Cardioprotective effects of the novel curcumin analogue C66 in diabetic mice is dependent on JNK2 inactivation

Abstract Aim Diabetic cardiomyopathy is an independent cardiac injury that can develop in diabetic individuals. Our previous study showed that C66, a curcumin analogue, protects against diabetes‐induced cardiac damage. The present study sought to reveal the underlying mechanisms of C66‐mediated cardioprotection. Methods An experimental diabetic model was established using JNK2−/− and wild‐type (WT) mice. C66 (5 mg/kg) was administered orally every other day for 3 months. Body weight, plasma glucose levels, cardiac function, and structure were measured. Masson trichrome and TUNEL staining were used to assess myocardial fibrosis and apoptosis, respectively. mRNA and protein levels of inflammation, fibrosis, oxidative stress, and apoptosis molecules were measured by quantitative PCR and Western blot, respectively. Results Neither C66 treatment nor JNK2 knockout affected body weight or plasma glucose levels. Cardiac inflammation, fibrosis, oxidative stress, and apoptosis were increased in WT diabetic compared to WT control mice, all of which were attenuated by C66 treatment. However, these pathological and molecular changes induced by diabetes were eliminated in JNK2−/− diabetic mice compared to JNK2−/− control mice, and C66 treatment did not further affect these parameters in JNK2−/− diabetic mice. Conclusions Our results indicate that C66 ameliorates diabetic cardiomyopathy by inhibiting JNK2 relative pathways.

Oxidative cellular damage contributes to diabetic cardiomyopathy pathogenesis. C-Jun N-terminal kinases (JNKs) are members of the mitogen-activated protein kinase (MAPK) family and are major signal transducers mediating the physiological response to cellular stressors. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a transcription factor that controls redox balance through regulating expression of many antioxidant and anti-inflammatory genes. 2 It has been reported that JNK inactivation can significantly activate Nrf2 and subsequently attenuate diabetes-induced inflammation, fibrosis, and oxidative stress. 3 Furthermore, JNK2 deficiency can protect the liver from oxidative injury induced by haemorrhage and resuscitation. 4 Diabetes-induced oxidative stress can trigger JNK activation, which is considered a critical factor in the pro-apoptotic signalling pathway. 5 Foxo3a, a key downstream molecule of JNK activation, can modulate various cellular processes, including cell survival, oxidative stress, and apoptosis. 6 Other studies have reported that Foxo3a is also regulated by the phosphoinositide 3-kinase (PI3K)/Akt pathway. Activated Akt phosphorylates and inactivates Foxo3a, traps Foxo3a in the cytoplasm, and impairs its transcriptional activity, 7 promoting cell survival. In contrast, if Akt is inhibited, Foxo3a can be activated by JNK2 and enter the nucleus with increased transcriptional activity 8 to induce apoptosis. Therefore, Foxo3a-induced apoptosis may depend on a competitive interaction with JNK2 and PI3K/Akt. C66, a new curcumin analogue, has anti-inflammatory, anti-fibrotic, antioxidative, and anti-apoptotic properties in diabetic mice, and also significantly suppresses JNK activity. 9,10 In addition, we found that C66 has a high affinity for JNK2 in rat renal proximal tubular cells under conditions of high glucose, suggesting that C66 may directly target JNK2. 11 Therefore, we suggested that the protective effects of C66 in diabetes-induced myocardial injury are mediated by inhibiting JNK2 function. To test this hypothesis, we established a type 1 diabetic model in JNK2 gene knockout (JNK2 −/− ) and wildtype (WT) mice and investigated whether JNK2 depletion abolishes the beneficial effects of C66 on cardiac injury.

| Blood glucose and echocardiography
Plasma glucose levels were measured using a glucometer (Accu- Chek; Roche, Basel, Switzerland) at 8:00 AM every 2 weeks. At 68 days after STZ injection, transthoracic echocardiography was performed under general anaesthesia using a small animal echo system

| Histology
Hearts were fixed in 4% paraformaldehyde, paraffin-embedded, and cut into 4-μm-thick sections. The sections were deparaffinized and stained with haematoxylin and eosin and Masson's trichrome to determine morphology and myocardial fibrosis, respectively.

| Real-time quantitative PCR
Total RNA was extracted from heart tissues using the AxyPrep ™ multisource total RNA kit (Axygen Scientific, Inc., Hangzhou, China).
RNA was reverse transcribed to cDNA using the TransScript All-in-One first-strand cDNA synthesis SuperMix (Transgen Biotech, Inc., Beijing, China). Real-time PCR was performed using the TransStart Top Green qPCR SuperMix (Transgen Biotech, Inc.) and the ABI 7300 Real-Time qPCR system. All PCR experiments were performed in triplicate. The primers were purchased from Sangon Biotech (Shanghai, China) and the sequences are listed in Table 1.

