| Version | Summary | Created by | Modification | Content Size | Created at | Operation |
|---|---|---|---|---|---|---|
| 1 | Heather Armstrong | + 3055 word(s) | 3055 | 2021-02-25 08:02:33 | | | |
| 2 | Rita Xu | -1595 word(s) | 1460 | 2021-03-05 05:33:30 | | |
Many studies performed to date have implicated select microbes and dietary factors in a variety of cancers, yet the complexity of both these diseases and the relationship between these factors has limited the ability to translate findings into therapies and preventative guidelines.
Although cancer development is known to be complex and related, in many cases, to a diverse array of factors (genetic and environmental), with well-described impacts of longitudinal exposures, there are still critical gaps in the knowledge of what some of these exposures are and how they impact cancer biology. Diet has long been considered a critical and, importantly, modifiable factor in many different biological processes, including cancer. Recent advances in measuring dietary intake and its effect on biological processes (e.g., using metabolomics), and especially the interaction between diet and gut microbes, have opened the way to a myriad of papers implicating diet, the gut microbiome, and related metabolites in the pathogenesis, course, and response to therapy of major cancers. A better definition of how diet is related to cancer biology is especially attractive, as diet is rarely utilized in cancer therapy or prevention and it can provide a safe alternative for intervention in a variety of other chronic conditions [1][2][3][4][5]. However, the potential for diet to affect cancer development and progression remains to be fully elucidated. We focus on diet−microbiome interactions in this setting, as not only do dietary factors and nutrition have profound effects on the health of host cells, but they also affect the human microbiome [6], which in turn is closely linked to cancer.
These observations are especially relevant to the gut: emerging evidence continues to demonstrate that nutritional states directly drive intestinal adaptation, resulting in altered signaling mechanisms within adult intestinal stem cells relevant to intestinal tumour formation [7][8][9][10]. Some correlations to chronic conditions, such as inflammatory diseases, may help inspire future research. The link between inflammation and concomitant tumour development in cancers was first suggested by Virchow in 1863 [11][12], and today roughly 20% of cancers are thought to directly result from chronic inflammation [13]. Likely one of the most well studied examples to date is the link between inflammatory bowel diseases (IBD) and colorectal cancer (CRC) [14][15][16][17]. Notably, select dietary factors including saturated fats, red meat, and refined carbohydrates have been suggested to display pro-inflammatory properties and diet has been shown to play a key role in up to 40% of all cancers [18][19][20][21]. These dietary factors are further involved in modulating the gut microbiome, which in turn is involved in regulating both gut physiology and immune response [22][23][24]. This model of IBD−CRC has further demonstrated the critical role of microbes in inflammatory pathways due to their ability to modulate inflammatory cytokines (e.g., tumour necrosis factor (TNF), interleukin (IL)-1, IL-6), which are similarly dysregulated in both IBD and CRC [25][26][27][28]. While dietary factors have clear effects on organs of the intestinal tract, absorption of these factors results in systemic effects on a number of organ systems, which have been implicated in the progression of a variety of cancers, as detailed in later sections (Figure 1).
Figure 1. Cancer promoting and preventative interactions of dietary factors and organ systems. Intake of dietary factors, as demonstrated only by more recent research studies, can affect cancer progression through positive (green) or negative (red) effects on organ systems, as indicated. Details are further highlighted in Table 1.
While diet is considered an environmental exposure, it is difficult to measure accurately in humans for several reasons. In contrast to cigarette smoking, for example, which can be expressed as pack-years, food contains thousands of molecules, is not easily quantified, varies geographically, and humans are notoriously inaccurate in reporting consumption [29]. Although it would probably be best to define dietary exposures using food patterns [30], most current research has focused on specific nutrients. A series of recent review articles highlighting the studies examining cancer in relation to specific dietary patterns labelled Mediterranean [31], Western, Ketogenic [32], or other such common dietary patterns [33], demonstrate that the nutrients or food groups that are specific to these diets appear to underscore potential mechanisms behind the positive or negative effects correlated with these diets. This section will present some examples of links between these specific nutrients and cancer (Figure 1).
