Gut bacteria and cancer

Abstract

Microbiota on the mucosal surfaces of the gastrointestinal (GI) tract greatly outnumber the cells in the human body. Effects of antibiotics indicate that GI tract bacteria may be determining the fate of distal cancers. Recent data implicate dysregulated host responses to enteric bacteria leading to cancers in extra-intestinal sites. Together these findings point to novel anti-cancer strategies aimed at promoting GI tract homeostasis.

1 Introduction

Breast cancer is the most frequently diagnosed cancer in women [1–3]. It has been known for some time that antibiotics and anti-inflammatory drug therapy, such as aspirin, alter relative risks for breast cancer in women [4–6]. However, the etiopathogenic factors leading to breast malignancy are not well understood [5, 7, 8]. During studies of gastrointestinal (GI) tract inflammation, it was discovered that certain gut commensal bacteria trigger not only colonic tumors but also mammary and prostate gland tumors in susceptible mouse models [9, 10]. More recently, human milk-borne microbes were found to inhibit mammary neoplasms in predisposed mice [11], with effects transcending several generations [12]. This raises the intriguing possibility that our microbial passengers may unveil novel targets for cancer prevention and therapy.

1.1 Cancer development is a multi-factorial process

Cancers of tissues including the colon and breast are attributable to complex interactions between cells surviving genetic damage and their micro- and macro-environments [13–16]. This continuous interplay eventually leads to the formation of indestructible cancer cell clones through a natural selection process [14]. Immune and stromal cells, cytokines, proteases and hormones are now acknowledged as major environmental contributors in the natural history of cellular malignant transformations [13, 14]. Consequently, the immune system status [17], the metabolic profile [18] and the psychological condition of the host [19], which influence each other at the whole organism level, could be viewed as external determinants of the perturbed ecosystem of abnormal cells with neoplastic potential.

Studies in animal models of cancer increased the understanding of the multistep evolution of dysplasia and pre-neoplasia to cancer [11, 20–22]. Several of these studies have also highlighted the fact that early neoplastic lesions are less autonomous in their growth than previously thought [11, 22–26]. Instead, their thriving and evolution depends on their micro- and macro-environment[13–15, 17, 27, 28]. This finding raises interesting possibilities for cancer prevention. Indeed, accidental gene mutations occurring during the lifetime of a human being are countless [15, 29]. Therefore, most people develop focal dysplastic and pre-neoplastic lesions during their lifetimes. These lesions rarely develop into cancer. However, co-existing local or systemic smoldering inflammatory disturbances of homeostasis have a trophic effect on them, promote their development, and greatly increase the chances of carcinogenesis [15, 17, 23].

Taken together these recent conceptual advances in tumor biology suggest that immune system elements, hormones and psychosomatic factors may determine the fate of pre-neoplastic lesions towards progression to cancer through interrelated mechanisms. This raises an important question whether effective modalities that could contribute towards shaping an overall systemic homeostatic, non-tumor promoting status may exist.

Recent findings using mouse models suggest this may be possible. It appears that supplementation with certain gastrointestinal bacteria initiates multifaceted systemic events that overlap with basic pro-carcinogenic signaling [12, 27, 28, 30–34]. In this case, bacteria apparently suppress the evolution of early neoplastic lesions to cancer in epithelia locating distally from the gut, such as those of mammary gland, by down-regulating the systemic inflammatory index in the form of pro-inflammatory cytokine levels and inflammatory cells [11]. Beneficial Firmicutes bacteria including Lactobacillus spp are likewise able in mice to interrupt metabolic disorders such as obesity and induce a reproductive fitness-matching hormonal milieu with youthful testosterone, free thyroxin (T4), and oxytocin levels [35–37]. Interestingly, these same hormones have been connected with healthful mentality and anxiolytic effects [38–40]. As pivotal elements of the gut microbiota-brain axis, certain gastrointestinal tract bacteria are being introduced as psychobiotics due to their potential to counteract depression and promote a sense of well-being [41].

2 Our gut reactions: a balancing act that shapes systemic immune tone

The GI tract encompasses the largest surface of the human body where microbial products interact with the immune system. It has become clear that balance of systemic health is routinely enforced by activities of CD4+ T regulatory (T_REG) cells along mucosal surfaces. These lymphocytes have evolved to play a sophisticated balancing act of allowing host protective immune responses during acute inflammatory responses, while later regaining suppressive roles that limit deleterious pathological sequellae of chronic smoldering inflammation [42–44]. Recent evidence highlights the important developmental and functional associations of intestinal microbiota with T_REG cells [45–48]. Both in vitro data and lymphocyte titration experiments in preclinical models have revealed that homeostatic potency of T_REG [ie., ability of T_REG to restore homeostasis after environmental insults] is modulated by prior intestinal bacterial challenges [9, 23, 49–55]. These studies on T_REG cells complement other data showing an array of different effects of gut bacteria on systemic innate and adaptive immunity [56] thus solidifying a pivotal role for gut microbiota in shaping systemic immune tone and responses [Figure 1].

