PI3K/AKT/mTOR pathway

PI3K/AKT/mTOR pathway. This intracellular signaling cascade is a central regulator of fundamental cellular processes including growth, proliferation, and survival. Its discovery and elucidation involved key researchers like Lewis C. Cantley, who identified Phosphoinositide 3-kinase. Due to its critical functions, dysregulation of this pathway is implicated in a wide spectrum of human diseases, most notably Cancer, driving extensive efforts in therapeutic development.

Overview

The pathway integrates signals from extracellular growth factors and hormones, such as Insulin and Insulin-like growth factor 1, received by receptor tyrosine kinases like the Insulin receptor. This integration allows the cell to coordinate anabolic processes and inhibit Apoptosis in response to favorable conditions. Its discovery was pivotal in understanding oncogenic transformation, with foundational work conducted at institutions like the Beth Israel Deaconess Medical Center and Harvard Medical School. The pathway's output is tightly controlled by key negative regulators, most famously the PTEN tumor suppressor.

Components and activation

The primary trigger is the binding of a ligand to a receptor, such as the Epidermal growth factor receptor, which recruits and activates class I Phosphoinositide 3-kinase. This enzyme phosphorylates the lipid PIP2 to generate PIP3 at the Plasma membrane. The production of PIP3 is antagonized by PTEN, which performs the reverse reaction. PIP3 then serves as a docking site for pleckstrin homology domain-containing proteins, including AKT and its activator PDK1. Full activation of AKT often requires phosphorylation by the mTORC2 complex.

Role in cellular processes

Once activated, AKT phosphorylates numerous downstream targets to promote cell survival by inhibiting pro-apoptotic proteins like BAD and the FOXO family of transcription factors. It drives cell cycle progression by affecting regulators such as p27. A major effector is the mTORC1 complex, which AKT activates by inhibiting the TSC1/TSC2 complex and PRAS40. mTORC1 is a master regulator of Anabolism, stimulating Protein synthesis via S6K1 and 4E-BP1, and promoting Lipogenesis and Glycolysis while inhibiting Autophagy.

Dysregulation and disease

Hyperactivation of this pathway is one of the most common events in human Cancer, found in malignancies such as Breast cancer, Prostate cancer, and Glioblastoma. This can occur via gain-of-function mutations in PIK3CA, the gene encoding the catalytic subunit of Phosphoinositide 3-kinase, amplification of AKT1, or loss-of-function mutations or deletion of PTEN, as often seen in PTEN hamartoma tumor syndrome. Beyond oncology, pathway dysfunction is linked to metabolic diseases like Type 2 diabetes, neurological disorders including Alzheimer's disease, and conditions of aberrant growth like Lymphangioma.

Therapeutic targeting

Given its role in disease, the pathway is a major focus for drug development. mTOR inhibitors like Sirolimus (rapamycin), Everolimus, and Temsirolimus are approved for cancers such as Renal cell carcinoma and Breast cancer, and for preventing organ rejection in Transplant medicine. Phosphoinositide 3-kinase inhibitors, including Idelalisib for Chronic lymphocytic leukemia and Alpelisib for Breast cancer, target specific isoforms. AKT inhibitors like Ipatasertib are in clinical trials, often in combination with agents like Paclitaxel. Research at centers like the Memorial Sloan Kettering Cancer Center continues to explore resistance mechanisms and combination therapies.

Category:Cell signaling Category:Oncology Category:Molecular biology