Effects of AhR activation on the Wnt pathway and CK2 subunits
Although there are hereditary risk factors strongly associated with breast cancer, only a small percentage of breast tumors can be attributed to these. Instead, it is believed that 85-90% of breast cancers are primarily of environmental origin. Polycylic aryl hydrocarbons (PAHs) are environmental carcinogens derived from combustion including fossil fuels. PAHs bind to a cellular aryl hydrocarbon receptor (AhR) that mediates downstream events leading to cellular transformation. Previous work in our laboratory used the prototypical PAH 7,12-dimethylbenz[a]anthracene (DMBA) to form tumors in mouse mammary glands that had constitutive AhR activation, increased Wnt signaling, and strong induction of the CK2 subunit CK2α (Currier, Solomon et al. 2005). Wnt, an important developmental pathway, is implicated in several cancers (Dominguez, Sonenshein et al. 2009). CK2 is a highly promiscuous, constitutively active serine/threonine kinase that is over-expressed in every cancer that has been examined for its presence (Meggio and Pinna 2003). Data from the DMBA-induced mouse tumors demonstrated that these factors may be involved in carcinogen-induced breast cancer, but their role in tumor development is uncertain. The hypothesis underlying this project was that CK2 and the Wnt pathway would be involved in early changes in mouse mammary and liver tissues in animals exposed to DMBA. We used qPCR, Western blotting, and immunohistochemistry to look at changes in Wnt pathway components and known Wnt-dependent genes, and CK2 subunits in mammary and liver tissues one and two weeks after DMBA exposure. Liver tissue was analyzed along with mammary gland tissue because the liver is the site of DMBA metabolism. Our results showed no change in liver or mammary gland morphology at these time points. There was induction of the AhR gene targets cyp1a1 and cyp1b1 in liver tissue but not in mammary gland. Liver also had evidence of Wnt pathway activation. Mammary glands did not have a strong AhR response but did show Wnt induction at two weeks post-exposure, suggesting that DMBA was affecting the liver before the mammary glands. CK2α had an unexpected early decrease in protein expression at one week in liver, which at two weeks resolved to the same levels as control tissue. In mammary glands, CK2α expression levels were the same as control at one week and decreased at two weeks, again suggesting a slower response than liver. Interestingly, CK2β was markedly overexpressed in mammary glands at the two week time-point. These results suggest there is a role for both Wnt and CK2 in early DMBA-generated tissue changes. It is still unclear if these pathways are separately affected by DMBA or if one initiates the other. Further experimentation, possibly in cell culture using inhibitors and siRNA, are called for to better understand these findings.
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