Retinol binding protein expression is induced in HepG2 cells by zinc deficiency.

:Zinc (Zn) deficiency is often associated with low plasma vitamin A (retinol) concentrations. It has been suggested that the reduction in plasma retinol is secondary to reduced liver retinol binding protein (RBP) synthesis. In the present study, RBP expression was determined in HepG2 cells cultured in either Zn adequate media or chelated media containing varying concentrations of Zn. Levels of RBP mRNA increased in a time- and Zn concentration-dependent manner such that 0.5 microM Zn-treated cells exhibited a >7.5-fold increase while cells treated with 15 microM Zn were increased 2.9-fold at 72 h compared to controls. RBP protein also progressively increased by 72 h to levels >8-fold and 3-fold higher than controls, in 0.5 microM and 15 microM Zn-treated cells, respectively. The increase in RBP occurred without any change in DNA concentration between groups through 72 h. The Zn deficiency-induced elevations in RBP transcript levels could be reversed within 24-48 h of repletion in Zn adequate media. Thus, the reductions in plasma retinol observed in Zn deficiency are in part a direct consequence of the deficiency.

Satre MA ，Jessen KA ，Clegg MS ，Keen CL ... -  《FEBS LETTERS》

Regulation of plasma retinol binding protein secretion in human HepG2 cells.

Retinol binding protein (RBP) is the plasma transport protein of retinol. Mobilization of RBP from the liver stores is stimulated by retinol. During vitamin A deficiency, RBP secretion is specifically inhibited while its rate of biosynthesis is unaffected. As a consequence, RBP, as apoprotein, accumulates inside the endoplasmic reticulum (ER) of the hepatocyte, and a new elevated steady-state concentration is reached. We have studied the role of degradation on the regulation of RBP metabolism in retinol deficient HepG2 cells and determined the intracellular site where RBP degradation takes place. Pulse-chase experiments show that RBP half-life is ca.9 h in retinol-depleted cells. RBP degradation is slow and is insensitive to the treatment with NH4Cl, which inactivates lysosomal proteases and to the drug brefeldin A, which prevents protein export from the ER. The data obtained suggest that RBP degradation occurs, at least in part, in a pre-Golgi compartment. 2-Mercaptoethanol, at millimolar concentration, induces RBP secretion, suggesting a possible role for sulfhydryl-mediated apo-RBP retention by resident ER proteins.

Tosetti F ，Ferrari N ，Pfeffer U ，Brigati C ，Vidali G ... -  《EXPERIMENTAL CELL RESEARCH》

Effects of acute inflammation on plasma retinol, retinol-binding protein, and its mRNA in the liver and kidneys of vitamin A-sufficient rats.

The acute inflammatory response to tissue injury and infection is associated with low concentrations of plasma retinol and its specific transport proteins, retinol-binding protein (RBP) and transthyretin (TTR). To examine the kinetics and mechanism of hyporetinemia, we have induced acute inflammation with lipopolysaccharide (LPS, from Pseudomonas aeruginosa) in rats with adequate stores of vitamin A. Twenty-four h after treatment with LPS (50 micrograms i.p. per 100 g body weight) or saline and food withdrawal, plasma retinol equalled 0.72 +/- 0.06 mumol/L (mean +/- SEM) in five LPS-treated rats versus 1.35 +/- 0.1 mumol/L in five saline-treated rats (P < 0.01). Plasma, liver, and kidney RBP and TTR concentrations were also significantly reduced, but liver and kidney retinol concentrations did not differ between treatment groups. The relative abundance of RBP mRNA in liver (LPS treatment compared to saline treatment) was reduced as early as 12 h (0.44 +/- 0.15, n = 4 pairs, P < 0.02), and continued to be reduced at 24 h (0.57 +/- 0.12, n = 5 pairs, P < 0.02). In the kidney this ratio did not change significantly due to LPS treatment. The relative abundance of cellular retinol-binding protein (CRBP) mRNA in liver and kidney also was not affected by LPS treatment. We infer from these data that inflammation-induced hyporetinemia results from a reduction in the hepatic synthesis of RBP and secretion of the retinol-RBP complex. Moreover, the results imply that plasma retinol concentration is a poor indicator of vitamin A status during inflammation.

