Targeting SIRT3 to prevent cardiac hypertrophy

Anti-hypertrophic and anti-fibrotic

NAD-dependent deacetylase sirtuin-3, mitochondrial also known as SIRT3 is a protein that in humans is encoded by the SIRT3 gene sirtuin (silent mating type information regulation 2 homolog) 3. SIRT3 is a member of the mammalian sirtuin family of proteins, which are homologs of the Sir2 protein found in yeast. Like SIRT1 and SIRT2, SIRT3 exhibits NAD+- dependent deacetylase activity, this dependence on NAD+ directly links their activity with the metabolic status of the cell.

The activity of SIRT3 depends on the availability of NAD+, a required cofactor for functional deacetylase activity of the enzyme. The ratio of NAD+ to NADH is directly linked to cellular energy status, implicating SIRT3 as a key metabolic sensor (1).

SIRT3 has received considerable attention for its role in metabolism and aging and there have been a number of research studies into Honokiol and its influence on the levels of SIRT3 present in the system. These studies are of interest as until recently the only other known method to increase SIRT3 was via caloric restriction and exercise. This makes targeting SIRT3 a potential caloric restriction mimetic, with the various benefits to mitochondrial function, aging, and neurodegeneration that go with it (2).

A 2015 study focused on SIRT3 in relation to its effect on mitochondrial function and how cell stress resistance is influenced by SIRT3 levels. Their results suggest a potential therapeutic target for protecting against age-related neurodegenerative diseases and cognitive decline by increasing levels of SIRT3 (3).

The Sirtuin family is widely considered to increase lifespan across various species, though it remains unknown if sirtuins can robustly reverse age-related degeneration. There is however an increasing amount of scientific data demonstrating a number of health benefits and some research suggesting Honokiol could be a potential therapeutic agent for age-related diseases.

Cardiac hypertrophy is the thickening of the heart muscle (myocardium) which results in the decrease of size of the chambers of the heart, including the left and right ventricles. A common cause of cardiac hypertrophy is hypertension (high blood pressure) and narrowing of the aortic valve (heart valve stenosis).

Fibrosis increases as we age and is the formation of excess fibrous connective tissue in an organ or tissue in a reparative or reactive response to injury or pathological state.

Today we are going to take a look at an interesting honokiol study from 2015 and see if we can reach a conclusion regarding the influence of Honokiol over SIRT3 (5). The study deals with cardiac hypertrophy and fibrosis, both consequences of aging.

Honokiol blocks and reverses cardiac hypertrophy in mice by activating mitochondrial Sirt3

In a 2015 study researchers showed that honokiol was able to increase in-vitro SIRT3 expression almost two-fold and that it was also present in the mitochondria. The findings of the research team also shows that honokiol binds to SIRT3 and upregulates its activity as the diagram below explains.

As we can see from this diagram, in cardiomyocytes honokiol increases the level of SIRT3 expression which promotes the deacetylation of mitochondrial targets, including MnSOD. This in turn leads to the reduced creation of reactive oxygen species (ROS) and thusly reduced levels of cellular oxidative stress.

The activated SIRT3 also activates peroxisome proliferator-activated receptor γ coactivator 1a (PGC1α). PGC1α is a positive regulator of mitochondrial biogenesis and respiration, adaptive thermogenesis, gluconeogenesis as well as many other metabolic processes and is an essential metabolic pathway central to maintaining homeostasis as well as important in aging (6). Increasing levels of PGC1α activates the SIRT3 gene promoter, which leads to an increased synthesis of Sirt3 mRNA transcripts. The resulting increase in SIRT3 activity then prevents cardiac hypertrophy by inhibiting ROS production and Akt (also known as Protein kinase B (PKB)) activation as the same research showed in an earlier study (7).

In-vivo results

Whilst the above is interesting, these are as previously noted the results from in-vitro testing. The same research team also conducted in-vivo testing in mice and demonstrated that honokiol was also able to increase SIRT3 levels as a result of supplementing.

