Cell signaling and signal transduction

Cell signaling and signal transduction: Signal transduction is the process of transmitting a signal from a cell-surface receptor to intracellular targets. In most cases, the signal is amplified as it moves through the cell. The initial signal is typically a hormone, growth factor, cytokine, neurotransmitter, drug, etc. The following patterns of signal transmission are seen at the cellular level:

i) G protein coupled receptors or GPCR: G proteins are intracellular signaling proteins that bind GTP (hence the name). They also have intrinsic GTPase activity, meaning they can hydrolyze GTP to GDP.

Heterotrimeric (three-component) G proteins consist of:

• An alpha subunit (G alpha), which binds and hydrolyzes GTP
• Beta and gamma subunits, which inhibit G alpha but can also participate in signaling reactions

GPCRs are plasma membrane receptors with seven transmembrane-spanning regions. They are associated with heterotrimeric G proteins.

• In the unstimulated state, G alpha is bound to GDP and is associated with G beta and gamma, forming an inactive heterotrimer.
• Ligand binding to the GPCR causes a conformational change that leads G alpha to release GDP and bind GTP.
• The G alpha-GTP complex then dissociates from the G beta/gamma subunits and can interact with downstream enzymes.

Three types of G alpha subunits are present: Gs, Gq and Gi. Gs and Gq activate downstream enzymes, while Gi inhibits downstream enzyme activity. These subunits act through specific enzymes to generate second messengers that carry the signal forward.

Enzymes and second messengers associated with G alpha subunits

G alpha subunit: Gs

• Enzyme associated: Adenylyl cyclase is activated
• Second messengers: cAMP increases
• Downstream effects: Activates Protein kinase A

G alpha subunit: Gq

• Enzyme associated: Phospholipase C is activated
• Second messengers: Both IP3 and DAG increase
• Downstream effects: IP3 binds to and opens calcium channels in the endoplasmic reticulum, increasing intracellular calcium; Calcium + DAG activate protein kinase C; DAG may be converted to arachidonic acid to produce prostaglandins

G alpha subunit: Gi

• Enzyme associated: Adenylyl cyclase is inhibited
• Second messengers: cAMP decreases
• Downstream effects: Indirectly opens G protein-coupled inwardly-rectifying potassium channels (GIRKs), increasing K+ entry into the cell and causing hyperpolarization

Eventually, the enzyme phosphodiesterase (PDE) converts cAMP to AMP, which terminates the effects of increased cAMP. Methylxanthines such as theophylline and caffeine inhibit PDE, causing a sustained increase in intracellular cAMP levels.

ii) Catalytic receptors: Most catalytic receptors are single transmembrane protein chains whose cytoplasmic tail has catalytic activity. In other words, the receptor itself functions as an enzyme.

Catalytic receptors with intrinsic tyrosine kinase activity are used by insulin, TGF, EGF, and PDGF. Ligand binding causes autophosphorylation of tyrosine residues on the cytoplasmic tail. These phosphotyrosines then serve as docking sites for adaptor proteins such as Ras and JAK/STATs (signal transducers and activators of transcription).

In some cases, the phosphatidylinositol kinase (PIK) pathway is activated by a catalytic receptor. This pathway is important for cell growth and survival.

• PIK binds to phosphotyrosines on the receptor tail.
• PIK acts on the membrane phospholipid phosphatidylinositol di-phosphate (PIP2) and converts it to phosphatidylinositol triphosphate (PIP3).
• PIP3 docks Akt (Protein Kinase B), which then phosphorylates and inactivates the pro-apoptotic factor Bad, preventing apoptosis.
• PIP3 is finally inactivated by PTEN, which terminates the pathway.

Effects of PTEN gene mutations

• Increase the risk of developing breast, prostate, and endometrial cancers, astrocytomas, glioblastoma, melanoma, etc
• Cause poor responsiveness to trastuzumab (Herceptin)
• Bannayan-Riley-Ruvalcaba syndrome: PTEN mutations, macrocephaly, multiple non-cancerous tumors and hamartomas, and dark freckles on the penis in males
• Cowden Syndrome: PTEN mutations, multiple hamartomas, and an increased risk of developing certain cancers, particularly breast cancer, thyroid cancer, and endometrial cancer

The insulin receptor specifically causes tyrosine phosphorylation of IRS (Insulin Receptor Substrates) 1, 2, 3, and 4. Downstream signals are transmitted through multiple pathways (signal-splitting), including Ras, MAPK, JAK/STATs, and PIK, leading to widespread effects. IRS 3 is expressed in fat cells, beta cells of the pancreas, and liver.

Anti-epidermal growth factor receptor (EGFR) drugs

• The receptor for EGFR is a catalytic receptor with intrinsic tyrosine kinase activity. It belongs to the Erb family, which also includes the HER2/neu receptor implicated in breast cancers.
• Erlotinib, gefitinib, and lapatinib are tyrosine kinase inhibitors that inhibit EGFR receptor activity.
• Cetuximab and panitumumab are monoclonal antibodies to the EGFR receptor.

iii) MAP kinase pathway: This pathway uses a kinase cascade, where each kinase phosphorylates (and activates) the next kinase in the sequence.

• MAP-kinase kinase kinase activates MAP-kinase kinase.
• MAP-kinase kinase activates MAP kinase.
• MAP kinase (ERK) acts on intracellular targets that affect cell growth and differentiation, immune systems, etc.

The Ras signal transduction protein uses the MAP kinase pathway for its effects. Ras is a small GTP-binding protein located in the plasma membrane. Receptors with tyrosine kinase activity (such as insulin receptors) activate Ras by promoting GTP binding. Activated Ras then activates MAP-kinase kinase kinase.

iv) Ligand gated ion channels: Some plasma membrane ion channels also function as receptors for signaling molecules. When the signaling molecule binds, the channel opens and ions pass through. This changes the membrane potential (depolarization) and can generate an action potential. Examples include nicotinic, GABA-A, and 5HT3 receptors.

v) Steroid receptors: Steroid effects result from membrane initiated steroid signalling (MISS) and nuclear initiated steroid signalling (NISS).

MISS involves signaling through Ras, G proteins, tyrosine kinases, growth factor receptors, etc., culminating in phosphorylation of target proteins by kinases. Effects are seen within seconds to minutes.

NISS involves the steroid ligand binding to its intracellular or intranuclear receptor. Steroid binding activates the receptor, and the complex acts as a transcription factor to initiate transcription. Effects are seen within hours to days.

Intracellular steroid receptors may be cytoplasmic or nuclear.

• Nuclear receptors are seen with Vitamins A and D, retinoids, thyroid hormone, progesterone, and estrogen.
• Cytoplasmic steroid receptors are seen with testosterone, glucocorticoids, and mineralocorticoids. After steroid binding, cytoplasmic receptors translocate to the nucleus.

Activated hormone-receptor complexes bind to HREs (Hormone Response Elements) on DNA in promoter or enhancer regions with the help of zinc-finger motifs. The DNA-binding domain of steroid hormone receptors contains zinc finger motifs that include cysteine.