than controls. Because there was no change in the levels of antioxidant enzymes on Western blots, this beneficial effect is likely due to the direct antioxidant properties of P. leucotomos.
There is abundant evidence that UVB radiation-induced immunosuppression is a major contributing factor to UVB-induced skin cancers in mice.15 Investigators sought to determine the effect of P. leucotomos on the immune system. They explored whether oral P. leucotomos placed in the drinking water would reverse direct immunosuppressive effect of UVB that occurs locally in the skin or the general immunosuppression that acts systemically following UVB exposure, or both.16 To test for the local effects of P. leucotomos on skin exposed to UVB, mice were exposed to UVB radiation daily for 4 days and were then sensitized by topical application of the experimental contact allergen oxazolone directly on the radiated site. To test for effects of P. leucotomos on UVB-induced systemic immunosuppression, a separate group of mice was exposed to 10,000 J/cm2 UVB radiation once and 3 days later were sensitized with oxazolone at a site not exposed to UVB. Compared to mice receiving plain drinking water, animals that had received P. leucotomos exhibited significantly less local and systemic UVB radiation-induced immune suppression. Thus, oral administration of P. leucotomos reverses UVB immunosuppression regardless whether it is local or systemic. In their studies, P. leucotomos also inhibited UV mediated depletion of Langerhans cells.17 While recent observations indicate that the primary role of epidermal Langerhans cells may be to generate regulatory T-cells, many studies have shown a clear association between UVB-induced depletion of Langerhans cells and local UVB-induced immunosuppression.
In addition to affecting the immune system, UV radiation has also been shown to produce other adverse systemic effects including oxidative stress.18 8-oxo-dG is a pre-mutagenic marker of oxidative damage to DNA and is caused by the UV-induced generation of reactive oxygen species. 8-oxo-dG positive cells were reduced by approximately 59% at 24 hours and by 79% at 48 hours in P. leucotomos-treated animals compared to control animals. These findings support the concept that P. leucotomos reduces oxidative DNA damage. Two weeks after UV exposure, mutations in P. leucotomos-fed-mice were approximately 25% less than those from mice treated with UV alone.11 In other studies, when hairless rats were pretreated with an oral extract of P. leucotomos (30 mg/kg) each day for 7 days prior to UV exposure, less glutathione oxidation in both blood and epidermis was observed, providing evidence for a potent systemic antioxidant effect.1
In Vitro Human Studies
While many of the studies performed on mice cannot be conducted in humans, many in vitro studies using human cell cultures have demonstrated the photoprotective and beneficial effects of P. leucotomos.
One study demonstrated that pretreatment of human keratinocytes with P. leucotomos inhibited solar-simulator-UV-mediated increase of tumor necrosis factor-alpha and nitric oxide production and prevented nitric oxide synthase induction.20 Other effects observed were suppression of solar simulated radiation induced transcriptional activation of nuclear factor-kappaB (NFkB) and activator protein-1 (AP-1), both of which have been implicated in UVB-induced skin carcinogenesis. Pretreatment with P. leucotomos was cytoprotective against UV-induced damage resulting in increased cell survival.
P. leucotomos extract has been shown to be photoprotective against UV-induced cell damage. Fibroblasts obtained from healthy volunteers were treated with an extract of P. leucotomos and then exposed to UV-A and UV-B radiation.21 P. leucotomos maintained fibroblast survival and proliferative capacity in a dose-dependent manner. P. leucotomos treatment of human fibroblasts had a protective effect on the morphological changes that occur following UV exposure. Specifically, there was less disorganization of F-actin-based cytoskeletal structures and a reduction in the coalescence of the tubulin cytoskeleton. Moreover, there was a more regular distribution of integrin and cadherin adhesion molecules in UV irradiated cells following UV exposure.21 In that study, the protective effects of P. leucotomos following UV exposure in terms of proliferation and survival was also observed with the human HaCaT keratinocyte cell line. The ability of P. leucotomos treatment to protect against UV-induced death of human fibroblasts has been demonstrated by others as well.22
A major UV-absorbing chromophore in the stratum corneum is trans-urocanic acid (t-UCA). When the skin is exposed to ultraviolet radiation, trans-urocanic acid undergoes photoisomerization to its cis- conformation. Cis-urocanic acid is a well-studied mediator of UVB-induced immune suppression.23 In vitro studies demonstrated that P. leucotomos was also to inhibits UV-radiation-induced photoisomerization of t-UCA to cis-UCA. Trans-UCA will decompose in the presence of such oxidizing agents as H2O2, and titanium dioxide (TiO2). P. leucotomos treatment blocked that effect.22
Interestingly, an extract from the related fern Polypodium decumanum has been used to treat psoriasis and related immunological disorders. Using human neutrophils, the extract demonstrated inhibitory activity against platelet activating factor,24 which is a known mediator of UVB-induced immunosuppression.25 The specific compound responsible for this effect has been identified as 1,2-di-O-palmitoyl-3-O-(6-sulpho-alpha-D-quinovopyranosyl)-glycerol.26 In an in vitro model using human leukocytes P. decumanum was had the ability to inhibit the formation of the inflammatory mediator leukotriene B4, which is present in abnormally high quantities in psoriatic skin.27
