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Original Article

Korean J Physiol Pharmacol 2024; 28(3): 285-294

Published online May 1, 2024

Copyright © Korean J Physiol Pharmacol.

Fibroblast-derived interleukin-6 exacerbates adverse cardiac remodeling after myocardial infarction

Hongkun Li1,2 and Yunfei Bian1,2,*

1Key Laboratory of Cardiovascular Medicine and Clinical Pharmacology of Shanxi Province, 2Department of Cardiology, The Second Affiliated Hospital of Shanxi Medical University, Taiyuan 030001, Shanxi, China

Correspondence to:Yunfei Bian

Author contributions: H.L. performed the experiments, and collected and analyzed the data. Y.B. conceived and designed the experiments.

Received: December 4, 2023; Revised: March 13, 2024; Accepted: March 14, 2024

Myocardial infarction is one of the leading causes of mortality globally. Currently, the pleiotropic inflammatory cytokine interleukin-6 (IL-6) is considered to be intimately related to the severity of myocardial injury during myocardial infarction. Interventions targeting IL-6 are a promising therapeutic option for myocardial infarction, but the underlying molecular mechanisms are not well understood. Here, we report the novel role of IL-6 in regulating adverse cardiac remodeling mediated by fibroblasts in a mouse model of myocardial infarction. It was found that the elevated expression of IL-6 in myocardium and cardiac fibroblasts was observed after myocardial infarction. Further, fibroblast-specific knockdown of Il6 significantly attenuated cardiac fibrosis and adverse cardiac remodeling and preserved cardiac function induced by myocardial infarction. Mechanistically, the role of Il6 contributing to cardiac fibrosis depends on signal transduction and activation of transcription (STAT)3 signaling activation. Additionally, Stat3 binds to the Il11 promoter region and contributes to the increased expression of Il11, which exacerbates cardiac fibrosis. In conclusion, these results suggest a novel role for IL-6 derived from fibroblasts in mediating Stat3 activation and substantially augmented Il11 expression in promoting cardiac fibrosis, highlighting its potential as a therapeutic target for cardiac fibrosis.

Keywords: Cardiac fibrosis, Fibroblasts, Interleukin-6, Interleukin-11, Myocardial infarction

Cardiovascular diseases (CVDs), a leading global cause of impairment of human health, impose a heavy economic burden on society and compromise the overall quality of life. Cardiac fibrosis is an essential cardiological alteration triggered by CVDs such as myocardial infarction, which leads to deleterious cardiac remodeling in the later stages, provoking ventricular systolic and diastolic dysfunction, and ultimately, heart failure. Thus, it is possible that elucidating the endogenous regulatory mechanisms governing cardiac fibrosis may contribute to the identification of potential new targets for therapeutic intervention after myocardial infarction.

Although myocardial dysfunction induced by heart disease is usually associated with impaired cardiomyocyte function, excessive cardiac fibrosis is consistently a major cause of end-stage cardiac injury [1]. Myocardial fibrosis is a complex process resulting from abnormal heart tissue healing, eventually resulting in the formation of non-functional scars that impede the activity of the entire heart. Fibroblast activation is considered to be the most dominant pathological process in the development of cardiac fibrosis [2]. Targeting fibroblast activation would be a potentially effective approach for anti-cardiac fibrosis therapy. Therefore, elaboration of the activation mechanism of fibroblasts will contribute to the proposal of therapeutic approaches for cardiac fibrosis.

Myocardial infarction can trigger the production of large amounts of inflammatory cytokines, including interleukin-6 (IL-6). Immune cells such as macrophages are the most dominant cell type for IL-6 production. As a major proinflammatory cytokine, IL-6 plays an important regulatory role in the pathologic alterations induced by myocardial infarction [3]. In clinical practice, elevated serum levels of the proinflammatory cytokine IL-6 are a poor prognostic indicator of future cardiac events and cardiac morbidity [4,5]. However, the molecular regulatory mechanisms by which IL-6 affects the progression of heart disease still need to be further clarified.

