Degradation of PKB/Akt protein by inhibition of the VEGF receptor/mTOR pathway in endothelial cells.
An intact VEGF receptor/PI3K/PKB/Akt signaling cascade protects endothelial cells from apoptotic stress-stimuli and mediates the formation of new blood vessels in pathological conditions such as cancer. Therefore, downregulation of this signaling cascade is of clinical interest for antiangiogenic cancer therapy. In this report, we demonstrate that VEGF controls the protein stability of the serine-threonine kinase PKB/Akt via inhibition of PKB/Akt protein degradation. VEGF deprivation or blockage of the VEGF signal transduction cascade with the VEGF receptor tyrosine kinase inhibitor PTK787/ZK222584 resulted in a specific decrease of the PKB/Akt protein level and subsequent cellular restimulation with VEGF rescued its stability. Real-time quantitative RT-PCR analysis demonstrated that VEGF does not regulate PKB/Akt gene expression. On the other hand, broad range inhibitors of caspases and the proteasome complex prevented VEGF-dependent downregulation of the PKB/Akt protein level indicating that PKB/Akt protein stability is regulated by VEGF-controlled proteolysis. Inhibition of the VEGF receptor and PKB/Akt-downstream PIK-related mTOR-kinase by rapamycin also neutralized the VEGF-protective effect in an PKB/Akt gene expression-independent way but results in proteolysis-dependent reduction of PKB/Akt protein stability. These results demonstrate a novel regulatory mechanism of the activated VEGF receptor/mTOR-signal transduction pathway to control the protein stability of PKB/Akt and survival threshold in endothelial cells.
Riesterer
Oliver
O
Department of Radiation Oncology, University Hospital Zurich, CH-8091 Zurich, Switzerland.
Zingg
Daniel
D
Hummerjohann
Jörg
J
Bodis
Stephan
S
Pruschy
Martin
M
D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism; D016923:Cell Death / Q000235:genetics; D002470:Cell Survival / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004730:Endothelium, Vascular / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D004795:Enzyme Stability; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism; D010793:Phthalazines / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex; D011494:Protein Kinases / Q000187:drug effects / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D011725:Pyridines / Q000494:pharmacology; D040262:Receptors, Vascular Endothelial Growth Factor / Q000378:metabolism*; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology
D061945:Caspase Inhibitors; D015853:Cysteine Proteinase Inhibitors; D009097:Multienzyme Complexes; D010793:Phthalazines; D011518:Proto-Oncogene Proteins; D011725:Pyridines; D042461:Vascular Endothelial Growth Factor A; C404768:vatalanib; D011494:Protein Kinases; D040262:Receptors, Vascular Endothelial Growth Factor; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; D020169:Caspases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D020123:Sirolimus