Liver sinusoidal endothelial cells show reduced scavenger function and downregulation of Fc gamma receptor IIb, yet maintain a preserved fenestration in the Glmpgt/gt mouse model of slowly progressing liver fibrosis

Liver sinusoidal endothelial cells (LSECs) are fenestrated endothelial cells with a unique, high endocytic clearance capacity for blood-borne waste macromolecules and colloids. This LSEC scavenger function has been insufficiently characterized in liver disease. The Glmpgt/gt mouse lacks expression of a subunit of the MFSD1/GLMP lysosomal membrane protein transporter complex, is born normal, but soon develops chronic, mild hepatocyte injury, leading to slowly progressing periportal liver fibrosis, and splenomegaly. This study examined how LSEC scavenger function and morphology are affected in the Glmpgt/gt model. FITC-labelled formaldehyde-treated serum albumin (FITC-FSA), a model ligand for LSEC scavenger receptors was administered intravenously into Glmpgt/gt mice, aged 4 months (peak of liver inflammation), 9-10 month, and age-matched Glmpwt/wt mice. Organs were harvested for light and electron microscopy, quantitative image analysis of ligand uptake, collagen accumulation, LSEC ultrastructure, and endocytosis receptor expression (also examined by qPCR and western blot). In both age groups, the Glmpgt/gt mice showed multifocal liver injury and fibrosis. The uptake of FITC-FSA in LSECs was significantly reduced in Glmpgt/gt compared to wild-type mice. Expression of LSEC receptors stabilin-1 (Stab1), and mannose receptor (Mcr1) was almost similar in liver of Glmpgt/gt mice and age-matched controls. At the same time, immunostaining revealed differences in the stabilin-1 expression pattern in sinusoids and accumulation of stabilin-1-positive macrophages in Glmpgt/gt liver. FcγRIIb (Fcgr2b), which mediates LSEC endocytosis of soluble immune complexes was widely and significantly downregulated in Glmpgt/gt liver. Despite increased collagen in space of Disse, LSECs of Glmpgt/gt mice showed well-preserved fenestrae organized in sieve plates but the frequency of holes >400 nm in diameter was increased, especially in areas with hepatocyte damage. In both genotypes, FITC-FSA also distributed to endothelial cells of spleen and bone marrow sinusoids, suggesting that these locations may function as possible compensatory sites of clearance of blood-borne scavenger receptor ligands in liver fibrosis.

The lysosomal transporter MFSD1 is essential for liver homeostasis and critically depends on its accessory subunit GLMP

Lysosomes are major sites for intracellular, acidic hydrolase-mediated proteolysis and cellular degradation. The export of low-molecular-weight catabolic end-products is facilitated by polytopic transmembrane proteins mediating secondary active or passive transport. A number of these lysosomal transporters, however, remain enigmatic. We present a detailed analysis of MFSD1, a hitherto uncharacterized lysosomal family member of the major facilitator superfamily. MFSD1 is not N-glycosylated. It contains a dileucine-based sorting motif needed for its transport to lysosomes. Mfsd1 knockout mice develop splenomegaly and severe liver disease. Proteomics of isolated lysosomes from Mfsd1 knockout mice revealed GLMP as a critical accessory subunit for MFSD1. MFSD1 and GLMP physically interact. GLMP is essential for the maintenance of normal levels of MFSD1 in lysosomes and vice versa. Glmp knockout mice mimic the phenotype of Mfsd1 knockout mice. Our data reveal a tightly linked MFSD1/GLMP lysosomal membrane protein transporter complex.

Lysosomes are specialized, enclosed compartments within cells with harsh chemical conditions where enzymes break down large molecules into smaller component parts. The products of these reactions are then transported out of the lysosome by transporter proteins so that they can be used to build new molecules that the cell needs.

Despite their importance, only a few lysosomal transporters have been thoroughly studied. A protein called MFSD1 had previously been identified as a potential lysosomal transporter, but its precise role has not been described.

Now, Massa López et al. have characterized the role of MFSD1, by genetically modifying mice so they could no longer make the transporter. These mice developed severe liver damage. In particular, a specific type of cell that is important for lining blood vessels in the liver, seemed to be lost in these mice. Older MFSD1 deficient mice also had more tumors in their livers compared to normal mice.

