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Over the past several decades, research on the action of bioactive constituents of plants (ie, phytochemicals) has focused predominantly on their cancer-preventive properties. Evidence for a protective role for vegetables is strongest and most consistent for vegetables in the Cruciferae (Brassicaceae) plant family, such as broccoli, cabbage, Brussels sprouts and cauliflower.1–4 Cruciferous vegetables are characterised by a unique phytochemistry: their high content of the sulfur-containing glucosinolates or β-thioglucoside N-hydroxysulfates with a side chain and sulfur-linked β-d-glucopyranose moiety.5 Hydrolysis of glucosinolates by a β-thioglucosidase (myrosinase), either in the plant or by gut bacteria, results in the formation of biologically active compounds such as isothiocyanates and indoles.6 These bioactive compounds are hypothesised to be responsible for the chemoprotective effects conferred by a high cruciferous vegetable intake.
Isothiocyanates may exert their protective effects through several distinct mechanisms. Of the isothiocyanates, sulforaphane, which is found in broccoli and broccoli sprouts at particularly high levels, has been the most extensively studied. Considerable attention has focused on sulforaphane as a “blocking” agent—that is, its ability to modulate the nuclear factor-erythroid-2-related factor 2 (Nrf2)/Kelch-like ECH-associated protein 1 (Keap1) pathway.7 Sulforaphane reacts with specific thiol groups on Keap1, promoting Nrf2 dissociation from Keap1, and allowing subsequent activation of antioxidant response element (ARE)-driven gene expression of phase II biotransformation enzymes (glutathione S-transferases, UDP-glucuronosyltransferases, etc.).7 However, sulforaphane acts by other mechanisms too. It induces cell cycle arrest and apoptosis in cancer cells, in part through inhibition of histone deacetylase.7 Further, studies in several cancer cell lines suggest that sulforaphane treatment can elicit a diverse range of mitogen-activated protein kinase (MAPK) responses and outcomes, which may influence promotion and inhibition of carcinogenesis and which appear to be tissue specific.7 In addition, sulforaphane has been shown to reduce nuclear factor-κB (NF-κB) activity and affect expression of NF-κB-mediated genes encoding adhesion molecules, inflammatory cytokines, growth factors and antiapoptotic factors.8 These additional mechanisms suggest that sulforaphane may be effective during the postinitiation stages of carcinogenesis—that is, as a “suppressing” agent.9
A major factor that limits effectiveness of chemotherapy in patients with advanced cancer or particularly aggressive tumours is resistance to the treatment. One strategy to overcome intrinsic and acquired resistance and enhance chemotherapy efficacy is the concomitant use of chemopreventive agents that are by themselves non-toxic, but may result in better response rates than either reagent alone.10–12 Sulforaphane has shown promise in this regard in several cell lines and with several therapeutic agents. For example, co-administration of sulforaphane and doxorubicin in mouse fibroblasts of differing p53 status enhanced the therapeutic efficacy of doxorubicin.12 Treatment of a 5-fluorouracil-resistant salivary gland adenoid cystic carcinoma high metastatic cell line (ACC-M) and low metastasis cell line (ACC-2) with sulforaphane and 5-fluorouracil in combination led to synergistic inhibition of cell growth and a decreased expression of nuclear NF-κB p65 protein.13 In prostate cancer cell lines, sulforaphane enhanced the therapeutic potential of tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) in PC-3 cells and sensitised TRAIL-resistant LNCaP cells.14 Shankar et al14 also showed that, in athymic nude mice inoculated with PC-3 cells, the combination of sulforaphane (administered by gavage at 40 mg/kg) and TRAIL, compared with a single agent alone, was more effective in inhibiting prostate tumour growth. TRAIL is considered to be a tumour-selective apoptosis-inducing cytokine and a promising candidate for cancer prevention and treatment because it has no effect on normal cells.15 At the same time, improving sensitivity to TRAIL in resistant cells by downregulating constitutive Akt and NF-κB is recognised as an important aspect of TRAIL therapy.14
