The human HDAC (histone deacetylase) family, a well-validated anticancer target, 
plays a key role in the control of gene expression through regulation of 
transcription. While HDACs can be subdivided into three main classes, the class 
I, class II and class III HDACs (sirtuins), it is presently unclear whether 
inhibiting multiple HDACs using pan-HDAC inhibitors, or targeting specific 
isoforms that show aberrant levels in tumours, will prove more effective as an 
anticancer strategy in the clinic. To address the above issues, we have tested a 
number of clinically relevant HDACis (HDAC inhibitors) against a panel of rhHDAC 
(recombinant human HDAC) isoforms. Eight rhHDACs were expressed using a 
baculoviral system, and a Fluor de Lystrade mark (Biomol International) HDAC 
assay was optimized for each purified isoform. The potency and selectivity of 
ten HDACs on class I isoforms (rhHDAC1, rhHDAC2, rhHDAC3 and rhHDAC8) and class 
II HDAC isoforms (rhHDAC4, rhHDAC6, rhHDAC7 and rhHDAC9) was determined. MS-275 
was HDAC1-selective, MGCD0103 was HDAC1- and HDAC2-selective, apicidin was 
HDAC2- and HDAC3-selective and valproic acid was a specific inhibitor of class I 
HDACs. The hydroxamic acid-derived compounds (trichostatin A, NVP-LAQ824, 
panobinostat, ITF2357, vorinostat and belinostat) were potent pan-HDAC 
inhibitors. The growth-inhibitory effect of the HDACis on HeLa cells showed that 
both pan-HDAC and class-I-specific inhibitors inhibited cell growth. The results 
also showed that both pan-HDAC and class-I-specific inhibitor treatment resulted 
in increased acetylation of histones, but only pan-HDAC inhibitor treatment 
resulted in increased tubulin acetylation, which is in agreement with their 
activity towards the HDAC6 isoform.