In the Fingolimod neuroblastoma cell line BE C, a G0 G1 arrest has been detected after siRNA mediated knockdown of HDAC8. This G0 G1 arrest induced by HDAC8 knockdown was associated with p21 mRNA upregulation. In contrast, no effect on the cell cycle was observed in the hepatocellular carcinoma cell lines BEL 7402 and Hep G2. This observation fits with our own marginal effects after siRNA mediated HDAC8 knockdown. The level of apoptosis induction in BEL 7402 and Hep G2 cells after siRNA mediated targeting of HDAC8 were comparable to the increase of the subG1 fraction in individual urothelial carcinoma cell lines after targeting of HDAC8. Concerning the use of inhibitors, effects of pharmaco logical inhibition on cell cycle distribution by c2 were, as expected, only minor.

In contrast, pharmacological in hibition by c5 or c6 resulted in a significant albeit low increase of the sub G1 fraction in two out of five cell lines and in an apparent G2 M arrest in four out of five cell lines. Consequently, p21 increased in two cell lines and thymidylate synthase decreased in all but one. Conclusions HDAC8 is deregulated in UCCs resulting in variable mRNA and protein expression levels. Suppression and pharmacological inhibition of HDAC8 had significant, but overall minor impacts on cell proliferation, clonogenic growth and migration. These effects were comparable to findings in other cancer entities. Furthermore, pharmacological inhibition of HDAC8 induced a G2 M arrest. However, those effects were observed only at drug concentrations probably not appropriate for the use in patients.

Neither HDAC8 mRNA nor protein ex pression levels were reliable predictive marker for sen sitivity to HDAC8 inhibition. In summary, HDAC8 on its own does not seem to con stitute a promising drug target in bladder cancer. Whether selective HDAC8 inhibition may synergize with either conventional chemotherapeutics or further targeted anti tumoral compounds remains to be further explored. Inter estingly, AV-951 in this respect, the compounds c5 and c6 which are efficient inhibitors of HDAC8 may have additional cellular targets which need to be further elucidated. Background Myb binding protein 1a was originally identi fied as a transcription co repressor that could bind to the negative regulatory domain the c myb protoonco gene product. Mybbp1a has the LXXLL motifs that often mediate interactions between nuclear receptors and their cofactors. Mybbp1a has also been shown to interact with a number of other transcription factors, including PGC 1, RelA/p65, Prep1, Aire, and CRY1, and exert inhibitory effect on their transactivation activity. These findings are highly suggestive of a context dependent co repressor function of Mybbp1a in RNA Pol II transcription.