HF is an emerging epidemic that is increasing strongly in prevalence as the population ages, with severe morbidity and a 5-year mortality rate of 50%(35). The burden of HF as a population-wide burden is highlighted by the fact that lifetime risks are pretty high regardless of sex, ethnicity, or region(2). In recent years, the multiple roles of biomarkers for HF have been validated to predict the future occurrence of HF, to identify its presence when fully developed, to stratify the risk of affected patients, and to serve as biological tools to guide treatment potentially. Although the pivotal role of necroptosis in various heart diseases has been emphasized, there are fewer studies of necroptosis in prognostic, diagnostic, and therapeutic value in the HF field. This study was a systematic bioinformatics analysis to find biomarkers of necroptosis in HF, which could offer reliable directions for future specific studies of HF and novel opportunities for developing effective diagnoses and therapies.
Based on the public GEO database, we have first screened 51 NRDEGs. Only eight genes were differentially expressed between the HF and normal groups, namely FLOT1, DAPK1, KLHDC10, FLOT2, FAS, UCHL1, TNFAIP3, and HSPA5, which expressions were all down-regulated in the HF group. Furthermore, we utilized FLOT2, FAS, and FLOT1 to construct a risk prediction model by the LASSO algorithm and multiple logistic analysis, which aims to assess the risk of HF in patients. The AUC was 0.749 in the training set and the AUC was 0.787 in the test set, suggesting that the model has relatively good accuracy and reliability. The AUC values for BNP and NT-proBNP to predict ischemic HF were 0.770 and 0.754 respectively(36, 37). To compare with such established and mature biomarkers in related HF risk assessment, like BNP and NT-proBNP, this risk prediction model based on three necroptosis-related genes has a certain clinical potential. Overall, a multi-biomarkers strategy could better reflect different HF pathways to improve clinical predictive performance.
FLOT1(flotillin-1) and FLOT2(flotillin-2), which belong to the SPFH superfamily, are ubiquitously expressed, highly conserved raft-associated integral membrane proteins(38). FLOT1 and FLOT2 share the same domain architecture and show a sequence identity of about 50% for the human protein(39). Even though flotillins are known to involve various cellular events, for instance, cell adhesion, cargo endocytosis, protein sorting and recycling, and cell migration, studies of their roles in cardiovascular disease are rare(40, 41). Flotillins are primarily known for their role in cancer by promoting tumorigenesis and metastasis(42). FLOT2 has altered in cardiac intercalated discs of patients with arrhythmogenic cardiomyopathy and DCM, thus suggesting its potential role in cardiac remodeling(43). FLOT1 with an excellent diagnostic value in DCM-caused HF has been confirmed in another study(44). Moreover, FLOT1 and FLOT2 were closely associated with cholesterol absorption. FLOT2-dependent exosomal cholesterol secretion was described by Strauss et al. as a novel cellular cholesterol homeostasis mechanism(45), while cholesterol absorption is strongly linked with various cardiovascular diseases. In our study, further correlation analysis showed that CFLAR is positively correlated with FLOT1, which might suggest that FLOT1 and CFLAR are both potential regulatory targets for HF. Fas as the tumor necrosis factor (TNF) receptor family member, which is encoded by the FAS gene, is a type I membrane protein that is produced by a variety of cells(46). The binding of Fas to its ligand FasL to form the FasL/Fas pathway plays a major role in immune function, particularly in activation-induced cell death(47). FasL/Fas pathway has been shown to provide an important mechanism for cardiomyocyte death following ischemia/reperfusion. Existing evidence strongly suggests FasL/Fas signaling is associated with progressive DCM, congestive HF, and ischemic HF in humans(48–50). Fas/FasL signaling represents a novel therapeutic target for HF.
