The present study demonstrated that high level of urinary total ITC was associated with a statistically significant, 30–35% decrease in risk of gastric cancer in middle-aged or older men in Shanghai, China. This protective effect of ITC on gastric cancer was stronger in men possessing the homozygous deletion polymorphisms of either GSTM1 or GSTT1 gene. These results showed a direct link between specific degradation products of cruciferous vegetables (i.e., ITCs) and reduced risk of developing gastric cancer in humans.
Few laboratory studies have examined the chemopreventive properties of ITC on gastric tumorigenesis. Feeding sulforaphane, an ITC that is abundant in broccoli, to mice following dosing of benzo[a
]pyrene significantly reduced the number of carcinogen-induced gastric tumors in experimental animals.18
There are a limited number of epidemiologic studies that investigated the effect of cruciferous vegetables on gastric cancer risk in humans. The results are mixed.44
In a hospital-based case-control study conducted in Japan, high consumption of broccoli and Chinese cabbage was associated with a borderline statistically significant, reduced risk of gastric cancer.45
A case-cohort study in Hawaiian men of Japanese ancestry demonstrated a statistically nonsignificant, inverse association between cruciferous vegetable consumption and gastric cancer risk after adjusting for age and smoking.46
A recent case-control study in Hawaii failed to find a statistically significant inverse association between cruciferous vegetable consumption and gastric cancer in H.pylori/
CagA-negative individuals, but suggested a decreasing risk with increasing intake of cruciferous vegetables among H.pylori/
CagA-positive individuals (Ptrend
In the present study, we used a validated urinary biomarker for ITC intake and demonstrated a statistically significant inverse association between urinary ITC and the risk of developing gastric cancer.
The modification of ITC chemoprevention of gastric cancer by polymorphisms of GSTM1
is biologically plausible. GST-catalyzed conjugation with glutathione aids in the elimination of not only environmental carcinogens but also of anticarcinogenic substances in the diet, such as ITCs. Conjugation of ITC with glutathione, a reaction catalyzed by GSTs, constitutes the major route of ITC metabolism.33, 48
ITC are among the GST substrates that are most rapidly conjugated.48
Thus, individuals with abolished GST enzymes, resulting from the homozygous deletion of GSTM1
gene, would derive more beneficial effect of dietary ITC on the inhibition of carcinogenesis. Within Chinese populations in Shanghai, China and Singapore, we have demonstrated the greatest cancer risk reduction associated with cruciferous vegetable consumption among individuals carrying null or low activity GST
A recent feeding study showed a statistically significant, elevated plasma levels of sulphoraphane metabolites in GSTM1 null
individuals compared to their GSTM1 non-null
counterparts who consumed comparable amounts of broccoli, supporting the notion that subjects lacking functional enzymes have higher in vivo exposure to ITC and may derive greater beneficial effect from consumption of cruciferous vegetables.49
Paradoxically, the same feeding study and another one-dose broccoli feeding study also showed higher urinary excretion of ITC in GSTM1 null
subjects compared to GSTM1 non-null
subjects, 49, 50
suggesting the complexity of ITC metabolism in humans.
The present study confirms our previous findings that history of infection with H. pylori
is associated with a significantly increased risk of gastric cancer,7
particularly among individuals with relatively long duration of follow-up. These results were consistent with findings by others.51
Infection with H. pylori
is an established risk factor for gastric cancer in humans7, 8, 13,51
and it is possible that the observed inverse association between urinary ITC levels and gastric cancer risk may be due to the bactericidal properties of ITC. Several laboratory studies have shown that ITC have various levels of antibacterial activity, and sulforaphane, a specific ITC, possesses bactericidal properties against both antibiotic sensitive and resistant strains of H. pylori
Sulforaphane could inhibit both extracellular and intracellular forms of H. pylori
in in vitro
and in vivo
While the exact mechanism of how ITC inhibits the H. pylori
bacteria remains unknown, it is suspected that the inhibitory effects of ITC on H. pylori
growth may be related to its intracellular accumulation in mammalian cells.18
Although not statistically significant, recent data from Hawaii suggests an inverse association between cruciferous vegetable intake and risk of gastric cancer among H. pylori/
CagA-positive but not H. pylori/
consistent with the notion of an antibacterial mechanism for the chemoprotection effect of cruciferous vegetables on gastric cancer.