| Western blot
Homogenized heart tissues were lysed and protein concentrations were measured using the Enhanced BCA Protein Assay Kit (Beyotime, Inc., Shanghai, China) . Lysates were separated by SDS-PAGE and then transferred onto 0.45 μm polyvinyldene difluoride membranes. Each membrane was pre-incubated in tris-buffered saline (TBS) containing 5% non-fat milk for 1 hour at room temperature and then incubated with specific primary antibodies overnight at 4°C. After three washes in TBS with Tween 20, the membranes were incubated with secondary antibodies conjugated to horseradish peroxidase, and immunoreactive bands were visualized using the enhanced chemiluminescence reagent (Immobilon ECL Ultra Western HRP Substrate; Millipore Sigma, Inc., Billerica, MA, USA). Image J was used to analyse protein band density.

| TUNEL assay
Tissues were fixed with formaldehyde, and antigen retrieval was performed by heat mediated citrate buffer, pH6. The tissues were incubated with 0.5% tritonX-100 for 15 minutes at room temperature, and 1% BSA for 30 minutes at 37°C. The tissues were incubated with sarcomeric α-actinin antibody at 1/200 dilution for 16 hours at 4°C. Secondary antibody was used at 1: 350 (coloured red). Apoptosis was measured using a TUNEL staining kit (DeadEnd TM Colorimetric TUNEL System; Promega, Inc., Madison, WI, USA) according to the manufacturer's instructions. Briefly, paraffin sections were dewaxed and incubated with TdT and fluorescein-labelled dUTP for 60 minutes at 37°C.
Nuclei were counterstained with DAPI. Images were acquired using confocal microscopy. Results were analysed using Image J. All assays and analyses were performed in a blind manner.

| Statistical analysis
Data are presented as means ± SD for each group. Comparisons among groups were performed using one-way ANOVA with post hoc pairwise repetitive comparisons using Tukey test with Origin 8.0 Lab data analysis and GraphPad Prism 6.0 graphing software. Statistical significance was considered as P < 0.05.

| Body weight, blood glucose, and heart weight
Body weights of both WT ( Figure 1A) and JNK2 −/− ( Figure 1B) diabetic mice were significantly decreased by the fourth week. Blood glucose levels of both WT ( Figure 1C) and JNK2 −/− ( Figure 1D) diabetic mice were significantly increased by the second week. Body weights and blood glucose levels were similar between WT and JNK2 −/− diabetic mice following C66 treatment, suggesting that C66 does not affect weight gain and glycaemic control. At 3 months, heart weight to tibia length ratios were lower in the WT diabetic mice compared to the WT control group (P < 0.05), and C66 treatment slightly increased this ratio ( Figure 1E). In contrast, there was no difference in the heart weight to tibia length ratios between the diabetic and control JNK2 −/− mice, and C66 treatment did not alter the ratio in either group ( Figure 1E).

| Cardiac remodelling and dysfunction
Cardiac structure and function were evaluated by transthoracic echocardiography (Figure 2A-F). The systolic functional parameters, EF and FS, were significantly lower in WT diabetic mice compared to WT control mice (both P < 0.05, Figure 2A

| Phosphorylation of JNK
The phosphorylation levels of JNK are representative of its activity. Phosphorylated (p)-JNK was significantly reduced in the hearts of WT  diabetic mice compared to the WT control mice (P < 0.05, Figure 3), and this affect was reversed by C66 treatment (P < 0.05, Figure 3), indicating that C66 can inhibit JNK activity. However, there was no significant change in p-JNK between JNK2 −/− diabetic and control mice ( Figure 3). C66 treatment did not affect p-JNK levels in diabetic JNK2 −/− mice ( Figure 3). These results indicate that JNK knockout decreases JNK activity in the hearts of diabetic mice and that C66 inhibits diabetes-induced p-JNK through inactivating JNK2.

| Cardiac oxidative stress
Reactive oxygen species (ROS) production is increased in diabetes, resulting in enhanced oxidative stress, lipid peroxidation, and protein oxidization, which contribute to diabetic cardiomyopathy pathogenesis. We found that MDA cardiac levels were significantly higher in the WT diabetic mice compared to the WT control mice, which were reversed by C66 treatment (P < 0.05, Figure 6A). Nrf2 was significantly down-regulated, while HO-1, NQO-1 and SOD-1 were slightly up-regulated, in the WT diabetic mice compared to WT control mice, all of which were increased after C66 treatment (all P < 0.05, Figure 6C-H). There were no significant difference in the expression of these four molecules between JNK2 −/− diabetic mice and JNK2 −/− mice administered C66 ( Figure 6C-H).

