Wednesday, December 7, 2011

Dietary Methyl Donors and Progression of Prostate Carcinogenesis...

Carducci's lab at Hopkins have just published a study looking at the effect of excess dietary methyl donors, for example folic acid, methionine, choline, B12 and betaine - on prostate cancer progression in mice.  As this is pretty much what we do in the lab, I was excited -- we have previously published (Tomaszewski et al.) on the very high levels of serum folate in our prostate cancer patients, and the significant increased cancer cell proliferation in patients with high versus low serum folate -- this study concludes that in mice, and in cell culture (in vitro) high levels of folic acid and / or methyl donors, does not result in an increase in tumor growth rate or the ability of a demethylating agent to cause demethyation of the AR promoter, or the Reprimo promoter.  In fact, they find a protective effect of the supplemented diet on the development of higher grades of prostate cancer in the hi-myc mouse model.  The paper is well-written and has an unbiased appraisal of the literature - unfortunately I don't think they can make the conclusions that they are -- I think some of the experiments would be worth doing, but using relevant diets - you'll see what I mean below -

1)  the xenograft model using DU145 and PC3 prostate cancer cells
The cells were grown in mice being fed either the Teklad 18% protein diet, or the same diet supplemented with methyl donors, including 15mg/kg folic acid.  Teklad's base diet has 3mg/kg folic acid from the added vitamin mix, and an estimated further 1mg/kg from natural folates -- found in the brewer's yeast, corn, wheat etc. that the diet is made from.  2mg/kg is the basal level of folic acid for rodent diets, so this diet starts out with twice as much.  The supplemented diet has 15mg/kg folic acid on top of this, so is closer to 19mg/kg.  The cup can only get so full guys!  Figure 1c has the ki67 labelling index for the xenograft tumors (both DU145/PC3 together?) -- on the regular and methyl supplemented diet -- yikes!  is that 55% proliferating at any one time in the saline groups?  Is it possible that the cells literally can not grow any faster than they already do on the "regular" diet?  In humans, the proliferation index for prostate cancer is considered high when it is around 5% -- so how do the circulating folate levels compare to humans for these mice?  Well the ones on the so called "regular" diet have a mean plasma folate of around 200nM, the supplemented group has 340nM (mean values, calculated from the ng/ml value x 2.27).  In our lab studies, patients range from <10nM, to our patient with the highest serum folate we've seen, somewhere around 160nM.  Just because the cells aren't growing faster doesn't mean supplemental folic acid is not driving prostate cancer proliferation in humans --  in addition, the statistics used in this manuscript don't seem to mesh with the data as presented -- t-tests are for normal distributions, but as seen in Fig 4 - the distributions aren't all normal - the plasma folates in the methyl supplemented group have outliers -- the mann-whitney rank sum would be better -- although from guestimating the numbers, it would still be significant (as expected).  The problem is many of the other analyses that are not significant have just 4 data points, and as it seems this manuscript wasn't seen by a statistician, we can not be sure that there was sufficient power to make the non significant determination. 

2)  Cell culture assay
The next issue is the media used for the cell culture assay that showed that myc was expressed similarly in regular media, versus folic acid supplemented media.  We assume it was folic acid supplemented, but it is not mentioned in the methods section and could have been another folate.  Anyhow, DMEM has 9 micromolar folic acid in the standard formulation -- and the authors supplemented that with another 226 micromolar, for a comparison of 9 vs 235 micromolar folic acid containing DMEM. Smiraglia's lab at Roswell has shown that the proliferative capacity of prostate cancer cell lines in response  to folic acid is maxed out somewhere around 200 nanomolar.  Yep, 1000-fold less than the 235 micromolar being tested here, and not relevant to the mouse model above, either.

Why do I care?  The last thing I want to do is criticize someone's freshly minted manuscript -- in fact, I usually won't comment on a paper unless I think it is well done -- however we are talking about figuring out what levels of folic acid intake are safe in cancer patients, and once someone gets an idea out into the scientific mainstream that "supplemental folic acid is safe, does not cause increased proliferation of cancer cells and may even reduce cancer", it is very hard to get the idea back again - and that affects funding and publishing decisions.  So while I think that the idea was great, and I'm glad to see that the methylation inhibitor assay was not affected by huge folate doses (although would it be more effective in a low-folate environment?) - this paper, at least to my mind, does not answer the question that it set out to answer.  But hey, don't believe me -- read it for yourself: 

Cancer Prev Res (Phila). 2011 Dec 2. [Epub ahead of print]

Progression of Prostate Carcinogenesis and Dietary Methyl Donors: Temporal Dependence.

Shabbeer S, Williams SA, Simons BW, Herman JG, Carducci MA.


Insufficient dose of dietary methyl groups are associated with a host of conditions ranging from neural tube defects to cancer. On the other hand, it is not certain what effect excess dietary methyl groups could have on cancer. This is especially true for prostate cancer (PCa), a disease that is characterized by increasing DNA methylation changes with increasing grade of the cancer. In this three-part study in animals, we look at (i) the effect of excess methyl donors on the growth rate of PCa in vivo, (ii) the ability of 5-aza-2'-deoxycytidine, a demethylating agent, to demethylate in the presence of excess dietary methyl donors and (iii) the effect of in utero feeding of excess methyl donors to the later onset of PCa. The results show that when mice are fed a dietary excess of methyl donors, we do not see (i) an increase in the growth rate of DU-145 and PC-3 xenografts in vivo, or (ii) interference in the ability of 5-aza-2'-deoxycytidine to demethylate the promoters of Androgen Receptor or Reprimo of PCa xenografts but (iii) a protective effect on the development of higher grades of PCa in the "Hi-myc" mouse model of PCa which were fed the increased methyl donors in utero. We conclude that the impact of dietary methyl donors on PCa progression depends upon the timing of exposure to the dietary agents. When fed before the onset of cancer, i.e. in utero, excess methyl donors can have a protective effect on the progression of cancer.
[PubMed - as supplied by publisher]

1 comment:

  1. Thanks for the review! Could this be a potential journal club article? I'm planning to read this tomorrow. I'm surprised it didn't come through my NCBI search!