Enforced expression of the antiapoptotic Bcl-2 family protein Mcl-1 promotes lymphomagenesis in the mouse; however, the functional role of Mcl-1 in human B-cell lymphoma remains unclear. We demonstrate that Mcl-1 is widely expressed in malignant B-cells, and high-level expression of Mcl-1 is required for B-lymphoma cell survival, since transfection of Mcl-1-specific antisense oligodeoxynucleotides was sufficient to promote apoptosis in Akata6 lymphoma cells. Mcl-1 was efficiently cleaved by caspases at evolutionarily conserved aspartic acid residues in vitro, and during cisplatin-induced apoptosis in B-lymphoma cell lines and spontaneous apoptosis of primary malignant B-cells. Overexpression of the Mcl-1 cleavage product that accumulated during apoptosis was sufficient to kill cells. Therefore, Mcl-1 is an essential survival molecule for B-lymphoma cells and is cleaved by caspases to a death-promoting molecule during apoptosis. In contrast to Mcl-1, Bcl-2 and Bcl-X L were relatively resistant to caspase cleavage in vitro and in intact cells. Interfering with Mcl-1 function appears to be an effective means of inducing apoptosis in Mcl-1-positive B-cell lymphoma, and the unique sensitivity of Mcl-1 to caspasemediated cleavage suggests an attractive strategy for converting it to a proapoptotic molecule.

The total synthesis of spiruchostatin A was accomplished, unambiguously confirming its structure. Key steps included the use of the Nagao thiazolidinethione auxiliary for a diastereoselective acetate aldol reaction and as an activated acylating agent for amide formation, and macrolactonization by the Yamaguchi protocol. Spiruchostatin A is shown to have biological activity similar to that of FK228, a potent histone deacetylase (HDAC) inhibitor in clinical trials. The spiruchostatin A analogue, epimeric at the beta-hydroxy acid, is inactive, highlighting the importance of stereochemistry at this position for interactions with HDACs.

The FK228 and spiruchostatin bicyclic depsipeptide natural products are among the most potent histone deacetylase (HDAC) inhibitors known. Although FK228 is in advanced clinical trials, the complexity of the natural products has precluded mechanistic studies and the discovery of structure-activity relationships. By total synthesis, we have prepared the first depsipeptide analogues. Our results prove that the dehydrobutyrine residue in FK228 is not essential, and other residues can be substituted without loss of HDAC inhibitory activity. Conformational restriction by the macrocyclic scaffold is important, as a linear peptide was inactive. The intramolecular disulfide formed with a cysteine side chain can be removed provided the zinc-binding thiol is protected to ensure good cellular availability. Like the natural products, the analogues are selective against class I isoforms, with nanomolar inhibition of class I HDAC1 and significantly less potency against class II HDAC6.Eukaryotic DNA is packaged together with histone and nonhistone proteins into the higher order structure of chromatin. Chromatin proteins feature extensive posttranslational modification 1 including methylation, acetylation, phosphorylation, ubiquitinylation, sumoylation, and poly-ADPribosylation. These dynamic alterations are believed to constitute a 'histone code' 2 that mediates protein-DNA and protein-protein interactions in chromatin and thereby ultimately regulates gene transcription. Consequently, the modulation of the chromatin histone code offers opportunities for targeting diverse disease states at a higher level of intervention than individual signal transduction pathways. Among the enzymes involved in chromatin remodeling, zinc-dependent histone deacetylases (HDACs a ) are currently at the most advanced stage in drug discovery, with one FDA approval and numerous other compounds in clinical development. These metalloenzymes catalyze the hydrolysis of acetyllysine residues back to lysine. Since the latter, unlike acetyllysine, is protonated at physiological pH, this switch has a dramatic effect on chromatin structure and recruitment of binding partners. Furthermore, reversible lysine acetylation is now recognized 3 as a general posttranslational modification with important functional consequences in nuclear and cytoplasmic proteins unrelated to chromatin, thus increasing the therapeutic importance of this event.HDAC inhibitor design has concentrated 4 on the class I and class II zinc-dependent enzymes, comprising HDACs 1-11 in the human genome. These hydrolases share a highly conserved catalytic domain containing an active site zinc ion, and the majority of inhibitors fall into five broad structural classes (Figure 1): short chain aliphatic carboxylic acids, hydroxamic acids, benzamides, cyclic peptides, and the depsipeptides. HDAC inhibitors are typically substrate mimics of the linear acetyllysine side chain with a zinc-binding group replacing the scissile acetamide and a 'cap' at the other end extending beyond the enzyme substrate-bi...