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Non-dialysable protein-bound uremic toxins (PBUTs) contribute to the development of cardiovascular disease (CVD) in chronic kidney disease (CKD) and vice versa. PBUTs have been shown to alter sphingolipid imbalance. Dihydroceramide desaturase 1 (Des1) is an important gatekeeper enzyme which controls the non-reversible conversion of sphingolipids, dihydroceramide, into ceramide. The present study assessed the effect of Des1 inhibition on PBUT-induced cardiac and renal effects in vitro, using a selective Des1 inhibitor (CIN038). Des1 inhibition attenuated hypertrophy in neonatal rat cardiac myocytes and collagen synthesis in neonatal rat cardiac fibroblasts and renal mesangial cells induced by the PBUTs, indoxyl sulfate and p-cresol sulfate. This is at least attributable to modulation of NF-κB signalling and reductions in β-MHC, Collagen I and TNF-α gene expression. Lipidomic analyses revealed Des1 inhibition restored C16-dihydroceramide levels reduced by indoxyl sulfate. In conclusion, PBUTs play a critical role in mediating sphingolipid imbalance and inflammatory responses in heart and kidney cells, and these effects were attenuated by Des1 inhibition. Therefore, sphingolipid modifying agents may have therapeutic potential for the treatment of CVD and CKD and warrant further investigation. Refereed/Peer-reviewed

Cardiac fibrosis and myocyte hypertrophy play contributory roles in the progression of diseases such as heart Failure (HF) through what is collectively termed cardiac remodelling. The phosphoinositide 3- kinase (PI3K), protein kinase B (Akt) and mammalian target for rapamycin (mTOR) signalling pathway (PI3K/Akt- mTOR) is an important pathway in protein synthesis, cell growth, cell proliferation, and lipid metabolism. The sphingolipid, dihydrosphingosine 1 phosphate (dhS1P) has been shown to bind to high density lipids in plasma. Unlike its analog, spingosine 1 phosphate (S1P), the role of dhS1P in cardiac fibrosis is still being deciphered. This study was conducted to investigate the effect of dhS1P on PI3K/Akt signalling in primary cardiac fibroblasts and myocytes. Our findings demonstrate that inhibiting PI3K reduced collagen synthesis in neonatal cardiac fibroblasts (NCFs), and hypertrophy in neonatal cardiac myocytes (NCMs) induced by dhS1P, in vitro. Reduced activation of the PI3K/Akt- mTOR signalling pathway led to impaired translation of fibrotic proteins such as collagen 1 (Coll1) and transforming growth factor β (TGFβ) and inhibited the transcription and translation of tissue inhibitor of matrix metalloproteinase 1 (TIMP1). PI3K inhibition also affected the gene expression of S1P receptors and enzymes such as the dihydroceramide delta 4 desaturase (DEGS1) and sphingosine kinase 1 (SK1) in the de novo sphingolipid pathway. While in myocytes, PI3K inhibition reduced myocyte hypertrophy induced by dhS1P by reducing skeletal muscle α- actin (αSKA) mRNA expression, and protein translation due to increased glycogen synthase kinase 3β (GSK3β) mRNA expression. Our findings show a relationship between the PI3K/Akt- mTOR signalling cascade and exogenous dhS1P induced collagen synthesis and myocyte hypertrophy in primary neonatal cardiac cells.

Protein-bound uremic toxins (PBUTs) have adverse effects on vascular function, which is imperative in the progression of cardiovascular and renal diseases. The role of sphingolipids in PBUT-mediated vasculo-endothelial pathophysiology is unclear. This study assessed the therapeutic potential of dihydroceramide desaturase 1 (Des1) inhibition, the last enzyme involved in de novo ceramide synthesis, to mitigate the vascular effects of the PBUT indoxyl sulfate (IS). Rat aortic rings were isolated and vascular reactivity was assessed in organ bath experiments followed by immunohistochemical analyses. Furthermore, cultured human aortic endothelial cells were assessed for phenotypic and mechanistic changes. Inhibition of Des1 by a selective inhibitor CIN038 (0.1 to 0.3 μM) improved IS-induced impairment of vasorelaxation and modulated immunoreactivity of oxidative stress markers. Des1 inhibition also reversed IS-induced reduction in endothelial cell migration (1.0 μM) by promoting the expression of angiogenic cytokines and reducing inflammatory and oxidative stress markers. These effects were associated with a reduction of TIMP1 and the restoration of Akt phosphorylation. In conclusion, Des1 inhibition improved vascular relaxation and endothelial cell migration impaired by IS overload. Therefore, Des1 may be a suitable intracellular target to mitigate PBUT-induced adverse vascular effects. Refereed/Peer-reviewed

Aims Indoxyl sulfate (IS), a protein-bound uremic toxin, is implicated in endothelial dysfunction, which contributes to adverse cardiovascular events in chronic kidney disease. Apoptosis signal regulating kinase 1 (ASK1) is a reactive oxygen species-driven kinase involved in IS-mediated adverse effects. This study assessed the therapeutic potential of ASK1 inhibition in alleviating endothelial effects induced by IS. Main methods IS, in the presence and absence of a selective ASK1 inhibitor (GSK2261818A), was assessed for its effect on vascular reactivity in rat aortic rings, and cultured human aortic endothelial cells where we evaluated phenotypic and mechanistic changes. Key findings IS directly impairs endothelium-dependent vasorelaxation and endothelial cell migration. Mechanistic studies revealed increased production of reactive oxygen species-related markers, reduction of endothelial nitric oxide synthase and increased protein expression of tissue inhibitor of matrix metalloproteinase 1 (TIMP1). IS also increases angiopoietin-2 and tumour necrosis factor α gene expression and promotes transforming growth factor β receptor abundance. Inhibition of ASK1 ameliorated the increase in oxidative stress markers, promoted autocrine interleukin 8 pro-angiogenic signalling and decreased anti-angiogenic responses at least in part via reducing TIMP1 protein expression. Significance ASK1 inhibition attenuated vasorelaxation and endothelial cell migration impaired by IS. Therefore, ASK1 is a viable intracellular target to alleviate uremic toxin-induced impairment in the vasculature.