Massa López et al. next examined what happened to other lysosomal proteins in the MFSD1 deficient mice, and found that these mice had strikingly low levels of a protein called GLMP. To better understand the relationship between GLMP and MFSD1, another strain of genetically modified mice was analyzed, this time missing GLMP. Mice without GLMP were found to have very similar liver problems to those observed in the mice lacking MFSD1. Moreover, the GLMP deficient mice had low levels of the MFSD1 protein.

Further experiments demonstrated that MFSD1 and GLMP physically interact with each other: GLMP seemed to protect MFSD1 from being degraded in the harsh internal environment of the lysosome. Thus both GLMP and MFSD1 were needed to form a stable lysosomal transporter.

Characterizing MFSD1 is important for scientists attempting to understand how the lysosomal membrane and transporters work. Moreover, these findings may shed light on how defects in lysosomal transporters contribute to metabolic disease.

Age-dependent development of liver fibrosis in Glmp (gt/gt) mice

Background

Mice lacking glycosylated lysosomal membrane protein (Glmp^gt/gt mice) have liver fibrosis as the predominant phenotype due to chronic liver injury. The Glmp^gt/gt mice grow and reproduce at the same rate as their wild-type siblings. Life expectancy is around 18 months.

Methods

Wild-type and Glmp^gt/gt mice were studied between 1 week and 18 months of age. Livers were analyzed using histological, immunohistochemical, biochemical, and qPCR analyses.

Results

It was shown that Glmp^gt/gt mice were not born with liver injury; however, it appeared shortly after birth as indicated by excess collagen expression, deposition of fibrous collagen in the periportal areas, and increased levels of hydroxyproline in Glmp^gt/gt liver. Liver functional tests indicated a chronic, mild liver injury. Markers of inflammation, fibrosis, apoptosis, and modulation of extracellular matrix increased from an early age, peaking around 4 months of age and followed by attenuation of these signals. To compensate for loss of hepatocytes, the oval cell compartment was activated, with the highest activity of the oval cells detected at 3 months of age, suggesting insufficient hepatocyte proliferation in Glmp^gt/gt mice around this age. Although constant proliferation of hepatocytes and oval cells maintained adequate hepatic function in Glmp^gt/gt mice, it also resulted in a higher frequency of liver tumors in older animals.

Conclusions

The Glmp^gt/gt mouse is proposed as a model for slowly progressing liver fibrosis and possibly as a model for a yet undescribed human lysosomal disorder.

Electronic supplementary material

The online version of this article (doi:10.1186/s13069-016-0042-4) contains supplementary material, which is available to authorized users.

Increased glucose utilization and decreased fatty acid metabolism in myotubes from Glmp(gt/gt) mice

Glycosylated lysosomal membrane protein (GLMP) has been reported to enhance the expression from a peroxisome proliferator-activated receptor alpha (PPARα) responsive promoter, but also to be an integral lysosomal membrane protein. Using myotubes established from wild-type and Glmp(gt/gt) mice, the importance of GLMP in skeletal muscle was examined. Glmp(gt/gt) myotubes expressed a more glycolytic phenotype than wild-type myotubes. Myotubes from Glmp(gt/gt) mice metabolized glucose faster and had a larger pool of intracellular glycogen, while oleic acid uptake, storage and oxidation were significantly reduced. Gene expression analyses indicated lower expression of three PPAR-isoforms, a co-regulator of PPAR (PGC1α) and several genes important for lipid metabolism in Glmp(gt/gt) myotubes. However, ablation of GLMP did not seem to substantially impair the response to PPAR agonists. In conclusion, myotubes established from Glmp(gt/gt) mice were more glycolytic than myotubes from wild-type animals, in spite of no differences in muscle fiber types in vivo.

