How does sunshine make you tan?

In honor of the heat wave that finally broke last weekend, let’s talk about sunshine.  Aside from visible light, sunlight is also made up of infrared radiation and ultraviolet (UV) radiation.  We experience the infrared radiation as heat, but humans have a more complicated relationship with UV radiation.  As you were probably told during your childhood, having enough vitamins is an essential part of being healthy – and UV radiation is a major source of Vitamin D.  On the other hand, high exposure to UV radiation is associated with increased risk of skin cancer, and it contributes to skin aging.  That’s right – spending too much time in the sun can literally increase the rate at which your skin ages.  UV radiation is also what causes you to tan (or burn), particularly in the summer when the earth gets more direct sunlight.  So how exactly does UV radiation cause you to tan while also increasing your skin cancer risk?

Even though the most obvious change from being in the sun is the way your skin gets darker (or redder), the underlying cause is at a much smaller scale than what we can see with the naked eye.  UV radiation affects the DNA inside your skin cells.  As explained in an earlier post, DNA is comprised of two strands that form the two sides of a “ladder” that twists to form the DNA double helix.

There are four basic “pieces,” called bases, that are strung together to make DNA.  These bases are adenine (A), thymine (T), guanine (G), and cytosine (C).  Your DNA is made up of millions of these four bases, and the order in which they are strung together is unique to each person.  The bases are physically attached to a backbone made of sugar and phosphate (represented by the two black strands in the figure below).

 

The bases are usually attached only to the backbone – think of charms dangling from a charm bracelet.  The charms are held in one long string via their attachment to the bracelet, but they aren’t connected to one another directly – an eagle charm hanging next to a paintbrush dangles freely from the bracelet and isn’t attached to the paintbrush with an additional string.  UV radiation causes a very specific form of damage to DNA: it causes bonds to form between two thymines (T) or two cytosines (C) when they are next to each other on the same strand (see below).  This is called a thymine or cytosine dimer.

This dimer is problematic for a couple of reasons.  First, it distorts the shape of the DNA molecule, which destabilizes the DNA helix (instability is generally bad, regardless of what causes it).  Additionally, the DNA can’t be accurately copied if this extra bond is in the way because the copying machinery won’t recognize the two individual bases when they’re stuck together.  You acquire plenty of these extra bonds between adjacent T’s or C’s whenever you go outside.  Usually, the cell cuts out the damaged DNA and inserts fresh bases.  (In the name of scientific accuracy, I have to point out that the diagram below isn’t quite accurate – the cell cuts out approximately 30 bases in this process, which I didn’t fully depict.)

 

Unfortunately, if you spend too much time in the sun, your body can’t absorb all that UV radiation without the clean-up machinery becoming overtaxed.  When there are more mutations than the body can handle, these extra bonds between thymines or cytosines aren’t all fixed before the cell copies its DNA.  Most of the time, the cell can still properly copy the DNA even if it gets stuck at one of these mutations while copying the DNA.  If there is simply too much damage, the skin cell will undergo cell death – which is what happened when skin peels after getting burned.

The problem is that there is always a small chance that the cell will incorrectly copy the DNA, avoiding cell death while creating a mutation in its DNA sequence.  One way to increase cancer risk is to increase the number of mutations in a cell – eventually, something important could be mutated that allows the cell to become cancerous.

The take home message is that UV radiation causes inappropriate bonds (dimers) to form between two thymines or two cytosines when they are next to each other, and too many of these can result in DNA mutations, which increases cancer risk.

But how does UV radiation make you tan?  The major pigment that determines skin color is called melanin, which absorbs UV radiation in order to protect our body from UV radiation damage.  In the presence of UV radiation, melanin becomes darker, which results in the immediate tanning affects you can see after a day by the pool.  At the same time, the presence of DNA damage (the thymine and cytosine dimers) triggers the production of more melanin over the course of the next couple of days.  More melanin means less DNA damage next time, since the melanin protects from DNA damage.  As a result, there’s a two-fold effect when you go out in the sun: you have a fast tanning effect from the melanin that was in your cells when you went outside the first time, and then you get tanner the next time you go outside because there is more melanin present in your cells, meaning that more dark pigment is produced.

