Medical School · Year 1 · Foundations · includes a quiz and discussion video

Lecture 16: Signal Transduction Pathways

Unit 1.1: Foundations of Medicine & Medical Sciences

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the general principles of signal transduction including ligand-receptor interactions and signal amplification
2. Explain the structure and function of G-protein coupled receptors (GPCRs) and their downstream signaling cascades
3. Describe receptor tyrosine kinase (RTK) signaling including the RAS-MAPK and PI3K-AKT pathways
4. Identify second messenger systems including cAMP, calcium, and phosphoinositides
5. Explain how signaling pathway dysregulation contributes to disease, particularly cancer
6. Identify therapeutic drugs that target signaling pathways

Principles of Signal Transduction

Overview

Cells do not exist in isolation. They must sense their environment, communicate with neighboring cells, and respond to signals from distant tissues. Signal transduction is the process by which extracellular signals are converted into intracellular responses—a chain of molecular events that begins when a signaling molecule (ligand) binds its receptor and culminates in changes to cell behavior, including altered gene expression, metabolic activity, movement, or survival. Understanding signal transduction is essential for modern medicine because dysregulation of signaling pathways underlies many diseases, and many of our most effective drugs work by modulating these pathways.

General Features

Signal transduction exhibits several key features that enable precise control of cellular responses. Specificity arises from the complementary fit between ligands and their receptors, ensuring that cells respond only to appropriate signals. Amplification means that a single ligand molecule can trigger a cascade of events, with each step producing many activated molecules—this allows tiny amounts of hormone to produce large cellular effects. Modularity refers to the reuse of signaling components in different contexts; the same protein kinase, for example, may participate in multiple pathways in different cell types. Feedback regulation, both positive and negative, fine-tunes responses and enables oscillations or switch-like behavior. Finally, integration allows cells to combine multiple simultaneous signals to produce a unified response appropriate to the overall context.

Types of Signaling

Cells communicate over different distances. Endocrine signaling involves hormones released into the bloodstream that travel throughout the body to reach distant target cells; insulin released from pancreatic beta cells affecting liver, muscle, and fat is a classic example. Paracrine signaling acts locally, with secreted factors affecting nearby cells; growth factors released during wound healing exemplify this mode. Autocrine signaling occurs when cells respond to signals they themselves produce, creating feedback loops; this is common in cancer cells that produce their own growth factors. Juxtacrine signaling requires direct cell-cell contact, as in Notch pathway signaling where both the ligand and receptor are membrane-bound proteins on adjacent cells.

<image>Panel A: Cell with an extracellular signal (hexagon) binding to a membrane receptor (Y-shaped or serpentine), triggering an intracellular cascade showing amplification (one molecule activates 10, then 100, then 1000 as expanding arrays). Panel B: Multiple cellular responses from the cascade: gene expression (nucleus with DNA transcribed), metabolic changes (enzyme diagrams), cytoskeletal reorganization (actin filaments), and cell survival/death decisions. Panel C: Endocrine signaling (bloodstream carrying hormones to distant organs) and paracrine signaling (cells releasing factors to nearby cells). Panel D: Autocrine signaling (a cell releasing and receiving its own signal) and juxtacrine signaling (two adjacent cells with membrane-bound ligand and receptor touching).</image>

Receptor Types

Classification by Location

Receptors fall into two broad categories based on where they reside. Cell surface receptors span the plasma membrane and detect water-soluble (hydrophilic) signals that cannot cross the lipid bilayer. These include G-protein coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), and ligand-gated ion channels. Intracellular receptors reside in the cytoplasm or nucleus and bind lipophilic signals—steroid hormones, thyroid hormone, vitamin D, and retinoids—that can diffuse across the plasma membrane. Upon ligand binding, intracellular receptors typically act directly as transcription factors.

Major Receptor Classes

G-protein coupled receptors (GPCRs) are the largest family, with approximately 800 members in humans. They are characterized by seven membrane-spanning domains and signal through heterotrimeric G proteins to regulate second messenger production. Approximately 35% of all FDA-approved drugs target GPCRs.

Receptor tyrosine kinases (RTKs) have intrinsic enzyme activity—their intracellular domains are tyrosine kinases that become activated upon ligand binding. They signal through cascades including the RAS-MAPK and PI3K-AKT pathways.

