Posttranscriptional regulation of albumin gene expression in Xenopus liver: evidence for an estrogen receptor-dependent mechanism

Mechanisms of endonuclease-mediated mRNA decay

Endonuclease cleavage was one of the first identified mechanisms of mRNA decay but until recently it was thought to play a minor role to the better-known processes of deadenylation, decapping, and exonuclease-catalyzed decay. Most of the early examples of endonuclease decay came from studies of a particular mRNA whose turnover changed in response to hormone, cytokine, developmental, or nutritional stimuli. Only a few of these examples of endonuclease-mediated mRNA decay progressed to the point where the enzyme responsible for the initiating event was identified and studied in detail. The discovery of microRNAs and RISC-catalyzed endonuclease cleavage followed by the identification of PIN (pilT N-terminal) domains that impart endonuclease activity to a number of the proteins involved in mRNA decay has led to a resurgence of interest in endonuclease-mediated mRNA decay. PIN domains show no substrate selectivity and their involvement in a number of decay pathways highlights a recurring theme that the context in which an endonuclease function is a primary factor in determining whether any given mRNA will be targeted for decay by this or the default exonuclease-mediated decay processes.

Polysomal ribonuclease 1 exists in a latent form on polysomes prior to estrogen activation of mRNA decay

Estrogen induces a global change in the translation profile of Xenopus hepatocytes, replacing serum protein synthesis with production of the yolk protein precursor vitellogenin. This is accomplished by the coordinate destabilization of serum protein mRNAs and the transcriptional induction and subsequent stabilization of vitellogenin mRNA. Previous work identified an endonuclease activity whose appearance on polysomes correlated with the disappearance of serum protein mRNAs. This enzyme, polysomal ribonuclease 1 (PMR1), is a novel member of the peroxidase gene family. The current study examined the association of PMR1 with its mRNA targets on polysomes and mRNPs. The highest amount of polysome-bound PMR1 was observed prior to estrogen induction of mRNA decay. Its distribution on sucrose density gradients matched the absorbance profile of polysome-bound mRNA, suggesting that PMR1 forms a latent complex with mRNA. Following dissociation with EDTA the 62 kDa PMR1 sedimented with a larger complex of >670 kDa. Estrogen induces a 22-fold increase in unit enzymatic activity of polysome-bound PMR1, and a time-dependent loss of PMR1 from polysomes in a manner that mirrors the disappearance of albumin mRNA. These data suggest that the key step in the extensive estrogen-induced change in mRNA decay in Xenopus liver is activation of a latent mRNA endonuclease associated with its target mRNA.

A synthetic oestrogen antagonist, tamoxifen, inhibits oestrogen-induced transcriptional, but not post-transcriptional, regulation of gene expression

Oestrogen (E2) regulates the expression of its target genes at transcriptional and post-transcriptional levels. To clarify the mechanism of E2-induced post-transcriptional regulation, with attention to the involvement of the oestrogen receptor (ER), we studied the effect of tamoxifen (TAM), a synthetic E2 antagonist that inhibits ER-mediated transcription, on E2-induced transcriptional and post-transcriptional regulation of the chicken ovalbumin (OVA) gene in chick oviducts. Run-on analysis with oviduct nuclei isolated from E2-treated chicks showed that TAM treatment completely blocked E2-induced transcription of the OVA gene within 24 h without affecting ER gene expression. Likewise, the rate of transcription fell to below the limit of detection after E2 withdrawal from the chicks. Reflecting the transcription rate, OVA mRNA accumulated linearly in E2-treated chicks, and E2 withdrawal caused a rapid loss of OVA mRNA. However, in the chicks treated with TAM and E2, OVA mRNA was degraded slowly over 48 h with a half-life of 24 h, suggesting that TAM does not inhibit E2-induced mRNA stabilization. Moreover, E2-induced mRNA stabilization was observed even when transcription of the OVA gene was blocked by a transcription inhibitor. Western-blot analysis showed that the remaining OVA mRNA was translatable. Thus the present study indicates that E2 regulates expression of the OVA gene via distinct pathways at transcriptional and post-transcriptional levels.

Purification and characterization of an estrogen-regulated Xenopus liver polysomal nuclease involved in the selective destabilization of albumin mRNA

A previous report from this laboratory described an estrogen-regulated endoribonuclease activity on Xenopus liver polysomes which had properties one might expect for a messenger ribonuclease involved in the regulated destabilization of albumin mRNA (Pastori, R. L., Moskaitis, J. E., and Schoenberg, D. R. (1991) Biochemistry 30, 10490-10498). This report describes the purification and properties of this ribonuclease. The purified nuclease fraction contained a doublet of 62 and 64 kDa and a small amount of a 40-kDa peptide. In situ analysis on both denaturing and nondenaturing gels using an albumin transcript as substrate showed all three proteins possess nuclease activity. Peptide mapping and Western blot with a polyclonal antiserum showed the 62- and 64-kDa peptides to be isoforms, and the 40-kDa peptide to be a degradation product of the larger species. Two-dimensional gel electrophoresis further separated the 62- and 64-kDa species into three pairs of proteins, with isoelectric points of 9.6, 9.8, and 9.8. The purified ribonuclease rapidly degraded a full-length albumin transcript, yet had no effect on either a full-length albumin antisense transcript or full-length ferritin transcript. A number of properties of the purified nuclease were characterized, including the effects of salt, divalent cations, EDTA, sulfhydryl reagents, and temperature. Treatment of the polysomal nuclease with micrococcal nuclease had no effect, indicating that this enzyme does not require an RNA cofactor for activity. Finally, primer extension mapped the major cleavage site to an overlapping repeated sequence APyrUGA, with cleavage between and adjacent to the two pyrimidine residues generating fragments with 5'-hydroxyls.

