The predominant cAMP-stimulated 3.5 kb StAR mRNA contains specific sequence elements in the extended 3′UTR that confer high basal instability

Abstract

cAMP stimulation of rodent steroidogenic cells produces two StAR transcripts, a major 3.5 kb and a minor 1.6 kb mRNA, differing only in their 3′ untranslated regions (3′ UTR). They exhibit very different responses to stimulation and removal of 8-Br-cAMP, with the 3.5 kb form increasing and declining much more rapidly than the 1.6 kb form. The 3′ end of the 3.5 kb StAR mRNA contains three conserved AU-rich element (AURE) motifs that mediate fast mRNA turnover in over 900 genes in the human genome. In this paper, we explore post-transcriptional regulation in steroidogenic and non-steroidogenic cells using expression vectors containing StAR or luciferase with different StAR 3′ UTRs. We show that the basal steady-state levels of StAR or luciferase protein and mRNA are five to eight times lower with the 3′ UTR of 3.5 kb StAR compared with that of the 1.6 kb 3′ UTR. Examination of transcript stability by direct mRNA transfection showed only a 1.5-fold increase in the rate of cytoplasmic decay of the 3.5 kb mRNA relative to the 1.6 kb mRNA. However, the long 3′ UTR caused a fivefold decrease in the rate of appearance of mature cytoplasmic mRNA despite transcription from the same promoter. This is attributed to less efficient nuclear processing of immature transcripts prior to export to cytoplasm. Selective 3′ UTR sequence substitutions, deletions, and mutations showed that this loss of expression is produced additively by specific sequences in a 700-base basal instability region and by non-specific length effects. These mechanisms are selectively enhanced in steroidogenic cells. The AURE contribute a smaller basal destabilization effect selective for steroidogenic cells that is removed by their mutations. Inclusion of introns in the 3.5 kb StAR vector enhances StAR expression, suggesting the effects of introns complexes on nuclear processing. Br-cAMP provides an additional means to rapidly modulate StAR expression independent of transcription by attenuating the nuclear and cytoplasmic instability mechanisms within the extended 3′ UTR.