Phosphatidylinositol Signaling

Phosphatidylinositol signaling uses a family of membrane phospholipids — the phosphoinositides — as both regulated second-messenger sources and as docking marks that recruit signaling proteins to specific membranes. Reversible phosphorylation of the inositol headgroup generates distinct lipid species that organize processes from growth-factor signaling to membrane traffic.

Definition

Phosphatidylinositol signaling is the regulated phosphorylation, hydrolysis, and dephosphorylation of inositol phospholipids in cell membranes to produce second messengers and lipid docking sites that control intracellular signaling.

Scope

This topic covers the structure and interconversion of phosphoinositides, the two best-characterized branches of their signaling — phospholipase C generation of soluble second messengers and PI3-kinase generation of 3-phosphorylated lipids — and how lipid-binding domains read these signals. Downstream disease links are reference context only.

Core questions

• How are different phosphoinositide species generated and interconverted?
• How do phosphoinositides act both as second-messenger precursors and as membrane address labels?
• How is the PI3-kinase branch coupled to downstream effectors?

Key concepts

• Phosphatidylinositol and its phosphorylated derivatives (PIPs)
• Phospholipase C, IP3, and diacylglycerol
• Phosphoinositide 3-kinase (PI3K)
• PIP3 as a membrane docking signal
• PH-domain recruitment of effectors
• Lipid phosphatases (e.g., the PTEN reaction) as counter-regulators

Mechanisms

Phosphatidylinositol carries an inositol headgroup whose hydroxyls can be reversibly phosphorylated at several positions, producing a set of distinct phosphoinositide species that mark different membranes and recruit specific proteins (Di Paolo & De Camilli, 2006). In one branch, phospholipase C cleaves phosphatidylinositol 4,5-bisphosphate into the soluble messenger inositol trisphosphate (IP3), which releases intracellular calcium, and membrane-retained diacylglycerol, which activates protein kinase C. In a second branch, phosphoinositide 3-kinase phosphorylates the 3-position of the inositol ring to generate 3-phosphorylated lipids such as PIP3 (Vanhaesebroeck et al., 2001). PIP3 acts as a docking signal that recruits pleckstrin-homology-domain proteins to the membrane, coupling the lipid signal to downstream kinases; the AKT/PKB network is a principal reader of this signal and integrates it into growth and survival outputs (Cantley, 2002; Manning & Toker, 2017). Lipid phosphatases reverse these modifications and so terminate or shape the signal.

Clinical relevance

The PI3K-AKT axis is one of the most studied growth-and-survival signaling networks, and its components are frequently examined in cancer research because their dysregulation can sustain proliferative signaling (Cantley, 2002; Manning & Toker, 2017). This entry describes the pathway as reference knowledge and does not provide diagnostic or treatment guidance.

Evidence & guidelines

The topic is based on biochemical and cell-biology reviews of phosphoinositide metabolism and the PI3K-AKT pathway (Vanhaesebroeck et al., 2001; Cantley, 2002; Di Paolo & De Camilli, 2006; Manning & Toker, 2017) rather than clinical guidelines.

History

Inositol phospholipids were recognized as a signaling system in the 1980s with the phospholipase C / IP3 / diacylglycerol pathway, and the discovery of phosphoinositide 3-kinase and its 3-phosphorylated products in the late 1980s and 1990s opened the now-central PI3K branch of lipid signaling (Vanhaesebroeck et al., 2001; Cantley, 2002).

Key figures

• Lewis Cantley
• Bart Vanhaesebroeck
• Pietro De Camilli
• Brendan Manning

Seminal works

• vanhaesebroeck-2001
• cantley-2002
• dipaolo-2006

Frequently asked questions

What is the difference between the PLC and PI3K branches of phosphoinositide signaling?

The phospholipase C branch hydrolyzes PIP2 into the soluble messenger IP3 and membrane diacylglycerol, while the PI3K branch phosphorylates inositol lipids at the 3-position to create membrane docking sites such as PIP3.

Why are phosphoinositides described as membrane address labels?

Different phosphorylated species are enriched on different membranes and are recognized by specific lipid-binding domains, so they help recruit signaling proteins to the correct location.