Functional roles and therapeutic implications of the histone chaperone FACT complex in hepatocellular carcinoma

Abstract

The high-order chromatin structure determines the genome architecture and gene expression of eukaryotic cells. The nucleosome, which consists of 146bp of DNA wrapping around a histone octamer (one H3–H4 tetramer and two H2A–H2B dimers), is the fundamental building block of chromatin. Active disassembly, reassembly and repositioning of nucleosomes render spatial and temporal controls of chromatin accessibility for RNA polymerases and regulatory factors. These dynamic changes are regulated by several factors, including ATP-dependent chromatin remodelers, histone-modifying enzymes and an increasing list of histone chaperones.

Histone chaperones are a group of proteins participated in the dynamic nucleosome assembly and disassembly processes during DNA replication, DNA repair and gene transcription. However, their roles in cancer development remain poorly understood. In this study, we interrogated the expression of all known histone chaperones in human liver cancer (hepatocellular carcinoma, HCC) and found that the two subunits of the FACT (Facilitates Chromatin Transcription) complex, SSRP1 and SUPT16H, were remarkably upregulated in HCC and associated with poor survivals of HCC patients. We further utilized CRISPR-based gene activation and knockout systems to demonstrate the oncogenic functions of the histone chaperone FACT complex with in vitro and in vivo models. Over expression of the FACT complex promoted HCC growth and metastasis, while knockout of the FACT complex suppressed tumor initiation and progression.

Interestingly, we unprecedentedly revealed an indispensable role of the FACT complex in HCC oxidative stress and hypoxia responses. Cancer cells are undergoing various intrinsic and extrinsic stresses trigged by metabolic programming, altered microenvironment and drug treatment. Cancer cells adapt to oxidative stress and hypoxia through master regulator NRF2 (NFE2-related factor 2) and HIFs (Hypoxia-inducible factors) to initiate the transcription of a myriad of genes that enabling cancer cells survive under stress conditions. Oxidative stress and hypoxia relieved the FACT complex from E3 ligase KEAP1 and VHL mediated protein ubiquitination and degradation, respectively. The stabilized FACT complex translocated into the nucleus and mediated nucleosome disassembly that enables the expeditious passage of RNA Polymerase II through chromatin during transcription elongation. Knockout of the FACT complex impeded transcription elongation and abolished the induction of oxidative stress and hypoxia responses genes that are crucial for cancer cell to survive under stress conditions. Metabolic tracing confirmed that loss of the FACT complex suppressed glycolytic flux and impaired lactate extrusion, leading to intracellular acidification and apoptosis of hypoxic cancer cells.

Therapeutically, Curaxin, a small-molecule inhibitor of the FACT complex effectively suppressed HCC growth both in vitro and in vivo. Treatment of Curaxin also blocked the gene expressions for oxidative stress and hypoxia responses in cancer cells. In addition, hepatic artery ligation surgery and tyrosine kinase inhibitors treatment adversely induce intratumoral hypoxia and ROS in HCC, while co-treatment of Curaxin remarkably hinders the growth of the hypoxic tumors when compared to single treatment.

In summary, this study delineated stress adaptation functions of the histone chaperone FACT complex in oxidative stress and hypoxia responses. Meanwhile, this study highlighted the therapeutic value of the FACT complex inhibitors in HCC treatment.