Submitted on July 27, 2007
Accepted on March 17, 2008
Polyunsaturated fatty acids block platelet-activating factor-induced phosphatidylinositol 3 kinase/Akt -mediated apoptosis in intestinal epithelial cells
Jing Lu1, Michael S. Caplan1, Dan Li2, and Tamas Jilling1*
1 Pediatrics, ENH Research Institute, Feinberg School of Medicine, Northwestern University, Evanston, Illinois, United States
2 Pediatrics, ENH Research Institute, Evanston, Illinois, United States
* To whom correspondence should be addressed. E-mail: tjilling{at}northwestern.edu.
We have shown earlier that platelet-activating factor (PAF) causes apoptosis in enterocytes via a mechanism that involves Bax translocation to mitochondria, followed by caspase activation and DNA fragmentation. Herein we report that in rat small intestinal epithelial cells (IEC-6) these downstream apoptotic effects are mediated by a PAF-induced inhibition of the phosphatidylinositol 3 kinase (PI3-kinase)/Akt signaling pathway. Treatment with PAF results in rapid dephosphorylation of Akt, PDK1, and the YXXM p85 binding motif of several proteins and redistribution of Akt-PH domain-GFP, i.e., an in vivo PIP3 sensor, from membrane to cytosol. The pro-apoptotic effects of PAF were inhibited by both n-3 and n-6 polyunsaturated fatty acids, but not by a saturated fatty acid palmitate. Indomethacin, an inhibitor of prostaglandin biosynthesis, did not influence the baseline or PAF-induced apoptosis, but 2-bromo-palmitate, an inhibitor of protein palmitoylation, inhibited all of the pro-apoptotic effects of PAF. Our data strongly suggest that an inhibition of the PI3-kinase/Akt signaling pathway is the main mechanism of PAF-induced apoptosis in enterocytes and that polyunsaturated fatty acids block this mechanism very early in the signaling cascade independently of any effect on prostaglandin synthesis, and probably directly via an effect on protein palmitoylation.
