Functional characterization of heterogeneous nuclear ribonuclear protein C1/C2 in vitamin D resistance: a novel response element-binding protein.
Clinically apparent hereditary vitamin D-resistant rickets (HVDRR) usually results from a loss of function mutation in the vitamin D receptor (VDR). We recently described a human with the classical HVDRR phenotype but normal VDR function. Hormone resistance resulted from constitutive overexpression of heterogeneous nuclear ribonucleoprotein (hnRNP) that competed with a normally functioning VDR-retinoid X receptor (RXR) dimer for binding to the vitamin D response element (VDRE). Here we describe the purification, molecular cloning, and expression of this vitamin D resistance-causing, competitive response element-binding protein (REBiP) hnRNP C1/C2. When overexpressed in vitamin D-responsive cells, cDNAs for both hnRNPC1 and hnRNPC2 inhibited VDR-VDRE-directed transactivation (28 and 43%, respectively; both p < 0.005). By contrast, transient expression of an hnRNP C1/C2 small interfering RNA increased VDR transactivation by 39% (p < 0.005). Chromatin immunoprecipitation of nucleoproteins bound to the transcriptionally active 1,25-dihydroxy vitamin D-driven CYP24 promoter revealed the presence of REBiP in vitamin D-responsive human cells and indicated that the normal pattern of 1,25-dihydroxy vitamin D-initiated cyclical movement of the VDR on and off the VDRE is legislated by competitive, reciprocal occupancy of the VDRE by hnRNP C1/C2. The temporal and reciprocal pattern of VDR and hnRNPC1/C2 interaction with the VDRE was lost in HVDRR cells overexpressing the hnRNP C1/C2 REBiP. These observations provide further evidence for the functional importance of REBiP as a component of the multiprotein complex involved in the regulation of vitamin D-mediated transcription. In particular, chromatin immunoprecipitation data suggest that, in addition to its RNA-processing functions, hnRNP C1/C2 may be a key determinant of the temporal patterns of VDRE occupancy.
Chen
Hong
H
Division of Endocrinology, Diabetes and Metabolism, Burns and Allen Research Institute, Cedars-Sinai Medical Center, UCLA School of Medicine, Los Angeles, California 90048, USA.
Hewison
Martin
M
Adams
John S
JS
D000595:Amino Acid Sequence; D000818:Animals; D002843:Chromatin / Q000737:chemistry; D003001:Cloning, Molecular; D034482:Heterogeneous-Nuclear Ribonucleoprotein Group C / Q000737:chemistry* / Q000378:metabolism; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D006801:Humans; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D051381:Rats; D018167:Receptors, Calcitriol / Q000235:genetics; D012279:Rickets / Q000235:genetics / Q000378:metabolism; D013250:Steroid Hydroxylases / Q000235:genetics; D014158:Transcription, Genetic; D014807:Vitamin D / Q000037:antagonists & inhibitors*; D065668:Vitamin D3 24-Hydroxylase
D002843:Chromatin; D034482:Heterogeneous-Nuclear Ribonucleoprotein Group C; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D018167:Receptors, Calcitriol; D014807:Vitamin D; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase