Effect of UVB radiation emitted from the narrowband TL‐01 lamp (311 nm) on the calcitriol synthesis in organotypic cultures of keratinocytes

The skin is the only tissue yet known in which the complete UVB-induced pathway from 7-dehydrocholesterol (7-DHC) to hormonally active calcitriol (1α,25-dihydroxyvitamin D3) occurs under physiological conditions. It is well known that both calcitriol and UVB radiation exert potent antipsoriatic effects. We speculate that the therapeutic effect of UVB radiation can be attributed to UVB-triggered cutaneous synthesis of calcitriol for which the optimum wavelength was a range of 300 ± 3 nm in vitro and in vivo. On the other hand, the narrowband Philips TL-01 lamp which is commonly used as UVB source for therapeutical treatment of psoriasis, has a maximum spectral irradiance at around 311 nm. The aim of this study was to investigate the calcitriol-inducing potential of the TL-01 lamp in organotypic cultures of keratinocytes supplemented with 25 μM 7-DHC at different radiant exposures (125–1000 mJ/cm2). We found that the maximum calcitriol-generating capacity of the TL-01 lamp at 500 mJ/cm2(corresponding to 2.1 SED [Standard Erythema Dose]) and 16 h after irradiation still amounts to approximately 45 percent of that of monochromatic radiation at 300 nm and 30 mJ/cm2. We conclude from our findings that irradiation with the narrowband TL-01 lamp in therapeutic dose range can affect calcitriol synthesis in epidermal keratinocytes. Thus, the antipsoriatic effect observed after TL-01 lamp exposures may be, at least partially, explained by the known action of newly-synthesized calcitriol on the epidermal cell proliferation and differentiation.