mTOR Core · Lesson 07 · Intermediate · 25 min
Growth Factor Signalling
The question
How does a signal from outside the cell reach mTORC1?
What you should be able to do
After this lesson you should be able to
- Explain how a lipid second messenger creates a docking site and why that counts as signal transduction.
- Distinguish the two routes by which AKT reaches mTORC1, and state what each one acts on.
- Interpret an experiment that inhibits ‘mTOR’ when the same kinase sits on both sides of AKT.
- Evaluate a claim that a compound promotes muscle growth when the measurement was a phosphorylation.
Research skill Pathway reasoning
The core idea
Nutrients tell a cell what it has. Growth factors — hormones like insulin — tell it what the rest of the body wants.
The route is well known: the hormone lands on a receptor, the receptor switches on an enzyme called PI3K, PI3K changes a fat molecule in the membrane, and that change pulls in a kinase called AKT.
From AKT there are two ways into mTORC1, and both work by removing a brake — one brake outside the complex (the TSC complex from Lesson 04) and one sitting inside it.
The mechanism, in outline
The step worth pausing on is the fat molecule, PIP3. Signalling here is not a relay of one enzyme switching on the next; it is a change in the membrane that creates a landing spot.
That is also why PTEN, the enzyme that reverses that change, is one of the most commonly broken safety genes in human cancer.
AKT needs two phosphate marks to work fully, and one of them is added by mTORC2. So mTOR appears twice in the same picture — once helping to switch AKT on, once as the thing AKT releases.
Any experiment that blocks 'mTOR' without saying which assembly, at what dose, for how long, is cutting this picture in more than one place.
The branch, with and without its phosphatase
The growth-factor route as far as the inhibitory subunit inside mTORC1, plus PTEN — the phosphatase that reverses PI3K and is lost in many tumours. The lipid step is drawn as a link rather than a node; the nodes and effects come from the Atlas pathway model.
PIP3 high · AKT active · PRAS40 released · mTORC1 active
The normal fed response. AKT phosphorylates PRAS40 and the inhibitory subunit lets go of the complex — a release of restraint from inside, alongside the TSC route from Lesson 04.
- Growth hormone / IGF-1 axis
- Endocrine input acting through receptor tyrosine kinases, IRS proteins and PI3K.The 'there is food and it is safe to grow' hormone signal.Compressed here into one node; the axis spans GH→hepatic IGF-1→IGF1R/InsR→IRS→PI3K, and dwarf-mouse longevity phenotypes sit on it.
- PTEN
- Lipid phosphatase that converts PIP3 back to PIP2.Erases the signal PI3K writes.PTEN does not inhibit the PI3K enzyme; it degrades PI3K's product. Haploinsufficient tumour suppressor with dose-dependent phenotypes.
- PI3K
- Class I PI3K phosphorylates PIP2 to PIP3 at the plasma membrane.Makes the membrane signal that pulls Akt in.p110/p85 heterodimer; PIK3CA is one of the most frequently mutated oncogenes in human cancer. Its output is a lipid, not a phosphoprotein — which is why PTEN reverses it.
- Akt/PKB
- AGC kinase requiring PIP3 recruitment plus two phosphorylations to be fully active.The main 'grow' relay from growth factors.T308 by PDK1 and S473 by mTORC2. Recruitment and activation are separate events — a distinction the older Atlas diagram blurred.
- PRAS40
- Raptor-binding inhibitor displaced by Akt phosphorylation.A plug in mTORC1's substrate slot.AKT1S1. Competes with substrate for the Raptor TOS-motif site; its displacement is a substrate-access mechanism, not a change in kinase catalytic rate.
- mTORC1
- mTOR–Raptor–mLST8 (+PRAS40, DEPTOR); switched on only at the lysosome.The growth decision itself.Coincidence detector: nutrients supply location via the Rags, growth factors supply activation via Rheb. Neither alone is sufficient — the single most important idea in the pathway.
This is a simplified teaching model with a fixed set of states, not a simulation. It shows the direction each control pushes the pathway, not how much, how fast, or what any particular cell would do.
