es and activates p53 which then blocks cellular proliferation. Alternatively, ARF inhibits cellular growth in a p53-independent manner by 9349566 regulating ribosomal biogenesis through nucleophosmin. Arf-deficient mice develop undifferentiated sarcomas, lymphomas, and carcinomas with a mean life expectancy of,9 months. Mice lacking both Arf and p53 exhibit a tumor spectrum distinct from that observed in single knockout animals, implying that ARF and p53 can act independently to suppress tumor formation. Not surprisingly, expression of oncogenes in Arf-/- transgenic mice affects the timing and characteristics of the tumors that arise. For example, Arf-/mice transgenic for Em-myc developed aggressive B cell lymphomas with a dramatic acceleration in onset and increased mortality when compared to Arf+/+ Em-myc mice. Additionally, c-myc overexpression in Arf-/- bone marrow resulted in the rapid development of acute myeloid leukemia. ARF possesses activities that are independent of its role as a tumor suppressor. Arf-deficient mice succumb to blindness soon after birth. In this case, the lack of programmed cell death within the hyaloid vasculature leads to an accumulation of perivascular cells and destruction of the retina and lens. We have previously described that Arf deficiency leads to increased osteoclast function and accelerated differentiation through nucleophosmin, independent of p53. We now report that Arf-deficient mice also develop enhanced osteoblast function and that the December 2010 | Volume 5 | Issue 12 | e15755 ARF Regulates Bone Biology and Osteosarcoma co-regulation of OB and OC in the absence of ARF leads to constitutively increased bone BIBW 2992 remodeling in vivo. The role of ARF in bone remodeling carries implications for cancer biology. p53, which is regulated by ARF, has also been demonstrated to regulate OB function. Moreover, the bone microenvironment is known to support tumor growth in leukemia, myeloma and solid tumor bone metastasis. To pursue the implications of accelerated bone remodeling on tumor development we bred Arf-/- mice with mice expressing the viral oncogene Tax. Since the lymphoid tumors that arise in Tax mice cause significant bone loss , we hypothesized that crossing Arf-/- mice with Tax mice would exacerbate tumorigenesis and tumor mediated osteolysis. Unexpectedly, the Tax+Arf-/- mice became highly susceptible to development of spontaneous osteosarcoma. Osteosarcoma is the most common primary bone malignancy, with a peak incidence in adolescence, a time of high bone remodeling, and 5-year survival rates in patients with metastatic or relapsed disease of,20%. p53 mutations and Rb loss is common in human OS and recent mouse models have provided clear evidence that OBrestricted deletion of p53 and Rb leads to OS. ARF loss is also common in human OS. We found that the OS that spontaneously arose in Tax+Arf-/- mice recapitulates the malignancy described in the p53/Rb OB-specific deletion models. We further hypothesized that the increased bone remodeling in Arf-/- mice contributed to OS development and provide evidence that zoledronic acid, a bisphosphonate inhibitor of bone resorption, prevented or delayed onset of OS in Tax+Arf-/- mice. 
These data demonstrate that ARF regulates bone remodeling through cell autonomous effects on both OB and OC and introduce Tax+Arf-/mice as a new model of spontaneous OS. This discovery implicates bisphosphonates as novel therapeutics for targeted prevention of OS after resection of p
