Despite significant progress in diagnostics and
therapeutics, over 50 thousand patients die from
colorectal cancer annually. Hence, there is urgent need for new lines of treatment.
Triptolide, a natural compound isolated from the Chinese herb Tripterygium wilfordii, is effective against multiple
cancers. We have synthesized a water soluble analog of
triptolide, named
Minnelide, which is currently in phase I trial against
pancreatic cancer. The aims of the current study were to evaluate whether
triptolide/
Minnelide is effective against
colorectal cancer and to elucidate the mechanism by which
triptolide induces cell death in
colorectal cancer. Efficacy of
Minnelide was evaluated in subcutaneous xenograft and liver
metastasis model of
colorectal cancer. For mechanistic studies,
colon cancer cell lines HCT116 and HT29 were treated with
triptolide and the effect on viability,
caspase activation,
annexin positivity,
lactate dehydrogenase release, and cell cycle progression was evaluated. Effect of
triptolide on E2F transcriptional activity,
mRNA levels of E2F-dependent genes, E2F1-
retinoblastoma protein (Rb) binding, and
proteins levels of regulator of G1-S transition was also measured.
DNA binding of E2F1 was evaluated by
chromatin immunoprecipitation assay.
Triptolide decreased
colon cancer cell viability in a dose- and time-dependent fashion.
Minnelide markedly inhibited the growth of
colon cancer in the xenograft and liver
metastasis model of
colon cancer and more than doubles the median survival of animals with liver
metastases from
colon cancer. Mechanistically, we demonstrate that at low concentrations
triptolide induces apoptotic cell death but at higher concentrations it induces cell cycle arrest. Our data suggest that
triptolide is able to induce G1 cell cycle arrest by inhibiting transcriptional activation of E2F1. Our data also show that
triptolide downregulates E2F activity by potentially modulating events downstream of
DNA binding. Therefore, we conclude that
Triptolide and
Minnelide are effective against
colon cancer in multiple pre-clinical models.