Vitamins A, C, and D have been demonstrated to play an anticarcinogenic role in a range of studies [34]. A recent meta-analysis of random controlled trials concluded that Vitamin D significantly reduced mortality (13%) but did not have an impact on cancer incidence, with the exception of colorectal and ovarian cancers [35]. Furthermore, vitamin A has been shown to play a significant role in cell proliferation and differentiation [36] and studies demonstrate an inverse relationship between vitamin A and bladder, colorectal, and liver cancers [37][38][39]. These studies suggest that the effects of vitamins may occur through enhanced DNA repair, antioxidant effects, or immunomodulation of host cells [40][41]. The in-depth role of vitamins has previously been well reported in relation to cancer [42][43].
Sodium and potassium have long been thought to play a role in the development of cancer as, for example, patients with decreased potassium levels, or hypokalemia (<3.5mmol/L), associated with cell aging, obesity, alcoholism, and stress, display increased rates of cancer; in contrast patients with hyperkalemia (>5.3 mmol/L) and diseases associated with increased potassium levels, such as Parkinson and Addison disease, display reduced cancer rates [34]. The concentrations of sodium and potassium also interestingly play a role in regulating the effects of calcium, resulting in a series of varied results in studies investigating the tumorigenicity of calcium [34]. Furthermore, almost 50% of cancer patients have been shown to display hyponatremia (<135 mmol/L sodium), although this may also be secondary to the cancer itself [44].
The sodium salt of propionic acid, sodium propionate, is often used as a food preservative in bakery products and has been shown to exert an antitumour effect through the mitogen-associated protein kinase (MAPK) signalling pathway in breast cancer xenograft models. [45] This example illustrates the complexity between food preservatives, microbes, and cancer, as most are observed to be negatively associated. Nitrates have also been well-described to be associated with colorectal cancer [46], while sorbate and benzoate correlate with the occurrence of breast cancer [47]. Processed foods and the use of food preservatives have dramatically increased over the last few decades and many have attempted to link this with increases in cancer [48]. Food preservatives are a diverse group of ingredients and therefore we have decided to only highlight a few in this review. Most studies focus on negative associations between preservatives and cancer, but to balance the literature, we choose to describe here some studies that display protective effects.
Capsaicin has long been thought to display anti-inflammatory, antioxidant, anti-proliferative, metabolic, and cardioprotective effects and, interestingly, has been shown to affect microbes within the gut too [49]. Decursin, an extract from the Angelica gigas root, has been shown to display potent anticancer activity in cell line models of lung and colon cancer, along with Lewis lung carcinoma allograft mouse models of tumour growth [50]. A recent study has shown the capacity of decursin to promote HIF-1α degradation within the proteasome, therefore improving T cell activation and antitumour effects within the tumour microenvironment [50].
A subset of dietary fibres, collectively known as β-glucans, are found in a variety of food groups, from mushrooms and other fungi, to wheat, oats, and barley. Interestingly, raw and roasted barley rich in β-glucan has been shown to provide chemoprotective effects via inhibition of growth and promotion of apoptotic pathways [51]. Similar studies examining the effects of raw and roasted oat flakes, following fermentation with human fecal microbes, demonstrated that the fermentation supernatants, with reduce pH and increased butyrate, significantly decreased growth and increased apoptosis of colon adenoma cells [52]. These findings suggest that food products, mostly made of specific grain β-glucans, harbour chemoprotective potential.
The flower buds of adaptogenic plants, typically found in Chinese traditional medicines, including Gardenia jasminoides, Sophorae japonica, and Lonicerae japonicae, demonstrated a significant effect on reducing polyp burden, along with lowering expression of oncogenic signaling molecules in mice [53]. These changes were associated with a reduction in pathobiont microbes including Helicobacter pylori, along with an increase in beneficial microbes, including the key short chain fatty acid (SCFA) producers, Akkermansia, Barnesiella, Coprococcus, Lachnoclostridium, and Ruminococcus [53]. A diet high in seaweed has been linked with promotion of Bacteroides plebeius, which is involved in the breakdown of the seaweed Sargassum wightii to polysaccharides (SWP1 and SWP2); this significantly reduces cell proliferation and induces apoptosis in human breast cancer cell lines [54][55]. Seaweeds’ health benefits have been associated with the polyphenols, polysaccharides, sterols, and bioactive molecules that have been shown to play a role in anti-inflammatory and anticancer effects [56].