Induction of Cytosolic Triiodo-L-Thyronine (T₃) Binding Protein (CTBP) by T₃ in Primary Cultured Rat Hepatocytes

Abstract

Cytosolic 3, 5, 3'-triiodo-L-thyronine (T₃)-binding protein (CTBP) plays an important role in the regulation of intracellular T₃ translocation from cytoplasm to the nuclear T₃ receptor. We examined whether the CTBP activity could be induced by T₃ or not in cultured hepatocytes prepared from thyroidectomized rats. CTBP activity was not detected in primary cultured hepatocytes from thyroidectomized rats. However, the protein was induced by the addition of T₃ to the culture medium. The increase in the activity of CTBP was time dependent and the maximal level was obtained by 48h in the presence of 300nM T₃. CTBP activity was also increased by retinol (35μM) or by 1, 25-(OH)₂-vitamin D₃ (10nM). On the other hand, the activity of malic enzyme (ME) was induced by the addition of T₃ to the culture medium. The maximal activity of ME was obtained by 48h in the presence of 300nM T₃. The increase in ME activity was also induced by retinol or 1, 25-(OH)₂-vitamin D₃. These results suggested that not only ME activity but also CTBP activity is induced by T₃. Further, retinol and vitamin D₃ have similar effects on the induction of CTBP activity and ME activity.