Critical role of mitochondria-mediated apoptosis for JNJ-26481585-induced antitumor activity in rhabdomyosarcoma.

JNJ-26481585 is a second-generation histone deacetylase inhibitor with broad-range efficacy and improved pharmacodynamic properties. In the present study, we investigated the therapeutic potential of JNJ-26481585 and its molecular mechanisms of action in rhabdomyosarcoma (RMS). Here, we report that JNJ- 26481585's anticancer activity critically depends on an intact mitochondrial pathway of apoptosis. JNJ-26481585 induces apoptosis and also inhibits long-term clonogenic survival of several RMS cell lines at nanomolar concentrations that cause histone acetylation. Importantly, JNJ-26481585 significantly suppresses tumor growth in vivo in two preclinical RMS models, that is, the chorioallantoic membrane model and a xenograft mouse model. Mechanistically, we identify activation of the mitochondrial pathway of apoptosis as a key event that is critically required for JNJ-26481585-mediated cell death. JNJ-26481585 upregulates expression levels of several BH3-only proteins including Bim, Puma and Noxa, which all contribute to JNJ-26481585-mediated apoptosis, as knockdown of Bim, Puma or Noxa significantly inhibits cell death. This shift toward proapoptotic Bcl-2 proteins promotes activation of Bax and Bak as a critical event, as genetic silencing of Bax or Bak protects against JNJ-26481585-induced apoptosis. Intriguingly, rescue experiments reveal that JNJ-26481585 triggers Bax/Bak activation independently of caspase activation and activates caspase-9 as the initiator caspase in the cascade, as Bcl-2 overexpression, but not the broad-range caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD.fmk) blocks JNJ-26481585-induced Bax/Bak activation and caspase-9 cleavage. In conclusion, JNJ-26481585 exerts potent antitumor activity against RMS in vitro and in vivo by engaging mitochondrial apoptosis before caspase activation and represents a promising therapeutic for further investigation in RMS.

Histone acetyltransferase inhibitors block neuroblastoma cell growth in vivo.

We have previously described novel histone acetyltransferase (HAT) inhibitors that block neuroblastoma cell growth in vitro. Here we show that two selected pyridoisothiazolone HAT inhibitors, PU139 and PU141, induce cellular histone hypoacetylation and inhibit growth of several neoplastic cell lines originating from different tissues. Broader in vitro selectivity profiling shows that PU139 blocks the HATs Gcn5, p300/CBP-associated factor (PCAF), CREB (cAMP response element-binding) protein (CBP) and p300, whereas PU141 is selective toward CBP and p300. The pan-inhibitor PU139 triggers caspase-independent cell death in cell culture. Both inhibitors block growth of SK-N-SH neuroblastoma xenografts in mice and the PU139 was shown to synergize with doxorubicin in vivo. The latter also reduces histone lysine acetylation in vivo at concentrations that block neoplastic xenograft growth. This is one of the very few reports on hypoacetylating agents with in vivo anticancer activity.