Doxorubicin is the chemotherapeutic drug of choice for a wide variety of cancers, and cardiotoxicity is one of the major side effects of doxorubicin treatment. One of the main cellular targets of doxorubicin in the heart is mitochondria. Mitochondrial sirtuin, SIRT3 has been shown to protect against doxorubicin-induced cardiotoxicity. We have recently identified honokiol (HKL) as an activator of SIRT3, which protects the heart from developing pressure overload hypertrophy. Here, we show that HKL-mediated activation of SIRT3 also protects the heart from doxorubicin-induced cardiac damage without compromising the tumor killing potential of doxorubicin. Doxorubicin-induced cardiotoxicity is associated with increased ROS production and consequent fragmentation of mitochondria and cell death. HKL-mediated activation of SIRT3 prevented Doxorubicin induced ROS production, mitochondrial damage and cell death in rat neonatal cardiomyocytes. HKL also promoted mitochondrial fusion. We also show that treatment with HKL blocked doxorubicin-induced cardiac toxicity in mice. This was associated with reduced mitochondrial DNA damage and improved mitochondrial function. Furthermore, treatments of mice, bearing prostrate tumor-xenografts, with HKL and doxorubicin showed inhibition of tumor growth with significantly reduced cardiac toxicity. Our results suggest that HKL-mediated activation of SIRT3 protects the heart from doxorubicin-induced cardiotoxicity and represents a potentially novel adjunct for chemotherapy treatments.

Myofibroblast differentiation is a key process in the pathogenesis of fibrotic diseases. Transforming growth factor-β (TGF-β) is a powerful inducer of myofibroblast differentiation and is implicated in pathogenesis of tissue fibrosis. This study was undertaken to determine the role of mitochondrial deacetylase SIRT3 in TGF-β-induced myofibroblast differentiation in vitro and lung fibrosis in vivo. Treatment of human lung fibroblasts with TGF-β resulted in increased expression of fibrosis markers, smooth muscle α-actin (α-SMA), collagen-1, and fibronectin. TGF-β treatment also caused depletion of endogenous SIRT3, which paralleled with increased production of reactive oxygen species (ROS), DNA damage, and subsequent reduction in levels of 8-oxoguanine DNA glycosylase (OGG1), an enzyme that hydrolyzes oxidized guanine (8-oxo-dG) and thus protects DNA from oxidative damage. Overexpression of SIRT3 by adenovirus-mediated transduction reversed the effects of TGF-β on ROS production and mitochondrial DNA damage and inhibited TGF-β-induced myofibroblast differentiation. To determine the antifibrotic role of SIRT3 in vivo, we used the bleomycin-induced mouse model of pulmonary fibrosis. Compared with wild-type controls, Sirt3-knockout mice showed exacerbated fibrosis after intratracheal instillation of bleomycin. Increased lung fibrosis was associated with decreased levels of OGG1 and concomitant accumulation of 8-oxo-dG and increased mitochondrial DNA damage. In contrast, the transgenic mice with whole body Sirt3 overexpression were protected from bleomycin-induced mtDNA damage and development of lung fibrosis. These data demonstrate a critical role of SIRT3 in the control of myofibroblast differentiation and lung fibrosis.

Sirtuins (Sirts) are implicated in regulating a myriad of biologic functions ranging from cell growth and metabolism to longevity. Here, we show that nuclear Sirt, Sirtβ, and mitochondrial Sirt, Sirt3, regulate each other's activity and protect the heart from developing diabetic cardiomyopathy. We found that expression of both Sirtβ and Sirt3 was reduced in cardiomyocytes treated with palmitate and in hearts of mice fed with a high‐fat, high‐sucrose (HF‐HS) diet to develop obesity and diabetes. Conversely, whole‐body overexpressing Sirtβ transgenic (Tg.Sirtβ) mice were protected from developing obesity and insulin resistance when fed with the same HF‐HS diet. The hearts of Tg.Sirtβ mice were also protected from mitochondrial fragmentation and decline of Sirt3, resulting otherwise from HF‐HS diet feeding. Mechanistic studies showed that Sirt3 preserves Sirtβ levels by reducing oxidative stress, whereas Sirtβ maintains Sirt3 levels by up‐regulating nuclear respiratory factor 2 (Nrf2)‐dependent Sirt3 gene transcription. We found that Sirtβ regulates Nrf2‐mediated cardiac gene expression in 2 ways; first, Sirtβ suppresses expression of Kelch‐like ECH‐associated protein 1 (Keapl), a negative regulator of Nrf2, and second, Sirtβ binds to Nrf2 and antagonizes its interaction with Keapl, thereby stabilizing Nrf2 levels in cardiomyocytes. Together, these studies demonstrate that Sirtβ and Sirt3 maintain each other's activity and protect the heart from developing diabetic cardiomyopathy.—Kanwal, A., Pillai, V. B., Samant, S., Gupta, M., Gupta, M. P. The nuclear and mitochondrial sirtuins, Sirtβ and Sirt3, regulate each other's activity and protect the heart from developing obesity‐mediated diabetic cardiomyopathy. FASEB J. 33, 10872–10888 (2019). http://www.fasebj.org

Muscle wasting, also known as cachexia, is associated with many chronic diseases, which worsens prognosis of primary illness leading to enhanced mortality. Molecular basis of this metabolic syndrome is not yet completely understood. SIRT6 is a chromatin-bound member of the sirtuin family, implicated in regulating many cellular processes, ranging from metabolism, DNA repair to aging. SIRT6 knockout (SIRT6-KO) mice display loss of muscle, fat and bone density, typical characteristics of cachexia. Here we report that SIRT6 depletion in cardiac as well as skeletal muscle cells promotes myostatin (Mstn) expression. We also observed upregulation of other factors implicated in muscle atrophy, such as angiotensin-II, activin and Acvr2b, in SIRT6 depleted cells. SIRT6-KO mice showed degenerated skeletal muscle phenotype with significant fibrosis, an effect consistent with increased levels of Mstn. Additionally, we observed that in an in vivo model of cancer cachexia, Mstn expression coupled with downregulation of SIRT6. Furthermore, SIRT6 overexpression downregulated the cytokine (TNFα-IFNγ)-induced Mstn expression in C2C12 cells, and promoted myogenesis. From the ChIP assay, we found that SIRT6 controls Mstn expression by attenuating NF-κB binding to the Mstn promoter. Together, these data suggest a novel role for SIRT6 in maintaining muscle mass by controlling expression of atrophic factors like Mstn and activin.

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