Various miRNAs may play a significant role in the pathological ventricular remodeling induced by CKD. The miR-30 family is highly expressed in the heart and modulates cardiomyocytes autophagy, apoptosis and oxidative stress, either under normal or pathological conditions [14].

In a recent work, Bao et al. investigated the role of miR-30 in promoting LVH in a model of CKD Sprague-Dawley rats [15]. These rats exhibited lower miR-30 expression, which was inversely related to the entity of renal damage. miR-30 transfection was able to attenuate cardiac apoptosis, thereby suggesting a pivotal role of this miRNA in CKD-induced hypertrophy. Of note, such as inhibition was likely to depend on an inhibition of the calcineurin pathway, a well-known powerful driver of cardiac hypertrophy in various disease models [16].

Several pathogenic factors may inhibit cardiac miR-30 expression, such as oxidative stress, inflammation and hemodynamic disorders [14], the majority of which are also hallmarks of CKD. In this view, miR-30 could thus represent a final common pathway to all these different mechanisms leading to LVH, thereby asking the question as to whether this miRNA could also represent a potential therapeutic target for CKD patients with LVH.

Cardiomyocyte apoptosis plays a crucial role in pathological cardiac remodeling, particularly in the genesis and worsening of asymmetric patterns of LVH, such as eccentric hypertrophy. Similar to apoptosis, pyroptosis is programmed cell death related to inflammation, and it is mediated by caspase-1 activation followed by the cellular processing of proforms of the inflammatory cytokines IL-1β and IL-18 into their active forms [17]. However, compared to other mechanisms of cell death, such as apoptosis and necroptosis, pyroptosis is morphologically and mechanistically distinct. In fact, while apoptosis is induced by caspases 2, 8 and 9 and developed by the effector caspases 3, 6 and 7, pyroptosis is mainly driven by caspase-1 activation. The initial phase of pyroptosis is not distinguishable from apoptosis, while differences occur when the inflammatory process takes over. Macrophage tissue infiltration is commonly found in cardiomyopathies, and macrophage-derived exosomes induce inflammation [18] and promote programmed cell death [19].

miR-155 is an acknowledged promotor of interstitial inflammation and pyroptosis when an extensive cardiac remodeling occurs such as, for instance, after diffusing ischemic heart damage; miR-155 is highly expressed by macrophages that infiltrate the interstitium and can be transferred into cardiomyocytes, thereby regulating their survival [20]. In a landmark study [21], Wang et al. investigated the possible role of miR-155 in regulating cardiomyocyte pyroptosis and hypertrophy in a rat model of CKD with superimposed UCM. Increased levels of caspase-1, IL-1β and IL-18 at the uremic heart level confirmed the implication of pyroptosis in the genesis of UCM. Moreover, a remarkable increase in miR-155 expression was found, particularly in exosomes of infiltrated macrophages. In line with these observations, selective blockades of the caspase-1 pathway attenuated the overall severity of UCM, leading to a dramatic reduction in myocardial hypertrophy and interstitial fibrosis with a following decrease of heart size, heart weight and heart weight-to-body weight ratio and a significant improvement in the Ejection fraction (EF). A similar response was observed in miR-155 knockout models or after administration of selective miR-155 inhibitors while the transfection of macrophage-derived exosomes into uremic miR-155^−/− mice restored the UCM picture.

Unlike miR-155, miR-26a was found to be a “beneficial” modulator of cardiac alterations driven by CKD. In another study, Wang et al. [22] demonstrated the capacity of this miRNA to significantly improve peripheral muscle wasting and atrophy, two well-acknowledged consequences of sustained uremia, as well as to reduce fibrosis and hypertrophy at the cardiac level. Despite CKD contributing to UCM and muscle atrophy because of multiple factors, including metabolic acidosis, inflammation and increased oxidative stress, insulin resistance is recognized to play an equally important role. Downregulation of the Insulin/IGF-1 pathway signaling observed in CKD induces muscle wasting by increasing protein degradation through ubiquitin proteasome pathway activation [23]. Interestingly miR-26a expression was found to be almost suppressed on the heart and muscle levels. Transfection of exosomes enriched with miR-26a improved cardiac and muscle sensitivity to insulin by activating Akt and, in turn, by inactivating the FoxO1 and GSK-3 pathways. No less important, miR-26a transfection reduced the circulating levels of the profibrotic factors TGF-β1 and PTEN. Hence, this miRNA may be a candidate as a serious therapeutic target for reverting UCM in CKD.