Cytokine receptors lack intrinsic kinase activity but associate with JAK kinases that phosphorylate STAT transcription factors. Receptor serine/threonine kinases (exemplified by TGF-β receptors) phosphorylate SMAD proteins. Ion channel receptors open in response to ligand binding, allowing ion flux that changes membrane potential. Nuclear receptors act as ligand-activated transcription factors directly binding DNA.

G-Protein Coupled Receptors (GPCRs)

Structure

GPCRs are also called seven-transmembrane (7TM) receptors because their single polypeptide chain crosses the membrane seven times, forming a characteristic serpentine structure. The extracellular loops and N-terminus participate in ligand binding, while the intracellular loops and C-terminus interact with heterotrimeric G proteins. This receptor family senses an enormous range of signals: photons (rhodopsin in the retina), odorants, hormones (epinephrine, glucagon), neurotransmitters (dopamine, serotonin), and local mediators. Their physiological importance is reflected in their status as the target of approximately one-third of all pharmaceutical drugs.

Heterotrimeric G Proteins

GPCRs signal through heterotrimeric G proteins, named for their ability to bind guanine nucleotides (GDP and GTP). Each G protein consists of three subunits: α, β, and γ. The α subunit binds GDP in the inactive state; upon activation, it exchanges GDP for GTP and dissociates from the βγ dimer. Both the GTP-bound α subunit and the free βγ dimer can activate downstream effectors.

Different α subunits couple to different effectors. Gαs (stimulatory) activates adenylyl cyclase, increasing cAMP production. Gαi (inhibitory) inhibits adenylyl cyclase, decreasing cAMP. Gαq activates phospholipase C, generating the second messengers IP₃ and diacylglycerol (DAG). These different coupling mechanisms explain how different GPCRs can produce different cellular responses.

The GPCR Activation Cycle

The GPCR signaling cycle proceeds through defined steps. First, ligand binding induces a conformational change in the receptor that allows it to function as a guanine nucleotide exchange factor (GEF) for the associated G protein. The receptor catalyzes exchange of GDP for GTP on the α subunit. GTP binding induces conformational changes that cause the α subunit to dissociate from the βγ dimer. Both the activated α subunit and the free βγ dimer can now activate downstream effectors.

Signaling terminates when the α subunit hydrolyzes GTP to GDP through its intrinsic GTPase activity, a process accelerated by GTPase-activating proteins (GAPs) called RGS proteins (regulators of G-protein signaling). The GDP-bound α subunit then reassociates with βγ, reforming the inactive heterotrimer ready for another cycle of activation.

<image>Panel A: Inactive GPCR (serpentine receptor) with inactive heterotrimeric G protein (G-alpha-GDP bound to G-beta-gamma) on its cytoplasmic face, and a ligand (small hexagon) binding the extracellular portion causing conformational change. Panel B: GDP-GTP exchange on the G-alpha subunit (GDP departing, GTP arriving) as the receptor acts as a GEF, followed by active G-alpha-GTP dissociating from G-beta-gamma. Panel C: Both G-alpha-GTP and free G-beta-gamma activating their respective effectors (adenylyl cyclase for G-alpha-s, ion channels for G-beta-gamma), with time estimates for each step. Panel D: Signal termination with G-alpha hydrolyzing GTP to GDP (GTPase activity), becoming G-alpha-GDP and reassociating with G-beta-gamma to return to the inactive state, completing the cycle.</image>

The cAMP Signaling Pathway

One of the best-characterized GPCR pathways is the cAMP cascade. When Gαs activates adenylyl cyclase (an enzyme embedded in the plasma membrane), this enzyme catalyzes conversion of ATP to cyclic AMP (cAMP). cAMP is the second messenger—it transmits the signal from the receptor-G protein complex at the membrane to targets throughout the cell.

The primary target of cAMP is protein kinase A (PKA), a serine/threonine kinase. In its inactive state, PKA is a tetramer of two catalytic subunits bound to two regulatory subunits. cAMP binds the regulatory subunits, causing them to release the catalytic subunits, which become active and phosphorylate numerous substrates. PKA activates phosphorylase kinase (initiating glycogen breakdown), phosphorylates the transcription factor CREB (activating cAMP-responsive genes), and modifies ion channels and metabolic enzymes.

The signal terminates when phosphodiesterases hydrolyze cAMP to AMP, and phosphatases remove the phosphates added by PKA.

