High pressure liquid chromatographic detection of intracellular retinoid binding proteins from cultured cell and tumor cytosols

Structure-affinity relationships of retinoids with embryonic cellular retinoic acid-binding protein

Separation and quantitation of cellular retinoic acid-binding protein (CRABP) in embryonic and fetal hamster tissues was accomplished with high-performance size-exclusion chromatography. Binding affinity of 26 retinoids was established by in vitro displacement of high specific activity all-trans-[3H2]retinoic acid from fetal CRABP. The CRABP concentration in presomite-to-early somite (Day 8) hamster embryos was 1.9 pmol/mg cytosolic protein and increased to 7.5 pmol/mg protein in Day 13 fetuses; CRABP concentrations subsequently declined as gestation progressed. CRABP was located primarily in fetal brain and skin (5.8 +/- 0.3 and 2.2 +/- 0.1 pmol/mg protein, respectively), whereas only trace concentrations were found in fetal liver, placenta, and maternal uterus. Retinoids that could displace all-trans-retinoic acid from CRABP had a free acid at the polar terminus (or were carboxylate esters that were readily hydrolyzed to the corresponding free acid) and had a hydrophobic ring at the distal position. The ligand specificity of the CRABP studied here suggests that this protein was analogous to the CRABP I isoform. The in vitro binding affinities of teratogenic retinoids that competed for embryonic CRABP failed to correlate directly with relative teratogenic potency. In some instances, the latter observation can be related to extensive in vivo biotransformation of retinoids to multiple teratogenic metabolites and to retinoid persistence in the embryo. Three analogs containing a free carboxy terminus, SRI 5898-21, SRI 7323-78, and SRI 6153-40, were identified with high teratogenic potency but failed to bind fetal hamster CRABP. The structure-activity and binding data of the analogs studied here indicate that many, if not most, teratogenic retinoids (or their acidic metabolites) bind with embryonic/fetal CRABP, but the present data question the role for CRABP in their teratogenic mechanism of action.

Localization of the cellular retinoic acid binding protein (CRABP) gene relative to the acute promyelocytic leukemia-associated breakpoint on human chromosome 15

A human genomic fragment comprising the cellular retinoic acid binding protein (CRABP) gene was isolated. By using a panel of somatic cell hybrids, this gene could be assigned to human chromosome 15. Subsequently, a possible involvement of the CRABP gene in translocation (15;17) (q22;q11) positive acute promyelocytic leukemia (APL) was investigated. Although transposition of the CRABP gene could be demonstrated, we did not observe any gross CRABP rearrangement in a series of primary APL patients, nor in the acute myeloblastic leukemia cell line HL-60. Thus, the observed lack of CRABP expression in these leukemic cells may not be caused by disruption of its gene. CRABP maps to the region 15q22-qter.

Affinity purification of retinoic acid-binding proteins using immobilized 4-(2-hydroxyethoxy)retinoic acid

An affinity chromatographic matrix that purifies cellular retinoic acid-binding protein to near homogeneity from rat testes cytosol has been developed. The three-step procedure includes an acid precipitation, a batch treatment with CM Bio-Gel, and affinity chromatography on 4-(2-hydroxyethoxy)retinoic acid coupled to epoxy-activated Sepharose 6B. The binding protein was purified approximately 8500-fold based on total soluble testicular protein and with a recovery in excess of 80%. In addition, further enhancement of the purity of the protein can be attained by size-exclusion HPLC to increase purification to 21,000-fold. The recovered protein has an apparent M(r) 14,300 as determined by size-exclusion HPLC and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein is isolated in the apo-form and retains its ability to bind retinoic acid as evidenced by the binding of [3H]retinoic acid. An apparent retinoic acid-binding protein of M(r) 18,000 has also been isolated from rat testes nuclei by the affinity chromatography step. The affinity phase has been used for 6 months without any detectable loss in its ability to purify cellular retinoic acid-binding protein.

A slab gel electrophoresis technique for measurement of plasma retinol-binding protein, cellular retinol-binding and retinoic-acid-binding proteins in human skin

At least four different proteins that bind retinoids could be present in a vitamin A target tissue like the skin. In order to separate cellular retinoid-binding proteins (CRBP and CRABP) from serum retinol-binding protein (RBP) and albumin, a one-step procedure was devised. The technique is based on slab polyacrylamide gel electrophoresis (PAGE) of the extracted proteins incubated with tritiated retinoids. The procedure was used to study binding proteins in the skin. The results show that epidermal extracts (the epithelial part of the skin) contain no RBP activities whereas dermal extracts (the mesenchymal part of the skin) contain 1.6 +/- 0.81 pmol/mg protein of RBP. This technique further showed higher levels of CRABP in both epidermal (9.05 +/- 1.16 pmol/mg protein) and dermal (1.5 +/- 0.54 pmol/mg protein) extracts than those previously determined by other less specific techniques. On the other hand CRBP levels were found to be lower in the two tissues (epidermis 0.2 +/- 0.1 pmol/mg and dermis 0.12 +/- 0.05 pmol/mg protein). New conditions to measure specifically CRABP with the charcoal/dextran technique could be developed and analyzed by the PAGE technique; a dissociation constant of 13.7 nM was then calculated for epidermal CRABP. This PAGE technique appears to be the most appropriate method for the study of retinoid-binding proteins including RBP in human skin.

Purification and partial characterization of cellular retinoic acid-binding protein from human placenta

Cellular retinoic acid-binding protein (CRABP) has been purified to homogeneity from human placenta by a series of procedures, including acetone powder extraction, gel filtration on Sephadex G-50, and ion-exchange chromatography on DEAE-cellulose and on SP-Sephadex. Cellular retinol-binding protein (CRBP) was isolated concurrently. CRABP was purified 75,400-fold, based on total soluble acetone powder extract of placenta. The protein is a single polypeptide chain with a molecular mass of 14,600 Da, estimated by sodium dodecyl sulfate (SDS) gel electrophoresis or gel filtration, and has an isoelectric point of 4.78 (apo-CRABP, 4.82). On analysis of absorption and fluorescence spectra, the protein was seen to exhibit an absorption peak at 350 nm, fluorescence excitation maxima at 350 and 370 nm, and a fluorescence emission maximum at 475 nm. Human CRABP was immunologically distinct from human CRBP and serum retinol-binding protein.

A high-performance liquid chromatographic technique that separates cellular retinol binding protein from cellular retinoic acid binding protein

A one-step procedure to detect cellular [3H]retinol and [3H]retinoic acid binding proteins (CRBP and CRABP) from rat testis cytosolic extract was devised. The procedure is based on anion-exchange high-performance liquid chromatography of the cytosolic fraction on columns of Mono Q, which permits elution of CRABP and CRBP at 12 and 22 min, respectively.