive regulator of TSH receptor gene expression and binds to an element with a GXXXXG motif. The S box of the mouse major histocompatibility class II gene has multiple GXXXXG motifs and can also bind SSBP-1. The S box is one of four highly conserved elements on the 5′-flanking region of class II genes that are necessary for interferon-γ (IFNγ) to overcome the normally suppressed state of the gene and induce aberrant class II expression. In this report we show that SSBP-1, when overexpressed in FRTL-5 thyroid cells, is a positive regulator of human leukocyte antigen (HLA)-DRα class II gene expression, as is IFNγ or the class II trans-activator (CIITA). This is evidenced by increased exogenous promoter activity, increased endogenous RNA levels, and increased endogenous antigen expression after transfecting full-length SSBP-1 complementary DNA together with a HLA-DRα promoter-reporter gene chimera into TSH-treated FRTL-5 thyroid cells whose endogenous SSBP-1...