Abstract
Photobiomodulation-based (LLLT) therapies show tantalizing promise for treatment of skin diseases. Confidence in this approach is blighted however by lamentable inconsistency in published experimental designs, and so complicates interpretation. Here we interrogate the appropriateness of a range of previously-reported treatment parameters, including light wavelength, irradiance and radiant exposure, as well as cell culture conditions (e.g., serum concentration, cell confluency, medium refreshment, direct/indirect treatment, oxygen concentration, etc.), in primary cultures of normal human dermal fibroblasts exposed to visible and near infra-red (NIR) light. Apart from irradiance, all study parameters impacted significantly on fibroblast metabolic activity. Moreover, when cells were grown at atmospheric O2 levels (i.e. 20%) short wavelength light inhibited cell metabolism, while negligible effects were seen with long visible and NIR wavelength. By contrast, NIR stimulated cells when exposed to dermal tissue oxygen levels (approx. 2%). The impact of culture conditions was further seen when inhibitory effects of short wavelength light were reduced with increasing serum concentration and cell confluency. We conclude that a significant source of problematic interpretations in photobiomodulation reports derives from poor optimization of study design. Further development of this field using in vitro/ex vivo models should embrace significant standardization of study design, ideally within a design-of-experiment setting.
Highlights
Photobiomodulation (PBM) or low-level light therapy (LLLT)[1] is a rapidly expanding field, showing encouraging results for treatment of cutaneous disorders[2, 3] and a wider range of health conditions[4,5,6]
Our results demonstrate that wavelength (p < 0.001) and radiant exposure (p < 0.001) of light had the strongest impact on the metabolic activity of human dermal fibroblast (DF)
On average, shorter wavelengths (450, 500 and 530 nm) have a large impact on fibroblast metabolic activity, while long wavelengths (>550 nm) show negligible effects within the tested radiant exposures when cells were cultured at 20% environmental oxygen
Summary
Photobiomodulation (PBM) or low-level light therapy (LLLT)[1] is a rapidly expanding field, showing encouraging results for treatment of cutaneous disorders[2, 3] and a wider range of health conditions[4,5,6]. Therapeutic action of PBM is attributed to non-thermal photochemical or photobiological action of light via interaction with a range of endogenous photoreceptors and chromophores contained in human skin[3, 15, 16]. Photoreceptors and mediators of the photobiological action of visible and NIR light include cytochrome c oxidase, nitrosated and flavo- proteins, opsins and ion-gated channels[3, 17,18,19]. A remarkably wide range of optical parameters has been applied to both in vitro and ex vivo model systems (variable across 2 orders of magnitude), where negative, positive and neutral outcomes are reported for visible and near-infrared (NIR) light despite very similar optical settings[3, 8, 9, 11]. Clarity on how one particular optical setting truly impacts on cell behavior is further compromised in published reports that don’t even disclose parameters applied or if they do sadly report them incorrectly[3]
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