Blue Curing Light Exposure Alters RANKL:OPG Ratios in PDLFs

Objective:  Short, discrete doses of blue light (400-500 nm) induce cytoprotective responses that potentially could affect bone homeostasis.  Blue light exposure enhances cellular levels of  the phase 2 protein, heme oxygenase-1 (HO-1).  This protein has been reported to participate in regulation of the important bone metabolism regulators Receptor Activator of NFkappaB Ligand (RANKL) and osteoprogeterin (OPG).  High RANKL:OPG ratios favor bone resorption, while low ratios favor new bone synthesis.  The goal of this research was to investigate the effects of blue light exposure on the response of periodontal ligament fibroblasts to neuropeptide-induced RANKL and OPG expression.

Method: Cultured human periodontal ligament fibroblasts (PDLFs) were exposed to 0 or 45 J/cm² (90 sec) doses of blue light delivered from a quartz-tungsten-halogen dental curing light (VIP, Bisco, 600 mW/cm²),  then left to incubate for an additional 2 h before  Substance P (10^-6M) was added to all cultures.  Parallel cultures were harvested 48 h later for either RNA or protein analysis.  RNA samples underwent quantitative  RT-PCR analysis using primers specific for the cytoprotective protein HO-1, and the bone metabolism regulators RANKL and OPG.  Proteins from whole cell lysates were separated by SDS-PAGE, transferred to PVDF membranes, and analyzed using immunoblot detection and Odyssey® quantification of RANKL, OPG, and HO-1. 

Result: Light pre-treatment of PDLFs resulted in a 1.8-fold increase in HO-1-specific RNA transcripts, suggesting a sustained cytoprotective response to the light exposure.  Moreover, light-treated cells exhibited a ~9.7-fold increase in OPG-specific RNA transcripts with a concurrent 2.6-fold decrease in RANKL. Immunblot analysis resulted in similar changes that were less prominent, but remained significant (p<0.05).

Conclusion: These data suggest that blue light initiates a cytoprotective response that ultimately shifts the RANKL:OPG ratio away from bone resorption.  This effect may eventually  prove useful in the clinical treatment of periodontitis.