Chapter 11: Nutrients, hormones, and gene interactions (Book, main)

Question 1

What do migration studies demonstrate about the influence of diet on cancer?

Answer 1

That an increase in colorectal cancer rates in originally low-risk ethnic groups after migration to high-risk location is seen.

Question 2

Humans can synthesize some required antioxidants, others must be obtained by eating fruit and vegetables. What are the four major groups of dietary antioxidants-phytochemicals?

Answer 2

Vitamin C, isoprenoids (e.g. vitamin E), phenolic compounds (flavonoids) and organosulfur compounds.

Question 3

What are three main aspects of our diet that can be considered as causative factors of cancer?

Answer 3

1) Any given food is a complex substance that can carry harmful factors (genotoxic agents) in addition to nutritional value. The consumption of food provides a route for chemical carcinogens to be delivered to the body.
2) Lack of a particular essential nutrient may enhance the risk of cancer
3) Worldwide health issues, such as obesity and chronic alcohol consumption, cause cancer.

Question 4

Why is there a suggestion that the source of all food should be properly labelled to allow for consumer choice and therefore creation of competition for the production of good products?

Answer 4

Because differences in the source of food can have varying consequences (like the fact that farmed salmon has more contaminants than wild salmon).

Question 5

Explain this picture (don’t learn by heart)

Answer 5

MTHFR an enzyme that regulates the balance between nucleotide synthesis and DNA methylation can convert 5,10-methylene THF to 5-methyl THF (precursors of respectively DNA synthesis and methylation). For DNA synthesis, and specifically for the synthesis of the nucleotide dTMP, 5,10-methylene together with dUMP is converted to dTMP. 5-methyl THF together with homocysteine produces methionine, which regenerates SAM, the methyl donor for DNA methylation.

Question 6

If you consider this given picture. How can a folate deficiency result in cancer?

Answer 6

When there’s low folate, the reactants for the synthesis of dTMP and methionine (5,10-methylene THF and 5-methyl THF respectively) are present in low amounts. So for DNA synthesis, dUMP will accumulate, which will cause mutations. And for DNA methylation, there’s a lack of methionine, therefore a lack of regeneration of SAM which causes hypomethylation. Hypomethylation and mutation can lead to cancer.

Question 7

Adipose tissue is an endocrine organ that can affect other tissues: it can release free fatty acids, peptides, hormones and steroid hormones. What mechanisms can result in an increased risk of cancer (in regard to the increased amounts of adipose tissue in an obese person)?

Answer 7

Altered sex hormone metabolism, increased production of fat cells (adipocytes), adipokine hormones, increased insuling signaling pathways and dietary alterations in the gut microbiota.

Question 8

What has been demonstrated in a metabolite of cholesterol (27HC)?

Answer 8

That it acts as a ligand for the estrogen receptor and liver X receptor, and thus may share some mechanisms used by estrogen in cancer such as estrogen-responsive genes (in mouse model, 27H increased growth and metastasis).

Question 9

How can adipokines and inflammation as a result of obesity result in cancer?

Answer 9

Obesity causes a chronic inflammatory response with increases in tumor-promoting cytokines, such as IL-6 and TNF.

Question 10

How can obesity promote liver cancer?

Answer 10

By causing a change in the gut microbiome (more Gram-positive bacteria), which results in an increase of a bacterial metabolite. This metabolite (deoxycholic acid) can circulate to the liver and cause DNA damage. Chronic DNA damage in liver cells triggers cell senescence that is characterized by the secretion of inflammatory and tumor-promoting factors.

Question 11

In short: how can chronic alcohol consumption result in cancer?

Answer 11

Alcohol is metabolized by the enzyme alcohol dehydrogenase to form acetaldehyde that can directly bind to DNA, forming DNA adducts that cause mutations.

Question 12

What is the most direct strategy for preventing the initiation of cancer and for slowing down the progression of disease?

Answer 12

The ability to block DNA damage caused by ROS and/or carcinogens.

