Variation of the expression of different Histone deacetylases (HDACs) and the functional effects of HDAC-inhibitors in hepatocellular carcinoma

Histone deacetylases (HDACs) comprise in humans currently 18 members divided in 4 classes. Application of HDAC inhibitors (HDACi) appears as promising therapeutic strategy in different types of cancer including hepatocellular cacrinoma (HCC). However, detailed information about distinct HDAC expression and functional mechanisms of HDACi-action in HCC are lacking.

The aim of this study was comprehensively analyze HDAC expression and to functionally analyze the effect of 3 different HDACi in HCC.

Methods and Results:

Quantitative RT-PCR analysis revealed significantly increased expression of (i) HDAC 1/2/3/8 (class I); (ii) HDAC 4/5/7/9 (class IIa); (iii) HDAC 6/10 (class IIb) and (iiii) HDAC 11 (class IV) in 4 human HCC cell lines (Hep3B, HepG2, PLC, HuH7) and 11 human HCC tissues compared to primary human hepatocytes (PHH). Biochemical analysis showed significantly higher HDAC-activity in HCC cells compared to PHH. In human HCC samples, expression levels of individual HDACs varied significantly but particularly HDACs of classes IIa showed significant correlations, i.e. it appeared that there are "high" and "low" HDAC-expressers. TCGA (The Cancer Genome Atlas) data set analysis of 377 HCC patients revealed that high HDAC1/2/8, (Class I), HDAC4/7/9 (Class IIa) or HDAC6 (Class IIb) correlated with poor patient survival. Next, we analyzed the effects of 3 different HDACi: SAHA (irreversible inhibitor of predominantly Class-I), trichostatin A (TSA; reversible inhibitor of foremost Class-IIb; less effective against Class-I, with the exception of HDAC5), and trapoxin (TPX; irreversible pan-inhibitor acting at nanomolar concentrations). All 3 HDACi caused dose-dependent toxicity in HCC cells, while the same doses did not exhibit any toxicity in PHH. Functional analysis of HDACi in sub-toxic doses showed a dose-dependent reduction of proliferation, with TSA and TPX inducing a complete growth arrest, while SAHA inhibited proliferation only approximately 50%. Furthermore, all 3 HDACi reduced the migratory potential and clonogenicity of HCC cells but also this with different efficacy. Moreover, combination with HDACi enhanced the efficacy of sorafenib in killing sorafenib susceptible cells. Furthermore, treatment with HDACi reestablished sorafenib sensitivity in resistant HCC cells.

Conclusion:

HDACs are significantly but differently increased in HCC cells and tissues and promote different facets of tumorigenicity of HCC cells in vitro. HDACi showed qualitatively similar but quantitatively different inhibitory effects on HCC cells in vitro, which may be exploit to develop more targeted therapeutic approaches. Further, combination with HDACi with sorafenib appears as novel approach to enhance efficacy of sorafenib and to break sorafenib resistance of HCC cells.