| Histone deacetylases (HDACs) are considered to be among the most promising targets in drug development for cancer therapy, and first-generation histone deacetylase inhibitors (HDACi) are currently being tested in phase I/II clinical trials. A wide-ranging knowledge of the role of HDACs in tumorigenesis, and of the action of HDACi, has been achieved. However, several basic aspects are not yet fully understood. Investigating these aspects in the context of what we now understand about HDACi action both in vitro and in vivo will further improve the design of optimized clinical protocols.Histone deacetylases (HDACs) have been intensively scrutinized over the past few years for two main reasons. First, they have been linked mechanistically to the pathogenesis of cancer, as well as several other diseases. Second, small-molecule HDAC inhibitors (HDACi) exist that have the capacity to interfere with HDAC activity and can therefore achieve significant biological effects in preclinical models of cancer. These findings justified the introduction of HDACi into clinical trials. These initial clinical trials have just ended and show encouraging results. At this stage it is crucial to evaluate whether our current knowledge on the mechanisms of tumorigenesis that are linked to HDACs, and on the mechanisms of tumour sensitivity to HDACi, is firm enough to improve the design of further clinical trials. Recent structural and chemical data have emerged that will help in the design of novel HDACi with more desirable properties than the existing ones. However, key areas of investigation that might help to further illuminate the design of successful HDACi-based cancer therapy remain poorly explored. Novel findings indicate that our understanding of how HDACi work will probably change significantly, establishing a new paradigm in the field of intelligent drug design with broad implications for the design of targeted therapies in cancer and possibly other diseases.HDACs: enzymes looking for substrate(s) Four HDAC classes have been identified (FIG. 1a). One of them (class 3 or the so-called sirtuins, from the yeast protein Sir2) constitutes a structurally unrelated, NADdependent subfamily, and will not be considered here; neither will the class 3-specific HDACi, which are less characterized than those for the other classes 1 .An extensive phylogenetic analysis of HDACs has been performed 2,3 . HDACs are members of an ancient enzyme family found in animals, plants, fungi and bacteria. It is thought that HDACs evolved in the absence of histone proteins. Indeed, eukaryotic HDACs can deacetylate non-histone as well as histone substrates, and some HDACs reside in the cytoplasm (where histones are synthesized and acetylated for proper assembly, without the intervention of HDACs) and in mitochondria (where histones are absent).Are HDACs, then, truly HDACs 4 ? We postulate that key HDAC substrates might not be histones, but instead belong to the growing list of acetylated non-histone proteins (FIG. 1b and Supplementary information S1 ...

Early tumorigenesis is associated with the engagement of the DNA-damage checkpoint response (DDR). Cell proliferation and transformation induced by oncogene activation are restrained by cellular senescence. It is unclear whether DDR activation and oncogene-induced senescence (OIS) are causally linked. Here we show that senescence, triggered by the expression of an activated oncogene (H-RasV12) in normal human cells, is a consequence of the activation of a robust DDR. Experimental inactivation of DDR abrogates OIS and promotes cell transformation. DDR and OIS are established after a hyper-replicative phase occurring immediately after oncogene expression. Senescent cells arrest with partly replicated DNA and with DNA replication origins having fired multiple times. In vivo DNA labelling and molecular DNA combing reveal that oncogene activation leads to augmented numbers of active replicons and to alterations in DNA replication fork progression. We also show that oncogene expression does not trigger a DDR in the absence of DNA replication. Last, we show that oncogene activation is associated with DDR activation in a mouse model in vivo. We propose that OIS results from the enforcement of a DDR triggered by oncogene-induced DNA hyper-replication.

Gene mutations in invertebrates have been identified that extend life span and enhance resistance to environmental stresses such as ultraviolet light or reactive oxygen species. In mammals, the mechanisms that regulate stress response are poorly understood and no genes are known to increase individual life span. Here we report that targeted mutation of the mouse p66shc gene induces stress resistance and prolongs life span. p66shc is a splice variant of p52shc/p46shc (ref. 2), a cytoplasmic signal transducer involved in the transmission of mitogenic signals from activated receptors to Ras. We show that: (1) p66shc is serine phosphorylated upon treatment with hydrogen peroxide (H2O2) or irradiation with ultraviolet light; (2) ablation of p66shc enhances cellular resistance to apoptosis induced by H2O2 or ultraviolet light; (3) a serine-phosphorylation defective mutant of p66shc cannot restore the normal stress response in p66shc-/- cells; (4) the p53 and p21 stress response is impaired in p66shc-/- cells; (5) p66shc-/- mice have increased resistance to paraquat and a 30% increase in life span. We propose that p66shc is part of a signal transduction pathway that regulates stress apoptotic responses and life span in mammals.