Forkhead transcription factors of the O class, such as FoxO1, FoxO3, FoxO4 and FoxO6, regulate several genes expression involved in various cell processes, such as proliferation, inflammation, apoptosis and pyroptosis. Activation of FoxO1 and FoxO3a plays a main role in cardiac hypertrophy [24] but unlike FoxO1, FoxO3 levels are notably reduced in uremic hearts. Overexpression of FoxO3a in uremic hearts ameliorates pyroptosis and improves myocardial hypertrophy, interstitial fibrosis area and cardiac function, confirming that FoxO3a decrease is likely to be implied in the pathologic cardiac remodeling occurring in UCM as well [21].

The Mir-212/132 is a miRNA family known to promote pressure-overload-induced LVH and heart failure through the repression of FoxO3 [25]. In a milestone study, Sarkozy et al. [26] found an increased left ventricular expression of miR-212 in rats with CKD as compared to sham. However, FoxO3 expression and phosphorylation levels were unchanged in this model. Bearing in mind that an increased phospho-FoxO3/total-FoxO3 ratio is typical of pressure-overload-induced LVH, the molecular pathways leading to LVH could therefore be different according to the origin of cardiac damage, as well as its mutual interactions. Unexpectedly, this study did not find changes in the levels of other miR-212 targets, such as ERK1, ERK2, Mef2a, AMPK, Sirt1 and PTEN in CKD animals. Hence, other unknown interactions could have played a role in regulating the downstream process of LVH as well as cardiac fibrosis in the course of chronic renal damage.

The impact of a panel of cardio-specific miRNAs on the PPARα signaling, as well as on other intracellular pathways, was further investigated in another experimental model of CKD following chronic heart disease (CHD), also known as type-2 cardio-renal syndrome (CRS) [31]. Interestingly, in this study, CHD was induced by pulmonary artery constriction (PAC), an experimental model of pulmonary hypertension that ends with the induction of a pathological, dysfunctional remodeling that affects mostly the right ventricle (RV). Indeed, pulmonary hypertension (PH) is a highly pervasive condition among CKD patients, also representing an independent predictor of adverse CV outcomes [32,33]. miR-205-5p and miR-208b-3p were significantly upregulated in RV specimens from the PAC group as compared with controls, while no differences were reported for miR-21a-3p. Conversely, other miRNAs such as miRNA-215, miRNA-150 and miR-26b-5p were significantly down-regulated. Molecular mechanisms of RV dysfunction were investigated through the construction of pairs of miRNAs to their target genes, in order to get functional information about the biological process implicated. Further analyses of upregulated mRNAs in RV tissue revealed positive regulation of the Ras protein signaling pathway, while downregulated mRNAs showed implications in the low-density lipoprotein receptor particle metabolic process, as well as in carboxylic and organic acid biosynthetic processes. KEGG analyses showed upregulated mRNAs in PAC mice paired with an increased PI-3K/Akt signaling and focal adhesion pathway, while downregulated mRNAs were implicated in the PPARα signaling pathway.

Na/K-ATPase signaling is another major driver of pathological cardiac fibrogenesis. This holds true particularly in UCM, in which a hyperstimulation of this pump is triggered by endogenous steroids, such as marinobufagenin (MBG), which accumulates in the bloodstream as uremic toxins [34,35]. Na/K-ATPase signaling involves downstream signaling proteins, such as Src [36], Akt and PKCs [37], which culminate in the enhancement of various pro-fibrotic pathways [38].

Drummond et al. showed the important relationship between Na/K-ATPase signaling and miR-29b-3p regulation in determining cardiac fibrosis in a rat model of 5/6 partial nephrectomy induced-CKD [39]. The authors found miR-29b-3p expression to be negatively regulated by the Na/K-ATPase signaling through activation of Src and NF-kB. Downregulation of this miRNA was more evident in cardiomyocytes, although it was observed in other interstitial populations as well. An increase in Src and NF-kB activation resulted in fibrosis enhancement in LV tissue samples, thus suggesting an antifibrotic role of miR-29b-3p by acting on the Na/K-ATPase signaling.

In another work [40], the same group confirmed the crucial role of miR-29b-3p in modulating collagen synthesis by cardiac fibroblasts induced by Na/K-ATPase activation [40]. Primary cardiac fibroblasts cultures were obtained from 5/6 Partial Nephrectomy (PNx) and MBG-infused rats. In LV specimens from both PNx and MBG-infused rats, miR-29b-3p expression was decreased respectively by 60% and 50%, while COL1A1 mRNA expression was increased. In vitro, cardiac fibroblasts cultures from adult rats were treated with two different Na/K-ATPase ligands, ouabain and MBG, both eliciting a significant decrease of miR-29b-3p levels and a significant increase in collagen protein expression. Hence, collectively, these data indicate that Na/K-ATPase signaling has a major role in uremia-induced cardiomyopathy through the downregulation of miR-29b-3p expression.

