PI3K/AKT/mTOR signaling

Cells use a complex intracellular signaling system to interpret and respond to various signals or stimuli. This communication network enables cells to react to cues from other cells, their environment, or internal sources. The PI3K/AKT, mTOR, and PTEN pathways are particularly significant, regulating cell growth, survival, and metabolism while frequently implicated in diseases like cancer.

PI3K/AKT Signaling

The PI3K/AKT pathway functions as a vital cellular communication route, directing responses based on environmental signals – from growth decisions to nutrient availability adjustments.

Activation and Role of PI3K

Activation occurs when growth factors bind to cell surface receptors, such as insulin binds the insulin receptor affecting glucose metabolism. PI3K activation produces PIP3 (phosphatidylinositol (3,4,5)-trisphosphate), a phospholipid messenger that acts as a beacon for AKT activation. (1)

AKT – The Central Coordinator

AKT (also referred to as protein kinase B or PKB) serves as a serine/threonine kinase coordinator. Once activated, AKT influences multiple cellular processes. It promotes survival by inhibiting pro-apoptotic proteins such as Bad, Bax and caspase 9, protecting cells from unnecessary apoptosis.

AKT regulates the cell cycle through interactions with cyclin D1 and p27Kip1 (cyclin-dependent kinase inhibitor 1B), maintaining balance between proliferation and growth arrest. Metabolically, AKT facilitates glucose uptake and utilization while modulating lipid metabolism, making it relevant to conditions like diabetes. (2)

PTEN – The Guardian of the PI3K/AKT Pathway

PTEN (phosphatase and tensin homolog) acts as a critical regulator, ensuring that the PI3K/AKT pathway's activities are kept in check. (3) Its primary function counteracts PI3K by dephosphorylating PIP3 back to PIP2, dampening AKT activation.

PTEN functions as a tumor suppressor, preventing uncontrolled cell growth and division. Dysfunctional PTEN leads to unchecked AKT activation, fostering excessive survival and proliferation. PTEN mutations have been identified in many advanced prostate cancers, emphasizing its protective role against cancer progression.

Regulation of PTEN

Post-translational modifications regulate PTEN's activity and stability. Phosphorylation decreases activity as a regulatory switch, while ubiquitination targets PTEN for degradation. Additionally, PTEN's localization within the cell, shifting between the cytoplasm and the nucleus, can influence its diverse roles, from controlling cell growth to maintaining DNA integrity.

PTEN and Other Pathways

Beyond the PI3K/AKT pathway, PTEN interacts with the MAPK/ERK pathway and the Wnt/β-catenin signaling pathway. It maintains chromosomal stability, participates in DNA repair processes, and influences cell migration and adhesion through the FAK-Paxillin pathway.

mTOR – The Master Growth Regulator

mTOR (mechanistic target of rapamycin) is a pivotal protein kinase that orchestrates cell growth, proliferation and survival. As a master regulator, it determines whether cells grow and divide or conserve resources based on nutrient availability, energy status, and growth factors.

mTOR and its Regulatory Complexes

mTOR operates through two complexes: mTORC1 and mTORC2. mTORC1 is particularly sensitive to nutrient availability, especially amino acids. When nutrients are abundant, mTORC1 activates, promoting protein synthesis, ribosome biogenesis, and nutrient uptake. The TSC1/2 complex (tuberous sclerosis complex 1 and 2) acts as a critical negative regulator. Low growth factors or compromised cellular energy cause TSC1/2 to inhibit mTORC1, conserving resources. However, PI3K/AKT signals can inhibit TSC1/2, leading to mTORC1 activation. Scarce nutrients diminish mTORC1 activity, pushing cells into conservation mode.

mTORC2, though less understood, controls the actin cytoskeleton and provides full AKT activation, linking it to broader PI3K/AKT signaling.

Interplay Between PI3K/AKT and mTOR

The PI3K/AKT pathway directly influences mTOR, particularly mTORC1. AKT can promote mTORC1 activity, leading to increased protein synthesis and cell growth. Conversely, PTEN suppresses mTOR activity indirectly by inhibiting PI3K/AKT, emphasizing pathway interconnectedness.

Impact on Human Health and Therapeutic Potential

These pathways influence numerous diseases, especially cancer. Dysregulation – such as PI3K/AKT overactivation from PTEN dysfunction – drives tumor growth and therapy resistance. (4) Aberrant mTOR signaling has been linked to metabolic disorders and neurodevelopmental diseases like autism and epilepsy.

Understanding these pathways enables therapeutic interventions. Drugs like rapamycin, which targets mTOR, and inhibitors for PI3K and AKT are in clinical use or trials. (5) mTOR's pivotal role makes it a prime therapeutic target. The sensitivity of mTORC1 to rapamycin is utilized in treatments, including organ transplant rejection prevention and cancer therapies. mTOR inhibitors are researched for treating tuberous sclerosis and aggressive cancers.

References

1. Miricescu D, Totan A, Stanescu-Spinu II, et al. PI3K/AKT/mTOR Signaling Pathway in Breast Cancer: From Molecular Landscape to Clinical Aspects. Int J Mol Sci. 2020;22(1):173. https://doi.org/10.3390/ijms22010173
2. Altomare DA, Khaled AR. Homeostasis and the importance for a balance between AKT/mTOR activity and intracellular signaling. Curr Med Chem. 2012;19(22):3748-62. https://doi.org/10.2174/092986712801661130
3. Papa A, Pandolfi PP. The PTEN-PI3K Axis in Cancer. Biomolecules. 2019;9(4):153. https://doi.org/10.3390/biom9040153
4. Ortega MA, Fraile-Martínez O, et al. Signal Transduction Pathways in Breast Cancer: The Important Role of PI3K/Akt/mTOR. J Oncol. 2020;2020:9258396. https://doi.org/10.1155/2020/9258396
5. Porta C, Paglino C, Mosca A. Targeting PI3K/Akt/mTOR Signaling in Cancer. Front Oncol. 2014;4:64. https://doi.org/10.3389/fonc.2014.00064