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

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Abstract
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 59-flanking region of class II genes that are necessary for interferon-g (IFNg) 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)-DRa class II gene expression, as is IFNg or the class II trans-activator (CIITA). This is evidenced by increased exogenous promoter activity, increased en- dogenous RNA levels, and increased endogenous antigen expression after transfecting full-length SSBP-1 complementary DNA together with a HLA-DRa promoter-reporter gene chimera into TSH-treated FRTL-5 thyroid cells whose endogenous SSBP-1 levels are low. IFNg reverses the ability of TSH to decrease endogenous SSBP-1 RNA levels. Also, whereas SSBP-1 transfection does not cause any increase in IFNg- induced exogenous promoter activity, transfection of SSBP-1 and CIITA additively increases endogenous class II RNA levels to levels measured in cells treated with IFNg. Further, competition studies show that SSBP-1 binding is necessary for formation of the double strand protein/ DNA complexes that are seen in electrophoretic mobility shift assays when the class II 59-flanking region is incubated with extracts from IFNg-treated FRTL-5 cells and that have been previously associated with IFNg-induced aberrant class II expression. These data suggest that SSBP-1 is involved in the action of IFNg to overcome the normally suppressed state of the class II gene; it functions together with CIITA, whose expression is independently increased by IFNg. The effect of SSBP-1 as a positive regulator of class II promoter activity is lost in cells maintained without TSH, in which endogenous SSBP-1 RNA levels are already high in the absence of aberrant class II gene expression. These data suggest that high levels of endogenous SSBP-1 are insufficient to cause aberrant class II expression, but, rather, TSH or IFNg treatment additionally modulates the cell, albeit differently, such that transfected or endogenous SSBP-1, respectively, can express its positive regulatory activity. The effect of TSH is consistent with reports indicating that TSH enhances the ability of IFNg to increase class II gene expression despite the fact IFNg increases endogenous SSBP-1 to only the same levels as in cells untreated with TSH. Finally, the effect of SSBP-1 as a positive regulator is lost when GXXXXG motifs, which exist on both the coding and noncoding strands of the S box, are mutated. Consistent with this, mutation and oligonucleotide competition studies show that GXXXXG motifs are necessary for either strand of the S box to bind protein/DNA complexes containing SSBP-1 in FRTL-5 cell extracts or to bind to re- combinant SSBP-1. They also suggest that the SSBP-1-binding sites on either strand of the HLA-DRa S box are functionally distinct. We con- clude from these data that the positive regulatory action of SSBP-1 on class II gene expression involves GXXXXG motifs on each strand of the highly conserved S box of the class II 59-flanking region. As SSBP-1 is modulated by IFNg and is involved in class I and TSH receptor as well as class II gene expression in FRTL-5 cells, the sum of the data supports the hypotheses that common transcription factors regulate all three genes, and their altered activities may contribute to the development of autoimmunity. (Endocrinology 139: 2300-2313, 1998)

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