Tissue distribution and subcellular localization of retinol-binding protein in normal and vitamin A-deficient rats.

Levels of retinol-binding (RBP), the plasma transport protein for vitamin A, were measured by radioimmunoassay in sera and in a large number of tissues from both normal and vitamin A-deficient rats. The tissues included liver, kidney, fat, muscle, brain, eye, salivary gland, thymus, lung, heart, intestine, spleen, adrenal, testes, thyroid, and red blood cells. The RBP levels in tissues other than serum, liver, and kidneys varied from 12 mug/g of tissue for normal spleen to an undetectable level in red blood cells. Much of the RBP in the tissues with low levels may have been due to residual serum in the samples. In general, except for liver, RBP levels were lower in tissues from vitamin A-deficient rats than in those from normal rats. In normal rats, the liver, kidney, and serum levels were 30 plus or minus 4 (mean plus orminus SEM), 151 plus or minus 22, and 44 plus or minus 3 mug/g, respectively. In vitamin A-deficient rats, the liver RBP level was about three times the normal level whereas the kidney and serum levels were about one-fifth the normal values. When normal liver homogenates were fractionated by centrifugation, 67% of the RBP was recovered in the microsomal fraction and only 9% was found in the soluble 105,000 g supernate. In contrast, 76% of the RBP in homogenates of normal kidneys was in the soluble fraction. Similar results were obtained with deficient livers and kidneys. Incubation with deoxycholate released the liver RBP into the soluble fraction. RBP is produced in the liver and removed from the blood by the kidneys. The levels of RBP in normal and deficient liver, serum, and kidney appear to reflect the relative rates of RBP secretion and turnover.     

An unhydrolyzable analogue of N-(4-hydroxyphenyl)retinamide. synthesis and preliminary biological studies.

The synthesis of a nonhydrolyzable, carbon-linked analogue (4-HBR) of the retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) using Umpolung methods is described. Preliminary studies of biological activity show 4-HBR is similar to 4-HPR in its actions although a potentially relevant and desirable difference is its reduced suppression of plasma vitamin A levels. These results show that 4-HPR does not have to be hydrolyzed to retinoic acid to produce its chemotherapeutic effects.     

Hydrolysis of 4-HPR to atRA occurs in vivo but is not required for retinamide-induced apoptosis

The retinamide, N-(4-hydroxyphenyl)retinamide (4-HPR), has shown promising anti-tumor activity, but it is unclear whether this compound is hydrolyzed to all-trans retinoic acid (atRA) and if so, whether this plays any role in its chemotherapeutic activity. To address this issue, the ability of 4-hydroxybenzylretinone (4-HBR), a carbon-linked analog of 4-HPR, to support growth in vitamin A-deficient (VAD) animals and to activate an atRA-responsive gene in vivo was compared to 4-HPR and atRA. Further, the non-hydrolyzable 4-HBR analog was used to determine whether the presence of the labile amide linkage in 4-HPR is essential for the induction of apoptosis in cultured MCF-7 breast cancer cells. Studies in VAD rats showed that 4-HPR, like atRA, supports animal growth and induces CYP26B1 mRNA expression in lung whereas 4-HBR does not. Analysis of plasma from 4-HPR- and atRA-treated VAD animals revealed the presence of atRA whereas it was not detected in plasma from animals given 4-HBR. To determine whether hydrolysis to atRA is necessary for apoptosis induced by 4-HPR in MCF-7 breast cancer cells, morphological and biochemical assays for apoptosis were performed. 4-HBR, like 4-HPR, induced apoptosis in MCF-7 cells. Apoptosis was not induced even at high concentrations of atRA, showing that 4-HPR and 4-HBR act in cells via a distinct signaling pathway. These results show that although limited hydrolysis of 4-HPR occurs in vivo, the ability to liberate atRA is not required for these 4-hydroxyphenyl retinoids to induce apoptosis in MCF-7 breast cancer cells. Thus the non-hydrolyzable analog, 4-HBR, may have significant therapeutic advantage over 4-HPR because it does not liberate atRA that can contribute to the adverse side effects of drug administration in vivo.     