Question 13

Microconstituents found in fruit and vegetables play a role in blocking DNA damage caused by ROS and/or carcinogens. They do this directly or indirectly. How is this accomplished?

Answer 13

• Directly it is accomplished by free radical scavengers
• Indirectly it is accomplished by regulating the expression of genes that code for phase I and phase II metabolizing enzymes in the body.

Question 14

How is the carcinogen aflatoxin B1 (AFB1) metabolized (with the help of phase I and II metabolizing enzymes in 3 steps)?

Answer 14

• AFB1 undergoes oxidation by phase I cytochrome P450. AFB1-8-9-epoxide is formed.
• AFB1-8-9-epoxide is conjugated to glutathione by phase II enzyme glutathione S-transferase. AFB1-glutathione is formed.
• AFB1-glutathione can be detoxified and easily excreted.

Question 15

In short: several microconstituents in fruits and vegetables acts as antioxidants that scavenge ROS. What two microconstituents are known for this? Describe their function/characterisations.

Answer 15

• Vitamin C is water-soluble and can donate an electron to a free radical (making it harmless). Vitamin C radical is formed when scavenging ROS, but vitamin C reductase can regenerate vitamin C.
• Vitamin E is fat-soluble and can therefore help in membranes, where vitamin C cannot.

Question 16

What is the ARE?

Answer 16

The ARE is the antioxidant response element in the promotor region of genes encoding detoxification and antioxidant enzymes. It can be activated by food antioxidants (sulforaphane, EGCC) but also in response to reactive electrophilic intermediates and H2O2 (i.e. carcinogens).

Question 17

This question is also in the deck about the lecture. But as a reminder: Briefly explain the function of the following components in regard to the regulation the gene expression of metabolizing enzymes:

1. Antioxidants
2. Carcinogens (H2O2 and reactive elecrophilic intermediates)
3. Antioxidant Response Element (ARE)
4. Nrf2
5. KEAP1
6. Maf

Answer 17

1. Antioxidants can regulate the gene expression of metabolizing enzymes (by inhibiting Nrf2 degradation).
2. Carcinogens can do the same as the antioxidants.
3. ARE is a response element in the promotor region of genes for metabolizing enzymes.
4. Nrf2 is a transcription factor that can bind to ARE to induce gene expression
5. KEAP1 contains cysteines that can sense the redox state of the cell. It can then steal Nrf2 from DNA, causing degradation of Nrf2.
6. Maf is a cofactor and when Nrf2 and Maf are bound, gene expression is induced.

Question 18

This question is also in the deck about the lecture, but as a reminder: In a normal non-stressed cell KEAP1 pulls Nrf2 from the ARE in the genes of the metabolic enzymes and tags it for degradation, so that transcription cannot be induced. How can carcinogens or antioxidants activate this Nrf2-Are signaling pathway and with this the metabolic enzymes?

Answer 18

Carcinogens or antioxidants can bind to the cysteine residues in KEAP1. This inhibits the function of KEAP1 so that Nrf2 stays on the ARE of the metabolic enzyme genes. Nrf2 then binds to cofactor Maf which causes induction of gene expression.

Question 19

We’ve already concluded that the most direct strategy for prevention of cancer and slowing down its’ progression is blocking DNA damage caused by ROS and/or by carcinogens through the use of free radical scavenging of protective enzymes. What two other mechanisms of particular vegetables in cancer prevention are there?

Answer 19

Modulation of apoptosis and/or cell proliferation.

Question 20

Garlic contains organosulfur, ajoene and allicin. Three compounds that all can contribute to prevention of cancer. Shortly describe these.

Answer 20

1. Organosulfur can induce phase II enzymes and scavenging.
2. Ajoene induces apoptosis ((in leukemic patients)) through activation of caspases-3 and -8 and peroxide.
3. Allicin can inhibit proliferation of human mammary endometrial, and colon cancer cells.

Question 21

Through what mechanism is a tumor able to affect the immune response against the tumor?

