Genetic deletion of osteopontin in TRAMP mice enhances the growth of aggressive prostate cancer with feature of human neuroendocrine tumors

Osteopontin (OPN) is a secreted glycoprotein, belonging to the non-structural extracellular matrix (ECM), and its over expression in human prostate cancer cells has been associated with disease progression, androgen independence and metastases. Nevertheless the pathophysiology of OPN in prostate tumorigenesis has never been studied. We crossed TRansgenic Adenocarcinoma of the Mouse Prostate (TRAMP) mice with OPN deficient (OPN-/-) mice and followed tumor onset and progression in these double mutants. Ultrasound examination detected the early onset of a spherical homogeneous tumor in about 60% of OPN-/- TRAMP mice that seldom occurs in parental TRAMP mice. Histology and immunohistochemistry characterized these tumors for being Tag positive but negative for AR, highly proliferative and endowed of neuroendocrine (NE) features. Gene expression profiling showed up-regulation of genes involved in tumor progression, cell cycle and neuronal differentiation in OPN-deficient versus -sufficient TRAMP tumors. Down-regulated genes included key genes of TGF pathway, and a role for TGF in NE differentiation of prostate cancer was also confirmed at the protein level. Furthermore, NE genes and particularly those characterizing early prostatic lesions of OPN-deficient mice were found to correlate with those of human NE tumours. These data underscore a novel role of OPN at early stages of prostate cancer growth, protecting against the development of aggressive NE tumors. Total RNA obtained from prostate tumors from 18 and 30 weeks old TRAMP mice, compared to RNA extracted from prostate tumors and prostate tissue from Osteopontin-deleted TRAMP mice