Liquid chromatography method for quantifying N-(4-hydroxyphenyl)retinamide and N-(4-methoxyphenyl)retinamide in tissues

A simple and accurate high-performance liquid chromatography (HPLC) method was developed to measure levels of N-(4-hydroxyphenyl)retinamide (fenretinide, 4-HPR) and its main metabolite N-(4-methoxyphenyl)retinamide (4-MPR) in tissue. Following ultrasonic extraction of fresh tissue in acetonitrile (ACN), 4-HPR and 4-MPR were measured by HPLC with UV absorbance detection at 340 nm, using isocratic elution with ACN, H(2)O, and acetic acid. N-(4-ethoxyphenyl)retinamide (4-EPR) was employed as an internal standard. The 4-HPR and 4-MPR recovery in bovine liver or bovine brain tissue samples spiked with known amounts of 4-HPR and 4-MPR ranged from 93 to 110%. The detection limit of the method was 50 ng/ml. The method was tested on actual samples from an athymic (nu/nu) mouse carrying a subcutaneous tumor xenograft originating from SMS-KCNR neuroblastoma cells. The tissues were harvested and analyzed following a 3 day long treatment with intraperitoneal injections of 4-HPR/Diluent-12. 4-HPR and the metabolite 4-MPR were detected and quantitated in the tested tissues including tumor, liver, and brain. This method can be used to quantify 4-HPR and 4-MPR in different tissues to determine the bioavailability of 4-HPR.     

Fenretinide inhibits vitamin A formation from β-carotene and regulates carotenoid levels in mice

N-[4-hydroxyphenyl]retinamide, commonly known as fenretinide, a synthetic retinoid with pleiotropic benefits for human health, is currently utilized in clinical trials for cancer, cystic fibrosis, and COVID-19. However, fenretinide reduces plasma vitamin A levels by interacting with retinol-binding protein 4 (RBP4), which often results in reversible night blindness in patients. Cell culture and in vitro studies show that fenretinide binds and inhibits the activity of β-carotene oxygenase 1 (BCO1), the enzyme responsible for endogenous vitamin A formation. Whether fenretinide inhibits vitamin A synthesis in mammals, however, remains unknown. The goal of this study was to determine if the inhibition of BCO1 by fenretinide affects vitamin A formation in mice fed β-carotene. Our results show that wild-type mice treated with fenretinide for ten days had a reduction in tissue vitamin A stores accompanied by a two-fold increase in β-carotene in plasma (P < 0.01) and several tissues. These effects persisted in RBP4-deficient mice and were independent of changes in intestinal β-carotene absorption, suggesting that fenretinide inhibits vitamin A synthesis in mice. Using Bco1^?/? and Bco2^?/? mice we also show that fenretinide regulates intestinal carotenoid and vitamin E uptake by activating vitamin A signaling during short-term vitamin A deficiency. This study provides a deeper understanding of the impact of fenretinide on vitamin A, carotenoid, and vitamin E homeostasis, which is crucial for the pharmacological utilization of this retinoid.     

The isoelectric fractionation of hen''s-egg ovotransferrin

1. ATP sulphurylase was assayed in various organs from vitamin A-deficient and pair-fed control rats at different stages of deficiency. Activity decreased slightly in the liver and markedly in the adrenal gland. Striking differences in liver activity were observed between pair-fed control and ad libitum-fed animals. This observation suggested that diet (apart from vitamin A) strongly influenced the activity of ATP sulphurylase. 2. Total starvation caused a severe decrease in activity in liver within 48hr. This was due to a lack of protein intake. 3. By feeding groups of vitamin A-deficient and pair-fed control rats on a diet containing 80% protein, the specific activity of the liver ATP sulphurylase was maintained in the pair-fed control group at the normal level of an ad libitum-fed rat, whereas it decreased by 25% (statistically significant at P<0.01) in the deficient rat. On a 20%-protein diet, there were no significant differences between vitamin A-deficient and pair-fed control rats. These relationships held also for enzyme activity expressed per g. of liver, per total liver and per g. of DNA. There were no differences in liver protein or DNA concentration between vitamin A-deficient and control rats on either protein intake. 4. Control rats on a 20%-protein diet had liver specific enzyme activities about one-half of those in control rats on an 80%-protein diet, as well as lower liver protein concentrations. 5. It is concluded that, when the effect of protein deprivation on ATP sulphurylase is separated from the effect of vitamin A deficiency, a lowering of the enzyme activity caused by the vitamin deficiency is demonstrable.     

Hepato-renal transport of phenolsulfonphthalein in analbuminemic rats: albumin is essential for hepatic compensatory elimination of a nephrophilic ligand in animals with renal dysfunction

To investigate a possible function of plasma albumin in partitioning organic anions into bile and urine, phenolsulfonphthalein (PSP) was administered intravenously and its in vivo fate was studied in normal and analbuminemic mutant rats (NAR). No significant change in the rate of PSP disappearance was observed in bilaterally nephrectomized normal rats. However, biliary excretion of the injected dye increased remarkably in nephrectomized normal rats. Intravenously injected PSP disappeared very rapidly from the circulation of NAR. Thus, the plasma clearance and distribution volume of PSP were significantly larger in NAR than in normal rats. Bilateral nephrectomy also failed to decrease the plasma clearance and distribution volume of the dye in NAR. In striking contrast to the experiments in normal rats, bilateral nephrectomy did not increase the biliary secretion of PSP in NAR. When PSP bound to equimolar albumin was injected into bilaterally nephrectomized NAR, the biliary excretion of PSP increased significantly with concomitant decrease in both plasma clearance and distribution volume of the dye. These results indicate that, in cases of renal transport dysfunction, albumin plays a critical role in hepatic compensatory excretion of PSP, a nephrophilic organic anion, whose molecular weight (MW 354) is close to the threshold value for partitioning a ligand to the eliminatory routes in liver and kidney of a rodent.     

Immunocytochemical studies on the localization of plasma and of cellular retinol-binding proteins and of transthyretin (prealbumin) in rat liver and kidney

The immunocytochemical localization of cellular retinol-binding protein (CRBP), of plasma retinol-binding protein (RBP), and of plasma transthyretin (TTR) was studied in rat liver and kidney. The studies employed normal rats, retinol-deficient rats, and rats fed excess retinol. Antisera were prepared in rabbits against purified rat CRBP, RBP, and TTR. The primary antibodies and goat anti-rabbit IgG were purified by immunosorbent affinity chromatography, using the respective pure antigen coupled to Sepharose as the immunosorbent. This procedure effectively removed cross-reactive and heterophile antibodies, which permitted the specific staining and localization of each antigen by the unlabeled peroxidase-antiperoxidase method. CRBP was found to be localized in two cell types in the liver, the parenchymal cells and the fat-storing cells. Diffuse cytoplasmic staining for CRBP was seen in all the parenchymal cells. Much more intense staining for CRBP was seen in the fat-storing cells. The prominence of the CRBP-positive fat- storing cells changed markedly with vitamin A status. Thus, these cells were most prominent, and appeared most numerous, in liver from rats fed excess retinol. Both RBP and TTR were localized within liver parenchymal cells. The intensity of RBP staining increased markedly in retinol-deficient rat liver, consistent with previous biochemical observations. With the methods employed, specific staining for RBP or TTR was not seen in cells other than the parenchymal cells. In the kidney, all three proteins (CRBP, RBP, and TTR) were localized in the proximal convoluted tubules of the renal cortex. Staining for RBP was much more intense in normal kidney than in kidney from retinol- deficient rats. These findings reflect the fact that RBP in the tubules represents filtered and reabsorbed RBP. The pattern of specific staining for CRBP among the various tubules was very similar to that seen for RBP on adjacent, serial sections of kidney. The function of CRBP in the kidney is not known.     