Hyperactivation of the Renin-angiotensin-aldosterone system (RAS) is another worsening factor, as well as a putative driven by cardiac fibrosis in UCM. Angiotensin-II (Ang-II), the last active compound generated in the pathway, exerts its actions through two distinct G-protein-coupled receptors, type 1 (AT1R) and type 2 (AT2R), expressed in different tissues, including the heart. Ang-II has a known effect on collagen synthesis and in the fibronectin stimulating TGF-β1 pathway [41,42]. Classical RAS activation also induces ADAM17, which promotes inflammation and fibrosis through the release of pro-fibrotic and pro-inflammatory cytokines. Previous studies have shown that cardiac fibrosis induced by Ang-II can be prevented by the administration of the small angiotensin-derived peptides Ang (1–7). Moreover, the overexpression of the angiotensin-converting enzyme 2 (ACE2) in mice reverses not only cardiac fibrosis, but also cardiac hypertrophy, while a lower ACE2 expression resulted in the progression of cardiac and renal fibrosis [43].

RAS inhibitors such as ACE-inhibitors (ACEi) and Ang-II receptor blockers (ARBs) are notoriously endowed with anti-fibrotic properties, which make these drugs as best-practice treatment for retarding cardiac and kidney fibrosis in various heart and renal diseases. Evidence nowadays exists that such an antifibrotic effect might be attributable, at least in part, to their capacity to modulate gene transcription of certain miRNAs directly or indirectly involved in cardiac fibrosis and pathological hypertrophy [44].

For instance, in a diabetic nephropathy (DN) mice model, the capacity of the valsartan to ameliorating cardiac fibrosis is partly linked to the inhibition of miR-21 expression [45]. Previous studies evidenced that miR-21 levels in cardiac tissue could be an indirect indicator of cardiac function [46]. Western blotting analysis after transfection experiments demonstrated that miR-21 over-expression significantly decreased the expression of the matrix metalloproteinase-9 (MMP-9), a powerful antifibrotic interstitial protein while, predictably, miR-21 inhibition led to an enhancement of MMP-9 expression. Therefore, miR-21 directly downregulates MMP-9, contributing in this way to cardiac fibrosis in the course of DN. Of note, ARB administration decreased miR-21 expression both in cardiac tissue and in the bloodstream, suggesting that the ARB-mediated suppression of miR-21 expression could be one of the underlying mechanisms of diabetic cardiomyopathy in DN mice. Additionally, this study found the urinary albumin to creatinine ratio (ACR) to be strictly related to miR-21 cardiac levels; this observation paired well with the consolidated assumption that in heart failure with a preserved ejection fraction rate, albuminuria has an independent association with cardiac remodeling [47].

In another study, ACE-inhibition with ramipril was able to prevent cardiac damage in a rat model of cardiac damage following acute kidney injury (AKI) [48]. In these animals, AKI was followed by an increase in miR-212 and miR-132 expression, two miRNAs which are well acknowledged for their pro-fibrotic effect based on the suppression of the antihypertrophic FoxO3 target gene [49]. However, the administration of ramipril blunted such an increase, also eliciting a hyperexpression of miR-133 and miR-1, two acknowledged cardioprotective and anti-fibrotic miRNAs [50]. The target genes of miR-133, such as the profibrotic COL1A1 and the proapoptotic Caspase-9, were upregulated in AKI rats and downregulated in the ACE inhibitor treatment group. By the same token, ramipril administration induced miR-1 upregulation, which caused inhibition of the miR-1 target profibrotic gene FN1 [51] and the antiapoptotic BCL2 gene [52], thereby contributing through another pathway to fibrosis attenuation.

Beyond the RAS, many other hormonal factors may make a significant contribution to UCM worsening. Vitamin D plays an essential role in calcium homeostasis and bone metabolism, but it is also endowed with myriad pleiotropic effects on other tissues and systems [53,54]. Vitamin D receptor activators (VDRAs) have shown anti-inflammatory and antifibrotic effects through the inhibition of specific signaling factors, such as NF-kB, TGF-β1, collagen I and ADAM 17. Hence, activation of Vitamin D-mediated pathways, including Klotho overexpression, prevent worsening of kidney and cardiac damage by inhibiting fibrosis [55]. Importantly, the VDRA paricalcitol attenuates LVH and cardiac fibrosis by reducing ECM proteins synthesis and TGF-β1 expression and by increasing the activity of the MMP-1 collagenase [56,57].

