Biology of Cancer - Lectures 9 and 10

Diet and cancer

Diet and cancer

• Obesity

• Free radicals

• DNA adducts

Why was HFCS created?

• 1970s: shortage of gas, so scientists wanted to use corn to generate ethanol and mix it with gas

• This process was expensive and bad for the environment

• A ton of corn was being produced and needed to be used→ farmers desperate for a solution

• An idea emerged to convert corn to high fructose corn syrup; revolutionized processing of food in US (added to every single food type)

Why has the rate of obesity in the US increased from the 1960s to the 2020s?

Due to high corn fructose syrup

HFCS and portions matter

• HFCS is derived from corn starch

• Inexpensive and abundant, partially due to corn subsidies

• Portions have gotten way bigger since the 1950s

• HFCS is addictive and used as a filler in foods

• 75% of all food in US contain HFCS

Why is sugar (fructose) bad for us?

• Fructose has no marker in the bloodstream to flag the rise in fructose levels

• Fructose goes into the liver undigested and unused

• Some fructose goes to liver and gets converted into glycogen but most are converted into fat

• Adipocytes (fat cells) release leptin

How is glucose regulated in the body?

• When eating, glucose levels in body rise and insulin kicks in and makes it go back down

  • Insulin recognizes the spike in glucose levels and reduces it

• When the pancreas releases insulin, insulin opens up gates to get glucose from the bloodstream into different tissues

  • Leftover glucose becomes glycogen

What is the role of leptin in the control of food intake?

• Adipocytes (fat cells) release leptin

• Leptin triggers the brain→ do I have enough energy for a task, or should I consume food to get more energy?

• When fat cells are empty→ no leptin signal, want to eat to get more energy

• When fat cells are full→ leptin signal, don’t need to eat

• Too much fat in the system→ have high levels of leptin and become leptin resistant

  • Always want to consume, even if you’re not hungry

The Princeton Experiment

• Two groups of mice→ one group had a regular diet and the other group had a HFCS diet

• Male rats ballooned in size→ animals with access to HFCS gained 48% more weight than those eating a normal diet

Story of the leptin resistant kid

• Young kid from London who used to eat 6000 calories a day

• Got a blood test and found out that he had a mutated leptin gene

• He was always leptin deficient, so he had no way of knowing if he was hungry or not

• Got leptin treatment and was fine

How do people become obese?

They become leptin resistant and are always hungry

How does obesity increase an individual’s risk of cancer?

• Obese individuals have

  • Increased levels of IGF (insulin growth factor)→ increases cell proliferation and decreases cell apoptosis

  • Huge amount of leptin

  • Increased inflammation

  • Increased levels of estrogen

• All of these factors are substantiated by the presence of free radicals

  • Ex: the more IGF you have, the more free radicals you have

What are estrogens?

• Hormones important to female body

• Can break down to their byproduct estrone (a free radical)

Estrogen and breast cancer

• Young girls getting periods early because of hormones and HFCS in blood

• In a normal menstrual cycle, high levels of estrogen cause breast cell proliferation

  • Low levels of estrogen cause breast cell death

• High levels of estrogen increase cell proliferation and chance of accumulating a mutation

• Free radicals can cause mutations

• High levels of estrogen generate free radical estrones that will damage DNA, causing cancer

Association of childbearing and risk of cancer

• Childless women have a higher risk of cancer because their estrogen levels remain high

• Having a child reduces estrogen levels, reducing the risk of cancer

Beneficial effects of estrogen

• Breast→ programs milk production

• Liver and heart→ controls cholesterol

• Uterus→ prepares for fetus

• Bone→ preserves strength

Harmful effects of estrogen

• Breast→ increases cancer risk

• Uterus→ increases cancer risk

Free radicals

• Molecules that can grab electrons (they have unpaired electrons)

• Molecule is getting destabilized, can oxidize lipids in the membrane, making these cells get damaged and die

• Most important→ damage DNA, causing mutations

Linkage between obesity, free radicals, and cancer

Obesity causes increased levels of free radicals, which damage DNA, causing cancer

Obesity and cancer risk

• Fat tissue produces excess amounts of estrogen

• Obesity increases level of insulin and IGF-1 (hyperinsulinemia or insulin resistance), which may promote the development of certain tumors

• Fat cells produce hormones, leptin, which is more abundant in obese people, seems to promote cell proliferation

