Mitogen-activated protein kinase inhibits 1,25-dihydroxyvitamin D3–dependent signal transduction by phosphorylating human retinoid X receptor α
Cynthia Solomon, … , John H. White, Richard Kremer
Cynthia Solomon, … , John H. White, Richard Kremer
Published June 15, 1999
Citation Information: J Clin Invest. 1999;103(12):1729-1735. https://doi.org/10.1172/JCI6871.
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Mitogen-activated protein kinase inhibits 1,25-dihydroxyvitamin D3–dependent signal transduction by phosphorylating human retinoid X receptor α

Abstract

Human retinoid X receptor α (hRXRα) is a member of the nuclear receptor family of transcriptional regulators. It regulates transcription through its association with several heterodimeric partners, including the vitamin D3 receptor (VDR). Signaling through the VDR is essential for normal calcium homeostasis and has been shown to inhibit the proliferation of cancer cells derived from a number of tissues. Here we show that phosphorylation of hRXRα in ras-transformed human keratinocytes through the activated Ras–Raf–mitogen-activated protein kinase (Ras-Raf-MAP kinase) pathway results in attenuated transactivation by the VDR and resistance to the growth inhibitory action of 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] and RXR-specific agonist LG1069 (4-[1-(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl-2-naphtalenyl) ethenyl]-benzoic acid). Phosphorylation of hRXRα occurs at serine 260, a consensus MAP kinase site. Inhibition of MAP kinase activity or point mutagenesis of serine 260 of hRXRα reverses the observed resistance to 1,25(OH)2D3 and LG1069. Thus, hRXRα is a downstream target of MAP kinase, and its phosphorylation may play an important role in malignant transformation.

Authors

Cynthia Solomon, John H. White, Richard Kremer

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Figure 3

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MAP kinase–dependent phosphorylation of hRXRα in HPK1A and HPK1Aras cell...
MAP kinase–dependent phosphorylation of hRXRα in HPK1A and HPK1Aras cells. (a) HPK1A cells were transfected with activated or inactivated MAPKK expression plasmids and were treated with increasing concentrations of 1,25(OH)2D3 and 5% charcoal-stripped FBS. CAT activity was assayed after 24 hours, as in Figure 1a. Asterisks indicate a significant difference from vehicle-treated control values, whereas open circles indicate a significant difference between cells overexpressing active and inactive MAPKK. (b) Nuclear extracts prepared from these cells were immunoprecipitated with an anti-RXRα antibody, followed by SDS-PAGE and Western blotting. The blots were treated with an anti-phosphoserine antibody or an anti-RXRα antibody to confirm equal loading of the protein as indicated. The diamond indicates the presence of the hRXRα protein.