Lack of the Lysosomal Membrane Protein, GLMP, in Mice Results in Metabolic Dysregulation in Liver

Ablation of glycosylated lysosomal membrane protein (GLMP, formerly known as NCU-G1) has been shown to cause chronic liver injury which progresses into liver fibrosis in mice. Both lysosomal dysfunction and chronic liver injury can cause metabolic dysregulation. Glmp^gt/gt mice (formerly known as Ncu-g1^gt/gtmice) were studied between 3 weeks and 9 months of age. Body weight gain and feed efficiency of Glmp^gt/gt mice were comparable to wild type siblings, only at the age of 9 months the Glmp^gt/gt siblings had significantly reduced body weight. Reduced size of epididymal fat pads was accompanied by hepatosplenomegaly in Glmp^gt/gt mice. Blood analysis revealed reduced levels of blood glucose, circulating triacylglycerol and non-esterified fatty acids in Glmp^gt/gt mice. Increased flux of glucose, increased de novo lipogenesis and lipid accumulation were detected in Glmp^gt/gt primary hepatocytes, as well as elevated triacylglycerol levels in Glmp^gt/gt liver homogenates, compared to hepatocytes and liver from wild type mice. Gene expression analysis showed an increased expression of genes involved in fatty acid uptake and lipogenesis in Glmp^gt/gt liver compared to wild type. Our findings are in agreement with the metabolic alterations observed in other mouse models lacking lysosomal proteins, and with alterations characteristic for advanced chronic liver injury.

Loss of lysosomal membrane protein NCU-G1 in mice results in spontaneous liver fibrosis with accumulation of lipofuscin and iron in Kupffer cells

Human kidney predominant protein, NCU-G1, is a highly conserved protein with an unknown biological function. Initially described as a nuclear protein, it was later shown to be a bona fide lysosomal integral membrane protein. To gain insight into the physiological function of NCU-G1, mice with no detectable expression of this gene were created using a gene-trap strategy, and Ncu-g1^gt/gt mice were successfully characterized. Lysosomal disorders are mainly caused by lack of or malfunctioning of proteins in the endosomal-lysosomal pathway. The clinical symptoms vary, but often include liver dysfunction. Persistent liver damage activates fibrogenesis and, if unremedied, eventually leads to liver fibrosis/cirrhosis and death. We demonstrate that the disruption of Ncu-g1 results in spontaneous liver fibrosis in mice as the predominant phenotype. Evidence for an increased rate of hepatic cell death, oxidative stress and active fibrogenesis were detected in Ncu-g1^gt/gt liver. In addition to collagen deposition, microscopic examination of liver sections revealed accumulation of autofluorescent lipofuscin and iron in Ncu-g1^gt/gt Kupffer cells. Because only a few transgenic mouse models have been identified with chronic liver injury and spontaneous liver fibrosis development, we propose that the Ncu-g1^gt/gt mouse could be a valuable new tool in the development of novel treatments for the attenuation of fibrosis due to chronic liver damage.

Human NCU-G1 can function as a transcription factor and as a nuclear receptor co-activator

Background

Novel, uncharacterised proteins represent a challenge in biochemistry and molecular biology. In this report we present an initial functional characterization of human kidney predominant protein, NCU-G1.

Results

NCU-G1 was found to be a highly conserved nuclear protein rich in proline with a molecular weight of approximately 44 kDa. It is localized on chromosome 1 and consists of 6 exons. Analysis of the amino acid sequence revealed no known transcription activation domains or DNA binding regions, however, four nuclear receptor boxes (LXXLL), and four SH3-interaction motives in addition to numerous potential phosphorylation sites were found. Two nuclear export signals were identified, but no nuclear localization signal. In man, NCU-G1 was found to be widely expressed at the mRNA level with especially high levels detected in prostate, liver and kidney. Electrophoretic mobility shift analysis showed specific binding of NCU-G1 to an oligonucleotide representing the footprint 1 element of the human cellular retinol-binding protein 1 gene promoter. NCU-G1 was found to activate transcription from this promoter and required presence of the footprint 1 element. In transiently transfected Drosophila Schneider S2 cells, we demonstrated that NCU-G1 functions as a co-activator for ligand-activated PPAR-alpha, resulting in an increased expression of a CAT reporter gene under control of the peroxisome proliferator-activated receptor-alpha responsive acyl-CoA oxidase promoter.

Conclusion

We propose that NCU-G1 is a dual-function protein capable of functioning as a transcription factor as well as a nuclear receptor co-activator.