Here, we demonstrated that fibroblasts are also a major source of IL-6 after myocardial infarction. Damage-associated molecular patterns (DAMPs) triggered by myocardial infarction can induce the production of IL-6 in fibroblasts. Fibroblast-specific IL-6 knockout effectively ameliorates adverse cardiac remodeling and prevents cardiac dysfunction due to myocardial infarction. Importantly, we found that IL-6 mediates increased expression of IL11 through activation of the signal transduction and activation of transcription (STAT)3 signaling pathway, which in turn promotes fibroblast activation that leads to excessive cardiac fibrosis.


Male C57BL/6 mice (20−23 g, 6–8 weeks) were obtained from The Animal Center of Shanxi Medical University. Il6fl/fl mice have been described previously. To direct fibroblast-specific Il6 deletion in mice, Il6fl/fl mice were crossed with homozygousCol1a2-CreERT mice (Jackson's Lab) to generate Il6fl/fl; Col1a2-CreERT progenies. Mice were injected with tamoxifen dissolved in corn oil (70 mg/kg/day, intraperitoneal injection, for 5 consecutive days) at 8 weeks of age to induce CreERT activity and facilitate Cre/loxP-mediated gene deletion in Cre-positive fibroblasts. All mice were housed under pathogen-free conditions with a room temperature of 22°C ± 2°C, a relative humidity of 55% ± 5%, and a light/dark cycle of 12/12 h.All experimental procedures conducted in this study were approved by the Experimental Animal Ethics Committee of Shanxi Medical University (DW2024028) and were following the Guide for the Care and Use of Laboratory Animals (NIH, 8th Edition, 2011).

Construction of the murine myocardial infarction model

Myocardial infarction models were constructed by permanent left anterior descending (LAD) artery ligation as previously described. In brief, mice were anesthetized with 1% pentobarbital (50 mg/kg), and subsequently extubated and mechanically ventilated using a small rodent ventilator. After opening the left chest cavity, the left ventricle and left auricle were exposed and the LAD was permanently ligated with 8-0 sutures. Finally, the chest wall and skin incisions were closed with 5-0 sutures. The sham-operated group underwent a similar procedure without ligating the artery. Cardiac function was evaluated using the Vevo3100 High-Resolution Imaging System (FUJIFILM) at designated postoperative time points. Left ventricular ejection fraction (LVEF), left ventricular shortening rate (LVFS), and cardiac output were obtained in each group under double-blinded conditions and a final statistical analysis was performed.

Cardiac fibroblasts isolation

Cardiac fibroblasts were isolated from adult mice as described previously. Briefly, the ventricles of 2–4-week-old mice were cut into approximately 1 mm3 pieces. The tissues were digested with 0.1% trypsin and 0.05% collagenase type II for 10 min at 37°C in a water bath. The supernatant was collected and the digestion was stopped by adding a Dulbecco’s Modified Eagle Medium containing 10% fetal bovine serum. At the same time, the remaining tissues were subjected to another round of digestion until there was almost no tissue left. The supernatant was centrifuged (1,000 rpm, 5 min) and the cells were resuspended. After 1 h of incubation of the cell suspension in a culture dish, the supernatant was discarded and a new medium was added, giving the adherent cells as cardiac fibroblasts.

siRNA transfection

Lipofectamine 3000 (Invitrogen) was used for siRNA transfection experiments according to the manufacturer's instructions. Briefly, Lipofectamine and siRNA were first separately diluted into Opti-MEM medium. The pre-diluted siRNA solution was mixed with Lipofectamine solution at a ratio of 1:1, and the mixture was allowed to incubate at room temperature for 15 min before being added dropwise to the cell culture medium. The total volume of the mixture added was 10% of the total volume of the final medium. Further experiments were carried out after 72 h of transfection. The sequence of siRNA for Il11: sense: GCUGUUCUCCUAACCCGAUTT; anti-sense: AUCGGGUUAGGAGAACAGCTT.

Histological analysis

Mice were euthanized by cervical dislocation after inhalation anesthesia with 3% isoflurane. Subsequently, hearts were removed, fixed in 4% paraformaldehyde for 24 h, and then cut into 6 µm thick sections. Masson trichrome staining, immunohistochemical staining and immunofluorescence staining were performed according to standard protocols. Stained images were obtained by light microscopy.

