Role of Caspases, Bid, and p53 in the Apoptotic Response Triggered by Histone Deacetylase Inhibitors Trichostatin-A (TSA) and Suberoylanilide Hydroxamic Acid (SAHA)

Histone deacetylase activity is potently inhibited by hydroaximc acid derivatives such as suberoylanilide hydroxamic acid (SAHA) and trichostatin-A (TSA). These inhibitors specifically induce differentiation/apoptosis of transformed cells in vitro and suppress tumor growth in vivo. Because of its low toxicity, SAHA is currently evaluated in clinical trials for the treatment of cancer. SAHA and TSA induce apoptosis, which is characterized by mitochondrial stress, but so far, the critical elements of this apoptotic program remain poorly defined. To characterize in more detail this apoptotic program, we used human cell lines containing alterations in important elements of apoptotic response such as: p53, Bcl-2, caspase-9, and caspase-3. We demonstrate that caspase-9 is critical for apoptosis induced by SAHA and TSA and that efficient proteolytic activation of caspase-2, caspase-8, and caspase-7 strictly depends on caspase-9. Bcl-2 efficiently antagonizes cytochrome c release and apoptosis in response to both histone deacetylase inhibitors. We provide evidences that translocation into the mitochondria of the Bcl-2 family member Bid depends on caspase-9 and that this translocation is a late event during TSA-induced apoptosis. We also demonstrate that the susceptibility to TSA- and SAHA-induced cell death is regulated by p53

Phloem cytochemical modification and gene expression following the recovery of apple plants from apple proliferation

Recovery of apple trees from apple proliferation was studied by combining ultrastructural, cytochemical, and gene expression analyses to possibly reveal changes linked to recovery-associated resistance. When compared with either healthy or visibly diseased plants, recovered apple trees showed abnormal callose and phloem-protein accumulation in their leaf phloem. Although cytochemical localization detected Ca2+ ions in the phloem of all the three plant groups, Ca2+ concentration was remarkably higher in the phloem cytosol of recovered trees. The expression patterns of five genes encoding callose synthase and of four genes encoding phloem proteins were analyzed by quantitative real-time reverse transcription- polymerase chain reaction. In comparison to both healthy and diseased plants, four of the above nine genes were remarkably upregulated in recovered trees. As in infected apple trees, phytoplasma disappear from the crown during winter, but persist in the roots, and it is suggested that callose synthesis/deposition and phloem-protein plugging of the sieve tubes would form physical barriers preventing the recolonization of the crown during the following spring. Since callose deposition and phloem-protein aggregation are both Ca2+-dependent processes, the present results suggest that an inward flux of Ca2+ across the phloem plasma membrane could act as a signal for activating defense reactions leading to recovery in phytoplasma-infected apple trees.L'articolo é disponibile sul sito dell'editore: http://www.apsjournals.apsnet.or

Caspase-dependent Regulation of Histone Deacetylase 4 Nuclear-Cytoplasmic Shuttling Promotes Apoptosis

Histone deacetylases (HDACs) are important regulators of gene expression as part of transcriptional corepressor complexes. Here, we demonstrate that caspases can repress the activity of the myocyte enhancer factor (MEF)2C transcription factor by regulating HDAC4 processing. Cleavage of HDAC4 occurs at Asp 289 and disjoins the carboxy-terminal fragment, localized into the cytoplasm, from the amino-terminal fragment, which accumulates into the nucleus. In the nucleus, the caspase-generated fragment of HDAC4 is able to trigger cytochrome c release from mitochondria and cell death in a caspase-9–dependent manner. The caspase-cleaved amino-terminal fragment of HDAC4 acts as a strong repressor of the transcription factor MEF2C, independently from the HDAC domain. Removal of amino acids 166–289 from the caspase-cleaved fragment of HDAC4 abrogates its ability to repress MEF2 transcription and to induce cell death. Caspase-2 and caspase-3 cleave HDAC4 in vitro and caspase-3 is critical for HDAC4 cleavage in vivo during UV-induced apoptosis. After UV irradiation, GFP-HDAC4 translocates into the nucleus coincidentally/immediately before the retraction response, but clearly before nuclear fragmentation. Together, our data indicate that caspases could specifically modulate gene repression and apoptosis through the proteolyic processing of HDAC4