K-ras-mediated increase in cyclooxygenase 2 mRNA stability involves activation of the protein kinase B

Cyclooxygenase (COX) 2 expression is regulated via the Ras signaling pathwa y, and induction of mutated Ras rapidly increases COX-2 levels in intestina l epithelial cells, Protein kinase B (Akt/PKB) is an important effector of Ras signaling and a critical component of Ras-mediated transformation. Here we investigate the role of Akt/PKB in K-Ras-mediated induction of COX-2. R at intestinal epithelial cells (IEC-6) were transfected with an inducible K -Ras(Val12) cDNA (IEC-iK-Ras cells). Addition of 5 mM isopropyl-1-thio-beta -D-galactopyranoside induced the expression of K-Ras(Val12), followed by i ncreased activity of extracellular signal-regulated kinase and Akt/PKB. COX -2 levels were dramatically increased after induction of K-Ras(Val12). Inhi bition of MAPK/ERK kinase activity by PD 98059 completely blocked the K-Ras -mediated induction of COX-2, whereas inhibition of PI3K/Akt/PKB activity w ith LY 294002 or by expressing a dominant negative Akt (Akt-K179M) partiall y blocked the induction of COX-2 by K-Ras. Transient transfection of cells with phosphatidylinositol 3-kinase and Akt expression vectors revealed that PI3/Akt/PKB activity predominantly regulates the stability of COX-2 mRNA. Thus, Akt/PKB activity is involved in K-Ras-induced expression of COX-2 and stabilization of COX-2 mRNA largely depends on the activation of Akt/PKB.