COFFEE AND CANCER

The effects of coffee consumption on genitourinary cancers have not provided definitive associations in epidemiological studies, and research in this area is ongoing.

A recent study examined the activities of the phytochemicals in coffee and concluded that their overall effects were not sufficient to account for the required radical scavenging anti-inflammatory activity observed in human and laboratory animal studies. It was proposed that the activation of nuclear factor erythroid 2 (Nrf2) and its protective pathways (Figure 2) play a major role in mediating the beneficial health effects of coffee [102]. For example, coffee/coffee phytochemicals induce or activate Nrf2 in cells under some oxidative stress [102,103,104,105]. Among the important Nrf2-regulated genes associated with antioxidant activity are glutathione peroxidase haem oxygenase-1, glutathione reductases, superoxide dismutase, quinone oxidoreductase, and several thioreductases. Nrf2 exists as a cytosolic dimer with Keap2, and it also interacts with Cullin 3-based ubiquitin ligase and this complex maintains basal cytosolic levels of the Nrf2-Keap heterodimer. A combination of factors, including those that inhibit Keap-Nrf2 interactions or enhance Keap degradation, results in the nuclear uptake of Nrf2, which binds to small musculoaponeurotic fibrosarcoma (sMaF) protein to form the Nrf2-sMaF heterodimer, which then interacts with the cis-acting antioxidant response element (ARE) in target gene promoters to activate gene expression. This results in the activation of antioxidant genes (e.g., glutathione reductase, glutathione peroxidase), multiple redox family genes, (e.g., catalase, haem oxygenase (1)), anti-inflammatory genes (e.g., interleukins, interferons, tumor necrosis factor), drug metabolism enzymes (e.g., epoxide hydrolase, UDP–glucuronyl transferases, CYP1B1) and many other genes/pathways. All of these induced, Nrf2-dependent pathways/genes play diverse roles as cellular protective proteins. In addition, several reports demonstrate that the AhR and its ligands co-operatively enhance Nrf2 pathways [105,106,107,108]. One of these pathways involves the aryl hydrocarbon receptor (AhR)-dependent induction of CYP1A1 and CYP1A1-dependent substrate metabolism, which, in turn, enhances Nrf2 expression. The AhR also directly binds to the Nrf2 promoter and induces the levels of this protein [106].

 

The protective properties of Nrf2 are dependent on the induction of Nrf2 and Nrf2-dependent genes, as illustrated in Figure 2, and there is evidence that many of the constituents in coffee, including chlorogenic acids, phenolics, caffeine, cafestrol, and kahweol, are inducers of Nrf2 [109,110,111,112,113,114,115,116,117,118,119]. Moreover, a recent study showed that aqueous coffee extracts induced Nrf2 and Nrf2-dependent genes in various cell lines, and the co-operative role of the coffee-induced activation of AhR/Nrf2 had been reported in several in vitro and in vivo studies involving coffee extracts [120,121,122,123,124,125]. The activation of Nrf2 and Nrf2 protective genes by coffee and its individual components is consistent with their activity to protect nontransformed cells from oxidative stress. However, there is also evidence that the cytotoxicity of coffee components can occur by multiple pathways, including the disruption of the mitochondrial membrane, and this results in a loss of mitochondrial membrane potential and ROS-dependent cytotoxicity [125,126,127,128,129,130,131] (Figure 3). This pathway has been observed for many ROS-inducing phytochemicals and includes the inactivation of cMyc and cMyc-regulated microRNAs (miRs), the induction of ZBTB genes, which repress Sp1-, Sp2-, and Sp3-regulated genes/pathways [131]. Thus, coffee components activate cell context-dependent antioxidant and oxidative stress pathways that are primarily but not exclusively linked to chemopreventive and chemotherapeutic pathways in non-cancer and cancer cells, respectively.