That said, the best way to protect your body is to wear sunscreen and avoid getting burned.  Experts haven’t pinpointed exactly how much sunlight is too much, but they’re definitely in agreement that sunburn is always bad.  So enjoy the weekend, and if you’re planning to get some tanning done, now you’ll appreciate how it works.

Related articles

• Science of Summer: What Causes Sunburns? (livescience.com)

Carcinogens: Everyday things that do (or don’t) cause cancer

Cell phones, cigarette smoke, radiation, asbestos, artificial sweeteners.  The list of carcinogens (things that cause cancer) found in popular media goes on and on.  Sure, we like to joke within the cancer research community that everything causes cancer, but that’s not actually true.  Many things, such as cigarette smoke and UV radiation, are well-documented causers of cancer; other things, such as cell phones and artificial sweeteners, are victims of misinformation and sensationalism.

Let’s go back to the basics before we tackle the myths and facts of commonly evoked carcinogens.  In order to become cancerous, a normal cell must undergo several changes first.  Cancer cells have to be able to grow all the time; they need to be able to survive with low levels of food and oxygen; they need to be immortal.  They acquire these changes when the cell’s DNA is altered, or mutated.  Only after acquiring several of these mutations will a normal cell become truly “cancerous.”  Our cells are constantly acquiring mutations, but normally there are “repairmen” in the cell whose job it is to notice mutations and fix them; if the mutation can’t be fixed, the repairmen call in exterminators who force the cell to undergo cell death.

There’s a fixed number of these repairmen in the cell whose job it is to monitor the integrity of your DNA.  So what happens if the number of mutations in a cell skyrockets?  The repairmen can handle only a certain number of mutations at a time, meaning that some mutations are bound to slip under the radar if there are too many mutations.  A house with a leaky faucet or a broken latch on the window won’t come crashing down around you; similarly, a mutation here or there probably won’t cause the cell to become cancerous.  But as the house falls into disrepair it eventually becomes unlivable, and a cell that acquires too many mutations can become cancerous if it evades death.

The hazard symbol for carcinogenic chemicals.

So back to carcinogens.  Carcinogens work by damaging DNA or metabolic processes in the cell, thus increasing the workload a cell’s repairmen have to handle – and consequently increasing the odds that a cell could become cancerous.  Known carcinogens include radiation from sunlight, tanning beds, X-rays, and radioactive materials; cigarette smoke inhaled directly or second-hand; air pollution; and significant exposure to asbestos.  High alcohol consumption clearly increases the risk for several types of cancer beyond the commonly associated liver cancer.  Hormones can also influence cancer risk: hormone therapy prescribed at menopause has been shown to increase drastically the risk of breast cancer in women (fortunately, it’s not recommended treatment anymore).  In all of these cases, there is clear evidence that cancer is directly caused by exposure to the carcinogen.  No such clear evidence exists for other popular “cancer-causing” buzzwords.  Several rigorous studies have concluded that cell phones do not cause brain cancer, even though they emit small levels of radiation near the ear.  The same is true of artificial sweeteners, which have not been linked to any increased risk for developing cancer.

This list only covers a sliver of the long list of carcinogens, most of which are less commonly encountered than those listed above.  If you’re interested in what the rest of them are, they’re covered extensively here.  I chose the handful of carcinogens above based on a quick survey of friends and an internet search of carcinogens in the news, so feel free to comment and let me know if there are any other truths or myths about carcinogens floating around that I neglected to address.

A final aside about cancer risk: lifestyle also has important ramifications.  Consumption of red meats, eating fruits and vegetables, level of exercise, obesity, working nightshifts/sleeping during the day, and the age at which a woman has her first child can all influence one’s cancer risk, but that’s another post’s worth of information and will have to wait for another week.