Translational upregulation of folate receptors is mediated by homocysteine via RNA-heterogeneous nuclear ribonucleoprotein E1 interactions
Aśok C. Antony, … , Hiremagalur N. Jayaram, Sally P. Stabler
Aśok C. Antony, … , Hiremagalur N. Jayaram, Sally P. Stabler
Published January 15, 2004
Citation Information: J Clin Invest. 2004;113(2):285-301. https://doi.org/10.1172/JCI11548.
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Translational upregulation of folate receptors is mediated by homocysteine via RNA-heterogeneous nuclear ribonucleoprotein E1 interactions

Abstract

Cellular acquisition of folate is mediated by folate receptors (FRs) in many malignant and normal human cells. Although FRs are upregulated in folate deficiency and downregulated following folate repletion, the mechanistic basis for this relationship is unclear. Previously we demonstrated that interaction of an 18-base cis-element in the 5′-untranslated region of FR mRNA and a cystolic trans-factor (heterogeneous nuclear ribonucleoprotein E1 [hnRNP E1]) is critical for FR synthesis. However, the molecular mechanisms controlling this interaction, especially within the context of FR regulation and folate status, have remained obscure. Human cervical carcinoma cells exhibited progressively increasing upregulation of FRs after shifting of folate-replete cells to low-folate media, without a proportionate rise in FR mRNA or rise in hnRNP E1. Translational FR upregulation was accompanied by a progressive accumulation of the metabolite homocysteine within cultured cells, which stimulated interaction of the FR mRNA cis-element and hnRNP E1 as well as FR biosynthesis in a dose-dependent manner. Abrupt reversal of folate deficiency also led to a rapid parallel reduction in homocysteine and FR biosynthesis to levels observed in folate-replete cells. Collectively, these results suggest that homocysteine is the key modulator of translational upregulation of FRs and establishes the linkage between perturbed folate metabolism and coordinated upregulation of FRs.

Authors

Aśok C. Antony, Ying-Sheng Tang, Rehana A. Khan, Mangatt P. Biju, Xiangli Xiao, Qing-Jun Li, Xin-Lai Sun, Hiremagalur N. Jayaram, Sally P. Stabler

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Figure 1

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Analysis of FR protein, FR mRNA, and hnRNP E1 in HeLa-IU1-HF cells propa...
Analysis of FR protein, FR mRNA, and hnRNP E1 in HeLa-IU1-HF cells propagated in low-folate media over 12 weeks. (a) Western blot analysis of the expression of FR protein as a function of time after shift of HeLa-IU1-HF cells to low-folate media. Each data point (fold increase of FR protein as a function of time) represents the mean ± SD of four independent evaluations. The Western blot data shown represent one of the four studies. (b) Determination of the cell surface receptor number by [3H]folate-binding to intact cells and Scatchard analysis as a function of time after shifting of HeLa-IU1-HF cells to low-folate media. Each data point represents the mean ± SD of four independent evaluations. (c) Northern blot analysis of FR mRNA relative to β-actin mRNA expression as a function of time after shifting of HeLa-IU1-HF cells to low-folate media. Signals from Northern blot analysis of 10 μg of total cellular RNA from cells for the indicated times were quantitated by densitometry. The quantity of the FR mRNA was normalized to that of β-actin mRNA. Each data point (fold increase of FR mRNA as a function of time) represents the mean ± SD of four independent evaluations. The Northern blot data shown represent one of the four studies. (d) Northwestern gel analysis of the FR mRNA–binding hnRNP E1 from HeLa-IU1-HF cells and HeLa-IU1-LF cells probed with FR-α mRNA cis-element (3). The FR-α RNA sequence 5′-CUCCAUUCCCACUCCCUG-3′ was labeled by in vitro transcription. Densitometric analysis of the signals is shown in the panel.