Wednesday, May 28, 2014

Sunscreen and p53: A Case Study

In my recent blog post, I discussed the p53 gene, and experiments testing sunscreens effect on p53 in mice. In this blog post, I will discuss a study testing sunscreen’s relation to p53 protein production done on humans. From 1992-1996, the NambourSkin Cancer Prevention Trial conducted a study to test the association of time spent outdoors and the expression of p53, as well as its relation to the use of sunscreen. Researchers predicted that sunscreen counteracts p53 production, and carried out their research on over 160 random participants for 6 months. There were four different treatment groups: 1. Application of SPF 16 sunscreen daily + beta-carotene supplementation, 2. Application of SPF 16 sunscreen daily + placebo tablets, 3. Beta-carotene only, 4. Placebo only. Beta-carotene is a type of pigment called a carotenoid, and produces Vitamin A, which helps prevent cell damage. It’s thought to be possibly effective for people who are sensitive to sun exposure in order to help prevent sunburn, but the exact efficiency is not known.
 Figure 1. Proportion of p53-positive cells in the whole epidermis (n=139) of participants’ dorsal hand.

Based on the above graph, it seems like there’s a high frequency of individuals with <5% of their cells being p53 positive, and that the frequency for p53 positive cells becomes relatively low after 25%. What this graph leaves unclear is what the frequency represents—is it measured by number of individuals who had that amount of p53 positive cells? Does it carry forward? For example, if a person is 80% positive, will they be counted into the frequency for 0-5, 5-10, 10-15, etc. in addition to 75-80? If so, this could be why the data looks so skewed toward a higher frequency for lower levels of p53 positive cells. The study says that 61% of participants had over 5% of p53 cells and 22% had over 20%, but 17% had <5%, and so I would think that the frequencies would correlate with these numbers but it doesn’t seem like it does. Not to mention, does it make a difference that they took biopsies from the hand and not other areas of the body? Different parts of the body will most likely have different susceptibilities to sunburn or sensitivity to UV exposure due to their cell makeup, so does using the hand also skew the data

Table 1 shows that there is a significant difference in having p53 positive cells when comparing males to females (p<0.01), as well as females smokers to male smokers (p<0.05). One thing the researchers could control if they were to repeat the study would be to not accept past smokers, whose history could affect the results of the trial. It is indicated that the subsequent analysis were adjusted for sex and smoking status, which could also affect the data by making p53 percentages higher even though they might not be attributed to UV exposure. According to the table, it seems like other variables such as age, skin color, skin type, and cancer history did not significantly influence the proportion of p53-positive cells.

Table 3 (above) shows the effect of sunscreen on proportion of p53-positive cells. The study found that when sunscreen is the only variable used to compare participants, there is no significant difference in proportion of p53 positive cells. But they also conducted a multivariable model to take into account time spend outdoors, sex, smoking status, skin type, and skin color.

Overall, the researchers concluded that sunscreen is independently associated with lowering p53 protein production, and I somewhat agree with their argument. What makes the data difficult to interpret is the amount of external factors that could contribute. Also, the numbers in Table 3 suggest that there is a significant difference between the groups, but between never wearing sunscreen and wearing sunscreen 7 days per week, there appears to be the same percentage of p53 positive cells, but when comparing sunscreen use from 1-6 days, it appears that there is a lower proportion in the 5-6 range, which is confusing. Another downside to the experiment is that it was done in Australia, which is known as the “skin cancer capital of the world,”(3) and so another variable could be location of the homes of these individuals. The paper also did not differentiate results for the four different groups, and did not go into depth about why they also used beta-carotene. Wouldn’t that add another variable other than sunscreen when analyzing data? As a result, I don’t fully trust the conclusion of the paper because there are so many variables that could contribute to the experiment, and there are also parts of the experiment that aren’t fully explained, which makes it difficult to interpret the data and propose conclusions.

1.     Jolieke C. van der Pols, Chunxia Xu, Glen M. Boyle, Peter G. Parsons, David C. Whiteman, and Adele C. Green. Expression of p53 Tumor Suppressor Protein in Sun-exposed Skin and Associations with Sunscreen Use and Time Spent Outdoors: A Community-based Study Am. J. Epidemiol. (1 June 2006) 163 (11): 982-988 first published online April 19, 2006 doi:10.1093/aje/kwj137
2.     "Beta-carotene." U.S National Library of Medicine. U.S. National Library of Medicine, 19 July 2011. Web. 26 May 2014. <>.
3.     "SunSmart." Cancer Council Australia. Cancer Council Australia, 19 Nov. 2013. Web. 25 May 2014. <>.