RNA isolation, cDNA synthesis, and quantitative PCR

Total RNA was isolated from myocardial tissue or fibroblasts using Trizol reagent (Thermo Fisher Scientific, Inc.), and cDNA was synthesized using the Transcriptor First Strand cDNA Synthesis Kit (Roche Diagnostics) according to the manufacturer's instructions. qPCR was performed with SYBR Green PCR Master Mix (Roche Diagnostics) by using the LightCycler 480 System (Roche Diagnostics). All threshold cycle values were normalized to the GAPDH.

Western blot analysis

Total protein was extracted from left ventricular tissues using RIPA lysis buffer containing phenylmethylsulfonyl fluoride, protease inhibitors, and phosphatase inhibitors (Beyotime). Protein concentration was determined using the bicinchoninic acid method. Equal amounts of protein (20 µg) were separated by SDS-PAGE (10%) and transferred to nitrocellulose membranes (Millipore-Sigma). The membranes were blocked with 5% skimmed milk for 1 h at room temperature and then incubated overnight at 4°C with primary antibody. After incubation with secondary antibody conjugated to horseradish peroxidase (1:10,000 dilution; Proteintech) for 1 h at room temperature, protein band images were acquired using an ECL imaging system (Bio-Rad Laboratories, Inc.).


IL-6 protein level was determined by Mouse IL-6 Quantikine ELISA Kit (R&D, Catalog No. M6000B) according to the manufacturer's instructions.

Measurement of myocardial infarct size

2, 3, 5-triphenyl tetrazolium chloride (TTC) staining was used to assess cardiac infarct size. The hearts were isolated and cut into transverse slices uniformly 1 mm thick. The heart sections were infiltrated with phosphate-buffered saline (PBS) and incubated with 1% TTC (Sigma-Aldrich) for 30 min at 37°C. Sections were then fixed with 4% formaldehyde. The infarct area was measured with Image J software.

Statistical analysis

Data are presented as mean ± SD. Student's t-test was used to compare the differences between the two groups. All data were analyzed using GraphPad software, and p < 0.05 was considered statistically significant.

Myocardial infarction induces IL-6 production in fibroblasts

Macrophages are typically thought to be the main source of producing IL-6 after myocardial infarction. To reveal whether cardiac fibroblasts are also involved in the secretion of IL-6 after myocardial infarction, we examined mRNA and protein levels of IL-6 derived from cardiac fibroblasts in infarcted myocardial tissue. We observed significantly elevated mRNA levels of IL-6 in myocardial tissues 3 days after myocardial infarction (Fig. 1A). Consistently, IL-6 production was also significantly upregulated in myocardial tissue (Fig. 1B). We similarly found a significant increase in IL-6 production in peripheral blood from mice with myocardial infarction (Fig. 1C). We found that the expression of IL-6 was also significantly elevated in fibroblasts from myocardial tissue after myocardial infarction (Fig. 1D). Myocardial infarction results in tissue and cell hypoxia, and we next explored whether hypoxia activates fibroblast IL-6 production. Cardiac fibroblasts from wild-type mice were subjected to hypoxia, and we found that hypoxia significantly promoted the expression of IL-6 mRNA and protein levels (Fig. 1E, F). Furthermore, we found that the damage-associated molecule pattern S100A8 can also induce cardiac fibroblasts to secrete IL-6 (Fig. 1G, H).

Figure 1. Heart injury induces fibroblast interleukin-6 (IL-6) production. (A) Q-PCR analysis of Il6 mRNA expression in myocardial tissues from WT mice after MI or sham operation. (B) ELISA of IL-6 production in myocardial tissue homogenates from WT mice after MI. (C) ELISA of IL-6 production in peripheral blood from mice with myocardial infarction. (D) Q-PCR analysis of Il6 mRNA expression in fibroblasts isolated from WT mice myocardial tissues at day 3 after MI. (E) Q-PCR analysis of Il6 mRNA expression in isolated fibroblasts under hypoxia condition for 1 day. (F) ELISA of IL-6 production in supernatants of isolated fibroblasts under hypoxia condition for 1 day. (G) Q-PCR analysis of Il6 mRNA expression in isolated fibroblasts stimulated with S100A8 for 12 h. (H) ELISA of IL-6 production in supernatants of isolated fibroblasts stimulated with S100A8 for 12 h. n = 5 per group. Data presented as mean ± SD. Unpaired Student’s t-test was performed. WT, wildtype; MI, myocardial infarction; Ctrl, control.