• Obese people often have chronic low-level, or “subacute,” inflammation, which has been associated with increased cancer risk

• Other possible mechanisms include altered immune responses, effects on the nuclear factor kappa beta system, and oxidative stress

Obesity and inflammation

• Obesity (having too many fat cells) also causes chronic low grade inflammation

• Have an increased number of cytokines such as TNF, IL18, CD40L, and IL6 (inflammatory mediators that cause inflammation)

Cooking and risk of cancer

• Heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs) are chemicals formed when muscle meat, including beef, pork, fish, and poultry, is cooked using high-temperature methods, such as pan frying or grilling directly over an open flame

• The formation of HCAs and PAHs is influenced by the type of meat, the cooking time, the cooking temperature, and the cooking method

• Exposure to high levels of HCAs and PAHs can cause cancer in animals; however, whether such exposure causes cancer in humans is unclear

• HCA and PAH formation can be reduced by avoiding direct exposure of meat to an open flame or a hot metal surface, reducing the cooking time, and using a microwave oven to partially cook meat before exposing it to high temperatures

Examples of carcinogens (HCA and PAH) that causes DNA adducts

Benzo(a)pyrene, estrone

DNA adducts

• Molecules generated when fat is heated

• Looks like a nucleotide

• Example: guanine will bond to it, thinking the molecule is a nucleotide→ causes a bulking effect

• Generates a major kink in the DNA

• DNA polymerase will add a wrong base or skip over it

Activation of carcinogens

• Many important carcinogens require metabolic activation

• Multiple enzyme systems involved in activation

• Linkage from

  • Chemical activation

  • Adducts

  • Specific mutations

  • Cancer

Process of benzo(a)pyrene becoming carcinogenic

• Metabolizes into a reactive intermediate

• Liver has a detoxification procedure called a glutathione conjugate

  • Enzymes that help detoxify the DNA adduct

• If detoxification does not occur because the liver is overloaded

  • Adduct formation occurs, but can undergo DNA repair

• If DNA is not repaired, mutations will occur causing genetic damage and a loss of growth control→ causes cancer

Human digestive system

• Less acidic and long intestines

• No fiber

• Not built to consume all this fat and meat

  • Food sits in intestines for around 4 hours

  • Can only digest 40% of food we eat

  • Microbiome trying to digest the rest, gets exhausted, generate toxins and free radicals that travel throughout the body

Animal digestive system

• Very acidic and short intestine

• Lots of fiber

Effects of eating meat on digestive system

• Decrease species of microbiomes causes a decrease in immunity

• Increase chance of free radicals

Effects of eating vegetables and fruits

• Have fibers that help remove products from system much quicker

• Clear digestive system on a regular basis, healthier microbiomes

• Have anti-oxidants→ species with lots of electrons; will give electrons to free radicals

Signal transduction

Lets cells communicate (outside to inside or one cell to another)

Signal pathways

Consist of receptors (proteins) embedded in a cell’s plasma membrane

Ligand

A protein that binds to a receptor; one ligand can activate many receptors

How does EGF (epithelial growth factor) work?

• Binds to EGFR (epithelial growth factor receptor)

• EGF will change its shape when it binds, activating the receptor

• An amino acid such as tyrosine will become phosphorylated in the cytoplasmic domain

What are the three amino acids that can get phosphorylated?

• Serine

  • Ex: if a serine is phosphorylated, receptor serine kinases

• Threonine

  • Ex: if a threonine is phosphorylated, receptor threonine kinases

• Tyrosine

  • Ex: if a tyrosine is phosphorylated, receptor tyrosine kinases

What phosphorylates things?

Kinases

EGF and the MAP kinase pathway

• When EGF binds to a monomer (has two receptors) on the surface of a cell, the two receptors come into close proximity and bind together to dimerize

• As EGF binds to the EGFR, a tyrosine in the cytoplasmic domain is phosphorylated (everything after this step is part of the MAP kinase pathway)

• Once the tyrosine is phosphorylated, GRB2 binds to it

• GRB2 then activates SOS

• RAS protein (a protooncogene) is activated for cell proliferation

• RAS can’t cause cell proliferation so it conveys that message to RAF1

• RAF1 activation transfers cell proliferation message to MEK

• MEK sends message to ERK 1 and 2 (all previous messages occurring in cytoplasmic domain)

• ERK 1 and 2 go into the nucleus of the cell

  • Acts as a transcription factor in nucleus, causing increased gene expression of genes that help with cell proliferation