ABCA1, ABCG1 and SR-BI: hormonal regulation in primary rat hepatocytes and human cell lines

Background

Scavenger receptor type B class I (SR-BI), ABC transporter A1 (ABCA1) -and G1 (ABCG1) all play important roles in the reverse cholesterol transport. Reverse cholesterol transport is a mechanism whereby the body can eliminate excess cholesterol. Here, the regulation of SR-BI, ABCA1, and ABCG1 by dexamethasone (a synthetic glucocorticoid) and insulin were studied in order to gain more insight into the role of these two hormones in the cholesterol metabolism.

Results

By use of real time RT-PCR and Western blotting we examined the expression of our target genes. The results show that SR-BI, ABCA1 and ABCG1 mRNA expression increased in response to dexamethasone while insulin treatment reduced the expression in primary rat hepatocytes. The stimulatory effect of dexamethasone was reduced by the addition of the anti-glucocorticoid mifepristone. In HepG2 cells and THP-1 macrophages, however, the effect of dexamethasone was absent or inhibitory with no significant change in the presence of mifepristone. The latter observation may be a result of the low protein expression of glucocorticoid receptor (GR) in these cell lines.

Conclusion

Our results illustrates that insulin and glucocorticoids, two hormones crucial in the carbohydrate metabolism, also play an important role in the regulation of genes central in reverse cholesterol transport. We found a marked difference in mRNA expression between the primary cells and the two established cell lines when studying the effect of dexamethasone which may result from the varying expression levels of GR.

Bile acids reduce SR-BI expression in hepatocytes by a pathway involving FXR/RXR, SHP, and LRH-1

Hepatic SR-BI mediates uptake of circulating cholesterol into liver hepatocytes where a part of the cholesterol is metabolised to bile acids. In the hepatocytes, bile acids reduce their own synthesis by a negative feedback loop to prevent toxic high levels of bile acids. Bile acid-activated FXR/RXR represses expression of CYP7A1, the rate-limiting enzyme during bile acid synthesis, by inducing the expression of SHP, which inhibits LXR/RXR and LRH-1-transactivation of CYP7A1. The present paper presents data indicating that CDCA suppresses SR-BI expression by the same pathway. As previously reported, LRH-1 induces SR-BI promoter activity. Here we show that CDCA or over-expression of SHP inhibit this transactivation. No FXR-response element was identified in the bile acid-responsive region of the SR-BI promoter (-1200bp/-937bp). However, a binding site for LRH-1 was characterised and shown to specifically bind LRH-1. The present study shows that also the SR-BI-mediated supply of cholesterol, the substrate for bile acid synthesis, is feedback regulated by bile acids.

Glucocorticoid response and promoter occupancy of the mouse LXRalpha gene

The liver X receptors alpha and beta (LXRalpha and LXRbeta) are members of the nuclear receptor superfamily of proteins which are highly expressed in metabolically active tissues. They regulate gene expression of critical genes involved in cholesterol catabolism and transport, lipid and triglyceride biosynthesis, and carbohydrate metabolism in response to distinct oxysterol intermediates in the cholesterol metabolic pathway. Several LXR target genes have been identified, but there is limited information on how expression of the LXRs themselves is controlled. In this study we have characterized the upstream flanking region of the mouse LXRalpha gene. Transient transfections show that the LXRalpha promoter is able to drive transcription of a luciferase reporter gene, however, the transcriptional potential of the promoter in the cell lines used was low. The -2143 to -1513 region of the promoter mediates repression of reporter gene activity in all cells analyzed and multiple DNA-protein interactions were detected in this region by DNase I footprinting. The Zta, Ets, and Hes1 transcription factors were all shown to mediate alterations in reporter gene activity driven by LXRalpha promoter deletion constructs. These factors have been linked to cell cycle and differentiation processes suggesting that expression of LXRalpha might be under control of signalling mechanisms regulating cell proliferation. Several putative binding sites of the glucocorticoid receptor (GR) were identified in the LXRalpha promoter and transient cotransfections of the GR and LXRalpha promoter deletion constructs induced reporter gene activity. Addition of dexamethasone, a GR agonist, abolished this effect suggesting cross talk between GR and LXR signalling.

