SREBP-1 integrates the actions of thyroid hormone, insulin, cAMP, and medium-chain fatty acids on ACCα transcription in hepatocytes

In chick embryo hepatocytes, activation of acetyl-CoA carboxylase-alpha (ACCalpha) transcription by 3,5,3'-triiodothyronine (T3) is mediated by a cis-acting regulatory unit (-101 to -71 bp) that binds the nuclear T3 receptor (TR) and sterol regulatory element-binding protein-1 (SREBP-1). SREBP-1 directly interacts with TR on the ACCalpha gene to enhance T3-induced transcription. Here, we show that treating hepatocytes with T3 or insulin stimulates a 4-fold increase in the concentration of the mature, active form of SREBP-1. When T3 and insulin are added together, a 7-fold increase in the mature SREBP-1 concentration is observed. Time course studies indicate that the T3-induced increase in mature SREBP-1 abundance is closely associated with changes in ACCalpha transcription and that the mechanism mediating the effect of T3 on mature SREBP-1 is distinct from that mediating the effect of insulin. Transfection analyses indicate that inhibition of ACCalpha transcription by cAMP or hexanoate is mediated by ACCalpha sequences between -101 and -71 bp. Treatment with cAMP or hexanoate suppresses the increase in mature SREBP-1 abundance caused by T3 and insulin. These results establish a new interaction between the SREBP-1 and TR signaling pathways and provide evidence that SREBP-1 plays an active role in mediating the effects of T3, insulin, cAMP, and hexanoate on ACCalpha transcription.