Cancer chemopreventive activity of brassinin, a phytoalexin from cabbage

Abstract

Brassinin [3-(S-methyldithiocarbamoyl)aminomethyl indole], a phytoalexin first identified as a constituent of cabbage, was synthesized and evaluated for cancer chemopreventive activity. Dose-dependent inhibition of 7,12-dimethylbenz[a]anthracene (DMBA)-induced preneoplastic lesion formation was observed with mouse mammary glands in organ culture, as was dose-dependent inhibition of DMBA-induced mouse skin tumors that were promoted by treatment with 12-O-tetradecanoylphorbol-13-acetate. Cyclobrassinin is a biologically derived product of the oxidative cyclization of brassinin, and was as active as the parent compound in inhibiting the formation of preneoplastic mammary lesions in culture; however, 2-methylbrassinin was not significantly active in this process. Therefore, oxidative cyclization may be an effective metabolic activation step. As judged by these tumor inhibition studies in conjunction with potential to induce phase II enzymes in mice or cell culture, brassinin may be effective as a chemopreventive agent during both the initiation and promotion phases of carcinogenesis. This is the first report documenting the chemopreventive potential of structurally novel indole-based phytoalexins that are naturally occurring in cruciferous vegetables, and the synthetic route described herein has proven amenable for scale-up production. The bifunctional structural nature of brassinin, bearing both an indole nucleus and a dithiocarbamoyl-aminomethyl moiety, is notably similar to the individual structural elements of other known chemopreventive agents such as indole-3-carbinol or benzylisothiocyanate. The favorable biological activity demonstrated by the compound may originate from the presence of these two moieties.

A key in vivo antitumor mechanism of action of natural product-based brassinins is inhibition of indoleamine 2,3-dioxygenase

“…QR is a phase II enzyme that detoxifies mutagenic carcinogens, and as an inducer of this enzyme, brassinin would be predicted to act as an 'anti-initiator'. However, the same study found that, in the two stage DMBA/TPA skin carcinogenesis model, brassinin acted instead as an 'anti-promoter' with no apparent impact on the initiation stage of carcinogenesis (Mehta et al, 1995). Unlike QR, IDO is likely to affect the promotional stage of carcinogenesis and, indeed, TPA is a powerful proinflammatory contact-sensitizer that would be expected to substantially elevate levels of IFNg, the principle cytokine inducer of IDO, in the draining lymph nodes (Thomson et al, 1993).…”

“…While a variety of possible molecular mechanisms have been proposed to explain this activity, none has been directly validated in vivo. In particular, brassinin is cited as an inducer of quinone reductase (QR) based on data from a mouse mammary gland organ culture model in which incubation with 220 mM brassinin for 3 days resulted in a fourfold increase in QR activity (Mehta et al, 1995). QR is a phase II enzyme that detoxifies mutagenic carcinogens, and as an inducer of this enzyme, brassinin would be predicted to act as an 'anti-initiator'.…”

“…Brassinin ([3-(S-methyldithiocarbamoyl)aminomethyl indole]), first isolated from Chinese cabbage inoculated with Pseudomonas chichorii (Takasugi et al, 1986(Takasugi et al, , 1988, belongs to a group of sulfur-containing, tryptophan-derived phytoalexins that are unique to crucifers (Mezencev et al, 2003). Brassinin has been shown to inhibit the formation of carcinogen-induced preneoplastic lesions in mouse mammary gland organ culture and to suppress papilloma formation in the classical two-stage DMBA/TPA skin carcinogenesis model (Mehta et al, 1995). Recently, we have shown that brassinin is a micromolar inhibitor of the IDO enzyme and have evaluated the IDO inhibitory activity of a large set of derivatized variations of the brassinin core structure in order to investigate structure-activity relationships (Gaspari et al, 2006).…”

Spirocyclization reactions and antiproliferative activity of indole phytoalexins 1-methoxybrassinin and its 1-substituted derivatives

“…17 In addition, brassinin (5b) has been reported to exhibit dose-dependent inhibition of DMBA-induced and TPA-promoted skin carcinogenesis. 19 Izutani et al demostrated that brassinin (5b) inhibits cell growth in human colon cancer cells by arresting the G1 phase via inceased expression of p21 and p27. 20 Spiroindoline phytoalexins and their derivatives exhibit an antiproliferative effect against human cancer cell lines.…”

“…20 Spiroindoline phytoalexins and their derivatives exhibit an antiproliferative effect against human cancer cell lines. 15,[21][22][23][24][25][26] Brassinin (5b) and its synthetic derivative 5-bromobrassinin (5c) act as bioavailable competitive inhibitors of indoleamine 2,3-dioxygenase (IDO), a tryptophan-catabolizing enzyme that promotes tumor escape via a mechanism of immune tolerance. 27,28 1-Methoxybrassinin (5a) displayed significant antiproliferative effect on intramolecular amastigotes of Trypanosoma cruzi and demonstrated a higher potency than shown by the currently used antichagasic agents (nifurtimox, benznidazol).…”