Inorganic phosphate as a signaling molecule: a potential strategy in osteosarcoma treatment

Inorganic phosphate (Pi) is an essential nutrient to living organisms. It plays a key role in diverse biological processes, including osteoblast differentiation and skeletal mineralization. Maintenance of proper Pi homeostasis is a critical event, as any deviation from that state can lead to several acute and chronic disease states and influence the ageing process and lifespan. Serum Pi level is maintained within a narrow range through a complex interplay between intestinal absorption, exchange with intracellular and bone storage pools, renal tubular reabsorption and depends mainly on the activity of Na/Pi cotransporters. Pi is abundant in the diet and intestinal absorption of Pi is efficient and minimally regulated. The kidney is a major regulator of Pi homeostasis and can increase or decrease its Pi reabsorptive capacity to accommodate Pi need. Relevantly, Pi is emerging as an important signalling molecule capable of modulating multiple cellular functions by altering signal transduction pathways, gene expression and protein abundance in many cell types. However, little is known about the initial events involving the detection of changes in serum or local Pi concentrations and the subsequent downstream regulation cascade. Previously, we provided evidence that Pi inhibits proliferation and aggressiveness of human osteosarcoma U2OS cells identifying adenylate cyclase, beta3 integrin, Rap1, ERK1/2 as proteins whose expression and function are relevantly affected in response to Pi. More recently, we demonstrated that Pi is capable also of inducing sensitization of osteosarcoma cells to doxorubicin in a p53-dependent manner and through a mechanism involving ERK1/2 down-regulation. This review summarizes the current knowledge regarding inorganic phosphate as a novel specific signaling molecule in bone and other cell types in mammals and discuss how targeting Pi levels at local sites might represent a potential strategy for improving osteosarcoma therapy.