PDK1, PDK2 (phosphoinositide-dependent kinase 1 and 2).
Akt/PKB then phosphorylates downstream targets.
Akt/PKB Phosphorylation
Targets include:
GSK-3β (glycogen synthase kinase 3 beta).
HIF-1α (hypoxia-inducible factor 1-alpha).
Bad (BCL2 associated agonist of cell death).
Leads to:
Proliferation.
Angiogenesis.
Inhibition of apoptosis.
Conceptual Representation of a Signaling Network
Wide input layer (multiple RTKs).
Small number of core processes (PI-3K signaling, MAPK signaling, Ca2+ signaling).
Wide output layer (transcriptional responses and cytoskeletal changes).
Feedback loops exist between core processes and input/output layers.
Feedforward regulation by core processes (e.g., MAPK signaling) of immediate early gene products.
System control occurs between the input and output layers.
Intrinsic Activity of Signaling Molecules
May be modulated by:
Noncovalent modification (e.g., binding of GTP, phospholipid, Ca2+).
Receptor dimerization.
Covalent modification (e.g., phosphorylation).
Proteolytic cleavage.
Concentration of a signaling molecule is modulated by:
Transcriptional regulation.
Protein stability.
Involves intracellular localization and kinetics.
Activation of dormant signaling molecules.
Modular architecture with afferent (incoming) and efferent (outgoing) signals.
Thought Questions (and potential answers)
Why is autocrine signalling an intrinsically destabilising force for a normal tissue?
Autocrine signaling bypasses normal regulatory mechanisms that control cell proliferation and ensures tissue homeostasis. It allows cells to proliferate independently of external cues, disrupting tissue architecture and potentially leading to uncontrolled growth and tumor formation.
Each growth factor elicits its own, quite characteristic set of biological responses in cells. How might you alter a cell so that its biological responses to one growth factor (e.g. EGF) are characteristic instead of the responses that it usually makes after being exposed to another growth factor (e.g. PDGF)?
To alter a cell's response to growth factors, you could modify its receptor repertoire or downstream signaling pathways. For example, you could overexpress the EGF receptor and introduce components of the EGF signaling pathway into cells that normally respond to PDGF. Alternatively, you could disrupt or inhibit signaling molecules specific to the PDGF pathway, redirecting the cell's response towards EGF-like behavior.
The responsiveness of a cell to exposure of growth factor are usually attenuated after a period of time (e.g. half an hour), after which time it loses this responsiveness. Given what you have already learnt about growth factor receptors, what mechanisms might be employed by a cell to reduce its responsiveness to growth factors?
Several mechanisms can attenuate a cell's responsiveness to growth factors over time. Receptor internalization and degradation (endocytosis) can reduce the number of receptors on the cell surface. Also receptor dephosphorylation by phosphatases can reverse the activating phosphorylation events. Furthermore, the induction of negative feedback loops, where downstream signaling molecules inhibit upstream components of the pathway, can dampen the response. Finally desensitization mechanisms can be involved too.
What lines of evidence can you cite to support the notion that growth factor receptor firing following ligand binding is often dependent on the dimerisation of a receptor?
Several lines of evidence support the idea that growth factor receptor firing depends on receptor dimerization. Structural studies have shown that ligand binding often induces receptors to dimerize, bringing their intracellular kinase domains into close proximity. Mutating receptors to prevent dimerization impairs their ability to autophosphorylate and activate downstream signaling pathways. Overexpression of truncated receptors that can bind ligand but lack kinase domains can act as dominant negatives, inhibiting signaling by endogenous receptors by preventing dimerization.