Osteoprotegerin (OPG) is a secreted glycoprotein and a member of the tumor 
necrosis factor receptor superfamily. It usually functions in bone remodeling, 
by inhibiting osteoclastogenesis through interaction with a receptor activator 
of the nuclear factor κB (RANKL). Transglutaminases-2 (Tgase-2) is a group of 
multifunctional enzymes that plays a role in cancer cell metastasis and bone 
formation. However, relationship between OPG and Tgase-2 is not studied. 
Therefore, we investigated the involvement of 12-O-Tetradecanoylphorbol 
13-acetate in the expression of OPG in MG-63 osteosarcoma cells. Interleukin-1β 
time-dependently induced OPG and Tgase-2 expression in cell lysates and media of 
the MG-63 cells by a Western blot. Additional 110 kda band was found in the 
media of MG-63 cells. 12-O-Tetradecanoylphorbol 13-acetate also induced OPG and 
Tgase-2 expression. However, an 110 kda band was not found in TPA-treated media 
of MG-63 cells. Cystamine, a Tgase-2 inhibitor, dose-dependently suppressed the 
expression of OPG in MG-63 cells. Gene silencing of Tgase-2 also signifi cantly 
suppressed the expression of OPG in MG-63 cells. Next, we examined whether a 
band of 110 kda of OPG contains an isopeptide bond, an indication of Tgase-2 
action, by monoclonal antibody specifi c for the isopeptide bond. However, we 
could not fi nd the isopeptide bond at 110 kda but 77 kda, which is believed to 
be the band position of Tgase-2. This suggested that 110 kda is not the direct 
product of Tgase-2's action. All together, OPG and Tgase-2 is induced by IL-1β 
or TPA in MG-63 cells and Tgase-2 is involved in OPG expression in MG-63 cells.