Involvement of the matrix protein SPARC in the dynamic interaction between tumour and host cells

Tumour growth depends on the crosstalk between malignant and surrounding stromal cells (fibroblasts, osteoblasts, endothelial cells and inflammatory cells). Malignant cells secrete soluble proteins that reach neighbouring stromal cells, forcing them to provide a suitable 3D environment for their growth and spreading (metastasis). Different studies, including expression array analysis, identified the matrix protein SPARC as a marker of poor prognosis in different types of cancer.

Growth factors, like TGFbeta and PTHrP, play a role in tumour progression by modulating protein expression. However, little is known of how these external factors modulate the intracellular trafficking machinery to stimulate secretion of proteins such as SPARC.
The main aim of the project is to elucidate how aberrant growth factor signalling alters protein secretion. To investigate the effects of TGFbeta and PTHrP on SPARC secretion, we plan to use tumour cells and primary osteoblastic cells known to naturally express SPARC. We will study secretion of endogenous SPARC by immunoprecipitation as well as by measuring secretion of Green Fluorescent Protein-SPARC chimerae in response to treatment with TGFbeta and PTHrP. TGFbeta and PTHrP signalling most likely stimulates GTP exchange factors (GEF) that are specific for the small GTPases of the ARF and Rab family. We will therefore study SPARC secretion before and after knockdown of selected GTP-exchange proteins with small interfering RNA.