Obesity is a chronic metabolic disease that requires the development of effective therapeutic strategies to be easily implemented worldwide. Plant-based food interventions are promising therapeutic approaches for the management of obesity and associated comorbidities such as fatty liver. In this sense, plant microRNAs (miRNAs) have emerged as key bioactive molecules in cross-kingdom communication and could be involved in the beneficial effects of plants on human health. The present work aimed to evaluate the effect of plant miRNAs on the modulation of physiological features altered during obesity, particularly on inflammation and metabolism, with a special focus on the evaluation of gene expression changes. This research was conducted using in vitro cell culture models of macrophages, hepatocytes, and adipocytes, and human faecal and serum samples from volunteers subjected to an acute intake of plant-based foods.
In Chapter 1, we identified by Next Generation Sequencing (NGS) a myriad of miRNAs in plant food products. It was found that miR156e, miR159 and miR16262, selected to estimate the general bioavailability of plant miRNAs, resisted degradation during cooking processes. Moreover, we demonstrated that an acute consumption of plant products increased the amount of these miRNAs in the human gut, although they were not detected in serum.
In Chapter 2, we demonstrated that plant miR482f and miR482c-5p downregulated the expression of inflammation-related human genes (CLEC7A and NFAM1, and TLR6 respectively), some of them had been previously found to be dysregulated during obesity (CLEC7A and TLR6), in THP-1 monocytes differentiated into macrophages. In addition, these plant miRNAs had an impact on the gene expression profile of anti- and pro-inflammatory cytokines (i.e., IL10, TNF, and IL1B), and preliminary results suggested that they could enhance IL-10 protein secretion levels. In addition, miR482f and miR482c-5p were found to be present in edible plants, resist degradation during cooking processes, and were bioavailable in the human gut, but not serum.
In Chapters 3 and 4, we unveiled that plant miR8126-3p, miR8126-5p and miR6262 inhibited the expression of metabolic-related human genes, which have been associated with obesity pathophysiology (QKI, MAPKAPK2, and RXRA, respectively) and had an impact of the expression profile of key metabolic genes (i.e., PPARA and SREBF1), in an in vitro model of lipid accumulation (HepG2 hepatocytes treated with free fatty acids). In addition, plant miR8126 isoforms attenuated lipid accumulation increase in hepatocytes. Plant miR6262 also downregulated the expression of the predicted target RXRA in hMADS adipocytes differentiated into brown-like adipocytes and modulated the expression of genes related to metabolism and thermogenesis (CIDEA, CPT1M, and PLIN1). However, we did not find a biological function for plant miR6262 neither in hepatocytes nor adipocytes.
Overall, the results of the present thesis show that plant miRNAs are cross-kingdom gene expression regulators with the capacity to modulate the expression of mammalian (human) genes dysregulated during obesity and improve the inflammatory and metabolic gene expression profile. Moreover, this work shows that the bioavailability of plant miRNAs in humans could be achieved through acute plant intake, but eventually restricted to the gut levels, suggesting that other strategies should be developed to promote their absorption and potentially reach peripheral tissues and organs. This work lays the foundations for further studies exploring the potential role of miR482f, miR482c-5p, miR8126 (-3p and -5p), and miR6262 as an underlying mechanism by which plant foods influence obesity outcome in humans, and thus, their therapeutic potential to restore metabolic and inflammatory homeostasis in obesity.
