Inhibition of protein kinase Cbeta by enzastaurin enhances radiation cytotoxicity in pancreatic cancer.
OBJECTIVE
Aberrant activation of protein kinase Cbeta (PKCbeta) by pancreatic cancer cells facilitates angiogenesis and tumor cell survival. Targeting PKCbeta with enzastaurin, a well-tolerated drug in clinical trials, would be expected to radiosensitize pancreatic tumors through direct antitumor and antivascular effects.


METHODS
We tested the hypothesis that enzastaurin radiosensitizes pancreatic cancer cells in culture and in vivo through inhibition of PKCbeta. We analyzed pancreatic cancer xenografts for growth delay and microvessel density after treatment with enzastaurin, radiation, or both. We determined the effect of radiation and enzastaurin on glycogen synthase kinase 3beta, a mediator of cell death in culture and in vivo.


RESULTS
At concentrations attained in patients, enzastaurin reduced levels of active PKCbeta measured by phosphorylation at Thr(500) in culture and in xenografts. Enzastaurin alone did not affect pancreatic cancer cell survival, proliferation, or xenograft growth. However, enzastaurin radiosensitized pancreatic cancer cells in culture by colony formation assay. Enzastaurin alone decreased microvessel density of pancreatic cancer xenografts without appreciable effects on tumor size. When combined with radiation, enzastaurin increased radiation-induced tumor growth delay with a corresponding decrease in microvessel density. Enzastaurin inhibited radiation-induced phosphorylation of glycogen synthase kinase 3beta at Ser(9) in pancreatic cancer cells in culture and in tumor xenografts, suggesting a possible mechanism for the observed radiosensitization.


CONCLUSIONS
Enzastaurin inhibits PKCbeta in pancreatic cancer cells in culture, enhancing radiation cytotoxicity. Additional antivascular effects of enzastaurin were observed in vivo, resulting in greater radiosensitization. These results provide the rationale for a clinical trial in locally advanced pancreatic cancer combining enzastaurin with radiation.
Spalding
Aaron C
AC
Department of Radiation Oncology The University of Michigan Medical School, 1500 East Medical Center Drive, Ann Arbor, MI 48109, USA. spalda@med.umich.edu
Watson
Richard
R
Davis
Mary E
ME
Kim
Alex C
AC
Lawrence
Theodore S
TS
Ben-Josef
Edgar
E
D000818:Animals; D038362:Glycogen Synthase Kinase 3 / Q000037:antagonists & inhibitors / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D006801:Humans; D007211:Indoles / Q000494:pharmacology*; D051379:Mice; D008819:Mice, Nude; D010190:Pancreatic Neoplasms / Q000201:enzymology* / Q000473:pathology; D010766:Phosphorylation; D011493:Protein Kinase C / Q000037:antagonists & inhibitors*; D064546:Protein Kinase C beta; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D011836:Radiation Tolerance / Q000187:drug effects*; D014407:Tumor Cells, Cultured; D023041:Xenograft Model Antitumor Assays
D007211:Indoles; D047428:Protein Kinase Inhibitors; C000605506:GSK3B protein, human; D000071679:Glycogen Synthase Kinase 3 beta; C000605507:Gsk3b protein, mouse; D011493:Protein Kinase C; D064546:Protein Kinase C beta; D038362:Glycogen Synthase Kinase 3; C504878:enzastaurin