Human Gut Microbiome in Heart Failure: Trying to Unmask an Emerging Organ

1. Introduction

Heart failure (HF) is a severe and harmful syndrome and although many diagnostic and therapeutic efforts have been made, an effective, holistic management has not yet been reached. The reported data suggest a HF prevalence ranging from 1% to 2% in adults [1,2,3], whereas the HF incidence seems to be higher and increases with age exceeding 10% for those > 70 years old [3,4]. Importantly, the mortality rate is high [5,6] and is expected to further increase due to the population rise, ageing, senescence, coexisting morbidities, and probably the lack of an holistic prevention and management [7]. Thus, although HF is common, morbidity and mortality rates remain high [8]. Of note, despite the fact that major determinants of syndrome severity, namely prolonged activation of neurohormonal systems, inflammation, and free radical production have been recognized and relevant treatment has been implemented, there are several important issues to be resolved. Indeed, when referring to the HF process, diverse additional factors adversely affecting body homeostasis should be considered [9,10]. It has long been recognized [11,12] that HF by inducing gut ischemia and congestion may alter the gut microbiota (the community of gut micro-organisms themselves) and intestinal permeability stimulating immune and inflammatory processes [13,14] leading to a further deterioration of cardiac function [15,16]. Moreover, as gut microbiota regulates energetic function of several organs, including the heart, its derangement may be associated with multiorgan dysfunction [17,18,19].

2. Bidirectional Relationship between the Heart and the Gut

It is well known that neurohormonal activation in HF (activation of the sympa-thetic nervous system [SNS] and renin-angiotensin-aldosterone system [RAAS]) is initiated as a compensatory response to hemodynamic instability (decreased cardiac output and increased filling pressures) but eventually leads to multiorgan hypoperfusion and dysfunction (liver, kidneys, gut etc.). In this regard, gut ischemia and edema promote gut barrier dysfunction accompanied by an intestinal leak of microbes and/or their products, facilitating their entrance into the circulation and causing a low-grade inflammation together with a relevant immune response (Figure 1).

Conversely, the derangement of microbiota and development of one with abnormal endocrine and homeostatic regulatory characteristics [20] facilitates insulin resistance, obesity, metabolic syndrome etc. [21] suggesting that gut microorganism composition is strongly associated with cardiovascular diseases [16,17,22,23]. Changes in gut microbiota and its products share various pathways with those of HF, activating directly or indirectly the immune, neurohumoral, and inflammatory processes [24,25,26] and , therefore, contributing to left ventricular (LV) remodeling and myocardial fibrosis [15,27,28]. On the other hand, some of the HF microbiome (the collective genomes of the gut micro-organisms) patterns are similar to those observed in diverse clinical settings such as cardiometabolic disease [27] and inflammatory bowel disease, and several other chronic diseases [13,28]. Thus, considering the emerging role of human gut microbiota and its bidirectional association with the heart, the exploration and better understanding of this emerging “organ” is mandatory [29,30,31,32,33].

3. Understanding Gut Microbiota

There are 10 times the more microbial cells in the human gut than in the whole body, totaling approximately 100 trillion micro-organisms, which represent as many as 5000 different species, and weigh roughly 2 kg [34]. Gut microbiota composition includes bacteria, viruses, fungi, and parasites with the main species of bacteria being Prevotella, Ruminococcus, Bacteroidetes, and Firmicutes. In the typical human adult, Firmicutes are the most abundant, followed by Bacteroidetes and Actinobacteria [6] and the ratio between the bacterial species Bacteroidetes and Firmicutes seem to play an important role in health and disease. Bacteria within the gut microbiome are involved in harvesting energy from food, balancing the beneficial and opportunistic bacterial composition, and manufacturing neurotransmitters, such as serotonin, enzymes, and vitamins [35].

At the time of birth, the intestinal tract is rather sterile, but becomes rapidly colonized by trillions of non-pathogenic organisms affected mainly by environmental factors. The composition of this new ‘organ’ is not uniform and is personalized and, therefore, differs from individual to individual, depending on the host genetic variation, diet, lifestyle, xenobiotics, and medications [36,37,38,39]. In fact, in different individuals being under the same dietary regimen, blood glucose levels vary depending on microbiota composition [40]. It seems that microbiota is altered by dietary habits [41] but it is unknown whether dietary interventions can modify cardiovascular risk by affecting microbiota composition [28]. Nevertheless, the production of short-chain fatty acids from gut microbiota improves intestinal barrier function, modulates blood pressure, inhibits inflammation, and contributes to the regulation of the epigenome balance and immunity response as well. Acetate-producing bacteria seem to play a pivotal role in cardiac hypertrophy and fibrosis [41]. Interestingly, it seems that gut composition contributes to the body mass index [42] and to the development of autoimmune disease in humans [43]. The bile acid pool, which is altered in patients with HF, closely interacts with gut microbiota [44] and exhibits a negative chronotropic effect on myocardium [45,46], which has been attributed to the inverse relation of the bile acid to the β-adrenoreceptor activity and affinity [47]. Indeed, it has been demonstrated that the bile acid receptor «orphan nuclear receptor-FXR» through nuclear factor –kB, [48,49] promotes cardiac hypertrophy [50,51], facilitates apoptosis through mitochondrial signaling [52], and modulates metabolism and inflammation [53].

