Understanding the role of SPARC in tumour-stroma interactions using 2D and 3D culture models

Munasinghe, Dandagama Mudiyanselage Hansanie Amanda Thathsaranie (2017) Understanding the role of SPARC in tumour-stroma interactions using 2D and 3D culture models. (PhD thesis), Kingston University, .


Pancreatic ductal adenocarcinoma (PDAC) is the deadliest of all solid tumours. The aggressive nature of the tumour, a strong tumour-supportive environment (stroma), late diagnosis, and poor responsiveness to chemotherapy contribute to a poor patient prognosis that has remained largely unchanged over the last forty years (5-year survival of less than 5%). The tumour-stroma is a dynamic network of stromal cells, infiltrating immune/inflammatory cells, architectural extracellular matrix (ECM) proteins and matricellular proteins. Secreted protein acidic and rich in cysteine (SPARC) is matricellular protein secreted by stromal cells that mediates cell-cell and cell-matrix interactions. Aberrant SPARC expression is observed in many types of tumour but, controversially, SPARC is anti-proliferative. However, clinically there is a strong association between stromal SPARC expression and poor patient prognosis. This study aims to elucidate SPARC's seemingly controversial roles in PDAC by assessment of its protein structure, binding partners and function in PDAC cell proliferation, apoptosis, adhesion and migration. In this study, we demonstrate that SPARC is only anti-proliferative under low serum conditions. Under physiological conditions where SPARC is exposed to serum components, SPARC promotes the proliferation of PDAC cells. This data indicates that non-physiological conditions are responsible for the inconsistency in data on SPARC function between 'in vitro' studies and clinical observations. Furthermore, we identify plasma fibronectin (pFN) as a serum component that can regulate SPARC's function in PDAC cell proliferation. Importantly, FN depletion from serum not only prevents the proliferation promoting effects of SPARC in PDAC cells, but also switches the effect of SPARC such that it instead induces PDAC cell apoptosis. No evidence was found to suggest a strong direct interaction between SPARC and FN, suggesting that the effect of FN on SPARC function may be mediated indirectly. Together, this data suggest that it may be possible to manipulate SPARC, which is highly expressed in aggressive PDAC tumours, such that it works against the tumour for PDAC therapy. We have also found evidence that SPARC is differentially glycosylated in specific cell types and this may contribute to the tissue-specific effects of SPARC reported in literature. However, it is also possible that tissue-specific levels of SPARC modulating proteins such as FN play a role. SPARC has been reported to modulate invasion and metastasis in many tumours. Furthermore, SPARC is often labelled as a de-adhesive protein due to its known effects on endothelial cells 'in vitro'. However, we found that SPARC has no effect on initial attachment of PDAC cells or on cell migration, but instead promotes strong late-stage adhesion. This suggests that the primary role of SPARC in PDAC is to promote adhesion and initiate anchorage-dependent growth, rather than promoting invasion and metastais.

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