The researchers tested mice on mice that had undergone induced cardiac hypertrophy and found that supplementation with honokiol was able to increase SIRT3 levels significantly in comparison to a similar control group without supplementation. The study showed that the increased level of SIRT3 activity was due to the reduced acetylation of the mitochondrial SIRT3 substrates, MnSOD and OSCP, thus halting the development of cardiac hypertrophy.

The other important discovery by the research team was the ability of honokiol to inhibit cardiac fibrosis via a SIRT3 dependent mechanism. They found that cardiac fibroblasts treated with honokiol resulted in a dose dependent reduction of cell proliferation. The researchers noted there was no appreciable level of toxicity, suggesting that the ability of honokiol to reduce proliferation is not based on a cytotoxic action. The researchers first tested in-vitro using SIRT3 knockout fibroblasts which do not express SIRT3, these fibroblasts rapidly differentiate into myofibroblasts, and application of honokiol has no effect on this process, this strongly suggested that honokiol relies on SIRT3 for its anti-fibrotic effect.

To confirm this in-vivo the research team treated SIRT3 knockout mice with honokiol and it failed to protect them from developing cardiac hypertrophy and fibrosis.

Further confirmation of this was also demonstrated, where they treated mice with honokiol and saw they developed significantly less fibrosis compared to control mice. You can see from the study data here that the mice subjected to TAC (transverse aortic constriction) for 28 days to induce cardiac hypertrophy have high levels of fibrosis, however, in TAC mice treated with honokiol those levels are reduced significantly. The control mice and honokiol test group (without TAC) also have similar low levels of fibrosis.

Conclusion

Taken together these study results show that honokiol is able to increase SIRT3 levels both in-vivo and in-vivo as well as helping to prevent cardiac hypertrophy and organ fibrosis. As Sirt3 regulates many aspects of mitochondrial function and as oxidative stress is a primary cause for the development of various pathology, it is plausible that honokiol could be beneficial in the management of a wide variety of diseases. Thus honokiol is of considerable interest to the research community as a therapy for various diseases and for its potential as a dietary supplement as a preventative.

 

References

(1) Kim, S. C., Sprung, R., Chen, Y., Xu, Y., Ball, H., Pei, J., … & Grishin, N. V. (2006). Substrate and functional diversity of lysine acetylation revealed by a proteomics survey. Molecular cell, 23(4), 607-618.

(2) Kincaid, B., & Bossy-Wetzel, E. (2013). Forever young: SIRT3 a shield against mitochondrial meltdown, aging, and neurodegeneration. Frontiers in aging neuroscience, 5, 48.

(3) Cheng, A., Yang, Y., Zhou, Y., Maharana, C., Lu, D., Peng, W., … & Bohr, V. A. (2016). Mitochondrial SIRT3 mediates adaptive responses of neurons to exercise and metabolic and excitatory challenges. Cell metabolism, 23(1), 128-142.

(4) Brown, K., Xie, S., Qiu, X., Mohrin, M., Shin, J., Liu, Y., … & Chen, D. (2013). SIRT3 reverses aging-associated degeneration. Cell reports, 3(2), 319-327.

(5) Pillai, V. B., Samant, S., Sundaresan, N. R., Raghuraman, H., Kim, G., Bonner, M. Y., … & Gupta, M. P. (2015). Honokiol blocks and reverses cardiac hypertrophy in mice by activating mitochondrial Sirt3. Nature communications, 6.

(6) Sahin, E., & DePinho, R. A. (2012). Axis of ageing: telomeres, p53 and mitochondria. Nature reviews Molecular cell biology, 13(6), 397-404.

(7) Sundaresan, N. R., Gupta, M., Kim, G., Rajamohan, S. B., Isbatan, A., & Gupta, M. P. (2009). Sirt3 blocks the cardiac hypertrophic response by augmenting Foxo3a-dependent antioxidant defense mechanisms in mice. The Journal of clinical investigation, 119(9), 2758-2771.