RESUMEN
Transferrin, the iron-transport protein of vertebrate serum, is synthesized mainly in the liver, from which it is secreted into the blood. Transferrin is also synthesized in oligodendrocytes and is an early marker of their differentiation. We have analyzed the regulation of transferrin expression in HOG cells, a human oligodendrocyte cell line. Transferrin expression was correlated with the appearance of oligodendrocyte differentiation markers when cells were exposed to differentiation medium. In contrast to the protein expressed in hepatocytes or in Sertoli cells, transferrin was secreted by neither HOG cells nor immature rat primary oligodendrocytes in vitro. Moreover, transferrin appears to be localized in the cytosol and not in the secretory compartment, as is expected for secreted proteins. This transferrin localization was correlated with the synthesis of a specific transcript, resulting from an alternative splicing, which leads to the elimination of the signal peptide sequence. These results suggest the existence of a functional difference between transferrin synthesized in the brain and in other organs such as liver and testis. They are in accordance with the hypothesis that transferrin plays a specific role, other than iron transport, in oligodendrocyte maturation and in the myelination process.
Asunto(s)
Empalme Alternativo/fisiología , Diferenciación Celular/fisiología , Regulación de la Expresión Génica/fisiología , Oligodendroglía/metabolismo , Transferrina/metabolismo , Animales , Secuencia de Bases , Diferenciación Celular/efectos de los fármacos , Línea Celular , Medios de Cultivo/farmacología , Regulación de la Expresión Génica/efectos de los fármacos , Humanos , Datos de Secuencia Molecular , Oligodendroglía/citología , Oligodendroglía/efectos de los fármacos , Ratas , Transferrina/efectos de los fármacos , Transferrina/genéticaRESUMEN
We previously reported that the biosynthesis of phospholipids in the avian retina is altered by light stimulation, increasing significantly in ganglion cells in light and in photoreceptor cells in dark. In the present work, we have determined that light significantly increases the incorporation of [3H]glycerol into retina ganglion cell glycerophospholipids in vivo by a Fos-dependent mechanism because an oligonucleotide antisense to c-fos mRNA substantially blocked the light-dark differences. We also studied in vitro the enzyme activities of phosphatidate phosphohydrolase (PAPase), lysophosphatidate acyl transferase (AT II), and phosphatidylserine synthase from retinas of chickens exposed to light or dark. Higher PAPase I and AT II activities were found in incubations of retinal ganglion cells from animals exposed to light; no increase was observed in preparations obtained from light-exposed animals treated with the c-fos antisense oligonucleotide. No light-dark differences were found in phosphatidylserine synthase activity. These findings support the idea that a coordinated photic regulation of PAPase I and AT II is taking place in retina ganglion cells. This constitutes a reasonable mechanism to obtain an overall increased synthesis of glycerophospholipids in stimulated cells that is mediated by the expression of Fos-like proteins.
Asunto(s)
Fosfatidato Fosfatasa/metabolismo , Proteínas Proto-Oncogénicas c-fos/biosíntesis , Células Ganglionares de la Retina/enzimología , Animales , Pollos , Oscuridad , Activación Enzimática , Técnica del Anticuerpo Fluorescente , Glicerol/metabolismo , Isoenzimas/metabolismo , Luz , Oligonucleótidos Antisentido/química , Oligonucleótidos Antisentido/farmacología , Radioisótopos de Fósforo , Proteínas Proto-Oncogénicas c-fos/genética , ARN Mensajero/química , Células Ganglionares de la Retina/efectos de la radiación , TritioRESUMEN
Retina photoreceptor and ganglion cells isolated from chicks that in vivo were exposed to light have a different phospholipid labeling capacity than those from chicks in the dark. In the light exposed animals, the phospholipid labeling in the ganglion cells is higher (Delta% 45, p<0.005) than in those maintained in the dark, whereas in the photoreceptor cells, the opposite occurs, that is, the phospholipid labeling is higher in the dark than in light. The light-dark differences for phospholipid labeling correlate with the expression of c-fos: when c-fos expression increases (both in mRNA and in c-Fos protein content), phospholipid labeling increases concomitantly. That is, in ganglion cells, c-fos expression and the phospholipid synthesis is higher in light with respect to dark, whereas in photoreceptor cells, c-fos expression and phospholipid synthesis is higher in dark with respect to light. Moreover, when an oligonucleotide antisense to c-fos is administered intraocularly prior to separating the animals into light and dark, no differences in c-fos expression and, consequently, no differences in phospholipid synthesis are found between animals in light and dark. Taken together, these results point to a novel mechanism by which rapid genomic responses to cell stimulation are converted to longer lasting changes in the cell components.
Asunto(s)
Genes fos/efectos de la radiación , Luz , Fosfolípidos/biosíntesis , Células Fotorreceptoras/metabolismo , Proteínas Proto-Oncogénicas c-fos/biosíntesis , Células Ganglionares de la Retina/metabolismo , Animales , Pollos , Oscuridad , Regulación de la Expresión Génica/efectos de la radiación , Inmunohistoquímica , Células Fotorreceptoras/citología , Células Fotorreceptoras/efectos de la radiación , Células Ganglionares de la Retina/citología , Células Ganglionares de la Retina/efectos de la radiación , Transcripción Genética/efectos de la radiaciónRESUMEN
Retinal ganglion cells isolated from chicks that in vivo were exposed to light have a higher phospholipid labeling capacity than those obtained from animals in the dark. Actinomycin D or a mixture of protein synthesis inhibitors or of antisense oligonucleotides to c-fos plus c-jun injected intraocularly 1 hr prior to the stimulation period, abolished the light-dark differences for phospholipids but not for gangliosides. Light stimulation induced the formation (and/or stabilization) of c-fos mRNA and of the protein c-Fos, indicating that immediate early gene induction, and consequently the synthesis of the protein(s) encoded, is essential to increase the synthesis of phospholipids but not of gangliosides. These results suggest a novel mechanism by which immediate early genes engram neural cells, modifying specifically the metabolism of cell constituents producing long-lasting changes in the cells.