Lifestyle habits affect gut microbiota. Exercise reduces cytokine levels [54], sleep disorders affect microbiota community, and stress alters intestinal permeability [55]. Working conditions, sexual habits and physical interactionσ also affect microbiota composition [56,57,58]. Since gut microbiota is influenced by several factors, the gut composition is characterized both by stability and dynamic variation which is an obstacle for its use as a biomarker [59,60,61,62]. Unfortunately, the main body of relevant evidence originates from animal models rendering extrapolations to humans of doubtful validity [28,63]. Whether longitudinal control of microbiota variations can give us information regarding HF and its management remains to be elucidated. Despite these limitations, there is evidence suggesting that a diet rich in fatty acids (e.g. choline, L-Carnitine) can be metabolized and converted by gut microbiota and hepatic enzymes to trimethylamine N-oxide (TMAO), a product associated with atherogenesis, platelet dysfunction, thrombotic events, coronary artery disease, stroke, HF and ultimately death [64,65,66,67,68,69,70]. In patients with chronic HF the increase in the levels of TMAO is associated with ventricular dysfunction and decreased survival [71,72,73,74]. A study that included 1,208 patients with chronic HF after myocardial infarction reported that the major adverse cardiovascular event (MACE) risk increased with an elevation in TMAO levels, and this positive correlation became more significant when TMAO levels were higher than the median [75]. In the same study TMAO was also found to be an independent predictor of all-cause mortality after adjusting for traditional risk factors. The same seems to be true in acute HF, although for the time being there are reports only originating from experimental models [76,77]. Finally, a meta-analysis of 12 studies involving 13,425 participants reported that compared to low-level TMAO, elevated TMAO was correlated with MACEs and all-cause mortality in HF and that consistent results were obtained in all examined subgroups as well as in the sensitivity analysis [78].

In patients with HF several comorbidities are also present, including kidney dysfunction, iron deficiency and anemia, diabetes, electrolyte disorders, and obesity that may affect microbiota composition. Concerning kidney dysfunction, a relation between kidney function, cardiovascular disease and gut microbiota has been suggested [79,80,81]. Iron deficiency and anemia, which are associated with high cardiovascular and all-cause mortality [82,83], are to some extent controlled by gut microbiota [84,85,86,87,88]. The increased morbidity and mortality observed in diabetes, insulin resistance, and obesity is partially dependent on gut microbiota status. Of note, obesity, sedentary lifestyle, genetic susceptibility [89] and ultimately gut microbiota dysbiosis (“imbalance” in the gut micro-organism community that is associated with disease) seem to contribute to the development of T2DM [90,91] . A study, which compared 291 non-diabetic Danish individuals with 75 individuals with T2DM, showed that the increased levels of branched-chain amino acids in diabetic individuals correlated with a gut microbiota, suggesting that microbiota configurations contribute to the development of insulin resistance and pointed out that targeting these microbial clusters may have the potential to diminish insulin resistance and reduce the incidence of common metabolic and cardiovascular disorders[92].

4. Gut Microbiota as a Diagnostic Marker

According to the National Institute of Health (NIH), a biomarker is defined as “a characteristic that is objectively measured and evaluated as an indicator of normal biologic processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention” [93]. Alterations of gut flora are linked to several human diseases, including gastrointestinal disorders [94], ischemic stroke [95], allergies [96,97] , inflammation [98,99,100], cancer [101,102,103,104] and cardiovascular disease [105,106,107]. For example, gut microbiota derangement is linked to ST-elevation myocardial infarction [108] and can be used in the setting of a relevant prediction model [109]. Thus, it is reasonable to search for gut microbiota alterations per se or its products as diagnostic disease biomarkers [110,111,112].

As previously mentioned, there is a relationship between gut microbiota, neuro-hormonal activity, inflammation, and free oxygen production, the steadfast underpinnings of HF [30,31,33]. It is reasonable, therefore, based on this relationship to test microbiota-based biomarkers in the diagnosis and management of HF.