Cardiac fibrosis and myocyte hypertrophy are hallmarks of the cardiac remodelling process in cardiomyopathies such as heart failure (HF). Dyslipidemia or dysregulation of lipids contribute to HF. The dysregulation of high density lipoproteins (HDL) could lead to altered levels of other lipid metabolites that are bound to it such as sphingosine-1- phosphate (S1P). Recently, it has been shown that S1P and its analogue dihydrosphingosine-1-phosphate (dhS1P) are bound to HDL in plasma. The effects of dhS1P on cardiac cells have been obscure. In this study, we show that extracellular dhS1P is able to increase collagen synthesis in neonatal rat cardiac fibroblasts (NCFs) and cause hypertrophy of neonatal cardiac myocytes (NCMs). The janus kinase/signal transducer and activator (JAK/STAT) signalling pathway was involved in the increased collagen synthesis by dhS1P, through sustained increase of tissue inhibitor of matrix metalloproteinase 1 (TIMP1). Extracellular dhS1P increased phosphorylation levels of STAT1 and STAT3 proteins, also caused an early increase in gene expression of transforming growth factor-β (TGFβ), and sustained increase in TIMP1. Inhibition of JAKs led to inhibition of TIMP1 and TGFβ gene and protein expression. We also show that dhS1P is able to cause NCM hypertrophy through S1P-receptor-1 (S1PR1) signalling which is opposite to that of its analogue, S1P. Taken together, our results show that dhS1P increases collagen synthesis in cardiac fibroblasts causing fibrosis through dhS1P-JAK/STAT-TIMP1 signalling.

Dihydrosphingolipids refer to sphingolipids early in the biosynthetic pathway that do not contain a C4-trans-double bond in the sphingoid backbone: 3-ketosphinganine (3-ketoSph), dihydrosphingosine (dhSph), dihydrosphingosine-1-phosphate (dhS1P) and dihydroceramide (dhCer). Recent advances in research related to sphingolipid biochemistry have shed light on the importance of sphingolipids in terms of cellular signalling in health and disease. However, dihydrosphingolipids have received less attention and research is lacking especially in terms of their molecular mechanisms of action. This is despite studies implicating them in the pathophysiology of disease, for example dhCer in predicting type 2 diabetes in obese individuals, dhS1P in cardiovascular diseases and dhSph in hepato-renal toxicity. This review gives a comprehensive summary of research in the last 10-15 years on the dihydrosphingolipids, 3-ketoSph, dhSph, dhS1P and dhCer, and their relevant roles in different diseases. It also highlights gaps in research that could be of future interest.

Background Cardiac remodelling is a dynamic process whereby structural and functional changes occur within the heart in response to injury or inflammation. Recent studies have demonstrated reactive oxygen species sensitive MAPK, apoptosis signal-regulating kinase 1 (ASK1) plays a critical role in cardiac remodelling. This study aims to determine the effectiveness of small molecule ASK1 inhibitors on these processes and their therapeutic potential. Methods Neonatal rat cardiac fibroblasts (NCF) were pre-treated with ASK1 inhibitors, G2261818A (G226) and G2358939A (G235), for 2 hours before stimulated with 100 nM angiotensin II (AngII), 10 μM indoxyl sulphate (IS) or 10 ng/ml transforming growth factor β1 (TGFβ1) for 48 hours. Neonatal rat cardiac myocytes (NCM) were pre-treated with G226 and G235 for 2 hours before being stimulated with 100 nM AngII for 60 hours, 10 μM IS, 10 ng/ml interleukin 1β (IL-1β) or tumour necrosis factor α (TNFα) for 48 hours. 3H-proline and 3H-leucine incorporation was used to assess collagen turnover and hypertrophy, respectively. Pro-fibrotic, pro-hypertrophic and THP-1 inflammatory cytokine gene expressions were determined by RT-PCR. Results Both G226 and G235 dose-dependently attenuated AngII-, IS-, IL-1β- and TNFα-stimulated NCM hypertrophy and hypertrophic gene expression, IS-, AngII- and TGFβ1-stimulated NCF collagen synthesis and AngII- and TGFβ1-stimulated pro-fibrotic gene expression. Inhibition of ASK1 by G226 and G235 inhibited lipopolysaccharides-stimulated inflammatory cytokine gene expression in THP-1 cells. Conclusions Selective ASK1 inhibition confers anti-hypertrophic and anti-fibrotic effects in cardiac cells, and anti-inflammation in monocytic cells. ASK1 inhibitors may represent novel therapeutic agents to alleviate cardiac remodelling post cardiac injury where hypertrophy, fibrosis and inflammation play critical roles.