RNA-seq results have identified 3102 DEGS, 1135 DE-lncRNAs, and 52 DE-miRNAs between DCM mice and control. Among the DEGs, we found that up-regulated DEGs are mainly associated with oxygen metabolism, diabetes, and other pathways related to metabolic disease, consistent with previous studies [41, 42]. Conversely, down-regulated DEGs are correlated to the immunity-related pathway. In addition, we further performed immune cell infiltration analysis and revealed that activation of CD8+ T cells plays a predominant role in the development of DCM. Therefore, the development of diabetes is associated with abnormalities in the immune system. However, the role of immune cells in diabetic myocarditis is still unclear. T lymphocyte infiltration into the myocardium has been observed in a left coronary artery occlusion-induced murine myocardial infarction model and a transverse aortic constriction-induced murine pressure overload model [43]. Tang et al. [44] reported that CD8+ T cells in ischemic failing human hearts may contribute to the progression of heart failure. Abdullah et al. [45] showed that conditional T-cell sS1p1 knockout mice that exhibited sustained deficiency of both CD4+ and CD8+ T cells, had improved cardiac function and alleviated cardiac fibrosis after 11 weeks of diabetic induction, indicating that T cell Ss1p1 activation exacerbates fibrosis under hyperglycemia. Although current knowledge supports that CD4+ T cells play a more important role in the development of DCM, mainly via the subtype of CD4+Foxp3+ T cells [46], we speculate that CD8+ T cells may be involved in the development of DCM via the following mechanisms: 1) CD8+ T cell can directly damage cardiomyocyte via its cytotoxicity effect; 2) CD8+ T cell can regulate macrophage migration via stimulating the production of nitric oxide; 3) CD8+ T cell can up-regulate CD11b, CD64, and CD62L on neutrophils mainly through the secretion of inflammatory factors and resultingly maintain their survival.
The current study focuses on individual diabetic complications. We analyzed the differences between tissues affected by diabetic complications by comparing them with public databases. As expected, there were differences in pathways and immune cell infiltration for each complication abnormality. We further clarified the key genes and key modules responsible for DCM by WGCNA, which were mostly differentially expressed in DCM and normally expressed in other diabetic complications.
Furthermore, we identified and validated 5 hub genes (Tnnc1, Pln, Fabp3, Popdc2, and Trim63) from the DCM-related key module. Mutations in Tnnc1, a complex that is known as Cardiac Troponin C and contains the component, troponin C, was confirmed to be associated with hypertrophic or dilated cardiomyopathy [47, 48]. Pln is a 52-aa SR membrane protein expressed abundantly in cardiac muscle and a crucial regulator of cardiac function by modulating the rate of cardiac relaxation and size of the SR Ca2 + store [49, 50]. Increased expression of Pln has been identified to reduce cardiac contractility and correlates with the over-expression of NF-SLN, raising the possibility that induced expression of SLN in human hearts can impair cardiac function [51, 52]. Jia et al. [53] also reported that the expression of Pln was significantly increased in a time-dependent manner in diabetic groups. Fatty acid-binding protein 3 (Fabp3) participates in cell metabolism by binding free long-chain fatty acids (LCFAs) and transporting them for cell metabolism [54]. Fabp3-defect exacerbates cardiac hypertrophy and heart dysfunction, but over-expression of Fabp3 can up-regulate the phosphorylation of the MAPK signaling pathway and decrease phosphorylated Akt levels, which may account for the augmentation of apoptosis and remodeling after myocardial infarction [55, 56]. Popdc2, one of the Popeye domain-containing (Popdc) gene families, was highly expressed particularly in the sinoatrial node of the mouse and represented as a novel arrhythmia gene for cardiac conduction disorders [57, 58]. A recent study [59] showed that Popdc2 was a fasting-induced gene, which suggests that the abnormal expression of popdc2 may be related to blood glucose. Trim63, also known as MuRF1, was significantly increased not only in cardiac muscle of diabetic mice, but also in diabetic limb muscle and STZ-Diabetes [60, 61]. Previous studies have demonstrated that abnormalities in these genes were strongly associated with the development of diabetes or cardiovascular disease. Our results classify these genes as key factors in the pathogenesis of DCM and potential drug targets for DCM treatment. However, there are only a few reports on the regulation of these genes with miRNA or ceRNA, except Trim63 [62].
In this study, we first reported the potential regulatory network among DCM hub genes, DCM-related miRNAs, and ceRNA networks. Most of these miRNAs, such as miR-3064-5p [63], miR-690 [64], miR-1195 [65, 66], miR-696 [67, 68], miR-708-3p [69], miR-7225-5p [70], miR-466 (including miR-466b-3p, miR-466c-3p, miR-466p-3p, miR-466a-3p, miR-466e-3p, miR-466o-3p) [71], have been reported to be involved in diabetes or cardiovascular disease. This supports our speculation that these miRNAs may play an important role in DCM. On the other hand, most lncRNAs in the ceRNA network have yet identified to be participating in cardiovascular pathology. More in-depth research will be worth conducting to address the involvement of lncRNAs.