Abstract
Cytosolic 3,5,3′-triiodo-l-thyronine-binding protein plays pivotal roles in the regulation of intracellular
3,5,3′-triiodo-l-thyronine concentration in vivo. The expression of the protein, which is identical
to μ-crystallin, is regulated by various factors. To elucidate the mechanisms of its
expression, we evaluated the promoter transactivity and insulin signaling via the
AP-1 site in the promoter. The isolated 600 bp human and 1976 bp mouse 5′-flanking
regions were cloned in a luciferase reporter plasmid. The luciferase activity was
estimated in GH3, dRLh-84, HEK293, and insulin receptor-overexpressing CHO-IR cells.
The effects of 12-O-tetradecanoylphorbol 13-acetate and insulin on μ-crystallin mRNA expression were
evaluated in various cells. The region between −200 and the transcriptional start
site was crucial for constitutive expression in μ-crystallin-expressing dRLh-84 cells.
This region contained an AP-1 site. 12-O-Tetradecanoylphorbol 13-acetate increased the level of μ-crystallin mRNA expression
in HEK 293 cells. The compound also increased luciferase activity through the promoter.
Mutation in the AP1 site diminished the response to the compound. The promoter was
also activated by insulin treatment in CHO-IR cells. Insulin treatment increased μ-crystallin
mRNA expression in Raw264.7 cells, but decreased in HEK293, P19, and dRLH-84 cells.
The expression of μ-crystallin was regulated through the AP-1 site in the promoter.
The signals related to AP-1 activation, such as insulin signaling may have diverse
effects on μ-crystallin mRNA expression.
Key words
TPA - thyroid hormone - binding protein - insulin - gene expression
