Activation of the thyroid-stimulating hormone β-subunit gene by LIM homeodomain transcription factor Lhx2

Although there is evidence that the LIM homeodomain transcription factor, Lhx2, can stimulate transcription of the glycoprotein hormone alpha-subunit gene, the role of Lhx2 in regulating TSH beta-subunit has not been established. In the present studies, the ability of Lhx2 to regulate transcription of the TSH beta-subunit gene was examined. In the thyrotrope-derived T alpha T1 cell line, Lhx2 expression was found to be induced by treatment with either TRH or cAMP, consistent with the possibility that Lhx2 may play a role in mediating the ability of this signaling pathway to stimulate TSH gene expression. Transient, forced overexpression of Lhx2 stimulated activity of a TSH beta-subunit reporter gene. Deletion studies provided evidence that the - 177 to - 79 region of the TSH beta-subunit promoter was necessary for stimulation of reporter gene activity by Lhx2. Agel mobility shift assay provided the evidence that Lhx2 can bind to this region of DNA. DNase I footprinting studies demonstrated that two distinct regions of the TSH beta promoter, - 118 to - 108 and - 86 to - 68, are protected by Lhx2 from nuclease digestion. These regions contain repeats of the sequence, 5'-(G/T) CAAT(T/A)- 3'. Mutation of this sequence, especially in the - 86 to - 68 region, substantially decreased Lhx2 responsiveness of the TSH beta-subunit reporter gene. In addition, a DNA fragment containing the - 177 to - 79 region of the TSH beta promoter was found to confer Lhx2 responsiveness to a minimal promoter. These results provide multiple lines of evidence consistent with a role for Lhx2 in modulating expression of the TSH beta-subunit gene.