Question

1. In addition to energy needs, cancer cells also need metabolic intermediates for biosynthesis. Explain how increased reliance on glycolysis would be beneficial in this respect.
2. Glycolysis offers a growth advantage to cancers growing under hypoxic conditions. Why might this be the case? (Hint: what is the product of anaerobic glycolysis?)
3. A decrease in oxidative phosphorylation also leads to a decrease in reactive oxygen species (ROS). Why might this be an advantage for cancer cells?

Answer 1

1. Tumor cells live in an hypoxic or low oxygen environment. Hence, they undergo anaerobic respiration to obtain ATP. In anaerobic, pyruvate obtained by glycolysis is converted to lactic acid and NAD+ is regenerated from NADH/ This NAD+ is used again as reducing power for glycolysis. This NAD+ will help increase glycolysis. Tumor cells are also capable of aerobic respiration, where ATP is generated by TCA cycle coupled to oxidative phosphorylation.

PFK-1 is an important regulator of glycolytic flux. In cancer cells, PFK-2 is expressed in high amounts which leads to production of Fructose 2,6 bisphosphate. This F-2,6 bP overcome negative feedback regulation of PFK-21 by ATP. Thus, glycolysis can occur even in high ATP levels. Pyruvate kinase M2 is expressed in cancer cells over PKM1 isoform which converts PEP to Pyruvate. In cancer cells, this enzyme is allosterically and covalently inhibited by fructose 1, 6 bisphosphate. Phosphorylation of PMK2 at lysine 105 inhibits the tetramer formation of PMK2, the active form of PMK2. Increased glycolysis also inactivates PMK2 by glycosylation. Hence, all glycolytic intermediates are used for growth rather than converted to pyruvate. NADH is produced by increased reliance on pentose phosphate pathway as glycolytic intermediates are accumulated. Thus, glycolysis actually helps in generation of glycolytic intermediates.

2. Tumor cells grow and divide in hypoxic environment. This is due to increase stabilization of HIF-1 alpha under low oxygen conditions. HIF 1 alpha is known to increase the glycolysis and lactate production via lactate dehydrogenase. Lactate is also an activator of HIF-1 alpha in tumor cells. Lactate dehydrogenase, major enzyme in anaerobic glycolysis, in turn is activated by HIF1 alpha. Hypoxia upregulates the expression of many enzymes involved in glycolysis. This lactate will prevent the growth of normal cells due to acidosis. Acidosis leads to secretion of H+ ions in the surrounding which helps in increasing invasiveness. In cancer cells, the decreased pH will increase histone deacetylation. Acetate release can be exported by monocarboxylate transporter in tumor cells along with protons. Thus, within cancer cell PH is effectively regulated, but affects normal cells. Lactate also suppresses lymphocyte function and response of immune system against tumor cells. Tumor cells produce lactate even in presence of oxygen, an effect known as Warburg effect.

Further, increased anaerobic glycolysis provides glycolytic intermediates for growth. The glycolytic intermediates such as 3 phosphoglycerate can be channels to pentose phosphate pathway, an anabolic pathway for generating NADH.

3. Mitochondria carry out oxidative phosphorylation in presence of oxygen. Oxygen is used as a terminal electron acceptor. Increased oxidative phosphorylation will allow electrons to be accepted by oxygen leading to generation of reactive oxygen species. Superoxide radicals are generated via oxidative phosphorylation, which are usually converted to H2O2 by superoxide dismutase in cytoplasm. H2O2 can be degraded by glutathione peroxidase to water. However, if superoxide radical production increases, they can accumulate in cells.

Tumor cells prefer anaerobic glycolysis even in presence of oxygen. Hence, less ROS are generated in tumor cells. Reduced production of hydrogen peroxide (ROS) is known increase cellular proliferation mostly due to reduced oxidative stress. Hydroxyl radicals oxidize DNA bases, and induce single stranded and double stranded breaks. This leads to formation of DNA lesions. Tumors usually have defective tumor suppressor proteins. Hence, they cannot effectively repair these lesions and will undergo apoptosis. However, decrease ROS production will reduce such DNA lesions, allowing cells to escape programmed cell death and form tumors. ROS can also inactivate enzymes and proteins in cells. Further, ROS can react with lipids, which undergo peroxidation forming compounds like alondialdehyde, 2-alkenals and 4-hydroxy-2-alkenals, which may affect their growth. Reduced ROS prevent lipid peroxidation and also inactivation of enzymes.