Glucocorticoid receptors in leukemias, lymphomas and myelomas of young and old

In this paper we have briefly reviewed the nature of leukemias and lymphomas in the old and the young. We surveyed in general the ways in which lymphoid cells and other hematologic elements respond to glucocorticoids, mentioning that there may be direct or indirect effects on their growth by these ligands. We have reviewed the current general model for the action of glucocorticoids in all cells, namely the fact that the actions of these steroids are mediated to a large extent through binding with ligand-activated transcription factors, their receptors. The growing wealth of detail about the nature of the interaction of these receptors with regulatory sites in the genome is discussed. Finally, we have described our results with lines of tissue culture cells representing clones from a typical leukemia of the young, and of myeloma, a typical hematologic malignancy of the elderly. Several features of the effects of glucocorticoids on these cells point up areas that would be pertinent to explore in aging and in the relationship of hematologic diseases to survival and response to therapy in the older versus the younger patient.

Differential regulation and polyadenylation of transferrin mRNA in Xenopus liver and oviduct

Estrogen destabilizes transferrin mRNA in male Xenopus liver in the same manner as observed for albumin and gamma-fibrinogen. The present study examined estrogen regulation of transferrin gene expression in female Xenopus liver and oviduct. In female Xenopus liver estrogen causes the same enhanced degradation of transferrin mRNA from the cytoplasm as seen in males. In contrast, transferrin is induced 3- to 4-fold in both oviduct nuclear and cytoplasmic RNA. The similar increase in transferrin RNA in both preparations suggests a transcriptional mechanism is responsible for this stimulation. Therefore, transferrin expression is differentially regulated in these tissues by the same hormone. Previous experiments showed that Xenopus serum albumin mRNA has a very short (17 residue) poly(A) tail that may play a role in its hormone-regulated instability. Transferrin mRNA has a similarly short poly(A) tail in liver of both male and female Xenopus. Estrogen has no effect on transferrin polyadenylation in liver. Similarly short poly(A) is found on transferrin mRNA from estrogen-deprived oviducts in explant culture. However, addition of estradiol to the medium results in the appearance of a 50-200 nucleotide poly(A) concurrent with induction. Therefore, transferrin mRNA is differentially polyadenylated in Xenopus liver and oviduct. In the latter tissue polyadenylation is under hormonal control.

The estrogen-regulated destabilization of Xenopus albumin mRNA is independent of translation

Protein synthesis inhibitors have been shown to increase the stability of a number of labile mRNAs. In Xenopus laevis serum albumin mRNA is destabilized in the liver cell cytoplasm following estrogen administration. The present study examined the effect of translation inhibitors on this process. The initiation inhibitor 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide causes accumulation of albumin mRNA in 20-80S mRNP particles whereas the elongation inhibitor cycloheximide causes albumin mRNA to accumulate in polysomes. Neither inhibitor blocked the disappearance of albumin mRNA from liver cell cytoplasm when added with estradiol to the medium of liver explant cultures. We conclude that unlike a number of labile mRNAs the instability of Xenopus albumin mRNA following estradiol is independent of translation.

Steroid mediated lysis of lymphoblasts requires the DNA binding region of the steroid hormone receptor

Glucocorticoids kill certain types of lymphoblasts, but the mechanisms are unknown. It is clear that sufficient numbers of functional glucocorticoid receptors are required to mediate lysis, but whether they do so through the classical model of steroid hormone activation and modulation of gene expression has not been established. In this report we have asked which region(s) of the steroid receptor are important for mediating lysis in leukemic T lymphoblasts. CEM-ICR 27 leukemic lymphoblasts, a clone of CEM cells which lack functional glucocorticoid receptors and therefore are neither lysed by dexamethasone nor capable of showing glutamine synthetase induction, were provided with steroid receptors by DNA transfections of various receptor gene constructs. We measured steroid mediated lysis, receptor number and induction of glutamine synthetase in the transfected cells. Our results provide evidence that the lysis mechanism in the ICR27 lymphoblasts is restored when functional receptor number is restored. The DNA binding region specifying high affinity for GRE sites is required. Lysis is mediated by any steroid that allows for activation of the receptor containing such a region. Our data support the view that steroid-mediated cell death occurs by a process requiring direct interaction of steroid-receptor complexes with the genome.