Defense Date

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Biochemistry

First Advisor

Andrew Larner

Abstract

The Jak/Stat signaling cascade mediates cell proliferation, differentiation, survival, apoptosis and immune responses. Aberrant activation of this pathway mediates neoplastic transformation and abnormal growth of many malignancies including breast cancer, the most common cancer among women, and the second leading cause of cancer deaths in women in United States. The mechanism by which the Jak/Stat pathway modulates the pathogenesis of breast cancer is unclear. This dissertation elucidates roles of Jak/Stat members that mediate the pathogenesis of breast cancer. For these studies, we used 4T1 mouse mammary tumor cells as a model which mimics human breast cancer. First, we investigated the role of Tyk2 tyrosine kinase in the pathogenesis of breast cancer. Here we show for the first time that compared with wild type mice, Tyk2 -/- mice show increased tumor growth rate as well as metastatic disease and splenomegaly when inoculated with 4T1 breast cancer cells. Such increased tumorigenicity was associated with a significant decrease of IFNg production in 4T1 tumor-bearing Tyk2 deficient mice T cells compared with wild type (WT) mice. We demonstrated that NK cells or CD8+ T cells control tumor growth in both Tyk2-/- and WT mice, but neither Tyk2-/- NK cells alone nor Tyk2-/- CD8+ T cells alone do not contribute to enhanced tumor growth and metastatic disease of Tyk2-/- mice. Tumor-bearing Tyk2-/- mice have increased level of myeloid-derived suppression cells than tumor-bearing mice. Tyk2-/- MDSCs have a slight increase in suppression of T cell proliferation. Since elevated phosphorylation of Stat3 has been seen in human and murine breast cancer, and expression of Stat3 in the mitochondria (mitoStat3) appears to have important affects on cell growth, we studied the ability of Stat3 targeted to the mitochondria (MLS Stat3) to influence growth and metastasis of 4T1 cells. We show that a serine mutant of Stat3 expressed in the mitochondria (Stat3 S727A) inhibits the ability of 4T1 tumor cells to grow and metastasize. In contrast, a serine to aspartic acid mutant of Stat3 (S727D) enhances tumorigenesis. We found that expression of mitochondrial-targeted Stat3 does not affect cell growth rate in cell culture under normal conditions, however in low glucose, the serine to alanine mutant shows reduced growth rate and ability to invade. Moreover, we found that expression of mitochondrial-targeted Stat3 protects cells from hypoxia. Our data indicate that serine phosphorylation of mitochondrial-localized Stat3 is required for cell transformation. In summary, our studies provided new insights into the role of Stat3 in breast cancer and suggest new therapeutic targets for the treatment of this disease.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

February 2010

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