Tobacco smoking, polymorphisms in carcinogen metabolism enzyme genes, and risk of localized and advanced prostate cancer: results from the California Collaborative Prostate Cancer Study

Introduction

Tobacco smoking is associated with an increased risk of several cancers, yet its carcinogenic role in the prostate is not clearly established [1, 2]. Whereas several epidemiologic studies do not support an association [2], a meta-analysis of 24 prospective cohort studies reported an estimated overall increased risk of PCa- and PCa-related mortality associated with tobacco smoking [1]. A 2009 review of the epidemiological literature further supported an association between tobacco smoking and aggressive PCa [3]. Possible factors contributing to inconsistencies in the literature include the heterogeneity in study designs, the varying smoking status definitions, the lack of details on smoking history and cessation, and lack of consideration of stage and tumor grade. Moreover, risk of PCa associated with tobacco smoking may also differ by race/ethnicity. Most studies examining the association between PCa risk and tobacco smoking predominantly include non-Hispanic White men, with only a few in African-Americans (AA) [4, 5] or Hispanics [6].

Tobacco smoke contains a wide variety of chemical carcinogens, including polycyclic aromatic hydrocarbons (PAHs), aromatic amines, heterocyclic amines (HCAs), and N-nitroso compounds (NOCs) [7]. The prostate gland is able to metabolize many of these chemicals into activated carcinogens [8–11], suggesting a plausible link between tobacco smoking and prostate carcinogenesis. In support of this, a prior study has reported associations between tobacco smoking and the presence of PAH-DNA adducts in the prostate, which varied by race and were modified by genetic variants involved in PAH metabolism [12]. To date, few studies have evaluated polymorphisms in tobacco carcinogen metabolism enzymes as possible modifiers of the association between tobacco consumption and PCa risk, in particular among different racial/ethnic populations [5, 13, 14].

In this study, we evaluated associations between tobacco smoking and risk of localized and advanced PCa using data from the California Collaborative Prostate Cancer Study, a large population-based case–control study in non-Hispanic White, AA, and Hispanic men. We also considered the role of polymorphisms in selected genes that code for tobacco carcinogens metabolism enzymes (GSTP1, PTGS2, CYP1A2, CYP2E1, EPHX1, CYP1B1, UGT1A6, NAT2, GSTM1, and GSTT1) as potential modifiers of the relationship between tobacco smoking and PCa risk.

Materials and Methods

The California Collaborative Prostate Cancer Study was conducted in Los Angeles County (LAC) and in the San Francisco Bay area (SFBA) and used similar protocols and a common structured questionnaire administered in person. The characteristics of the study population and participation rates have been previously described [15, 16]. Briefly, newly diagnosed PCa cases were identified through the LAC and Greater Bay Area cancer registries. At both study sites, patients with intracapsular PCa were classified as localized cases and patients with extracapsular extension of the tumor, and/or extension into adjacent surrounding tissue, or regional lymph nodes, or metastasis to other areas of the body, were classified as advanced cases (Surveillance, Epidemiology, and End Result [SEER] 1995 clinical and pathologic extent of disease codes 41–85).

Results

Socio-demographic and lifestyle characteristics, including tobacco smoking, of cases and controls are presented in Table 1. When compared to controls, localized and advanced cases were more likely to report a family history of PCa. Localized cases were of lower SES than controls. Among controls, 67% had ever smoked tobacco and 18% were current smokers during the reference year. They consumed, on average, about a pack of cigarettes a day and smoked for an average of 28.2 years. Tobacco smoking characteristics by PCa stage and race/ethnicity are presented in Table S1. No substantial differences in smoking characteristics were seen among races/ethnicities. Among controls, 65% of non-Hispanic Whites were ever tobacco smokers and smoked 30.7 pack-years compared to 73% of AA who smoked 27.4 pack-years and 70% of Hispanics who smoked 24.6 pack-years.

Characteristics of PCa cases by smoking status (never smoker, quit >21 years ago, quit ≤21 years ago, current smoker) are presented in Table S2. When compared to never and former smokers, current smokers had a lower BMI (P = 0.002), lower SES (P = 0.001), were more likely to be non-Hispanic White or AA (P < 0.001), were of younger age at PCa diagnosis (P < 0.001), had higher levels of alcohol consumption during their lifetime (P < 0.001), consumed more meat cooked at high temperature (P < 0.001), had lower total vegetable consumption (P < 0.001), and lower total fruit intake (P < 0.001). When compared to former smokers, current smokers were more likely to smoke a pack or less (P < 0.001) and more likely to smoke for >29 years (P < 0.001).

Discussion

In this study, ever smoking was found to be associated with localized PCa risk, particularly among non-Hispanic Whites. Quitting smoking was also associated with localized PCa. In contrast, being a current smoker was associated with risk of advanced PCa among non-Hispanic White men. For both localized and advanced PCa, the association with smoking was modified by a polymorphism in the carcinogen metabolism CYP1A2 gene. Overall, our findings lend support to a role for tobacco smoking in PCa risk after taking into account both PCa stage and race/ethnicity in the analyses.

