The Caspase-3/PKCδ/Akt/VEGF-A Signaling Pathway Mediates Tumor Repopulation during Radiotherapy

Purpose: Tumor repopulation is known as a major cause of treatment failure and/or tumor recurrence after radiotherapy. The underlying mechanism remains unclear. Our previous study demonstrated that irradiated apoptotic cells mediated tumor repopulation, in which caspase-3 played an important role. Herein, we investigated downstream effectors of caspase-3 involved in this process. Experimental Design: A dominant-negative protein kinase C delta (DN_PKC delta) mutant that could not be cleaved by caspase-3 and therefore could not be activated by irradiation-induced apoptosis was constructed. DN_PKC delta stably transduced tumor cells were compared with wild-type tumor cells for their growth stimulation effects in in vitro and in vivo tumor repopulation models. Downstream effectors of caspase-3 and PKC delta were investigated. The role of PKC delta was further verified in human colorectal tumor specimens. Results: Inactivation of caspase-3 or caspase-7 attenuated tumor repopulation and weakened PKC delta cleavage. Both DN_PKC delta and PKC delta inhibitors restrained tumor repopulation both in vitro and in vivo. Phosphorylated Akt was attenuated in caspase-3-, caspase-7-, or PKC delta-inactivated tumor cells. Furthermore, expression of vascular endothelial growth factor (VEGF)-A but not hypoxia-inducible factor 1 alpha (HIF1 alpha) was decreased in WPKC delta- or Akt-inactivated tumor cells. In addition, inhibition of p-Akt, HIF1 alpha, VEGF-A, or VEGF-A receptor reduced tumor repopulation significantly. Finally, increased nuclear translocation of PKC delta in colorectal tumor specimens was associated with worse patient prognosis. Conclusions: The caspase-3/PKC delta/Akt/VEGF-A axis is involved in tumor repopulation and could be exploited as a potential target to enhance the efficacy of radiotherapy.