“Prostate cancer could be a sexually transmitted disease caused by a common infection,” The Independent reports.
Researchers have found evidence of a link between the cancer and trichomoniasis – a common parasite that is passed on during unprotected sexual contact.
A laboratory study found the parasite produces a protein similar to a human protein that is necessary for the immune system to function. However, the human protein had also been shown to be involved in the growth of cancers, as it causes inflammation.
This is of potential concern as trichomoniasis causes no noticeable symptoms in up to half of men. These men may then be subject to chronic inflammation without realising it.
The study found that in the laboratory setting, the protein from the parasite acted on human blood cells and benign and cancerous prostate cells in a similar way to the human protein. The researchers conclude that infection with the parasite, in combination with other factors, might trigger inflammatory pathways that could lead to cancer growth.
It is important to note that this early study did not involve any men with benign enlargement of the prostate or prostate cancer. Further research will be required to investigate whether there is a clear link between trichomoniasis and prostate cancer.
It could be the case that trichomoniasis is just one of a series of risk factors rather than a single definitive cause.
Where did the story come from?
The study was carried out by researchers from the University of California, Los Angeles, the Università degli Sutdi di Sassari, Italy and the Instituto de Investigaciones Biotecnológicas-Instituto Tecnológico de Chascomύs, Argentina. It was funded by the National Institutes of Health Grants, the Microbial Pathogenesis Training Grant, a Warsaw Fellowship, a Graduate Division Dissertation Year Fellowship, a Medical Scientist Training Program Grant, Fondazione Banco di Sardegna Grant and a Regione Autonoma della Sardegna Grant.
No conflicts of interest were reported.
The study was published in the peer-reviewed medical journal PNAS.
While the general content of the reporting by BBC News and The Independent was accurate, their headlines (“Prostate cancer 'may be a sexually transmitted disease'”) were probably a bit over-the-top given the preliminary nature of the research. Though both organisations included quotes from Cancer Research UK pointing out that it is too early to add prostate cancer to the list of cancers that have been found to have an infectious cause, such as cervical cancer.
We cannot say with any conviction that prostate cancer is a sexually transmitted infection. Other known risk factors for prostate cancer include age, ethnicity and family history. This arguably suggests that the disease may arise due to a combination of complex risk factors.
What kind of research was this?
This was a laboratory study investigating the potential role of a parasite which causes a common sexually transmitted infection in humans (trichomoniasis), in prostate cancer.
Inflammation plays a part in cancer development and growth. Previous research has found that levels of a protein that stimulate inflammation in humans (called human macrophage migration inhibitory factor, HuMIF), is increased in prostate cancer.
The parasite Trichomonas vaginalis produces a protein called Trachomonas vaginalis macrophage migration inhibitory factor (TvMIF) that is broadly similar to HuMIF. So the researchers wanted to investigate if TvMIF was associated with increased risk of prostate cancer.
What did the research involve?
Several experiments were conducted in the laboratory to examine the effects of the TvMIF protein on human immune cells and prostate cancer cells.
The study first determined whether the trichomonas vaginalis parasites secretes TvMIF in humans during infections. They did this by measuring whether people with the infection had produced antibodies to TvMIF compared to people without infection. They took blood samples from 111 people with trichomoniasis infection and 79 people who reported no infection.
They then examined whether TvMIF causes similar inflammatory reactions as HuMIF, by mixing it with the white blood cells of three human donors.
Lastly, they investigated the effect of TvMIF on cell growth, division and invasion of other cells – all aspects of cancer. They did this by adding either HuMIF or TvMIF to benign (non-cancerous) prostate cells taken from men with benign enlargement of the prostate in one experiment, and to prostate cancer cells in another experiment.
What were the basic results?
TvMIF increased the growth and invasion of benign and cancerous prostate cells in the laboratory:
- TvMIF increased the growth and division of benign prostate cells by 20% and prostate cancer cells by 40%, similar to the effects of the human protein, HuMIF.
- TvMIF increased the spread of benign and cancer cells by 30%.
TvMIF had the same effect on human white blood cells as HuMIF, triggering several different inflammatory pathways including some that have previously been shown to be active in cancers.
People infected with trichomoniasis create antibodies against the TvMIF, which shows that trichomoniasis does secrete this protein during infections in humans.
Blood tests from people infected with trichomoniasis showed that antibodies had been produced against TvMIF in 57% of people who had the infection compared to 11% of people who had not. Infected males had a much higher incidence of antibodies (79%) than infected females (30%).
How did the researchers interpret the results?
The researchers say that “together, these data indicate that chronic T. vaginalis infections may result in TvMIF-driven inflammation and cell proliferation, thus triggering pathways that contribute to the promotion and progression of prostate cancer”.
This study found that trichomoniasis infection appears to produce a protein that is similar to a human protein involved in inflammation. Inflammatory processes are associated with the growth and development of cancer cells. So this study shows that trichomoniasis may have a potential role in the development of prostate cancer.
However, this was a laboratory study and did not involve anyone with prostate cancer. Although the TvMIF protein had similar effects on the immune pathways and growth of the cancer cells as HuMIF, this is not proof that trichomoniasis causes prostate cancer.
It is interesting that antibodies to TvMIF were only present in 57% of people with T. vaginalis infection, and actually present in 11% of people who did not report current or previous infection. Possible reasons for this include the accuracy of the test – there may be other similar proteins which react the same to the test, or people may have had mild infections which they did not notice.
A further unusual aspect is that only 30% of women who had the infection had antibodies to TvMIF.
What is not clear from this study is whether there is a link between those people who have the antibody to TvMIF and prostate cancer.
Further research will be required to investigate whether there is a clear link between trichomoniasis and prostate cancer.
Despite the uncertainty of the current situation, the study does reinforce the benefits of using a condom during sex. Condoms are the most reliable method of protecting both partners from sexually transmitted infections.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
BBC News, 20 May 2014
The Independent, 20 May 2014
Links to the science
PNAS. Published online May 19 2014