Identification of a transforming growth factor-beta1/bone morphogenetic protein 4 (TGF-beta1/BMP4) response element within the mouse tissue transglutaminase gene promoter.

Tissue transglutaminase is a calcium-dependent, protein cross-linking enzyme that is highly expressed in cells undergoing apoptosis. The expression of tissue transglutaminase is regulated by a variety of molecules including retinoids, interleukin-6, and transforming growth factor-beta1 (TGF-beta1). Retinoid and interleukin-6 inductions of tissue transglutaminase expression are mediated by specific cis-regulatory elements located within the first 4.0 kilobase pairs of the promoter of the gene. The present studies were designed to identify the molecular mechanisms mediating the regulation of tissue transglutaminase gene expression by TGF-beta family members. Transient transfection of Mv1Lu cells with transglutaminase promoter constructs demonstrated that 0.2 nM TGF-beta1 maximally induced the activation of the promoter through a 10-base pair TGF-beta1 response element (TRE; GAGTTGGTGC) located 868 base pairs upstream of the transcription start site. This same element mediated an inhibitory activity of TGF-beta1 on the transglutaminase promoter in MC3T3 E1 cells. The TRE through which TGF-beta1-regulated the activity of the transglutaminase promoter was necessary and sufficient for bone morphogenetic protein 2- (BMP) and BMP4-dependent inhibition of the tissue transglutaminase promoter. The TGF-beta1, BMP2, and BMP4 regulation of the transglutaminase promoter activity was similar to the responses we observed for the endogenous transglutaminase activity of Mv1Lu and MC3T3 E1 cells. For BMP2 and BMP4, this regulation was paralleled by a decrease in tissue transglutaminase mRNA in MC3T3 E1 cells. The results of these experiments suggest that TGF-beta1, BMP2, and BMP4 regulation of mouse tissue transglutaminase gene expression requires a composite TRE located in the 5'-flanking DNA.

The complex between retinol and retinol-binding protein is formed in the rough microsomes of liver following repletion of vitamin A-depleted rats.

Retinol-binding protein (RBP), the plasma transport protein for vitamin A, is primarily synthesized in the rough endoplasmic reticulum of the liver. RBP then passes through the smooth endoplasmic reticulum and into the Golgi apparatus where vesicles form and transport the protein to the cell membrane. When rats were depleted of their vitamin A stores, RBP accumulated in the liver microsomes, particularly in the rough microsomes. To identify the organelle(s) where retinol initially binds to RBP, vitamin A-depleted rats were given an i.v. injection of [3H]retinol suspended in Tween 40. After intervals of 2, 3, 4, 5, 6, 8, 10, 15 and 20 min, liver fractions enriched in rough and smooth microsomes and Golgi apparatus were prepared. The retinol/RBP complex (holoRBP) was detected in the rough microsomes within 3 min post injection. HoloRBP later appeared in the smooth microsomes and Golgi fraction, and then the serum at time intervals consistent with the known secretion rate for RBP. HoloRBP was detected in the rough microsomes at all times after 3 min, whether or not the complex was present in the other subcellular fractions. Thus, the holoRBP complex can form in the rough endoplasmic reticulum of the liver.

Multiple retinoids alter liver bile salt-independent retinyl ester-hydrolase activity, serum vitamin A and serum retinol-binding protein of rats.

Liver bile salt-independent retinyl ester hydrolase (BSI-REH) has been suggested to play a significant role in the hydrolysis of chylomicron derived retinyl esters. Studies were conducted to investigate the individual effects of N-(4-hydroxyphenyl)retinamide (HPR), retinoic acid, 13-cis-retinoic acid, Acitretin and Temarotene on BSI-REH, serum retinol, and serum retinol-binding protein (RBP) concentrations. We have demonstrated that micromolar concentrations of HPR, retinoic acid, 13-cis-retinoic acid or Acitretin significantly reduced the in vitro hydrolysis of retinyl palmitate. In contrast, Temarotene stimulated retinyl palmitate hydrolysis by BSI-REH. Retinoic acid and 13-cis-retinoic acid produced transient, but significant, depressions of both serum retinol and RBP concentrations, when the individual retinoids were administered orally to rats. The duration of the depression was shorter than we previously observed with acute HPR administration. Furthermore, Acitretin appeared to function with bimodal activity, producing significant depression of serum retinol at 2 h and 24 h. No effect of Acitretin or Temarotene on serum RBP concentration was observed. The alterations observed in BSI-REH activity, serum retinol and RBP concentrations provide evidence that these retinoids can alter liver retinyl ester hydrolysis, but the effects observed on serum retinol concentration can only be partially explained by the BSI-REH activity.