In addition to its inhibition on H. pylori
, ITC may protect against the development of gastric cancer through other mechanisms. ITC can influence the metabolism of procarcinogens via inhibition of phase I enzymes such as cytochrome P450 (CYP) enzymes, which are involved in the activation of procarcinogens.52
In NNK-treated rats, phenethyl ITC selectively inhibits specific CYP enzymes, most likely, CYP2A3 and CYP2A13.53, 54
Experimental studies have shown that ITC could block the formation of carcinogen-induced DNA adducts in rats and mice.55–57
Administration of benzyl ITC prior to dosing of carcinogen almost completely inhibited the development of benzo(a
)pyrene-induced forestomach tumors in mice.58
Also, ITC can induce phase II enzymes such as GSTs, which can enhance the conjugation of activated carcinogens with glutathione, thus increasing their urinary excretion.59
Finally, the chemopreventive effect of ITC on cancer may be mediated through its ability to induce apoptosis. It has been demonstrated that dietary ITC can induce apoptosis in the gastrointestinal tract,60
thereby preventing colonal expansion of damaged cells and eventual formation of cancer in the stomach.
The present study had several strengths. We used a validated biomarker to assess total ITC exposure.33
We adjusted for the potential confounding effects of H. pylori
infection, cigarette smoking, alcohol drinking, and serum antioxidants on the ITC-gastric cancer association. The prospective cohort study design ruled out the possibility of recall bias on exposure. For most cases, baseline blood and urine samples were collected many years prior to cancer diagnosis. Thus, the concern over a possible impact of clinical manifestation of cancer on the metabolism of ITC, resulting in altered levels of ITC in urine, among cases with a short follow-up duration is minimized. The long duration of follow-up (up to 16 years) allowed the present study to examine the long-term protective effect of ITC against the development of gastric cancer. The almost complete follow-up for incident cancer and death minimized the potential bias on results due to the loss to follow-up.
There are also several potential limitations to this study. One cannot presume that ITC level in a randomly-timed, single void urine sample correlates with usual intake of dietary ITC in an individual. However, we previously demonstrated among Chinese in Singapore, a population that shares a similar cultural and dietary heritage as our study population in Shanghai, a close and statistically significant correlation between dietary ITC ascertained from a validated food frequency questionnaire and total ITC levels in a randomly-timed spot urine.33
In the Shanghai Cohort Study, intake of specific cruciferous vegetables was not solicited at baseline. Therefore, the association between dietary ITC and gastric cancer risk cannot be assessed in this report. In a cohort study, it is advantageous to assess exposure at multiple time points prior to disease occurrence. However, in the case of biomarkers, it is rarely feasible due to the high cost and logistical complexity in collecting biospecimens from large numbers of cohort participants. Using this same biomarker approach based on a single spot urine, we have established an inverse association between dietary intake of ITC and lung cancer in this cohort study.28
Our findings were subsequently confirmed by others.30, 61
It is biologically plausible that the protective effect of ITC on gastric cancer is mediated through the former’s inhibitory actions on H. pylori
. However, only 12 cases in the present analysis can be considered truly H. pylori
negative (see ). Thus, this study is incapable of examining whether there exists an interaction effect between ITC and H. pylori
infection on gastric cancer.
Smokers consumed less amounts of cruciferous vegetables,62
and thus showed lower levels of urinary ITC than nonsmokers (0.97 versus 1.15 µmol/g creatinine; P
= 0.04). The lower consumption of cruciferous vegetables in smokers could potentially explain the weaker inverse association between ITC and gastric cancer in this subgroup versus the never smokers.
Cruciferous vegetables contain precursors of ITC and other bioactive compounds including indole-3-carbinol.16
Indole-3-carbinol has shown a tumor inhibitory effect in mice forstomach.63
The observed inverse association between urinary total ITC and gastric cancer risk may merely reflect the chemoprotective effects of other compounds that are present in cruciferous vegetables.
In summary, dietary ITC, as measured by urinary biomarkers of total ITC, may protect against the development of gastric cancer. Individuals with homozygous deletion of GSTM1 and GSTT1 genes may derive a greater beneficial effect of ITC on gastric carcinogenesis.