Laser photobiomodulation (PBM) or low-level laser therapy (LLLT) is a form of phototherapy recognized worldwide for its expansive use in medicine. PBM/LLLT has the ability to enhance enzymatic activity and mitochondrial transmembrane potential that increase the availability of energy and the signal transduction, which promotes cell proliferation. This study primarily investigates the appropriateness of a range of treatment parameters, including light wavelength, irradiance (doses) and exposure time of PBM/LLLT in proliferation of cultured human monocytic leukaemia cell line THP-1(Tamm-Horsfall Protein 1), as well as DNA under in vitro conditions. A secondary objective was to exploit the beneficial effect of PBM/LLLT to reduce the harmful impacts caused by exposure to ultraviolet (UV) radiation. Cells were irradiated with near infrared (NIR) diode laser at 850 nm with doses ranging from 0 – 26.8 J/cm2. After irradiation, cells were incubated for 12 hr and 24 hr to allow time for proliferation. Comet assay was conducted to evaluate genotoxicity of the irradiated cells. Trypan blue exclusion test and MTT assay (3-4-5-dimethy-2.5 thiazol-2.5 diphenyl tetrazolium bromide) were used to estimate cytotoxicity, Exposure to NIR diode laser revealed the beneficial effect of PBM/LLLT at low doses (< 5 J/cm2), which increased the viability and proliferation of THP-1 cells, which conformed by increasing ATP synthesis and the activity of mitochondria. Comet assay showed no significant difference between irradiated and non-irradiated cells at low doses and showed no DNA damage. The photobiomodulation effect of low doses, in particular 0.6 J/cm2 and 1.2 J/cm2, managed to reduce the damage to THP-1 cells after exposure to UV radiation by decreasing cell apoptosis, accelerate DNA damage repair and increase cell survival. While, the exposure for high doses (> 5 J/cm2), showed different results involved decrease in the number of THP-1 cells and viability, in addition to a considerable increase in DNA damage, which emphasised by decrease in the level of ATP synthesis and reducing the mitochondrial activity. These findings demonstrated the photobiomodulation effect of NIR diode laser through modulating various pathways such as ATP synthesis and mitochondrial activity in monocyte cells and DNA.

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