AKT1 Gene Function
AKT Serine/Threonine Kinase 1
Overview
AKT1 is the central hub of the PI3K/AKT pathway, constitutively activated in cancers through PIK3CA mutations, PTEN loss, or the AKT1 E17K PH domain mutation (~5% of HR+ breast cancers) that enables membrane tethering independent of PIP3. Active AKT coordinates cell survival (BAD, FOXO3a phosphorylation), proliferation (MDM2→p53 suppression, GSK3β→cyclin D1 stabilisation), and metabolism (mTORC1 via TSC2). Capivasertib (pan-AKT allosteric inhibitor) received 2023 FDA approval in combination with fulvestrant for AKT1/PIK3CA/PTEN-altered HR+ breast cancer; median PFS in the biomarker-altered population was 7.3 months versus 3.1 months with placebo plus fulvestrant. Resistance develops through mTORC1→IRS-1 feedback relief and RAS/MAPK bypass signalling.
Molecular Mechanism
Mechanism Summary
AKT1 integrates PIP3 signals from PI3K — translating growth factor receptor activation into coordinated programmes of cell survival (BAD and FOXO3a inhibition), proliferation (MDM2 and GSK3β phosphorylation enabling cyclin D1 and MYC accumulation), and anabolic metabolism (mTORC1 activation via TSC2 phosphorylation). The AKT1 E17K pleckstrin homology domain mutation causes constitutive plasma membrane tethering independent of PIP3 levels, driving AKT activity in ~5% of HR+ breast cancers. Capivasertib (pan-AKT allosteric inhibitor) received 2023 FDA approval in combination with fulvestrant for AKT1/PIK3CA/PTEN-altered HR+ breast cancer; median PFS in the biomarker-altered population was 7.3 months versus 3.1 months with placebo plus fulvestrant in CAPItello-291.
Step-by-Step Mechanism
PI3K-generated PIP3 at the inner plasma membrane leaflet recruits AKT via its pleckstrin homology (PH) domain, causing a conformational change that relieves PH-kinase interdomain autoinhibition.
Membrane-co-localised PDK1 phosphorylates AKT T-loop residue Thr308, achieving partial activation (~20% maximal). mTORC2 (RICTOR complex) phosphorylates the hydrophobic motif Ser473, achieving full catalytic activation.
Active AKT phosphorylates BAD at Ser136, preventing BAD from binding and inhibiting BCL2/BCL-XL. This shifts the BCL2-family balance toward survival and blocks intrinsic apoptosis.
AKT phosphorylates TSC2 (tuberin) at Thr1462, inactivating the TSC1/TSC2 GAP complex, allowing RHEB-GTP to activate mTORC1. mTORC1 then phosphorylates S6K1 and 4EBP1 to drive cap-dependent translation.
AKT phosphorylates GSK3β at Ser9, inactivating it. This prevents GSK3β-mediated phosphorylation and degradation of cyclin D1 and MYC, sustaining G1 cell cycle entry.
AKT phosphorylates FOXO1/FOXO3a at Thr24/Ser256, causing cytoplasmic sequestration by 14-3-3 proteins, blocking FOXO-driven transcription of BIM, PUMA, and p27KIP1 — pro-apoptotic and anti-proliferative targets.
Upstream Regulators
Generates PIP3 for AKT PH domain recruitment to plasma membrane
T-loop Thr308 phosphorylation; partial AKT activation
Hydrophobic motif Ser473 phosphorylation; required for full AKT activation and FOXO inhibition
Downstream Targets
BCL2/BCL-XL sequestration of BAD → apoptosis suppression
p53 ubiquitination and degradation
mTORC1 activation → protein synthesis
Cyclin D1 and MYC stabilisation → G1 progression
Cytoplasmic sequestration → BIM/PUMA suppression
Key Post-Translational Modifications
T-loop activation; partial kinase activity
Full kinase activation; substrate selectivity for FOXO
Proteasomal degradation; feedback regulation
Disease Mechanism
AKT is constitutively activated in cancers through PIK3CA mutations, PTEN loss, AKT1 E17K mutation, or HER2 amplification. Capivasertib combined with fulvestrant is approved (2023 FDA) for AKT1/PIK3CA/PTEN-altered HR+/HER2− breast cancer; median PFS in the CAPItello-291 biomarker-altered population was 7.3 months versus 3.1 months with placebo plus fulvestrant. Why AKT inhibition faces resistance: (1) pathway inhibition can relieve S6K1→IRS-1 negative feedback, causing compensatory RTK and PI3K reactivation. (2) RAS→RAF→MEK→ERK signalling provides a parallel proliferation route that can reduce dependence on AKT. (3) Mechanism-based toxicities — including hyperglycaemia, rash, and diarrhoea — can require dose modification. (4) Clinical benefit is biomarker selected and should not be generalised to tumours without qualifying AKT1, PIK3CA, or PTEN alterations.
