Date of Graduation
Doctor of Philosophy (PhD)
Francois X. Claret
Mary E. Edgerton
Zahid H. Siddik
HER2-positive breast cancer, which is characterized by the over-expression of the HER2 onco-protein, accounts for approximately 20% of all breast cancer cases. Trastuzumab (Herceptin), the first targeted therapy approved for HER2-positive disease, potently prevents the activation of signaling pathways downstream of HER2 and significantly improves patients’ outcomes. However, resistance to trastuzumab is inevitable; such resistance can occur through reduced expression of PTEN protein.
Jab1 is over-expressed in 50% of primary cancers and 90% of metastatic tumors. Our lab previously showed that depletion of Jab1 in combination with trastuzumab treatment up-regulated PTEN in mouse xenografts refractory to trastuzumab. PTEN was not detected in the control Jab1 knockdown. However, how Jab1 modulated trastuzumab responses and affected PTEN function was incompletely understood. The overall goal of my project was to identify the role of Jab1 in regulating PTEN and in contributing to trastuzumab resistance in HER2-positive breast cancer. I demonstrated that Jab1 mediated the post-translational regulation of PTEN by associating with PTEN and facilitating its degradation. I also found that the C-terminal end of Jab1 was required for Jab1 to induce degradation of PTEN. Furthermore, I showed that proteasome inhibitors failed to prevent PTEN degradation induced by Jab1 over-expression in breast cancer cells. Instead, the combination of lysosomal protease inhibitors - E64D and pepstatin A - significantly impaired the ability of Jab1 to degrade PTEN. Further, I showed that silencing Jab1 increased trastuzumab’s inhibitory effects on cell proliferation. In contrast, the introduction of Jab1 into breast cancer cells conferred resistance to trastuzumab.
Taken together, my findings suggest that Jab1 negatively regulates PTEN and promotes trastuzumab resistance in HER2-positive breast cancer.
HER2-positive, breast cancer, Jab1/CSN5, PTEN, trastuzumab, resistance, ubiquitin, proteasome, lysosome, chaperon-mediated autophagy