Overview of metabolism in cancer

Primo N. Lara, Jr. Eric Jonasch (Editors). kidney cancer (2 ed). Springer International Publishing (2015) → Русский

Proliferating cells require a steady source of nutrients to meet their demand for energy (ATP) and to serve as building blocks for macromolecules (lipids, proteins, nucleic acids, etc.) that are essential components of newly formed cells. Cancer cells are particularly adept at diverting available nutrients toward pathways conducive to their agenda of dysregulated proliferation [5]. This is accomplished, at least in part, by extensive reorganization of cellular metabolism to ensure (1) a constant supply of critical intermediates required for biosynthesis and (2) the generation of sufficient energy to fuel this process [6, 7]. While our understanding of the metabolic alterations in cancer cells is still evolving, considerable progress has been made toward this end in the past decade.

Glycolysis, the Krebs cycle, and the Warburg effect

Glucose is both the primary source of cellular ATP and a major contributor of carbon for macromolecule synthesis. Mammalian cells convert glucose into pyruvate in a multistep process referred to as glycolysis. In normal cells, in the presence of adequate oxygen, pyruvate is diverted to the mitochondrial Krebs cycle (tricarboxylic acid cycle, TCA), leading to the generation of ATP via oxidative phosphorylation.

Fig. 6.2. The genes known to cause kidney cancer, VHL, MET, FLCN, FH, SDH, TSC1, and TSC2 share the common feature in that each is involved in various pathways that regulate cell growth, proliferation, or nutrient metabolism pathways (Permission granted from Elsevier) (Linehan et al.)

However, under anaerobic conditions, ATP is generated by a process that results in conversion of pyruvate to lactate in the cytoplasm. While the latter process is able to meet cellular bioenergetic requirements as long as the availability of glucose is not limiting, it is a far less efficient means of generating ATP than is oxidative phosphorylation. In addition to its essential role in ATP synthesis, the Krebs cycle also plays an important role in macromolecule synthesis with intermediates such as citrate, oxaloacetate, and a-ketoglutarate serving as critical components of biosynthetic pathways leading to generation of lipids, nucleotides, and proteins.

The concept of altered metabolism in cancer is not new; as early as the 1920s, the German scientist and subsequent Nobel laureate Otto Warburg noted that compared to normal cells, tumor cells (derived from ascitic fluid in his original experiments) consume larger amounts of glucose and generate excessive amounts of lactic acid. Based on these observations, he concluded that cancer cells utilize glycolysis preferentially as a means of ATP synthesis (i.e., aerobic glycolysis or the “Warburg Effect”) [8, 9]. He further posited that this phenomenon was the result of an intrinsic defect in mitochondrial function and oxidative phosphorylation, forcing cells to turn to aerobic glycolysis. Although there is overwhelming evidence of the Warburg effect in a wide range of cancers, Warburg’s suggestion that this results from mitochondrial dysfunction is supported only in a small minority of tumors, including at least two subsets of kidney cancer [10, 11]. Instead, in most cancers where the Warburg effect is evident, the reliance on aerobic glycolysis for ATP generation appears to be an attempt to spare Krebs cycle intermediates for preferential use as substrates for biosynthetic pathways [12]. Regardless, aerobic glycolysis appears to be essential for tumor proliferation and survival in many human cancer models and is a valid and promising target for therapeutic intervention.

