STAT5 binding contributes to lactational stimulation of promoter III expressing the bovine acetyl-CoA carboxylase α-encoding gene in the mammary gland

Activity of acetyl-CoA carboxylase (ACC)-alpha is rate limiting for de novo synthesis of fatty acids. The encoding gene is expressed by three different promoters. We characterized promoter III (PIll) from cow, previously only known from sheep. Quantitation of transcripts by RNAse protection assays and real time PCR revealed that PIlI is primarily expressed and strongly induced (similar to28-fold) in the lactating mammary gland. PIll transcripts are expressed in mammary epithelia] cells (MEC) as shown by in situ hybridization. A2999 bp segment of the PIlI promoter conferred prolactin and dexamethasone inducibility to a luciferase reporter gene in stably transfected mouse MEC cells. Lactogenic induction was abolished if a unique signal transducer and activator of transcription (STAT)-binding site at position -797 was inactivated by two point mutations. An oligonucleotide probe harboring this STAT-site specifically bound nuclear proteins from the lactating mammary gland. Binding was abolished by those two point mutations and super-shift analyses showed that STAT5A factors are present in this complex. Hence, prolactin, acting through STAT5, contributes to the activation of ACC expression in the milk producing cells of the lactating mammary gland. We discuss that STAT5 might be important in determining the milk composition by coordinating fatty acid and protein synthesis during lactation.