Adaptive and constitutive activations of antioxidant-producing metabolic pathways in hepatocellular carcinoma

Abstract

Liver cancer, or hepatocellular carcinoma (HCC), is a major health burden worldwide. HCC remains highly resistant to conventional treatments and patients ultimately succumb to liver failure and other HCC-associated complications. HCC cells proliferate at accelerated rates which demand a large amount of anaplerotic nutrients and antioxidants. Increased anaplerotic reactions provide building blocks to support HCC cell division. However, the roles of antioxidants in cancer especially HCC have been controversial. Cancer cells persistently exhibit an increased level of natural metabolic by-product reactive oxygen species (ROS) causing oxidative stress. A long-standing perception of ROS as oncogenic substance dominated the scientific field in the past decades. ROS was found to increase DNA mutations driving cancer cell transformation. This concept has been significantly reformed by a number of research groups including us. We and others have proposed and provided evidence that excessive ROS detrimentally damages the integral components of cancer cells leading to cancer cell death. Cancer cells, in order to survive, acquire increased antioxidant-producing capacity to counteract ROS. There are three major aims of this study. 1) I aimed to provide the first comprehensive overview on the impact of the core antioxidant-producing pathways in HCC. 2) I aimed to identify in detail the molecular mechanisms, adaptive or inherently activated, that drive these antioxidant-producing pathways in HCC. 3) Harnessing knowledge from the above two aims, we endeavoured to identify novel druggable metabolic vulnerabilities in HCC to shed new translational insights into novel HCC therapeutic regimens. NADPH is the key and only molecule that returns thioredoxin (TXN) into reduced state, allowing TXN to stabilize ROS and oxidized molecules. NADPH is generated mainly by pentose phosphate pathway (PPP), folate cycle, and malate enzyme 1 & 3 (ME1 & ME3). Our research group has previously reported that PPP is an important metabolic pathway in HCC and demonstrated that PPP blockade sensitized HCC cells to targeted therapy, sorafenib. PPP is only the tip of the iceberg of the complicated metabolic network but this seminal study in HCC has opened an uncharted research avenue for me to investigate the regulation, impact, clinical and therapeutic implications of all NADPH-producing pathways in HCC development. I unprecedentedly identified that the NADPH-producing pathways and the TXN system are regulated distinctly by two mechanisms. Specifically, methylenetetrahydrofolate dehydrogenase 1-like (MTHFD1L), a crucial mitochondrial enzyme of the folate cycle, cytosolic ME1, and cytosolic thioredoxin reductase 1 (TXNRD1) of the TXN system were all transcriptionally activated by NRF2 which is adaptively stabilized by oxidative stress. The mitochondrial ME3 was however found to be constitutively up-regulated by BRD4-dependent super-enhancers. Inhibition of any components in adaptive and constitutively active antioxidant systems led to dramatic induction of ROS and hindered HCC growth in vitro and in vivo. Excitingly, HCC cells’ sensitivities towards sorafenib treatment were observed to be enhanced. This study highlights the paradoxical utilization of ROS as therapeutic method to combat HCC. My identification of two distinct mechanisms which function alongside to provide HCC cells different protective layers against continuous ROS exposure and sudden episodes of ROS insult.