PDA or pancreatic ductal adenocarcinoma belongs to the class of pancreatic cancer that represents the second most common cause of cancer-related death worldwide. It is found that more than 95 % of the patients with PDA harbor activating mutations in the Kirsten Rat Sarcoma Viral Oncogene Homology (KRAS).
KRAS mutation is one of the challenging targets for chemotherapeutics. Therefore, a new approach has been established that alters the glutamine metabolism in the PDA driven by the KRAS mutation. For example, PDA cells generate plenty of ribose from the non-oxidative phase of the pentose phosphate pathway to synthesize nucleotides. Because of that, the production of NADPH through the oxidative phase of the pentose phosphate pathway halts.
Glutamine metabolism in PDA
To compensate for the production of NADPH and redox state of the cell, PDA cells utilize glutamine through a pathway that requires three different enzymes. These enzymes are mitochondrial glutaminase (GLS1), mitochondrial glutamate oxaloacetate transaminase-2 (GOT2) and cytosolic glutamate oxaloacetate transaminase-1 (GOT1). However, this is in contrast with the metabolism of glutamate derived from glutamine that provides a carbon skeleton to the citric acid cycle.
Therefore, inhibition of enzymes of glutamine metabolism can inhibit the PDA cell growth significantly without causing cytotoxicity. This result reveals that the induction of redox balance with concurrent inhibition of KRAS-dependent glutamine metabolism pathway may provide a new approach to induce tumor-selective killing.
Targeting glutamine metabolism in PDA
To inhibit the glutamine metabolism in PDA cells, researchers have devised a small molecule that can inhibit the GLS1 (e.g., bis-2-(5-phenylacetamido-1,2,4-thiadiazol-2-yl)ethyl sulfate (BPTES) and CB-839). GLS1 catalyzes the first step of the glutamine metabolic pathway in which glutamine is converted to glutamate and its inhibition in the culture of the PDA cells results in the blockage of the glutamine metabolism with no cytotoxicity.
In addition to this, GLS1 inhibitors are also potent inhibitors of cell proliferation in cell culture models. However, they have slightly less effect on tumor growth in the cancer model.
Activation of NADPH: quinone oxidoreductase-1
Therefore, to increase the specificity and efficacy of the GLS1 inhibition, researchers combined BPTES with β-lapachone (β-lap). β-lap is a targeted cancer chemotherapeutic agent that causes tumor-selective reactive oxygen species (ROS) formation through NADPH: quinone oxidoreductase-1 (NQO1) and is expressed in many types of cancers including PDA.
β-lap is a substrate for the oxidoreduction reactions catalyzed by NQO1 and produces two moles of the superoxides per mole of NADPH. This reaction leads to a rapid formation of a futile cycle inside the NQO1 that causes the formation of large amounts of ROS. Production of a large amount of ROS causes oxidative DNA damage including H2O2-mediated DNA single-strand breaks.
To repair these damages, poly (ADP-ribose) polymerase PARP becomes hyperactivated and generates extensive free branched poly (ADP-ribose) (PAR) polymers. Hyperactivated PARP decreases the NAD+ and ATP pools significantly and overwhelms the ability of the DNA repair mechanism to repair the β-lap induced DNA damage.
Simultaneous activation of NADPH: quinone oxidoreductase-1 and inhibition of glutamine metabolism
β-lap and GLS1 inhibitors have a distinct mechanism but are highly complementary to each other. β-lap induces the selective ROS formation in the tumor by expressing a high level of NQO1 while GLS1 inhibition directs the PDA cancer cells for death by lowering antioxidant (NADPH) pools derived from glutamine metabolism and sensitivity cells to ROS damage.
In an in vivo pre-clinical trial on the model of PDA, researchers have shown that the increase in the dependency of PDA cells on glutamine metabolism can be targeted specifically by exposing those cells to both chemotherapeutics (BPTES) and β-lap. Therefore, the use of β-lap and BPTES (GLS1 inhibitors) together leads to a synergistic NQO1 and PARP-dependent cancer cell death and also allows the use of lower doses and shorter treatment time for both agents.
Reference: Cancer and Metabolism (Targeting glutamine metabolism sensitizes pancreatic cancer to PARP-driven metabolic catastrophe induced by β-lapachone)
Article DOI: 10.1186/s40170-015-0137-1