Objective: To investigate the inhibitory effect of nanoparticle-mediated endostatin gene therapy on hepatocellular carcinoma xenografts combined with local hyperthermia utilizing heat-inducible promoter.
Methods: Heat-inducible HSP70B promoter and fusion gene of Endo/EGFP were cloned into pcDNA3.1 (+) plasmid, thus obtaining recombinant plasmid of pcDNA3.1 (+)/HSP70-Endo/EGFP using restriction endonucleases BglII/HindIII and EcoRI/SalI. The nanoparticles polylactide-grafted dextran copolymer (DEX-g-PLA) encapsulating the recombinant plasmid DNA were prepared by the method of emulsification and evaporation of organic solvent, and the surface shape of nanoparticles was observed by transmission electron microscope. Human hepatocellular cells of the lines HepG2 and ECV304 were cultured and transfected with the recombinant plasmid utilizing the nanoparticles. Following thermal induction at 37 degrees C, 39 degrees C, 41 degrees C, 43 degrees C, and 45 degrees C for 30 min, the expression of enhanced green fluorescent protein (EGFP) was detected by fluorescence microscope and flow cytometry. The concentration of endostatin protein in the supernatant was tested by ELISA, and the growth inhibition on the HepG2 and ECV304 cells was tested by MTT method. Balb/c nude mice were inoculated with HeG2 cells and then randomly divided into 2 groups to undergo intra-tumor injection of nanoparticles (heated or not heated), Lipofectamine 2000. Mice were used as controls without intra-tumor injection. Four weeks the mice were killed to observe the tumor inhibition rate.
Results: The nanoparticles encapsulating recombinant plasmid were of round or elliptical shape 90 approximately 120 nm in diameter. The efficiency of gene transfection mediated by nanoparticles was about 30.65%. The expression of Endo/EGFP gene in the HepG2 cells was up-regulated along with the increase of temperature, peaked at 43 degrees C (with the EGFP expression level 3.3 times as that at 37 degrees C). The concentration of endostatin protein in the supernatant of the 43 degrees C group was (177 +/- 28) microg/L, significantly higher than that of the 37 degrees C group [(41 +/- 10) microg/L]. MTT results indicated that endostatin inhibited the growth of ECV304 cells with a inhibition rate of 96.3% at the time point of 72 h in the 43 degrees C group, however, it did not show influence on HepG2 cells no matter what was the temperature The tumor inhibition rate in the mice of endostatin with thermal induction group was 58.5%, significantly higher than that of the 37 degrees C group (34.9%, P < 0.05).
Conclusion: Low temperature thermal induction enhances the expression and secretion of endostatin in hepatocellular cells transfected by nanoparticles, and inhibits the growth of hepatocellular carcinoma xenografts.