Quantitation of the Noncovalent Cellular Retinol-Binding Protein, Type 1 Complex Through Native Mass Spectrometry

Altered RBP1 Gene Expression Impacts Epithelial Cell Retinoic Acid, Proliferation, and Microenvironment

Vitamin A is an essential diet-derived nutrient that has biological activity affected through an active metabolite, all-trans retinoic acid (atRA). Retinol-binding protein type 1 (RBP1) is an intracellular chaperone that binds retinol and retinal with high affinity, protects retinoids from non-specific oxidation, and delivers retinoids to specific enzymes to facilitate biosynthesis of RA. RBP1 expression is reduced in many of the most prevalent cancers, including breast cancer. Here, we sought to understand the relationship between RBP1 expression and atRA biosynthesis in mammary epithelial cells, as well as RBP1 expression and atRA levels in human mammary tissue. We additionally aimed to investigate the impact of RBP1 expression and atRA on the microenvironment as well as the potential for therapeutic restoration of RBP1 expression and endogenous atRA production. Using human mammary ductal carcinoma samples and a series of mammary epithelial cell lines representing different stages of tumorigenesis, we investigated the relationship between RBP1 expression as determined by QPCR and atRA via direct liquid chromatography-multistage-tandem mass spectrometry-based quantification. The functional effect of RBP1 expression and atRA in epithelial cells was investigated via the expression of direct atRA targets using QPCR, proliferation using Ki-67 staining, and collagen deposition via picrosirius red staining. We also investigated the atRA content of stromal cells co-cultured with normal and tumorigenic epithelial cells. Results show that RBP1 and atRA are reduced in mammary tumor tissue and tumorigenic epithelial cell lines. Knock down of RBP1 expression using shRNA or overexpression of RBP1 supported a direct relationship between RBP1 expression with atRA. Increases in cellular atRA were able to activate atRA direct targets, inhibit proliferation and inhibit collagen deposition in epithelial cell lines. Conditions encountered in tumor microenvironments, including low glucose and hypoxia, were able to reduce RBP1 expression and atRA. Treatment with either RARα agonist AM580 or demethylating agent Decitabine were able to increase RBP1 expression and atRA. Cellular content of neighboring fibroblasts correlated with the RA producing capacity of epithelial cells in co-culture. This work establishes a direct relationship between RBP1 expression and atRA, which is maintained when RBP1 expression is restored therapeutically. The results demonstrate diseases with reduced RBP1 could potentially benefit from therapeutics that restore RBP1 expression and endogenous atRA.

Development of Native MS Capabilities on an Extended Mass Range Q-TOF MS

Native mass spectrometry (nMS) is increasingly used for studies of large biomolecules (>100 kDa), especially proteins and protein complexes. The growth in this area can be attributed to advances in native electrospray ionization as well as instrumentation that is capable of accessing high mass-to-charge (m/z) regimes without significant losses in sensitivity and resolution. Here, we describe modifications to the ESI source of an Agilent 6545XT Q-TOF MS that is tailored for analysis of large biomolecules. The modified ESI source was evaluated using both soluble and membrane protein complexes ranging from ~127 to ~232 kDa and the ~801 kDa protein chaperone GroEL. The increased mass resolution of the instrument affords the ability to resolve small molecule adducts and analyze collision-induced dissociation products of the native complexes.

Aberrant retinoic acid production in the decidua: Implications for pre-eclampsia

Fine-tuning of the endometrium during the evanescent 'window of implantation' relies upon an array of diverse and redundant signaling molecules, particularly the ovarian steroids E2 and P4, but also growth factors, eicosanoids, and vitamins including the vitamin A compounds (retinoids). Pregnancy complications such as preeclampsia (PE) can result from aberrations in the production or function of these molecules that arise during this critical period of decidual development. Such aberrations may be reflected by incomplete decidualization, reduced spiral artery modification, and/or loss of immune tolerance to the developing fetus. Our understanding of the role of the active retinoid metabolite all-trans retinoic acid (RA) in maintaining immune balance in certain tissues, along with data describing its role in decidualization, present a compelling argument that aberrant RA signaling in the decidua can play a significant role in the etiology of PE. Recent findings that decidualization and expression of the anti-angiogenic gene product, 'soluble fms-like tyrosine kinase-1' (sFLT1) are negatively correlated and that sFLT1 expression is directly inhibited by RA, provide additional evidence of the critical role of this retinoid in regulating early vascular development in the decidua. This review provides insight into the production and function of RA in the decidua and how modifications in its metabolism and signaling might lead to certain pregnancy disorders such as PE.

Study on the Structural Effect of Maltoligosaccharides on Cytochrome c Complexes Stabilities by Native Mass Spectrometry

Noncovalent interactions between ligands and targeting proteins are essential for understanding molecular mechanisms of proteins. In this work, we investigated the interaction of Cytochrome c (Cyt c) with maltoligosaccharides, namely maltose (Mal II), maltotriose (Mal III), maltotetraose (Mal IV), maltopentaose (Mal V), maltohexaose (Mal VI) and maltoheptaose (Mal VII). Using electrospray ionization mass spetrometry (ESI-MS) assay, the 1:1 and 1:2 complexes formed by Cyt c with maltoligosaccharide ligand were observed. The corresponding association constants were calculated according to the deconvoluted spectra. The order of the relative binding affinities of the selected oligosaccharides with Cyt c were as Mal III > Mal IV > Mal II > Mal V > Mal VI > Mal VII. The results indicated that the stability of noncovalent protein complexes was intimately correlated to the molecular structure of bound ligand. The relevant functional groups that could form H-bonds, electrostatic or hydrophobic forces with protein's amino residues played an important role for the stability of protein complexes. In addition, the steric structure of ligand was also critical for an appropriate interaction with the binding pocket of proteins.