Objective      To determine the effect of RNA interference (RNAi) of apurinic/apyrimidinic endonuclease (APE1) in multiple myeloma cell line U266 on the differentiation of a human monocytic cell line THP-1 into osteoclast-like cells in a contactless co-culture system.       Methods      Constructed APE1 siRNA expression vector Ad5v2 APE1 siRNA was used to transfect the U266 cells. The protein levels of APE1 and RANKL were detecte

d by Western blotting. A co-culture system of THP-1 cells and U266 cells was established with Transwell insert culture dish. The co-culture system were divided into 3 groups:THP-1+U266 group

THP-1+U266

APE1 siRNA

group and the control group (THP-1 cells). Osteoclast-like cells were identified through tartrate-resistant acid phosphatase (TRAP) staining. The mRNA levels of Cathepsin K and V-ATPase were examined by RT-PCR. Osteoclast bone resorption was measured by pit assay.       Results      U266 cells transfected with Ad5v2 APE1 siRNA had significantly lower protein expression of APE1 and RANKL than the untransfected cells.THP-1 cells differentiated into the osteoclast-like cells with the elevated mRNA expression of Cathepsin K and V-ATPase（

P

0.05）. The number of osteoclast-like cells was decreased significantly in the THP-1+U266

APE1 siRNA

group

as well as the mRNA levels of Cathepsin K and V-ATPase （

P

0.05）.       Conclusion      Inhibiting APE1 expression in U266 cells in a co-culture system suppresses the differentiation of THP-1 cells into osteoclast-like cells

which may be related to APE1 down-regulating RANKL in U266 cells.