Every state of this model
| Growth factors | PTEN | Readout | What it means |
|---|---|---|---|
| ON | present | PIP3 high · AKT active · PRAS40 released · mTORC1 active | The normal fed response. AKT phosphorylates PRAS40 and the inhibitory subunit lets go of the complex — a release of restraint from inside, alongside the TSC route from Lesson 04. |
| OFF | present | PIP3 low · AKT quiet · PRAS40 clamping · mTORC1 quiet | No ligand, no PIP3, no AKT, and PRAS40 stays where it is. Note what is doing the inhibiting here: not an active off-signal but an unopposed brake. |
| ON | lost | PIP3 high and sustained · AKT active · mTORC1 active | PTEN removes the phosphate PI3K adds, so losing it leaves PIP3 elevated and the branch running. This is one of the most frequently altered routes in human tumours — and mTORC1 sits downstream of the alteration rather than causing it. |
| OFF | lost | PIP3 elevated without ligand · AKT active · mTORC1 active | The state that makes the point: with the phosphatase gone, the branch no longer needs the hormone. A pathway that has stopped listening to its input is a reasonable one-line description of what an oncogenic lesion does. |
The same wiring, seen in an animal
Most of the above comes from cells in a dish. The clearest whole-animal example here is muscle: in mice, this axis is needed for muscle to grow and can counteract muscle loss.
That moves the claim from 'these proteins interact' to 'this matters for a whole tissue in a living animal' — and a muscle result is still a muscle result.
Why this branch is the one that goes wrong in cancer
The PI3K–AKT axis is among the most frequently altered signalling routes in human tumours, whether through activating changes in PI3K itself or loss of PTEN. It is tempting to read that as 'mTORC1 activation causes cancer', and the corpus does not support the sentence in that form: mTORC1 sits downstream of these alterations, most of the mechanistic work is in cell lines, and drugs that inhibit mTORC1 have had a mixed record against tumours driven by upstream changes — partly for reasons that are the subject of Lesson 08.
What does the evidence say?
These are Atlas studies, with the Atlas's own evidence tier. Each card links to the full record — nothing here restates it.
Review establishing PI3K as a central signaling hub controlling cell survival, metabolism, and growth downstream of growth-factor receptors and upstream of Akt and mTOR.
Study page →Akt directly phosphorylates and inactivates TSC2, disrupting the TSC1-TSC2 complex and releasing its inhibition of mTOR - the link between growth-factor/insulin signaling and mTORC1 activation.
Study page →The rictor-mTOR complex (mTORC2) directly phosphorylates Akt/PKB on Ser473, regulating cell survival.
Study page →Identified PRAS40 as the missing insulin-controlled brake INSIDE mTORC1.
Study page →Spatial control of the TSC complex integrates insulin and nutrient inputs at the lysosome.
Study page →Akt/mTOR signalling is necessary and sufficient to drive skeletal-muscle hypertrophy and counteract atrophy in vivo.
Study page →Almost all of the wiring in this lesson was mapped in cultured cells using strong, acute stimulation with insulin or growth factors. How the same branch behaves at physiological hormone concentrations, in tissue, and across the daily feeding cycle is much less well described — and the muscle work shows that tissue context changes the outcome.
Work through it
Each of these asks you to commit to something — a prediction, a reading of two studies, a design — before it answers. Everything here is in the page, so nothing is lost if you would rather just read it.
From a phosphorylation site to a phenotype
| D Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex | C Akt/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy and can prevent muscle atrophy in vivo | |
| Model system | Human cells; Drosophila | Mouse; muscle |
| Perturbation | Human cells and Drosophila: the rictor–mTOR complex tested for kinase activity toward AKT. | Mouse skeletal muscle in vivo: the AKT–mTOR axis raised and lowered during hypertrophy and atrophy. |
| Readout | AKT phosphorylation at Ser473 — a single site, assigned to a single complex. | Muscle mass and fibre size over weeks, with the axis shown to be required for hypertrophy and able to counteract atrophy. |
What do both studies support?
That the AKT–mTOR axis is load-bearing rather than incidental: it carries a specific biochemical activity and it is needed for a tissue-level outcome in an animal.
Where do they differ?
In the distance between the readout and the claim. The first measures a phosphorylation site in cells — precise, mechanistic, and silent about whether anything grows. The second measures tissue mass in a living animal — the outcome anyone actually cares about, at the cost of not knowing which molecular step carried it. Tier D and tier C here mark that trade, not a ranking.
What experiment would help next?
The bridging experiment is a tissue-specific, acute manipulation of one step in the axis in an adult animal, with both the phosphorylation and the mass measured in the same animals over the same period.