In this issue of Gut, Kallifatidis et al (see page 949) show, using several approaches, that sulforaphane in combination with TRAIL may be a promising strategy for targeting treatment-resistant pancreatic tumour-initiating cells (TICs).16 Pancreatic adenocarcinoma is an aggressive malignancy, often diagnosed late in the course of the disease, and very difficult to diagnose and treat.17 To date, systemic treatment for pancreatic cancer, either a single agent or combinations of agents, has shown only modest benefits18; however, growing understanding of the molecular pathogenesis of pancreatic cancers, and the complex heterogeneous signals associated with them, may help to target these pathways specifically, overcome chemoresistance and improve patient outcome. In their study, Kallifatidis et al demonstrate that treatment resistance of TICs in pancreatic cancer cell lines was due to increased binding of transactivation-competent NF-κB complexes, which prevented induction of apoptosis. Treatment with three different chemopreventive agents, sulforaphane, wogonin and resveratrol, abrogated this resistance. All three compounds were able to resensitise TICs by interfering with NF-κB activity; however, this effect was strongest with sulforaphane. They went on to use sulforaphane in nude mice injected with MIA-PaCa2 cells and showed that sulforaphane could selectively potentiate apoptotic effects in pancreatic TICs in vivo without exhibiting marked cytotoxic side effects on normal cells.
This study provides intriguing support for a role for sulforaphane in suppression of pancreatic TICs and potentiation of antitumour effects of TRAIL, effects which were achieved with intraperitoneal doses of 4.4 mg sulforaphane/kg/day in immunodeficient mice. The potential to achieve such an effect in humans will require consideration of the route of administration, dose and possibly drug–phytochemical interactions19; however, taken together with the reported efficacy of sulforaphane by oral gavage in the prostate cancer orthotopic model,14 these findings are certainly grounds for optimism. To date, few human studies have used doses of free isothiocyanates; rather, most studies have used oral doses of glucosinolates, which can result in wide ranges of isothiocyanate bioavailability: 1–45% recovery of the administered dose as urinary isothiocyanate metabolites.20 Recovery of a dose of free isothiocyanates is higher and in the range of 70–80%, probably because gut bacterial metabolism is not required for availability.21 Plasma concentrations of sulforaphane and its thiol conjugates can reach maximal concentrations (Cmax) of 2–6 μM within 2–3 h of dosing; however, isothiocyanates have relatively short plasma half-lives, and are undetectable in plasma after 24 h of consumption.21 In clinical trials, isothiocyanates extracted from broccoli sprouts appear to be safe and well tolerated; however, there has been little systematic evaluation of high doses and long-term administration of isothiocyanates in human studies. One clinical phase I study evaluated the safety, tolerance and metabolism of orally dosed broccoli sprout extracts as glucosinolates and isothiocyanates in healthy volunteers over a 7-day period.6 No significant or consistent subjective or objective abnormal events (ie, potential toxicities) were observed with 25 μmol isothiocyanate (equivalent to ∼4.4 mg as sulforaphane) per person per day. In order to move sulforaphane into large-scale clinical trials, further research into the metabolism, bioavailability and the impact of this compound on the relevant pathways in humans is needed.
In summary, there is growing experimental evidence to support the efficacy of sulforaphane and other phytochemicals in downregulating the activation of NF-κB through several different mechanisms, and these compounds are recognisd for their potential to be developed as anti-inflammatory and chemopreventive agents for prevention of cardiovascular disease and cancer.22 The work of Kallifatidis et al extends the potential use of sulforaphane to the chemotherapeutic realm. It demonstrates the use of sulforaphane as a supplement to chemotherapy, namely as an NF-κB-inhibitor, sensitising pancreatic cancer cells to TRAIL-induced apoptosis.
Competing interests: None.
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