According to data from 2020, gastric cancer (GC) is responsible for the fifth and fourth greatest incidence and mortality rates worldwide, respectively(Reference Sung, Ferlay and Siegel1). The prevalence rate of GC varies geographically(Reference Katoh and Ishikawa2). The highest incidence rate of GC was recorded in East Asia(Reference Katoh and Ishikawa2,Reference Rahman, Asombang and Ibdah3) . Although South Korea has experienced a decreasing trend in incidence and mortality since 1999(Reference Hong, Won and Lee4), GC remains a significant concern.
Several studies have been conducted to explore various risk factors associated with GC development(Reference Yusefi, Bagheri Lankarani and Bastani5). Although infection with Helicobacter pylori (H. pylori) has been well recognised in relation to GC progression, the role of other risk factors in the aetiology of GC still needs to be determined(6). A healthy diet is indicated to be a key source of important vitamins and minerals, and its importance has drawn much attention in recent years(Reference Richa and Sageena7).
Previous epidemiological studies documented that minerals may have certain roles in GC pathogenesis. For example, sodium consumption has a detrimental effect on GC even with the intake of intermediate levels(Reference Pelucchi, Tramacere and Bertuccio8). A higher haem iron intake was reported to be associated with an elevated GC risk, whereas non-haeme iron was suggested to be a protective factor against GC risk(Reference Tran, Gunathilake and Lee9). A beneficial effect against gastric carcinogenesis was also found for Ca and Mg(Reference Shah, Dai and Zhu10).
Potassium is an essential mineral that is mainly derived from dietary sources such as fruits, vegetables, beans and lentils. The Western diet is a consequence of the global spread of the Western lifestyle, which is characterised by low fruit and vegetable consumption and high processed food consumption. Consequently, there is an imbalance in Na and K intake, where high Na and low K intake have been reported(Reference Weaver11). To date, few studies have been conducted to elucidate the link between low potassium consumption and cancer. Potassium intake was found to be a preventive nutrient for colorectal cancer(Reference Kune, Kune and Watson12,Reference Meng, Sun and Yu13) . Similarly, an appropriate potassium intake served as a protective factor against lung cancer risk(Reference You, Zhang and He14). In contrast, available evidence regarding a protective effect of potassium against other cancers is limited, and GC is no exception. To the best of our knowledge, only one study has been conducted to explore the association of potassium with GC. However, a non-significant association was observed(Reference Pelucchi, Tramacere and Bertuccio8), whereas potassium was suggested to have a preventive effect on GC development by a nutrition survey(Reference Correa, Cuello and Fajardo15). Thus, there is a paucity of evidence related to the relationship of potassium intake with GC risk. In addition to dietary factors such as dietary potassium intake, it is necessary to focus on inflammation-related factors that are strongly related to gastric carcinogenesis.
It has been reported that there are positive correlations between inflammatory cytokines specifically in individuals with H. pylori infection and GC risk(Reference Bockerstett and DiPaolo16). TNF-α is one of the proinflammatory cytokines that stimulates other cytokines and mediates the cytokine cascade causing inflammation(Reference Khan, Mandal and Jawed17). There is a causal association between the production of TNF-α and cancer tumorigenesis(Reference Oshima, Ishikawa and Yoshida18). The regulation of TNF-α production occurs at the level of transcription, and polymorphisms in the TNF-α promoter region are indicated in relation to TNF-α production(Reference Li, Wang and Gu19). The association between TNF-α promoter polymorphisms and GC risk was indicated in a previous meta-analysis(Reference Xu, Kong and Zhao20). One of the TNF-α promoter polymorphisms is the G (guanine) > A (adenine) (rs1800629) SNP located at position -308, which is associated with TNF-α production(Reference Li, Wang and Gu19). In detail, a higher transcriptional activity was observed for the A allele compared to the G allele(Reference Wilson, Symons and McDowell21,Reference Kroeger, Carville and Abraham22) . For example, in comparison with the presence of the G allele at -308 of the TNF-α promoter, the presence of the A allele increased the transcriptional level twofold(Reference Kroeger, Carville and Abraham22). Notably, a previous study emphasised that TNF-α production may be regulated by potassium (K+) and K+ channels by activated human culture-derived macrophages(Reference Qiu, Campbell and Breit23). Furthermore, differences in genetic variants and dietary patterns may account for different GC risks among individuals. Thus, the discrepancy in GC susceptibility may be explained by the interaction between genes and diet(Reference Kim, Cho and Choi24). Based on this biological mechanism, we hypothesised that there may be an interaction between dietary potassium intake and TNF-α rs1800629 in gastric carcinogenesis.