The β-adrenergic response exemplifies this pathway. When epinephrine binds β-adrenergic receptors on heart cells, the resulting cAMP/PKA activation increases heart rate and contractility—the "fight-or-flight" response. β-blockers (propranolol, metoprolol) are widely used cardiovascular drugs that antagonize these receptors, reducing heart rate and blood pressure.

<image>Panel A: Beta-adrenergic receptor (serpentine) binding epinephrine at the plasma membrane, coupled to G-alpha-s which activates adenylyl cyclase (cylinder spanning the membrane) to convert ATP to cAMP (cyclic structure shown). Panel B: PKA in two states: inactive (regulatory and catalytic subunits together as a tetramer) and active (cAMP binding to regulatory subunits releasing free catalytic subunits). Panel C: Active PKA phosphorylating multiple substrates: phosphorylase kinase (glycogen breakdown), CREB (gene transcription at the nucleus), and cardiac proteins (increased heart rate). Panel D: Signal termination by phosphodiesterase degrading cAMP to AMP, and a beta-blocker (drug symbol) shown blocking the receptor to illustrate pharmacological intervention.</image>

The Phospholipase C Pathway

GPCRs coupled to Gαq activate phospholipase C (PLC), an enzyme that cleaves the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP₂) into two second messengers: inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG).

IP₃ is water-soluble and diffuses through the cytoplasm to the endoplasmic reticulum, where it binds IP₃ receptors—calcium channels in the ER membrane. Opening these channels releases stored calcium into the cytoplasm, rapidly raising cytoplasmic calcium concentration from approximately 100 nM to over 1 μM. Calcium then acts as a second messenger, binding calmodulin and other calcium-binding proteins to activate downstream effectors including calcium/calmodulin-dependent kinases (CaMKs), calcineurin (a phosphatase important in T cell activation), and nitric oxide synthase.

DAG remains in the membrane, where it cooperates with calcium to activate protein kinase C (PKC). PKC is a family of serine/threonine kinases with diverse cellular roles including cell proliferation, differentiation, and survival. The PKC family includes conventional isoforms (requiring both DAG and calcium), novel isoforms (requiring DAG but not calcium), and atypical isoforms (requiring neither).

<image>Panel A: GPCR coupled to G-alpha-q activating phospholipase C (PLC) at the membrane, which cleaves PIP2 (shown with phosphate head groups) into IP3 (released into cytoplasm as small triangular molecule) and DAG (remaining in membrane as lipid with two tails). Panel B: IP3 pathway showing IP3 diffusing to the ER, binding IP3 receptors (channels in ER membrane) to release calcium (dots flooding into cytoplasm), with calcium binding calmodulin (dumbbell shape) and activating CaMKs, calcineurin, and NOS. Panel C: DAG pathway showing DAG recruiting PKC from the cytoplasm to the membrane where it is activated by both DAG and calcium, with active PKC phosphorylating various substrates. Panel D: Integration of both IP3/calcium and DAG/PKC pathways emphasizing how they converge through PKC activation and cooperate to produce the full cellular response.</image>

Receptor Tyrosine Kinases (RTKs)

Structure

Receptor tyrosine kinases are single-pass transmembrane proteins with an extracellular ligand-binding domain, a single membrane-spanning helix, and an intracellular domain containing tyrosine kinase activity. Important RTKs include the epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), insulin receptor, and vascular endothelial growth factor receptor (VEGFR). These receptors typically bind growth factors that regulate cell proliferation, differentiation, survival, and metabolism.

Activation Mechanism

RTK activation follows a common pattern. Ligand binding induces receptor dimerization—two receptor molecules come together. The proximity of the two intracellular kinase domains allows them to phosphorylate each other on specific tyrosine residues, a process called trans-autophosphorylation. These phosphotyrosines serve as docking sites for downstream signaling proteins.

Proteins containing SH2 (Src homology 2) domains or PTB (phosphotyrosine-binding) domains recognize and bind specific phosphotyrosine motifs. Different phosphotyrosines recruit different signaling proteins, enabling a single activated receptor to engage multiple downstream pathways simultaneously. Key recruited proteins include Grb2 (an adaptor linking to the RAS-MAPK pathway), the p85 subunit of PI3K (activating the PI3K-AKT pathway), and PLCγ (generating IP₃ and DAG).