Drivers of the Warburg effect in human cancer

The precise mechanisms that promote aerobic glycolysis in cancer cells are a matter of ongoing debate. It is becoming increasingly clear that there is no single unifying mechanism driving the glycolytic shift across all cancers, with distinct processes identified in disparate tumor types. Activation of several oncogenes as well as inactivation of a variety of tumor suppressor genes has been implicated in promoting the glycolytic phenotype. Inactivation of the Krebs cycle enzymes fumarate hydratase or succinate dehydrogenase in some forms of familial kidney cancer offers the best examples of direct interruption of oxidative phosphorylation and obligate use of aerobic glycolysis for ATP generation [10, 11]. The PI3K/Akt/mTOR pathway is an important regulator of cell growth and proliferation and plays a key role in nutrient sensing and cellular responses to growth factors [13]. Activation of this pathway has been described in several cancers and can lead to upregulation of glycolysis by several mechanisms, including increased influx of glucose and other nutrients, transcriptional activation of key glycolytic enzymes, and enhanced translation of a number of proteins essential to the process [14, 15]. Similarly, amplification of the proto-oncogene MYC has been shown to activate glycolysis via upregulation of enzymes regulating this process (including PKM-2, hexokinase, and LDH-A) as well as upregulation of transmembrane glucose transporters such as GLUT-1 [16, 17]. Activation of the NRF2 oxidative stress response pathway is a feature of several cancers including nonsmall cell lung cancer and some forms of papillary RCC; recent evidence suggests that this pathway promotes metabolic reprogramming for cancer cells by enhancing glycolysis as well as glutaminolysis [18, 19].

Hypoxia-inducible factor-1 (HIF-1) is an important component of the cellular oxygensensing apparatus and plays a key role in the regulation of glycolysis in response to hypoxia. In normoxic conditions, key proline residues in the alpha subunit of HIF-1 are hydroxylated by prolyl hydroxylase. The VHL gene encodes the VHL protein which binds to hydroxylated HIF-1α, targeting the latter for ubiquitin-mediated degradation [20, 21]. In hypoxic conditions, prolyl hydroxylase is inhibited, permitting an HIF-1α to accumulate. Stabilization of HIF-1α enables the cell to respond to hypoxic and low-iron conditions by transcriptional upregulation of a number of genes critical for cancer proliferation and activation of aerobic glycolysis including vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), glucose transporter 1 (GLUT1), phosphofructokinase-2 (PFK2), and pyruvate dehydrogenase (PDH). In many human cancers, HIF-1α is stabilized by a variety of mechanisms independent of ambient oxygen tension. One of the best studied mechanisms mediating this pseudohypoxic HIF response is seen in VHL-deficient tumors. Due to VHL inactivation in these tumors, HIF-1α is no longer appropriately targeted for ubiquitinmediated degradation even when oxygen is readily available, a phenomenon that is seen in both VHL-associated and sporadic ccRCC [22]. VHLindependent upregulation of HIF-1 has also been described, particularly in RCC variants where disruption of the Krebs cycle leads to accumulation of substrates such as fumarate and succinate, which impede the hydroxylation of the alpha subunits of HIF-1 [11, 23]. Additionally, translational upregulation of HIF-1α has been described with activation of the mammalian target of rapamycin (mTOR) signaling, which can occur by a variety of mechanisms [24, 25].

Glutaminolysis and reductive carboxylation

Glutamine is an additional important nutrient substrate in tumor cells, contributing to the generation of citrate and acetyl coenzyme A for lipid synthesis as well as acting as a nitrogen donor for amino acid/ protein synthesis. A key step in glutamine metabolism is its deamidation by glutaminases to generate glutamate. While this reaction is bidirectional in normal cells, in tumor cells, deamidation of glutamine to glutamate is favored, partly a result of the overexpression of glutaminases and suppression of glutamine synthetase [26, 27]. Glutamate is converted to a-ketoglutarate which enters the Krebs cycle and can be converted to oxaloacetate in one of two ways. In tumors with an intact Krebs cycle and electron transport chain, mitochondrial oxidative metabolism is the predominant pathway used. However, in instances where oxidative metabolism is impaired (e.g., mutations in FH or SDH), an alternative pathway of reductive carboxylation is employed to generate both oxaloacetate and acetyl CoA [28]. In the latter instances, glutamine is the major carbon source for fatty acid synthesis [28]. Glutaminolysis also allows tumor cells to meet the NADPH demands of growth and supports enhanced fatty acid synthesis, nucleotide biosynthesis, and maintenance of the glutathione pool [12].