Rosales FJ ，Ritter SJ ，Zolfaghari R ，Smith JE ，Ross AC ... -  《JOURNAL OF LIPID RESEARCH》

Dietary restriction alters retinol and retinol-binding protein metabolism in aging rats.

Recent studies reported that retinoid metabolism was influenced by long-term dietary restriction (DR) in rats. Because plasma retinol was decreased in rats subjected to DR, it was thought that this dietary manipulation may have an effect on retinol-binding protein (RBP) metabolism. Thus, the aim of this study was to assess retinoids, RBP, and transthyretin (TTR) levels in plasma and liver of young (3 months), adult (12 months), and old (22 months) female Sprague-Dawley rats fed ad libitum (AL) or subjected to a 40% DR, enriched (DR+), or not (DR), with vitamins and minerals. Results indicate that hepatic total retinoid concentrations and content increased with age in all the groups. DR+ rats showed higher hepatic retinoid concentrations than age-matched AL and DR rats. Adult and old DR and DR+ rats exhibited significantly lower plasma RBP-retinol and higher total retinoic acid levels than corresponding controls, although these parameters were not influenced by aging. Liver RBP levels were also decreased in DR and DR+ rats when compared to respective AL controls. There was a slight age-related decline in plasma TTR levels in DR and DR+ rats which was not associated with modifications in liver TTR levels. Hepatic gene expression of RBP and TTR, as evaluated by Northern blot hybridization, did not change with age or diet, suggesting that the lower levels of plasma RBP-retinol and liver RBP in vitamin A-sufficient rats subjected to DR may reflect post-transcriptional alterations and/or accelerated degradation of hepatic RBP. The elevated plasma levels of retinoic acid may represent an adaptive mechanism developed by DR rats to maintain retinoid-dependent functions.

Chevalier S ，Blaner WS ，Azais-Braesco V ，Tuchweber B ... -  《JOURNALS OF GERONTOLOGY SERIES A-BIOLOGICAL SCIENCES AND MEDICAL SCIENCES》

Mouse retinol binding protein gene: cloning, expression and regulation by retinoic acid.

A full-length cDNA clone encoding the retinol binding protein (RBP) was isolated from a mouse liver cDNA library by hybridization screening. The nucleotide sequence of murine RBP is 85 and 95% homologous to that of human and rat RBP, respectively, with a deduced amino acid sequence > or = 83% homologous to both species. Analysis of the tissue expression pattern of RBP mRNA in the female mouse indicated relatively abundant expression in the liver, with lesser amounts in extrahepatic tissues including adipose, kidney, spleen and uterus, suggesting that these tissues may have a significant role in retinol homeostasis. Mouse liver cell RBP regulation by retinoids was also investigated. Both all-trans retinoic acid (AT-RA) and 9-cis retinoic acid (9c-RA) induced RBP mRNA expression in a dose- and time-dependent manner. Maximal levels (up to 4-fold above controls) were observed at > or = 48 h following treatment of both mouse hepatoma cells in vitro and in vivo in mice receiving a single, oral dose of either retinoid. Interestingly, 9c-RA was more potent at RBP induction in both in vivo and in vitro systems. Given the extent and temporal pattern of RBP induction, we suggest that the RA-mediated increase in liver RBP is part of a cellular protection mechanism. Increased levels of RBP would facilitate sequestration and possibly cellular export of RA in cells receiving prolonged exposure to high levels of RA, thus minimizing toxicity.

Jessen KA ，Satre MA 《MOLECULAR AND CELLULAR BIOCHEMISTRY》