<image>Panel A: Two monomeric RTK receptors (each with extracellular domain, single transmembrane helix, and intracellular kinase domain) separated and inactive with no phosphotyrosines, and a dimeric growth factor ligand approaching. Panel B: Activated state with the ligand bound, bringing the two receptors together as a dimer, with kinase domains facing each other and arrows indicating trans-autophosphorylation (each kinase phosphorylating the other), producing multiple phosphotyrosines (P-Tyr). Panel C: Signaling proteins with SH2 domains docking at specific phosphotyrosines: Grb2 (adaptor leading to RAS) and Shc (another adaptor), each color-coded with arrows to their downstream pathways. Panel D: Additional docking proteins: p85 of PI3K (leading to AKT pathway) and PLC-gamma (leading to calcium signaling), illustrating how a single activated receptor simultaneously engages multiple downstream pathways.</image>

The RAS-MAPK Pathway

Components

The RAS-MAPK pathway is the prototypical mitogen-activated protein kinase cascade, driving cell proliferation and differentiation in response to growth factors. RAS is a small GTPase that acts as a molecular switch—active when bound to GTP, inactive when bound to GDP. RAF (also called MAPKKK, or MAP kinase kinase kinase) is the first kinase in the cascade. MEK (MAPKK) is the second kinase, notable for its dual specificity—it phosphorylates both tyrosine and threonine residues on its substrate. ERK (MAPK) is the final kinase, which translocates to the nucleus to phosphorylate transcription factors.

Pathway Mechanism

When an RTK is activated, the adaptor protein Grb2 binds to specific phosphotyrosines through its SH2 domain. Grb2 also has SH3 domains that bind SOS (Son of Sevenless), a guanine nucleotide exchange factor (GEF) for RAS. SOS recruitment to the membrane brings it into contact with RAS, where it catalyzes exchange of GDP for GTP, activating RAS.

Active RAS-GTP undergoes conformational changes that allow it to bind and recruit RAF to the plasma membrane, where RAF becomes activated through a complex process involving phosphorylation and conformational changes. Active RAF phosphorylates and activates MEK. Active MEK phosphorylates and activates ERK on both threonine and tyrosine residues.

Active ERK has many substrates, both in the cytoplasm and nucleus. ERK translocates into the nucleus where it phosphorylates transcription factors including Elk-1 and contributes to stabilization and activation of Myc. These transcription factors drive expression of genes promoting cell cycle progression and proliferation.

RAS as Molecular Switch

RAS exemplifies the molecular switch concept that governs many signaling proteins. Like the α subunit of heterotrimeric G proteins, RAS cycles between active (GTP-bound) and inactive (GDP-bound) states. GEFs (like SOS) accelerate the activating GDP→GTP exchange. GAPs (GTPase-activating proteins, like NF1) accelerate the inactivating GTP→GDP hydrolysis. The balance between GEF and GAP activity determines the proportion of RAS in the active state.

RAS mutations are among the most common oncogenic mutations in human cancer. Mutations at codons 12, 13, or 61 impair GTPase activity, locking RAS in the active GTP-bound state. This constitutive activation drives persistent proliferative signaling even in the absence of growth factors. KRAS mutations occur in approximately 90% of pancreatic cancers, 45% of colorectal cancers, and 30% of lung adenocarcinomas.

<image>Panel A: Activated RTK (dimer with phosphotyrosines) recruiting Grb2 (adaptor with SH2 and SH3 domains) via SH2-phosphotyrosine interaction, with Grb2's SH3 domains binding SOS (elongated protein) which contacts RAS at the membrane (lipid tail anchor). Panel B: Detailed inset showing RAS transitioning from GDP-bound (inactive, gray) to GTP-bound (active, green) with SOS catalyzing the exchange, and active RAS recruiting RAF to the membrane where RAF becomes activated by phosphorylation. Panel C: Three-tiered kinase cascade (MAPKKK to MAPKK to MAPK): RAF phosphorylates MEK, MEK phosphorylates ERK on both Tyr and Thr residues (dual phosphorylation indicated). Panel D: ERK translocating to the nucleus (arrow through nuclear envelope) to phosphorylate transcription factors (Elk-1, contributing to Myc activation), with target genes (cyclin D, Fos) listed and "Proliferation" as the outcome.</image>

The PI3K-AKT Pathway

Components

The PI3K-AKT pathway promotes cell survival and growth. Phosphatidylinositol 3-kinase (PI3K) is a lipid kinase that phosphorylates membrane phosphoinositides. PIP₃ (phosphatidylinositol 3,4,5-trisphosphate) is the lipid second messenger generated by PI3K. PDK1 (phosphoinositide-dependent kinase 1) is recruited to the membrane by PIP₃ and phosphorylates AKT. AKT (also called protein kinase B, PKB) is the central kinase, often described as a "node" because of its many substrates. PTEN (phosphatase and tensin homolog) is a lipid phosphatase that opposes PI3K by converting PIP₃ back to PIP₂.

