Environmental Pollutants and AHR pathway in Uterine Leiomyoma

Uterine leiomyomas (LM, fibroids) disrupt uterine function and cause recurrent pregnancy loss, excessive uterine
bleeding, and anemia in 15-30% of reproductive-age women. No long-term medical treatment is available.
Understanding how a LM develops is essential for identifying new non-surgical treatments. MED12 mutations
(mut-MED12) occur in 70% of all LM and drive LM growth in a steroid hormone-dependent manner. Pre- and
peri-menopausal women are widely exposed to endocrine disrupting chemicals (EDCs), e.g., phthalates, which
are found in many consumer products and associated with a number of reproductive diseases. How a mut-
MED12 influences LM growth and whether this process is enhanced by exposure to EDCs remain unknown. We
recently found that tryptophan (Trp) 2,3-dioxygenase (TDO2) gene expression is strikingly upregulated in mut-
but not wild-type (wt)-MED12 LM. TDO2 catalyzes a critical step in Trp breakdown to kynurenine (Kyn), an
endogenous ligand for the aryl hydrocarbon receptor (AHR) whose activation stimulates the expression of pro-
growth genes to promote cell survival and proliferation in various tissues. Kyn levels are markedly higher in LM
vs myometrium, particularly in mut-MED12 LM. Trp and Kyn treatments of LM cells activated AHR and increased
cell survival; blocking the Trp-Kyn-AHR pathway by siRNA knockdown or inhibitors of TDO2 or AHR abolished
these effects, with mutant LM cells showing higher sensitivity to the treatments. Epidemiological studies have
shown positive associations between LM and exposure to di(2-ethylhexyl) phthalate (DEHP). In vitro, mono(2-
ethyl-5-hydroxyhexyl) phthalate (MEHHP), a major metabolite of DEHP, stimulated the expression of Trp
transporters (LAT1 and LAT2), increased Trp uptake and Kyn production, activated AHR, and promoted LM cell
survival. In addition, progesterone receptor is crucial for the expression of AHR and its nuclear translocator
ARNT. Thus, the Trp-Kyn-AHR pathway appears to be a hub at which mut-MED12, steroid hormone action, and
EDC effects converge enabling LM growth. Our overall hypothesis is that increased Trp uptake and metabolism,
Kyn production, and AHR pathway activation, as a result of mut-MED12 or high exposure to the environmental
pollutant DEHP, promote cell survival and proliferation and lead to LM growth. Using a xenograft mouse model
and genome-wide studies, we will test our hypothesis in the following Aims: (1) Define the functional role of the
Trp-Kyn-AHR pathway in LM tumorigenesis. Hypothesis: elevated expression of the TDO2 enzyme in mut-
MED12 LM causes Kyn overproduction that activates AHR and promotes proliferation and survival of smooth
muscle cells and tumor growth. (2) Determine whether DEHP stimulates LM growth via activation of the Trp-
Kyn-AHR pathway. Hypothesis: exposure to DEHP and its metabolite MEHHP upregulates the expression of Trp
transporters to increase its uptake, resulting in increased Kyn production and AHR activation leading to LM cell
survival and tumor growth. The study will link, for the first time, abnormal amino acid metabolism and LM growth,
opening a new avenue for translational research and the development of novel therapeutics for LM.

Grant Number: 5R01ES034753-03
NIH Institute/Center: NIH
Principal Investigator: Serdar Bulun