INDUCTION OF CYTOSOLIC TRIIODO-L-THYRONINE (T(3)) BINDING-PROTEIN (CTBP) BY T(3) IN PRIMARY CULTURED RAT HEPATOCYTES

Cytosolic 3, 5, 3'-triiodo-L-thyronine (T3)-binding protein (CTBP) pla ys an important role in the regulation of intracellular T3 translocati on from cytoplasm to the nuclear T3 receptor. We examined whether the CTBP activity could be induced by T3 Or not in cultured hepatocytes pr epared from thyroidectomized rats. CTBP activity was not detected in p rimary cultured hepatocytes from thyroidectomized rats. However, the p rotein was induced by the addition of T3 to the culture medium. The in crease in the activity of CTBP was time dependent and the maximal leve l was obtained by 48 h in the presence of 300 nM T3. CTBP activity was also increased by retinol (35 muM) or by 1,25-(OH)2-vitamin D3 (10 nM ). 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 48 h in the presence of 300 nM T3. The increase in ME activity was also induced by retinol or 1,25-(OH)2-vitamin D3. These r esults suggested that not only ME activity but also CTBP activity is i nduced by T3. Further, retinol and vitamin D3 have similar effects on the induction of CTBP activity and ME activity.