Selective inhibition of HDAC8 decreases neuroblastoma growth in vitro and in vivo and enhances retinoic acid-mediated differentiation

Abstract

For differentiation-defective malignancies, compounds that modulate transcription, such as retinoic acid and histone deacetylase (HDAC) inhibitors, are of particular interest. HDAC inhibitors are currently under investigation for the treatment of a broad spectrum of cancer diseases. However, one clinical drawback is class-specific toxicity of unselective inhibitors, limiting their full anticancer potential. Selective targeting of individual HDAC isozymes in defined tumor entities may therefore be an attractive alternative treatment approach. We have previously identified HDAC family member 8 (HDAC8) as a novel target in childhood neuroblastoma. Using small-molecule inhibitors, we now demonstrate that selective inhibition of HDAC8 exhibits antineuroblastoma activity without toxicity in two xenograft mouse models of MYCN oncogene-amplified neuroblastoma. In contrast, the unselective HDAC inhibitor vorinostat was more toxic in the same models. HDAC8-selective inhibition induced cell cycle arrest and differentiation in vitro and in vivo. Upon combination with retinoic acid, differentiation was significantly enhanced, as demonstrated by elongated neurofilament-positive neurites and upregulation of NTRK1. Additionally, MYCN oncogene expression was downregulated in vitro and tumor cell growth was markedly reduced in vivo. Mechanistic studies suggest that cAMP-response element-binding protein (CREB) links HDAC8- and retinoic acid-mediated gene transcription. In conclusion, HDAC-selective targeting can be effective in tumors exhibiting HDAC isozyme-dependent tumor growth in vivo and can be combined with differentiation-inducing agents.

The HDAC6/8/10 inhibitor TH34 induces DNA damage-mediated cell death in human high-grade neuroblastoma cell lines

“…4e). Low-micromolar doses of TH34 enhanced ATRA-induced morphological features of differentiation in a similar manner as we had previously described with the selective HDAC8 inhibitor PCI-34051 (Rettig et al 2015) (Supplementary Fig. 3a).…”

“…HDAC8 has been identified to play a role in maintenance of an undifferentiated cellular phenotype (Rettig et al 2015). We thus investigated whether TH34 treatment would enhance expression of neuronal differentiation markers.…”

Structure of ‘linkerless’ hydroxamic acid inhibitor-HDAC8 complex confirms the formation of an isoform-specific subpocket

“…While they both appear to bind to the catalytic zinc ion, Compound 4 is more sterically hindered about the alpha carbon, which may hinder its binding in the active site. While the docking studies predict that neither Compounds 1 nor 2 bind to the catalytic zinc ion, both have modest IC 50 values (20.0 and 14.0 µM, respectively), and Compound 2 was recently used to inhibit the growth of neuroblastoma cells, as discussed above (Rettig et al, 2015). …”

“…These compounds were indeed potent inhibitors of HDAC8, and demonstrated selectivity for HDAC8 over HDACs 1 and 6. A recent study verified the use of Compound 2 for the selective inhibition of HDAC8 in the treatment of neuroblastoma cell growth (Rettig et al, 2015). Compound 2 was well tolerated in mouse models, while broad-spectrum Vorinostat caused severe side effects, including diarrhea and weight loss.…”

HDAC8: a multifaceted target for therapeutic interventions

“…In neural crest-derived neuroblastoma, HDAC8 expression correlates with advanced tumor stage and poor outcome, and selective inhibition induces differentiation, shown by the outgrowth of neurite-like structures [44]. HDAC8 inhibitors showed advantages over pan-HDAC inhibition with regard to toxicity and efficacy in a preclinical neuroblastoma model [21,50,51]. Targeting of other HDAC isotypes in neuroblastoma results in completely different phenotypes (apoptosis or blocked autophagy) [52][53][54].…”