NOTE from GMWatch's Claire Robinson:

A new in-vitro study in human cells (below) shows that glyphosate, the main chemical ingredient of Roundup herbicide, induces the growth of human breast cancer cells via estrogen receptors. The study found that even low, environmentally relevant doses stimulated estrogenic activity. The researchers also found that there was an additive estrogenic effect between glyphosate and genistein, a phytoestrogen in soybeans.

This study is especially interesting because I couldn't help noticing in my trawl through the German government's report on the industry dossier of tests on glyphosate submitted for its 2002 EU approval that industry's own studies showed that treatment with glyphosate, including at the lower doses tested, increased tumour incidence in lab animals. But Germany dismissed the findings on the grounds that the tumours did not increase in a straight line as the dose increased (linear dose-response).

This is incorrect reasoning, based on outdated scientific concepts. Scientists have published papers since the 1990s showing that in the case of certain toxins, the toxic effect does not predictably increase in a straight line upwards as the dose increases (the “ski slope” pattern). Instead, some toxins can have a strong effect at a low dose and a weaker effect at a higher dose. Other weird and wonderful dose-response curves can take the shape of a “U” or even a mountain range. Such nonlinear dose-response curves are often found with substances that affect the hormonal system.

More about this here:
http://www.combat-monsanto.co.uk/spip.php?article775

In addition, epidemiological studies link Roundup exposure to multiple myeloma and Non-Hodgkin's lymphoma, both types of cancer. References can be found here (click on the references link at the bottom of the page):
http://earthopensource.org/index.php/4-health-hazards-of-roundup-and-glyphosate/4-1-myth-roundup-is-a-safe-herbicide-with-low-toxicity
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Glyphosate induces human breast cancer cells growth via estrogen receptors
Thongprakaisang S, Thiantanawat A, Rangkadilok N, Suriyo T, Satayavivad J.
Food Chem Toxicol. 2013 Jun 8. pii: S0278-6915(13)00363-3. doi: 10.1016/j.fct.2013.05.057. [Epub ahead of print]
http://www.ncbi.nlm.nih.gov/pubmed/23756170

Abstract
Glyphosate is an active ingredient of the most widely used herbicide and it is believed to be less toxic than other pesticides. However, several recent studies showed its potential adverse health effects to humans as it may be an endocrine disruptor. This study focuses on the effects of pure glyphosate on estrogen receptors (ERs) mediated transcriptional activity and their expressions. Glyphosate exerted proliferative effects only in human hormone-dependent breast cancer, T47D cells, but not in hormone-independent breast cancer, MDA-MB231 cells, at 10-12 to 10-6 M in estrogen withdrawal condition. The proliferative concentrations of glyphosate that induced the activation of estrogen response element (ERE) transcription activity were 5-13 fold of control in T47D-KBluc cells and this activation was inhibited by an estrogen antagonist, ICI 182780, indicating that the estrogenic activity of glyphosate was mediated via ERs. Furthermore, glyphosate also altered both ERα and βexpression. These results indicated that low and environmentally relevant concentrations of glyphosate possessed estrogenic activity. Glyphosate-based herbicides are widely used for soybean cultivation, and our results also found that there was an additive estrogenic effect between glyphosate and genistein, a phytoestrogen in soybeans. However, these additive effects of glyphosate contamination in soybeans need further animal study.

Copyright © 2013. Published by Elsevier Ltd.