Fibroblasts derived IL-6 promotes myocardial infarction-induced cardiac injury

To further reveal the regulatory role of IL-6 derived from fibroblasts in the pathological process induced by myocardial infarction, we constructed fibroblast-specific Il6 knockout mice. We assessed the knockdown efficiency of Il6 in cardiac fibroblasts and the results showed a significant decrease in Il6 expression in fibroblasts isolated from knockout cardiac myocardial tissues (Fig. 2A and Supplementary Fig. 1A). Besides, we observed a dramatic decrease in IL-6 production in fibroblast-specific Il6 knockout mice heart (Supplementary Fig. 1B).

Figure 2. Fibroblast-specific interleukin-6 (IL-6) knockout alleviates cardiac dysfunction after MI. (A) Q-PCR analysis of Il6 mRNA expression in fibroblasts isolated from KO or littermate control WT mice. (B) Representative TTC staining images in myocardial tissues from Il6 KO or WT mice at day 14 after MI. (C, D) Echocardiographic measurement of LVEF and LVFS of Il6 KO or WT mice under sham or MI operation for 3 days. (E) Echocardiographic measurement of cardiac output in Il6 KO or WT mice. n = 5 per group. Data presented as mean ± SD. Unpaired Student’s t-test was performed. MI, myocardial infarction; KO, knockout; WT, wildtype; TTC, 2, 3, 5-triphenyl tetrazolium chloride; LVEF, left ventricular ejection fraction; LVFS, left ventricular shortening rate.

Myocardial infarct size was demonstrated by TTC staining. As shown in Fig. 2B, cardiac fibroblast Il6 knockout showed a significantly decreased infarct size compared to wild-type control mice. We then evaluated the effect of fibroblast-derived IL-6 on cardiac function after myocardial infarction. As shown in Fig. 2C and D, cardiac fibroblast Il6 knockout markedly improves cardiac dysfunction resulting from myocardial infarction. In addition, fibroblast Il6 knockout also prevents the decrease in cardiac output (Fig. 2E).

Fibroblasts derived IL-6 exacerbates the fibrotic response after myocardial infarction

Fibroblasts are the predominant cell type that mediates the onset of cardiac fibrosis, and we further explored the role of fibroblast IL-6 knockdown in regulating cardiac fibrosis after myocardial infarction. Fibrotic areas were assessed by Masson's trichrome staining, and the results showed that fibroblast IL-6 knockout exhibited a significant reduction in the accumulation of extracellular matrix (ECM) in the myocardium after myocardial infarction (Fig. 3A and Supplementary Fig. 1C). Sirus Red staining also indicated decreased accumulation of collagen components (Supplementary Fig. 1D). We further evaluated the activation of fibroblasts through the detection of α-smooth muscle actin expression. As shown in Fig. 3B and Supplementary Fig. 1E, fibroblast IL-6 knockout significantly inhibited the activation of fibroblasts induced by myocardial infarction.

Figure 3. Fibroblast-specific interleukin-6 (IL-6) knockout alleviates adverse cardiac remodeling after MI. (A) Representative Masson’s Trichrome staining images in myocardial tissues from Il6 KO or WT mice at day 14 after MI (×1.25). (B) Representative immunofluorescence staining of α-SMA in myocardial tissues from Il6 KO or WT mice after MI (×400). (C) Q-PCR analysis of Col1a1, Col3a1, Tgfb and Postn mRNA levels in myocardial tissues from Il6 KO or WT mice after MI. n = 5 per group. Data presented as mean ± SD. Unpaired Student’s t-test was performed. MI, myocardial infarction; KO, knockout; WT, wildtype; α-SMA, α-smooth muscle actin.

In addition, the expression of fibrosis-related genes is dramatically decreased following the deletion of the IL-6 gene in fibroblasts after myocardial infarction (Fig. 3C). Taken together, these results suggest that fibroblast IL-6 knockout prevents myocardial infarction-induced cardiac fibrosis.

