Importance of the most proximal GC box for activity of the promoter of human thyroid hormone receptor beta 1.

Abstract

We wish to localize the sequences required for transcriptional expression of the thyroid hormone receptor beta 1 (TR beta 1). Constitutive activity of the promoter of human thyroid hormone receptor beta 1 was assessed by transient transfection of deletion constructs attached to luciferase as reporter, into P19, GH3, HepG2, H19-7, and COS1 cells. A 40-base pair fragment of the beta 1 promoter including the TATA box induced minimal luciferase activity, which was considered basal activity. The activities of various lengths of the beta 1 promoter were estimated relative to the minimal promoter in five cell lines. The region between -130 and -40 was crucial for constitutive activity in all cell lines. Further deletion analysis in HepG2 cells showed that two regions mainly augmented the transcriptional activity of the minimal 40 base pair fragment. One region located at -115 to -93, which is highly GC-rich, included the most proximal of five putative GC boxes present in the whole 1325-base pair promoter. A second region contributing to expression of TR beta 1 in HepG2 cells is at -70 to -40. Mutation of the most proximal GC box strongly suppressed transactivity of the whole promoter in P19 and HepG2 cells. In contrast, mutations in the other GC boxes did not suppress transactivation in P19 cells and slightly suppress activation in HepG2 cells. In Schneider cells, which do not express Sp1, transactivity of the region distal to -40 is positively regulated by cotransfection with a vector expressing Sp1.(ABSTRACT TRUNCATED AT 250 WORDS)