Abstract C70: Trop2 regulates prostate tumorigenesis and stem cell self-renewal

Cancer cells share multiple characteristics with adult tissue stem cells such as self-renewal, proliferative capacity and differentiation. Several groups have identified Trop2, a type I trans-membrane glycoprotein, as a marker of stem/progenitor cells in adult tissues. Consistent with the relationship between stem cells and tumorigenesis, we have previously identified that Trop2hi cells are efficient targets for transformation in the prostate. Additionally, Trop2 is highly expressed in various cancers including advanced colorectal, pancreatic, prostate, ovarian and pulmonary carcinomas and is associated with unfavorable outcome. However the functional role of Trop2 in stem cell self-renewal, its relation to transformation, and the molecular mechanisms by which Trop2 transmits signals to regulate these processes remain unclear. We hypothesize that heightened Trop2 may promote the stem-like properties of self-renewal and proliferative activity in prostate cancer. We observe significantly elevated levels of Trop2 in human prostate cancer, lymph node metastases and in mouse models of prostate cancer. Over-expression of Trop2 induces anchorage-independent growth in vitro and murine prostatic intraepithelial neoplasia (mPIN) in vivo. In vitro sphere and in vivo regeneration assays demonstrate that Trop2 regulates stem/progenitor self-renewal in the prostate, suggesting that elevated levels of Trop2 in prostate cancer are selected for by promoting tumor cell self-renewal. Biochemical analysis reveals that Trop2 is activated by regulated intra-membrane proteolysis (RIP), resulting in shedding of the extracellular domain (ECD) and release of the intracellular domain (ICD) to the nucleus. RIP is required for Trop2 activity and cleavage mutants of Trop2 fail to enhance self-renewal in vitro. We generated expression constructs of Trop2 ICD and ECD to test their role in self-renewal and tumorigenesis. Secreted Trop2 ECD induces Trop2 cleavage and results in increased stem/progenitor cell proliferation. Trop2 ICD alone promotes self-renewal activity and transformation as measured by sphere formation and in vivo regeneration assays, respectively. Trop2 ICD is sufficient to induce mPIN lesions similar to previously reported mouse models of prostate cancer. These findings suggest that the ICD is the functional unit of Trop2. Trop2 ICD is involved in a signaling cascade that is dependent upon β-catenin to promote self-renewal. We find Trop2 ICD, but not the ECD, forms a complex with β-catenin. While β-catenin loss does not affect baseline sphere-forming activity, we find that over-expression of Trop2 requires the presence of β-catenin to enhance progenitor activity in vitro. Taken together, our study provides evidence that Trop2 is a key regulator of transformation and self-renewal in the prostate and its function depends on its activation by RIP and downstream signaling through β-catenin. Delineating the molecular mechanism of Trop2 action allows development of therapeutic strategies applicable to not only prostate cancer, but other epithelial malignancies that exhibit increased Trop2 expression. Such strategies include small molecules able to block Trop2 proteolysis, thus inhibiting Trop2 function. More specific therapies such as monoclonal antibodies that will interfere with Trop2 processing and consequent activation could also represent promising future anticancer therapies. Citation Format: Tanya Stoyanova, Andrew Goldstein, Houjian Cai, Justin M. Drake, Jiaoti Huang, Hong Zhang, Owen N. Witte. Trop2 regulates prostate tumorigenesis and stem cell self-renewal [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr C70.