Transcription of the acetyl-CoA carboxylase (ACC)-alpha gene is initiated from two promoters, promoter I (PI) and promoter II (PII) such that transcripts demonstrate heterogeneity in their 5' untranslated regions (UTR). Exons 1 and 2 (E1 and E2) are the primary exons in transcripts initiated from PI and PII respectively; E5 is the first coding exon present in all transcripts. In addition alternative exon splicing results in transcripts that either include or exclude a 47 nucleotide sequence corresponding to E4, such that E[1/4/5] and E[1/5] type transcripts result from PI activity, whereas transcripts containing E[2/4/5] or E[2/5] in the 5'UTR result from PII. In subcutaneous adipose tissue from non-pregnant non-lactating sheep approximately 60% of ACC-alpha transcripts are derived from PI, of which 85% are the E[1/5] type. Lactation resulted in an 88% reduction in total PI transcripts, of which the E[1/5] type was reduced 90% and the E[1/4/5] type 80%. By contrast lactation reduced the total levels of PII transcripts by only 50%. Culture of explants from the subcutaneous depot of lactating sheep with insulin plus dexamethasone for 72 h resulted in an 8-fold increase in both E[1/4/5] and E[1/5] types when compared with explants prior to culture. PII transcripts, by contrast, were increased 2-fold by culture in insulin plus dexamethasone and this was entirely attributed to an increase in the expression of the E[2/4/5] type. Dexamethasone acts to potentiate the action of insulin on PI and PII transcript abundance and this effect is greatest for PI transcripts. This study has demonstrated that repression of the ACC-alpha gene in adipose tissue during lactation is largely achieved through attenuation of PI transcript abundance and may be related, in part, to a change in the sensitivity of the apparatus that regulates PI transcript steady-state levels to insulin.
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