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THE ROLE OF GUT MICROBES IN THE PROTECTIVE EFFECTS OF POLYPHENOLS AND VITAMIN E FORMS AGAINST COLON INFLAMMATION
Ulcerative colitis is a chronic disease that affects more than 770,000 U.S individuals and the number will increase to 1 million by 2025, resulting in $7 billion cost to manage the disease. Ulcerative colitis is characterized by inflammation along the colon and is a risk factor for the deadly colitis-associated colon cancer (CAC). Emerging research shows that gut microbes, the microorganisms living in our intestine, regulate colon inflammation. Specifically, an imbalanced microbial community may promote the growth of pathogens that invade the host to cause or exacerbate colitis. Therefore, researchers have been searching for safe and cost-effective approaches to keep gut microbes balanced in a long run and thus to control colitis. To this end, my research investigates the microbial modulatory capacities of dietary phytochemicals including polyphenols and vitamin E forms, delineates the role of microbial interaction in their protective effects against colon inflammation and further utilizes such interactions to develop anti-colitis therapies. To address the research questions, I have performed three independent projects and discussed them separately in chapters 2-4.
The first project (chapter 2) focused on the anti-CAC and anti-colitis effects of grape polyphenols supplemented through a whole grape powder. Polyphenols are natural chemicals found in plants and have been shown to alleviate colon inflammation in both clinical and animal studies, but the underlying mechanisms are not completely understood. In particular, the role of microbial modulation in polyphenol-mediated benefits is not fully established. Here we hypothesized that, polyphenols may attenuate colon inflammation via interacting with gut microbes. Through two animal studies, we found that 10% grape powder (10GP) diet, which contains 0.033% polyphenols, attenuated colitis-associated tumorigenesis, and prevented disease-induced microbial dysbiosis. Moreover, 10GP diet only mitigated colitis in conventional animals, but not antibiotic-treated, gut microbe-depleted animals. Collectively, these two studies demonstrated that the interaction with gut microbes played a causative role in the protective effects of 10GP against colon inflammation.
Like polyphenols, vitamin E forms are also phytochemicals with phenolic structures and undergo liver metabolism followed by biliary excretion to the gut. In the second project, we investigated the anti-colitis effects of vitamin E-based synbiotics therapies. Previously, we found that d-tocotrienol 13-carboxychromanol (dTE-13’), a metabolite of the natural vitamin E form dTE, inhibited colitis-associated tumorigenesis in mice, modulated their gut microbiota and increased the relative abundance of a lactic acid bacterium, which is commonly used in food industry. Interestingly, a subspecies of this bacterium, named Lactococcus. lactis subsp. cremoris (L. cremoris), has been reported to attenuate ulcerative colitis in mice. Therefore, we reasoned that combining dTE-13’with L. cremoris may offer synergistic protection against ulcerative colitis by modulating gut microbes. Through two animal studies coupled with anaerobic cell culture, we found that combining L. cremoris with dTE-13’, not the parental dTE, showed superior anti-colitis effects, rendered gut microbes resistant to disease-associated dysbiosis and facilitated the microbial reduction of a double bond on dTE-13’ into dTE-13’ (2DB). Overall, these data suggested that dTE-13’ interacted with L. cremoris to benefit the host.
To further corroborate the microbial metabolism of vitamin E forms under in vivo settings, we launched the third project (chapter 4) where we compared the metabolites formation of dTE and dTE-13’ between antibiotic-treated mice that had reduced gut bacterial load and conventional ones. We found that in dTE-gavaged animals, antibiotics treatment decreased the fecal amounts of dTE and its metabolites by 61% and 98%, respectively, while increased dTE level in the adipose tissue. Similarly, in animals gavaged with dTE-13’, antibiotics treatment led to a 98% reduction in its downstream metabolites. More importantly, antibiotics treatment reduced the ratio of the parental dTE and dTE-13’ to their metabolites in feces, especially the reduction from dTE-13’ (3DB) to dTE-13’ (2DB), suggesting the active role of gut microbes in the metabolism of dTE and dTE-13’. This observation is consistent with the results from the anaerobic study performed in the second project.
In summary, we showed that grape polyphenols and vitamin E form-based synbiotics offered strong protection against colon inflammation and their interaction with gut microbes likely contributed to the observed benefits. In the study of grape polyphenols, we proved the causal role of gut microbes in polyphenol-mediated alleviation of colitis. In the subsequent study of vitamin E forms, we presented evidence that the superiority of the synbiotics might be rooted in the enhanced microbial metabolism of vitamin E forms. Together, these results supported the central role of gut microbes in the management of colitis and proposed two different classes of dietary phytochemicals that can manipulate gut microbes to benefit the host. Natural bioactive compounds like polyphenols and vitamin E forms are ideal candidates for long-term preventive measures as they have less side effects and are more cost-effective compared to drugs. Moreover, by understanding the targeting microbes of different phytochemical compounds, hopefully we will be able to customize phytochemical supplementation based on individual microbial profile and dietary habits. For instance, we may optimize the dosage and type used based on the microbes present in the gut, or add in probiotics to design more effective synbiotics just like the combination of dTE-13’ and L.cremoris.
California Table Grape Commission
Purdue Center for Cancer Research
- Doctor of Philosophy
- Nutrition Science
- West Lafayette