RESUMEN
Sustained expression of the Spi-1/PU.1 and Fli-1 oncoproteins blocks globin gene activation in mouse erythroleukemia cells; however, only Spi-1/PU.1 expression inhibits the inclusion of exon 16 in the mature 4.1R mRNA. This splicing event is crucial for a functional 4.1R protein and, therefore, for red blood cell membrane integrity. This report demonstrates that Spi-1/PU.1 downregulation induces the activation of TRIM10/hematopoietic RING finger 1 (HERF1), a member of the tripartite motif (TRIM)/RBCC protein family needed for globin gene transcription. Additionally, we demonstrate that TRIM10/HERF1 is required for the regulated splicing of exon 16 during late erythroid differentiation. Using inducible overexpression and silencing approaches, we found that: (1) TRIM10/HERF1 knockdown inhibits hemoglobin production and exon splicing and triggers cell apoptosis in dimethylsulfoxide (DMSO)-induced cells; (2) TRIM10/HERF1 upregulation is required but is insufficient on its own to activate exon retention; (3) Fli-1 has no effect on TRIM10/HERF1 expression, whereas either DMSO-induced downregulation or shRNA-knockdown of Spi-1/PU.1 expression is sufficient to activate TRIM10/HERF1 expression; and (4) Spi-1/PU.1 knockdown triggers both the transcription and the splicing events independently of the chemical induction. Altogether, these data indicate that primary Spi-1/PU.1 downregulation acts on late erythroid differentiation through at least two pathways, one of which requires TRIM10/HERF1 upregulation and parallels the Spi-1/PU.1-induced Fli-1 shutoff regulatory cascade.
Asunto(s)
Diferenciación Celular/genética , Células Eritroides/metabolismo , Hematopoyesis/fisiología , Antígenos de Histocompatibilidad/metabolismo , Proteínas Proto-Oncogénicas/metabolismo , Transactivadores/metabolismo , Animales , Proteínas Sanguíneas/genética , Proteínas Sanguíneas/metabolismo , Proteínas Portadoras/genética , Proteínas Portadoras/metabolismo , Línea Celular Tumoral , Supervivencia Celular/genética , Dimetilsulfóxido/farmacología , Regulación hacia Abajo/genética , Eritrocitos/citología , Eritrocitos/metabolismo , Células Eritroides/citología , Exones/genética , Hematopoyesis/genética , Hemoglobinas/biosíntesis , Antígenos de Histocompatibilidad/genética , Péptidos y Proteínas de Señalización Intracelular , Ratones , Proteínas de Microfilamentos , Proteína Proto-Oncogénica c-fli-1/genética , Proteína Proto-Oncogénica c-fli-1/metabolismo , Proteínas Proto-Oncogénicas/genética , Interferencia de ARN , Empalme del ARN/genética , Transducción de Señal/genética , Transactivadores/genética , Proteínas de Motivos Tripartitos , Regulación hacia Arriba/genéticaRESUMEN
Ferroportin [FPN; Slc40a1 (solute carrier family 40, member 1)] is a transmembrane iron export protein expressed in macrophages and duodenal enterocytes. Heterozygous mutations in the FPN gene result in an autosomal dominant form of iron overload disorder, type-4 haemochromatosis. FPN mutants either have a normal iron export activity but have lost their ability to bind hepcidin, or are defective in their iron export function. The mutant protein has been suggested to act as a dominant negative over the wt (wild-type) protein by multimer formation. Using transiently transfected human epithelial cell lines expressing mouse FPN modified by the addition of a haemagglutinin or c-Myc epitope at the C-terminus, we show that the wtFPN is found at the plasma membrane and in Rab5-containing endosomes, as are the D157G and Q182H mutants. However, the delV162 mutant is mostly intracellular in HK2 cells (human kidney-2 cells) and partially addressed at the cell surface in HEK-293 cells (human embryonic kidney 293 cells). In both cell types, it is partially associated with the endoplasmic reticulum and with Rab5-positive vesicles. However, this mutant is complex-glycosylated like the wt protein. D157G and G323V mutants have a defective iron export capacity as judged by their inability to deplete the intracellular ferritin content, whereas Q182H and delV162 have normal iron export function and probably have lost their capacity to bind hepcidin. In co-transfection experiments, the delV162 mutant does not co-localize with the wtFPN, does not prevent its normal targeting to the plasma membrane and cannot be immunoprecipitated in the same complex, arguing against the formation of FPN hetero-oligomers.