Direct activation of the epithelial Na⁺ channel by phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate produced by phosphoinositide 3-OH kinase

The phospholipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P ₂) is accepted to be a direct modulator of ion channel activity. The products of phosphoinositide 3-OH kinase (PI3K), PtdIns(3,4)P₂ and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P₃), in contrast, are not. We report here activation of the epithelial Na⁺ channel (ENaC) reconstituted in Chinese hamster ovary cells by PI3K. Insulin-like growth factor-I also activated reconstituted ENaC and increased Na⁺ reabsorption across renal A6 epithelial cell monolayers via PI3K. Neither IGF-I nor PI3K affected the levels of ENaC in the plasma membrane. The effects of PI3K and IGF-I on ENaC activity paralleled changes in the plasma membrane levels of the PI3K product phospholipids, PtdIns(3,4)P ₂/PtdIns(3,4,5)P₃, as measured by evanescent field fluorescence microscopy. Both PtdIns(3,4)P₂ and PtdIns(3,4,5)P ₃ activated ENaC in excised patches. Activation of ENaC by PI3K and its phospholipid products corresponded to changes in channel open probability. We conclude that PI3K directly modulates ENaC activity via PtdIns(3,4)P ₂ and PtdIns(3,4,5)P₃. This represents a novel transduction pathway whereby growth factors, such as IGF-I, rapidly modulate target proteins independent of signaling elicited by kinases downstream of PI3K.