Sunscreen & Skin Cancer: The Basics

Jacey Nishigushi and I are researching the inhibition of p53 mutations by sunscreens and the efficacy of p53 mutations as indicators of melanoma. In order to understand our eventual analysis, several key concepts must be discussed prior to exposure to our final project. We are planning to use our blog posts for this purpose, to consider the basics and build a solid foundation from which we can build off of.

UVA vs. UVB
Sun exposure is inevitable, as is resulting DNA damage. In an attempt to protect our skin from damaging effects, humans have come up with some sort of sun “screen” we are all told to put on without real knowledge about its protecting mechanism. So what is sunscreen? Sunscreens are products that consist of a combination of ingredients that help to prevent the skin from ultraviolet (UV) radiation. They are topical solutions that contain molecular compounds that can absorb, reflect, or scatter UV photons. UV radiation is one of the types of “lights” on the spectrum that we measure from the earth to the sun. Because its wavelengths are shorter than visible light, our naked eye cannot detect them. The two main classifications of UV are UVA and UVB, both of which enter through the atmosphere and come in contact with human skin. UV light can interact with the skin and can alter purines and pyrimidines, disrupt gene linkages, or even have deletion effects with the genome. Though DNA repair mechanisms exist, mistakes are made and failures happen, both which can lead to apoptosis or proliferation of abnormal cells.

UVA rays are accountable for about 95% of UV radiation experienced on Earth, as they penetrate through the air with equal intensity during all hours of light and all seasons of the year. UVA can pass through glass and still have the same effect. These rays reach the dermis of human skin, contributing to aging, wrinkling, and the development of melanoma. UVA is the prime emission of tanning beds, as its exposure to the skin causes the skin to darken in an attempt to prevent further DNA damage [See Jacey’s post about Melanin].

UVB rays are the main reason we get sunburnt and red after a long day outside, contributing to about 5% of UV radiation experienced on Earth. These rays do not penetrate as deep into the skin, mainly affecting the epidermal layers and playing a key role in the development of melanoma tumors. Unlike UVA, the intensity of UVB is dependent on time of day, season, and location on Earth. UVB is also unable to pass through glass and have a significant effect.

SPF

While walking down the sunscreen aisle of Target, I would often find myself buying a bottle of sunscreen with an SPF around 45 simply because I liked the number and I had heard sometime during my life that I should use SPF 15 or higher. So at the time, SPF 45 seemed like a safe choice. I really did not understand what the purpose of SPF was prior to writing this blog post, and I have a feeling that many of you are right where I was just a few days ago. SPF, or Sun Protection Factor, is not only a basic idea that you must understand before reading our final project, but also is a concept that is relevant to our day-to-day lives. So what is it? SPF is a mean of classifying the degree of protection that sunscreen provides against solar radiation, an in vivo assessment that offers quantitative results. According to the Skin Cancer Foundation, SPF 15 prevents reddening of the skin for 15 times longer than one would experience with unprotected skin. SPF 15 filters out about 93% of incoming UVB rays, while SPF 30 filters about 97% and SPF 50 98%. But what is the definition of “reddening” and what is happening on a molecular level with our skin and its interaction with UV light? Is a red color on our skin the first indicator of DNA damage or is there damage occurring prior to any visible change? After reviewing the components of sunscreen, we can analyze the molecular responses to its ingredients.

Chemical vs. Physical Protection

Active ingredients of sunscreen vary depending on its manufacturer and can be categorized as chemical or physical agents. Chemical sunscreens are made of several active ingredients, as no single chemical ingredient successfully blocks an entire UV spectrum. Most block a narrow section and, in combination with others blocking different small windows of the spectrum, create a broad spectrum protection for the skin. The majority of chemical agents block UVB rays. Chemical agents absorb the energy emitted from UV radiation before its contact with the skin. In order to prevent more long-term effects of UVA rays, UCSF dermatologists suggest using sunscreens with chemical and physical agents. There are two types of physical sunscreens: zinc oxide and titanium dioxide. These physical agents reflect or scatter the UV radiation before its contact with the skin. Let’s get more specific about the ingredients that make up what we know as “sunscreen”…

The Sunscreen Recipe

The table below lists the potentially active ingredients for FDA-approved sunscreens used today:

Donathan G. Beasley and Thomas A. Meyer published a paper in 2010 that examined the efficacy of avobenzone, ZnO, and TiO₂ in relation to providing UVA protection and illustrated the differences between various formulation methods used to maintain avobenzone’s structure during prolonged exposure to UV rays. Using in vitro methods, it was proven that formulas with TiO₂ provided lower levels of UVA attenuation as well as protection for human skin, both in comparison with sunscreens containing avobenzone or ZnO. Beasley and Meyer concluded that TiO₂ cannot be substituted for avobenzone or ZnO when the purpose is to provide high levels of UVA protection for the skin. Strategies used when formulating avobenzone into solution are carefully constructed so that avobenzone losses are minimized to the point where sustained protection is not affected by long exposure periods to UV rays. It interests me that no sunscreen ingredient has effects that remain static. I plan on discussing the Beasley and Meyer paper further in a subsequent post, so for now, let’s just focus on the basics. Until next time!



Sources:
Beasley, DG, and TA Meyer. "Characterization of the UVA protection provided by 
     avobenzone, zinc oxide, and titanium dioxide in broad-spectrum sunscreen 
     products." Am J Clin Dermatol, December 1, 2010. 

Epstein, John H., MD, and Stephen Q. Wang, MD, eds. "Understanding UVA and UVB." 
     Skin Cancer Foundation. Last modified 2014. Accessed April 18, 2014. 
     http://www.skincancer.org/prevention/uva-and-uvb/understanding-uva-and-uvb. 

Pelizzo, Maria, Edoardo Zattra, Piergiorgio Nicolosi, Andrea Peserico, Denis 
     Garoli, and Mauro Alibac. "In Vitro Evaluation of Sunscreens: An Update for 
     the Clinicians." ISRN Dermatology, November 27, 2012. 

The Skin Cancer Foundation. "Sunscreens Explained." Skin Cancer Foundation. Last 
     modified 2014. Accessed April 18, 2014. http://www.skincancer.org/ 
     prevention/sun-protection/sunscreen/sunscreens-explained. 

UC Regents. "Sunblock." UCSF School of Medicine. Last modified June 10, 2011. 
     http://www.dermatology.ucsf.edu/skincancer/General/prevention/Sunscreen.aspx.