1. ¹Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, Ohio, USA
2. ²The Digital Imaging Core, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, Ohio, USA
3. ³Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, Massachusetts, USA
4. ⁴Department of Dermatology, Cleveland Clinic Foundation, Cleveland, Ohio, USA

Correspondence: Dr Edward V. Maytin, ND-20, Biomedical Engineering, Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, Ohio 44195, USA. E-mail: maytine@ccf.org

⁵Current address: Department of Dermatology, Niigata University School of Medicine, Asahimachi-dori 1-757, Niigata City, Niigata 951-8510, Japan.

Received 20 June 2006; Revised 21 July 2006; Accepted 29 August 2006; Published online 19 October 2006.

Abstract

Photodynamic therapy (PDT) with 5-aminolevulinic acid (ALA) is based upon the intracellular synthesis of protoporphyrin IX (PpIX), which absorbs light and targets metabolically active cells. We tested the hypothesis that levels of PpIX within keratinocytes might be increased by vitamin D (Vit D), a differentiation-promoting hormone. Vit D promoted terminal differentiation in monolayer cultures of rat epidermal keratinocytes (REKs), but high PpIX signals were found only in stratifying islands. To simulate a normal epidermis, REKs were grown in organotypic cultures. The presence of Vit D (10^-10 M for 4 days) led to heightened expression of terminal differentiation markers (stratum corneum, K10, and loricrin). PpIX levels, at 4 hours after addition of ALA (1 mM), were significantly increased in the Vit D-preconditioned cultures by confocal fluorescence microscopy and semiquantitative image analysis. Maximal PpIX induction was seen at (Vit D) 10^-12–10^-10 M. Phototoxic cell killing after exposure to 635 nm light was significantly higher in Vit D-preconditioned cultures. No differences in apoptotic markers between Vit D and control cultures were seen, suggesting that Vit D augments photodynamic cell death via alternative pathways (e.g., necrosis). In summary, Vit D may be useful as a biological enhancer of ALA-based PDT.