Abstract
High-carbohydrate feeding and triiodothyronine (T3) increase the abundance of acetyl-CoA carboxylase-α (ACCα) mRNA in avian hepatocytes, whereas starvation, glucagon, and medium-chain fatty acids decrease the abundance of ACCα mRNA. These changes in ACCα mRNA levels are mediated by alterations in the rate of transcription of the ACCα gene. In liver, ACCα transcription is initiated from two promoters, promoter 1 and promoter 2, resulting in transcripts that contain heterogeneity in their 5′-untranslated regions. Here, we investigated the role of promoter 1 and promoter 2 in mediating nutrient- and hormone-induced changes in ACCα mRNA abundance by measuring the level of transcripts expressed from promoter 1 and promoter 2 using a ribonuclease protection assay. The results indicated that both promoter 1 and promoter 2 were regulated by starvation/refeeding in livers of intact chicks and by T3, glucagon, and medium-chain fatty acids in chick embryo hepatocyte cultures and that alterations in the activity of promoter 2 accounted for a greater proportion of the changes in total ACCα mRNA abundance caused by nutrient and hormone treatment. Five DNase-hypersensitive sites were also identified between −500 and +1 bp relative to the transcription start site of promoter 2 in livers of intact chicks and in chick embryo hepatocyte cultures. In transient transfection analyses, this region of DNase hypersensitivity conferred regulation of transcription by T3, glucagon, and medium-chain fatty acids in chick embryo hepatocytes. Data from this study demonstrate that diet-induced changes in the activities of promoter 1 and promoter 2 in livers of intact chicks are mimicked in chick embryo hepatocyte cultures by manipulating the concentrations of T3, glucagon and medium-chain fatty acids in the culture medium and that cis-acting sequences mediating the effects of nutrients and hormones on promoter 2 activity are located immediately upstream of the transcription start site of this promoter.
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