chapter 5 growth factors, receptors and cancer - 5. 2 ~ 5.9 - mar 27 & 29, 2007

Chapter 5

Growth Factors, Receptorsand Cancer

- 5. 2 ~ 5.9 -

Mar 27 & 29, 2007

Decisions about growth versus no-growth must be made for the welfare of the entire tissue and whole organism, not for the benefit of its individual component cells.

Figure 5.5a The Biology of Cancer (© Garland Science 2007)

5.2 the Src protein functions as a tyrosine kinase

A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation).

- Most of the kinases in the cell are serine /threonine kinases (add phosphate group on serine/threonine).

- More than 99% of the phosphoamino acids in normal cells are phosphothreonine or phosphoserine; phosphotyrosine constitutes as little as 0.05 to 0.1% of these cells’ total phosphoamino acids.

Figure 5.8 The Biology of Cancer (© Garland Science 2007)

Electrophoresis can be used to resolve 3 typesof phosphoamino acids – phosphotyrosine, phosphothreonine and phosphoserine

almost no phosphotyrosine

increased levelof phosphotyrosine


Src can phosphorylate more than 50 distinct proteins (substrate) in the cell

32P-labeled ATP was added into the cell lysates

Figure 5.7b The Biology of Cancer (© Garland Science 2007)

A protein kinase usually phosphorylates and modifies the functional state of a number of distinct substrate proteins

Akt: AKR mouse T-cell lymphoma (v-Akt)

PKB: protein kinase B

GSK: glycogen synthase kinase

HIF: hypoxia- inducible factor

Bad: BclxL/Bcl-2 -associated death promoter

5.3 The epidermal growth factor receptor (EGF-R) functions as a tyrosine kinase

- A variety of proteins involved in cell-to-cell signaling were found and sequenced.

- The 1st of the growth factors to be discovered was epidermal growth factor (EGF).

- EGF has mitogenic effects when applied to a variety of epithelial cell types.

- EGF (ligand) is able to bind to a surface protein (receptor) of the cells whose growth it stimulates.


Structure of the epidermis growth factor (EGF) receptor

tyrosine kinase domain


Structure of tyrosine kinase receptors

IGF-1: insulin-like growth factor-1, NGF: nerve growth factor, PDGF: platelet-derived growth factor, FGF: fibroblast growth factor, VEGF: vascular endothelial growth factor, Eph: ephrin

Table 5.1 The Biology of Cancer (© Garland Science 2007)

Growth factors and their tyrosine kinase receptors

HGF: hepatocyte growth factor, SF: scatter factorGDNF: glial cell derived neurotrophic factor Ret: Rearranged during transfection, SCF: stem cell factor


5.4 An altered growth factor receptor can function as an oncoprotein

- In 1984, the sequence of the EGF receptor was recognized to be closely related to the sequence of a known oncogene product v-ErbB from avian erythroblastosis virus.


Mutations in growth factor receptor gene can cause ligand-independent activation


Tumor cells may synthesize growth factor and creates an autocrine signaling

receptor: EGF-R

ligand: TGF-α

super-imposed

invasive human breast carcinoma


4.6 - In about 1/3 of glioblastomas examined, the EGF-R has been found to be decapitated, lacking most of its extracellular domain.

- In many lung cancers, the EGF-R mRNA lacks the coding sequences carried by exons 2 through 7.

5.5 A growth factor gene can become an oncogene: the case of sis

- In 1983, the B chain of platelet-derived growth factor (PDGF) was found to be closely related in sequence to the oncoprotein encoded by the v-sis oncogenes of simian sarcoma virus.

- PDGF stimulates growth of mesenchymal cells, such as fibroblasts, adipocytes, smooth muscle cells, and endothelial cells.

A B PDGF can also be AA or BB

Growth factors can act on cells via three ways


IL-6: interleukin-6, NRG: neuregulin, PRL: prolactin,

TGF: transforming growth factor, GRP: gastrin releasing peptide

(SCLC)

- Certain lung cancers produce 3 distinct growth factors and express their receptors:

1. transforming growth factor (TGF) - α / EGF-R

2. stem cell factor (SCF) / Kit

3. insulin-like growth factor (IGF) - 1 / IGF-1-R

- Kaposi’s sarcomas produce PDGF, TGF-β, IGF-1, angiogenin 2 (Ang2), CCL8, CXCL11, endothelin and express their receptors. At the same time, the causal agent of this disease, the human herpesvirus-8 (HHV-8) produces vIL-6 and macrophage inflammatory protein (vMIP).

5.6 Transphosphorylation underlies the operations of receptor tyrosine kinases (RTK)

How do growth factor receptors use their tyrosine kinase domains to emit signals in response to ligand binding?


Receptor dimerization following ligand binding


Transphosphorylation of the EGF receptor

A431 human vulve epidermoid carcinoma cell line

(overexpress EGF-R)

in

32PO4-containing medium

- EGF + EGF

Why does overexpression of growth factor receptors participate in the formation of cancers ?

1. When the receptor molecules are overexpressed, their high numbers cause them to collide frequently, and these encounters, like the dimerization events triggered by ligand binding, can result in trans-phosphorylation, receptor activation, and signal emission.

