Akt2, phosphatidylinositol 3-kinase, and PTEN are in lipid rafts of intestinal cells: role in absorption and differentiation

In intestinal Na absorptive cells, phosphatidylinositol 3-kinase (PI 3-K) is involved in rapid epidermal growth factor (EGF) stimulation of Na absorption by the brush border membrane (BBM) Na(+)/H(+) exchanger NHE3. However, how NHE3 is regulated by the PI 3-K pathway and the role of Akt2 are poorly defined. The localization of Akt, PI 3-K, and NHE3 was determined by either immunocytochemistry and/or membrane fractionation using OptiPrep density gradient centrifugation. In ileum, active total Akt was present most in the villi and basal layer of the crypts, and Akt2 was mostly in villi. In villus cells, PI 3-K and Akt2 were mostly at the apical surface at which they were present partially in lipid rafts (LR). EGF increased PI 3-K and active Akt2 in ileal BBM at the same time that it increased PI 3-K-dependent trafficking of NHE3 to BBM and stimulation of Na absorption. However, Akt2 was only active in the detergent soluble (DS) pool and not LR of ileal BBM, which correlated with the presence of PTEN in LR. In Caco-2 cells, while EGF stimulated BB NHE3, Akt2 was active in both LR and DS pools. This correlated with the lack of PTEN in the LR of Caco-2 membranes. Akt2 also correlated with epithelial cell differentiation. Akt2 amount and activity were greater in differentiated than undifferentiated Caco-2 cells. These results suggest that LR may play an important role in determining the function of PI 3-K/Akt2 signaling, including stimulation of intestinal Na absorption. These results also suggest that LR-associated Akt2 may be involved in enterocyte differentiation.