Protein Kinase Cζ-dependent LKB1 Serine 428 Phosphorylation Increases LKB1 Nucleus Export and Apoptosis in Endothelial Cells*

LKB1 is a serine-threonine protein kinase that, when inhibited, may result in unregulated cell growth and tumor formation. However, how LKB1 is regulated remains poorly understood. The aim of the present study was to define the upstream signaling events responsible for peroxynitrite (ONOO^-)-induced LKB1 activation. Exposure of cultured human umbilical vein endothelial cells to a low concentration of ONOO^- (5 μm) significantly increased the phosphorylation of LKB1 at Ser⁴²⁸ and protein kinase Cζ (PKCζ) at Thr⁴¹⁰. These effects were accompanied by increased activity of the lipid phosphatase PTEN, decreased activity and phosphorylation (Ser⁴⁷³) of Akt, and induction of apoptosis. ONOO^- enhanced Akt-Ser⁴⁷³ phosphorylation in LKB1-deficient HeLa S3 cells or in HeLa S3 cells transfected with kinase-dead LKB1. Conversely, ONOO^- inhibited Akt Ser⁴⁷³ phosphorylation when wild type LKB1 were reintroduced in HeLa S3 cells. Further analysis revealed that PKCζ directly phosphorylated LKB1 at Ser⁴²⁸ in vitro and in intact cells, resulting in increased PTEN phosphorylation at Ser³⁸⁰/Thr^382/383. Finally, ONOO^- enhanced PKCζ nuclear import and LKB1 nuclear export. We conclude that PKCζ mediates LKB1-dependent Akt inhibition in response to ONOO^-, resulting in endothelial apoptosis.