2. Alternatively, excessive receptor expression may make some cancer cells hyper-responsive to the low levels of growth factors that may be present in their surroundings.


deletion

amplificationpoint mutationtranslocation


Sidebar 5.7 Mutant forms of a single tyrosine kinase-R may play a causal role in very different types of cancer

GIST: gastrointestinal stromal tumor

AML: acute myelogenous leukemia

SCF stimulates the formation of various types of cells in the blood (hematopoiesis), as well as the development of a variety of nonhematopoietic cell types, including melanocytes and the cells mediating gut motility.

c-Kit is the receptor for stem cell factor (SCF)

a.a. substitutions or deletions

5.7 Yet other types of receptors enable mammalian cells to communicate with their environment

- Other types of receptors which contain no kinase domains also contribute to cancer formation.

(1) When the receptors dimerize in response to ligand binding, the associated Janus kinases (Jaks) phosphorylate and activate each other.

The activated Jaks then proceed to phosphorylate the C-terminal tails of the receptor molecules, thereby activating the receptors to emit signals.

Janus : Roman god of gates and doors 兩面神

http://en.wikipedia.org/wiki/Image:Janus-Vatican.JPG


Receptors with Jaks as associated tyrosine kinases

erythropoietin (EPO) receptor – regulates the development of erythrocytes.

thromboietin (TPO) receptor – controls the development of the precursors

of blood platelets, megakaryocytes.

interferon (IFN) receptor – delivers anti-viral signals.

Jak family:

Jak1, Jak2, Jak3, Tyk2

Tyrosine kinase 2


(2) Transforming growth factor β (TGF-β) receptors are heterodimers and their kinas domains phosphorylate serine and threonine rather than tyrosine residues.

→ suppress the proliferation of normal epithelial cells and promote the acquisition of invasive properties by transformed cells


(3) Notch receptor: After binding ligands (NotchL, Delta, Jagged), Notch is cleaved successively by two proteases. One of the resulting proteolytic Notch fragments, derived from its cytoplasmic domain, migrates to the nucleus, where it functions as part of a complex of transcription factors that activate expression of responder genes.

act as a transcription factor contribute to Ras-mediated cell transformation and morphogenetic processes.

cytoplasm


(4) Patched (Ptc) receptor: When the ligand Hedgehog (Hh) binds, Ptc moves away from a 2nd membrane-spanning protein called Smoothened (Smo).

Smoothened then signals to a cytoplasmic complex that releases a transcription factor, and translocates to the nucleus.

Mutant alleles of both Ptc and Smo have been found in the common basal cell carcinoma of

the skin.

(functionally inert)

of transcription


(5) Wnt (growth factor) can activate Frizzled (Frz, the receptor) and trigger a cascade of steps that shut down glycogen synthase kinase-3β (GSK-3β) firing, allowing its downstream substrate -catenin to escape degradation and to promote cell proliferation.

(adenomatous polyposis coli)


Signaling through G proteins by serpentine receptors

- seven-membrane-spanning

- signaling through G proteins (guanine nucleotide-binding proteins)

- also called G-protein-coupled receptors (GPCRs)

- contribute to the pathogenesis of a small number of human cancers

5.8 Integrin receptors sense association between the cell and the extracellular matrix


anchorage-independent growth

In the absence of attachment, many types of normal cells will activate a death program (apoptosis) that is called anoikis.


- Cells are not anchored directly to the glass or plastic surface of the dishes.

- Instead, they attach to a complex network of molecules, called extracellular matrix (ECM), which is usually found in the spaces between cells within most tissues.

- ECM is composed of glycoproteins, including collagens, laminins, proteoglycans, and fibronectin.

- Cells are able to sense whether or not they attach to the ECM.

- Such sensing depends on integrin receptors.

extracellular matrix


- Integrins constitute a large family of heterodimeric transmembrane cell surface receptors composed of α and β subunits.

- At least 18 α and 8 β subunits have been identified, with a total of 24 distinct integrins.


focal adhesion

actin fiber

(clustered integrins, cytoskeletons, associated proteins)

Integrin clustering connects ECM to cytoskeleton (focal adhesions) and activate signaling pathways

- cell migration, proliferation and survival (anti-apoptosis)


Organization of ECM-integrin-cytoskeleton

signals outside-in

signals inside-out

5.9 The Ras protein functions as a G protein

- The ras oncogene triggers many of the same changes in cells which are transformed by erbB (truncated EGF-R) or sis (PDGF-B).

- Could Ras be found somewhere downstream of erbB and sis ?

- Do the signals emitted by EGF-R and PDGF-R converge on some common molecule ?

EGF/EGFR-mediated Ras activation

SH2, SH3: Src homolog

GRB2: growth factor receptor- bound protein 2

Sos: son-of-sevenless

Sos is a guanosine exchange factor (GEF)

Guanosine diphosphateGuanosine triphosphate

EGF/EGFR-mediated Ras activation

Regulation of Ras (a GTPase) activity

GAP:

GTPase-activating(or accelerating) protein

GEF: guanine nucleotide exchange factor

Sos is a GEF, which catalyzes conversion of inactive GDP-boundRas to the active GTP-bound form.

Mammalian Ras proteins have been studied in great detail because mutants Ras proteins are associated with many types of human cancer. These mutant proteins, which bind but cannot hydrolyze GTP, are permanently in the “on” state and contribute to neoplastic transformation.

Most oncogenic, constitutively active Ras protein contain a mutation at position 12. Replacement of the normal glycine-12 with other amino acid blocks the functional binding of GAP, and in essence “lock” Ras in the active GTP-bound state.


The structure of the Ras protein


Alternative mechanisms of transformation by Ras

chapter 5 growth factors, receptors and cancer - 5. 2 ~ 5.9 - mar 27 & 29, 2007

Documents

hepatocyte growth factor

fibroblast growth factor

nerve growth factor

epidermal growth factor

growth factors

growth factor receptor

altered growth factor

plateletderived growth