Bone-related growth factors and zoledronic acid regulate the PTHrP/PTH.1 receptor bioregulation systems in MG-63 human osteosarcoma cells. Bisphosphonates are known to inhibit osteoclast-mediated bone resorption and osteoblast differentiation and are currently used in the treatment of Paget's disease, osteoporosis, metastatic and osteolytic bone disease and hypercalcaemia of malignancy. The parathyroid hormone-related peptide (PTHrP) and type 1 PTH/PTHrP receptor (PTH.1R) bioregulation systems mediate a wide range of local paracrine/autocrine and intracrine functions in various tissues and modify the actions of pharmaceutical agents on target tissues, both in vivo and in vitro. In addition, bone microenvironment-related growth factors, such as insulin-like growth factor 1 (IGF-1), transforming growth factor beta 1 (TGF beta 1), basic fibroblast growth factor (bFGF) and interleukin 6 (IL-6), can modify the actions of various pharmaceutical agents, including cytotoxic drugs in malignant cell lines. Whether IGF-1, TGF beta 1, bFGF, IL-6 and zoledronic acid affect the expressions of PTHrP and PTH.1R in MG-63 osteoblast-like osteosarcoma cells was investigated in this study. Relative quantitative-PCR (expression at mRNA level) and immunofluorescence analysis (localization of the expression at protein level) were employed to assess PTHrP and PTH.IR expressions. Our data showed that primarily IGF-1, TGF beta 1 and IL-6 (up to 25 ng/ml for 48 h) increased PTHrP mRNA expression and modified its perinuclear localization, while zoledronic acid (up to 100 microM for 48 h) inhibited cell proliferation and suppressed PTHrP expression in the MG-63 osteosarcoma cells. These growth factors were incapable of reversing the zoledronic acid decrease of the expression of PTHrP in the MG-63 cells, suggesting that zoledronic acid and the growth factors affect PTHrP expression via an independent intracellular signal transduction pathway in these cells. However, no appreciable modulation of the PTH.1R expression by IGF-1, TGF beta 1, bFGF, IL-6 or zoledronic acid was detected in MG-63 cells. Therefore, we conclude that PTHrP expression possibly mediates the action of bone microenvironment-related growth factors and of zoledronic acid in MG-63 cells.