The development of tumors in mice expressing transgenic polyomavirus middle T (MT) antigen in mammary cells has been intensively studied because it closely matches the progression of human breast cancer. Here I have examined the regulation of ERK by MT in mouse mammary gland epithelial cells. Stress responses are important in both the development and treatment of cancer. MT suppresses the strong activation of ERK by many stress conditions (serum starvation, oxidative stress, UV radiation, hypoxia or drugs causing ER stress). This is an unexpected result, since MT activates RAS, an upstream activator of ERK. This suppression is specific in that MT does not affect all aspects of response to a particular stress. MT affects ERK activation at multiple points: involvement of the ERK kinase MEK1/2 and ERK phosphatases. MT genetic analysis and gain of function assays indicated PI3K/AKT is required but not sufficient for ERK suppression by MT. In addition, based on human mammary epithelial cells MCF10A experiments, MT also uses Y322 to regulate ERK dephosphorylation.

ERK suppression by MT subjected to stress is involved in multiple cell phenotypes. Suppression of ERK improves cell survival exposed to peroxide and reduces COX2 expression caused by hypoxia. Suppression of ERK caused NMuMG cells accumulated in G1 cycle while MT-NMuMG cells can overcome ERK suppression via constitutively activated PI3K pathway. More importantly, ERK suppression also contributes to MT regulation of cytokine expression and cytokine responses.

Because ERK, which can regulate cytokine response, is affected by MT, I looked more generally at the role of MT in regulating cytokine. I found MT induces cytokines such as IL-6, CCL2 and CCL5 production. MT attenuates ERK activation induced by TNF and sensitizes TNFα-induced apoptosis. MT enhances IL-6, CCL2 and CCL5 while reduces COX2 induction by TNFα. Gene Array study showed MT regulates cytokine expression at least at RNA level both under basal activities and after TNF treatment.

Together, this work gives a new insight of polyomavirus MT signaling in regulation of ERK in response to stress and emphasizes the importance of MT regulating cytokine expression and cytokine response.

In chapter IV, I described some experiments directed at the possibilities that there is a second target for the MT Y315 site. These experiments did not reach any final conclusions.