Phosphate wasting in oncogenic osteomalacia: PHEX is normal and the tumor-derived factor has unique properties

Oncogenic osteomalacia (OOM) is characterized by renal phosphate wasting an d abnormal metabolism of vitamin D, somewhat similar to the phenotype of X- linked hypophosphatemic rickets (HYP). DNA from OOM tumor cells was analyze d for mutations in the PHEX gene, which is mutated in HYP, Screening for mu tations by single-strand conformation polymorphism analysis and subsequent sequencing of all the exons revealed no mutations. Conditioned media from l ong-term cultures of OOM tumor cells were used to further characterize the physical properties of the phosphate-regulating factor and its mechanism of action, Inhibition of OK 3B2 cell renal phosphate transport by conditioned media was dose-dependent and maximal after 20 h, This time course differed from that of parathyroid hormone (PTH). The bioactivity was stable to mild acid and alkali treatment and freeze drying and was retained in the aqueou s phase following organic solvent extraction. The activity was not suppress ed by heat or by treatment with trypsin but was suppressed by the protease papain and had an apparent molecular weight of <5000, No change was detecte d in the expression of type II sodium/phosphate cotransporter (NaPi) mRNA i n OK 3B2 cells in response to conditioned media, unlike the reduction seen in Hyp mice. In the presence of colchicine or cytochalasin D, the inhibitor y response to conditioned media was reduced, similar to the effect of these agents on the response to PTH. Cycloheximide also suppressed the inhibitor y response of conditioned media, but not the response to PTH. These studies indicate that mutations in the PHEX gene are unlikely to be responsible fo r OOM and suggest that the tumor-derived factor that inhibits phosphate upt ake is a small protein that does not downregulate type II NaPi mRNA, and re quires an intact cytoskeleton and protein synthesis for activity, (Bone 28: 430-439; 2001) (C) 2001 by Elsevier Science Inc. All rights reserved.