RESUMEN
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 C4 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 C4 PAF analogs recovered after UVB exposure. We irradiated HAPC directly and found, even in the absence of a photosensitizer, fragmentation to C4-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.
Asunto(s)
Éteres Fosfolípidos/química , Factor de Activación Plaquetaria/metabolismo , Piel/efectos de la radiación , Rayos Ultravioleta , Animales , Apoptosis , Calcio/metabolismo , Caspasa 3 , Caspasas/metabolismo , Línea Celular , Trasplante de Células , Cromatografía Líquida de Alta Presión , Células Epidérmicas , Epidermis/efectos de la radiación , Humanos , Inflamación , Queratinocitos/citología , Queratinocitos/efectos de la radiación , Ligandos , Luz , Lípidos/química , Espectrometría de Masas , Ratones , Ratones SCID , Modelos Químicos , Neutrófilos/metabolismo , Oxígeno/metabolismo , Fosfolípidos/química , Fosfolípidos/metabolismo , Fosforilación , Glicoproteínas de Membrana Plaquetaria/metabolismo , Receptores Acoplados a Proteínas G/metabolismo , Transducción de Señal , Factores de Tiempo , Tirosina/química , XenopusRESUMEN
In addition to releasing preformed granular proteins, polymorphonuclear leukocytes (PMNs) synthesize chemokines and other factors under transcriptional control. Here we demonstrate that PMNs express an inducible transcriptional modulator by signal-dependent activation of specialized mechanisms that regulate messenger RNA (mRNA) translation. HL-60 myelocytic cells differentiated to surrogate PMNs respond to activation by platelet activating factor by initiating translation and with appearance of specific mRNA transcripts in polyribosomes. cDNA array analysis of the polyribosome fraction demonstrated that retinoic acid receptor (RAR)-alpha, a transcription factor that controls the expression of multiple genes, is one of the polyribosome-associated transcripts. Quiescent surrogate HL60 PMNs and primary human PMNs contain constitutive message for RAR-alpha but little or no protein. RAR-alpha protein is rapidly synthesized in response to platelet activating factor under the control of a specialized translational regulator, mammalian target of rapamycin, and is blocked by the therapeutic macrolide rapamycin, events consistent with features of the 5' untranslated region of the transcript. Newly synthesized RAR-alpha modulates production of interleukin-8. Rapid expression of a transcription factor under translational control is a previously unrecognized mechanism in human PMNs that indicates unexpected diversity in gene regulation in this critical innate immune effector cell.