The clinical relevance and roles of NADH dehydrogenase (ubiquinone) 1 alpha subcomplex 4-like 2 in hepatocellular carcinoma progression

Abstract

Hepatocellular Carcinoma (HCC) is the fifth most common and the second most lethal malignancy in the world. Sorafenib is the only FDA-approved medication for advanced HCC with the average of 3 months survival benefit. Liver is a major metabolic organ, yet the detailed metabolic alterations driving HCC remain elusive. The rapidly growing nature of HCC results in oxygen deprivation, i.e. hypoxia, in regions of tumors with insufficient blood supply. Hypoxia stabilizes hypoxia-inducible factor (HIF) which transcriptionally activates a battery of genes that favor glycolysis over oxidative phosphorylation as the major ATP-producing pathway. HIF therefore acts as a master metabolic regulator which enables cancer cells to efficiently produce energy and macromolecules, as well as to counteract oxidative stress under hypoxic condition.

In this study, it was demonstrated that among all the mitochondrial subunits in the complex I of the electron transport chain (ETC), only the subunit NADH dehydrogenase (ubiquinone) 1 alpha subcomplex 4-like 2 gene (NDUFA4L2) was uniquely and drastically induced by hypoxia. Clinically, NDUFA4L2 was found to be substantially overexpressed in human HCC and other human solid cancers. Overexpression of NDUFA4L2 in human HCC was closely associated with more aggressive clinicopathological behaviors including presence of tumor microsatellite formation and absence of tumor encapsulation. Overexpression of NDUFA4L2 was also associated with poorer overall survival in HCC patients, indicating its potential role as a prognostic marker for HCC patients.

Mechanistically, ChIP assay confirmed binding of HIF-1α to the hypoxia response elements (HREs) in the promoter of NDUFA4L2. Genetic knockdown by short hairpin RNA (shRNA), knockout using gene editing by transcription activator-like effector nuclease (TALEN), and pharmacologic inhibition of HIF-1α(digoxin and sorafenib) consistently suppressed NDUFA4L2 expression in hypoxia, suggesting NDUFA4L2 was a direct transcriptional target of HIF-1α.

Functionally, genetic ablation of HIF-1α/NDUFA4L2 pathway in HCC cells increased mitochondrial complex I activity, mitochondrial membrane potential, and oxygen consumption. Moreover, knockdown of NDUFA4L2 drastically induced intracellular reactive oxygen species (ROS) and ROS-mediated apoptosis. Employment of mechanistically different antioxidants (N-acetyl-cysteine and ascorbic acid) significantly reduced the ROS level in NDUFA4L2 knockdown HCC cells.

More importantly, knockdown of NDUFA4L2 markedly suppressed HCC growth in vitro and in vivo, while antioxidant restored cell proliferative defect of NDUFA4L2 knockdown HCC cells. Furthermore, HIF inhibitors drastically induced intracellular ROS level and suppressed the growth of tumors that expressed high level of NDUFA4L2 in orthotopic HCC xenograft model.

Taken together, the findings have delineated the clinical relevance and oncogenic roles of NDUFA4L2 in promoting HCC survival under hypoxia. Targeting HIF-1_/NDUFA4L2 could dismantle the metabolic and survival adaptation of HCC cells, paving the way to new HCC therapeutic regimens involving HIF inhibitors.