Answer 21

Through the Warburg effect. By producing lactate, the tumor can alter the microenvironment to affect the immune repsonse against the tumor.

Question 22

How can acetyl CoA contribute to carcinogenesis?

Answer 22

Nutrients are converted to intermediary metabolites during metabolism. Glucose is converted into acetyl CoA during glycolysis, which is a universal donor for acetylation reactions (like histone acetylation). Cancer cells have an altered metabolism and can alter the abundance of metabolites like acetyl CoA, which can lead to transcription of genes that regulate cell proliferation.

Question 23

The family of NAD-dependent HDACs, called sirtuins, provides additonal links between metabolites, gene expression and cancer. They mediate responses to changes in the energy status of cells by using NAD+ as a co-substrate for enzyme function. How do these enzymes provide this link between metabolites, gene expression and cancer?

Answer 23

These enzymes deacetylate histones and other proteins such as transcription factors (e.g. p53). This means that transcription of genes like p53 is more likely to occur which is anti-cancerous. It was therefore demonstrated that overexpression of proteins like p53 are linked to anti-aging and anti-cancer effects.

Question 24

Resveratrol, a polyphenol in grapes and wine, has cancer preventative effects. How?

Answer 24

It inhibits the Warburg effect by activating SIRT1 (tumor suppressor).

Question 25

What two things contribute to the altered metabolism observed in cancer cells?

Answer 25

1. Genetic alterations in oncogenes and tumor suppressor genes 2. Response to hypoxia via HIF-1a

Question 26

Also AMP-activated protein kinase (AMPK) provides a link between energy metabolism and cancer. What is the function of AMPK and when is AMPK activated?

Answer 26

AMPK senses the energy state of a cell and is activated under conditions of increased AMP and decreased ATP in response to starvation, hypoxia, and exercise. When active is can stimulate ATP-generating pathways and inhibit ATP-consuming pathways.

Question 27

So what needs to happen in order for AMPK to be activated and fulfil its’ function?

Answer 27

Starvation/hypoxia/exercise -> increased AMP and decreased ATP -> AMP binds to γ-subunit of AMPK -> conformational change of AMPK -> potential phosphorylation site on α-subunit -> phosphorylation -> active AMPK -> genes that are involved in stimulating ATP-generating pathways and inhibiting ATP-consuming pathways.

Question 28

What other function (except regulating genes that are involved in stimulating ATP-generating pathways and inhibiting ATP-consuming pathways) does AMPK have?

Answer 28

It can activate p53 (where it can induce cell cycle arrest in response to low cellular energy).

Question 29

Both hypoxia, and oncogenic mutations in the absence of hypoxia, can activate HIF-1a. HIF-1a is important in VEGF transcription. But also has a function in glycolysis, how?

Answer 29

It upregulates the expression of enzymes needed for glycolysis and inhibits the expression of enzymes involved in Krebs cycle and oxidative phosphorylation.

Question 30

So is increased glycolysis observed in some tumor a consequence of mostly a lack of oxygen or more a consequence of upregulation of genes needed for glycolysis?

Answer 30

The latter, a consequence of upregulation of genes needed for glycolysis.

Question 31

What will polymorphism (C->T transition at nucleotide 677) in the MTHFR genes result in? Why does this lead to a 50% decreased risk of colorectal cancer (if you homozygotic for this polymorphism)?

Answer 31

It reduces its enzymatic activity of MTHFR. As a consequence, more 5,10-methyle THF is available and thus a lower chance of disrupting nucleotide and DNA synthesis.

Question 32

How (and why) can a polymorphism of the MTHFR gene (C->T transition at nucleotide 677) that reduces its enzymatic acitvity, result in an increased risk of cancer? This, even though homozygotes for this polymorphism normally have a 50% decreased risk of colorectal cancer.

Answer 32

When individuals with this polymorphism become folate-deficient. Under these conditions, methyl-THF becomes depletes and DNA methylation is altered in a manner characteristic for carcinogenesis.

