Functional genetic screen for genes involved in senescence: role of Tid1, a homologue of the Drosophila tumor suppressor l(2)tid, in senescence and cell survival

We performed a genetic suppressor element screen to identify genes whose inhibition bypasses cellular senescence. A normalized library of fragmented cDNAs was used to select for elements that promote immortalization of rat embryo fibroblasts. Fragments isolated by the screen include those with homology to genes that function in intracellular signaling, cellular adhesion and contact, protein degradation, and apoptosis. They include mouse Tid1, a homologue of the Drosophila tumor suppressor gene l(2)tid, recently implicated in modulation of apoptosis as well as gamma interferon and NF-kappaB signaling. We show that GSE-Tid1 enhances immortalization by human papillomavirus E7 and simian virus 40 T antigen and cooperates with activated ras for transformation. Expression of Tid1 is upregulated upon cellular senescence in rat and mouse embryo fibroblasts and premature senescence of REF52 cells triggered by activated ras. In accordance with this, spontaneous immortalization of rat embryo fibroblasts is suppressed upon ectopic expression of Tid1. Modulation of endogenous Tid1 activity by GSE-Tid1 or Tid1-specific RNA interference alleviates the suppression of tumor necrosis factor alpha-induced NF-kappaB activity by Tid1. We also show that NF-kappaB sequence-specific binding is strongly downregulated upon senescence in rat embryo fibroblasts. We therefore propose that Tid1 contributes to senescence by acting as a repressor of NF-kappaB signaling.     

Cross-hybridizing snake satellite, Drosophila, and mouse DNA sequences may have arisen independently

Previous reports have interpreted hybridization between snake satellite DNA and DNA clones from a variety of distant taxonomic groups as evidence for evolutionary conservation, which implies common ancestry (homology) and/or convergence (analogy) to produce the cross- hybridizing sequences. We have isolated 11 clones from a genomic library of Drosophila melanogaster, using a cloned 2.5-kb snake satellite probe of known nucleotide sequence. We have also analysed published sequence data from snakes, mice, and Drosophila. These data show that (1) all of the cross-hybridization between the snake, fly, and mouse clones can be accounted for by the presence of either of two tandem repeats, [GATA]n and [GACA]n and (2) these tandem repeats are organized differently among the different species. We find no evidence that these sequences are homologous apart from the existence of the simple repeat itself, although their divergence from a common ancestral sequence cannot be ruled out. The sequences contain a variety of homogeneous clusters of tandem repeats of CATA, GA, TA, and CA, as well as GATA and GACA. We suggest that these motifs may have arisen by a self-accelerating process involving slipped-strand mispairing of DNA. Homogeneity of the clusters might simply be the result of a rate of accumulation of tandem repeats that exceeds that of other mutations.      

Large T antigens of simian virus 40 and polyomavirus efficiently establish primary fibroblasts.

Recombinant retroviruses that transduce the simian virus 40 (SV40) large T antigen or the polyomavirus large T antigen as well as encoding resistance to antibiotic G418 were used to investigate whether these genes alone were sufficient for immortalization of primary cells. The results provided definitive evidence that either viral gene can efficiently establish primary fibroblasts. The capability of the SV40 large T antigen to establish primary fibroblasts was undiminished by a mutation that alters its binding to sequences within the origin of replication. Surprisingly, most of the primary cells established by the expression of the SV40 large T antigen did not have a transformed phenotype. This suggests that transformation by SV40 is not simply due to a high level of expression of the SV40 large T antigen and stabilization of cellular p53.     

NMR investigations of protein-carbohydrate interactions: refined three- dimensional structure of the complex between hevein and methyl beta- chitobioside

The specific interaction of hevein with GlcNAc-containing oligosaccharides has been analyzed by1H-NMR spectroscopy. The association constants for the binding of hevein to a variety of ligands have been estimated from1H-NMR titration experiments. The association constants increase in the order GlcNAc-alpha(1-->6)-Man < GlcNAc < benzyl-beta-GlcNAc < p-nitrophenyl-beta-GlcNAc < chitobiose < p- nitrophenyl-beta-chitobioside < methyl-beta-chitobioside < chitotriose. Entropy and enthalpy of binding for different complexes have been obtained from van't Hoff analysis. The driving force for the binding process is provided by a negative DeltaH0which is partially compensated by negative DeltaS0. These negative signs indicate that hydrogen bonding and van der Waals forces are the major interactions stabilizing the complex. NOESY NMR experiments in water solution provided 475 accurate protein proton-proton distance constraints after employing the MARDIGRAS program. In addition, 15 unambiguous protein/carbohydrate NOEs were detected. All the experimental constraints were used in a refinement protocol including restrained molecular dynamics in order to determine the highly refined solution conformation of this protein- carbohydrate complex. With regard to the NMR structure of the free protein, no important changes in the protein nOe's were observed, indicating that carbohydrate-induced conformational changes are small. The average backbone rmsd of the 20 refined structures was 0.055 nm, while the heavy atom rmsd was 0.116 nm. It can be deduced that both hydrogen bonds and van der Waals contacts confer stability to the complex. A comparison of the three-dimensional structure of hevein in solution to those reported for wheat germ agglutinin (WGA) and hevein itself in the solid state has also been performed. The polypeptide conformation has also been compared to the NMR-derived structure of a smaller antifungical peptide, Ac-AMP2.      

Rat embryo fibroblasts immortalized with simian virus 40 large T antigen undergo senescence upon its inactivation.

Introduction of simian virus 40 T antigen into rodent fibroblasts gives rise to cells that can proliferate indefinitely but are dependent upon it for maintenance of their growth once the normal mitotic life span has elapsed. Inactivation of T antigen in these immortalized cells causes rapid and irreversible cessation of growth. To determine whether this growth arrest is associated with entry into senescence, we have undertaken a genetic and biological analysis of conditionally immortal (tsa) cell lines derived by immortalizing rat embryo fibroblasts with the thermolabile tsA58 T antigen. This analysis has identified the following parallels between the tsa cells after inactivation of T antigen and senescent rat embryo fibroblasts: (i) growth arrest is irreversible; (ii) it occurs in G1 as well as G2; (iii) the G1 block can be partially overcome by stimulation with 20% fetal calf serum, but the G2 block cannot be overcome; (iv) 20% fetal calf serum induces c-fos, but c-myc is unaltered; and (v) fibronectin and p21(Waf1/Cip1/Sdi1) are upregulated upon growth arrest. These results suggest that T-antigen-immortalized fibroblasts are committed to undergo senescence but are prevented from undergoing this process by T antigen. Inactivation of T antigen removes this block and results in senescence of the cells. Thus, these cell lines may represent a powerful system for study of the molecular basis of entry into senescence.