Thyroidal control of hepatic release and metabolism of vitamin A

The inverse relationship that exists between thyroxine and the vitamin A level of plasma has been examined in chicken. Thyroxine treatment leads to a decrease in the level of vitamin A carrier proteins, retinol-binding protein and prealbumin-2 in plasma and liver. There is an accumulation of vitamin A in the liver, with a greater proportion of vitamin A alcohol being present compared to that of control birds. In thyroxine treatment there is enhanced plasma turnover of retinol-binding protein and prealbumin-2, while their rates of synthesis are marginally increased. Amino acid supplementation partially counteracts effects of thyroxine treatment. Amino acid supplementation of thyroxine-treated birds does not alter the plasma turnover rates of retinol-binding protein and prealbumin-2 but increases substentially their rates of synthesis. The release of vitamin A into circulation is interfered with in hyperthyroidism due to inadequate availability of retinol-binding protein being caused by enhanced plasma turnover rate not compensated for by synthesis.     

N-(4-hydroxyphenyl) retinamide inhibits cystogenesis by polycystic epithelial cell lines in vitro.

Primary tubular epithelial cells develop spherical monolayered cysts when cultured in collagenI matrix, a model that has been used to study the mechanism of cystogenesis. In an attempt to block cystogenesis, we have evaluated the effect of N-(4-hydroxyphenyl) retinamide (HPR), a synthetic derivative of retinoic acid, on both formation and growth of cysts in a human model of polycystic kidney cells. Number, dimension and submicroscopical characteristics of cysts were evaluated after 2 and 4 weeks from treatment with HPR. A marked inhibitory effect of HPR on cystogenesis was found at concentration of 1 microM, while a complete block was observed at concentration between 5 and 10 microM. Furthermore, treatment with HPR of already formed cysts resulted in their disruption. HPR at 10 microM also induced apoptosis of several tubular epithelial cell models suggesting a correlation between the two phenomena. Taken together these observations demonstrate that HPR blocks cystogenesis by polycystic kidney cells "in vitro" and that it also reverts the fate of already formed cysts. Apoptosis may be the mechanism which mediates the inhibitory effect on cystogenesis in this model.     

Lecithin:retinol acyltransferase from mouse and rat liver. CDNA cloning and liver-specific regulation by dietary vitamin a and retinoic acid

Lecithin:retinol acyltransferase (LRAT), present in microsomes, catalyzes the transfer of the sn-1 fatty acid of phosphatidylcholine to retinol bound to a cellular retinol-binding protein. In the present study we have cloned mouse and rat liver LRAT cDNA and tested the hypothesis that LRAT mRNA, like LRAT activity, is regulated physiologically in a liver-specific manner. The nucleotide sequences of mouse and rat liver LRAT cDNA each encode a 231-amino acid protein with 94% similarity between these species, and approximately 80% similarity to a cDNA for LRAT from human retinal pigment epithelium. Expression of rat LRAT cDNA in HEK293T cells resulted in functional retinol esterification and storage. RNA from several rat tissues hybridized with liver LRAT cDNA. However, LRAT mRNA was virtually absent from the liver of vitamin A-deficient animals, while being unaffected in intestine and testis. LRAT mRNA was rapidly induced by retinoic acid (RA) in liver of vitamin A-deficient mice and rats (P < 0.01). LRAT mRNA and enzymatic activity were well correlated in the same livers of rats treated with exogenous RA (r = 0.895, P < 0.0001), and in a dietary study that encompassed a broad range of vitamin A exposure (r = 0.799, P < 0.0001). Liver total retinol of <100 nmol/g was associated with low LRAT expression (<33% of control).We propose that RA, derived exogenously or from metabolism, serves as an important signal of vitamin A status. The constitutive expression of liver LRAT during retinoid sufficiency would serve to divert retinol into storage pools, while the curtailment of LRAT expression in retinoid deficiency would maintain retinol for secretion and delivery to peripheral tissues.     

