Maspin modulates adhesion of bladder carcinoma cells to vascular endothelium

Maspin belongs to the serpin family and has been shown to suppress tumor growth and metastasis in several tumor types. The role of maspin in bladder carcinoma has not been fully elucidated, and the object of this study was to investigate whether maspin contributes to bladder tumor adhesion to vascular endothelial cells (HUVEC). Expression of maspin-coding mRNA was evaluated in a panel of bladder carcinoma cell lines. Maspin distribution in maspin mRNA(high) versus maspin mRNA(low) cells was further analyzed by flow cytometry and confocal microscopy. Adhesion to HUVEC was measured in a coculture model and correlated with the surface-bound maspin. Maspin(high) (RT-4, RT-112) cell lines strongly attached to HUVEC, whereas maspin(low) (UMUC-3, MGH-U1) cell lines poorly adhered to HUVEC. Distinct cytoplasmic maspin accumulation and moderate surface-bound maspin was found in RT-4 cells. Blocking maspin surface receptors prevented tumor cell attachment to HUVEC, indicating that surface-bound maspin is responsible for triggering cell adhesion. PMA-triggered elevation of surface-bound maspin was accompanied by an enhanced adhesion capacity of RT-4 cells, compared to controls. Finally, exposing the bladder carcinoma cells to the differentiation-inducing agent valproic acid led to a surface-bound (but not cytoplasmic) maspin decrease, paralleled by a significant reduction in tumor cell binding to HUVEC. Surface-bound maspin directly controls bladder carcinoma cell adhesion to the vascular wall. Blocking this process may prevent transendothelial migration and tumor cell dissemination. Therefore, therapeutic down-regulation of surface-bound maspin might become an option to prevent tumor spread into distant organs.