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Members of the Drosophila HeT-A family of transposable elements are LINE-like retroposons that are found at telomeres and in centric heterochromatin. We recently characterized an active HeT-A element that had transposed to a broken chromosome end fewer than mine generations before it was isolated. The sequence arerangement of this element, called 9D4, most likely represents the organization of an actively transposing member of the HeT-A family. Here we assess the degree of divergence among members of the HeT-A family and test a model of telomere length maintenance based on HeT-A transposition. The region containing the single open reading frame of this element appears to be more highly conserved than the non-coding regions. The HeT-A element has been implicated in the Drosophila telomere elongation process, because frequent transpositions to chromosome ends are sufficient to counter-balance nucleotide loss due to incomplete DNA replication. The proposed elongation model and the hypothetical mechanism of HeT-A transposition predict a predominant orientation of HeT-A elements with their oligo (A) tails facing proximally at chromosome ends, as well as the existence of irregular tandem arrays of HeT-A elements at chromosome ends resulting from transposition of new HeT-A elements onto chromosome ends with existing elements. Twenty-nine different HeT-A fragments were isolated from directional libraries that were enriched in terminal DNA fragments. Sequence analyses of these fragments and comparisons with the organization of the HeT-A element, 9D4, fit these two predictions and support the model of Drosophila telomere elongation by transposition of HeT-A elements. [ABSTRACT FROM AUTHOR]

The D-group cyclins play a key role in the progression of cells through the G(1) phase of the cell cycle. Treatment of MCF-7 breast cancer cells with the cyclopentenone prostaglandin 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)) results in rapid down-regulation of cyclin D1 protein expression and growth arrest in the G(0)/G(1) phase of the cell cycle. 15d-PGJ(2) also down-regulates the expression of cyclin D1 mRNA; however, this effect is delayed relative to the effect on cyclin D1 protein levels, suggesting that the regulation of cyclin D1 occurs at least partly at the level of translation or protein turnover. Treatment of MCF-7 cells with 15d-PGJ(2) leads to a rapid increase in the phosphorylation of protein synthesis initiation factor eukaryotic initiation factor 2alpha (eIF-2alpha) and a shift of cyclin D1 mRNA from the polysome-associated to free mRNA fraction, indicating that 15d-PGJ(2) inhibits the initiation of cyclin D1 mRNA translation. The selective rapid decrease in cyclin D1 protein accumulation is facilitated by its rapid turnover (t(1/2) = 34 min) after inhibition of cyclin D1 protein synthesis. The half-life of cyclin D1 protein is not significantly altered in cells treated with 15d-PGJ(2). Treatment of cells with 15d-PGJ(2) results in strong induction of heat shock protein 70 (HSP70) gene expression, suggesting that 15d-PGJ(2) might activate protein kinase R (PKR), an eIF-2alpha kinase shown previously to be responsive to agents that induce stress. 15d-PGJ(2) strongly stimulates eIF-2alpha phosphorylation and down-regulates cyclin D1 expression in a cell line derived from wild-type mouse embryo fibroblasts but has an attenuated effect in PKR-null cells, providing evidence that PKR is involved in mediating the effect of 15d-PGJ(2) on eIF-2alpha phosphorylation and cyclin D1 expression. In summary, treatment of MCF-7 cells with 15d-PGJ(2) results in increased phosphorylation of eIF-2alpha and inhibition of cyclin D1 mRNA translation initiation. At later time points, repression of cyclin D1 mRNA expression may also contribute to the decrease in cyclin D1 protein.