We recently observed delayed onset of mammary tumors
in mice fed diets containing soy isoflavones (Jin
and MacDonald, 2002). The primary investigator of this project
was Zeming Jin, PhD who is currently a Post-Doctoral Fellow at the
MD Anderson Cancer Research Center. The study used transgenic mice,
MMTV-neu, purchased from Jackson Labs, that spontaneously develop
mammary tumors. This animal provides a model that correlates well
to human disease because about 20% of breast cancers have increased
neu/erbB2 expression. Female mice were fed diets containing no isoflavones
(control), genistein (250 mg/kg diet), daidzein (250 mg/kg diet)
or a mixture of isoflavones derived from soy, NovaSoy, which is
a commercial product provided to us by the Archer Daniels Midland
Company, Decatur, IL. The mice were fed the diets beginning at 7
weeks of age, they underwent one pregnancy with two weeks of lactation,
and were euthanized at about 56 weeks of age during the diestrous
phase. |
The mice fed genistein, daidzein or NovaSoy developed tumors about
4 weeks later than mice fed the control diet (Figure 1). |
|
| There was no effect of the diets on growth or final body weight,
however mice fed the isoflavones had significantly heavier uterus
plus ovary weights compared to the mice fed the control diet (Figure
2). |
|
| This suggests the isoflavones had an estrogenic effect on the animals.
Although the isoflavones caused a delay in the development of mammary
tumors, they were ineffective in preventing tumor growth. Hence, when
the study ended there was no difference in tumor number, weight or
total tumor weight due to diet (Figure 3). |
|
Genistein and daidzein are the primary isoflavones found in soy. These
compounds are structurally similar to the mammalian hormone estrogen
(Figure 4) and are biologically active in animals. Others have shown
that genistein and daidzein bind to estrogen receptors and elicit
biological responses. In some systems the response mimic that of estrogen
(agonist) but in other systems they oppose the effects of estrogen
(antagonist). The activity appears to be cell-specific, and is dependent
on both the concentration of the isoflavone and the level of endogenous
estrogen present. Genistein, but not daidzein, has been found to inhibit
tyrosine kinases as well. Tyrosine kinases are enzymes that phosphorylate
tyrosine residues of proteins, usually those associated with cell
signaling pathways of hormone receptors. Through phosphorylation and
dephosphorylation of signaling proteins, cellular events, such as
proliferation and apoptosis, are activated and inactivated. Hence,
genistein may affect intracellular events through interacting with
the estrogen receptor, or via altering tyrosine kinases of various
hormone signaling pathways. |
We have measured several tyrosine kinase proteins in the tumor tissue
obtained from the mice in our study in order to identify potential
mechanisms to explain the observed delay in tumor development. We
measured, using Western immunoblot, mitogen activated kinase (MAPK
1 and 2), neu, and Akt 1 in tumor samples from mice fed genistein
or control diets. There was a general trend for decreased expression
of MAPK 1,2 and neu in mice fed genistein compared to control (Figure
4) however the differences were not statistically significant. |
|
The concentration of phosphorylated Akt was suppressed in tumors from
mice fed genistein compared to control (Figure 5). These results suggest
that genistein may block phosphorylation of tyrosine kinases within
signaling pathways associated with cell proliferation and apoptosis.
|
|
| To further explore these findings we are using cell culture systems.
The BT-474 cell line is similar to the animal model (MMTV-neu) we
used because it overexpresses neu. We have observed decreased cell
proliferation by genistein in a dose-dependent manner in these cells
(Figure 6). |
|
We are continuing to explore the role of genistein in affecting signaling
pathways and the interdependence of the response to endogenous estrogen
availability. The primary investigator of this work is Mary Sakla, who
is working on her PhD in the Genetics Area Program at the University of
Missouri.
Another avenue of research is to define the effects of isoflavones on
enzymes associated with the metastatic progression of breast cancer. We
have initiated studies on the expression of MMP-3 by BT-474 cells following
exposure to genistein. The primary investigator of this project is Mona
Hdeib, a Biology major at the University of Missouri.
RELATED PUBLICATIONS:
Day JK, Besch-Williford C, Lubahn DB, MacDonald RS.
Dietary genistein increased DMBA-induced mammary adenomas in wild-type
but not ERaKO mice. Nutrition and Cancer, 39:226-232, 2001.
Day JK, Bauer AM, DesBordes C, Zhuang Y, Kim BE, Newton LG, Nehra V,
Forsee KM, MacDonald RS, Besch-Williford C, Huang TH,
Lubahn DB. Genistein alters DNA methylation patterns in mice. J Nutrition
132:2419S-2423S, 2002.
Breast Cancer | Prostate
Cancer | Colon Cancer | Biological
Role of Zinc
