Tyrosine Kinases and Signaling by the MAP Kinase and PI 3-Kinase Pathways

Receptor Tyrosine Kinases (RTKs): Structure and Activation

• RTK Structural Composition:   - Receptor tyrosine kinases are comprised of three distinct structural components:     - Extracellular ligand-binding domain: Located at the N-terminus, this domain is responsible for recognizing and binding specific signaling molecules (ligands).     - Transmembrane α-helix: A single segment that spans the plasma membrane, anchoring the receptor.     - Cytosolic C-terminal domain: Contains intrinsic tyrosine kinase activity, allowing the receptor to phosphorylate specific tyrosine residues.
• Primary Ligands and Mediators:   - RTKs serve as the major mediators for growth factor signaling. Key examples include:     - Epidermal Growth Factor (EGF)     - Platelet-Derived Growth Factor (PDGF)     - Insulin     - Nerve Growth Factor (NGF)
• The Mechanism of Activation:   - Ligand-Induced Dimerization: The binding of a growth factor to the extracellular domain induces a conformational change that causes two individual receptor polypeptide chains to come together, forming a dimer.   - Cross-phosphorylation (Autophosphorylation):     - Once dimerized, the two cytosolic kinase domains are brought into close proximity.     - The kinase domains cross-phosphorylate each other on specific tyrosine residues located within the cytosolic tail.
• Resultant Functionality:   - Activation: Phosphorylation increases the enzymatic activity of the kinase domains.   - Docking Site Creation: The phosphorylated tyrosine residues act as specific high-affinity binding sites for downstream signaling proteins.   - Signal Triggering: Activated RTKs initiate intracellular phosphorylation cascades that regulate critical cellular processes including metabolism, survival, differentiation, and proliferation.

SH2 Domain Recognition and Downstream Propagation

• The Role of SH2 Domains:   - Definition: The SH2 (Src Homology 2) domain is a modular protein domain specifically designed to recognize and bind to motifs containing phosphotyrosine.   - Molecular Adaptors: SH2 domains function as adaptors, linking the activated, phosphorylated receptor to an array of intracellular signaling proteins.
• Localization and Activation:   - The recruitment of SH2-containing proteins to the plasma membrane localizes them near the receptor.   - This positioning allows these proteins to be activated either by the receptor's kinase activity or by being placed in proximity to their specific targets at the membrane.
• Key Signaling Cascades Linked to RTKs:   - Through SH2 domain interactions, receptors connect to several pivotal pathways, including:     - Ras/MAP kinase pathway     - PI 3-kinase/Akt pathway     - Phospholipase C-γ ($PLC\gamma$) pathways

Nonreceptor Tyrosine Kinases and the Cytokine Receptor Superfamily

• Basic Mechanism of Cytokine Receptors:   - Unlike RTKs, cytokine receptors lack intrinsic enzymatic (kinase) activity.   - They function by associating with cytoplasmic nonreceptor tyrosine kinases, most notably the Janus kinase ($JAK$) family.   - Activation Process:     - Ligand binding (e.g., interleukins, cytokines) induces receptor dimerization.     - This dimerization brings associated $JAK$s into proximity, leading to cross-phosphorylation of the kinases themselves.     - The activated $JAK$s then phosphorylate specific tyrosine residues on the receptor, creating phosphotyrosine docking sites for downstream effectors.
• The JAK/STAT Pathway:   - STAT Proteins: Signal Transducer and Activator of Transcription proteins contain SH2 domains that allow them to bind to the phosphorylated cytokine receptor sites.   - Phosphorylation and Dimerization: Once the $STAT$ protein binds to the receptor, it is phosphorylated by the receptor-associated $JAK$.   - Nuclear Translocation: Phosphorylated $STAT$ proteins dimerize and move from the cytosol into the nucleus.   - Transcriptional Regulation: $STAT$ dimers function as transcription factors, directly binding to DNA to regulate genes involved in immune responses, growth, and differentiation.
• Physiological Examples:   - Interleukin-2 ($IL-2$): Triggers the development of leukocytes.   - Erythropoietin ($EPO$): Stimulates the bone marrow to produce red blood cells.

Integrin Signaling and Src Family Kinases

• Src Family Kinases:   - These include $Src$, $Fyn$, and other related kinases.   - They associate with multiple receptor types, including cytokine receptors, growth factor receptors, and integrins.   - Their primary roles involve signaling for cell migration, adhesion, survival, and proliferation.
• Integrin Signaling via Focal Adhesion Kinase ($FAK$):   - Integrin Clustering: Binding of integrins to the extracellular matrix (ECM) causes them to cluster together.   - FAK Autophosphorylation: Clustering triggers the activation of $FAK$ through autophosphorylation.   - Src Recruitment: $Src$ binds to the initial $FAK$ autophosphorylation site.   - Further Phosphorylation: $Src$ then phosphorylates $FAK$ on additional tyrosine residues.   - Downstream Coupling: These multiple phosphotyrosine sites serve as docking points for various signaling proteins, effectively linking physical cell adhesion to cytoskeletal reorganization and growth control.

