This is the third of 6 guest posts on infection and chronic disease.
Does chronic IL-6 levels lead to epigenetic changes in DNA methylation that contribute to this pathway?
By Matthew Fitzgerald
How can infection be a carcinogen?
How do infectious diseases lead to cancer, if at all, is still a highly debated area of research. Do infectious diseases change the genetic information by insertions, mutation, or do bacterial toxins act as carcinogens? Does inflammation lead to free radical damage and cancer? While all of these and more are possible causes, another potential mechanism is that infection could change the epigenetics of cells at the site of infection. What is epigenetics? It is how our genetic information is controlled to tell cells what genes should be expressed and what genes should be silenced. For example what tells a stem cell to become a heart cell or kidney cell? Both contain the exact same DNA but are different because they express different genes, this is epigenetics.
(More after the jump...)
Epigenetics and DNA methylation in Cancer
In genetics we learned that there are four bases that comprise DNA, however many researchers today suggest there is a fifth base. No this isn't the X-files or alien DNA it is methylated cytosine. These cytosines are methylated only at CpG sites in the genome. These CpG's are frequently found in clusters or "islands" in the promoter region of many genes. Methylation of these cytosines by enzymes called DNA methyltransferases has been shown to help regulate RNA transcription by blocking transcription factors or recruiting proteins to essentially close down the chromatin structure to shut down the gene. This elevated level of cytosine has been reported in almost all tumors and in many tumor suppressor genes such as p16, p14, Rb, Maspin, etc. How does this relate to infection? IL-6 has been reported to effect DNA methyltransferase enzymes and DNA methylation.
IL-6 in Infection, Inflammation and Cancer
IL-6 is a cytokine that plays many roles. It is produced at the site of inflammation and acts to regulate acute phase proteins after infection. It also plays a key role in chronic inflammation. It orchestrates the transition by the body from using neutrophils to macrophages and helps to stimulate T and B cells that favor a more chronic condition of inflammation. Chronic high levels of IL-6 have been reported in patients with numerous chronic diseases: HIV, Hepatitis B and C, arthritis and in many solid tumors such as pancreatic cancer, liver cancer, prostate cancer and others.
IL-6 and DNA Methylation
So it has been shown that chronic high levels of IL-6 is found in patients with numerous diseases such as infection and cancer. What does this high level of IL-6 mean and what could be its effects? Besides its stimulatory effects on cells IL-6 has recently been shown to alter DNA methylation. This could alter a cells gene expression pattern and help lead to transformation to a cancer cell. In a report by Hodge, et. al. in the journal Cancer Research they showed that IL-6 can upregulate DNA methyltransferase(DNMT1). This enzyme is responsible for maintaining the methylation pattern on genes and controlling their expression. They go on to show that high IL-6 expression was linked to the hypermethylation of p53 and a significant decrease in this important tumor suppressors expression. Other researchers have also shown that IL-6 can turn on or off other important genes through methylation, i.e. EGFR, microRNA-370.
Role of IL-6?
This pathway from infection to inflammation to cancer has been widely studied in numerous cancers namely HBV and others but the mechanism of progression to carcinogenesis is still debated. This blog tried to postulate on another possible mechanism that could link these three events. IL-6 is involved to some degree in each of these steps and the recent reports that IL-6 can effect expression through DNA methylation is very interesting. The famous two-hit hypothesis for cancer proposed by Knudson states that two hits such as mutations in oncogenes and silencing of tumor suppressor genes is necessary to develop cancer.
If IL-6 can silence tumor suppressor genes than the other hit could come from other infectious disease insults such as viral genetic insertion into the genome or production of free radicals as a result of chronic inflammation from infection. Too conclude IL-6 is elevated in the sera of numerous patients with infection and chronic disease. While the role of IL-6 in these cases is still being elucidated, IL-6 could have a significant impact at the site of chronic inflammation by altering the epigenetics or gene expression of important genes that lead to carcinogenesis.
While I have studied a couple of papers for this blog post I am by no means an immunologist and it is still too early to say if IL-6's effect on methylation is a large contributor to cancer. However, it is interesting to think about this cytokines role in chronic disease and the progression towards cancer because it can provide a link from infection to inflammation to cancer.
