Role of the Type III TGF-beta Receptor Cytoplasmic Domain in Breast Cancer Progression

<p>Breast cancer remains among the most common cancers of the developed world. Despite advances in treatment modalities, deaths due to breast cancer are the second leading cause of cancer death among women. The transforming growth factor-beta (TGF-&beta;) pathway is an important modulator of breast cancer progression, acting in a tumor suppressing fashion in early carcinogenesis but switching in a poorly understood fashion to a promoter of cancer progression in later stages. Mutations and loss of function of TGF-&beta; components are common across a variety of cancers. In particular, the expression of the type III TGF-&beta; receptor (T&beta;RIII) is decreased with cancer grade and clinical progression in prostate, lung, ovarian, and pancreatic cancers. In an effort to enhance our understanding of the biology of TGF-&beta; on carcinogenesis, this dissertation looks at the role of T&beta;RIII in breast cancer progression.</p><p>Through an examination of clinical specimens, loss of T&beta;RIII was seen at both the message and protein levels with increasing tumor grade. Analysis of correlated patient outcomes showed that low T&beta;RIII expression was predictive of a shorter time to recurrence, demonstrating clinical relevance for T&beta;RIII expression. The contribution of T&beta;RIII to tumor progression was further examined by examining known TGF-&beta; functions, including proliferation, apoptosis, migration, and invasion. T&beta;RIII had no effect on proliferation or apoptosis, but had a suppressive effect on metastasis <italic>in vivo</italic>, as mammary cancer cells stably expressing T&beta;RIII that were orthotopically injected exhibited lower metatstatic burden and local invasion. <italic>In vitro</italic>, breast cancer cells exhibited suppression of migration and invasion in transwell assays. Finally, soluble T&beta;RIII (sT&beta;RIII) was shown to recapitulate the suppressive effects on invasion.</p><p>To further explore other potential mechanisms by which T&beta;RIII may be mediating its tumor suppressive effects, I examined the contribution of the cytoplasmic domain of T&beta;RIII, which is known to be critical in the regulation of T&beta;RIII cell surface expression and downstream signaling. <italic>In vitro</italic>, I demonstrated that abrogation of the cytoplasmic domain attenuates the T&beta;RIII-mediated suppression of migration and invasion. T&beta;RIII's suppressive effects are also concomitant with loss of TGF-&beta; signaling, as abrogation of the cytoplasmic domain failed to attenuate TGF-&beta; signaling while the full length receptor was able to do so. <italic>In vivo</italic>, I also showed that in the absence of the cytoplasmic domain, T&beta;RIII is unable to suppress metastasis and local invasion. Finally, a closer dissection of the cytoplasmic domain revealed that abolishing the interaction of T&beta;RIII with the scaffolding protein GIPC also attenuated T&beta;RIII's ability to dampen TGF-&beta; signaling and invasion.</p><p>In sum, T&beta;RIII was established as a prognostic marker for recurrence-free survival of breast cancer patients and as a suppressor of metastasis, migration, and invasion. Furthermore, several mechanisms contribute to T&beta;RIII's tumor suppressive effects, namely the generation of sT&beta;RIII and the interaction of T&beta;RIII with GIPC. Taken together, these studies further demonstrate the importance of TGF-&beta; signaling in cancer biology, elucidate mechanisms by which T&beta;RIII suppresses breast carcinogenesis, and expand upon our understanding of the emerging roles of T&beta;RIII in regulating tumor biology in general.</p>Dissertatio