Pathway Mechanism

Upon RTK activation, the p85 regulatory subunit of PI3K binds to specific phosphotyrosines, recruiting the p110 catalytic subunit to the membrane. There, PI3K phosphorylates PIP₂ to generate PIP₃. This phospholipid second messenger serves as a membrane recruitment signal for proteins containing PH (pleckstrin homology) domains.

Both AKT and PDK1 have PH domains and are recruited to the membrane by PIP₃. The proximity of PDK1 to AKT at the membrane allows PDK1 to phosphorylate AKT on threonine 308. Full activation of AKT requires additional phosphorylation on serine 473, mediated by mTORC2 (mechanistic target of rapamycin complex 2). Fully active AKT then phosphorylates numerous substrates that promote survival and growth.

AKT Targets and Cellular Effects

AKT phosphorylates a diverse array of substrates to promote cell survival and growth. The pro-apoptotic protein BAD is phosphorylated by AKT, causing it to bind 14-3-3 proteins in the cytoplasm rather than inhibiting anti-apoptotic Bcl-2 family members—this promotes survival. FOXO transcription factors are phosphorylated by AKT, causing their exclusion from the nucleus and preventing transcription of pro-apoptotic and cell cycle arrest genes. GSK3 (glycogen synthase kinase 3) is phosphorylated and inhibited by AKT, relieving GSK3's inhibition of glycogen synthase and promoting glycogen synthesis. mTORC1 is activated downstream of AKT (through inhibition of TSC1/2), promoting protein synthesis and cell growth. The CDK inhibitor p27 is phosphorylated by AKT, causing its cytoplasmic retention and preventing cell cycle arrest.

PTEN as Tumor Suppressor

PTEN is a critical negative regulator of the PI3K pathway. By dephosphorylating PIP₃ to PIP₂, PTEN reverses the action of PI3K and turns off AKT signaling. Loss of PTEN function, through mutation, deletion, or epigenetic silencing, is common in cancer. Without PTEN, PIP₃ accumulates, AKT is constitutively active, and cells have enhanced survival, growth, and proliferation. PTEN loss is particularly common in prostate cancer, glioblastoma, and endometrial cancer.

<image>Panel A: Activated RTK with phosphotyrosines recruiting the p85-p110 PI3K complex at the membrane, with p110 converting PIP2 (two phosphates on inositol ring) to PIP3 (three phosphates), and PTEN shown reversing this reaction (red X over PTEN indicating loss in cancer). Panel B: PIP3 recruiting both PDK1 and AKT to the membrane via PH domains, with PDK1 phosphorylating AKT at T308 and mTORC2 phosphorylating AKT at S473 for full activation. Panel C: Fully active AKT phosphorylating multiple substrates: BAD (prevents apoptosis, survival), FOXO (excluded from nucleus, decreased apoptotic gene expression), and GSK3 (inactivated, increased glycogen synthesis). Panel D: Additional AKT substrates: TSC1/2 (inactivated, activating mTORC1 for protein synthesis/growth) and p27 (cytoplasmic retention, cell cycle progression), with overall outcomes of survival, growth, and metabolism listed.</image>

The JAK-STAT Pathway

Cytokine Receptor Signaling

Many cytokines signal through receptors that lack intrinsic kinase activity. These receptors instead associate with cytoplasmic tyrosine kinases of the JAK (Janus kinase) family—JAK1, JAK2, JAK3, and TYK2. The principal substrates of JAKs are the STAT (signal transducer and activator of transcription) proteins, which serve as both signal transducers and transcription factors.

Pathway Mechanism

Cytokine binding induces receptor dimerization or oligomerization, bringing the associated JAKs into proximity. The JAKs trans-phosphorylate and activate each other. Activated JAKs then phosphorylate tyrosine residues on the receptor's cytoplasmic tail, creating docking sites for STAT proteins. STATs bind these phosphotyrosines through their SH2 domains and are themselves phosphorylated by the JAKs on a conserved tyrosine residue.

