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Exposing human retinal pigmented epithelial cells to red light in vitro elicits an adaptive response to a subsequent 2-μm laser challenge

The objective of this study was to elucidate cellular mechanisms of protection against laser-induced thermal killing utilizing an in vitro retina model. When exposed to a 1-sec pulse of 2-μm laser radiation 24 hr after illuminating hTERT-RPE cells with red light (preconditioning), the cells are more resistant to thermal challenge than unilluminated controls (adaptive response). Results of efforts to understand the physiology of this effect led us to two genes: Vascular Endothelial Growth Factor C (VEGF-C) and Micro RNA 146a (miR-146a). Transfecting wild type (WT) cells with siRNA for VEGF-C and miR-146a mRNA resulted in knockdown strains (VEGF-C(KD) and miR- 146a(-)) with 10% and 30% (respectively) of the constitutive levels expressed in the WT cells. To induce gene expression, WT or KD cells were preconditioned with 1.44 to 5.40 J/cm2, using irradiances between 0.40 and 1.60 mW/cm2 of either 671-nm (diode) or 637-nm (laser) radiation. Probit analysis was used to calculate threshold damage irradiance, expressed as ED50, between 10 and 100 W/cm2 for the 2-μm laser pulse. In the WT cells there is a significant increase in ED50 (p 0.05) with the maximum response occurring at 2.88 J/cm2 in the preconditioned cells. Neither KD cell strain showed a significant increase in the ED50, although some data suggest the response may just be decreased in the knockdown cells instead of absent. So far the response does not appear to be dependent upon either wavelength (637 vs. 671 nm) or coherence (laser vs. LED), but there is an irradiance dependence.