In a rat model of CKD, VDRAs administration was able to attenuate uremia-induced cardiac fibrosis by regulating miR-29b, miR-30c and miR-133b expression [58]. The levels of these miRNAs were reduced in the LV of CKD rats, while calcitriol administration partially prevented this decrease. Mechanistic analyses revealed that such miRNAs specifically targeted the profibrotic genes COL1A1, MMP-2 and CTGF, which suggested a potential therapeutic application as antifibrotic agents.

Like Vitamin-D, thyroid hormones (THs) are also renowned for their pleiotropic effects on several organs including the CV system. A reduction in circulating THs may enhance various mechanisms underlying myocardial damage in the course of uremia [59,60]. The usual hormonal pattern in CKD individuals is represented by low or low–normal plasma levels of triiodothyronine (T3) and thyroxine (T4) with normal thyroid stimulating hormone (TSH). Additionally, peripheral resistance to THs activity may also be present [61]. THs regulate expression of hypertrophy-associated proteins such as the α-myosin heavy chain (α-MHC) and the β-myosin heavy chain (β-MHC) [62], as well as collagen deposition [63] and collagen removal [64]. Abnormal myocardial remodeling may thus occur, driven by a decreased expression of α-MHC and an overexpression of β-MHC.

miR-208, which is selectively expressed in myocardial tissue, modulates α-MHC expression and myocardial fibrosis in response to various stimuli, thereby acting as an important regulator of heart remodeling [65]. miR-208 was previously studied in several models of LVH, such as thoracic aorta banding (TAB), in which this miRNA played a crucial role in shifting the synthesis of α-MHC to β-MHC. Yet, the presence of miR-208 appeared necessary but not sufficient to generate LVH or to modify the MHC pattern, as additional mechanical stimuli are needed to produce such changes [66].

Interestingly, THs may control miR-208 activity at the pre-transcriptional level and the selective antagonism of miR-208 has been seen to be effective in improving cardiac function and survival in heart failure [67]. Accordingly, the ventricular expression of miR-208 was dramatically reduced in a mice model of CKD and UCM while MHC levels were abnormally increased [68]. Of note, the lower miR-208 expression paired well with the severity of LVH and fibrosis and the preventive administration of high doses of T4 was able to normalize miR-208 expression and MHC levels with a subsequent improvement in cardiac geometry and function with, however, no changes reported in kidney damage and function.

Gender-specific hormones are other well-acknowledged protagonists in the progression of renal disease, also exerting a significant impact on long-term CV outcomes [69,70]. Men with CKD usually display a faster tendency to ESKD and a higher incidence of CKD-related CVD; conversely, women show a higher age-adjusted prevalence of CKD and are less exposed to CV disease by the protective effects of gonadal hormones until menopause [71]. The latter is partly dependent on a favorable cardiac morphology and a generally more compliant and preserved CV system.

In a very interesting work, Paterson et al. investigated the sex-specific role of miR-146b-5p on the cardiac pathology in a rat model of CKD [72]. Following 5/6 Nx, cardiac hypertrophy was less pronounced in female miR-146b−/− rats as compared to the wild-type (females) while, on the contrary, renal dysfunction was apparently more pronounced. Such a phenotype was partly attenuated by a preventive ovariectomy. Conversely, the loss of miR-146b-5p had no effect on the hypertrophic response in male animals which, on the contrary, exhibited a significant LVH with chamber dilation following 5/6Nx when compared to miR-146b−/−, but not when compared to wild-type males. Complex in silico analyses evidenced that miR-146b-5p activity mostly targeted the expression of TGFB1 protein in the TGF-β pathway as well as that of Vimentin (Vim) and e-cadherin (Cdh1). Treatment with β-estrogens decreased the TGF-β-induced Vim expression, and pre-miR-146b treatment significantly reduced the expression of collagen in cells simultaneously treated with TGF-β and β-estrogens but not in those incubated with TGF-β alone, thus suggesting a sex hormones-dependent action of miR-146b-5p. Hence, miR-146b-5p in WT females may enhance the protective effects of estrogen, thereby explaining why miR-146b-5p suppression resulted in a dysregulation of gonadal hormones signaling and CKD worsening. Further studies are awaited to improve insights on the possible role of other miRNA families in modulating the effects of sex-hormones on cardiac pathological adaptations in the course of CKD.