The ERK MAP Kinase Pathway

• Overview of MAP Kinases:   - Mitogen-Activated Protein (MAP) kinase pathways are evolutionary conserved cascades that translate extracellular signals into specific cellular responses.
• The Sequential Activation Relay:   1. RTK Activation: Growth factor binding leads to RTK autophosphorylation.   2. Adaptor Recruitment: The adaptor protein $Grb2$ and a Guanine Nucleotide Exchange Factor ($GEF$), such as $SOS$, are recruited to the receptor.   3. Ras Activation: The $GEF$ stimulates $Ras$ to exchange $GDP$ (inactive state) for $GTP$ (active state).   4. Raf Activation: Active $Ras-GTP$ recruits and activates $Raf$ (a MAP kinase kinase kinase / $MAPKKK$).   5. MEK Activation: $Raf$ phosphorylates and activates $MEK$ (a MAP kinase kinase / $MAPKK$).   6. ERK Activation: $MEK$, a dual-specificity kinase, phosphorylates $ERK$ (MAP kinase) on two specific residues: threonine-183 ($Thr-183$) and tyrosine-185 ($Tyr-185$).
• Regulation of Ras activity:   - $GEFs$: Promote activity by facilitating the exchange of $GDP$ for $GTP$.   - $GAPs$ (GTPase-activating proteins): Inhibit activity by stimulating the intrinsic GTPase activity of $Ras$, leading to $GTP$ hydrolysis back to $GDP$.

ERK Nuclear Signaling and Gene Induction

• Nuclear Translocation: Activated $ERK$ moves from the cytoplasm into the nucleus to target transcription factors.
• Immediate-Early Gene Induction:   - Elk-1 Phosphorylation: $ERK$ phosphorylates the transcription factor $Elk-1$.   - Complex Formation: Phosphorylated $Elk-1$ forms a complex with the Serum Response Factor ($SRF$).   - SRE Binding: This complex binds to the Serum Response Element ($SRE$) within the promoter regions of target genes.   - Rapid Expression: This process activates "immediate-early" genes such as $c-Fos$ and $Egr-1$, which are expressed within minutes of stimulation.
• Secondary Response: The protein products of immediate-early genes subsequently act as activators for "secondary response" genes, driving long-term changes in cell metabolism and growth.

Additional MAP Kinase Pathways and Scaffold Proteins

• JNK and p38 Pathways:   - Mammalian cells contain multiple MAP kinase pathways beyond $ERK$, notably $JNK$ ($c-Jun$ N-terminal kinase) and $p38$ MAP kinase.   - Small GTPase Regulation: These pathways are activated by the $Rho$ subfamily of proteins ($Rac$, $Rho$, and $Cdc42$) rather than $Ras$.   - Stimuli: These pathways are typically triggered by inflammatory cytokines or environmental stress (e.g., osmotic shock, UV radiation).   - Outcomes: They generally lead to inflammation, cell cycle arrest, or apoptosis ($cell death$).
• Scaffold Proteins:   - Scaffold proteins insulate distinct kinase cascades to prevent crosstalk and ensure efficiency.   - Example: The $KSR$ scaffold protein binds $Raf$, $MEK$, and $ERK$ simultaneously, organizing them into a discrete functional module.

The PI 3-Kinase/Akt and mTOR Pathways

• PI 3-Kinase/Akt Activation:   - Recruitment: PI 3-kinase is recruited to activated RTKs via its SH2 domain.   - Lipid Phosphorylation: PI 3-kinase phosphorylates the 3rd position of the inositol ring of phosphatidylinositol 4,5-bisphosphate ($PIP_2$), converting it to phosphatidylinositol 3,4,5-trisphosphate ($PIP_3$).   - Akt Recruitment: $Akt$ (Protein Kinase B) binds to $PIP_3$ at the membrane using its Pleckstrin Homology ($PH$) domain.   - Akt Activation: $Akt$ is phosphorylated and activated by $PDK1$ and $mTORC2$, both of which are also recruited by $PIP_3$.
• Targets of Akt:   - Bad: Phosphorylation of $Bad$ inhibits its pro-apoptotic function, promoting cell survival.   - GSK-3: $Akt$ inhibits $GSK-3$ via phosphorylation. When active (not inhibited), $GSK-3$ phosphorylates the translation initiation factor $eIF2B$, metabolic enzymes, and transcription factors.
• Regulation of FOXO Transcription Factors:   - FOXO Function: In the absence of growth factors, $FOXO$ (e.g., $FOXO1$, $FOXO3$) resides in the nucleus, inducing genes for cell cycle arrest and apoptosis.   - Akt Inhibition of FOXO: Growth factor stimulation leads to $Akt$ phosphorylating $FOXO$. This creates a binding site for the cytosolic chaperone protein $14-3-3$.   - Nuclear Export: $14-3-3$ sequesters $FOXO$ in the cytoplasm, preventing it from activating pro-death genes.
• The mTOR Pathway:   - $mTOR$ functions downstream of $Akt$.   - It serves as a master regulator that integrates signals regarding nutrients, energy status, and growth factors.   - Functions: Stimulates protein synthesis and overall cell growth while simultaneously inhibiting autophagy (cellular self-digestion).