The aryl hydrocarbon receptor as an antitumor target of synthetic curcuminoids in colorectal cancer.

BACKGROUND: Curcumin has proven to be a potent antitumor agent in both preclinical and clinical models of colorectal cancer (CRC). It has also been identified as a ligand of the transcription factor known as the aryl hydrocarbon receptor (AHR). Our laboratory has identified the AHR as a mechanism which contributes to both tumorigenesis in a mouse model of inflammatory CRC as well an apoptotic target in vitro. Curcumin's role as an AHR ligand may modulate its effects to induce colon cancer cell death, and this role may be enhanced via structural modification of the curcumin backbone. We sought to determine if the two piperidone analogs of curcumin, RL66 and RL118, exhibit more robust antitumor actions than their parent compound in the context of colorectal cancer in vitro. Moreover, to ascertain the ability of curcumin, RL66 and RL118 to activate the AHR and evaluate if this activation has any effect on CRC cell death.
MATERIALS AND METHODS: DLD1, HCT116, LS513, and RKO colon cell lines were propagated in vitro. Natural curcumin was obtained commercially, whereas RL66 and RL118 were synthesized and characterized de novo. Multiwell fluorescent/luminescent signal detection was used to simultaneously ascertain cell viability, cell cytonecrosis, and relative amounts of apoptotic activity. AHR activity was measured with a dual luciferase reporter gene system. Stable expression of small interfering RNA interference was established in the HCT116 cell lines to create AHR "knock down" cell lines.
RESULTS: Both RL66 and RL118 proved to be more potent antitumor agents than their parent compound curcumin in all cell lines tested. The majority of this cell death was due to induction of apoptosis, which occurred earlier and to a greater degree following RL66 and RL118 treatment as opposed to curcumin. Also, RL66 and RL118 were found to be activators of AHR, and a portion of their ability to cause cell death was dependent on this induction. Curcumin was found unable to activate the AHR, and levels of AHR messenger RNA did not change their effects on cell death.
CONCLUSIONS: Piperidone analogs of curcumin exhibited enhanced antitumor effects in vitro as opposed to their parent compound. Even more, this enhanced cell death profile may be partially attributed to the ability of these compounds to activate the AHR. Further study of synthetic curcumin analogs as chemopreventives and chemoadjuncts in CRC is warranted. Also, more generally, the AHR may represent a potential putative target for novel anticancer agents for CRC.