Although the activation of Nrf2 plays an important role in alleviating oxidative stress, the increased levels of Nrf2 in cancer cells are linked to malignant progression and drug resistance [132]. Coffee extract-induced Nrf2 and the similar effects of other phytochemical-enriched diets may be beneficial in normal cells that require low levels of ROS; however, in cancer cells, this can result in enhanced carcinogenesis [132,133]. Nrf2 levels in cancer cells are enhanced due to overexpressed mutations that activate Nrf2 or mutations in Keap, and this enhances the pro-oncogenic pathways and the development of resistance to anticancer agents [133,134]. For example, in hepatic progenitor cells, Nrf2 induces malignant transformation due to the activation of the wnt-β-catenin pathway [135]; suppression of Nrf2 by histone lysine methyltransferase SETDB2 inhibits the progression of lung adenocarcinoma cells, whereas the decreased SETDB2 in these cells enhances Nrf2-mediated tumorigenesis [136]. It has also been reported that the chemically or genetic-mediated downregulation of Nrf2 increases ROS-medicated cell death and reverses some drug resistance [137,138,139]. Interestingly, coffee extracts contain trigonelline, which inhibits Nrf2 activity and thereby modulates the potentially beneficial and harmful effects of NRF2. The inhibitor of Nrf2 trigonelline and other coffee components inhibit cancer cell growth, migration, and drug resistance [140,141,142,143,144,145]. In contrast, Nrf2 inhibitors, such as trigonelline, can interfere with Nrf2-mediated antioxidant activity and thereby reverse some of the protective effects of Nrf2. Thus, coffee and some of its key chemical components (chlorogenic acid and phenolics trigonelline) and pathways (Nrf2 and ROS) exhibit cell context-dependent opposing effects. This suggests that other cell-specific factors must also contribute to the regulation of other pathways/genes that are associated with the health benefits of this beverage. Moreover, the issue of coffee and other phytochemicals and their beneficial effects need to be further evaluated with respect to cancer, where Nrf2 may not be beneficial.

As indicated above, Nrf2 and the AhR are co-operatively activated by coffee extracts, and studies in our laboratories have primarily focused on the direct effects of coffee extracts as AhR ligands in the intestine (Figure 4A) [146]. AhR plays an important role in multiple organs/tissues, and there is extensive evidence that AhR is involved in aging and is related to diseases, suggesting that AhR and its ligands can affect these conditions [147,148,149,150,151]. Aqueous coffee extracts induced the Ah- responsive CYP1A1, CYP1B1, and UGT1A1 genes in Caco-2 colon cancer cells and YAMC mouse colonocytes, and these responses were abrogated in their corresponding AhR knockout (AhR-KO) cell lines. In addition, Ah responsiveness was primarily observed in a chloroform extract of aqueous coffee extracts, and chromatographic and cell culture analysis indicated that caffeine was not an AhR-active compound. The chloroform extracts from coffee were separated by this layer chromatography into three bands: a less polar band, a caffeine band, and a polar band. The extracts of the polar and less polar bands were AhR-active [146]. A comparison between aqueous extracts of unroasted and roasted ground coffee demonstrated that the high levels of AhR-active extracts in brewed coffee were primarily due to the roasting process. In vivo studies showed that coffee extracts inhibited DSS-induced colonic inflammation and inhibited the growth of organoids enriched in colonic stem cells. This latter response is consistent with anticancer activity since colonic stem cells are precursors for the development of colon tumors. We observed some of the same responses for the alkaloid norharman(β-carboline), which is one of the AhR agonists in coffee extracts that contributes to the AhR-dependent responses observed in the study. These results for coffee were observed in wild type but not AhR-KO cells and animal models and are consistent with previous studies showing a protective role for AhR and its ligands in colonic inflammation and cancer [152,153,154,155,156,157]. The identities of most AhR ligands in coffee extracts have not been determined; however, it should be pointed out that AhR and its ligands can be both beneficial and harmful, and these responses are organ/tissue-context-dependent [158]. The beneficial effects of AhR ligands in colon cancer are well supported; however, AhR is also a pro-oncogenic factor in other tumor types, including head and neck cancer [159]. Thus, at least some of the health-promoting effects of coffee are mediated through its binding and activation in AhR signaling.