TMAO is the most studied microbiota biomarker, showing a correlation with HF functional class [71,72,73,113] and along with B-type natriuretic peptide with mortality either in chronic [113] or acute HF [76]. Interestingly, the correlation between TMAO and mortality remains even after adjustment for the natriuretic peptide levels [71,114]. Additionally, TMAO can be used as an index of mortality/hospitalization risk in HF patients with preserved ejection fraction presenting with restrictive physiology pattern [71,76,115,116,117]. Finally, it seems that TMAO can be used as an index of cardiac fibrosis and contractility, platelet reactivity and endothelial function [13].

Short-chain fatty acids augment mitochondrial DNA protection and regulate ATP concentration controlling, thus, the energetic needs of several organs, including the heart [17,116,118,119,120]. They are inversely correlated with outcome in HF patients with reduced ejection fraction [30] and can be used as markers of cardiac fibrosis and hypertrophy [41,121,122], vascular tone [41,121,122], gut barrier function [123], and insulin sensitivity [124]. Lipopolysaccharides ( endotoxins), which are elevated in decompensated HF [125], play a crucial role in gut barrier function, inflammation, cardiac contractility, insulin resistance, and endothelial function.

Phenylacetyl glutamine (PAGln) along with phenylacetylglycine (PAGly) are gut microbiota metabolites, which act through G-protein coupled receptors, and are involved in platelet function and thrombosis leading, therefore, to cardiovascular disease [29,126]. Their presence in blood samples is related to increased reactive oxygen production and apoptosis, decreased cell viability and myocardial contraction, and high rates of thrombotic events [126,127,128]. The increased free radical production activates the enzyme calmodulin kinase II (CaMKII) and the ryanodine receptor 2 (RyR2) inducing a proarrhythmic status characterized by cardiomyocyte apoptosis and electrical remodeling [129,130]. Indeed, a recently conducted study demonstrated that the plasma PAGln levels are significantly elevated in atrial fibrillation, suggesting that PAGln may be a promising therapeutic target in this clinical setting [128].

Based on the above it is tempting to suggest the use of gut microbiota or their metabolites either in feces or in blood samples as biomarkers of cardiovascular involvement. However, there are several limitations, mainly because the normal microbiota has not been adequately defined [33]. Additionally, both gut microbiota composition and its products as well as HF are influenced by age [131]. Moreover, there is a database limitation for studying the human gut microbiome [132] and the coupling of taxonomy and function in microbiome in not well defined. It is hoped that these discrepancies could be resolved [133] by using artificial intelligence, 16S rRNA gene sequencing or even whole metagenome shotgun sequencing is not known [134,135].

5. Gut Microbiota and Medications

There is a bidirectional relationship between gut microbiota and drugs since microbiota can be altered by drug action and conversely the microbiota can modify the pharmacokinetic properties of the drugs. Resistance to aspirin [136] along with other platelet aggregation inhibitors due to microbiota action has been documented [137]. Further, most of the drugs used in HF, including β-blockers, angiotensin receptor blockers, angiotensin converting enzyme inhibitors, calcium channel blockers, statins, and the more recently introduced SGLT2 inhibitors [138], can alter gut microbiota composition [137,139,140], which in turn may modify drug action and ultimately affect HF management. Although there is a well-documented bidirectional relationship between gut microbiota and medications, the exact mechanisms underlying this interaction have not been delineated. However, there is some evidence to suggest that lifestyle modifications including exercise and Mediterranean diet along with the use of pre- or probiotics might beneficially alter the gut microbiota environment [113,141,142,143]. Current evidence, however, is insufficient and new paths of research are required to explore new approaches for treatment optimization. Sequencing gut microbial genome could be an option, but it is still under investigation [108,116,144,145]. In the mean-time antibiotics, bile – acid sequestrants, non-lethal microbial inhibitors, fecal microbiota transplantation, etc. [32,146] might be used along with the necessary lifestyle changes.

Figure 2. Common pathophysiological pathways between gut microbiota and the heart in heart failure. Mediterranean diet, exercise and possibly the use of probiotics may attenuate these dangerous interactions.

Figure 2. Common pathophysiological pathways between gut microbiota and the heart in heart failure. Mediterranean diet, exercise and possibly the use of probiotics may attenuate these dangerous interactions.

