Incrimination of Heterogeneous Nuclear Ribonucleoprotein E1 (hnRNP-E1) as a Candidate Sensor of Physiological Folate Deficiency

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 defi...

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Bibliographic Details
Published in:The Journal of biological chemistry 2011-11, Vol.286 (45), p.39100-39115
Main Authors: Tang, Ying-Sheng, Khan, Rehana A., Zhang, Yonghua, Xiao, Suhong, Wang, Mu, Hansen, Deborah K., Jayaram, Hiremagalur N., Antony, Aśok C.
Format: Article
Language:English
Subjects:
Disulfides - metabolism
DNA-Binding Proteins
Folate Metabolism
Folate Receptor
Folate Receptor 1 - biosynthesis
Folate Receptor 1 - genetics
Folic Acid - metabolism
Folic Acid Deficiency - genetics
Folic Acid Deficiency - metabolism
HeLa Cells
Heterogeneous-Nuclear Ribonucleoproteins - genetics
Heterogeneous-Nuclear Ribonucleoproteins - metabolism
Homocysteine
Homocysteine - genetics
Homocysteine - metabolism
Humans
IRES-trans-activating Factor
Metabolism
Posttranscriptional RNA Operon
Protein Binding
Protein Chemical Modification
Protein Structure, Tertiary
Receptor Regulation
RNA, Messenger
RNA-binding Protein
RNA-Binding Proteins
RNA-Protein Interaction
Translation Regulation
Up-Regulation
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Summary: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. Background: How do cells sense folate deficiency and then somehow restore folate homeostasis? Results: Accumulated intracellular homocysteine covalently binds heterogeneous nuclear ribonucleoprotein-E1 (hnRNP-E1) to open its high affinity mRNA-binding site and accommodate folate receptor (FR) mRNA; this triggers up-regulation of FR. Conclusion: hnRNP-E1 fulfills criteria as a cellular sensor of physio
ISSN:0021-9258
1083-351X
DOI:10.1074/jbc.M111.230938