Akt/PTEN Signaling Mediates Estrogen-Dependent Proliferation of Primordial Germ Cellsin Vitro

Testicular tumors in humans are reported to be significantly increasing in incidence. Embryo exposure to environmental estrogens has been proposed as one of the possible underlying causes. In mice, genetic, immunological, and experimental evidence suggest that germ cell testicular tumors may derive from primordial germ cells (PGCs), the embryonic precursors of gametes. Here we show that relatively high concentrations of estrogens stimulate mouse PGC growth in vitro through the somatic cells of the gonadal ridges. Moreover, we found that estrogens stimulate the transcription of the Steel gene and the production of c-Kit ligand in gonadal somatic cells, and that this growth factor is likely to be responsible for the observed stimulation of PGC growth via an Akt/PTEN pathway. Finally, we show that estrogen stimulation of gonadal somatic cells in culture, in combination with PTEN down-regulation in PGCs and the presence of leukemia inhibitory factor in the culture medium, result in high frequency of PGC transformation in tumorigenic cells. Based on these results, we present a novel experimental in vitro model for tumorigenic germ cell transformation and identify molecular pathways likely involved in development of germ cell tumors after estrogen exposure.

Oxysterol-activated LXRalpha/RXR induces hSR-BI-promoter activity in hepatoma cells and preadipocytes

SR-BI mediates exchange of cholesterol between HDL and cells, and is a crucial factor in the transport of excessive cellular cholesterol from extrahepatic tissues to the liver ("reverse cholesterol transport") and, therefore, also for cholesterol homeostasis. Hepatic SR-BI mediates transfer of HDL-cholesterol to the hepatocytes where cholesterol may be metabolised to bile acids. LXR and SREBP are key factors in the regulation of cholesterol metabolism. The purpose of the present study was to determine whether these transcription factors are involved in the regulation of SR-BI. Here we show that LXRalpha/RXR and LXRbeta/RXR induce SR-BI transcription in human and murine hepatoma cell lines, and in 3T3-L1 preadipocytes independently of SREBP-1. The LXR/RXR response was mapped within -1,200 to -937 of the promoter region. Gel mobility shift analysis confirmed that the putative LXR response element bound LXRalpha/RXR and LXRbeta/RXR heterodimers.

Different regulation of the LXRalpha promoter activity by isoforms of CCAAT/enhancer-binding proteins

LXRs have recently been shown to regulate key enzymes in cholesterol degradation, reverse transport of cholesterol from peripheral cells, cholesterol uptake and lipogenesis. The LXRalpha promoter was thus studied to investigate if LXRalpha gene expression is under the regulation of transcription factors involved in adipogenesis. We report that the C/EBP transcription factor interacts with the promoter of the LXRalpha gene. In in vitro footprinting experiments, protein extracts from several tissues gave footprints covering a putative C/EBP recognition site. Transfection experiments and EMSA showed a direct effect of these transcription factors on the LXRalpha promoter. C/EBPalpha upregulated expression of the reporter gene in an NIH 3T3-L1 preadipocyte cell line, while C/EBPbeta and C/EBPdelta had no effect. In liver hepatoma Fao II and Cos-7 kidney cells, both C/EBPalpha and C/EBPbeta downregulated expression of the reporter gene while C/EBPdelta induced activity, indicating that the functional consequences of C/EBP isoform interactions with the LXRalpha promoter are dependent on the cellular context. Monitoring of the LXR mRNA levels during adipose tissue differentiation showed that LXRbeta is constitutively expressed during the entire differentiation process while LXRalpha is induced upon addition of differentiation mix.

Members of the nuclear factor 1 family reduce the transcriptional potential of the nuclear receptor LXRalpha promoter

Expression of the LXRalpha nuclear receptor in liver is predicted to affect cholesterol and lipid metabolism. Here we show that a short fragment from the LXRalpha gene promoter spanning the region from -144 to +43 relative to the mRNA initiation site can drive transcription of a reporter gene. Under basal conditions, in vitro DNase I footprinting demonstrated interaction between nuclear proteins and an NF1 recognition site in close vicinity to the transcriptional initiation. Both supershift, mutational analyses in EMSA and transfections provided evidence that the NF1 (nuclear factor I) transcription factor interacts with the LXRalpha promoter. All four members of the NF1 family were found to suppress the transcriptional activity indicating a general inhibitory effect on LXRalpha expression. A similar regulation by NF1 was also observed when using a fragment from the LXRalpha promoter extending up to position -3033 therefore giving the inhibitory effect of NF1 a significant impact on LXRalpha gene expression.

