Signaling Molecules: Roles in Differentiation and Proliferation

Signaling Molecules: Roles in Differentiation and Proliferation

Signaling Molecules ●

Receptors – Ion channel linked receptors – Enzyme-linked receptors: single pass ● ●

intrinsic enzyme activities coupled to intracellular enzymes

– G-protein coupled: 7-pass serpentine – Cytoplasmic and nuclear receptors ● ●

Cytoplasmic transducers Nuclear targets

Ion Channel Linked Receptors ! ! !

Commonly called transmitter-gated ion channels or ionotropic receptors Involved in rapid synaptic signaling Classically defined by acetylcholine (ACh) receptor at neuromuscular junction ! Nerve impulse depolarizes axon, signal travels to nerve terminal leading to opening of voltage-gated Ca2+ channels, Ca2+ flows in and ACh is released ! ACh binds receptors on muscle cells leading to opening of cation channel and Na+ flows in

Receptors Associated with Enzyme Activity ●Y

kinases: 9 different classes

– eg. PDGF, FGF, insulin, EGF ●Y

phosphatases

– eg. CD45 ● guanylate

cyclases

– eg. natriuretic peptide receptors ● S/T

kinases

– TGF-β, activin and BMP ● lack

activity but coupled to Y-kinases

Receptors with Tyrosine Kinase Activity

Coupling insulin binding to multiple signal pathways

Intracellular Transducers ● S/T

kinases

– PKA, PKC, MAPK, ERK, Raf, etc. ● phospholipases

– PLCγ ● lipid

kinases

– PI3K, PKB/AKT

Receptor Coupling ● thru

SH2 domains, bind to Y-phos.

– PLCγ, PI3K, grb2, Shc – phosphatases (PTP) ● thru

SH3 domains

– grb2, Sos

SH-2 groups on proteins bind phosphotyrosine SH-3 groups on proteins bind proline-rich regions

G-Protein Coupling ● activation

of adenylate cylcase

– eg, glucagon receptor ● activation

of PLCγ

– eg, angiotensin, vasopressin, bradykinin ● activation

of phosphodiesterase

– transducin in photoreception

Glucagon and epinephrine as examples of cAMP-mediated signaling

G-Protein Regulators ● GAPs:

GTPase activating proteins

– eg, rasGAP – NF1 (neurofibromin) a tumor suppressor – BCR locus in CMLs and ALLs

G Protein cycle of activation and inactivation G Proteins contain intrinsic GTPase activity Accessory Factors GEF Guanine nucleotide Exchange Factor GAP GTPase Activating Protein GDI Guanine nucleotide Dissociation Inhibitor

Activation of Ras signaling pathway by an RTK Phosphotyrosine on receptor is bound by adaptor protein, which binds and activates Guanine Nucleotide Releasing Protein (GNRP) (also GEF). GNRP activates Ras; Ras activates a kinase cascade. Ras GTPase is activated by GAP, turning the pathway off.

Ras activation of MAP Kinase Once Ras is activated, it activates a series of kinases, beginning with the Raf kinase (MAPKKK) Raf activates MAPKK (MEK) MAPKK activates MAPK (ERK) MAPK activates transcription factors

Activation of STAT signaling Receptorassociated PTK is activated and phosphorylates STAT proteins (Signal Transduction and Activator of Transcription), which dimerize and translocate to nucleus where they function as transcription factors

Signaling by Transforming Growth Factor-β superfamily receptors Smad proteins phosphorylated (note S/T intrinsic kinase) by dimerized receptors; Smad complexes act as transcription factors

Cadherins Cell Adhesion Molecules

Activation of signal transduction via cell adhesion molecules

Integrin-mediated signaling through associated Tyr-PK

Another example of signaling through integrins via associated PTK

Convergence, Divergence, and Crosstalk in signal transduction pathways

Signal Divergence, Convergence, and Cross-talk from PDGF Receptor

Convergence of signals to the Ras pathway

Signal divergence from insulin receptor

Vasculogenesis and Angiogenesis Vasculogenesis involves formation of initial blood islands and construction of capillary networks Angiogenesis involves formation of new blood vessels.

Nitric Oxide (NO) signaling NO can act as a diffusible messenger

Intracellular Receptors ● Steroid/Thyroid

hormone super-family of

receptors ● contain ligand and DNA binding domains ● reside in cytoplasm, bind ligand then migrate to nucleus where they interact with specific DNA sequences

Steroid Hormone Receptors

!

Other binding proteins include: – Hsp70 – FKBP (immunophilin) – p23

!

Functions of the binding proteins are unclear – Receptor trafficking

!

Binding proteins dissociate upon ligand binding

Receptor-Binding Proteins

APC in GI Cancers ! FAP

due to germline mutations in APC ! Somatic (sporadic) mutations in the APC gene initiate colorectal malignancy ! APC mutations in 63% of colonic adenomas ! 99% of germline and 100% of sporadic APC mutations lead to truncated protein

Significance of Signal Transduction to Disease: Tumorigenesis ● ●

Cancer cells contain genetic damage many of the genes encoding signaling molecules have been shown to be capable of causing cancer if disrupted in some way

APC Function ! ! ! !

APC associates with β-catenin, a protein involved in the Wnt signaling pathway APC induces β-catenin phos. leading to its degradation β-catenin acts as transcription factor, induces genes like MYC β-catenin also linked to intercellular adhesion molecules, e.g. E-cadherin

Wnt signaling β-catenin bound in complex which includes GSK-3; this kinase phosphorylates βcatenin leading to its degradation. Activation of disheveled causes inhibition of GSK-3. β-catenin builds up, is released from complex, and translocates to nucleus where it forms part of a transcription factor complex to activate gene expression

Chronic Myeloid Leukemia !

CML results from a translocation between chromosomes 9 and 22 ! Result is termed the Ph+ chromosome

!

ABL gene on chromosome 9 placed next to BRC gene on 22 results in a fusion protein bcr-abl ! abl is a non-receptor Y-kinase ! bcr is member of the Rho family of small GTPases

! !

Similar translocation occurs in ALL CML is target of Y-kinase inhibitor Glivec

Non-Hodgkin’s Lymphoma ! ! ! ! ! !

80-90% of NHL show chromosomal aberrations t(8;14) most frequent, also 1st chromosome abnormality to be molecularly characterized Also found in Burkitt lymphoma MYC on 8 placed near immunoglobulin heavy chain gene enhancer on 14 Another common translocation is t(14;18) BCL2 gene on 18 is disrupted, bcl2 involved in control of apoptosis, disruption leads to loss of induced apoptosis