Mechanism of Lipid Radical Formation Following Exposure of Epidermal Homogenate to Ultraviolet Light

133

Ultraviolet B light (UVB) causes cutaneous inflammation and cell death, but the agents responsible are not defined. These studies examined the role of the platelet-activating factor (PAF) signaling system in UVB-mediated effects. Expression of the PAF receptor in the PAF receptor-negative epidermoid cell line KB augmented apoptosis in response to UVB irradiation. Overexpression of the PAF receptor in primary human keratinocytes also enhanced UVB-mediated apoptosis in vitro, and it enhanced apoptosis in an in vivo model of human keratinocytes grafted onto severe combined immune-deficient (SCID) mice. To define the mechanism by which UVB activates the PAF receptor, we used mass spectrometry to demonstrate significant amounts of the C 4 PAF analogs 1-alkyl-2-(butanoyl and butenoyl)-sn-glycero-3-phosphocholine, as well as native PAF in an epidermal cell line after UVB irradiation. Supplementing the cells with the precursor phospholipid 1-hexadecyl-2-arachidonoyl-sn-glycero-3-phosphocholine (HAPC) increased the amount of C 4 PAF analogs recovered after UVB exposure. We irradiated HAPC directly and found, even in the absence of a photosensitizer, fragmentation to C 4 -PAF receptor ligands. We conclude UVB photo-oxidizes cellular phospholipids, creating PAF analogs that stimulate the PAF receptor to induce further PAF synthesis and apoptosis. PAF signaling may participate in the cutaneous inflammation that occurs during photo-aggravated dermatoses.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Ultraviolet B Radiation Generates Platelet-activating Factor-like Phospholipids underlying Cutaneous Damage

Marathe¹,

Johnson

Billings

et al. 2005

133

“…It has been shown that exposure of epidermal homogenates to UV light-induced lipid peroxide formation via lipid radicals (7). The hydroxyl radical was found to generate lipid radicals as a result of ⅐OH-mediated hydrogen atom abstraction (8).…”

mentioning

confidence: 99%

Epidermal Peroxisome Proliferator-activated Receptor γ as a Target for Ultraviolet B Radiation

Zhang¹,

Southall²,

Mezsick³

et al. 2005

Ultraviolet B radiation (UVB) is a pro-oxidative stressor with profound effects on skin in part through its ability to stimulate cytokine production. Peroxisome proliferator-activated receptor ␥ (PPAR␥) has been shown to regulate inflammatory processes and cytokine release in various cell types. Since the oxidized glycerophospholipid 1-hexadecyl-2-azelaoyl glycerophosphocholine (azPC) has been shown to be a potent PPAR␥ agonist, this study was designed to assess whether the PPAR␥ system is a target for UVB irradiation and involved in UVB-induced inflammation in epidermal cells. The present studies demonstrated the presence of PPAR␥ mRNA and functional protein in human keratinocytes and epithelial cell lines HaCaT, KB, and A431. The treatment of epidermal cells with the PPAR␥-specific agonist ciglitazone or azPC augmented cyclooxygenase-2 expression and enzyme activity induced by phorbol 12-myristate-13-acetate or interleukin-1␤. Lipid extracts from the cell homogenate of UVB-irradiated, but not control, cells contained a PPAR␥-agonistic activity identified by reporter assay, and this activity up-regulated cyclooxygenase-2 expression induced by phorbol 12-myristate-13-acetate. Subjecting purified 1-hexadecyl-2-arachidonoyl-glycerophosphocholine to UVB irradiation generated a PPAR␥-agonistic activity, among which the specific PPAR␥ agonist azPC was identified by mass spectrometry. These findings suggested that UVB-generated PPAR␥-agonistic activity was due to the free radical mediated nonenzymatic cleavage of endogenous glycerophosphocholines. Treatment with the specific PPAR␥ antagonist GW9662 or expression of a dominant-negative PPAR␥ mutant in KB cells inhibited UVB-induced epidermal cell prostaglandin E 2 production. These findings suggested that UVB-generated PPAR␥ activity is necessary for the optimal production of epidermal prostaglandins. These studies demonstrated that epithelial cells contain a functional PPAR␥ system, and this system is a target for UVB through the production of novel oxidatively modified endogenous phospholipids.

“…Acute short term UVB absorption by keratinocytes results in oxidative stress and DNA damage (1)(2)(3). UVB can also induce cytokine production in keratinocytes including interleukin (IL)-1, 1 IL-6, IL-8, IL-10, and tumor necrosis factor ␣ (TNF-␣) (4 -6).…”

mentioning

confidence: 99%

Augmentation of Ultraviolet B Radiation-induced Tumor Necrosis Factor Production by the Epidermal Platelet-activating Factor Receptor

Dy¹,

Pei²,

Travers³

1999

Ultraviolet B radiation (UVB) has been shown to damage human keratinocytes in part by inducing oxidative stress and cytokine production. Indeed, UVB-induced production of the pro-inflammatory and cytotoxic cytokine tumor necrosis factor ␣ (TNF-␣) has been implicated in the epidermal damage seen in response to acute solar radiation. Though the lipid mediator platelet-activating factor (PAF) is synthesized in response to oxidative stress, and keratinocytes express PAF receptors linked to cytokine biosynthesis, it is not known whether PAF is involved in UVB-induced epidermal cell cytokine production. These studies examined the role of the PAF system in UVB-induced epidermal cell TNF-␣ biosynthesis using a novel model system created by retroviralmediated transduction of the PAF receptor-negative human epidermal cell line KB with the human PAF receptor (PAF-R). Treatment of PAF-R-expressing KB cells with the metabolically stable PAF-R agonist carbamoyl-PAF resulted in increased TNF-␣ mRNA and protein, indicating that activation of the epidermal PAF-R was linked to TNF-␣ production. UVB irradiation of PAF-R-expressing KB cells resulted in significant increases in both TNF-␣ mRNA and protein in comparison to UVB-treated control KB cells. However, UVB treatment up-regulated cyclooxygenase-2 mRNA levels to the same extent in both PAF-R-expressing and control KB cells. Pretreatment with the antioxidant vitamin E or the PAF-R antagonists WEB 2086 and A-85783 inhibited UVB-induced TNF-␣ production in the PAF-R-positive but not control KB cells. These studies suggest that the epidermal PAF-R may be a pharmacological target for UVB in skin.