Enhancement of Tumor Response by MEK Inhibitor in Murine HCa-I Tumors

Kim, Sung Hee; Seong, Jinsil

Abstract

PURPOSE: Extracellular signal-regulated kinase (ERK), which is part of the mitogen-activated protin kinase cascade, opposes initiation of the apoptotic cell death which is programmed by diverse cytotoxic stimuli. In this regard, the inhibition of ERK may be useful in improving the therapeutic efficacy of established anticancer agents.
MATERIALS AND METHODS
Murine hepatocarcinoma, HCa-I is known to be highly radioresistant with a TCD50 (radiation dose yield in 50% cure) of more than 80 Gy. Various anticancer drugs have been found to enhance the radioresponse of this particular tumor but none were successful. The objective of this study was to explore whether the selective inhibition of MEK could potentiate the antitumor efficacy of radiation in vivo, particularly in the case of radioresistant tumor. C3H/HeJ mice bearing 7.5~8 mm HCa-I, were treated with PD98059 (intratumoral injection of 0.16 microgram in 50 microliter).
RESULTS
Downregulation of ERK by PD98059 was most prominent 1h after the treatment. In the tumor growth delay assay, the drug was found to increase the effect of the tumor radioresponse with an enhancement factor (EF) of 1.6 and 1.87. Combined treatment of 25 Gy radiation with PD98059 significantly increased radiation induced apoptosis. The peak apoptotic index (number of apoptotic nuclei in 1000 nuclei X100) was 1.2% in the case of radiation treatment alone, 0.9% in the case of drug treatment alone and 4.9%, 5.3% in the combination treatment group. An analysis of apoptosis regulating molecules with Western blotting showed upregulation of p53, p21WAF1/CIP1 and Bcl-XS in the combination treatment group as compared to their levels in either the radiation alone or drug alone treatment groups. The level of other molecules such as Bcl-XL, Bax and Bcl-2 were changed to a lesser extent.
CONCLUSION
The selective inhibition of MEK in combination with radiation therapy may have potential benefit in cancer treatment.