cell biology lecture 16

What is the core role of insulin and glucagon in liver glucose control? ⚖️

Glucagon and insulin act in opposition: glucagon raises cAMP to promote glucose release during fasting, while insulin suppresses cAMP to promote glucose storage after feeding.

How does cAMP link hormonal state to liver metabolism? 🔗

High cAMP (fasting) activates PKA to drive glycogen breakdown and gluconeogenesis, while low cAMP (fed state) promotes glycogen synthesis and glucose storage.

Why are GLP-1 agonists clinically useful? 💊

They reduce glucagon secretion and increase glucose-dependent insulin release, improving blood glucose control in type 2 diabetes.

What type of receptor is the insulin receptor? 🧬

The insulin receptor is a receptor tyrosine kinase (RTK) with intrinsic kinase activity that signals via tyrosine phosphorylation.

Describe the structure of the insulin receptor and why it matters 🏗️

It has an extracellular insulin-binding domain, a single-pass transmembrane region, and an intracellular tyrosine kinase domain that triggers phosphorylation cascades.

What happens when insulin binds its receptor? 🔑

Insulin binding causes receptor dimer rearrangement, activation of the kinase domain, autophosphorylation, and recruitment of signalling proteins.

What are the general properties of RTKs? 📡

RTKs are single-pass receptors that dimerise, autophosphorylate tyrosines, recruit signalling proteins, and regulate metabolism, growth, survival, and proliferation.

Why was tyrosine phosphorylation an important discovery? 🧠

It revealed a new signalling mechanism used by oncogenes and laid the foundation for cancer biology and targeted therapies.

Describe RTK activation by dimerisation 🔗

Ligand binding induces RTK dimerisation → kinase domains cross-phosphorylate → phosphorylation stabilises the active state → downstream signalling begins.

What is the role of the activation loop in RTKs? 🚪

In inactive RTKs the activation loop blocks the catalytic site; phosphorylation moves the loop, opening the active site and enabling signalling.

What do SH2 domains do? 🧲

SH2 domains bind phosphorylated tyrosines in specific sequence contexts, ensuring signalling specificity.

What do PTB domains do? 📍

PTB domains bind specific amino acids that may be phosphorylated or unphosphorylated, helping recruit signalling proteins to RTKs.

Explain the writer–reader–eraser model ✍️

Kinases write tyrosine phosphorylation, SH2/PTB proteins read it, and phosphatases erase it to control signalling duration.

Name the three main RTK downstream pathways 🛤️

RTKs activate PI3K–AKT (metabolism/growth), PLCγ (Ca²⁺/PKC), and RAS–MAPK (gene expression).

Why are adaptor proteins important in RTK signalling? 🧩

Adaptors like IRS1, GAB1, and FRS2 act as scaffolds that amplify and organise signalling when receptors cannot bind effectors directly.

How is RTK signalling terminated? 🛑

Phosphorylated RTKs recruit Cbl, become ubiquitinated, internalised, and degraded to prevent prolonged signalling.

Describe the PI3K–PIP₃ signalling reaction 🔁

PI3K phosphorylates PI(4,5)P₂ to produce PIP₃ at the membrane, initiating downstream signalling.

What is the structure of Class I PI3K? 🧠

It contains a p110 catalytic subunit and a p85 regulatory subunit that inhibits p110 until recruited to RTKs.

How is PI3K activated by RTKs? 🔓

p85’s SH2 domains bind YxxM AA on phosphorylated receptors,

relieving inhibition

and recruiting PI3K to the membrane.

Which enzymes terminate PIP₃ signalling? 🧹

PTEN removes the 3-phosphate and SHIP1/2 remove the 5-phosphate, reducing PIP₃ levels.

Why is PTEN important in cancer? ⚠️

PTEN is a tumour suppressor; its loss leads to excessive PIP₃ signalling and uncontrolled growth.

Why is PIP₃ considered a spatial signal? 📍

PIP₃ recruits PH-domain of signalling proteins to the membrane, telling signalling proteins where to act.

How is AKT activated? ⚡

AKT is recruited by PIP₃ and activated by PDK1 and mTORC2 phosphorylation.

What are the major effects of AKT signalling? 📈

AKT increases glucose uptake, glycogen and lipid synthesis, protein synthesis, and suppresses gluconeogenesis.

How does insulin increase glucose uptake in muscle and fat? 🚚

AKT phosphorylates AS160, activating RAB GTPases that drive GLUT4 vesicle fusion with the plasma membrane.

Why does PI3K signalling produce different outcomes in different tissues? 🧠

Different tissues express different downstream targets, so the same signal controls metabolism, storage, or proliferation depending on context.

Describe the PLCγ pathway downstream of RTKs 🔥

RTK-bound PLCγ is phosphorylated → autoinhibition relieved → PIP₂ split into IP₃ + DAG → Ca²⁺ release and PKC activation.

What is RAS and how does it function? 🔄

RAS is a small GTPase molecular switch that cycles between inactive GDP-bound and active GTP-bound states.

How is RAS activated downstream of RTKs? 🔑

RTKs recruit Grb2–SOS, and SOS acts as a GEF to load GTP onto RAS.

How is RAS inactivated? 🛑

GAPs such as NF1 accelerate GTP hydrolysis, returning RAS to its inactive state.

Describe the RAS–MAPK cascade 📊

RAS → RAF → MEK → ERK → phosphorylation of transcription factors controlling proliferation and survival.

How does RAS activate PI3K? 🔗

RAS directly binds the p110 catalytic subunit of PI3K, enhancing PIP₃ production.

Why are RAS mutations oncogenic? 🚨

Mutations (G12, G13, Q61) block GTP hydrolysis, locking RAS in the active state.

What is the structural basis of oncogenic RAS mutations? 🧬

Mutations prevent GAP arginine-finger insertion, stopping GTP hydrolysis and causing constant signalling.

What is the main take-home message of RTK signalling? 🎓

RTKs use tyrosine phosphorylation to control metabolism, growth, and survival, and failure to regulate these pathways leads to cancer and metabolic disease.