Role of the matricellular protein SPARC in breast tumor growth and metastatic dissemination

LORENA GABRIELA BENEDETTI; EDGARDO SALVATIERRA; LEANDRO GÜTTLEIN; CRISTOBAL FRESNO; ELMER FERNÁNDEZ; SABRINA MANSILLA; VANESA GOTTIFREDI; ANDREA S. LLERA; OSVALDO LUIS PODHAJCER

Chicago

Congreso; Annual Meeting of the American Association for Cancer Research; 2012

AACR

Breast cancer is the leading cause of death on women worldwide. Whereas surgical resection and adjuvant therapy can cure well-confined primary tumors, metastatic disease is largely incurable because of its systemic nature and the resistance of disseminated tumor cells to existing therapeutic agents. SPARC (Secreted Protein Acidic and Rich in Cystein) is a matricellular protein whose normal expression is associated with remodeling tissues. In this context SPARC was described as involved in cell cycle and proliferation, invasion, adhesion, migration, angiogenesis and apoptosis. Expression array technology among other approaches identified SPARC as marker of poor prognosis, very often associated with most aggressive tumors in the vast majority of human cancer types. However, there is confusing data regarding the role of this protein during the development and progression of breast tumors. In order to achieve a better understanding of the role of this protein during breast cancer progression, we used the murine metastatic breast cancer model, 4T1. We found that silencing SPARC in these epithelial cells, after transduction with a lentiviral vector carrying a siRNA against murine SPARC, results in the completely loss of their metastatic capability (p<0.001) and inhibits the in vivo growth capability (p<0.001) in an immunocompetent murine model. This was consistently found in three different siRNA-expressing 4T1-derived cell clones. In order to identify genes involved in this SPARC-induced behavior we designed two microarray studies to compare gene expression in the c18-4T1 SPARC-deficient clone and the control SCR-4T1 cells. The first array was aimed to compare gene expression of cells grown in vitro (in vitro array) while the second array was carried out using orthotopic tumors and their derived lung metastasis (in vivo array). When analyzing the in vitro array, the comparative ontological analysis indicated that SPARC knockdown alters mainly the transcription of cell cycle and extracellular matrix response genes. In agreement, functional assays confirmed that the downregulation of SPARC in tumor cells enhances the G1/S checkpoint. In the in vivo array, the alteration of the transcription of immune response genes became evident, indicating a strong contribution of SPARC to the tumor implantation response. Moreover, the changes in the extracellular matrix and immune response gene expression were evident both in the metastasis and in the primary tumor. In addition, we were able to identify a ?minimal SPARC-dependent metastasis signature?, found in early SPARC-expressing tumors but not in SPARC-deficient ones, including 22 genes with expression levels similar to those found in their derived metastasis. Validation of this signature by meta-analysis of previously published human tumor signatures might serve to further test the relevance of SPARC in breast cancer progression.