Activation of fibroblast STAT3 is required for IL-6-induced cardiac fibrosis

Fibroblast IL-6 enhances the extent of cardiac fibrosis after myocardial infarction, which may be related to its activation of the STAT3 signaling pathway. We further demonstrated whether fibroblast IL-6 knockdown impacted STAT3 activation. The activation of STAT3 was significantly increased in the myocardium after myocardial infarction (Fig. 4A). We also observed a significant enhancement in the expression of fibrosis-associated proteins after myocardial infarction (Fig. 4B). As shown in Fig. 4C, fibroblast IL-6 deletion markedly attenuated the activation of STAT3 after myocardial infarction. Correspondingly, we also observed a significant reduction in the expression of fibrosis-associated proteins in myocardial tissues of fibroblast-specific IL-6 knockout mice (Fig. 4D). Inhibition of STAT3 activation and decreased expression of fibrosis-associated proteins was also observed after treatment with STAT3 inhibitor in mice with myocardial infarction (Fig. 4E, F). These results indicate that STAT3 activation is critical for IL-6-induced cardiac fibrosis.

Figure 4. Fibroblast-derived interleukin-6 (IL-6) promotes cardiac fibrosis via the activation of STAT3. (A) Immunoblot analysis of phosphorylation levels of STAT3 in myocardial tissues from WT mice after MI. (B) Immunoblot analysis of Collagen I and Collagen III in myocardial tissues from WT mice after MI. (C) Immunoblot analysis of phosphorylation levels of STAT3 in myocardial tissues from Ii6 KO or WT mice after MI. (D) Immunoblot analysis of Collagen I and Collagen III in myocardial tissues from Ii6 KO or WT mice after MI. (E) Immunoblot analysis of phosphorylation levels of STAT3 in myocardial tissues from Ctrl or STAT3-IN-3 treated mice after MI. (F) Immunoblot analysis of Collagen I and Collagen III in myocardial tissues from Ctrl or STAT3-IN-3 treated mice after MI. Similar results were obtained from three independent experiments. Data presented as mean ± SD. Unpaired Student’s t-test was performed. STAT3, signal transduction and activation of transcription 3; WT, wildtype; MI, myocardial infarction; KO, knockout; Ctrl, control. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001.

IL-6 promotes the expression of Il11 to mediate the process of cardiac fibrosis

We further explored how STAT3 activation influences cardiac fibroblast activation. STAT3-Y640F is a functionally validated gain-of-function mutation. We reanalyzed the Gene Expression Omnibus database (GSE211111) and found that hyperactive STAT3 showed increased binding at the Il11 promoter (Fig. 5A). This result suggests that Il11 is probably a potential downstream target of activated STAT3. ChIP-PCR assays were performed to validate the direct enrichment of STAT3 at the Il11 gene promoter, and the results showed that STAT3 binds directly to the Il11 gene promoter region and that the binding is enhanced after myocardial infarction (Fig. 5B). We further observed increased expression of Il11 in the myocardium tissue after myocardial infarction (Fig. 5C). We also observed a significant elevation of Il11 expression in cardiac fibroblasts treated with IL-6 (Fig. 5D). The expression of Il11 triggered by myocardial infarction was significantly reduced after fibroblast-specific Il6 knockdown (Fig. 5E). The expression of Il11 was also significantly inhibited following the inhibition of STAT3 activity after myocardial infarction (Fig. 5F). In addition, the expression of fibrosis-related genes induced by IL-6 was significantly reduced after interfering with the expression of Il11 in cardiac fibroblasts (Fig. 5G). These results suggest that IL-6 derived from fibroblasts promotes the expression of IL-11, and the pro-fibrotic function of IL-6 is reliant on the production of IL-11.