Figure 1. A proposed model of gut microbe-induced extra-intestinal carcinogenesis.

Humans infected with pathogenic gut bacteria are at increased risk for inflammation and cancer. The compromised intestinal epithelial barrier during pathogenic infection leads to translocation of bacteria thereby triggering systemic activation of immune cells, culminating in an elevated septic and systemic inflammatory index and to cancer in distant sites such as breast tissue. Immunocompetent hosts have efficient T regulatory (Treg) cell responses in response to microbial challenges that help restore gut epithelial homeostasis.

Disruptive events in the GI tract also increase risk for microbial translocations [57] together with systemic immune cell trafficking. Microbial translocation from gut to mammary tissue has been postulated in breast cancer etiopathogenesis [ 74]. The subsequent increase of the systemic inflammatory index would be expected to increase likelihood of cancer in distal tissues. However, the bacterial translocation due to the compromised intestinal integrity caused by cancer treatment therapies has been shown to augment anti-tumor immunity networks of activated myeloid cells and T-lymphocytes. This beneficial effect was lost in germ-free mice or animals treated with antibiotics. Therefore, in this setting, bacterial translocation worked synergistically to certain cancer therapeutic regimens [32, 33, 56, 58]. This discrepancy is not surprising. The divergent effects of translocating bacteria-induced systemic immune responses on neoplastic disease outcomes reflect the multifaceted relationship of inflammation with cancer.

Taken together, there is abundant evidence that bacteria modulate cancer development and growth. Finding that bowel bacteria or their products promote a competent, healthy immune system provides an explanation for perplexing increases in cancers arising from epithelia in colon, breast and other sites in countries with more stringent hygiene practices [23, 59]. Along similar lines, chronic antibiotic therapy may disrupt constructive bacterial processes, ultimately leading to higher rates of breast cancer in women [4]. Systemic NSAID therapy has been linked with significant decreases in several types of cancer [60–63]. Thus, ways in which gut microbiota stimulate inherent host homeostatic properties are an attractive target for systemic good health approaches using probiotic bacteria or microbial product vaccines.

3 Cancer and Inflammation: Interleukin-6 and neutrophils

In the context of cancer, inflammation is widely believed to represent the body’s fight against tumor cells [64, 65]. Other data, however, suggests just the opposite; chronic smoldering inflammation may be a cause of cancer and is a powerful stimulus for tumor growth and invasion [66–68]. These opposing observations are not easily reconciled, and are most easily comprehended in the context of immune balance, with cancer arising during dysregulated attempts by the host animal to restore homeostasis after an insult.

Several lines of evidence support that pathogenic GI tract microbiota stimulate certain innate immune cells to enhance tumor formation throughout the body [9, 22, 69–73] [9, 74]. In order to identify the key immune cell players, prior studies have built upon reciprocal systemic relationships existing between neutrophils, mast cells and macrophages with cells of adaptive immunity [70, 75–81]. During homeostasis, these immune networks are persistently down-regulated by anti-inflammatory activities of CD4+ T_REG that bestow intestinal homeostasis [9, 52, 69]. In this context, a weakened T_REG-mediated inhibitory loop imparts carcinogenic consequences of elevated IL-6 and possibly IL-17, leading to more frequent inflammation-associated distal cancers [82]. In this context, neutrophils have been identified in animal models as an important factor in cancer initiation and development [24–26, 70] [83] [84–88]. A distant neoplastic effect of a commensal gut microbe was recently shown in FVB-Tg(C3-1-TAg)cJeg/JegJ mammary tissue by a neutrophil-mediated mechanism [75]. Importantly, systemic interplay between microbes, IL-6, and neutrophils was recently shown in human patients with breast cancer [77].

To the same extent that pathogenic gut bacteria can lead to carcinogenic events in distant tissues, it appears that beneficial bacteria may inhibit or even suppress carcinogenesis. A prototype beneficial microbe Lactobacillus reuteri was recently shown to rescue mice from age-associated obesity and the deleterious IL-6 and IL-17-rich smoldering systemic inflammatory obese status [10]. Obesity has been linked with postmenopausal mammary cancer [89–91]; thus, it was subsequently examined and shown that beneficial Lactobacillus sp microbes inhibit obesity-associated mammary carcinogenesis in mice [75]. Interestingly, the same anti-neoplastic effect occurred in the Her-2/Neu mouse, a genetically engineered mouse model of mammary cancer, in which the association between immunity and mammary carcinogenesis is less obvious. This animal model is transgenic for ErbB2 Epidermal Growth Factor [EGF] receptor and over-produces the protein HER-2; a condition that occurs in up to 30% of breast cancer patients and carries a poor prognosis [20].