Question 33

The UVB-vitamin D-cancer hypothesis suggests that sunlight is linked to a reduction in cancer risk due to the effects of vitamin D production. Epidemiological evidence has demonstrated that there is an increased risk of several cancer (prostate, colon, breast) in people living at higher latitudes (i.e. further away from the equator). Why does this epidemiological study still support the hypothesis?

Answer 33

People living at higher latitudes may be vitamin D deficient, because there’s less sunlight. Interestingly, prostate, colon and breast cells contain the enzyme needed to produce 1,25-dihydroxyvitamin D.

Question 34

Does circulating vitamin D correlate with a reduction in the risk for colorectal cancer?

Answer 34

Yes (30-40% reduction)!

Question 35

How is vitamin D a chemopreventative agent?

Answer 35

Active form of vitamin D (calcitrol/1,25-hydroxyvitamin D3) acts as a ligand for the vitamin D receptor. This receptor which is present in most cells of the body recognize the vitamin D response element in gene promotor regions and regulate transcription of its target genes. Vitamin D can inhibit growth and induce differentiation and apoptosis through this regulation.

Question 36

Think of ways vitamin D can inhibit cell growth and induce differentiation and apoptosis

Answer 36

• Inhibit cell growth -> vitamin D binds to the ligand-binding site of EGFR
• Apoptosis -> vitamin D can directly activate tumor suppressor genes like p21 (cell cycle arrest) through a vitamin D response element in their promotor regions. It can also induce redistribution of pro-apoptotic proteins BAK and BAX to the mitochondrium (intrinsic apoptotic pathway).

Question 37

What appears to play a central role in the initiation and progression of breast cancer?

Answer 37

Estrogens (estradiol and estrone).

Question 38

Why does increased amount of adipose tissue result in increased risk of breast cancer?

Answer 38

Adipose tissue use the enzyme aromatase to produce estrogen from androgens.

Question 39

There are two models for the mechanisms by which estrogens seem to contribute to breast cancer. What do these models suggest and how do they differ from each other?

Answer 39

• Model 1 suggests that estrogens promote proliferation of breast cells that contain estrogen receptors (ERs) through its mitotic activity. The high division rate allows less time for DNA repair, thus creating an opportunity for errors to occur during DNA replication (non-genotoxic).
• Model 2 suggests that estrogen and its metabolites are genotoxic. When a certain metabolite of estradiol is formed, it can bind to adenine or guanine bases. This results in unstable bonds and abasic sites.

Question 40

Effects of estrogen are mediated through ERs, ER-α and ER-β. What happens when estrogen binds to one of these receptors?

Answer 40

The receptors bind as dimers to the estrogen response element (ERE) and regulate estrogen-responsive genes.

Question 41

What is another gene that is involved in breast cancer? What does it normally do?

Answer 41

BRCA genes, the products are nuclear tumor suppressor proteins that play a role in transcriptional regulation, DNA repair and regulation of the cell cycle.

Question 42

There are two strategies for the design of drugs that target estrogen action. Explain these two strategies in short.

Answer 42

1. Designing drugs that antagonize the actions of estrogens by interacting with ERs in order to block growth in “ER-positive” tumors, called ER modulators (example is tamoxifen).
2. Designing drugs that interfere with estrogen synthesis, like drugs that target aromatase.

Question 43

What is the function of the drug tamoxifen?

Answer 43

It is a competitive inhibitor that alters the folding of the ligand-binding domain of the ER and blocks its ability to transactivate and initiate transcription of target genes.

Question 44

Why are aromatase inhibitors not an option for premenopausal patients?

Answer 44

In premenopausal patients aromatase is not the major estrogen producer.

Question 45

Does tamoxifen work better in comparison to aromatase inhibitors? Or is it the other way around?

Answer 45

Tamoxifen only blocks ER-mediated effects while aromatase inhibitors reduce total estrogen concentrations, and therefore block both ER mechanisms and non-receptor mediated genotoxic events.