Signal Transduction: How Cells Communicate Through Chemical Signals

General Principles of Cell Signaling

Cell signaling follows a common logic regardless of the specific signal or pathway involved. A signaling molecule (the ligand) binds to a receptor protein, triggering a conformational change in the receptor. This change activates intracellular signaling proteins, often through phosphorylation or the production of small molecule second messengers. The signal is amplified as it passes through the signaling cascade, so that a small number of activated receptor molecules can produce a large cellular response. Finally, the signal is terminated by specific mechanisms that deactivate signaling components, ensuring that the response is proportional to the stimulus and does not persist after the signal has been removed.

Signaling can occur over different distances. Endocrine signaling uses hormones that travel through the bloodstream to reach distant target cells. Paracrine signaling acts on nearby cells through locally diffusing molecules. Autocrine signaling occurs when a cell responds to signals it produces itself. Juxtacrine signaling requires direct cell-to-cell contact. Despite these differences in range, all four types use the same fundamental mechanism of ligand-receptor binding, signal transduction, and cellular response.

G-Protein Coupled Receptors

G-protein coupled receptors (GPCRs) are the largest family of cell-surface receptors in the human genome, with roughly 800 members. They share a common structure: seven transmembrane alpha-helices connected by intracellular and extracellular loops. GPCRs detect an enormous range of signals including hormones (epinephrine, glucagon), neurotransmitters (serotonin, dopamine), odorant molecules, tastants, and even photons of light (through the visual pigment rhodopsin).

When a ligand binds to the extracellular side of a GPCR, the receptor changes conformation and activates an associated heterotrimeric G protein on the intracellular side. The G protein consists of three subunits: alpha, beta, and gamma. In the inactive state, the alpha subunit binds GDP. Receptor activation causes the alpha subunit to exchange GDP for GTP and dissociate from the beta-gamma complex. Both the GTP-bound alpha subunit and the free beta-gamma complex can activate downstream effector proteins.

The most studied GPCR effector pathway involves adenylyl cyclase, the enzyme that converts ATP to cyclic AMP (cAMP). Stimulatory G proteins (Gs) activate adenylyl cyclase, raising cAMP levels. Inhibitory G proteins (Gi) inhibit it. cAMP activates protein kinase A (PKA), which phosphorylates target proteins to produce the cellular response. When epinephrine binds to beta-adrenergic receptors on liver cells, this pathway activates glycogen phosphorylase and stimulates glucose release into the blood, preparing the body for physical activity.

Another major GPCR effector is phospholipase C, activated by Gq-type G proteins. Phospholipase C cleaves the membrane lipid PIP2 into two second messengers: IP3, which releases calcium from the endoplasmic reticulum, and DAG, which activates protein kinase C (PKC). This pathway controls diverse cellular responses including smooth muscle contraction, secretion, and platelet activation.

Receptor Tyrosine Kinases

Receptor tyrosine kinases (RTKs) are cell-surface receptors that respond primarily to growth factors and play central roles in cell growth, differentiation, and survival. Major RTK ligands include epidermal growth factor (EGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), insulin, and nerve growth factor (NGF).

RTKs consist of an extracellular ligand-binding domain, a single transmembrane helix, and an intracellular domain with tyrosine kinase activity. Ligand binding causes two receptor molecules to dimerize (come together as a pair). The kinase domains of the paired receptors then phosphorylate each other on specific tyrosine residues, a process called autophosphorylation. These phosphorylated tyrosines serve as docking sites for intracellular signaling proteins that contain SH2 (Src homology 2) or PTB (phosphotyrosine-binding) domains.

One of the most important pathways downstream of RTKs is the Ras-MAPK pathway. The small GTPase Ras is activated at the membrane and triggers a cascade of protein kinases: Ras activates Raf (a MAP kinase kinase kinase), Raf phosphorylates and activates MEK (a MAP kinase kinase), and MEK phosphorylates and activates ERK (a MAP kinase). ERK then enters the nucleus and phosphorylates transcription factors that drive the expression of genes promoting cell growth and division.

Mutations that constitutively activate Ras or RTKs are found in a large fraction of human cancers. Approximately 30% of all human tumors contain activating mutations in Ras genes. Several successful cancer drugs target RTKs: imatinib (Gleevec) inhibits the BCR-ABL tyrosine kinase in chronic myeloid leukemia, trastuzumab (Herceptin) targets the HER2 receptor in breast cancer, and erlotinib inhibits the EGF receptor in lung cancer.