Phosphorylated STATs undergo a conformational change and dimerize through reciprocal SH2-phosphotyrosine interactions. STAT dimers translocate to the nucleus, where they bind specific DNA sequences and activate transcription of target genes. Different cytokines activate different combinations of JAKs and STATs, leading to distinct transcriptional programs.

Clinical Relevance

The JAK-STAT pathway has important clinical applications. The JAK2 V617F mutation (a valine-to-phenylalanine substitution at position 617) is present in virtually all cases of polycythemia vera and approximately half of cases of essential thrombocythemia and primary myelofibrosis. This mutation renders JAK2 constitutively active, driving uncontrolled proliferation of hematopoietic cells.

JAK inhibitors have been developed as targeted therapies. Ruxolitinib, a JAK1/2 inhibitor, is approved for myelofibrosis and polycythemia vera. Tofacitinib, a JAK1/3 inhibitor, is approved for rheumatoid arthritis, psoriatic arthritis, and ulcerative colitis, where it suppresses inflammatory cytokine signaling.

<image>Panel A: Cytokine receptor (two chains forming a dimer, each with JAK kinase on the cytoplasmic side) with a cytokine bridging the two receptor chains, and JAKs transphosphorylating each other and phosphorylating receptor tails to create phosphotyrosines. Panel B: STAT proteins (monomers with SH2 domains) docking at phosphotyrosines and being phosphorylated by JAKs, then dimerizing through reciprocal SH2-phosphotyrosine interactions. Panel C: STAT dimer translocating through the nuclear pore to bind specific DNA sequences and activate transcription of target genes (cytokine response genes, cell survival, proliferation), with SOCS proteins shown as a negative feedback mechanism induced by STATs to inhibit JAKs. Panel D: Inset showing the JAK2 V617F mutation location and its consequence of constitutive kinase activity driving myeloproliferative neoplasms, with clinical applications of JAK inhibitors noted.</image>

Nuclear Receptor Signaling

Characteristics

Nuclear receptors are a family of intracellular receptors that act as ligand-activated transcription factors. Their ligands are small, lipophilic molecules that can diffuse across the plasma membrane: steroid hormones (cortisol, aldosterone, estrogen, testosterone, progesterone), thyroid hormone (T3), vitamin D, and retinoids (vitamin A derivatives). Upon ligand binding, nuclear receptors bind specific DNA sequences called hormone response elements and regulate transcription of target genes.

Mechanism

The mechanism varies among receptor subtypes, but a general pattern emerges. The lipophilic ligand diffuses across the plasma membrane and encounters its receptor either in the cytoplasm (for some steroid receptors) or already bound to DNA in the nucleus (for thyroid hormone and retinoid receptors). Ligand binding induces conformational changes that alter the receptor's association with coregulatory proteins.

Unliganded nuclear receptors often associate with corepressor complexes that maintain chromatin in a repressed state. Ligand binding causes release of corepressors and recruitment of coactivators that promote chromatin opening and transcription. The effects of nuclear receptor activation are therefore primarily transcriptional, with a time course measured in hours rather than the seconds-to-minutes typical of cell surface receptor signaling.

Clinical Examples

Glucocorticoid receptors bind cortisol and related hormones, activating anti-inflammatory genes and repressing pro-inflammatory genes. Synthetic glucocorticoids (prednisone, dexamethasone) are among the most widely used anti-inflammatory drugs. Estrogen receptors bind estradiol and regulate genes involved in reproduction, bone metabolism, and cardiovascular function; they are therapeutic targets in breast cancer (tamoxifen is an estrogen receptor antagonist). Thyroid hormone receptors bind T3 and regulate metabolic genes; their dysfunction causes hypothyroidism or hyperthyroidism. Androgen receptors bind testosterone and dihydrotestosterone, regulating male characteristics and prostate growth; androgen receptor antagonists are used in prostate cancer treatment.

Signaling in Disease

Cancer as a Signaling Disease

Cancer can be viewed as a disease of dysregulated signaling. Normal cells require growth factor signals to proliferate, stop dividing when inhibitory signals are present, and die when survival signals are withdrawn. Cancer cells evade these controls, often through mutations that constitutively activate growth-promoting pathways or inactivate growth-inhibiting pathways.