Vitamin A deficiency causes oxidative damage to liver mitochondria in rats

Mitochondrial damage in rat liver induced by chronic vitamin A-deficiency was studied using three different groups of rats: (i) control rats, (ii) rats fed a vitamin A-free diet until 50 d after birth and (iii) vitamin A-deficient rats re-fed a control diet for 30 d. No statistical difference in body weight and food intake was found between control and vitamin A-deficient rats. Liver GSH concentration was similar in both groups. However, in vitamin A-deficient rats, the mitochondrial GSH/GSSG ratio was significantly lower and the levels of malondialdehyde (MDA) and 8-oxo-7, 8-dihydro-2'-deoxyguanosine (oxo8dG) were higher when compared to control rats. These values were partially restored in re-fed rats. The mitochondrial membrane potential of vitamin A-deficient rats was significantly lower than in control rats and returned to normal levels in restored vitamin A rats. Two populations of mitochondria were found in vitamin A-deficient rats according to the composition of membrane lipids. One population showed a similar pattern to the control mitochondria and the second population had a higher membrane lipid content. This report emphasizes the protective role of vitamin A in liver mitochondria under physiological circumstances.     

Cellular retinoid-binding proteins in reginerating rat liver: demonstration of a novel cellular retinoid-binding protein

Changes in the levels of liver cellular aetinol- and retinoic acid-binding proteins were studied after partial (about 70%) hepatectomy for 14 days in the rat. It was found that a novel binding protein designated F-type appears transiently in liver cytosol 3 days after the operation. The appearance of this protein coincides with the peak level of the alpha 1-fetoproteain. In contrast, cellular retinoic acid-binding protein was detected only the first day after hepatectomy, whereas no significant change was observed in the level of the cellular retinol-binding protein during the entire observation period. [3H]Retinol or [3H]retinoic acid complexed with serum retinol-binding protein injected intravenously into vitamin A-deficient rats 1 day after the hepatectomy was recovered 5 min or 20 min later bound specifically to cellular retinol- or retinoic acid-binding protein, respectively. The results presented here strongly suggest that each of the three cellular retinoid-binding proteins plays a distinct role in cell proliferation and differentiation.     

Vitamin A status regulates hepatic lecithin: retinol acyltransferase activity in rats

The activity of lecithin:retinol acyltransferase (LRAT) was determined in microsomes from the liver and small intestine of rats with differing vitamin A status. In animals depleted of retinol, as judged by undetectable liver vitamin A stores and low plasma retinol concentrations, hepatic LRAT activity was almost undetectable, whether assayed with retinol bound to cellular retinol-binding protein or solvent-dispersed retinol. In contrast, neither the activity of intestinal LRAT nor that of acyl-CoA:retinol acyltransferase in either liver or intestine differed from that of vitamin A-adequate rats. During the course of vitamin A depletion, liver LRAT activity fell progressively, nearly in parallel to the decrease in plasma retinol concentration. Oral repletion of vitamin A-depleted rats with 0.8 mg of retinol resulted in a very rapid restoration of plasma retinol concentration and full recovery of hepatic LRAT activity within 24 h, together with deposition of retinyl ester in the liver. These data strongly implicate LRAT activity in liver as responsible for the storage of hepatic retinyl esters. Retention of the intestine's capacity to esterify retinol during vitamin A deficiency provides a mechanism for capture of dietary vitamin A, while reduced hepatic LRAT activity may function to redirect retinol in liver from storage to other metabolic pathways.     