6. Gut Microbiota, Aging, Diet, Exercise Training and Supplements

Aging, an inevitable evolution in all species, is characterized by the progressive functional deterioration of multiple organs leading to dysfunctional tissues, with the cardiovascular system being no exception. Several studies, which have been performed in order to find an approach to extend life span, suggest that life duration depends on the type of diet, exercise, working environment, and pharmacological interventions [147,148]. It is well known that adherence to the Mediterranean diet provides a positive trajectory toward a healthy successful aging, with major potential benefits toward mental and cognitive health [149]. A study that included 153 subjects following the Mediterranean diet reported an increase in the level of fecal short-chain fatty acids, indicating a close relationship between this type of diet and a beneficial gut microbiota profile [150]. The effect of diet on microbiota and health was also demonstrated in another study that included 178 elderly subjects (> 65 years), which reported that the fecal microbiota composition significantly correlated with measures of frailty, comorbidity, nutritional status, markers of inflammation and with metabolites in fecal water [151]. In the same study the individual microbiota of people in long-stay care was significantly less diverse than that of community dwellers and loss of community-associated microbiota correlated with increased frailty. Finally, an experimental study in mice showed that high-fat, high-sugar diet promoted metabolic disease by depleting Th17-inducing microbes, and recovery of commensal Th17 cells restored protection. [152]. Thus, a diet with moderate protein consumption, low glycemic index, and abundance of foods rich in fibers and polyphenols, may promote normal gut symbiosis and hence healthy aging.

Along with healthy diet, several studies have suggested the beneficial effect of exercise on the intestinal flora [153]. Indeed, it has been shown that gut microbiota affect the exercise capacity both of trained and not trained individuals being, a regulatory factor on physiological function of skeletal muscles [154]. Further, regular exercise training beneficially affects human lipid profile, metabolic status, and immune activity reducing the risk for cardiovascular diseases [153,155,156]. Concerning HF, there are diverging data regarding the effect of diet on cardiac function [157,158]. Although there is a large number of studies that recommend the use of healthy diet, exercise training and in some cases the use of supplements, the evidence is not robust enough to strongly recommend this approach. However, it is a fact that whereas the consumption of non-refined fiber-rich foods, vegetables, fruits etc. promotes short chain fatty acid production, which is considered cardioprotective, meet consumption leads to TMAO production, which is considered harmful for various systems, including the cardiovascular [76,141]. Importantly, a relation between gut microbiota and mitochondria has been documented [159] according to which the gut environment regulates cell death by toxin secretion, targeting the mitochondria and host innate immune system and leading to chronic inflammation that in turn promotes dysfunction of various systems including the cardiovascular [17]. In this respect, by maintaining the gut microbiota “keeper” in track, control of mitochondrial function and minimization of harmful effects might be achieved. To answer important questions on these issues the PROMOTe (PROtein and Muscle in Older Twins, NCT04309292) study was designed [160] . This is a double blinded, randomized, placebo controlled, dietary intervention study in which volunteers will be recruited in twin pairs from TwinsUK cohort. Each pair will be randomized to either receive protein supplementation plus placebo or protein supplementation plus a gut microbiome modulator (prebiotic plus probiotic) and the intervention period will be 12 weeks. Clinical and biochemical measures will be taken at 0, and 12 weeks, with 2-monthly contact and gut microbiota composition will be measured, alongside a battery of physical assessments. Primary outcome will be muscle function measured using chair-rise time.

7. Future Directions

Gut dysbiosis (altered intestinal microbiota) is associated with several human diseases [161]. As a result, several relevant biomarkers have been proposed for early disease detection. However, due to the heterogeneity of the gut environment and the lack of definition of healthy gut microbiota, the current relevant biomarkers are imprecise, and , therefore, of doubtful significance, for disease classification [162,163,164,165,166,167]. Encouraging, however, were the results in studies employing machine learning and artificial intelligence for the differentiation between normal and abnormal gut microbiota as well as the prediction of treatment response in diverse diseases [168,169,170]. A recently published study, which systematically evaluated the cross-cohort performance of gut microbiota-based machine-learning classifiers for 20 diseases reported high predictive accuracies in intra-cohort validation, but low accuracies in cross-cohort validation, except for the intestinal diseases [171]. Other studies demonstrated that assessment of gut microbiota status using machine learning/artificial intelligence may be useful in staging and response to treatment of cancer [172,173]. Unfortunately, this is currently not the case in HF. It is anticipated, however, that computational methods assessing gut microbiota status will prove effective in HF early diagnosis, disease monitoring, and evaluation of treatment response, contributing to the better management of this lethal syndrome in the not so far future.

8. Conclusions

Gut microbiota is an emerging organ that exhibits a bidirectional association with the heart and deserves our attention. Diet, exercise, and the use of medications may modify gut microbiota composition and its interactions with several crucial pathophysiological mechanisms of HF. Whether evaluation of gut microbiota may prove useful for HF early diagnosis, monitoring, and management is currently a subject of intensive research.