The dietary phytochemical 3,3'-diindolylmethane induces G2/M arrest and apoptosis in oral squamous cell carcinoma by modulating Akt-NF-κB, MAPK, and p53 signaling

Abstract

In light of the growing incidence of oral cancer in Taiwan, this study is aimed at investigating the antitumor activity of 3,3'-diindolylmethane (DIM), an active metabolite of the phytochemical indole-3-carbinol (I3C), in oral squamous cell carcinoma (OSCC). DIM exhibited substantially higher antiproliferative potency than I3C in three OSCC cell lines with IC(50) values in SCC2095, SCC9, and SCC15 cells, respectively, of 22 versus 168μM, 25 versus 176μM, and 29versus 300μM. Flow cytometric analysis and Comet assay indicated that DIM suppressed the viability of SCC2095 cells by inducing apoptosis and G2/M arrest. Western blot analysis of various signaling markers revealed the ability of DIM to target pathways mediated by Akt, mitogen-activated protein (MAP) kinases, nuclear factor (NF)-κB, and p53, of which the concerted action underlined its antitumor efficacy. The concomitant inactivation of Akt and MAP kinases in response to DIM facilitated the dephosphorylation of the proapoptotic protein Bad at Ser-136 and Ser-112, respectively. Through endoplasmic reticulum (ER) stress, DIM stimulated the activation of p53 via Ser-15 phosphorylation, leading to increased expression of the BH3-only proapoptotic Bcl-2 members Puma and Noxa. Together, these changes decreased the mitochondrial threshold for apoptosis. G2/M arrest might be attributable to the suppressive effect of DIM on the expression of cyclin B1 and cdc25c. As many downstream effectors of the Akt-NF-κB pathway, including glycogen synthase kinase 3β, IκB kinase α, and cyclooxygenase-2, have been shown to promote oral tumorigenesis, the ability of DIM to inhibit this signaling axis underscores its chemopreventive potential in oral cancer.