EGF Regulation of PITP Dynamics Is Blocked by Inhibitors of Phospholipase C and of the Ras-MAP Kinase Pathway

Abstract

Phosphatidylinositol transfer proteins (PITP) function in signal transduction and in membrane traffic [1, 2]. Studies aimed at elucidating the mechanism of action of PITP have yielded a singular theme; the activity of PITP stems from its ability to transfer phosphatidylinositol (PI) from its site of synthesis to sites of cellular activity and to stimulate the local synthesis of phosphorylated forms of PI. The participation of various phosphoinositides in EGF signal transduction and in the trafficking of the EGF receptors is well documented [3]. Using fluorescence lifetime imaging microscopy (FLIM) to measure fluorescence resonance energy transfer (FRET) between EGFP-PITP proteins and fluorescently labeled phospholipids, we report that PITPα and PITPβ can dynamically interact with PI or PC at the plasma membrane when stimulated with EGF. Additionally, PITPβ is localized at the Golgi, and EGF stimulation resulted in enhanced FRET. Inhibitors of the PLC and the Ras/MAP kinase pathway were both able to inhibit the EGF-stimulated interaction of PITPα with PI at the plasma membrane. The mobility of PITP proteins was determined by using fluorescence recovery after photobleaching (FRAP), and EGF stimulation reduced the mobility at the plasma membrane. We conclude that the dynamic behavior of PITPα and PITPβ in vivo is a regulated process involving multiple mechanisms.