UV light causes everything from sunburn to skin cancer and the wrinkles and sagging that come with age. But what exactly does it do in our bodies to wreak all that havoc? Ultraviolet (UV) light is just a higher energy version of the light we see (visible light). And it’s that higher energy that makes the UV in sunlight damaging to our cells and tissues. To do any damage, UV light has to be absorbed. This happens at the molecular level. And it happens one electron at a time. When a single electron absorbs a photon of UV light, that electron goes into a higher energy state. An excited electron like that makes a molecule behave in different ways — sticking to things it shouldn’t stick to, changing shape, and generally messing with normal healthy cell business. Luckily we evolved in a world that’s saturated with UV light, so our cells have built-in repair kits for the damage UV radiation can do. But if the damage outweighs our capacity to repair, or our repair kits themselves get damaged, it’s hello liver spots, cataracts and — way too often — skin cancer. There are two kinds of UV radiation in sunlight — UVA and the higher energy UVB. And the trouble starts when they’re absorbed by our more important molecules — DNA, RNA or proteins. The pain. The heat. The redness. The judgemental looks from everyone who thought to dress appropriately. They’re all part of the sunburn experience. And — judgemental looks aside — they are all caused by UVB radiation. There are three types of UV radiation: UVA, B and C. The ozone layer absorbs all UVC and almost all UVB. Cancer and sunburn are likely to be result of UVB. Wrinkles are mostly the result of UVA. UV won’t burn you through a window, but will age you (glass absorbs all UVB, but not all UVA.) You can get sunburnt on a cloudy day as UVB radiation can penetrate clouds. Sunburn is the body’s way of healing from the damage caused by UV radiation. It’s your standard inflammation to clear away damaged cells, plus a little pain. Like all inflammations, it involves a bunch of messenger chemicals (cytokines) signalling for more blood to be delivered (causing redness and heat) and more white blood cells to clean up the mess. And the whole thing can be kicked off by UV damage to a single molecule — a small bit of RNA called U1. When a photon of UVB hits U1 RNA in the nucleus of a cell, it changes the RNA’s structure. That tiny tweak is enough to kick off the whole heat-redness sunburn cytokine festival. The pain part of sunburn is down to one particular messenger molecule, CXCL5. It attracts white blood cells into the skin’s dermal layer, activating pain fibres there. So it’s CXCL5 that makes us sensitive to pain for a couple of days after the event, which has made it a ‘molecule of interest’ for pharmaceutical companies looking for new pain-relief drugs. If you’ve had a skin cancer frozen off or cut out, you can blame kinky, mutant DNA caused by UVB light. Somewhere back in your past, a photon of UVB made two bits of your DNA that are next to one another stick together (a couple of thymines, or ‘T’s in DNA code). With those two T’s stuck together, that stretch of DNA suddenly had a kink in it. Like a zipper with a broken tooth, the kink distorts the shape of the DNA in that cell, making it harder for your cell’s enzymes to read and copy it properly. Luckily we’ve got an entire molecular system devoted to finding and fixing these kinks. More than 30 proteins work together to replace the affected DNA. It’s an incredible system, but it’s not foolproof, so over time you can get a build-up of missed errors. Errors in DNA are mutations, and if those mutations affect the cell’s ability to keep itself in check (like a mutation in a tumour-suppressing gene), you can get the kind of out-of-control growth of cells that make up a skin cancer. Worse still, if the mutation happens in a melanocyte (the skin cells containing the brown pigment melanin that colours our freckles, moles and tans) a far more dangerous cancer can result — melanoma. Direct attacks on DNA aren’t the only way UVB can cause mutations. It can also break proteins apart, and the high-energy molecules that result (radicals) are spectacular at attacking DNA. UVA gets in on the act too by creating high-energy, oxygen-based compounds that go on a rampage, ripping other molecules apart. Anti-oxidants in our cells do their best to soak up these destructive radicals, but it only takes one missed error in the wrong gene to need a date with your doctor and some liquid nitrogen.
Cataracts: UVA makes proteins sticky
The cloudy lens that makes it hard for cataract sufferers to see can also be caused by UV light. The lens in your eye is normally clear, because the cells that make it up don’t have a nucleus or other lumps to interfere with light — they mostly contain proteins, neatly aligned to let light through. But in 2014, researchers showed UVA light can trigger a chain reaction inside the lens that makes the proteins clump together, causing the cloudy look. Our eyes have a special set of proteins, called chaperones, that work against the clumping, but if the clumping outperforms the chaperones, cloudy cataracts are the result.
Wrinkles and generalised sagging? Thank you, UVA
UVA is almost entirely responsible for the wrinkles and other effects of ageing on our skin. While UVB is busy mutating DNA and setting off sunburn in the epidermis, UVA can penetrate more deeply into the dermis (the second layer of skin cells) where it converts firm, youthful skin to something that looks like mine. Anyone who’s spent more than $10 on a beauty product knows the key to firm skin is a protein called collagen. It comes in long fibres that form a mesh, giving structure to our flesh. UVA doesn’t directly attack the collagen, it’s sneakier than that. UVA activates receptors that produce the enemy of firm skin: matrix metalloproteinases. These enzymes have one job and they do it beautifully: breaking down collagen. And it doesn’t take much UV at all to get this going, so even without sunburn any parts of your skin that are exposed to the sun will age. As if the collagen attack wasn’t insult enough, UV radiation also interferes with the production of Vitamin A receptors on our skin cells. Vitamin A is critical for cell growth in our skin, but without functioning receptors for the vitamin to activate, our skin ends up thinning, and that’s something no amount of carrots can fix.
ABC News, 18 November 2015 ;http://www.abc.net.au/news/ ;