NR4A1, NR4A2 (Nurr1), and NR4A3 (Nor1) are orphan nuclear receptors and the immediate, early genes that respond to stress and play a role in maintaining cellular homeostasis and pathophysiology [160,161]. These receptors are often increased in the diseased cell types that are associated with stress, and this includes many solid tumors and derived cancer cell lines [162]. It was hypothesized that NR4A orphan nuclear receptors were the potential targets for anti-aging interventions [163] due to the functions of these receptors in the immune system and stress-related diseases, and this has now been confirmed by several studies [164,165,166,167]. For example, age-related cognitive impairment in mice results in decreased NR4A expression, and treatment with a bis-indole-derived NR4A2 ligand enhanced long-term spatial memory and rescued memory deficits in mouse models [166]. Age-related renal fibrosis was also suppressed by NR4A1 [165], and there are other reports showing that NR4A is involved in age-related diseases. Recent studies reported that polyphenolics, including flavonoids such as quercetin and kaempferol that occur in coffee, bind to NR4A1 in cancer cells, act as receptor antagonists, and inhibit pro-oncogenic NR4A1-regulated genes and pathways (Figure 4B) [168]. When using Rh30 rhabdomyosarcoma cells as a model, it was reported that quercetin and kaempferol decreased NR4A1-dependent transactivation, inhibited Rh30 cell growth, survival, and invasion, decreased tumor growth, and also decreased mTOR signaling and β1-integrin expression. All of these effects were also observed after NR4A1 knockdown or after treatment with other NR4A1 antagonists [168]. Ongoing studies have identified multiple polyphenols that are present in coffee extracts and in Mediterranean diets. Thus, it is likely that NR4A1 and its phytochemical ligands may play important roles in the health-promoting effects of coffee and phytochemical-enriched diets, and this needs to be further investigated.

Based on the health benefits of coffee consumption, an epigenome-wide association meta-analysis of DNA methylation with coffee drinkers was reported [169]. The data were generated from epigenome-wide association studies of coffee (and tea) consumption among 15,789 European and African American participants from 15 cohorts. An analysis of the data identified several CpGs that were related to coffee consumption, and these included the AHRR, F2RL3, FLJ43663, HDAC4, GFL1, and PHGDH genes. The relationship between these genes and the effects of coffee is uncertain, although it was suggested that PHGDH might be associated with protection from liver damage. In addition, AHRR represses AhR activation and could also affect AhR activity. The nuclear receptor NR4A1 regulates the methyltransferase gene G9A [168], and, possibly, the other methyltransferases and the epigenetic effects of coffee could be due, in part, to NR4A1 ligands, such as flavonoids and other polyphenolics in coffee extracts. Thus, the linkage between coffee extract-induced epigenetic changes as causal factors needs to be further investigated, along with parallel comparisons with animal models.

The gut microbial populations and their metabolites strongly influence intestinal health and also affect distal organs, and there is extensive evidence that the influence of diet on health may be due, in part, to changes in the gut microbiome [170,171,172,173]. The effects of coffee on the gastrointestinal system have recently been reviewed, and it is clear that coffee modulates the composition of intestinal microbiota and microbial metabolites [55]. Moreover, in mouse models, caffeine-induced sleep restriction affects the composition of the gut microbiome and fecal metabolites [174]. For example, the nondigestible polysaccharides in coffee are rapidly metabolized to short-chain fatty acids in the gut, and this results in increased levels of Bacteroides/Prevotella species [175,176]. Another study also reported higher levels of Bacteroides-Prevotella-Porphyromonas in high consumers of coffee [177], and this was also accompanied by lower levels of lipoperoxidation and the increased microbial production of short-chain fatty acids that are chemoprotective in the gut [178]. It was also observed in a human dietary study that a high fiber/coffee (no red meat) diet improved insulin sensitivity in type 2 diabetes and decreased the expression of the pro-inflammatory marker interleukin-18 [179]. It was reported that Prevotella induced IL-18 and other inflammatory cytokines in the colon of SPF mice, whereas inflammation (DSS), induced colonic inflammation, and cytokine production were inhibited by Prevotella, and this was consistent with the overall beneficial effects of Prevotella [180], which is enhanced in coffee drinkers. The results of several other studies indicate that coffee induces bacterial species and their metabolites that are known to have beneficial effects [181,182,183]. However, due to the variability in the diet-induced changes of the intestinal microbes and their metabolites [172,184], the role of these coffee-induced effects on changes in microbial populations and microbial metabolites is not yet fully understood.