In congruence with our findings, a population-based case–control study in the U.S. reported that current cigarette smoking was associated with an increased risk of PCa when compared to non-smoking [32]. In that study, PCa risk increased with increasing pack-years of cigarette smoking, something we did not observe in our study. Moreover, in contrast with our study, a stronger association was observed between pack-years and aggressive PCa, and quitting smoking was associated with reduced PCa risk. However, our observations of former smokers having an increased risk of localized PCa, and non-Hispanic White current smokers having an increased risk of advanced PCa, are consistent with findings from a 2010 meta-analysis of 24 prospective cohort studies showing that both former and current smokers had an increased risk of incident PCa, although stage and race/ethnicity were not accounted for in the meta-analysis [1]. A large cohort study including data from 10 European countries (EPIC), which considered stage and grade, reported an inverse association between localized and low-grade prostate cancer (PCa) among smokers, which is in contrast with our results that showed a positive association [33]. This study also reported no significant association with smoking among advanced and high-grade cases. In contrast, the Japan Public Health Center-based prospective study (JPHC study), which included over 48,000 men, and a study using the Shared Equal Access Regional Cancer Hospital (SEARCH) cohort both found a positive association between smoking and diagnosis of advanced PCa [34, 35]. Based on the available literature the 2014 Surgeon General's Report on smoking and tobacco use concluded that there is suggestive evidence showing smoking to be a risk factor of being diagnosed with advanced stage or high-grade PCa, which is a risk factor for progression and death [36]. Similar to 2010 meta-analyses mentioned above, this report highlighted the scarcity of studies that took into account stage and grade at diagnosis.

As with other tobacco-related cancers, a possible mechanism by which tobacco smoking might influence PCa risk is the action of tobacco-related carcinogens that could induce DNA damage in the prostate. These mutagenic carcinogens can be endogenously metabolized to their active forms, which upon reaching the target tissues can bind to DNA. Alternatively, they can be detoxified to less active forms that can be readily excreted from the body. Carcinogen metabolism enzymes are responsible for the detoxification or activation of mutagenic carcinogens and are coded by genes known to be variable in the population [37]. In this study, we found that the association between smoking status and PCa risk was modified by CYP1A2 -154A>C (rs762551), a gene that codes for an enzyme that plays a key role in the metabolism of many drugs, such as caffeine, and in the activation of various tobacco carcinogens, including HCAs and PAHs [38–42]. We observed that the association between current smoking status and PCa risk seemed restricted to carriers of one or two copies of the C allele. CYP1A2 is an inducible phase I metabolizing enzyme highly active in the liver, where it plays a predominant role in the activation of HCAs [40], such as those found in tobacco, to reactive species that can undergo further activation in the liver or detoxification. CYP1A2 mRNA is also expressed in prostate tissue [43–45]. CYP1A2 expression is variable in the general population and the CYP1A2 -154A>C polymorphism may partially explain the observed variability in CYP1A2 inducibility, with the protein coded by the A allele being correlated with higher enzymatic activity than the one coded by the C allele [42]. We have previously reported that the association between well-done cooked red meat, known to accumulate HCAs, and colorectal cancer was stronger among carriers of the C allele [46]. A meta-analysis of 19 case–control studies further showed that the CC genotype is associated with an increased risk of various types of cancer combined (including breast, colorectal, lung, pancreatic, ovarian, stomach, and bladder) and a significant increase in risk among Caucasians [47]. Furthermore, other studies showed that low activity of CYP1A2 was associated with risk of testicular cancer [48] and PCa [49]. A possible explanation for these findings is that slower activation of HCAs in the liver by CYP1A2 may allow HCAs to remain in the body's circulation longer, which could result in greater amounts of HCAs reaching other organs and tissues, such as the prostate. We cannot exclude, however, the possibility that our finding might be a false positive; Bonferroni adjustment of the CYP1A2 -154A>C by smoking interaction P-value by the number of SNPs/phenotypes tested (n = 10) would render an interaction P-value of borderline significance (P = 0.08). Since CYP1A2 is heavily involved in caffeine metabolism, we also considered frequency of coffee intake in the interaction models to take into account possible confounding, but ORs did not change by >10%.

Screening bias has been identified as a possible limitation in previous studies, and could be present if tobacco smoking patterns were correlated with PCa screening, specifically prostate-specific antigen (PSA) screening [3]. We evaluated potential confounding by PSA screening during the 5 years prior to the reference year among the cases and controls from the SFBA study site, for whom we had data on PSA screening. Including PSA screening in the model with other covariates did not change the OR estimates for any of the smoking variables by >10% and was therefore not included in the final model. Furthermore, there was no statistical difference in PSA screening when comparing cases with controls: 76% of controls, 80% of localized cases, and 71% of advanced cases reported previous PSA screening.

The overall strengths of this study include the utilization of a population-based study design with cases obtained from two SEER cancer registries, a large sample size of cases and controls, a multiethnic population that includes non-Hispanic White, AA, and Hispanic men, oversampling of advanced cases, and detailed information on lifestyle and other characteristics. Another strength is the consideration of genetic variation in tobacco carcinogen metabolism enzymes to examine gene by environment interactions. Among the limitations of this study is the inclusion of only a few selected functional SNPs for each candidate gene, which did not allow us to comprehensively consider all their genetic variation. Other limitations include the lack of data on environmental tobacco exposure, which may have introduced some exposure misclassification and finally, small sample sizes when stratifying the analyses by multiple factors (stage, race/ethnicity, age at diagnosis), which reduced our statistical power to detect possible associations.

In summary, our findings support a role for tobacco smoking and risk of PCa, and suggest that ever smokers, including those who quit, are at increased risk of having localized PCa, whereas current smokers have a statistically significant increased risk of advanced PCa. Moreover, our gene-environment analyses support a role for tobacco carcinogens in PCa risk, further strengthening an association between tobacco smoke and PCa risk.