REGULATORY MECHANISM OF RANKL GENE EXPRESSION

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Osteoclast differentiation factor (RANKL) is requisite for the formation and maintenance of osteoclasts from hematopoietic precursors. To clarify the mechanism of RANKL gene expression and osteoclastogenesis, mouse and human RANKL gene promoters were characterized. Both human and mouse RANKL gene promoter shares the common structure, inverted-TATA and CAAT boxes, Runx2/Cbfa-1 binding sites and vitamin D responsive element (VDRE). To elucidate the molecular mechanism of osteolytic bone metastasis, we assessed RANKL gene expression and osteoclastogenesis in osteolytic lesions of mouse experimental model and human bone specimen taken at the autopsy. In both mouse and human osteolytic lesion due to cancer metastasis, RANKL expression was observed on the stromal/osteoblastic cells close to the cancer cell nests. In the mouse model, PTHrP-producing tumor generated osteolytic lesion, whereas non-producing tumor rarely caused RANKL expression and induction of osteoclasts. We further analyzed the effects of PTHrP on RANKL gene transcription. By transient transfection studies using deletion constructs of mouse and human RANKL gene promoter, PTHrP upregulated the transcriptional activity through c-AMP responsive element (CRE) located close to VDRE in both mouse and human. EMSA showed specific protein DNA binding, and the supershift with anti-CREB1 and -ATF2 antibodies. Thus PTHrP induces osteoclastic bone resorption through the RANKL expression on stromal/osteoblastic cells, affording a bone microenvironment conducive to the survival of PTHrP-producing cancer cells.We have presented our data at various international as well as domestic meeting, and published scientific papers for academic journals.