The present study examined 1) whether the estrogen-regulated destabilization of albumin mRNA occurs in the nuclear or extranuclear fraction of the liver cell, and 2) whether the selective posttranscriptional regulation of albumin mRNA stability might result from covalent changes introduced in the processing or polyadenylation of the primary transcript. The disappearance of albumin mRNA after estrogen is restricted to the extranuclear fraction of the cell. Transient changes in steady state levels of the mature nuclear transcript were observed that mirrored the transient estrogen-induced changes previously reported for albumin gene transcription. When assayed 24 h after estrogen (when albumin RNA is virtually undetectable in the extranuclear fraction) the steady state levels of both the primary and mature albumin transcripts found in the nucleus were the same as observed in control animals. Estrogen had no effect on the splicing or selection of polyadenylation sites on the 3'-UTR as determined by high resolution gel analysis of the 3'-UTR and DNA sequencing of cDNA clones isolated from a liver library from an estrogen-treated male Xenopus. Most eukaryotic mRNAs have poly(A) tracts several hundred residues in length, and recent studies have demonstrated that a change in the stability of a number of mRNAs correlates directly with the degree of polyadenylation. Albumin contrasts sharply with this, first because it has an exceptionally short poly(A) tail of 17 residues, and second because the degree of polyadenylation is totally unrelated to its destabilization in response to estrogen. These findings indicate that a unique pathway is involved in the regulation of albumin RNA stability by estrogen in Xenopus.
Animals Base Sequence