Stress-defying molecules help cells fend off ageing

Eureka moments usually arrive before scientists write up their findings, but for Nadine Pernodet, it was the reverse. A biologist and senior vice president of Estée Lauder Companies (ELC) Research & Development division, Pernodet was writing up her lab’s results on a family of anti-ageing proteins called sirtuins when she came across a paper¹ describing how one of these proteins localizes to the cell’s cytoskeleton. A light bulb went off as Pernodet realized the potential link. “It just hit me,” she recalls. “Maybe that's the key to what we're looking for.”

She was right. Preliminary research by ELC seems to suggest that this sirtuin, called SIRT2, helps protect cells against the effects of ageing by maintaining their structural integrity and thus their function.² SIRT2 activity falls as we age, but Pernodet’s team’s ongoing work suggests that increasing it reduces the signs of cellular ageing and helps support skin’s natural mechanical properties.

These findings will be published soon, and are the latest in a series of discoveries Pernodet’s team has made about the role of sirtuins in the biology of the skin, and how they might be modulated with therapies to combat visible ageing.

“Holding back the years by altering sirtuin activity is a realistic prospect,” says Danica Chen, who studies sirtuins as a professor of metabolic biology, nutritional sciences and toxicology at the University of California, Berkeley.

Until now, however, most research interest has focused on their role in metabolism and stem cell biology, leaving the skin as an underserved area, she adds.

What are sirtuins?

Sirtuins are a family of enzymes that catalyse the removal of acyl groups from intracellular proteins. Scientific interest in them intensified in the early 2000s when discoveries linked sirtuin activity to metabolism and longevity.³

The founding member of the family, SIRT1, acts on histones in the nucleus to silence genes. A further six sirtuins have been found in mammals, including humans. They are involved in a wide range of processes, including genomic stability, inflammation, and metabolism, but their general function is to help cells resist stress.

“Cellular damage and stress is a general cause of ageing,” says Chen. “So what we need is something like the sirtuins that can sense stress and help improve cell survival and protection.”

Putting theory into action

In 2005, ELC started a collaboration with Leonard Guarente, a biologist at MIT, who made the seminal discoveries on sirtuins. The team focused on SIRT1, which at the time was the best characterized member of the family. It was known to silence genes by deacetylating histones and was reported⁴ to be activated by resveratrol, a small molecule found in grape skin.

Pernodet’s team found that in keratinocytes, SIRT1 promoted differentiation⁵ and inhibited cell proliferation, a key process in maintaining skin integrity. This is an example of how the team’s findings are not only exploring the little-known biology of sirtuins in the skin, but also feeding back to the sirtuin field more broadly, says Chen. Further unpublished experiments in skin cells hope to confirm findings made in other cell types that SIRT1 also reduced DNA damage, one of the hallmarks of ageing.

SIRT1 activity naturally declines with age, so the team developed a stable form of resveratrol, called resveratrate, to help restore more youthful levels of SIRT1 activity.

This aligns with the general aim of the field of anti-ageing research, which is to maximize overall wellness in later life. “The ultimate goal of ageing research is not about making us live for 500 years,” Chen says, “rather, our goal is to understand how to make the period of deteriorating health shorter and less severe.”

Meanwhile, Pernodet’s team set their sights on the remaining sirtuins. “Ageing is a multifactorial process,” she says. “Sirtuins are involved in so many critical pathways that we know are going to help to sustain and support healthy and youthful looking skin.”

Ageing and beyond

One driver of ageing is oxidative damage. This increases with age as the cell’s mitochondria produce more free radicals as a byproduct of cellular respiration. With this in mind, Pernodet focused on a mitochondrial sirtuin, SIRT3, which was already known⁶ to reduce oxidative stress.

The team showed that in skin cells, SIRT3 is sensitive to the environment. After UV exposure, SIRT3 expression reduced, resulting in less energy production and more damaging free radicals. They also showed, for the first time,⁷ that another mitochondrial sirtuin, SIRT4, is present in normal human keratinocytes. SIRT4 counterbalances the effects of SIRT3, allowing cells to maintain a balance between producing sufficient ATP and minimizing oxidative damage.

Pernodet’s team showed that UV light disrupted the cyclic pattern of SIRT3 and SIRT4 expression. “Supporting the normal activity of SIRT3 and SIRT4 will therefore help the skin resist environmental insults, especially UV,” says Pernodet.

Meanwhile, the team was exploring inflammation as a recognized theory of ageing. Once again, Pernodet and team realized the answer could lie in the SIRT family, this time SIRT6. This sirtuin regulates two further factors affecting ageing: genome stability, via control of telomere length and DNA repair; and inflammation. If its telomeres shorten too much, a cell loses its ability to divide, a state known as senescence. Senescent cells contribute to ageing in tissues by triggering oxidative damage and inflammation. SIRT6 damps down inflammation by helping to inhibit the activity⁸ of a protein called NF-κB.

To find out more, Pernodet’s team used a SIRT6 knockdown to inhibit SIRT6⁹ activity in cultured dermal fibroblasts, which resulted in a 400% increase in NF-κB expression, as well as increased cellular damage, both in the presence and absence of UV. In preliminary unpublished research, they then identified a way to boost SIRT6 activity, which slowed all of these processes in ageing cells. When this is written up, it will be the first demonstration of SIRT6’s action in skin cells, says Pernodet, who hopes their findings might also be applicable to inflammatory skin conditions.

The findings are yet to be published, but were presented¹⁰ at the Society for Investigative Dermatology's 2020 Annual Meeting Virtual Conference.

According to Pernodet, the future of sirtuins research in skin is finding ways to support the optimal activity of all of them together. Chen thinks that integrating work from basic and applied science will bring sirtuin-based therapeutics closer to reality.

“The field is ready,” says Chen. “It takes everyone interested in sirtuin biology, including ELC, to really put together all the data and expertise to make that happen.”