| Cardiomyocyte apoptosis
We used the TUNEL assay to analyse apoptosis of myocardial cells (cardiomyocytes were marked by sarcomeric α-actinin staining in red) and Western blot to measure expression of apoptosis-related proteins, including cleaved caspase-3, Bax, and Bcl-2. TUNEL staining showed that there were more apoptotic cells in the WT diabetic mice compared to the WT control mice, and C66 treatment decreased apoptosis (both P < 0.05, Figure 7A). Apoptosis was neither abundant nor different among the four JNK2 −/− groups ( Figure 7A). Interestingly, the hearts from the JNK2 −/− diabetic mice had significantly less apoptotic cells compared to the WT diabetic mice (P < 0.05, Figure 7A). Similar results were obtained for cleaved caspase-3, Bax, and Bcl-2 ( Figure 7B).

| PI3K/Akt/Foxo3a survival pathway
To investigate whether C66 affects activation of the PI3K/Akt/Fox-o3a pathway, we measured p-PI3K, p-Akt, and p-Foxo3a in heart tissue by Western blot. As depicted in Figure 8A-D, we found that diabetes down-regulated phosphorylation of all three proteins in WT mice, which was reversed by C66 treatment (all P < 0.05). However, these changes were not observed in the JNK2 −/− groups. In addition, there were significant increases in p-PI3K, p-Akt, and p-Foxo3a in the JNK2 −/− diabetic mice compared to the WT diabetic mice (all P < 0.05).

| DISCUSSION
The present study demonstrates that the protective effect of C66 in diabetes-induced cardiac injury is mediated by inhibiting JNK2 activity. Our data show that diabetes-induced cardiac inflammation, fibrosis, oxidative damage, and apoptosis can be prevented by C66 treatment and that JNK2 deficiency produces similar effects. These results suggest that C66 exerts its cardioprotective effects by inhibiting JNK2 function and activating the Akt/Foxo3a survival pathway.
This study defined the effects of JNK2 function inhibition on diabetes-induced myocardial damage. Specifically, we found that C66 can prevent increased phosphorylated JNK in WT diabetic mice.
However, phosphorylated JNK was not significantly increased in JNK2 −/− diabetic mice, and therefore C66 treatment has no effect on JNK2 −/− diabetic mice. In addition, C66 treatment prevented diabetes-induced myocardial remodelling, fibrosis, inflammation, oxidative damage, and apoptosis in WT but not JNK2 −/− diabetic mice.
These results indicate that C66 protects the heart from diabetic cardiomyopathy via targeting JNK2.
Our previous studies demonstrated that C66 inhibits inflammation, fibrosis, and oxidative stress, which is accompanied by JNK activity inhibition. 10,11 Evidence has shown that JNK2 activation increases metabolism-related inflammation, fibrosis, and oxidative stress. [12][13][14] Consistent with these findings, our present study demonstrated that both C66 treatment and JNK2 knockout markedly reduced diabetic-induced cardiac inflammation, fibrosis, and oxidative stress; however, C66 had no effect on JNK2 −/− diabetic mice. Thus, our data demonstrate that inhibition of JNK2 activity can protect against diabetes-induced myocardial injury, and C66 protects the heart from diabetic cardiomyopathy by inhibiting JNK2 activity.
Oxidative stress is a key mechanism by which diabetes induces diabetic cardiomyopathy. High glucose concentrations cause an imbalance between production and elimination of ROS, resulting in enhanced oxidative stress, which is accompanied by cardiac inflammation and fibrosis. 15 [16][17][18] It has been reported that high glucose can inhibit p-Akt and suppress nuclear translocation of Foxo3a, promoting Fox-o3a-dependent apoptosis in diabetic cardiomyopathy. 19 In addition, Foxo3a is regulated by JNK, which has been shown to induce Fox-o3a activity and nuclear localization by repressing p-Akt. 20 Moreover, Davila et al reported that JNK2 is necessary for Foxo3a activation. 8 We speculated that Foxo3a activation was competitively regulated by both JNK signalling (most likely by JNK2) and PI3K/Akt signalling pathways, both of which have been shown to be down-regulated in diabetes. The crosstalk between these signalling cascades and the inactivation of Foxo3a may be crucial factors in cardiac apoptosis. Diabetes downregulates the phosphorylation of PI3K, Akt, and Foxo3a, all of which were reversed by C66 in WT mice, but not in JNK2 -/mice, indicating that C66 activation of these enzymes is mediated by JNK2 activity inhibition.
JNK2 thus competes with PI3K/Akt to regulate the Foxo3a-depending survival pathway.
In summary, we found that the cardioprotective effects of C66 are dependent on JNK2. C66 protects against diabetic cardiomyopathy via inhibiting JNK2 activation, resulting in decreased cardiac inflammation, fibrosis, oxidative stress, and apoptosis. Furthermore, we found an interaction between the JNK and PI3K/Akt pathways in regulating the Foxo3a-dependent survival. These results will be valuable for developing strategies to prevent and treat diabetic cardiomyopathy.