Figure 5. STAT3 promotes the expression of Il11 in fibroblasts. (A) IGV analysis of STAT3 (GSE211111) signals in the gene locus of Il11. (B) ChIP analysis of STAT3 enrichment level at the Il11promoter in isolated fibroblasts after MI (n = 4 per group). (C) Q-PCR analysis of Il11 mRNA expression in myocardial tissues from wild-type mice after MI or sham operation (n = 5 per group). (D) Q-PCR analysis of Il11 mRNA expression in fibroblasts treated with IL-6 or control PBS for 12 h (n = 5 per group). (E) Q-PCR analysis of Il11 mRNA expression in myocardial tissues from KO or WT mice after MI or sham operation (n = 5 per group). (F) Q-PCR analysis of Il11 mRNA expression in myocardial tissues from Ctrl or STAT3-IN-3 treated mice after MI (n = 5 per group). (G) Q-PCR analysis of Col1a1, Col3a1, Tgfb and Postn mRNA levels in fibroblast transfection with Il11 siRNA or control siRNA followed by treatment with IL-6 for 12 h (n = 5 per group). Data presented as mean ± SD. Unpaired Student’s t-test was performed. STAT3, signal transduction and activation of transcription 3; IGV, integrative genomics viewer; ChIP, chromatin immunoprecipitation; MI, myocardial infarction; IL-6, interleukin-6; KO, knockout; WT, wildtype; Ctrl, control.

Acute inflammation plays a crucial role in diverse cardiac injuries, including the participation in the regulation of cardiac remodeling after myocardial infarction [6-8]. In the present study, we found that there is high IL-6 expression in cardiac fibroblasts triggered by myocardial infarction. Similarly, DAMPs released from injured myocardial tissue significantly induced cardiac fibroblasts the production of IL-6. Interestingly, we identified potential pro-fibrotic signaling pathways mediated by IL-6 downstream effectors using fibroblast IL-6-deficient mice. Mechanistically, IL-6 signaling aggravates myocardial infarction-induced cardiac fibrosis, at least in part, by activating the Stat3-IL-11 pathway. The present study identified for the first time that IL-6 promotes IL-11 expression through activation of the Stat3 signaling pathway, which led to the discovery of the precise role of IL-6 in cardiac fibrosis.

Cardiac fibroblasts account for approximately 20% of non-myocytes in the normal mouse heart and are considered to be the predominant cell type in the cardiac fibrosis progression [9]. Transforming growth factor (TGF)-β, triggered by myocardial infarction, is a critical cytokine that is capable of stimulating fibroblasts and accelerating the production of ECM in injured tissues. TGF-β binds to TGF-β receptors and mediates downstream small mothers against decapentaplegic (SMADs)-dependent signaling cascade activation [10]. Activation of the TGF-β receptor leads to phosphorylation of the transcription factors Smad2 and Smad3 [11]. Subsequently, phosphorylated Smad2, Smad3, and cytoplasmic Smad4 form a complex and translocate into the nucleus, ultimately leading to transcriptional activation of fibrosis-related genes. In addition to reliance on the TGF-β-Samds signaling pathway, many other bypass pathways influence the progression of cardiac fibrosis. One of the critical intracellular signaling pathways is the PI3K/Akt/protein kinase B signaling pathway. Previous studies have shown that Apelin-13 inhibits the PI3K/Akt axis, thereby attenuating fibrosis in heart failure rats and AngII-induced cardiac fibroblasts [12]. Moreover, lysyl oxidase-like protein II promotes fibroblast-to-myofibroblast transformation, collagen fiber production, and mechanical strength through the PI3K/Akt/mTOR pathway [13]. In addition, the mitogen-activated protein kinase (MAPK) signaling pathway also plays an important role in the regulation of cardiac fibrosis. For example, the calcium-activated chloride channel protein anoctamin-1 promotes cardiac fibroblast proliferation and ECM secretion through the MAPK pathway [14]. In addition, a large number of studies have confirmed that cardiac fibrosis is closely related to the activation of signaling pathways, such as p38/STAT3, wnt/β-catenin, and so on [15,16]. With the deepening of research, new regulatory mechanisms of cardiac fibrosis will be revealed continuously. Consistent with previous study [17], our study reveals that fibroblasts also secrete large amounts of the inflammatory cytokine IL-6 after myocardial infarction or in response to DAMPs following myocardial infarction and confirms the important role of IL-6 produced by fibroblasts. We found that interfering with fibroblast IL-6 production significantly reduced fibroblast activation and inhibited pro-fibrotic cytokine IL-11 release mediated by STAT3.