To our knowledge, the potential effect of dietary potassium intake on gastric carcinogenesis has been reported in a few previous studies, and the findings have been ambiguous. Additionally, an interactive effect between potassium intake and TNF-α rs1800629 on gastric carcinogenesis has not been investigated thus far. Therefore, we aimed to examine whether potassium intake is related to GC risk. Moreover, we wanted to observe whether the TNF-α rs1800629 SNP modifies this association.
Materials and methods
Study population
We recruited participants from the National Cancer Center (NCC) Hospital in Korea between March 2011 and December 2014 to conduct a case–control study. The details of participant recruitment are described elsewhere(Reference Kim, Lee and Choi25,Reference Hoang, Lee and Choi26) . Cases were identified as those who were diagnosed with GC within 3 months preceding enrollment, except participants with chronic diseases and who were pregnant or breastfeeding. Subjects without a history of cancer or chronic diseases who visited the Cancer Prevention and Detection Center in NCC for health-screening examinations were identified as controls. We used age (±5 years) and sex to match controls and cases at a ratio of 2:1. A total of 756 controls and 377 cases with available information on genotypes were included for analysis in our study. This study was conducted according to the guidelines laid down in the Declaration of Helsinki, and all procedures involving research study participants were approved by the Institutional Review Board of the National Cancer Center Korea (IRB No. NCC2021–0181). Written informed consent was obtained from all subjects/patients.
Data collection
The assessment of dietary intake of participants within 12 months prior to the interview was performed with a 106-item semiquantitative FFQ. The semiquantitative FFQ has been reported to be valid and reliable(Reference Ahn, Kwon and Shim27) and contains nine categories for food consumption frequency and three categories for portion size. Total energy and potassium intake were determined by using a Computer Aided Nutritional analysis program (CAN-PRO 5·0, Korean Nutrition Society). Dietary potassium (mg/d) for each participant was calculated by summing the amount of potassium obtained from consumed foods. Additionally, information on demographics and lifestyle was collected using a self-administered questionnaire.
Genotype identification
Detailed information on the genotyping and quality control steps is mentioned elsewhere(Reference Yang, Park and Lee28,Reference Park, Yang and Lee29) . Briefly, we used peripheral blood to extract genomic DNA. Genotyping was performed using the Affymetrix Axiom Exom 319 Array (Afymetrix Inc.) platform with 318 983 variants. Genotype imputation was performed using the Asian population (n 504) in the 1000 Genome haplotypes phase III integrated variant set release GRch37/hg19 (https://www.1000genomes.org/) as a reference panel. Genetic markers with deviation from Hardy–Weinberg equilibrium P values <1 × 10−10, a minor allele frequency < 0·05, and a low call rate (< 98 %) were discarded. We used SHAPEIT (v2.r837) for phasing and IMPUTE2 (2·3·2) for SNP imputation. After filtering for an INFO score over 0·6, quality control criteria were applied. Finally, TNF-α rs1800629 was selected as a candidate SNP for the analysis of our study.
Statistical analyses
Potassium intake was energy-adjusted using the residual method(Reference Hoang, Lee and Choi26). We used the distribution of controls to classify potassium intake into tertiles. The comparison of general characteristics of cases with controls was performed by using the χ2 test and t test for categorical and continuous variables, respectively. The calculation of OR and 95 % CIs was based on unconditional logistic regression models. The lowest tertile was considered the reference group. Furthermore, we determined the dose–response relationships of intake of dietary potassium in relation to GC risk by using the median value of each tertile of potassium intake to identify a test for trend. We used a dominant model to analyse genetic association. We used tertile categories of potassium intake to examine the impact of the interaction between dietary potassium intake and TNF-α rs1800629 on GC risk. Statistical interaction was determined using a likelihood ratio test of models with and without the interaction term (potassium ×SNP). SAS software (version 9.4, SAS Institute) was used for all statistical analyses, and a two-sided P value less than 0·05 was considered significant.