Biochemical and Molecular Responses to Loss of Renal Mass: Atpase and Cyclic Nucleotides: Changes in Renal Cyclic Nucleotides as a Trigger to the Onset of Compensatory Renal Hypertrophy

In adult male Wistar rats contralateral nephrectomy was followed, within 10 minutes, by a nearly twofold rise of the content of cGMP in renal tissue. 20 and 40 minutes after contralateral nephrectomy cGMP fell to one half its control level to rise again to its normal level within 90 minutes. The initial rise of the concentration of cGMP was accompanied by a simultaneous fall of the concentration of cAMP by about 30 percent: the cAMP concentration remained 10-20 percent below control level for approximately two hours and rose again to its initial level after three hours. Cross-circulation of a nephrectomized rat with an intact animal led to a sharp increase of cGMP in the kidneys of the latter with a peak at 10 minutes after initiating cross-circulation and also to a fall of the cAMP concentration. When the same nephrectomized donor rat was subsequently cross-circulated with one, or even two, intact receiver animals, similar short-lasting changes of cyclic nucleotide concentrations were recorded in the kidneys of all the receivers. When a normal kidney was transplanted to the neck of a rat, subsequent removal of one of its own kidneys did not result in any change in cyclic nucleotide content in either the remaining or the transplanted kidney. The data are interpreted to indicate that renal tissue produces a factor inhibiting renal growth which counteracts a circulating humoral kidney growth stimulating factor of unknown origin. An initial rise of cGMP and a fall of cAMP may trigger the subsequent stimulation of protein synthesis responsible for hypertrophy.     

Comparison of the effects of vitamin A and its analogs upon rabbit ear cartilage in organ culture and upon growth of the vitamin A-deficient rat

A study was conducted to explore the relationship between the effects of vitamin A upon cartilage and the biological role of vitamin A in maintaining growth and life. Retinol, retinoic acid, alpha-retinoic acid, and ROB-7699 (a cyclopentyl analog of retinoic acid) were highly effective in promoting the lysis of the extracellular matrix of cartilage grown in organ culture in vitro. Retinoic acid and its two analogs were quantitatively more active than was retinol in bringing about lysis of matrix and release of proteoglycan into the culture medium. A bioassay was then conducted to determine the ability of each compound to promote growth of vitamin A-deficient rats. In contrast to their effects upon cartilage, retinoic acid and its two analogs were considerably less active quantitatively than retinol in promoting growth of vitamin A-deficient rats. Moreover, the three acids tested showed graded biological activity in the growth bioassay, with alpha-retinoic acid showing reduced bioactivity (approx. one-fourth that of retinoic acid) and ROB-7699 being virtually inactive. The lysis of cartilage produced by these compounds was presumably caused by release of lysosomal enzymes as a result of the membrane-labilizing effects of the compounds. Thus, these membrane effects of the vitamin A-related compounds are poorly correlated with their biological growth-promoting activity. The alpha-ionone analogs of retinol and retinoic acid were able to maintain good health and growth of vitamin A-deficient rats, although their quantitative activity was low. Rats fed alpha-retinyl acetate showed high liver stores of alpha-retinyl esters and low levels of serum retinol-binding protein (similar to the levels seen in retinoic acid-fed rats). The biological activity of the alpha-ionone analogs was apparently not due to contamination with or conversion to the normal beta-ionone compounds.     

Utilization of vitamin A in rats with inflammation

Reductions in the concentration of retinol (vitamin A) in serum, lung and kidney were observed in rats subjected to inflammation-inducing treatments (turpentine oil injection of thermal injury). At the same time, the liver showed an almost normal vitamin A content. Feeding of retinol to vitamin A-depleted rats with inflammation revealed that intestinal absorption of retinol was still active in the inflamed state, and the livers of these rats showed good incorporation of retinol. The livers of normal and vitamin A-depleted rats subjected to the inflammatory treatments showed a normal RBP content (retinol-binding protein) and hepatic release of holo-RBP into the serum was not impaired functionally. These results suggest the possibility that the decreases of vitamin A in the lung, serum and kidney may be due primarily to enhanced local consumption of vitamin A related to the inflammation, rather than to a reduced supply of vitamin A from the liver or to decreased intestinal absorption. In bovine serum albumin (BSA)-sensitized rats produced by direct intubation of BSA into the lungs, the level of vitamin A in the lung decreased prior to that in the liver or serum, supporting the hypothesis that the decrease in vitamin A in the inflamed lungs of these rats may be due mainly to the consumption of vitamin A in the lung in response to inflammation.     

