Hypoxia decreases the osteogenic activity of human periodontal ligament cells

Objectives: Previously, we found that both bone resorption and hypoxia (low oxygen) occur on the compression side during orthodontic tooth movement. Periodontal ligament (PDL) cells have high activity of osteogenic molecules, such as alkaline phosphatase (ALP). To clarify the correlation between hypoxia and the osteogenic properties of PDL cells, we investigated the osteogenic properties of PDL cells stimulated by hypoxia and compressive forces.

Methods: Human PDL cells were obtained from healthy extracted human premolars. After a 3-day pre-culture in 20% O ₂ (normal), PDL cells were cultured in 1% or 5% O ₂ (hypoxia) under compression produced by differences in the amount of medium (5.0 or 8.0 g/cm ²). As an osteogenic marker, the ALP activity of the cell lysate was examined using p-nitrophenyl phosphate as the substrate. Total RNA was extracted from the cells after 0 and 4 days under hypoxia with or without compressive force. In addition, the expression of hypoxia-inducible 1 alpha (HIF1- α) and runt-related transcription factor 2 (Runx-2) mRNA was examined semi-quantitatively using RT–PCR.

Results: Both 1% and 5% hypoxia reduced the ALP activity of PDL cells. In 1% O ₂, the ALP activity was decreased (0.26 vs. 1.0 for the control) and the addition of 5.0 or 8.0 g/cm ² compressive force decreased the ALP activity synergistically (0.15 or 0.11, respectively, vs. 1.0 for the control). HIF1- α mRNA was expressed at constant levels in PDL cells with or without hypoxia. The Runx-2 mRNA level was lower with hypoxia than the control level (0.80-0.64 vs. 1.0).

Conclusions: These findings suggest that hypoxia and compressive force synergistically decrease the osteogenic activity of human PDL cells, which may result in the loss of the osteogenic property of PDL cells on the compression side during orthodontic tooth movement.