IL-6 is a pleiotropic inflammatory cytokine that plays an important regulatory role in a variety of diseases. Previous studies have shown that immune cells produce large amounts of IL-6 after myocardial infarction, which promotes the progression of myocardial infarction and accelerates the onset of heart failure [4]. In recent years, it has also been demonstrated that IL-6 plays an important regulatory role in the fibrotic process after cardiac injury [18,19]. Anti-myocardial infarction therapies targeting IL-6 are potentially efficacious, such as a single dose of the IL-6 receptor antagonist Tocilizumab, which attenuates the inflammatory response in patients with non ST elevation myocardial infarction (NSTEMI) [20]. Notably, tocilizumab also induces a selective and substantial increase in plasma IP-10 and MIP-1beta in NSTEMI [21], suggesting that the use of anti-IL-6 therapies is complicated by the diversity of functions of this cytokine. The effects of various IL-6-targeting strategies on the failing heart have not been systematically addressed. Therefore, a more in-depth exploration of the regulatory mechanisms of IL-6 provides a potential theoretical basis for precise anti-IL-6 therapy after myocardial infarction. Most of the existing studies have concluded that IL-6 is mainly produced by immune cells such as macrophages. Recently, it has also been shown that fibroblasts are also an important source of IL-6 after myocardial infarction. However, it remains unknown what role fibroblast-derived IL-6 plays in the function of fibroblasts [17]. Our study confirms that targeting fibroblast IL-6 expression also significantly improves cardiac dysfunction after myocardial infarction, providing evidence to support more precisely targeted therapy after infarction.

IL-11 is a member of the IL-6 cytokine family [20,22]. This cytokine was originally thought to be an anti-inflammatory cytokine. The Food and Drug Administration approved it for the treatment of thrombocytopenia in chemotherapy patients. Still, recent studies have discovered a plethora of other functions, particularly in developmental processes, inflammatory diseases, and certain types of fibrosis [23-27].

Previous studies have shown that IL-11 is a downstream effector of TGF-β-mediated cardiac fibrosis and is critical for the development of cardiac fibrosis [28]. Prolonged induction of IL-11 in the uninjured heart results in the persistent activation of fibroblasts, which contribute to cardiac fibrosis. The production of IL-11 is augmented in pressure-overload heart disease and is tightly associated with adverse cardiac remodeling [29]. Inhibition of IL-11 prevents fibroblast activation and reduces the level of fibrosis and adverse cardiac remodeling [28]. Apart from cardiac fibrosis, IL-11 signaling also contributes to liver fibrosis and idiopathic pulmonary fibrosis [30,31]. Consistent with previous findings, our findings demonstrate that fibroblast-derived IL-6 promotes the production of IL-11 and exacerbates the progression of cardiac fibrosis, identifying IL-6 as a novel upstream regulator of profibrotic IL-11 in cardiac fibroblasts. Importantly, activation of STAT3 is required for IL-6-induced IL-11 production. IL-11 binds to the membrane-bound IL-11 receptor, thereby activating intracellular signaling cascades, including the Jak/STAT pathway. However, research has shown that IL11 can prevent cardiomyocyte apoptosis by activating STAT3 signaling in cardiomyocytes, which in turn exerts a protective effect on cardiac function [32]. These different results may be due to the fact that IL-11 derived from fibroblasts is more inclined to activate fibroblasts themselves via the autocrine pathway. Activation of these molecular signals may establish a positive feedback regulatory mechanism and exacerbate cardiac fibrosis after myocardial infarction. Therefore, a more profound understanding of the mechanisms of these factors and their effector cells would be more beneficial to the treatment of CVDs.

In conclusion, we elucidated an interplay between IL-6 and IL-11 and proposed a novel mechanism for controlling excessive cardiac fibrosis. We identified the effect and the triggering mechanism of fibroblasts on IL-6 production in myocardial tissue after myocardial infarction. We demonstrate the potent ability of IL-6 derived from fibroblasts to promote cardiac fibrosis and adverse cardiac remodeling. This study provides new molecular mechanisms for our understanding of cardiac fibrosis, which will be critical for the development of effective cardioprotective therapies against cardiac fibrosis.

This study was supported by grants from the National Natural Science Foundation of China (82070472).

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