• In summary: EGF binds to EGFR dimerizing it→ tyrosine is phosphorylated→ GRB2→ SOS→ RAS→ RAF1→ MEK→ ERK 1 and 2

• In cancer, this process is exacerbated

  • Ex: instead of one signal, you get 100; too much phosphorylation, etc→ cause abnormal proliferation

EGFR structure

• Extracellular domain (at top)

• Transmembrane domain

• Intracellular domain

JAK-STAT pathway

• When a serine or threonine is phosphorylated

• When a ligand binds, the monomers become dimerized

  • Ligand changes shape and the receptor becomes active

• Trans-phosphorylation occurs

  • JAK phosphorylates TYK2 and TYK2 phosphorylates JAK

  • Serine or threonine is phosphorylated

• They phosphorylate STAT1 and STAT2

  • Dimerize and go into the nucleus

• Start transcription of genes: MYC, D2, D3 cyclins, BCL-X1, and RAS

EGFR signal transduction in tumor cells

PI3K is activated, which then activates AKT

Phospholipases and phospholipids in signal transduction

Phospholipases and phospholipids are involved in the processes of transmitting ligand-receptor induced signals from the plasma membrane to intracellular proteins, PLCγ and Phosphatidylinositol-3-Kinase (PI3K)

Phosphatases in signal transduction

• Tyrosine and serine/threonine phosphorylation with increased cellular growth, proliferation and differentiation

• Removal of phosphates required to turn off the proliferative signals

Phospholipase pathway

• Growth factor cytokine binds to a receptor, which gets phosphorylated

• Activates SRC, which binds to PI3K (everything after this is PI3 kinase pathway)

• PI3 kinase is hydrolyzed to PIP2

  • Inositol has multiple hydroxyl groups that can be phosphorylated besides the 1 position

  • PI kinase phosphorylates a phosphatidyl-inositol (PI) at position 5 and 4→ becomes phosphatidyl-inositol-(4,5) disphosphate (PIP2)

• PI3 kinase (PI3K) adds another phosphate to PIP2 at position 3→ becomes PIP3

• In summary: PI → PI kinase→ PIP2→ PI3K→ PIP3

• PIP3 binds to an extremely important protein called Akt (aka PKB)

  • Phosphorylated by PDK1 and PDK2 to turn on Akt

• Akt controls a lot of cellular activities

• Akt inhibits GSK-3B, an inhibitor of cell proliferation→ Akt increases cell proliferation

• Akt turns on HIF-1a, increasing angiogenesis

  • Helps tumor grow without oxygen and gives it blood

• Akt inhibits Bad, inhibiting apoptosis

• Mechanism to turn this off is P10, a tumor suppresor

  • Takes away a phosphate from PIP3 to make it PIP2 so that it can no longer bind to Akt

  • Akt can no longer be turned on

Hyperactivity of PI3K and cancer survival

• PI3K negative→ 80% of patients survive for 5 years

• PI3K positive→ 60% die at 18 months; 80% of patients die within 48 months

Deletion of PTEN and cancer patient survival

• Intact P10→ 55% of patients live 100 months

• P10 deletion→ 90% of patients after 28 months

• rNA is high→ better survival rate of patients

• rNa is low→ majority of patients die

Various types of receptor related proto-oncogenesis

• G-protein→ the G-proteins possess intrinsic GTPase activity that is regulated in conjunction with interaction with membrane-associated signal transducing receptors intracellular effector protein e.g. Ras

• G-protein coupled receptors→ seven membrane-spanning helices connected by three intracellular loops and three extracellular loops with an extracellular amino terminus and an intracellular carboxy terminus

  • 791 identified GPCRs I

G-protein coupled (GPCR) signaling

• When a ligand binds to the receptor, becomes phosphorylated and activated

• G-protein trimeric binds to receptor

• G-protein conformation changes, which releases GDP and GTP comes in

• a and GTP subunit separates from B and gamma unit

• a subunit binds with andenenal cyclase (AC), an enzyme

• Activation of AC causes ATP to break down into cAMP, generating PKA (protein kinase A)

• cAMP binds to PKA, making the subunits of PKA separate

• One subunit of PKA enters into cell and starts the process of cell proliferation

• In a cancerous situation, this process is problematic when there is too much cAMP

• Arrestin will turn off this process by binding to receptor so a subunit will no longer bind to receptor