A two-step transcriptional amplification system (
TSTA) was used to enhance the efficacy of suicide gene therapy for treatment of
prostate cancer. We designed a
TSTA system and constructed two types of plasmid: one containing
GAL4-VP16 fusion
protein under the control of a
tumor-specific promoter, the other containing
luciferase or herpes simplex virus
thymidine kinase (HSV-tk) under the control of a synthetic promoter. The
TSTA systems using nanoparticles based on
lipids were evaluated by measuring the amount of induced
luciferase activity as a function of prostate-specific membrane
antigen (PSMA) and
midkine (Mk) promoters, specific for LNCaP and PC-3
prostate cancer cells, respectively. In LNCaP cells that were PSMA-positive, the
TSTA system featuring the PSMA enhancer and promoter exhibited activity that was 640-fold greater than a system consisting of one-step transcription with the PSMA promoter. In contrast, this difference in activity did not occur in PSMA-negative PC-3 cells. In Mk-positive PC-3 cells, the
TSTA system with the Mk promoter exhibited a five-fold increase in activity over one-step transcription, but such activity was not induced in Mk-negative LNCaP cells. When using HSV-tk for suicide gene therapy,
TSTA systems featuring the PSMA or Mk promoter inhibited in vitro cell growth in the presence of
ganciclovir. Furthermore, the
TSTA system featuring the Mk promoter suppressed in vivo growth of PC-3
tumor xenografts to a greater extent than one-step transcription. These findings show that
TSTA systems can enhance PSMA and Mk promoter activities and selectively inhibit PC-3 cell growth in
tumors. This suggests that
TSTA systems featuring
tumor-specific promoters are suitable for
cancer treatment by gene therapy.