A new study in mice indicates that light sensitive proteins can sense sunlight on fat cells. It also found that too little natural light can change the behavior of fat cells and can increase the risk of metabolic syndrome.
Much has evolved over the entire evolution of life on Earth. Nevertheless, one important constant is the light coming from our sky. We can’t overestimate our reliance on this big plasma ball, 93 million miles away.
Because life under the light of our nearest star developed in all its ways, animals evolved to harness its pollution. Most notably, the eyes contain photoreceptors that detect light and share information with the brain, resulting in an image of the surroundings of a human.
The sunlight also sends us into a circadian sleep– wake cycle of about 24 hours. Practically every land animal on Earth uses the sun to maintain its regular rhythms.
A new study now published in the journal Cell Reports explains a new way that mammals ‘ lives could be affected by sunlight.
Light behind the eyes
Species can sense light by using proteins known as opsins. Melanopsin and neuropsin, which are found in certain retinal cells, are two of the most well-known among humans.
In other species, substances that detect light are located outside of the visual system. For example, frogs can spot light in their skin through cells called chromatophores.
Scientists believed until quite recently that animals only perceived light through their eyes. Yet this idea has now been reversed by several reports.
For example, one 2019 study found that neuropsin can detect light in rats ‘ skin and help them maintain their circadian rhythms. Still, evidence of so-called extraocular photo-reception is scarce in mammals.
A new study has investigated, for the first time, whether or not certain proteins on fat cells lying beneath the skin may also detect light.
Senior study author Richard Lang, Ph.D. — from Ohio’s Cincinnati Children’s Hospital Medical Center — explains, “This idea of light penetration into deep tissue is very new to many of my scientific colleagues too. But we and others found opsins located in a variety of types of tissue. This is only the beginning of the work.
Opsin 3 in fat cells
The researchers targeted opsin 3 (OPN3). This protein is present all over the body including in the brain, testis, liver, and kidneys.
They showed that OPN3 is also present in fat cells, or adipocytes, both human and mouse. They also showed, importantly, that light can travel deep enough through the skin of a rat to trigger OPN3 in the adipocytes.
A particular blue light wavelength (480 nanometers) stimulates OPN3. It happens in sunlight but not with artificial light.
The scientists used genetically engineered mice in their experiments which lack the gene coding for OPN3, called Opn3, in their adipocytes.
In the refrigerator
When mammals are in a cold environment including humans, the body adapts. The body burns fat apart from shivering to create heat. White adipose tissue is the primary energy source, and when we’re cold, brown adipose tissue is largely responsible for generating heat (without shivering).
The white adipose tissue releases free fatty acids and glycerol into the bloodstream in a process called lipolysis. Brown adipose fat then takes certain fatty acids and creates heat with them.
There is growing evidence to support the idea that activating brown adipose fat may protect against metabolic syndrome. Metabolic syndrome is a series of disorders involving high blood sugar, hypertension, irregular blood lipids, and excess body fat around the waist.
When the researchers exposed Opn3-deficient mice in their adipocytes to 39.2oF (4oC) cold temperatures, their cold response was compromised. Their core body temperatures were lower than those of intact Opn3 gene regulation mice.
Brown fat was not producing as much heat in the modified mice. Ironically, they did not burn off as much fat under cold conditions as regular mice when the researchers deprived those mice of food.
Testing the refrigerator light
The researchers put normal adult mice in a cold environment with full spectrum lighting in another experiment, and tracked their core body temperatures.
They turned off the blue light of the wavelength after 3 hours which triggers OPN3 but leaves the rest of the spectrum. Without the blue wavelength the core temperatures of the animals fall, providing more evidence that the metabolism is regulated by normal, full spectrum light.
In another set of experiments, the researchers raised mice under lights with intact Opn3 genes that lacked the specific wavelengths of blue light that usually activate OPN3. The authors of the study refer to this as the lighting conditions for “minus blue.”
The mice raised for minus blue lighting conditions, like the mice that lacked Opn3, did not respond effectively to cold temperatures and had lower core temperatures when in cold conditions. They also had larger adipocytes and at fasting, they did not lose weight.
Animals without an Opn3 usually used less energy and consumed less food and water. Although they were as successful as normal mice, they consumed less energy and brought higher fat levels.
In short, the authors of the study conclude that sunlight is important for the metabolism of healthy resources.
Looking forward
Though the scientists performed this research in mice, they believe there is probably a similar mechanism in humans. They do, of course, need to do more work to determine whether this is the case or not. We write:
“If the adipocyte pathway of light-OPN3 occurs in humans there are potentially large consequences for human health. Our modern lifestyle exposes us to abnormal lighting spectra, nightlight exposure, shift work and jet lag, all resulting in metabolic disruption.”
While scientists are at the very beginning of this line of research, the authors of the study theorize that “insufficient stimulation of the light-OPN3 adipocyte pathway is part of an explanation for the prevalence of metabolic deregulation in industrialized nations where artificial lighting has become the norm.”
Looking much further into the future—and hoping that such results can be replicated by other researchers—doctors may one day recommend “light therapy” for people at risk of developing metabolic syndrome.
That is all theoretical, as it stands, but it is definitely an interesting theory. For now, though, Lang says, “If people want to take away from this something personal, you definitely can’t go wrong by spending more time outdoors.”