What the growth-factor branch evidence supports
What this evidence supports
- That insulin and IGF-1 reach mTORC1 by relieving restraint at two points, one outside the complex and one inside it
- That the axis is required for skeletal-muscle hypertrophy in mice
What it does not establish
- That the same axis does the same thing in every tissue — a muscle result is a muscle result
- That signalling measured after strong acute stimulation describes what physiological hormone concentrations do
- That mTORC1 activation is sufficient for growth, which is a separate claim from being required for it
Why?
Nearly all of the wiring here was mapped in cultured cells given sharp stimuli. The one animal study raises the claim from mechanism to phenotype in one tissue, and that is exactly as far as it goes.
Think
mTORC2 phosphorylates AKT, and AKT relieves inhibition of mTORC1. Draw that as a circuit. What does it predict about an inhibitor that hits both complexes?
Ask what happens to the input when you remove the amplifier and the output at once.
Think first, then reveal
As a circuit, mTORC2 sits upstream of mTORC1 by way of AKT, so an inhibitor that reaches both should lower mTORC1 output by two routes at once: directly, and by weakening the AKT signal that would otherwise release TSC2 and PRAS40. That predicts a deeper and more sustained suppression than an mTORC1-only inhibitor — and a different side-effect profile, since AKT signalling does a great deal besides growth, including in glucose handling. Both predictions are broadly borne out in the corpus, and they are the reason 'mTORC1-selective' has been a design goal rather than a footnote. The catch is that selectivity claims depend on dose and duration, not on the molecule alone.
A paper reports that a compound ‘activates mTORC1’ in muscle cells and concludes that it will promote muscle growth. What would you want before accepting the second half?
Separate the readout from the outcome.
Think first, then reveal
Activating mTORC1 is a signalling readout, usually a phosphorylation of S6K1 or 4E-BP1; muscle growth is a tissue outcome measured over weeks. Between them lie several things the experiment did not test: whether the activation persists rather than being an acute spike, whether protein synthesis actually rises and by how much, whether degradation changes in parallel, whether the effect happens in an intact animal with an intact endocrine system, and whether feedback (Lesson 08) blunts the signal over time. The Bodine work is a useful contrast precisely because it went after necessity and sufficiency for the phenotype rather than for the phospho-signal.
Check yourself
Three questions, easiest first. Nothing is recorded and there is no pass mark — the explanation after each answer is the part worth reading.
Question 1 · Warm-upWhich lipid does PI3K produce to recruit AKT to the membrane?
PI3K phosphorylates PIP2 to make PIP3, and PIP3 is the docking site that brings AKT to the membrane. PTEN reverses that reaction, which is why losing PTEN raises signalling through this branch.
Show the answer
A — PIP3. PI3K phosphorylates PIP2 to make PIP3, and PIP3 is the docking site that brings AKT to the membrane. PTEN reverses that reaction, which is why losing PTEN raises signalling through this branch.
Question 2 · Step upAKT reaches mTORC1 by two routes. Which pair is correct?
Both are releases of restraint, one outside the complex and one within it. S6K1 and 4E-BP1 are substrates of mTORC1, not routes into it — a distinction worth keeping sharp.
Show the answer
B — It weakens the TSC complex, and it relieves PRAS40 inside mTORC1. Both are releases of restraint, one outside the complex and one within it. S6K1 and 4E-BP1 are substrates of mTORC1, not routes into it — a distinction worth keeping sharp.
Question 3 · HarderWhy does mTOR appear twice in this lesson's diagram, and what does that imply for an experiment using an inhibitor that blocks the mTOR kinase site?
The same kinase sits on both sides of AKT in two different complexes. An inhibitor that does not distinguish them removes an activator of AKT and a target of AKT simultaneously, which is why such results are hard to attribute to one complex.
Show the answer
B — mTORC2 phosphorylates AKT upstream while mTORC1 is the downstream target, so a kinase-site inhibitor cuts the circuit in two places at once. The same kinase sits on both sides of AKT in two different complexes. An inhibitor that does not distinguish them removes an activator of AKT and a target of AKT simultaneously, which is why such results are hard to attribute to one complex.
Go deeper
Follow a guided route through the mechanism:
- How does a cell learn that it is allowed to grow?This lesson as a route through the pathway map.
- Why does one kinase need two complexes?The other complex, which appears inside this branch as an activator of AKT.
Concepts introduced in this lesson
Receptor tyrosine kinase signalling · Lipid second messengers · Inhibitory subunits · Necessity versus sufficiency