title: Inhibition of heme oxygenase-1 increases responsiveness of melanoma cells to ALA-based photodynamic therapy.
abstract: Based on the observation that 5-aminolevulinic acid (ALA) induces the expression of heme oxygenase-1 (HO-1) in cultured melanoma cells, the role of HO-1 on the effectiveness of 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT) was examined. Transcriptional activation of the HO-1 gene is considered to be an adaptive response to oxidative and cellular stress and confers a protective capacity against cell and tissue injury, which could affect the responsiveness to ALA-PDT. A time-dependent accumulation (0-16 h) of protoporphyrin IX (PPIX) within melanoma cells was seen after incubation with ALA (0.5 mM ALA). Over the same time interval, a significant increase (up to 25-fold) in HO-1 protein expression was observed. Thus, the production and degradation of PPIX (via heme by HO-1) were simultaneously enhanced, leading to a reduced intracellular concentration of the photodynamically active substance PPIX. Diminishing HO-1 activity by the HO-1 inhibitor tin protoporphyrin IX (SnPPIX) significantly enhanced the formation of PPIX up to 1.8 fold. A further strong increase in HO-1 protein expression (up to 128-fold) was seen after ALA-PDT treatment. Induction of HO-1 is an essential step in the 'rescue response' of tumor cells. The pharmacological inhibition of HO-1 activity by SnPPIX leads to a considerable increase in the sensitivity of tumor cells to ALA-PDT treatment. At low radiation doses (0.42 J/cm(2)), the percentages of death cells increased significantly from 7.3+/-1.3% to 43.7+/-6.4%. This effect could be further intensified by cellular depletion of HO-1 mRNA by siRNA. The combination of pharmacological inactivation of HO-1 with gene silencing led to an increase in the death rate of up to 54.1+/-8.6%. The results presented indicate that HO-1 can play a protective role against ALA-PDT mediated cytotoxicity so that a specific inhibition of HO-1 activity and/or expression might be used to increase the efficacy of ALA-based photodynamic therapy.
authors.affiliation: zet Life Science Laboratory Linz, 4020, Linz, Austria. j.frank@zet.or.at
authors.forename: Jürgen
authors.initials: J
authors.lastname: Frank
authors.forename: Mohammad Reza
authors.initials: MR
authors.lastname: Lornejad-Schäfer
authors.forename: Harald
authors.initials: H
authors.lastname: Schöffl
authors.forename: Andrea
authors.initials: A
authors.lastname: Flaccus
authors.forename: Christine
authors.initials: C
authors.lastname: Lambert
authors.forename: Hans K
authors.initials: HK
authors.lastname: Biesalski
mesh_terms: D000622:Aminolevulinic Acid / Q000627:therapeutic use*; D045744:Cell Line, Tumor; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D051547:Heme Oxygenase-1 / Q000037:antagonists & inhibitors* / Q000096:biosynthesis; D006801:Humans; D008545:Melanoma / Q000188:drug therapy* / Q000473:pathology; D008665:Metalloporphyrins / Q000494:pharmacology; D010778:Photochemotherapy; D011524:Protoporphyrins / Q000096:biosynthesis / Q000494:pharmacology; D034741:RNA, Small Interfering / Q000494:pharmacology
chemical_list: D004791:Enzyme Inhibitors; D008665:Metalloporphyrins; D011524:Protoporphyrins; D034741:RNA, Small Interfering; D000622:Aminolevulinic Acid; C028025:protoporphyrin IX; C032628:tin protoporphyrin IX; C496541:HMOX1 protein, human; D051547:Heme Oxygenase-1