Results
Demographic characteristics of the study participants
In comparison with healthy individuals, GC patients were more likely to be infected with H. pylori, and a high proportion of patients were current smokers (92·6 % v. 61·4 % and 30·8 % v. 20·4 %, P < 0·001, respectively). Similarly, they exhibited a higher rate of first-degree family history of GC (20·4 % v. 12·6 %, P < 0·001). In contrast, lower proportions of physical activity, level of education, income and occupation were observed in cases than those in controls (36·1 % v. 56·1 %, 23·1 % v. 51·8 %, 23·3 % v. 32·7 % and 17·2 % v. 19·0 %, respectively, P < 0·001). GC cases consumed significantly lower amounts of dietary potassium than controls (P < 0·001) (Table 1).
Table 1. General characteristics of the study participants
* χ2 test for categorical variables and t test for continuous variables were applied.
† Mean ± sd was presented for continuous variables.
Potassium intake and GC risk
In comparison with subjects in the low tertile group of potassium intake, subjects in the high tertile group showed a lower GC risk. This significant association was observed in both the univariate model and model adjusted for possible confounders; OR (95 % CI) were 0·56 (0·41, 0·77), P for trend < 0·001 and 0·63 (0·45, 0·89), P for trend = 0·009, respectively. Importantly, the significant inverse associations between dietary potassium intake and GC risk remained for both male (OR = 0·65 (0·42, 0·99), P for trend = 0·042) and female (OR = 0·54 (0·29, 0·99), P for trend = 0·048) populations in the fully adjusted model (Table 2).
Table 2. Association of tertiles of dietary potassium intake with GC risk
GC, gastric cancer.
Model 1: unadjusted model; Model 2: adjusted for age, BMI, first-degree family history of GC, smoking status, alcohol consumption, regular exercise and marital status; Model 3: additionally adjusted for H. pylori infection. In the total subjects, models 2 and 3 were additionally adjusted for sex.
Associations of the TNF-α rs1800629 polymorphism with GC risk
The three genotypes of TNF-α rs1800629 were GG, AG and AA and were in Hardy–Weinberg equilibrium (P = 0·077). A dominant model of the TNF-α rs1800629 SNP was used to examine its association with GC risk. We observed a non-significant association of TNF-α rs1800629 with gastric carcinogenesis. The OR (95 % CI) in the unadjusted and adjusted models were 0·98 (0·68, 1·41) and 1·01 (0·68, 1·49), respectively. Non-significant associations were also observed for both sexes; OR (95 % CI) were 1·19 (0·74, 1·92) and 0·73 (0·36, 1·49) for males and females, respectively (Table 3).
Table 3. TNF rs1800629 genetic polymorphisms and risk of GC in the dominant model
GC, gastric cancer.
Model 1: unadjusted model; Model 2: adjusted for age, BMI, first-degree family history of GC, smoking status, alcohol consumption, regular exercise and marital status; Model 3: additionally adjusted for H. pylori infection. In the total subjects, models 2 and 3 were additionally adjusted for sex.
The interactive effect of the TNF-α rs1800629 polymorphism and potassium intake on gastric carcinogenesis
Table 4 presents the interactive effect of the TNF-α rs1800629 genetic polymorphism and potassium intake on gastric carcinogenesis. High potassium intake was found to be inversely associated with GC risk among subjects who carried the homozygous wild-type allele (GG) regardless of confounding adjustment (Model 3: OR = 0·63 (95 % CI: 0·43, 0·91)). Based on the sex stratification, this preventive effect seemed to be limited to females (OR = 0·40 (95 % CI: 0·20, 0·78)) with a significant interaction (P interaction = 0·041).
Table 4. Interactive effect of the TNF-α rs1800629 polymorphism and potassium intake on gastric carcinogenesis
Model 1: unadjusted model; Model 2: adjusted for age, BMI, first-degree family history of GC, smoking status, alcohol consumption, regular exercise and marital status; Model 3: additionally adjusted for H. pylori infection. In the total subjects, models 2 and 3 were additionally adjusted for sex.
OR, odds ratio; CI, confidence interval.
Discussion
In the present case–control study, we observed a negative association between potassium intake and GC risk. Additionally, TNF-α genetic polymorphism was observed to have an effect modification on this association. In detail, a protective effect of potassium against GC seemed to be greater in subjects who had higher potassium intake and carried the TNF-α rs1800629 homozygous wild-type allele (GG), especially among females.
