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Transl Oncol. 2008 December; 1(4): 195–201.
PMCID: PMC2582168
Enzastaurin, an inhibitor of PKCβ, Enhances Antiangiogenic Effects and Cytotoxicity of Radiation against Endothelial Cells1,2
Aaron C Spalding,* Benjamin D Zeitlin, Kari Wilder-Romans,* Mary E Davis,* Jacques E Nor, Theodore S Lawrence,* and Edgar Ben-Josef*
*The University of Michigan Medical School, Department of Radiation Oncology, Ann Arbor, MI 48109-0010, USA
The University of Michigan Dental School, Department of Cariology, Restorative Sciences, and Endodontics, Ann Arbor, MI 48109-0010, USA
Address all correspondence to: Aaron C. Spalding, MD, PhD, Division of Radiation Oncology, Department of Radiologic Sciences, St. Jude Children's Research Hospital, 332 North Lauderdale Street, Memphis, TN 38105-2794. E-mail: acspalding1/at/gmail.com
Received July 30, 2008; Revised September 14, 2008; Accepted September 16, 2008.
Abstract
PURPOSE: Angiogenesis plays an important role in pancreas cancer pathobiology. Pancreatic tumor cells secrete vascular endothelial growth factor (VEGF), activating endothelial cell protein kinase C beta (PKCβ) that phosphorylates GSK3β to suppress apoptosis and promote endothelial cell proliferation and microvessel formation. We used Enzastaurin (Enz) to test the hypothesis that inhibition of PKCβ results in radiosensitization of endothelial cells in culture and in vivo. MATERIALS/METHODS: We measured PKCβ phosphorylation, VEGF pathway signaling, colony formation, and capillary sprout formation in primary human dermal microvessel endothelial cells (HDMECs) after Enz or radiation (RT) treatment. Microvessel density and tumor volume of human pancreatic cancer xenografts in nude mice were measured after treatment with Enz, RT, or both. RESULTS: Enz inhibited PKCβ and radiosensitized HDMEC with an enhancement ratio of 1.31 ± 0.05. Enz combined with RT reduced HDMEC capillary sprouting to a greater extent than either agent alone. Enz prevented radiation-induced GSK3β phosphorylation of serine 9 while having no direct effect on VEGFR phosphorylation. Treatment of xenografts with Enz and radiation produced greater reductions in microvessel density than either treatment alone. The reduction in microvessel density corresponded with increased tumor growth delay. CONCLUSIONS: Enz-induced PKCβ inhibition radiosensitizes human endothelial cells and enhances the antiangiogenic effects of RT. The combination of Enz and RT reduced microvessel density and resulted in increased growth delay in pancreatic cancer xenografts, without increase in toxicity. These results provide the rationale for combining PKCβ inhibition with radiation and further investigating such regimens in pancreatic cancer.