Pneumolysin-induced autophagy contributes to inhibition of osteoblast differentiation through downregulation of Sp1 in human osteosarcoma cells.

Background Information: The 53kDa protein pneumolysin (PLY) is the main virulence factor of Streptococcus pneumoniae, a leading cause of invasive pneumococcal diseases. PLY forms pores in cholesterol-containing membranes, thereby interfering with the function of cells. Bone destruction is a serious matter in chronic inflammatory diseases such as septic arthritis and osteomyelitis. S. pneumoniae is increasingly being recognized as a common cause of septic arthritis, but its pathogenesis is poorly defined.

Method: We examined the effect of PLY on osteoblast differentiation and its mechanisms of action. The effect of PLY on osteoblast differentiation was evaluated by qRT-PCR, ALP activity assay, flow cytometric analysis, and Western blotting. We also examined the role of PLY-induced autophagy in osteoblast differentiation using RNA interference analysis.

Results: PLY inhibited osteoblast differentiation by decreasing the expression of osteoblast marker genes such as Runx2 and OCN, along with ALP activity. ROS production was increased by PLY during osteoblast differentiation. PLY induced autophagy through ROS-mediated regulation of AMPK and mTOR, which downregulated the expression of Sp1 and subsequent inhibition of differentiation. Treatment with autophagy inhibitors or Atg5 siRNA alleviated the PLY-induced inhibition of differentiation.

Conclusion: The results suggest that PLY inhibits osteoblast differentiation by downregulation of Sp1 accompanied by induction of autophagy through ROS-mediated regulation of the AMPK/mTOR pathway.

General Significance: This study proposes a molecular mechanism for inhibition of osteoblast differentiation in response to PLY.