Transcriptional control at regulatory checkpoints by histone deacetylases: molecular connections between cancer and chromatin

Cancer cells exhibit a set of unique properties that distinguish them from their normal counterparts. Among these features are increased growth rates, loss of differentiation, escape from cell death pathways, evasion of anti-proliferative signals, a decreased reliance on exogenous growth factors and escape from replicative senescence. Acquisition of these features by malignant cells requires impairment of normal cellular control mechanisms. Over the past few years, it has become increasingly apparent that an important subset of the molecular changes commonly found in cancer cells involves inappropriate regulation of gene expression. This review will address regulatory pathways whose disruption contributes to the malignant phenotype. The failure to deacetylate and thus repress transcription by the Class I histone deacetylases HDAC1 and HDAC2 due to disruption of the Rb family of proteins has been firmly established as a mechanism leading to increases in growth rate and cellular proliferation. Recent data suggest that this regulatory circuit also executes G(1) checkpoint arrest downstream of DNA damage, cellular senescence and contact inhibition. In contrast to this failure to deacetylate, it now seems probable that changes in differentiation status may result in part from inappropriate deacetylation and concomitant transcriptional repression mediated by the Class II histone deacetylases. This inappropriate deacetylation by HDAC4, HDAC5 and HDAC6 follows their relocalization from the cytoplasm to the nucleus. Thus, multiple classical features of cancer cells can be manifested by improper histone deacetylation.