Islet response to photobiomodulation varies with the nature of the stress: insights from hypoxia–reperfusion

Abstract

Photobiomodulation (PBM) has been shown to protect pancreatic islets under inflammatory or metabolic stress, but its effects during hypoxia-reperfusion remain unclear. We evaluated PBM applied as preconditioning or during hypoxia-reperfusion in MIN6 cells and rodent islets. Hypoxia significantly reduced viability and glucose-stimulated insulin secretion, while PBM failed to prevent these alterations and did not modify mitochondrial parameters. These findings indicate that PBM efficacy is stress-dependent and suggest limited benefit in the context of hypoxia-reperfusion, where mitochondrial dysfunction appears minimal in islet cells.

Dear Editor,

We report a distinct biological effect of PBM during hypoxia-reperfusion stress in pancreatic islets, differing its effects under inflammatory or substrate deprivation stresses [1]. Although, islet transplantation is an established cell therapy for unstable type 1 diabetes. up to 50% of transplanted islets died after portal vein infusion due to various stresses. Following intraportal transplantation, islets experience a transient period of stress, including severe hypoxia due to the disruption of their native vascularization and the delay required for revascularization within the hepatic environment. Experimental models commonly reproduce this early post-transplant condition using oxygen concentrations between 1% and 2% [2, 3]. We demonstrated the safety profile of PBM on islets in vitro and in vivo [4], and subsequent studies showed that PBM preconditioning protect islets from inflammatory or substrate deprivation stress [1, 5]. In the present report, we investigated the effect of either PBM preconditioning or concomitant PBM application during hypoxic stress on islet viability, function, and mitochondrial activity.

An insulin-secreting cell line (MIN6, AddexBio) and rat pancreatic islets were subjected to hypoxia (1% O₂) for 24 h (MIN6) or 16 h (islets), followed by 2 h culture in non-hypoxic (21% O₂) medium. These exposure durations were selected to induce a 15–25% decrease in viability. They were exposed to PBM (LED incoherent light, wavelength of 670 nm, continuous output power, 2.8 mW/cm², parameters previously used with positive effect regarding substrate deprivation and cytokine stress) [2] either for 24 h before hypoxia or during the hypoxia and reperfusion periods. Analyses (viability, glucose-stimulated insulin secretion assay, mitochondrial superoxide content, and mitochondrial membrane potential (MMP)) were performed immediately afterwards using previously described methods. Results are expressed as mean ± standard error of the mean, and analyses were made with the Jamovi software (version 2.25).

Hypoxia significantly decreased the viability of MIN6 and islets compared with controls (65.8 ± 4.9% vs. 82.5 ± 2.0%, p = 0.008, and 73.9 ± 7.5% vs. 96.7 ± 1.7%, p < 0.001, respectively). PBM did not prevent the decrease in viability of either MIN6 cells or islets (Fig. 1A-B). Hypoxia also impaired GSIS and reduced the insulin stimulation index compared with controls (0.92 ± 0.11 vs. 1.47 ± 0.20, p < 0.001, and 0.82 ± 0.29 vs. 2.07 ± 0.37, p = 0.002, respectively). PBM did not prevent the reduction in insulin stimulation index in either model (Fig. 1C-D). Hypoxia did not significantly affect mitochondrial superoxide content compared with controls (p = 0.95) or PBM treatment (Fig. 1E). Hypoxia did not alter MMP compared with controls (p = 0.36) or PBM (Fig. 1F).

Fig. 1.

Effect of PBM on viability and insulin secretion from hypoxic stressed MIN6 cells and rat islets. MIN6 cells are placed in hypoxic medium (1% O2) for 24 h, rat islets for 16 h, followed by 2 h of incubation in non-hypoxic medium (21% of O2). Cells and islets are exposed to 2.8 mW/cm² PBM illumination for 24 h before hypoxia or during hypoxia and reperfusion. (A) Effect of PBM on viability from hypoxic stressed MIN6 cells (n = 5). (B) Effect of PBM on viability from hypoxic stressed islets (n = 7). (C) Effect of PBM on the insulin stimulation index of hypoxic stressed MIN6 cells. The stimulation index is 100*(high glucose insulin secretion/low glucose insulin secretion) (n = 6). (D) Effect of PBM on insulin stimulation index from hypoxic stressed islets (n = 5). (E) Effect of PBM on superoxide content from hypoxic stressed MIN6 cells (n = 3). (F) Effect of PBM on mitochondrial membrane potential from hypoxic stressed MIN6 cells (n = 3). PBM: photobiomodulation. Results are expressed as mean ± SEM, One-way ANOVA Welch’s with Games-Howell post-hoc test, * p < 0.05, ** p < 0.01, *** p < 0.001

It is known that hypoxic-reperfusion stress alters the viability and function of islets. However, with regard to hypoxic-reperfusion stress on islets or beta-cells, PBM did not improve insulin secretion or viability in either MIN6 or islets. These results contrast with the protective effect of PBM on myoblasts/fibroblasts subjected to hypoxia [6]. One possible explanation is that the cellular response to PBM depends on the metabolic characteristics of the target tissue and the nature of the stress. Ischemia-reperfusion injuries seem to have little connection to mitochondrial metabolic impairment in beta cells (INS1), as indicated by minimal alterations in the MMP and ROS production [7].

Importantly, hypoxia–reperfusion in our model did not significantly alter MMP or mitochondrial ROS. Because PBM is thought to exert its biological effects primarily through the modulation of mitochondrial metabolism, particularly through cytochrome c oxidase activity and ROS signaling, the absence of detectable dysfunction during hypoxia may explain the lack of response observed here.

Although this study focused on an in vitro hypoxia model and a single PBM protocol exposure, future work including broader functional analyses (mitochondrial respiration, ATP production, or metabolic flux) or other PBM protocols exposure may provide further mechanistic insight. These findings suggest that PBM may be more effective under conditions associated with marked mitochondrial impairment, as observed in our previous studies involving inflammatory stress or substrate deprivation.

Abbreviations

PBM

photobiomodulation

MMP

mitochondrial membrane potential

ROS

Reactive oxygen species

Author contributions

Q.P conducted the experiments. All the authors wrote the manuscript.

Funding

This work was funded by the “Région Auvergne-Rhône-Alpes”, the Edmond J.Safra Foundation, the “Fond de Dotation-Clinatec” and its sponsors, CEA, UGA, CHUGA.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.