Second Messengers

Second messengers are small, rapidly diffusible molecules that relay and amplify signals from receptors to intracellular targets. Because they are small and mobile, they can spread the signal quickly throughout the cell.

Cyclic AMP (cAMP) is produced by adenylyl cyclase and degraded by phosphodiesterase. It activates protein kinase A, which phosphorylates serine and threonine residues on target proteins. cAMP-mediated signaling controls glycogen metabolism, heart rate, and gene expression through the transcription factor CREB (cAMP response element-binding protein).

Calcium ions (Ca2+) are perhaps the most versatile second messenger. Resting cells maintain cytoplasmic calcium at extremely low levels (around 100 nanomolar), roughly 10,000-fold lower than the concentration outside the cell or within the endoplasmic reticulum. Signaling events open calcium channels, causing a rapid influx that activates calcium-binding proteins such as calmodulin. The calmodulin-calcium complex activates calmodulin-dependent protein kinases (CaM kinases) that regulate muscle contraction, neurotransmitter release, gene expression, and many other processes.

Phosphoinositides, phosphorylated derivatives of phosphatidylinositol, serve as both membrane-localized signals and precursors for soluble second messengers. PI3-kinase phosphorylates PIP2 to produce PIP3, which recruits proteins containing PH (pleckstrin homology) domains to the membrane, including the kinase Akt (protein kinase B). The PI3K-Akt pathway promotes cell survival, growth, and metabolism, and its dysregulation is one of the most common events in cancer.

Nuclear Receptors

Not all signaling molecules bind to cell-surface receptors. Steroid hormones (cortisol, estradiol, testosterone, aldosterone), thyroid hormones, retinoic acid, and vitamin D are small, hydrophobic molecules that can cross the plasma membrane directly and bind to intracellular receptors called nuclear receptors. These receptors function as ligand-activated transcription factors: when the hormone binds, the receptor undergoes a conformational change that allows it to bind to specific DNA sequences (hormone response elements) in the promoter regions of target genes, directly activating or repressing gene transcription.

Because nuclear receptor signaling works through changes in gene expression, its effects are slower (hours to days) but longer-lasting than the rapid, transient responses mediated by cell-surface receptors. Cortisol binding to the glucocorticoid receptor stimulates transcription of genes encoding gluconeogenic enzymes in the liver, producing a sustained increase in blood glucose during fasting. Estrogen binding to estrogen receptors drives the expression of genes involved in cell proliferation, which is why some breast cancers that express estrogen receptors are treated with anti-estrogen drugs like tamoxifen.

Signal Amplification and Termination

Signal transduction cascades amplify signals at multiple steps. A single activated receptor can activate many G proteins. Each G protein can stimulate adenylyl cyclase to produce many cAMP molecules. Each PKA molecule can phosphorylate many target proteins. This multiplicative amplification means that a few molecules of hormone binding to the cell surface can mobilize millions of substrate molecules inside the cell. In the epinephrine signaling cascade in liver cells, a single epinephrine molecule can trigger the release of approximately 10^8 glucose molecules from glycogen.

Signal termination is equally important. Without termination mechanisms, signals would persist indefinitely, leading to uncontrolled cellular responses. G proteins have intrinsic GTPase activity that hydrolyzes GTP back to GDP, returning the G protein to its inactive state. Phosphodiesterases degrade cyclic nucleotides. Protein phosphatases remove phosphate groups added by kinases, reversing phosphorylation-dependent activation. Receptor internalization and degradation reduce the number of receptors available on the cell surface. Each of these mechanisms ensures that signaling is transient and precisely controlled.

Cells also integrate information from multiple signaling pathways through cross-talk, where components of one pathway influence the activity of another. The Ras-MAPK and PI3K-Akt pathways, for example, are both activated by receptor tyrosine kinases and share upstream activators, yet they regulate distinct downstream targets. Scaffold proteins help organize signaling complexes and prevent unwanted cross-talk by physically anchoring specific kinases together, ensuring that signals are transmitted along the correct pathway. Disruption of pathway cross-talk and scaffold function can contribute to the uncontrolled signaling seen in cancer and other diseases.