Oxidative folding and assembly with transthyretin are sequential events in the biogenesis of retinol binding protein in the endoplasmic reticulum

Maternal Inheritance of a Recessive RBP4 Defect in Canine Congenital Eye Disease

Maternally skewed transmission of traits has been associated with genomic imprinting and oocyte-derived mRNA. We report canine congenital eye malformations, caused by an amino acid deletion (K12del) near the N terminus of retinol-binding protein (RBP4). The disease is only expressed when both dam and offspring are deletion homozygotes. RBP carries vitamin A (retinol) from hepatic stores to peripheral tissues, including the placenta and developing eye, where it is required to synthesize retinoic acid. Gestational vitamin A deficiency is a known risk factor for ocular birth defects. The K12del mutation disrupts RBP folding in vivo, decreasing its secretion from hepatocytes to serum. The maternal penetrance effect arises from an impairment in the sequential transfer of retinol across the placenta, via RBP encoded by maternal and fetal genomes. Our results demonstrate a mode of recessive maternal inheritance, with a physiological basis, and they extend previous observations on dominant-negative RBP4 alleles in humans.

Biochemical Basis for Dominant Inheritance, Variable Penetrance, and Maternal Effects in RBP4 Congenital Eye Disease

Gestational vitamin A (retinol) deficiency poses a risk for ocular birth defects and blindness. We identified missense mutations in RBP4, encoding serum retinol binding protein, in three families with eye malformations of differing severity, including bilateral anophthalmia. The mutant phenotypes exhibit dominant inheritance, but incomplete penetrance. Maternal transmission significantly increases the probability of phenotypic expression. RBP normally delivers retinol from hepatic stores to peripheral tissues, including the placenta and fetal eye. The disease mutations greatly reduce retinol binding to RBP, yet paradoxically increase the affinity of RBP for its cell surface receptor, STRA6. By occupying STRA6 nonproductively, the dominant-negative proteins disrupt vitamin A delivery from wild-type proteins within the fetus, but also, in the case of maternal transmission, at the placenta. These findings establish a previously uncharacterized mode of maternal inheritance, distinct from imprinting and oocyte-derived mRNA, and define a group of hereditary disorders plausibly modulated by dietary vitamin A.

Sequestration of the abrin A chain to the nucleus by BASP1 increases the resistance of cells to abrin toxicity

Abrin, a type II ribosome-inactivating protein, comprises A and B subunits wherein the A subunit harbours toxin activity and the B subunit has a galactose-specific lectin activity. The entry of the protein inside the cell is through the binding of the B chain to cell surface glycoproteins followed by receptor-mediated endocytosis and retrograde transport. A previous study from our laboratory showed that different cell lines exhibited differences of as great as ~200-fold in abrin toxicity, prompting the present study to compare the trafficking of the toxin within cells. Observations made in this regard revealed that the abrin A chain, after being released into the cytosol, is sequestered into the nucleus through interaction with a cellular protein of ~25 kDa, BASP1 (brain acid-soluble protein 1). The nuclear localization of the A chain is seen predominantly in cells that are less sensitive to abrin toxicity and dependent on the levels of BASP1 in cells. The sequestration by BASP1 renders cells increasingly resistant to the inhibition of protein synthesis by abrin and the nucleus act as a sink to overcome cellular stress induced by the toxin.

Signaling by vitamin A and retinol-binding protein in regulation of insulin responses and lipid homeostasis

Vitamin A, retinol, circulates in blood bound to serum retinol binding protein (RBP) and is transported into cells by a membrane protein termed stimulated by retinoic acid 6 (STRA6). It was reported that serum levels of RBP are elevated in obese rodents and humans, and that increased level of RBP in blood causes insulin resistance. A molecular mechanism by which RBP can exert such an effect is suggested by the recent discovery that STRA6 is not only a vitamin A transporter but also functions as a surface signaling receptor. Binding of RBP-ROH to STRA6 induces the phosphorylation of a tyrosine residue in the receptor C-terminus, thereby activating a JAK/STAT signaling cascade. Consequently, in STRA6-expressing cells such as adipocytes, RBP-ROH induces the expression of STAT target genes, including SOCS3, which suppresses insulin signaling, and PPARγ, which enhances lipid accumulation. RBP-retinol thus joins the myriad of cytokines, growth factors and hormones which regulate gene transcription by activating cell surface receptors that signal through activation of Janus kinases and their associated transcription factors STATs. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

High liver RBP4 protein content is associated with histological features in patients with genotype 1 chronic hepatitis C and with nonalcoholic steatohepatitis

BACKGROUND AND AIM

To investigate the hepatic expression of retinol-binding protein-4 (RBP4) in chronic hepatitis C (CHC) and nonalcoholic steatohepatitis (NASH) patients, and its association with biochemical and histological patterns of liver damage.

MATERIALS AND METHODS

Sixty-six genotype 1 CHC and 32 NASH patients were tested for hepatic RBP4 expression. Liver expression at immunostaining was scored as 0 (slight), 1 (mild), 2 (moderate), and 3 (intense). In addition, the mRNA and the quantitative protein expressions of RBP4 were tested by PCR and by western blot, respectively, in 12 NASH and 28 CHC patients. Twelve subjects undergoing elective cholecystectomy served as controls.

RESULTS

Ten (31%), 16 (50%) and 6 (19%) NASH patients, and 21 (32%), 31 (47%) and 14 (21%) CHC patients had scores of 1, 2 and 3, respectively. All control subjects scored 0. In both CHC and NASH liver RBP4 scores were directly related to western blot (p=0.001 and p=0.03), not to mRNA expression (p=0.77 and p=0.40). Older age (OR, 1.07; 95%CI, 1.01-1.13), RBP4 score (4.26; 1.27-14.21) and HOMA (2.26; 1.15-4.42) were independently associated with steatosis≥10% in CHC patients. In NASH lobular inflammation (OR, 3.77; 95%CI, 1.01-24.22) and RBP4 score (4.87; 1.003-23.65) were the only risk factors for fibrosis ≥2 at logistic regression analysis.

CONCLUSION

Hepatic storage of RBP4, unrelated to its expression, could cause liver damage both in NASH and CHC.