The effect of a diet high in potassium on cancer prevention was investigated in some previous studies. Existing evidence supports the hypothesis that high potassium intake may contribute to reducing cancer risk. For example, a protective effect of potassium against lung cancer was emphasised in a study of 165 409 participants from the Prostate, Lung, Colorectal and Ovarian Cancer Screening trial and the Women’s Health Initiative(Reference You, Zhang and He14). Another study indicated that high potassium intake may be considered a preventative factor for colorectal cancer occurrence(Reference Kune, Kune and Watson12). The aforementioned association was reinforced by a conclusion drawn from a meta-analysis of twenty-nine studies(Reference Meng, Sun and Yu13).
To date, available evidence indicating the role of potassium in GC prevention is limited. There was only a case–control study conducted to explore the potential effect of potassium on GC risk and found a non-significant association(Reference Pelucchi, Tramacere and Bertuccio8). In contrast, high potassium intake was suggested to have a preventive effect on GC development in our study. This is in agreement with a previous nutrition survey(Reference Correa, Cuello and Fajardo15). Consistent findings were also observed in an in vivo study, which reported that the incidence of GC was reduced significantly due to prolonged oral treatment with potassium(Reference Tatsuta, Iishi and Baba30). The possible biological mechanisms underlying this association may be proposed. First, intracellular ions and potassium can be released into the extracellular fluid by tumour necrosis. T-cell receptor-driven Akt-mTOR phosphorylation and effector programs may be impaired by an increase in intracellular potassium within T cells due to increased extracellular potassium. As a result, T-cell effector function is suppressed. Neoantigens, which are generated and recognised by tumour cells and T cells, respectively, and cancerous cells may be killed by T cells. In addition, T cells may produce chemicals that play a role in the regulation of immunity and protective effects against tumours(Reference You, Zhang and He14). Second, the effect of potassium on GC prevention may be linked to gastric acid secretion. Gastric acid is known to be associated with gastric carcinogenesis(Reference Tatsuta, Iishi and Baba30). It is important to note that potassium ions have a critical role in activating and catalysing gastric H+, K+-ATPase, leading to the secretion of acid(Reference Geibel31). Third, potassium may serve as an anti-tumour agent because it is essential for folding and stabilising G-quadruplexes(Reference Frajese, Benvenuto and Fantini32). Fourth, pancreatic cells need potassium to secrete insulin. As a result, hypokalaemia may lead to impaired insulin secretion and glucose intolerance(Reference Stone, Martyn and Weaver33). A higher risk of diabetes can be attributable to lower potassium intake, which was well recognised in previous studies(Reference Colditz, Manson and Stampfer34,Reference Chatterjee, Colangelo and Yeh35) . Notably, epidemiological studies are robust enough to support the causal link between diabetes and GC occurrence(Reference Lai, Park and Hartge36–Reference Tran, Lee and Gunathilake38). Thus, it has been established that diabetes may be a mediator for the link between low potassium intake and increased GC progression.
H. pylori is known to be an aetiology for gastric carcinogenesis. H. pylori and host genetic factors impact the inflammatory response and epithelial cell physiology and increase GC risk(Reference Zheng, Zhang and Zhang39). The TNF-α gene is a major cytokine related to H. pylori infection(Reference Yang, Ko and Cho40). It is reported to be related to chronic inflammation, autoimmunity, tumour progression and metastasis(Reference Cui, Zhang and An41). The role of TNF-α has been indicated to be associated with not only polymorphisms in the genes in relation to regulation of TNF-α production and effect but also polymorphisms in TNF itself(Reference El-Tahan, Ghoneim and El-Mashad42). TNF-α promoter polymorphisms were documented to affect the expression of this gene and are associated with GC susceptibility(Reference Zheng, Zhang and Zhang39). One of the TNF-α promoter polymorphisms is the G (guanine) > A (adenine) (rs1800629) polymorphism, which suggests an impact on the TNF-α level and susceptibility to GC(Reference Li, Wang and Gu19). However, the detrimental effect of TNF-α rs1800629 on GC development is still controversial. It was considered a potential contributor to gastric tumorigenesis and was associated with H. pylori infection in a previous study(Reference Zheng, Zhang and Zhang39). However, a significant association was limited to Caucasians, and a non-significant association was found for East Asians(Reference Li, Wang and Gu19). We found a similar association between TNF-α rs1800629 and GC risk, which is in agreement with a previous study in Korea(Reference Yang, Ko and Cho40).
