Effects of Erbium-Doped Yttrium Aluminum Garnet Laser Irradiation on the Proliferation and Alteration of Gene Expression in MLO-Y4 Osteocyte-like Cells

Objective: To determine how Erbium-doped yttrium aluminum garnet (Er:YAG) laser irradiation affects the proliferation and transcriptional programs of MLO-Y4 osteocyte-like cells. Background: Although Er:YAG lasers are widely used in periodontal hard tissue therapy, how they affect osteocytes remain elusive. Materials and Methods: MLO-Y4 cells were seeded before 72 h Er:YAG laser irradiation at energy fluences of 3.1, 6.0, and 8.9 J/cm 2 . Cell surface temperature and Hspa1a expression were measured to evaluate thermal effects. Cell proliferation and cytotoxicity were assessed by the Cell Counting Kit-8 (CCK-8) and lactate dehydrogenase (LDH) assays. RNA sequencing (RNA-seq) was performed 6 h after 6.0 J/cm 2 laser irradiation versus nonirradiated controls. Results: According to surface temperature and Hspa1a expression, no major thermal effects were observed at 6.0 J/cm 2 . The CCK-8 assay revealed significantly increased proliferative activity, although LDH levels were slightly but significantly increased at 6.0 J/cm 2 laser irradiation. RNA-seq identified 337 differentially expressed genes (DEGs). Expression of mechanical stimulation-associated genes, including Epha2 , Itga5 , and Tcf7 , was highly upregulated in the laser-irradiated cells. Kyoto Encyclopedia of Genes and Genomes pathway analysis revealed upregulation of the PI3K-Akt signaling pathway in the laser-irradiated cells. Upregulated DEGs belonged to the “euchromatin” cellular component Gene Ontology term in the laser-irradiated cells. Gene set enrichment analysis showed enrichment of the Transforming Growth Factor Beta (TGF-β) signaling gene set and downregulation of mechanistic target of rapamycin (mTORC1) signaling gene set in the laser-irradiated cells. Conclusion: Er:YAG laser irradiation at 6.0 J/cm 2 promotes MLO-Y4 osteocyte-like cell proliferation via altering gene expression, possibly by activating PI3K-Akt and TGF-β signaling while suppressing mTORC1 signaling.