Molecular Bio of Cancer Exam 2

What are pleiotropic effects?

When a single factor regulates multiple downstream targets in multiple pathways

What is cross-talk in signaling?

Pathways influencing each other via activation or inhibition.

How is signal amplification achieved?

Through kinase cascades (e.g., Ras → Raf → MEK → ERK)

What’s the difference between immediate vs delayed signaling responses?

Immediate = act on pre-existing transcription factors; Delayed = require new transcription factors

What are the three main Ras effector pathways?

MAPK, PI3K/AKT, RAL

What are SH domains in SRC?

SH1 = tyrosine kinase domain

SH2 = allows interaction with phosphorylated tyrosine kinases

SH3 = binds proline-rich regions/motifs of other proteins

How do adaptor proteins link RTKs to Ras?

Grb2 binds phospho-tyrosine (via SH2) and Sos (via SH3), activating Ras

What does activated Ras do to Raf?

Translocates Raf to the plasma membrane for activation.

Why is v-Raf constitutively active?

It lacks the N-terminal autoinhibitory domain.

What mutation is common in melanoma?

BRAF V600E (81% of cases).

How is melanoma with BRAF mutations treated?

Small molecule RAF inhibitors like Vemurafenib.

What are PI3K and AKT’s origins?

Both were first discovered as viral oncogenes.

What does AKT signaling promote?

Cell survival, growth, and metabolism.

What mutation gave Eero Mäntyranta his Olympic advantage?

Overactive EpoR mutation → excess RBCs.

How did Lance Armstrong enhance performance?

EPO doping

What does the suffix “-nib” mean in cancer drugs?

Small-molecule kinase inhibitor (e.g., imatinib)

What does the suffix “-mab” mean?

Monoclonal antibody drug and radiolabeled antibodies (e.g., trastuzumab)

What does the suffix “-mids/Imids” mean?

immunomodulatory drugs—enhance the ability of immune cells to kill abnormal cells

What does the suffix “-mibs” mean?

small molecules that work inside cancer cells to slow proliferation and increase apoptosis

What does the suffix “-nabs” mean?

nanoparticle albumin-bound drug, drug formations are bonded to albumin as a delivery vehicle

What is thalidomide used for in cancer today?

Myeloma and myelofibrosis (off-label)

What role do GAP proteins play in Ras signaling?

GAPs stimulate GTP hydrolysis, converting Ras-GTP → Ras-GDP, inactivating Ras.

What role do phosphatases play in signaling?

they remove phosphate groups from kinases, shutting down signaling

What does the sevenless mutation in Drosophila cause?

Loss of the 7th photoreceptor cell in the eye

What human receptor is the Drosophila sevenless gene homologous to?

EGFR (Epidermal Growth Factor Receptor)

What adaptor proteins link Sevenless/EGFR to Ras activation?

Grb2 and Sos (GEF = Guanine Exchange Factor)

Why are proteins often described as modular?

They contain multiple domains that mediate different interactions (e.g., SH2, SH3, kinase domains)

What marks a receptor for SH2-domain recruitment?

Tyrosine phosphorylation

What is the normal function of Raf?

A serine/threonine kinase and Ras effector in the MAPK pathway.

Why are BRAF and RAS mutations usually not found together in cancers?

Because they act in the same pathway, so one activating mutation is usually enough

What drug targets BRAF mutations in melanoma?

Vemurafenib

What problem can arise with RAF inhibitors like Vemurafenib?

Resistance develops, often requiring combination therapy

What is the main function of PI3K?

Produces PIP3, a docking site for signaling proteins like AKT.

What does AKT activation promote?

Cell growth, survival, and metabolism (anti-apoptotic effects).

What tumor suppressor opposes PI3K activity?

PTEN, which dephosphorylates PIP3 → PIP2.

What is the RAL pathway downstream of Ras important for?

Membrane trafficking and transcriptional regulation.

What is feedback amplification?

When a downstream signal boosts its own pathway activity

What is feedback inhibition?

When a downstream signal inhibits an upstream step to prevent overactivation

How is Ras activated?

by binding GTP, promoted by Sos (a GEF)

How is Ras inactivated?

By hydrolyzing GTP → GDP, accelerated by GAPs

What happens structurally to Ras when GTP binds?

Effector loop changes conformation, allowing interaction with downstream effectors (Raf, PI3K, Ral)

What are the three tiers of kinases in the MAPK cascade?

Raf (MAPKKK), MEK (MAPKK), ERK (MAPK)

What does ERK activation lead to?

