Retinoids increase alpha-1 acid glycoprotein expression at the transcriptional level through two distinct DR1 retinoic acid responsive elements

A synthetic retinoic acid receptor agonist Am80 ameliorates renal fibrosis via inducing the production of alpha-1-acid glycoprotein

Renal fibrosis is a major factor in the progression of chronic kidney disease and the final common pathway of kidney injury. Therefore, the effective therapies against renal fibrosis are urgently needed. The objective of this study was to investigate the effect of Am80, a synthetic retinoic acid receptor (RAR) agonist, in the treatment of renal interstitial fibrosis using unilateral ureteral obstruction (UUO) mice. The findings indicate that Am80 treatment suppressed renal fibrosis and inflammation to the same degree as the naturally-occuring retinoic acid, all-trans retinoic acid (atRA). But the adverse effect of body weight loss in Am80-treated mice was lower compared to the atRA treatment. The hepatic mRNA levels of alpha-1-acid glycoprotein (AGP), a downstream molecule of RAR agonist, was increased following administration of Am80 to healthy mice. In addition, increased AGP mRNA expression was also observed in HepG2 cells and THP-1-derived macrophages that had been treated with Am80. AGP-knockout mice exacerbated renal fibrosis, inflammation and macrophage infiltration in UUO mice, indicating endogenous AGP played an anti-fibrotic and anti-inflammatory role during the development of renal fibrosis. We also found that no anti-fibrotic effect of Am80 was observed in UUO-treated AGP-knockout mice whereas atRA treatment tended to show a partial anti-fibrotic effect. These collective findings suggest that Am80 protects against renal fibrosis via being involved in AGP function.

The hepatic orosomucoid/α1-acid glycoprotein gene cluster is regulated by the nuclear bile acid receptor FXR

The α-1-acid glycoprotein/orosomucoids (ORMs) are members of the lipocalin protein family. Encoded by 3 polymorphic genes in mouse (2 in man, 1 in rat), ORMs are expressed in hepatocytes and function as acute-phase proteins secreted in plasma under stressful conditions. In addition to their role of nanocarrier, ORMs are involved in several pathophysiological processes such as immunosuppression, cardioprotection, and inflammatory bowel disease. The nuclear bile acid receptor farnesoid X receptor (FXR) regulates bile acid homeostasis and lipid and glucose metabolism and is an important modulator of enterohepatic functions. Here we report that hepatic FXR deletion in mice affects the expression of several members of the lipocalin family, among which ORMs are identified as direct FXR target genes. Indeed, a FXR response element upstream of the mouse Orm1 promoter was identified to which hepatic, but not ileal, FXR can bind and activate ORM expression in vitro and in vivo. However, ORMs are regulated in a species-specific manner because the ORM cluster is regulated by FXR neither in human nor rat cell lines. Consistent with these data, chromatin immunoprecipitation sequencing analysis of the FXR genomic binding sites did not detect any FXR response element in the vicinity of the human or rat ORM gene cluster. Thus, bile acids and their cognate nuclear receptor, FXR, are regulators of ORM expression, with potential implications for the species-specific metabolic and inflammation control by FXR because the expression of the proinflammatory genes in epididymal white adipose tissue was dependent on liver FXR activation.

Plasma alpha1-acid glycoprotein can be used to adjust inflammation-induced hyporetinolemia in vitamin A-sufficient, but not vitamin A-deficient or -supplemented rats

We examined the association between alpha(1)-acid glycoprotein (AGP), all-trans-retinol (retinol), and albumin concentrations in a longitudinal animal model of IL-6-induced inflammation. Vitamin A-sufficient (VAS) male Sprague-Dawley rats were administered recombinant human IL-6 [n = 4, 65 mug/(kg.d)] or PBS (n = 4) continuously for 7 d via osmotic minipumps. Plasma samples were obtained daily and concentrations of retinol, AGP, albumin, and total protein were measured. Compared with both baseline and controls, retinol and albumin decreased (P < 0.05), AGP increased (P < 0.05), and total protein concentrations were unaffected in IL-6-treated rats. In vitamin A-deficient (VAD) rats, AGP concentrations were significantly lower at all time points and increased only to one-third of that in VAS rats. The AGP cut-off value indicative of inflammation was 0.11 g/L (i.e., 95% upper limit of baseline concentrations). After 20.5 h, there was an inverse linear correlation between AGP concentrations and the relative change in retinol to baseline (y = -0.18x + 0.48, r = -0.84, P < 0.001). However, changes in AGP and albumin were not correlated (P = 0.94). The application of this function to retinol concentrations in rats from separate experiments showed that hyporetinolemia cannot be adjusted using plasma AGP in VAD or vitamin A-supplemented rats. In conclusion, correcting inflammation-induced hyporetinolemia using an acute-phase protein requires longitudinally derived data, knowledge of vitamin A status, and a common underlying mechanism of change.