TNF-α production is associated with cancer progression(Reference Oshima, Ishikawa and Yoshida18). Promoter polymorphisms were indicated in relation to elevated TNF-α production(Reference Li, Wang and Gu19). Macrophages are known to play an important role in immunity and inflammation due to bioactive molecule secretion. A previous study suggested certain roles of K+ and K+ channels in the regulation of TNF-α production by activated human culture-derived macrophages(Reference Qiu, Campbell and Breit23). Thus, we hypothesised that there is an interactive effect of potassium intake with TNF-α rs1800629 on GC carcinogenesis. Our findings suggest a difference in the protective effect of potassium against gastric carcinogenesis according to host genetic factors. A significant effect seems to be observed in those carrying the homozygous wild-type allele of TNF-α rs1800629 (GG). Importantly, a biological interaction between potassium intake and TNF-α rs1800629 was found in our study. Possible mechanisms for the interaction may be explained as follows. Potassium was indicated to regulate TNF-α production. In detail, phorbol myristate acetate-induced cytokine production may be inhibited by the effect of blockade of K+ channels through mechanisms regarding translation or post-translation. This effect is duplicated with an increase in extracellular K+(Reference Qiu, Campbell and Breit23). Additionally, potassium plays a role in TNF-induced apoptosis and gene induction. TNF receptor triggering leads to reduced intracellular spermine, which impacts the activity of potassium channels and intracellular potassium concentrations, enhances the activity of caspases and increases cell death(Reference Penning, Denecker and Vercammen43). Another possible mechanism can be proposed. Potassium cyanate is thought to induce apoptosis in colorectal cancer cell lines. Notably, potassium cyanate is a mediator of TNF-α release in these cells via activation of nuclear factor kappa B(Reference Yang and Chang44). Overall, our study suggests a biological interaction between potassium intake and TNF-α rs1800629. However, a significant interaction was found only for females. Higher expression of inflammatory genes was observed in females than in males due to sex hormones(Reference Klein and Flanagan45). Thus, different eating habits and sex hormones may account for the difference in the interactive effect between males and females(Reference Kim, Lee and Choi46). This interaction should be elucidated in further studies.
This study is one of few studies aiming to determine the protective effect of high potassium intake on the progression of GC. Importantly, our study represents the first attempt to demonstrate an impact of an interactive effect between potassium intake and genetic polymorphisms in proinflammatory genes on GC risk. Additionally, we used a validated and reliable semiquantitative FFQ to collect information on nutrient intake. Information on general characteristics, especially possible confounders, was collected by trained personnel. As a consequence, the quality of our data was relatively higher. Although the associations for subtypes of GC were not assessed in our study due to the limited number of cases, cases in our study well reflect the trend in Asia, where the majority of cases are non-cardia GC(6). Furthermore, the statistical power of genotype associations may be affected by the small number of variant allele carriers. Although we tried to tackle case–control study-related limitations, selection bias and recall bias may occur. Finally, although there would be a possibility to have effect from other genes, except TNF-α, that might be helpful in reaching an effective conclusion. However, we did not consider the effect from other probable genes or combinations of genes in our current study.
In conclusion, our study emphasised a protective effect of high potassium intake against GC carcinogenesis. Additionally, we drew a concept regarding an interaction between dietary potassium intake and TNF-α rs1800629. In detail, the preventative effect of potassium depended on the individual’s genetic background. A greater effect seems to be exhibited for TNF-α rs1800629 homozygous wild-type allele carriers, especially females. This evidence suggests that we should consider individual genotypes to develop strategies for GC prevention.
Acknowledgements
This work was supported by International Cooperation & Education Program (NCCRI NCCI 52 210–52 211, 2020) of National Cancer Center, Korea and grants from National Cancer Center, Korea (1 910 330) and National Research Foundation of Korea (2021R1A2C2008439).
Formal analysis, T. T. T., J. L.; Preparation of original draft, T. T. T.; Writing review and editing, M. G., J. K.; Data curation, I. J. C., Y-I. K., J. K.; Investigation, I. J. C. and Y-I. K.; Methodology, I. J. C., Y-I. K. and J. K.; Funding acquisition, J. K.; Project administration, J. K.; Supervision, J. K. All authors have critically reviewed and approved the final version of the manuscript submitted for publication.
The authors declare that they have no conflicts of interests.