Regulation of transcription, protein synthesis, and chromatin remodeling

What is the role of scaffolding proteins like KSR?

They organize Raf, MEK, and ERK to increase signaling efficiency and specificity

protozoan

Unicellular protists have limited coordination between cells (amoebas, paramecia)

metazoans

multicellular animals with different cell types that require extensive coordination (zebrafish)

growth factors drive multiple aspects of cell biology including:

1. cell movement

2. division (mitogens—drive cell into mitosis)

3. growth

4. differentiation (stem cells —> specific ones)

5. survival

src

modulator—intracellular non-receptor tyrosine kinase (NRTK) whose activity is regulated by phosphorylation

leads to phosphorylation of numerous proteins (directly or indirectly) and can activate multiple pathways

kinase (phosphotransferase)

enzyme that transfers a high-energy phosphate from ATP to a protein substrate

human kinome contaos 518 genes

phosphatase

catalyze removal of phosphate groups from phosphorylated substrates by hydrolysis 

human phosphatome contains 218 genes 

what activates the NAPK cascade?

EGF ligand

Order of the MAPKKK Pathway

MAPKKK (RAF) —> MAPKK (MEK) —> MAPK ( ERK) —> cell proliferation

4 ways diverse ectodomains mediate receptor oligerization

1. ligand dimer couples receptor

2. ligand recruits another molecular that couples receptors together

3. ligand alters ectodomain structure to expose a dimerization domain

4. ligand monomer couples receptors together

what multiple mutation types can lead to dysregulated signaling?

1. Truncation mutants can result in ligand-independent signaling.

2. receptor overexpression can lead to ligand-independent signaling (mutation is not in the receptor coding region)

3. autocrine signaling—cells make/secrete self-stimulating growth factors

4. gene fusions—ectodomain swap that cuases constitutive receptor dimerization

5. point mutations—specific mutations that cause constitutive activation

receptor tyrosine kinases (RTKS)

examples: EGF, insulin, PDGF

Key features:

• tyrosine kinase encoded within cytoplasmic tail

• Ligand-mediated receptor dimerization leads to:

  • transphosphorylation of receptor tails

  • Phosphorylated tails serve as a docking site for downstream signaling factors

Cytokine receptors (JAK/STAT) (two receptor family types)

Type 1 examples: interleukins, GM-CSF, G-CSF

Type 2 examples: interleukins, interfeurons

Key features:

• noncovalenty attached (NRTK) of the JAK family

• after ligand-mediated receptor dimerization

  • transphosphorylation (activation of JAK members)

  • receptor tail phosphorylation, generating a docking site for downstream signaling factors

TGF-B receptors

33 receptor family members in 3 types

examples: TGF-B, activin, BMP4

Key features:

• encodes serine/threonine kinase within the cytoplasmic tail

• form homo- and hetero-oligomers

• after ligand- mediated receptor oligomerization

  • transphosphorylation, the receptor tail phosphorylation, generating a docking site for downstream signaling

frizzled (receptor in canonical Wnt signaling pathway) 

10 frizzled encoded in genome 

example: Wnt 

Key features:

• receptor is a 7 pass integral membrane protein 

• frizzled encodes a G-protien coupled receptor that complexes with LRP one of two integral membrane protiein family members 

• pathway activation disrupts the constitutive degradation of B-catenin in the canonical Wnt signaling pathway

G-protein coupled receptor

at least 831 different human genes

examples: Wnt, acetylcholine, opioids, vasopressin, oxytocin

Key features:

• receptor is a 7 pass integral membrane protein 

• Ligand binding promotes GEF activity of the receptor, which leads to the exchange of GTP for GDP in the Ga subunit of the heterotrimeric G protein complex

• heterotrimeric G protein subunits dissociate and regulate the activity of various downstream targets

Notch

4 receptor family members in the human genome

examples: delta-like

Key features:

• Ligand binding leads to receptor conformational change and two successive proteolytic cleavage events in Notch

• the cleaved Notch intracellular domain (NICD) then translocates to the nucleus and coordinates gene expression

Patched (hedgehog receptor)

2 patched genes in the genome

examples: Shh,Ihh,Dhh

key features:

• receptor encodes a 12-pass integral membrane protein

• patched ligand releases smoothened (SMO) repression, allowing SMO transport into cilia, where it prevents ligand binding of the transcriptional inducer, GLI

• uncleaved GLI translocation to the nucleus and promotes gene expression 

Integrins

17 alpha subunits and 8 beta subunits

examples: collagen, laminin, fibronectin, thrombospondin

key features:

• receptor consists of heterodimeric complexes of alpha and beta subunits

• different alpha/beta subunit receptor complexes dictate ligand binding specificity.

