SPP1 and osteosclerosis: Here, we found that PC3CD133+ cells showed higher expression of OPN than DU145CD133+ cells, suggesting that OPN elicits essential biological functions and may play a critical role of osteosclerosis during PC metastasis to bone.40, 41 OPN, also called secreted phosphoprotein 1 (SPP1), was initially identified as an extracellular matrix protein that inhibited formation and growth of hydroxyapatite crystals within the bone matrix and other organs.42 Osteoblasts are the major source of OPN in bone tissue.