Matthew Fitzgerald is an RAIII in the Free Radical and Radiation Biology program at the University of Iowa.
Costello and Vertino. 2002. Methylation matters: a new spin on maspin. Nature. 31:123-4. Link.
Hodge DL et al. 2001. Interleukin-6 Regulation of the Human DNA Methyltransferase (HDNMT) Gene in Human Erythroleukemia Cells. 276:39508-39511. Link.
Gabay C. 2006. Interleukin-6 and chronic inflammation. Arthritis Research & Therapy. 8(Suppl 2):S3. Link.
Porta et al. 2008. Circulating interleukin-6 as a tumor marker for hepatocellular carcinoma. Annals of Oncology. 19: 353-358. Link.
Hodge DR et al. 2005. Interleukin 6 Supports the Maintenance of p53 Tumor Suppressor Gene Promoter Methylation. Cancer Research. 65:4673-4682. Link.
Alpini GD et al. 2007. Genetic and epigenetic changes associated with
cholangiocarcinoma: From DNA methylation to microRNAs. World J Gastroenterol. 13: 6465-6469. Link.
Meng F. et al. 2008. Epigenetic regulation of microRNA-370 by interleukin-6 in malignant human cholangiocytes. Oncogene. 27:378-386. Link.
I do wonder if it isn't a touch misleading to think of presense of IL-6 as a hit the cell has to absorb... the presense of any number of mitogens, cytokines and inflammatory factors don't (in themselves) produce cancer in an intact genome (unless you count proliferation of T-cells as cancer...). I'd like to see some work on the epigenetics showing that methylation of p53 alone can produce cancer... given it's role as a suppressor of initation (as opposed to solely a suppressor of progression) I don't suppose you could fully exclude mutations at other sites though. I can't quite think of how to demonstrate *sufficiency* of p53 methylation. Plus, strictly speaking it would break the Knudson dogma.
Though I can certainly see development of experimental model systems where either p53 methylation or IL-6 exposure was *necessary* to push a cell over the edge. I suspect that's the direction the research will go next.
Do the classic infection associated cancers (like gastric with H. pylori, or bladder cancer with schistosomiasis) show crazy-high Il-6 levels? Or is it more a feature of metastasis prone nasty ones (like pancreatic)? Where in the carcinogenic process would it play the biggest roles?
Very interesting-inflammation related AKA "vaccine associated" fibrosarcomas in cats could be a good model for this process.
Matthew/Tara, you should consider submitting this post to the Cancer Research Blog Carnival.
Nice post Matthew. I too don't think that you can can consider IL-6 a hit. By using hit you are implying permanent DNA damage and mutation. It appears that epigenetic changes function more like tumor promoters, whereas by themselves there is no cancer but chronic inflammation may promote the development of tumors in "initiated" people. The good thing about epigenetic changes is that they are potentially reversible since they are under the control of enzymatic systems (you could potentially turn on the opposing enzyme through a drug) whereras DNA mutations are not reversible once replicated. So if IL-6 is promoting a subset of inflammation-mediated cancers you might be able to target the carcinogenic (epigenetic) aspect of the IL-6 pathway without impacting the immunological function of the pathway.
Thanks for all the great responses. As far as IL-6 being a hit in the Knudson hypothesis I agree it is not a hit but its down stream actions through DNA methylation could be considered a hit. I work on Maspin(tumor suppressor gene) and breast cancer and it is epigenetically silenced and when you re-express or re-activate it with a dna methyltransferase inhibitor(5-aza-Dc) you can prevent invasion of breast cancer cells and slow their growth. There are numerous reports in cancer of p16 and p21 being silenced both of which are vital to controlling the cell cycle. So I consider silencing a gene similar to a mutation because it is a loss of function of that gene. On a side note for those interested in the latest Lancet journal there is a study using an IL-6 receptor inhibitor and its benefits to those with rheumatoid arthritis. I am not suggesting this is epigenetic, it is just interesting that blocking IL-6 action can be done and it is beneficial.