RAS pathway mutations are extraordinarily common. KRAS mutations occur in approximately 25% of all human cancers, with especially high frequencies in pancreatic (90%), colorectal (45%), and lung (30%) adenocarcinomas. BRAF V600E, a mutation that constitutively activates this kinase, is present in approximately 50% of melanomas and a subset of colorectal cancers.

RTK alterations are also frequent. EGFR is amplified or mutated in glioblastoma and lung cancer. HER2 (ERBB2) is amplified in approximately 20% of breast cancers. VEGFR signaling promotes tumor angiogenesis.

PI3K pathway alterations include activating mutations in PIK3CA (the gene encoding the p110α catalytic subunit of PI3K), loss of PTEN, and AKT amplification. These occur frequently in breast, endometrial, and other cancers.

Targeted Therapies

Understanding signaling pathways has enabled development of targeted therapies that inhibit specific pathway components. Imatinib revolutionized treatment of chronic myelogenous leukemia by inhibiting the BCR-ABL fusion kinase; it also inhibits KIT and is effective in gastrointestinal stromal tumors. Trastuzumab is a monoclonal antibody targeting HER2, used in HER2-amplified breast cancer. EGFR inhibitors (gefitinib, erlotinib, osimertinib) are effective in EGFR-mutant lung cancers. Vemurafenib and dabrafenib inhibit BRAF V600E in melanoma; trametinib inhibits MEK downstream of RAF.

These targeted therapies have transformed cancer treatment, converting some previously fatal cancers to manageable chronic diseases. However, resistance commonly develops, often through mutations that bypass the blocked pathway component, highlighting the need for combination approaches and continued pathway research.

<image>Panel A: Schematic cell showing RTK-level oncogenic alterations: EGFR (amplification/mutation in lung, GBM) with gefitinib/erlotinib as targeted therapy, and HER2 (amplification in breast) with trastuzumab, plus RAS mutations (pancreatic, colorectal, lung) noted as "undruggable." Panel B: Downstream MAPK pathway alterations: BRAF V600E (melanoma) with vemurafenib/dabrafenib and MEK with trametinib, showing pathway flow from receptor through kinase cascade to nuclear transcription factors. Panel C: PI3K-AKT pathway alterations: PI3K (mutations in breast, endometrial) with alpelisib and PTEN (loss in prostate, GBM), with arrows to cell proliferation/survival outcomes. Panel D: Legend identifying symbols for activating mutations, gene amplification, loss of function, and drug targets, with overall pathway integration showing how multiple alterations converge on proliferation and survival.</image>

Signal Termination and Regulation

Mechanisms of Signal Termination

For signaling to be meaningful, it must be reversible. Cells have evolved multiple mechanisms to terminate signals. Receptor internalization and degradation remove activated receptors from the cell surface; ligand binding often triggers endocytosis, delivering receptors to lysosomes for degradation. Phosphatases remove the phosphate groups added by kinases, reversing their effects; examples include SHP phosphatases that dephosphorylate RTKs and dual-specificity phosphatases (DUSPs) that dephosphorylate ERK. GTPase activity converts active GTP-bound RAS and G proteins to inactive GDP-bound forms, as discussed previously.

Second messenger degradation terminates signals that operate through small molecule intermediates. Phosphodiesterases hydrolyze cAMP to AMP and cGMP to GMP. Inositol phosphatases degrade IP₃. Calcium is pumped back into the ER or out of the cell by calcium ATPases.

Negative Feedback

Many signaling pathways include negative feedback loops that limit signal duration and amplitude. In the JAK-STAT pathway, STAT-induced transcription includes the SOCS (suppressors of cytokine signaling) genes; SOCS proteins bind JAKs and inhibit their activity, creating a negative feedback loop. ERK phosphorylates SOS, reducing its GEF activity and limiting RAS activation. These feedback mechanisms help ensure that signals produce measured, transient responses rather than runaway activation.

Summary

• Signal transduction converts extracellular signals to cellular responses with amplification
• GPCRs signal through heterotrimeric G proteins and second messengers (cAMP, Ca²⁺, IP₃/DAG)
• RTKs activate RAS-MAPK (proliferation) and PI3K-AKT (survival) pathways
• Cytokine receptors use JAK-STAT pathway
• Pathway dysregulation is common in cancer, with many targeted therapies available
• Negative feedback and signal termination are essential for controlled responses

Key Terms

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