4 It starts earlier in life

Recently, much attention has been focused on the possibility that modulating intestinal flora early in life may provide life-long protection against cancer. Prior work [9, 51, 52, 74, 92] supports a model whereby prior enteric infections serve to suppress gut inflammation, consistent with the observations of Belkaid and Rouse (2005) involving immune competency and T_REG cells [50]. More recent evidence highlights the importance of intestinal microbiota during maturation of the immune system [45–48]. Data from preclinical models has substantiated this notion that host ability to restore homeostasis after environmental insults is modulated by prior intestinal bacterial exposures [9, 23, 49–55]. This makes sense involving a paradigm proposed by Kuchroo and co-workers [93, 94] showing that ability of T_REG to enforce homeostasis and inhibit immune-mediated diseases depending upon levels of inflammation, and IL-6 in particular. In this paradigm, elevated levels of IL-6 trigger a T helper type (Th)-17 host response that contributes to worsening systemic disease conditions. Taken together, these observations link the immune system, gastrointestinal infections, and seemingly divergent downstream phenotypes: allergies, autoimmune disease, and cancer. These observations suggest that T_REG may do more than block constructive anti-cancer responses [64, 65]. Following this reasoning, a model in which childhood infections protect from inflammatory diseases later in life, T_REG cell biology may help explain unanswered questions of cancer risk and modern lifestyle.

During early life in mammals, the immune system programming process is chaperoned by milk-borne lactic acid bacteria that apparently serve to protect nursing infants by stimulating anti-inflammatory cytokine IL-10 and T_REG cells along naïve mucosal surfaces. Modern lifestyle practices that remove natural exposures due to Caesarian births and bottle-feeding may compromise host ability to navigate future mucosal challenges. It is promising that administration of similar beneficial bacteria later in life may serve to prevent enteric inflammation [95, 96] as well as a wide variety of inflammation-associated systemic disorders [97]. For example, as a consequence of eating L. reuteri, aged mice displayed superb integumentary health with increased wound healing capacity, and resisted age-associated thyroid and testicular involution [10, 98]. Dosing mice orally with L. reuteri isolated from human milk was proven sufficient to down-regulate systemic inflammatory cytokines and neutrophil accumulations [37, 98], in addition to lowered risk for mammary cancer [78], in mouse models.

According to recent studies, diet and dietary microbes may have transgenerational cancer consequences involving integrated immune and hormonal factors [99]. De Assis et al have shown that descendant generations of female rats fed a high-fat diet or exposed to estrogen during pregnancy are more prone to carcinogen-induced mammary cancer. In those studies, the increased sensitivity to mammary chemical carcinogenesis was epigenetically inherited, since offspring rats had an altered mammary tissue DNA methylation pattern [99]. Interestingly, both treatments used to achieve this epigenetically-regulated effect connect with pivotal elements of one proposed gut-centric mechanism of remote cancer risk control involving gut microbiota and regulatory T-cells. High fat diets have been shown to alter gut microbial communities in both rodents [98][99] and human beings [100]. In mice, high-fat diet and the related obesity-type gut microbiota coincide with reduced numbers of T_REG cells in both mesenteric and mammary lymph nodes [78, 98]. In the offspring with maternal estrogen changes, the prenatal exposure to estrogens disrupts T-cell differentiation in the thymus and has long-term effects in its immune system [103] including reduced T_REG cells [104]. Disrupted thymic maturation of suppressive CD4+ T cells has been recently shown to be responsible for spontaneous cancer upon aging [55]. Estrogenic prenatal exposures have been associated with increased mammary cancer later in life, but the mechanism has not been fully elucidated [101] [102] [105]. It is intriguing to hypothesize that gut microbiota, hormones and dysfunctional thymus, during the perinatal period are implicated with T_REG cells and unbalanced immune responses to explain increased cancer risk in subsequent generations. Indeed, a recent study showed that detrimental effects of the obesity-type gut microbiota were neutralized by enriching the microbiota of descendent mice with beneficial lactic acid bacteria [99].

5 Conclusion

It would follow logically that microbial measures aimed at restoring immune balance and reducing systemic inflammatory index would be beneficial for counteracting cancer. Yet, the roles of gut microbes may be far more complex, depending on the stage of the neoplastic disease or the specific aspects examined. Although it remains unclear precisely how microbes achieve these beneficial effects, this work highlights the need to exploit bacteria in novel cancer prevention and treatment strategies aimed at promoting intestinal homeostasis.