In the present study, it was shown that resveratrol induced both autophagy and apoptosis while inhibiting HSC activation. Furthermore, inhibiting autophagy with CQ and 3-MA or inhibiting apoptosis with Z-VAD-FMK reversed the antifibrotic effects of resveratrol. These results showed that resveratrol inhibited HSC activation by inducing autophagy and apoptosis in a dose-dependent manner, and the mechanism may be associated with the SIRT1 and JNK signaling pathways.
A large amount of evidence suggests that resveratrol exerts dose-dependent effects and even contradictory effects on various cell types or diseases. For example, in renal fibrosis, Liu et al. [11] showed that resveratrol activated antifibrotic or profibrotic effects on kidneys depending on the dose. In vivo, low-dose treatment with resveratrol (≤ 25 mg/kg) partly improved renal function, whereas high-dose treatment with resveratrol (≥ 50 mg/kg) had no antifibrotic effect and even exacerbated renal fibrosis. In addition, resveratrol at concentrations greater than 25–50 µM typically trigger growth arrest, senescence and/or apoptosis in different cell types. In contrast, concentrations less than 10 µM enhance the growth of log phase cell cultures and can rescue senescence in multiple types of human fibroblasts [42].
There are also conflicting reports on HSC activation due to different concentrations of resveratrol. Resveratrol (15 µM) upregulated the mRNA expression of α-smooth muscle actin (α-SMA) and amplified the profibrogenic effects of free fatty acids on LX-2 cells (human HSC line) [43]. In contrast, resveratrol decreased t-HSC/Cl-6 cell (rat HSC line) viability at very low concentrations (1.56 µM) at 24 h [28]. de Oliveira et al. [29] suggested that resveratrol at 1 and 10 µM did not alter the protein levels of α-SMA, collagen I and glial fibrillary acidic protein (GFAP) in GRX cells (mouse HSC line). Resveratrol at 10 and 50 µM decreased GRX cell migration and collagen I contraction. Consistent with previous studies, the results of the present study showed that resveratrol exerted dose-dependently supported or inhibited JS1 cell proliferation (Fig. 1a). Furthermore, the effect of resveratrol on HSC was not only dose-dependent but also time-dependent. Martins et al. [6] reported that treating GRX cells with 50 µM resveratrol for 24 h induced oxidative stress-related damage, drastically reducing cell viability, but this cytotoxicity seemed to be attenuated after 120 h. The complex effects of resveratrol on promoting or preventing HSC activation may be dependent on different cell types, concentrations, and durations of treatment.
In the present study, resveratrol inhibited the mRNA and protein expression of Col. I. However, the results were inconsistent when JS1 cells were treated with 10 µM resveratrol. As shown in Figs. 1a-c and 2a, 10 µM resveratrol promoted cell viability, increased LC3BII expression and significantly inhibited Col. I protein expression but had no effect on Col. I mRNA expression. Therefore, we concluded that resveratrol promoted intracellular Col. I degradation through autophagy rather than inhibiting its production in JS1 cells. Autophagy is an important protein degradation pathway. A large amount of research has shown that Col. I, which is the major component of the ECM, is degraded by the autophagy–lysosome system [44, 45]. For example, Kawano et al. [45] showed that internalized collagen accumulated in autophagy-deficient murine embryonic fibroblasts. Kim et al. [46] reduced Beclin1 expression through genetic disruption of Beclin1 or knockdown with specific siRNA in primary mouse mesangial cells and showed that this resulted in increased protein levels of Col. I. Moreover, autophagy exerts antifibrotic effects and enhances collagen degradation in intestinal fibroblasts [47], cardiac fibroblasts [48], and lung fibroblasts [49]. However, other studies reported contrasting results. Nakamura et al. [50] showed that autophagy facilitated type I collagen synthesis in periodontal ligament cells. Additionally, inhibiting autophagy promoted collagen degradation in pancreatic stellate cells [51]. The complex role of autophagy in promoting or inhibiting collagen degradation may be dependent on different cell types.
