PI, PIP & LPI
Phosphoinositides (PI) are minority components of cellular membranes in eukaryotes. PI’s are similar to other phospholipids, but the head group is the cyclic myo-inositol. This inositol head group has free hydroxyl groups at positions D2-D6 and the hydroxyl groups at D3, D4, and D5 are readily phosphorylated by lipid kinases. The seven combinatorially phosphorylated forms of the inositol head group result in informational content. These PI molecules act as acidic addresses identifying different membranes and providing instructions to proteins about how to behave with those membranes. These various phosphorylated inositol groups are found in the endoplasmic reticulum, early and late endosomes, trans-Golgi, secretory granules, and the plasma membrane.
Lysophosphatidylinositol (LPI) is believed to play an important role in several diseases. LPI can affect several cell functions such as cell growth, differentiation, and mobility in cell types such as cancer cells, endothelial cells, and nerve cells. G-protein coupled receptor 55 (GPR55) has been proposed as a potential LPI receptor. Evidence shows that LPI plays a key role in metabolic functions and possibly metabolic diseases.
Falkenburger BH, Jensen JB, Dickson EJ, Suh BC, Hille B. Phosphoinositides: lipid regulators of membrane proteins. J Physiol. 2010;588(Pt 17):3179-3185. doi:10.1113/jphysiol.2010.192153
Arifin SA, Falasca M. Lysophosphatidylinositol Signalling and Metabolic Diseases. Metabolites. 2016;6(1):6. Published 2016 Jan 15. doi:10.3390/metabo6010006