Vitamin A has diverse biological functions. It is transported in the blood as a complex with retinol binding protein (RBP), but the molecular mechanism by which vitamin A is absorbed by cells from the vitamin A-RBP complex is not clearly understood. We identified in bovine retinal pigment epithelium cells STRA6, a multitransmembrane domain protein, as a specific membrane receptor for RBP. STRA6 binds to RBP with high affinity and has robust vitamin A uptake activity from the vitamin A-RBP complex. It is widely expressed in embryonic development and in adult organ systems. The RBP receptor represents a major physiological mediator of cellular vitamin A uptake.

Plasma retinol-binding protein (RBP), the principal carrier of vitamin A in the blood, delivers vitamin A from liver, the site of storage, to distant organs that need vitamin A, such as the eye, brain, placenta, and testis. STRA6 is a high-affinity membrane receptor for RBP and mediates vitamin A uptake in these target organs. STRA6 is a 74-kDa multi-transmembrane domain protein that represents a new class of membrane transport protein.In this study, we used an unbiased strategy by analyzing >900 random mutants of STRA6 to study its structure and function, and we identified an essential RBP-binding domain in STRA6. Mutations in any of the three essential residues in this domain can almost completely abolish binding of STRA6 to RBP and its vitamin A uptake activity from holo-RBP without affecting its cell surface expression. We have also functionally characterized the mutations in human STRA6 that cause severe birth defects as well as several human polymorphisms. All STRA6 mutants associated with severe birth defects have largely abolished vitamin A uptake activity, consistent with the severe clinical phenotypes. In addition, we have identified a human polymorphism that significantly reduces the vitamin A uptake activity of STRA6. Interestingly, the residue affected by this polymorphism is located in the RBP-binding domain we identified, and the polymorphism causes decreased vitamin A uptake by reducing RBP binding. This study identifies an essential functional domain in STRA6 and a human polymorphism in this domain that leads to reduced vitamin A uptake activity. Plasma retinol-binding protein (RBP)2 is the principal carrier of vitamin A in the blood (1-4). It was first proposed in the 1970s that there exists a cell surface receptor that mediates vitamin A uptake from RBP on the retinal pigment epithelium and small intestine cells (5-10). During the past 3 decades, there has been mounting evidence for the existence of RBP receptors on diverse types of tissues, including the placenta (11-13), choroid plexus (12, 14), Sertoli cells and peritubular cells of the testis (12,(15)(16)(17)(18), macrophages (19), and skin (12,20). There are also indirect pieces of evidence for the existence of an RBP receptor. For example, in an unbiased search for a serum factor that stimulates the growth of B cells, it was found that the vitamin A-RBP complex (holo-RBP) is this factor (21). Using an unbiased strategy combining photo-cross-linking, high-affinity purification, and mass spectrometry, the RBP receptor was identified as STRA6, a multi-transmembrane protein of previously unknown function (22). STRA6 binds to RBP with high affinity and specificity and facilitates the transport of vitamin A into the cell. STRA6 was originally identified as a retinoic acid-stimulated gene in cancer cell lines (23, 24).STRA6 has not been systematically characterized at the structural and functional level and is not homologous to any protein with known function. In this study, we used two complementary approaches to study the structure and functi...

Purpose: Activity of ornithine decarboxylase (ODC), the first enzyme in polyamine synthesis, is required for normal growth and is elevated in many cancers, including colorectal cancer. We examined associations of the +316 ODC1 single nucleotide polymorphism (SNP) with colorectal cancer-specific survival among colorectal cancer cases, and then investigated its functional significance in colon cancer cells. Experimental Design: The study included 400 incident stage I-III colorectal cancer cases from the population-based University of California Irvine Gene-Environment Study of Familial Colorectal Cancer (diagnosed from 1994 to 1996 with follow-up through March 2008). The primary outcome was colorectal cancer-specific survival dependent on ODC1 (rs2302615) genotype (GG versus GA/AA). In human colon cancer cell lines, ODC1 allele-specific binding of E-box transcription factors was determined via Western blotting and chromatin immunoprecipitation assays. ODC1 allele-specific promoter activity was determined using promoter constructs in combination with vectors expressing either the transcriptional activator c-MYC or the repressor MAD1. Results: Genotype-specific survival differences were observed among colorectal cancer cases: compared with cases with the ODC1 GG genotype (hazards ratio, 1; reference) the adjusted colorectal cancer-specific survival hazards ratio was 2.02 (95% confidence interval, 1.17-3.50) for ODC1 GA/AA cases (P = 0.012). In colon cancer cells, the ODC1 SNP, flanked by two E-boxes, predicts ODC1 promoter activity. The E-box activator c-MYC and repressors MAD1 and MAD4 preferentially bind to ODC1 minor A-alleles, compared with major G-alleles, in cultured cells. Conclusions: These results have implications for conditional regulation of polyamine homeostasis and suggest a model in which the ODC1