Genetic alterations in Krebs cycle and its impact on cancer pathogenesis

Highlights

• Cancer cells exhibit alterations in oxygen sensing and energy metabolism.

• Mutations in genes encoding enzymes in TCA cycle are noted in many cancers.

• Abnormalities of Krebs cycle enzymes cause production of oncometabolites.

• Oncometabolites stimulate glutaminolysis, glycolysis and production of ROS.

• Genetic alterations in enzymes in TCA cycle are associated with EMT.

Abstract

Cancer cells exhibit alterations in many cellular processes, including oxygen sensing and energy metabolism. Glycolysis in non-oxygen condition is the main energy production process in cancer rather than mitochondrial respiration as in benign cells. Genetic and epigenetic alterations of Krebs cycle enzymes favour the shift of cancer cells from oxidative phosphorylation to anaerobic glycolysis. Mutations in genes encoding aconitase, isocitrate dehydrogenase, succinate dehydrogenase, fumarate hydratase, and citrate synthase are noted in many cancers. Abnormalities of Krebs cycle enzymes cause ectopic production of Krebs cycle intermediates (oncometabolites) such as 2-hydroxyglutarate, and citrate. These oncometabolites stabilize hypoxia inducible factor 1 (HIF1), nuclear factor like 2 (Nrf2), inhibit p53 and prolyl hydroxylase 3 (PDH3) activities as well as regulate DNA/histone methylation, which in turn activate cell growth signalling. They also stimulate increased glutaminolysis, glycolysis and production of reactive oxygen species (ROS). Additionally, genetic alterations in Krebs cycle enzymes are involved with increased fatty acid β-oxidations and epithelial mesenchymal transition (EMT) induction. These altered phenomena in cancer could in turn promote carcinogenesis by stimulating cell proliferation and survival. Overall, epigenetic and genetic changes of Krebs cycle enzymes lead to the production of oncometabolite intermediates, which are important driving forces of cancer pathogenesis and progression. Understanding and applying the knowledge of these mechanisms opens new therapeutic options for patients with cancer.

Introduction

Mitochondria are the power generator for cells. In the recent years, they have been discovered to possess more complex functions, especially in the pathogenesis of cancer [1]. Also, they are involved in programmed cell death (apoptosis) in multicellular organisms and regulate uncontrolled cell growth like cancer [2]. There are many therapeutic substances/drugs that can target mitochondria and act as anticancer, immunosuppressant and antiviral agents [3]. To make the best use of these recently available and newly designed drugs to target mitochondria in cancer treatment, we need to understand the mechanisms driven by the metabolites in mitochondria in the pathogenesis of cancer.

In normal cells, glucose processing begins with oxidative decarboxylation to generate energy and carbon dioxide (CO₂) [4]. A series of reactions, known as the tricarboxylic acid cycle (TCA cycle), citric acid cycle or Krebs cycle are responsible for this oxidation of glucose [4]. This cycle is the common metabolic pathway for fuel molecules such as glucose, fatty acids, and amino acids [4]. Most energy yielding molecules can enter into the TCA cycle as acetyl CoA and this cycle is the principal metabolic process for cells. Biomolecules, for example glucose and fatty acids which have the potential to be converted to acetyl groups or carboxylic groups ultimately enter the TCA cycle for aerobic metabolism, especially for catabolism [4]. In oxygenated conditions, pyruvate produced from glucose is converted to acetyl CoA and enters into the TCA cycle for complete oxidation [4].

In 1956, Warburg reported that cancer cells had different metabolic characteristics [5]. They mainly produce their energy in the form of ATP (adenosine tri-phosphate) by the process of glycolysis (a non-oxidative breakdown of glucose in the cytosol of cell) [5], [6]. Thus, the energy metabolism of cancer cells shifts to glycolysis from mitochondrial respiration. One possible reason that this may occur is that mutations in proto-oncogenes and tumour suppressor genes which could create pseudohypoxia. Pseudohypoxia is a condition where cells activate oxygen deprivation mechanisms despite the presence of optimal oxygen concentrations in cancer. This condition results in the accumulation of unusual metabolites and other abnormalities in cancer cells [7]. Recent advancements in genetic sequencing technologies have enabled researchers to discover mutations in genes encoding metabolic enzymes that may be involved in this process [8]. Studies demonstrated that mutation of the genes involved in energy metabolism (TCA cycle) caused unexpected enzymatic reactions and abnormal accumulation of metabolites, termedoncometabolites [8], [9] (Fig. 1). These oncometabolites in turn, regulate epigenetic mechanisms which control cell proliferation and differentiation [9].