title: Incrimination of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) as a candidate sensor of physiological folate deficiency.
abstract: The mechanism underlying the sensing of varying degrees of physiological folate deficiency, prior to adaptive optimization of cellular folate uptake through the translational up-regulation of folate receptors (FR) is unclear. Because homocysteine, which accumulates intracellularly during folate deficiency, stimulated interactions between heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and an 18-base FR-α mRNA cis-element that led to increased FR biosynthesis and net up-regulation of FR at cell surfaces, hnRNP-E1 was a plausible candidate sensor of folate deficiency. Accordingly, using purified components, we evaluated the physiological basis whereby L-homocysteine triggered these RNA-protein interactions to stimulate FR biosynthesis. L-homocysteine induced a concentration-dependent increase in RNA-protein binding affinity throughout the range of physiological folate deficiency, which correlated with a proportionate increase in translation of FR in vitro and in cultured human cells. Targeted reduction of newly synthesized hnRNP-E1 proteins by siRNA to hnRNP-E1 mRNA reduced both constitutive and L-homocysteine-induced rates of FR biosynthesis. Furthermore, L-homocysteine covalently bound hnRNP-E1 via multiple protein-cysteine-S-S-homocysteine mixed disulfide bonds within K-homology domains known to interact with mRNA. These data suggest that a concentration-dependent, sequential disruption of critical cysteine-S-S-cysteine bonds by covalently bound L-homocysteine progressively unmasks an underlying RNA-binding pocket in hnRNP-E1 to optimize interaction with FR-α mRNA cis-element preparatory to FR up-regulation. Collectively, such data incriminate hnRNP-E1 as a physiologically relevant, sensitive, cellular sensor of folate deficiency. Because diverse mammalian and viral mRNAs also interact with this RNA-binding domain with functional consequences to their protein expression, homocysteinylated hnRNP-E1 also appears well positioned to orchestrate a novel, nutrition-sensitive (homocysteine-responsive), posttranscriptional RNA operon in folate-deficient cells.
authors.affiliation: Department of Medicine, Indiana University School of Medicine, Indianapolis, Indiana 46202, USA.
authors.forename: Ying-Sheng
authors.initials: YS
authors.lastname: Tang
authors.forename: Rehana A
authors.initials: RA
authors.lastname: Khan
authors.forename: Yonghua
authors.initials: Y
authors.lastname: Zhang
authors.forename: Suhong
authors.initials: S
authors.lastname: Xiao
authors.forename: Mu
authors.initials: M
authors.lastname: Wang
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Hansen
authors.forename: Hiremagalur N
authors.initials: HN
authors.lastname: Jayaram
authors.forename: Aśok  C
authors.initials: A
authors.lastname: Antony
mesh_terms: D004268:DNA-Binding Proteins; D004220:Disulfides / Q000378:metabolism; D058975:Folate Receptor 1 / Q000096:biosynthesis* / Q000235:genetics; D005492:Folic Acid / Q000378:metabolism*; D005494:Folic Acid Deficiency / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells; D034441:Heterogeneous-Nuclear Ribonucleoproteins / Q000235:genetics / Q000378:metabolism*; D006710:Homocysteine / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D015854:Up-Regulation
chemical_list: D004268:DNA-Binding Proteins; D004220:Disulfides; C548696:FOLR1 protein, human; D058975:Folate Receptor 1; D034441:Heterogeneous-Nuclear Ribonucleoproteins; C094604:PCBP1 protein, human; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D006710:Homocysteine; D005492:Folic Acid