The Hole Story:
The Mouse Model of Cancer ctd.
Sunscreens and Immune Response
Quite a few years have been spent looking at different sunscreen ingredients, their effect when applied to the skin and particularly their effectiveness in protecting from cancer development. There is no doubt that the major sunscreens that are on the market today are very efficient in protecting from sunburn and from skin cancer outgrowth, says Reeve. But there have been a few experiments with Vivienne Reeve's mice in which tumours are still initiated despite the application of sunscreen, which means that potential cancer cells are still produced in the skin even in the presence of the sunscreen. The sunscreen seems to prevent the tumours from growing out within the same time period as they would grow out in an unprotected mouse.
Reeve is a bit concerned about this but there is not much she can do about it until new UV absorbing chemicals come onto the market. There is a need for more efficient and better sunscreen ingredients. The sunscreen industry seems to already know this and they have new products on the drawing board.
In the meantime another aspect of sunlight exposure that has come to light which isn't generally understood is that in addition to causing sunburn and skin cancer, UV radiation also suppresses immunity. It suppresses those kinds of immune responses of the body which are mediated by T lymphocytes. This is not so much antibody production as cell mediated immunity, which is the kind of immunity that our bodies use to counter the presence of cancer cells and also things like invading viruses and other pathogens.
From a whole swag of animal experiments we now know that UV radiation increases the infectivity of Herpes virus and of Candida and of a protozoan parasite called Leishmania, says Reeve. It also decreases abscess formation in mice, a sign of a healthy immune reaction. A similar example of this in humans is when people who are sensitive to cold sores are exposured to sunlight and 2 or 3 days later erupt in cold sores. This effect may not require sunburn and can sometimes result from quite small exposures of sunlight. It is an indicator of a temporary suppression of their immunity against the Herpes virus. The immune suppression allows the virus to be reactivated and the cold sore breaks out.
Dr Peter Hersey, from the University of Newcastle, has done quite a few studies on humans using sun-beds and artificial UV radiation in suntan parlours. He has also found that UV radiation suppresses immune reactions in humans. In animals it seems that the immune suppression caused by UV radiation is necessary for the outgrowth of the skin tumour cell. So UV light not only causes cancer cells to be produced but by interfering with immunity it also gives the cancer cells the opportunity to grow. So there is a compounding effect.
Reeve is interested to see whether sunscreens protect from this suppression of immunity and she looked at two different types of sunscreens which are examples of the two major UV absorbers used in Australia today. One sunscreen uses an ester of octyl para amino benzoic acid, PABA, while the other has an ester of cinnamate. Most of the experiments involved painting sunscreens on mice and then exposing them to artificial UV radiation. The hairless mouse used by Reeve is quite well recognised all over the world in photobiology laboratories and it has a normal immune system that responds in the normal way.
She has established in tissue culture several cell lines from carcinomas that have been induced in mice with UV radiation. By injecting those tumour cells back into the mouse she can watch whether the tumour will grow or won't grow. This will happen within 3 weeks and depending on what pre-treatment the mouse has had, such as UV radiation or immune suppressive drug or different diet, she can use these cultured tumour cells as a test of the mouse's immune function. If the mouse's immune function is healthy it will reject the tumour. But if the mouse is immune suppressed, then the injected tumour cell will survive and grow into a tumour at the site of the injection.
One of the sunscreens Reeve tested was totally protective against immune suppression while the other sunscreen failed to protect. The cinnamate is protective but the PABA type fails. Yet both of these sunscreens are equally protective from sunburn. She has yet to test them to compare for protection against skin cancer.
It certainly seems that we should be selective in which sunscreen we buy says Reeve, who thinks the PABA type sunscreens should be phased out of the market. In the United States the majority of sunscreens are cinnamate sunscreens and they are sometimes even advertised as having no PABA type ingredients. In Australia we seem to be one or two years behind America in this area although sunscreens are now appearing on the shelves here with labels saying they contain no PABA.
The PABA type sunscreens also have other disadvantages. They stain your clothing and they cause allergic reactions in a small number of people. There is also evidence that has been around for some years that they might cause DNA damage and they might also increase the DNA damage caused by sunlight. So for a whole number of reasons it is probably time we stopped using PABA type sunscreens.
There doesn't seem to be any direct problems with cinnamates at the moment. However, about 10 years ago a group at Sydney University in the School of Public Health and Tropical Medicine discovered that some batches of the cinnamate ingredient for sunscreen were contaminated with a mutagen, a chemical that causes genetic changes. They picked up the mutagen in some very sensitive tests they do for mutagenicity and of course all hell broke loose in the sunscreen industry. Threats were made and according to Reeve it was all very unpleasant. Nowadays when a formulater buys the cinnamate ingredient to make up the sunscreen it comes with an Ames Test certificate to say it's negative for mutagenicity. While it was never admitted publicly that there was a problem with the sunscreen, obviously there was one because each batch is now tested.
I think it was probably a dirty manufacturing technique because only some of the batches that had come off the production line were contaminated while others weren't, says Reeve. I think that we need to keep a watchful eye on these things. The sunscreen industry is huge and involves hundreds of millions of dollars in this country alone. When there are large amounts of money at stake it is very difficult to change the market because the chemical companies are very powerful. We need watchdogs because these are synthetic substances; foreign chemicals which are applied to the body.
We need to protect ourselves more in Australia against UV radiation. The increasing ozone depletion means we will need a general protection all over the body. But if you are going to slather your body and your face with sunscreen-containing topical substances you have got to be absolutely certain that they are safe and that they are actually protecting you. Reeve thinks that there is a period of development ahead in the sunscreen industry.
One of the important relationships between ozone depletion and sunscreens is that those wavelengths which are likely to increase as a result of ozone depletion, the short UVB wavelength, are precisely the part of the spectrum that suppresses the immune system. So this effect of sunlight on our immunity is likely to become relatively more important than the actual production of the skin cancer cell. As ozone depletion proceeds so will immune suppression.
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