MTOR Flashcards
- Describe the role of the PI3-kinase/Akt/mTOR pathway in normal cell function.
What a bullshit and vague learning objective anyways here goes:
-mTOR is part of the PI3-K/Akt/mTOR signaling system
- PI3-K (phosphoinositide-3 kinase) is
downstream of many growth factor receptors.
-Its regulatory subunit associates with receptors,
which upon activation trigger its association with other subunits as active enzyme.
-Active PI3-K then phosphorylates phosphoinositides, signaling lipids in the plasma membrane.
-The phosphorylation of these phosphoinositides is in turn regulated by PTEN (phosphatase and tensin
homolog), a phosphatase that dephosphorylates phosphoinositide-3P, preventing it from
activating PDK1 (phosphoinositide-dependent kinase 1).
-Active PDK1 phosphorylates Akt
(name derivation based on a breeding colony, also known as protein kinase B, PKB) on Thr308.
-Fully activated Akt (phospho-Thr308 and phospho-Ser473) phosphorylates the TSC1/2 (tuberous sclerosis) protein complex.
-which inactivates its inhibitory function on Rheb1 (Ras homolog enriched in brain).
-Upon release from TSC1/2 inhibition, activated Rheb1 in turn activates mTORC1.
-Note that fully activated Akt also requires phosphorylation on Ser473. -For many years a novel PDK2 was hypothesized that would phosphorylate that residue.
-Over the past decade, it became clear that mTORC2 is that kinase, i.e., it phosphorylates Ser473 to fully activate Akt. To further complicate the story, while TSC1/2 downregulates mTORC1 activity
through its inhibition of Rheb1, it activates mTORC2.
-mTORC1 activity is regulated by amino acid availability and energy status (AMP/ATP ratio).
This is part of the feedback loop of mTORC1, amino acid availability and autophagy.
-In the presence of high amino acids (and other nutrients), mTORC1 activity is high and it inhibits
autophagy.
-In low nutrients, mTORC1 activity drops, increasing autophagy.
-The amino acid
regulation of mTORC1 activity is independent of the PI3K/Akt pathway.
-Small GTPases (Rags)
are regulated by amino acid availability.
-In their activated state, they bind and activate mTORC1
by controlling its subcellular localization, moving it from a diffuse distribution to localization at the late endosome/lysosome membrane.
-Rags are localized to the lysosomal membrane by
positive regulators, the Ragulators, as is Rheb1, the mTORC1 activator.
-Rag activity is
downregulated by GATOR1, which controls the GTP loading of the Rags.
- Describe the role of the PI3-kinase/Akt/mTOR pathway in cancer.
-mTOR signaling regulates cell and tissue growth.
-Many negative regulators of mTOR are tumor suppressors.
-Mutations that inactivate PTEN or TSC1/2 induce uncontrolled cell growth.
-Activating mutations/duplications of Akt induce uncontrolled cell growth.
-These occur in tumors
in many tissues.
-Numerous tumors likely also have elevated mTOR activity secondary to other
changes.
- Discuss the role of mTOR inhibitors in clinical practice.
-The classic inhibitor of mTOR (mechanistic target of rapamycin) is—rapamycin (sirolimus).
-Rapamycin didn’t work well as an antifungal agent, as it had profound immunosuppressive and antiproliferative effects. Over time,it was found to act through mTOR. Hence the name.
But in terms of the science:
-Rapamycin binds to FKBP12 (FK506-binding protein
12), which binds to the FKBP12/rapamycin binding site on mTOR.
-This inhibits mTORC1
activity, likely by altering Raptor binding to the complex.
-This was originally thought to be
mTORC1-specific, but in some cells or after long term treatment and/or high dose, mTORC2 is also inhibited.
-This may result from longterm unavailability of mTOR itself to associate with the Rictor complex.
-Because of its immunosuppressive effects, rapamycin/sirolimus and other rapalogs were
developed as anti-rejection therapies.
-Rapamycin/sirolimus inhibits signaling that results in cell
growth.
-Thus, it is also used to reduce cell growth in cancers, etc., as temsirolimus and everolimus are approved for use in advanced renal cell carcinoma, advanced hormone-receptor
positive breast cancer and for subependymal giant cell astrocytoma in TSC.
-It does not appear to
have broad anticancer efficacy. This could result from the complexity of the signaling pathways
involved.
-For example, the degree to which mTORC2 is blocked by rapamycin is variable and
unpredictable. Also, inhibition of mTORC1 can increase the upstream signaling through Akt.
-The unique structure/function aspects of rapamycin may also result in incomplete longterm
inhibition, where some downstream signaling appears refractory to rapamycin inhibition.
-Thus, despite initial excitement about rapamycin as an anti-cancer drug, it has had limited success.
-Rapamycin/sirolimus extends lifespan in animal models. This may be through reduction of cancer incidence, immunosenescence, and/or cognitive impairment.
What are the NEGs:
-A negative of using rapamycin clinically has been its potential for increased susceptibility to
infectious disease.
-In transplantation patients, it may result in increased infection rates because of
its immunosuppressive effects.
-It has been suggested to increase risk of skin cancers and other cancers.
-It can also induce diabetes-like symptoms, resulting from hyperglycemia, decreased
glucose tolerance and insensitivity to insulin.
not one of her learning objectives directly but i feel as though it may come up because she spent so much time talking about this bullshit….so
what do mTORC1
what do mTORC2
GO!
mTORC1:
-mTORC1 is primarily known for its function regulating translation of RNAs in response to extracellular signaling.
-It does this through increasing ribosome production along with activation of translation activators, such as phosphorylating ribosomal protein S6 kinase, and inhibition of translation inhibitors, such as phosphorylating 4EBP1-3, which releases eIF4e to activate translation.
-It is also known for its inhibition of autophagy. It phosphorylates and inactivates several components of the autophagy system, much of which is localized near the lysosome.
-When amino acids are low, mTORC1 is inactivated, increasing autophagy.
-As amino acids
become available after autophagy, mTORC1 is reactivated.
mTORC2:
-mTORC2 is less well understood.
-It is involved in actin cytoskeleton organization, cell metabolism and cell survival.
-It regulates Akt activity, via phosphorylation at Ser473, but this does not impact mTORC1 activity but rather other Akt signaling pathways.
-mTORC2 associates
with an endoplasmic reticulum domain associated with mitochondria, and mTORC2 inactivation decreases mitochondrial function and cell survival.
-Its activity on the actin cytoskeleton is implicated in tumor cell migration and metastasis.
ALL togeher now:
-Both mTORC1 and mTORC2 regulate and are regulated by negative feedback loops. -mTORC1 and its phosphorylated substrate S6 kinase 1 feedback to downregulate insulin signaling by direct
phosphorylation.
-mTORC2 downregulates insulin signaling via activation of ubiquitination of proteins in the insulin pathway.
-Akt can downregulate mTORC2 signaling by phosphorylating
and dissociating proteins from the mTORC2 complex, while S6 kinase 1 can also phosphorylate
Rictor, to directly downregulate mTORC2 signaling.