Autophagy and apoptosis are well-controlled biological processes that determine the cell death or survival and the development of diseases. Accumulating evidence reveals that autophagy and apoptosis can cooperate, antagonize, or assist each other, and the crosstalk between these processes is complex [22]. As shown in Fig. 2b, both high and low concentrations of resveratrol induced autophagy and apoptosis successively at different times. The autophagic response occurs earlier than apoptotic cell death, placing autophagy upstream of apoptosis. Autophagy may be an adaptive stress response prior to apoptotic cell death. Furthermore, autophagy can induce or antagonize apoptosis [23]. Autophagy antagonizes apoptosis and promotes cell survival via a series of responses to damaged organelles, endoplasmic reticulum stress, DNA stability, or loss of nutrient and growth factor signaling pathways. Conversely, autophagic activation beyond a certain threshold may result in the collapse of cellular function, directly resulting in autophagic cell death or the execution of apoptotic cell death via common regulators [23]. In conclusion, resveratrol induced autophagy and apoptosis in JS1 cells, and crosstalk may occur between these processes. Resveratrol increased autophagic flux and may directly induce autophagic cell death, while the overactivation of autophagy contributes to apoptotic cell death.
As previously reported, the SIRT1 signaling pathway is involved in the regulation of autophagy and apoptosis and has been studied as part of the mechanism of resveratrol in various diseases [36–38]. Previous studies have shown that SIRT1 increases autophagic flux, and this effect may be mediated by the class III PI3K/Beclin1 and mTOR signaling pathways [34, 52, 53]. Moreover, p53 deacetylation by SIRT1 has been confirmed to play a significant role in inhibiting apoptosis [54]. As shown in Fig. 3a, our findings suggested that SIRT1 signaling induced autophagy to participate in the antifibrotic effects of resveratrol, which were probably attributable to autophagy-mediated promotion of Col. I degradation in HSCs. In addition, we observed that the SIRT1 inhibitor EX527 upregulated the expression of cleaved caspase3 induced by resveratrol in JS1 cells (Fig. 3a). That is, resveratrol inhibited apoptosis by inducing autophagy through the SIRT1 signaling pathway, which may be attributed to autophagy antagonizing apoptosis. Autophagy may partially counteract apoptotic cell death via the cell survival pathway at an earlier stage. Similar results were recently reported in which resveratrol induced SIRT1-dependent autophagy to prevent apoptosis in VSC4.1 motoneurons [55], intestinal epithelial cells [56] and human trophoblasts [57].
JNK signaling may play critical roles in many biological diseases by regulating various cellular processes, including inflammatory responses, differentiation, proliferation, death, and survival [58]. JNKs are among the most crucial MAPKs and include JNK1, JNK2, and JNK3. All three JNKs have been shown to be involved in stimulating apoptotic signaling [41]. A general mechanism through which JNKs modulate the apoptotic pathway involves stimulating the expression of proapoptotic genes and decreasing the expression of prosurvival genes via multiple transcription factors. In addition to their effect on gene expression, JNKs actively regulate both the intrinsic and extrinsic apoptotic pathways [41]. Furthermore, the JNK signaling pathway has also been reported to participate in autophagy regulation in response to various stress signals. JNK induces autophagic cell death by enhancing the expression of autophagy-related genes, including Atg5, Atg7, LC3, and Beclin1 [41]. In the present study, the role of the JNK pathway in resveratrol-induced autophagy and apoptosis was assessed. We observed that the JNK inhibitor SP600125 could attenuate autophagy and apoptosis in JS1 cells induced by resveratrol. Resveratrol exerted antifibrotic effects by mediating the JNK pathway and inducing apoptosis and autophagy in HSCs. A similar result was reported in cardiac cells. Xu et al. [59] reported that resveratrol protected cardiac cells by regulating the switch between autophagy and apoptotic machinery under diabetic conditions, and this effect was associated with JNK-mediated Beclin1-Bcl-2 dissociation. Bcl-2 is an important mediator of JNK-mediated autophagy and apoptosis activation. Activated JNK phosphorylates Bcl-2/Bcl-xL to release Beclin1 from the Beclin1-Bcl-2/Bcl-xL complex and promote autophagy. Sustained Bcl-2 phosphorylation not only leads to Beclin1-mediated autophagic cell death but also antagonizes the antiapoptotic activity of Bcl-2/Bcl-xL via a Bid-Bax-dependent mechanism [58, 60, 61].
In conclusion, the present study showed that resveratrol inhibited HSC activation by regulating autophagy and apoptosis by activating the SIRT1 and JNK signaling pathways. SIRT1 may be responsible for inducing autophagy, while JNK affects both autophagy and apoptosis. This study highlighted autophagy and apoptosis as therapeutic targets by which resveratrol can attenuate fibrosis. Additionally, these findings may provide a new framework for understanding the mechanism by which resveratrol inhibits HSC activation. However, autophagy and apoptosis may act independently on the SIRT1 and JNK pathways or may influence one another, which requires further study.