RESPONSE OF HEPATIC PROTEINS TO 3,5,3'-TRI-IODO-L-THYRONINE IN DIABETIC RATS

In order to study whether peripheral action of thyroid hormones is altered in insulin deficiency and to elucidate the biological consequences of alteration of the cytosolic 3,5,3'-tri-iodo-L-thyronine (T-3) binding protein (CTBP), we measured malic enzyme, T-3-responsive nuclear n protein, CTBP and nuclear thyroid hormone receptor in the liver and kidney of streptozotocin (STZ)-induced diabetic rats that were treated with or without insulin and/or a receptor-saturating dose of T-3. The following results were obtained. 1. Induction of malic enzyme by T-3 was apparently diminished in diabetic rats. However, supplementary injection of insulin enabled previously given Tg to take effect in diabetic rats. 2. T-3-responsiveness of other hepatic proteins (n protein and CTBP) was not altered by insulin in diabetic rats. 3. The level of n protein was increased by insulin in diabetic rats in vivo and in perfused rat liver, indicating that the hepatic n protein is a novel insulin-responsive protein. T-3 and insulin increased the level of n protein non-synergistically in diabetic rat liver. 4. Hepatic nuclear receptor levels were not altered in diabetic rats. 5. Hepatic CTBP levels were decreased in diabetic rats. This was not due to the toxic effect of STZ. Low CTBP level was only partially increased by insulin after 30 days of diabetic period. Renal CTBP levels were not altered in diabetic rats with or without insulin treatment. These results indicate that reduction of CTBP did not influence the hepatic response to a receptor-saturating dose of T-3, although CTBP may regulate the nuclear T-3 transport, and that fundamental action of a receptor-saturating dose of T-3 was not attenuated in diabetic rat liver.