Protein Kinase C-δ and Phosphatidylinositol 3-Kinase/Akt Activate Mammalian Target of Rapamycin to Modulate NF-κB Activation and Intercellular Adhesion Molecule-1 (ICAM-1) Expression in Endothelial Cells

We have shown that the mammalian target of rapamycin (mTOR) down-regulates thrombin-induced ICAM-1 expression in endothelial cells by suppressing the activation of NF-kappa B. However, the mechanisms by which mTOR is activated to modulate these responses remain to be addressed. Here, we show that thrombin engages protein kinase C (PKC)-delta and phosphattidylinositol 3-kinase (PI3K)/Akt pathways to activate mTOR and thereby dampens NF-kappa B activation and intercellular adhesion molecule 1 (ICAM-1) expression. Stimulation of human vascular endothelial cells with thrombin induced the phosphorylation of mTOR and its downstream target p70 S6 kinase in a PKC-delta- and PI3K/Akt-dependent manner. Consistent with this, thrombin-induced phosphorylation of p70 S6 kinase was defective in embryonic fibroblasts from mice with targeted disruption of PKC-delta (Pkc-delta(-/-)), p85 alpha and p85 beta subunits of the PI3K (p85 alpha(-/-)beta(-/-)), or Akt1 and Akt2 (Akt1(-/-)2(-/-)). Furthermore, we observed that expression of the constitutively active form of PKC-delta or Akt was sufficient to induce NF-kappa B activation and ICAM-1 expression, and that co-expression of mTOR suppressed these responses. In reciprocal experiments, inhibition/depletion of mTOR augmented NF-kappa B activation and ICAM-1 expression induced by PKC-delta or Akt. In control experiments, increasing or impairing mTOR signaling by the above approaches produced similar effects on NF-kappa B activation and ICAM-1 expression induced by thrombin. Thus, these data reveal an important role of PKC-delta and PI3K/Akt pathways in activating mTOR as an endogenous modulator to ensure a tight regulation of NF-kappa B signaling of ICAM-1 expression in endothelial cells.