Glycoprotein asporin as a novel player in tumour microenvironment and cancer progression

Background. Small leucine rich proteoglycans (SLRPs), major non-collagen components of the extracellular matrix (ECM), have multiple biological roles with diverse effects. Asporin, a member of the SLRPs class I, competes with other molecules in binding to collagen and affects its mineralization. Its role in cancer is only now being elucidated. Methods. The PubMed online database was used to search relevant reviews and original articles. Furthermore, altered asporin expression was analysed in publicly available genome-wide expression data at the Gene Expression Omnibus database. Results. Polymorphisms in the N-terminal polyaspartate domain, which binds calcium, are associated with osteoar-thritis and prostate cancer. Asporin also promotes the progression of scirrhous gastric cancer where it is required for coordinated invasion by cancer associated fibroblasts and cancer cells. Besides the enhanced expression of asporin observed in multiple cancer types, such as breast, prostate, gastric, pancreas and colon cancer, tumour suppressive effects of asporin were described in triple-negative breast cancer. We also discuss a number of factors modulating asporin expression in different cell types relevant for alterations toing the tumour microenvironment. Conclusion. The apparent contradicting tumour promoting and suppressive effects of asporin require further investigation. Deciphering the role of asporin and other SLRPs in tumour-stroma interactions is needed for a better understanding of cancer progression and potentially also for novel tumour microenvironment based therapies.