Biliary metabolites of all-trans-retinoic acid in the rat

Biliary metabolites from physiological doses of all-trans-[10-3H]retinoic acid were examined in normal and vitamin A-deficient rats. The bile from normal and vitamin A-deficient rats contained approximately 60% of the administered dose following a 24-h collection period. However, vitamin A-deficient rats show a 6-h delay in the excretion of radioactivity compared to normal rats. Retinoyl-beta-glucuronide excretion was particularly sensitive to the vitamin A status of the rats. In normal rats, retinoyl-beta-glucuronide reached a maximum concentration of 235 pmol/ml of bile 2 h following the dose and then rapidly declined. Vitamin A-deficient rats show a relatively constant concentration of this metabolite (100-150 pmol/ml of bile) over a 10-h collection period. Retinoic acid excretion was low in both normal and deficient rats. The concentration of retinotaurine, a recently identified biliary metabolite, was approximately equal to retinoyl-beta-glucuronide in normal rats and appeared in the bile 2 h later than the glucuronide.     

Anticarcinogenic effect of retinoids on 7,12-dimethylbenz(a)anthracene-induced mammary tumor induction, and its relationship to cyclic AMP-dependent protein kinase

Administration of 13-cis retinoic acid and N-(4-hydroxyphenyl) retinamide daily in the diet to female Sprague-Dawley rats beginning one day after intubation with 7,12-dimethylbenz(a)anthracene (DMBA) prolonged the latency periods and inhibited the percentage incidence of mammary tumors. A significant reduction in the total number of tumors was also evident. The inhibition of mammary tumor growth by retinoids was associated with a significant increase (3-fold) in cytosolic cAMP-binding and histone kinase activities. The increase of histone kinase activity was almost totally in the cAMP-dependent protein kinase Type II. Retinoic acid increased the amount of the regulatory subunit (R11) rather than altering its cAMP binding affinity. These results suggest that cAMP-dependent protein kinase Type II may be involved in mediating the retinoid action in the inhibition of mammary tumor growth in vivo.     

Retinol-binding protein mRNA is induced by estrogen in the kidney but not in the liver

Vitamin A is mobilized from the liver and transported in plasma as retinol bound to retinol-binding protein (RBP). In addition to the liver, several extrahepatic tissues including the kidney have been shown to contain RBP mRNA. A study was conducted to explore the role of sex hormones in the regulation of RBP mRNA levels in the kidney compared to those in the liver. Treatment of female rats with a single dose of testosterone or chronic treatment with testosterone had only a slight effect on the steady-state level of RBP mRNA in the kidney and the liver. However, treatment of male rats with estrogen caused an increase in the steady-state level of RBP mRNA in the kidney but not in the liver. A single injection of 17 beta-estradiol, either 1.0 or 0.1 micrograms/g body weight, resulted in a rapid rise in the level of RBP mRNA in the kidney which was maximal at 3-6 h (fivefold induction) after treatment. In addition, treatment of ovariectomized female rats with estrogen also resulted in a rapid rise in the accumulated level of RBP mRNA in the kidney while having no influence in the liver. Finally, studies using the anti-estrogen drug, hydroxytamoxifen, resulted in blockage of the estrogen-related induction of RBP mRNA in the kidney, suggesting that the induction of RBP mRNA in the kidney by estrogen may be mediated by the nuclear estrogen receptor. Taken together these data suggest that the regulation of RBP mRNA, levels in the liver and kidney, at least with respect to estrogen, is different.