Chimeric HDAC and the cytoskeleton inhibitor broxbam as a novel therapeutic strategy for liver cancer

Broxbam, also known as N-hydroxy-4-{1-methoxy-4-[4'-(3'-bromo-4',5'-dimethoxyphenyl)-oxazol-5'-yl]-2-phenoxy} butanamide, is a novel chimeric inhibitor that contains two distinct pharmacophores in its molecular structure. It has been previously demonstrated to inhibit the activity of histone deacetylases (HDAC) and tubulin polymerisation, two critical components required for cancer growth and survival. In the present study, the potential suitability of broxbam for the treatment of liver cancer was investigated. The effects of broxbam on cell proliferation and apoptosis, in addition to the underlying molecular mechanism of action, were first investigated in primary liver cancer cell lines Huh7, HepG2, TFK1 and EGI1. Real-time proliferation measurements made using the iCELLigence system and viable cell number counting following crystal violet staining) revealed that broxbam time- and dose-dependently reduced the proliferation of liver cancer cell lines with IC50 values <1 µM. In addition, a significant inhibition of the growth of hepatoblastoma microtumours on the chorioallantoic membranes (CAM) of fertilised chicken eggs by broxbam was observed according to results from the CAM assay, suggesting antineoplastic potency in vivo. Broxbam also exerted apoptotic effects through p53- and mitochondria-driven caspase-3 activation in Huh7 and HepG2 cells according to data from western blotting (p53 and phosphorylated p53), mitochondrial membrane potential measurements (JC-1 assay) and fluorometric capsase-3 measurements. Notably, no contribution of unspecific cytotoxic effects mediated by broxbam were observed from LDH-release measurements. HDAC1, -2, -4 and -6 expression was measured by western blotting and the HDAC inhibitory potency of broxbam was next evaluated using subtype-specific HDAC enzymatic assays, which revealed a largely pan-HDAC inhibitory activity with the most potent inhibition observed on HDAC6. Silencing HDAC6 expression in Huh7 cells led to a drop in the expression of the proliferation markers Ki-67 and E2F3, suggesting that HDAC6 inhibition by broxbam may serve a predominant role in their antiproliferative effects on liver cancer cells. Immunofluorescence staining of cytoskeletal proteins (α-tubulin & actin) of broxbam-treated HepG2 cells revealed a pronounced inhibition of tubulin polymerisation, which was accompanied by reduced cell migration as determined by wound healing scratch assays. Finally, data from zebrafish angiogenesis assays revealed marked antiangiogenic effects of broxbam in vivo, as shown by the suppression of subintestinal vein growth in zebrafish embryos. To conclude, the pleiotropic anticancer activities of this novel chimeric HDAC- and tubulin inhibitor broxbam suggest that this compound is a promising candidate for liver cancer treatment, which warrants further pre-clinical and clinical evaluation.