• ligand binding leads to conformational changes that induce assembly and coupling to the actin cytoskeleton in the cytoplasm, as well as other signaling events

What happens when RAS is inactive

promtes oncogenic activity

What pathways does RAS activate?

MAPK, P13K, RAL

RAS —> P13K = membrane trafficking and transcriptional regulation

RAS —> RAF = cell cycle and transcriptional regulation

RAS —> RAL = membrane trafficking and transcriptional regulation

Grb2 Adaptor Protein

SH2 domain binds receptor phosphotyrosine

SH3 domain binds proline-rich region of SOS

Shc Adaptor Protein

SH2 domain binds receptor phosphotyrosine

receptor phosphorylates Shc that then serves as a Grb2 docking site

SOS

GEF that activates RAS by replacing GDP with GTP

what does MAPK do

leads to chromatin remodeling, protein synthesis, transcription

RAS is bound to the membrane and relocates RAF to the membrane where it becomes activated

P13K

originally identified as an oncogene from ovarian sarcoma virus

AKT

originally identified as an oncogene from mouse lymphoma retrovirus

PTEN

phosphate and tensin homolog that dephosphorylates P13 downregulates AKT/PKB activity

PH

pleckstrin homolog domain binds PI (3,4,5)P

RAL

GTPase

regulated by GAPS and GEFs

RAS effector

regulates cytoskeletal dynamics

RALA and RALB play roles in proliferation, survival, metastasis of colon, pancreatic, skin, and bladder cancer

3 major roles of integrins

1. physically couple cells to the ECM

2. monitor cell-ECL adhesion (signaling)

3. Facilitate motility by cytoskeletal assembly/disassembly

anoikis

programmed cell death triggered when cells detach from the ECM

How do cancer cells evade anoikis

By expressing anti-apoptotic factors like Bcl-XL (e.g., in mammary carcinoma)

How many GPCR genes exist in humans?

800

What are biological responses of GPCR signaling?

Proliferation, survival, differentiation, migration/metastasis, angiogenesis, and ECM degradation.

What does “dual address” signaling mean?

Proteins function in two discrete cellular locations (e.g., cytoplasm and nucleus)

What is the main role of STAT proteins?

Signal transducers and activators of transcription that dimerize via SH2 domains when phosphorylated.

What are biological responses of JAK/STAT signaling?

Immunosuppression, proliferation, survival, migration/metastasis, and angiogenesis

What ligand activates Notch signaling?

Delta-like (DLL) proteins on neighboring cells

What happens when Notch is activated?

The receptor is cleaved; NICD (Notch intracellular domain) translocates to the nucleus to regulate gene expression

In what cancer is Notch frequently mutated?

Pediatric T-cell acute lymphoblastic leukemia (T-ALL)

How is NF-κB normally kept inactive?

Sequestered in the cytoplasm by IκB

What happens when IκB is degraded?

NF-κB translocates to the nucleus to activate gene expression

What are NF-κB’s roles in cancer?

Prevents apoptosis, promotes inflammation, angiogenesis, EMT, survival, and proliferation

What are the two modes of TGF-β signaling?

Canonical (SMAD) and non-canonical (non-SMAD)

How does TGF-β act differently in cell types?

Promotes mesenchymal cell proliferation but inhibits epithelial proliferation (anti-mitogenic)

What receptor does Hedgehog bind to?

Patched (PTCH)

What happens when Hedgehog binds PTCH?

Smoothened (SMO) enters cilia, prevents Gli cleavage, and activates transcription

What cancer is strongly linked to Hedgehog signaling?

Basal cell carcinoma (mutations in PTCH or SMO)

What is the key effector of canonical Wnt signaling?

β-catenin

What happens to β-catenin when Wnt is absent?

t is degraded in the cytoplasm

What happens to β-catenin when Wnt is present?

It avoids degradation, moves to the nucleus, and regulates gene expression

What does non-canonical Wnt signaling control?

Cell motility, polarity, cytoskeletal remodeling, and Ca²⁺-dependent transcriptional changes

What proteins regulate YAP/TAZ activity?

LATS1/2 kinases (they phosphorylate YAP/TAZ)

What happens to YAP/TAZ when phosphorylated?

They are degraded by the proteasome

What happens to hypophosphorylated YAP/TAZ?

They translocate to the nucleus and regulate gene expression.

Why is Hippo pathway dysregulation important in cancer?

Elevated nuclear YAP/TAZ is linked to tumor recurrence, therapy resistance, and poor prognosis