Translational upregulation of folate receptors is mediated by homocysteine via RNA-heterogeneous nuclear ribonucleoprotein E1 interactions
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
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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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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Quantitation of the extent of influx of extracellular homocysteine into ...
Quantitation of the extent of influx of extracellular homocysteine into HeLa-IU1 cells and the effects of accumulated homocysteine on the expression of FR mRNA and the biosynthesis of FR. (a) Determination of the concentration of intracellular homocysteine after incubation of HeLa-IU1–HF cells with 500 μM dl-homocysteine (white bars) or 1,000 μM dl-homocysteine (gray bars) for the indicated time. (b) Northern blots of total cellular mRNA (3 μg) from HeLa-IU1–HF cells exposed to 1,000 μM dl-homocysteine for various times and probed with FR mRNA and β-actin mRNA. (c–e) Stimulation of FR synthesis using [35S]cysteine (c and d) or [3H]leucine (e) in cultured HeLa-IU1–HF cells (c and e) and HeLa-IU1–LF cells (d) by various concentrations of homocysteine as a function of time. No dl-homocysteine (diamonds), 500 μM dl-homocysteine (squares), or 1,000 μM dl-homocysteine (triangles) was added to cultured cells during incubation with [35S]cysteine or [3H]leucine. [35S]FR or [3H]FR was immunoprecipitated with anti-FR antiserum and IgGsorb and radioactivity was measured. The data from three independent experiments (with each data point carried out in triplicate) were pooled. There was less than 10% variation from this mean among the three independent experiments.