SPARC, a matricellular protein that protects cancer from therapy

Santangelo, Alessandra (2014). SPARC, a matricellular protein that protects cancer from therapy. PhD thesis The Open University.



The matricellular protein SPARC (secreted protein acidic and rich in cysteine) is a master regulator of tissue stroma; it is expressed during tissue remodeling and repair and its impaired regulation has a role in fibrotic responses, in determining the composition of the tumor-associated stroma and, less expected, in regulating the immune response. The complexity of studying SPARC in cancer stems from its concomitant or alternative expression in tumor and/or stroma cells depending on the tumor histotypes. Extracellular matrix (ECM) gene profile of human breast carcinomas correlates SPARC expression with prognosis and response to therapy. In particular, a subgroup of breast tumors can be identified based on ECM gene signature headed by SPARC that correlates with tumor grade, bad prognosis and poor response to therapy. Epithelial to mesenchymal transition (EMT) has been associated to drug resistance and SPARC has been implicated in EMT. Nevertheless the source of SPARC and its path toward EMT and the associated drug resistance have not been identified yet. I have used a well-defined model of transgenic mouse mammary carcinoma expressing the mutated rat oncogene c-erB2 (HER-2/neu), under the mouse mammary turn or virus promoter, backcrossed with Sparc-/- mice to establish mammary carcinoma cell lines devoid of SPARC expression, in which SPARC can be restored by retroviral gene transduction. With this isogenic cell lines I have been able to show that SPARC expression in tumor cells induces EMT only in vivo, indicating that EMT is a micro environmental and not a cell autonomous process. Moreover, SPARC expressing tumors became resistant to treatment with Doxil (Doxil, a pegylated form of Doxorubicin). Investigating the environmental players responsible of EMT in SPARC transduced tumors, I found that myeloid cells and particularly the so called myeloid derived suppressor cells (MDSC) have a role in EMT and drug resistance depending on SPARC. SPARC does not change the number or ratio between the Ly6Ghigh and the Ly6Glow fractions of CD11b MDSC rather it determines the immunosuppressive phenotype of recruited myeloid cells. The data show that SPARC-forced expression increased the activation of pro-turn oral (CCL2, CCL5) and immunosuppressive (ArgI, NOS2, Cox-2) genes in monocyte-MDSC (the Ly6Glow fraction). Furthermore, the role of myeloid cells recruited in Doxil resistance and EMT has been proven adding bisphosphonate to Doxil during treatment. , Bisphosphonates have been shown to inhibit induction and function of myeloid suppressor cells. Indeed, Bisphosphonates addition reverted EMT, the immunosuppressive phenotype of myeloid cells and rendered SPARC-producing tumors sensitive to Doxil administration.

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