Simvastatin inhibits osteoclast differentiation induced by bone morphogenetic protein-2 and RANKL through regulating MAPK, AKT and Src signaling

The mevalonate pathway plays a crucial role in bone metabolism. Here we examined roles of simvastatin in osteoclast function and differentiation induced by RANKL and BMP-2 using mouse macrophage-like MLC-6 cells and human osteoclast precursor cells. MLC-6 cells expressed BMP type-I and -II receptors and Smads as well as osteoclast markers including TRAP, RANK, cathepsin-K, M-CSF receptor, MMP-9 and calcitonin receptor. Treatment with RANKL and BMP-2 acted synergistically to stimulate RANK, TRAP and cathepsin-K expression in MLC-6 cells. Simvastatin suppressed osteoclastic activity shown by increases in RANK, TRAP and cathepsin-K expression induced by RANKL and BMP-2. In contrast simvastatin alone had no effects on the osteoclastic markers in MLC-6 cells. Simvastatin activated ERK, SAPK/JNK and AKT pathways and inactivated Ras in MLC-6 cells. Simvastatin had no effect on BMP-induced Smad1/5/8 phosphorylation regardless of RANKL stimulation. Since chemical inhibition of ERK, SAPK/JNK and AKT increased TRAP and cathepsin-K expression induced by BMP-2 and RANKL, these pathways are functionally involved in inhibition of osteoclastic activity. In addition, Src phosphorylation induced by RANKL, which is involved in osteoclast differentiation, was suppressed by simvastatin. We further confirmed an inhibitory mechanism of simvastatin on osteoclast differentiation using human osteoclast precursor cells which express BMP receptor and Smad signaling machinery. Simvastatin also activated ERK pathways and inactivated Src phosphorylation in human osteoclasts differentiated by M-CSF and RANKL treatments. The inhibition of TRAP and RANK expression by simvastatin was reversed by ERK inhibition, whereas Src inhibitor enhanced simvastatin-induced suppression of osteoclast markers. Collectively, our data show that simvastatin inhibits osteoclastic differentiation through inhibiting Src as well as enhancing MAPK/AKT pathways.