We have constructed a PPI network and hub genes screening to identify more potential biomarker candidates related to necroptosis in HF. The top ten Hub genes were CALR, CCT2, PSMC3, RAB5A, FH, STIP1, OGDH, TGFBR2, SYNE2, and HSPD1. Stress-inducible phosphoprotein 1(STIP1) is an adaptor protein that bridges the gap between the folding of HSP70 and HSP90 and a secreted protein that regulates the growth of malignant cells(51). Oxidative stress has been directly linked to the pathophysiological mechanisms of HF. Nrf2 ablated mice have pathological susceptibility to exogenous oxidative, and STIP1 was reduced in its heart(52). We speculated that the downregulating of STIP1 caused increased oxidative stress to accelerate HF progress. In addition, glucocorticoids have been used to improve the curative effect of diuretics and natriuretic peptides in advanced HF. In the glucocorticoid pathway, STIP1 is a novel therapeutic target that is involved in the activation of the glucocorticoid receptor(53). Together the results hint that STIP1 downregulation may indicate early HF. On the other hand, STIP1 may become an indicator of oxidative stress levels in HF progress to apply in treatment.
TGFBR2 encodes the TGF-beta II receptor, which can activate the TGF-beta signaling pathway by specific ligand binding (54). HF patients had elevated plasma concentrations of TGF-beta1(55). TGF-beta1 signal and downstream Smads are essential for tissue fibrosis and matrix remodeling in multiple etiologies of HF. Direct evidence is provided by Rojas et al. that the specificity of the TGF-beta1 response can be influenced by the regulation of the expression level of TGFBR2(56). Hence, TGFBR2 may be a key target for alleviating ventricular remodeling in HF.
The HSPD1 gene encodes for a constitutively expressed mitochondrial heat shock protein Family D Member 1(HSPD1, also named Hsp60) chaperonin protein, which is responsible for the folding, transport, and quality control of mitochondrial matrix proteins and is essential for maintaining life(57).
The levels of circulatory HSPD1 increase during HF(58). Enomoto et al. demonstrated that the upregulation of the HSPD1 protein inhibited the activity of mitochondrial complex IV, increasing reactive oxygen species(ROS) concentration. Elevated ROS caused HF by significantly increasing the amount of mitophagy in the heart(59). This may suggest that the elevated circulating concentrations of HSPD1 may indicate early HF or exacerbation of HF.
Although the valuable NRDEGs were mined from the myocardial tissue databases, myocardial tissue samples from most HF patients are not readily available clinically, but blood samples are available for measuring the expression levels of NRDEGs. To validate the results further, we preliminarily explored the expression levels of these eight NRDEGs in collected human blood samples and found that the expression of TNFAIP3 and HSPA5 was lower in HF compared to the normal group, which is consistent with our previous bioinformatics analysis. Obviously, the differences in other NRDEGs need to be validated and explored by large-scale clinical trials. The TNFAIP3 gene encodes the ubiquitin-binding protein A20, a negative regulator of NF-κB(60). A host of studies confirmed that NF-κB activity was positively correlated with HF progression, and inhibition of NF-κB activity limits HF progression(61, 62). However, the direct pathological changes related to TNFAIP3/A20 in HF have been sparsely examined. Binding immunoglobulin protein (BiP), an endoplasmic reticulum chaperone from the Hsp70 family, is encoded by the HSPA5 gene(63). In an integrative meta-analysis of cellular and molecular transcriptomic landscape in nonfailing and HF human hearts, Luo et al. identified an altered mRNA transcription of HSPA5, associated with mitochondrial transaminases, in the left ventricle of the failing heart (64). Our initial findings indicated that TNFAIP3 and HSPA5 could be promising biomarkers and further increase TNFAIP3 and HSPA5 expression might be implicated in HF progression.
There are still some limitations to this research. First, the datasets used in the mining strategy only include DCM and ICM causing HF. Hence, there are a limited number of background diseases causing HF. Second, the animal experiment to further comprehensively verify the conclusions is inadequate. Third, the data were mined from a database of myocardial tissue, but for obvious reasons, only blood samples were collected for further validation. Moreover, the collected blood sample size was small, and the analysis originated from a single center. To sum up, a multi-center, randomized, controlled trial study and basic follow-up experiments are necessary for the future.