Streptozotocin-induced diabetes lowers retinol-binding protein and transthyretin concentrations in rats.

Retinol-binding protein (RBP) and transthyretin (TTR) in the plasma, liver and kidney, retinol in plasma, and total vitamin A in the liver were measured in rats 6 weeks after diabetes mellitus had been induced by streptozotocin (STZ). The diabetic rats gained 83% less weight despite consuming 45% more feed than the non-diabetic controls. Plasma and kidney concentrations of RBP and TTR were significantly lower in diabetic than in the non-diabetic control rats. Unlike the retinol carrier proteins, plasma albumin concentrations remained unaffected. Plasma concentrations of retinol were decreased while its hepatic levels increased in the diabetic animals. The depressed circulatory levels of retinol may reflect an altered metabolism of its transport proteins.

Retinol-binding protein secretion from the liver of N-(4-hydroxyphenyl) retinamide-treated rats.

N-(4-Hydroxyphenyl)retinamide (HPR; Fenretinide), a synthetic retinoid possessing antitumor activity, depresses plasma retinol and retinol-binding protein (RBP) concentrations. In study 1, the ability of retinol or HPR to induce RBP secretion from the livers of vitamin A-deficient rats was compared. A large apoRBP pool accumulated in the liver rough microsomes of these rats. Following retinol repletion, 80% of the accumulated RBP was rapidly secreted into the plasma. In contrast, HPR treatment only induced two-thirds of the RBP secretion observed with retinol. Prior colchicine treatment caused a large RBP accumulation in the Golgi-enriched fraction following retinol repletion. HPR plus colchicine treatment produced significantly less accumulation of RBP in the Golgi-enriched fraction than did retinol. In study 2, HPR treatment of vitamin A-adequate rats caused RBP to accumulate in the liver rough microsomes. When vitamin A-adequate rats were treated with colchicine, the concentration of RBP in the Golgi-enriched fraction increased 2.9-fold. However, significantly less RBP accumulated in the Golgi following HPR treatment. These studies demonstrate that HPR will induce liver RBP secretion, but to a lesser degree than retinol. Further, more RBP remained in the rough microsomes of HPR treated, vitamin A-adequate rats, indicating that HPR depressed the amount of RBP secreted.

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.

N-(4-hydroxyphenyl)retinamide (fenretinide) induces retinol-binding protein secretion from liver and accumulation in the kidneys in rats.

The chemopreventive retinoid N-(4-hydroxyphenyl)retinamide (HPR) depresses serum retinol and retinol-binding protein (RBP) concentrations. To study long-term effects of HPR on serum proteins, rats were fed a control diet or a diet containing HPR (737 mumol/kg diet) for 14 d. Serum retinol and RBP of HPR-treated rats decreased to 42 and 41%, respectively, of initial concentrations. Transthyretin, albumin and transferrin did not differ between treatments. Previous studies found that HPR decreased secretion of the retinol-RBP complex into plasma. To investigate acute effects of HPR on RBP metabolism, vitamin A-deficient rats were injected with HPR (51 mumol/kg body wt), retinol (0.52 mumol/rat) or Tween carrier only. Liver RBP concentrations in HPR- and retinol-treated rats were 45 and 18%, respectively, of concentrations in Tween-treated rats, indicating rapid RBP secretion. Tween- and HPR-treated rats maintained relatively constant serum RBP concentrations, whereas retinol-replete rats had 12-fold higher serum RBP after 150 min. Rats treated with HPR and rats treated with retinol had 29- and eightfold higher kidney RBP concentrations, respectively, than Tween-treated rats, indicating rapid clearance of RBP from plasma. We conclude that HPR affects RBP metabolism by inducing secretion of liver RBP into the bloodstream and rapid RBP accumulation in the kidney.