Osteoprotegerin (OPG) synthesized by osteoblasts is currently considered a crucial regulator to suppress the formation and function of osteoclasts. We previously showed that the synthesis of OPG is stimulated by
prostaglandin F2α (PGF2α) in the involvement of
p38 mitogen-activated protein kinase (MAPK), stress-activated
protein kinase/c-
Jun N-terminal kinase (SAPK/JNK) and p44/
p42 MAPK in osteoblast-like MC3T3-E1 cells. We also found that
Rho-kinase is involved in the signaling of PGF2α upstream of
p38 MAPK in these cells.
Tramadol is widely used to treat
chronic pain, such as
low back pain associated with
osteoporosis. We investigated whether or not
tramadol affects the OPG release induced by PGF2α in osteoblast-like MC3T3-E1 cells. The levels of OPG in the
conditioned medium were measured by an
enzyme-linked
immunosorbent assay. The
mRNA expression of OPG was determined with real-time reverse transcription polymerase chain reaction. The phosphorylation of target
protein was determined with a Western blot analysis. PGF2α induced the release and the
mRNA expression of OPG, which
tramadol significantly enhanced.
Morphine, a selective μ-
opioid receptor (MOR) agonist, also enhanced the PGF2α-induced OPG release. In addition,
naloxone, a MOR antagonist, suppressed the enhancement by
tramadol or
morphine of the PGF2α-induced OPG synthesis.
Tramadol upregulated the phosphorylation of SAPK/JNK and
p38 MAPK stimulated by PGF2α but not that of p44/
p42 MAPK or
myosin phosphatase targeting
protein (MYPT), a substrate of
Rho-kinase. The inhibitors of both
p38 MAPK and SAPK/JNK,
SB203580 and
SP600125, respectively, reduced the
tramadol amplification of OPG release stimulated by PGF2α. The present results strongly suggest that
tramadol enhances the synthesis of OPG stimulated by PGF2α through MOR in osteoblasts, and that the amplifying effect is exerted at upstream of
p38 MAPK and SAPK/JNK but downstream of
Rho-kinase.