Database References
Key Pathways
- ·PI3K-AKT signaling
- ·mTOR signaling
- ·Apoptosis regulation
- ·Cell cycle G1/S
- ·AKT inhibitor resistance
Disease Associations
- ·HR+ breast cancer (E17K)
- ·Prostate cancer
- ·Proteus syndrome (somatic E17K)
- ·Colorectal cancer
Research Activity
AKT1 is an actively studied target: about 10+ clinical trials that mention it are currently recruiting on ClinicalTrials.gov. Trial activity reflects research interest, not proven benefit — designs, endpoints and populations vary widely.
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Functional Partners
Common Questions About AKT1
What does AKT1 do in cancer?
AKT1 promotes cell survival and proliferation by phosphorylating substrates that inhibit apoptosis (BAD, FOXO3a), stimulate protein synthesis (mTORC1 via TSC2), and drive cell cycle entry (MDM2 activation, GSK3β inhibition). It is the central hub of the PI3K pathway — the most frequently activated oncogenic pathway in human cancer.
What is the AKT E17K mutation?
The AKT1 E17K substitution in the pleckstrin homology domain promotes constitutive membrane association and AKT activation independent of PIP3 levels. It occurs in ~5% of HR+ breast cancers and is a predictive biomarker for AKT inhibitors. Capivasertib combined with fulvestrant received FDA approval for AKT1/PIK3CA/PTEN-altered breast cancer in 2023.
What is the difference between AKT1, AKT2, and AKT3?
The three AKT isoforms share high structural homology but differ in tissue expression and substrate preferences. AKT1 is ubiquitous and primarily linked to cell survival; AKT2 is enriched in insulin-responsive tissues and metabolic regulation; AKT3 is brain-enriched and implicated in neurological cancers. AKT1 is the most commonly mutated isoform in solid tumours.
What is the AKT1 E17K mutation and how does it differ mechanistically from PIK3CA mutations?
AKT1 E17K substitutes a positively charged lysine for glutamate at position 17 in the pleckstrin homology (PH) domain. This charge reversal increases PH domain affinity for negatively charged phosphoinositides including PI(3,4)P2 and even PI(4,5)P2, enabling AKT1 membrane recruitment and activation independent of PIP3 levels. Unlike PIK3CA mutations that increase upstream PIP3 production to activate all downstream PH-domain-containing proteins, AKT1 E17K acts specifically and constitutively on AKT1 itself, making it a more targeted molecular event. The E17K mutation occurs in ~5% of HR+ breast cancers and is a predictive biomarker for AKT inhibitor response, independent of PIK3CA mutation status.
How does capivasertib work and in which patients is it approved?
Capivasertib is an oral, pan-AKT allosteric inhibitor that binds the PH domain of AKT when AKT is in its inactive conformation, preventing membrane recruitment and activation. Because it targets the regulatory PH domain rather than the catalytic kinase domain, it can inhibit both wild-type and E17K mutant AKT. Capivasertib combined with fulvestrant received FDA approval in November 2023 for HR+/HER2− advanced breast cancer with AKT1, PIK3CA, or PTEN pathway alterations who progressed on aromatase inhibitor ± CDK4/6 inhibitor. CAPItello-291 demonstrated a median PFS of 7.3 months versus 3.1 months (HR 0.60) in the biomarker-selected population — a 4.2-month improvement. Key toxicities include hyperglycaemia, rash, and diarrhoea from on-target PI3K pathway inhibition in normal tissues.
What is the relationship between AKT and mTOR, and why is it therapeutically relevant?
AKT activates mTORC1 by phosphorylating and inactivating the TSC1/TSC2 GAP complex, allowing RHEB-GTP to accumulate and activate mTORC1 at the lysosomal surface. mTORC2 (a separate mTOR complex) reciprocally phosphorylates AKT Ser473 for full AKT activation, creating a positive feedback loop between the two kinases. This bidirectional relationship has a critical therapeutic implication: when mTORC1 is inhibited (by everolimus or rapamycin analogues), the S6K1-driven negative feedback on PI3K is relieved, causing AKT Thr308 hyperphosphorylation. mTORC2 remains active under rapalog treatment, maintaining AKT Ser473 phosphorylation. The net result is that mTORC1 inhibition paradoxically activates AKT, limiting the therapeutic effect — the primary rationale for developing dual PI3K/mTOR inhibitors and AKT inhibitors.