Evidence for independent control at the mRNA and protein levels of cellular retinol binding protein 1 in rat Sertoli cells

Cellular retinol binding protein 1 (CRBP1) is the cytosolic carrier for retinol. It is expressed in many tissues, but the concentrations vary considerably. In Sertoli cells from immature rat testis, CRBP1 is highly expressed. The results of the present study show that regulation of CRBP1 expression at the protein level appears to be independent of regulation at the mRNA level. In Sertoli cells from prepubertal 19-day-old rats, CRBP1 mRNA is strongly induced for up to 72 h by the presence of serum factors. In contrast, treatment of the cells with cAMP analogues led to a rapid reduction in mRNA to quantities less than 5% of control values. However, the changes in CRBP1 mRNA did not lead to similar changes in the concentration of CRBP1 protein during 72 h of observation. Similarly, treatment of cells from 32- and 44-day-old rats with serum led to increased CRBP1 mRNA, whereas cAMP treatment resulted in a decrease in CRBP1 mRNA. Again, no changes were observed in the concentration of CRBP1 protein. Furthermore, co-incubation of Sertoli cells from 19-day-old rats with purified pachytene spermatocytes or round spermatids resulted in an increase in mRNA for CRBP1. However, comparable changes in CRBP1 protein concentrations were not observed. Neither cAMP nor serum changed the fraction of CRBP1 mRNA that was associated with polysomes. As a possible explanation for some of the results, pulse-chase experiments showed that the rate of CRBP1 degradation in cultured Sertoli cells is decreased by cAMP. It is proposed that these changes at the level of protein turnover contribute to the maintenance of stable concentrations of CRBP1 even when the corresponding mRNA concentrations vary markedly.

Evidence for independent control at the mRNA and protein levels of cellular retinol binding protein 1 in rat Sertoli cells

Cellular retinol binding protein 1 (CRBP1) is the cytosolic carrier for retinol. It is expressed in many tissues, but the concentrations vary considerably. In Sertoli cells from immature rat testis, CRBP1 is highly expressed. The results of the present study show that regulation of CRBP1 expression at the protein level appears to be independent of regulation at the mRNA level. In Sertoli cells from prepubertal 19-day-old rats, CRBP1 mRNA is strongly induced for up to 72 h by the presence of serum factors. In contrast, treatment of the cells with cAMP analogues led to a rapid reduction in mRNA to quantities less than 5% of control values. However, the changes in CRBP1 mRNA did not lead to similar changes in the concentration of CRBP1 protein during 72 h of observation. Similarly, treatment of cells from 32- and 44-day-old rats with serum led to increased CRBP1 mRNA, whereas cAMP treatment resulted in a decrease in CRBP1 mRNA. Again, no changes were observed in the concentration of CRBP1 protein. Furthermore, co-incubation of Sertoli cells from 19-day-old rats with purified pachytene spermatocytes or round spermatids resulted in an increase in mRNA for CRBP1. However, comparable changes in CRBP1 protein concentrations were not observed. Neither cAMP nor serum changed the fraction of CRBP1 mRNA that was associated with polysomes. As a possible explanation for some of the results, pulse-chase experiments showed that the rate of CRBP1 degradation in cultured Sertoli cells is decreased by cAMP. It is proposed that these changes at the level of protein turnover contribute to the maintenance of stable concentrations of CRBP1 even when the corresponding mRNA concentrations vary markedly.

Vitamin A-sensitive tissues in transgenic mice expressing high levels of human cellular retinol-binding protein type I are not altered phenotypically

The suggested function of cellular retinol-binding protein type I [CRBP(I)] is to carry retinol to esterifying or oxidizing enzymes. The retinyl esters are used in storage or transport, whereas oxidized forms such as all-trans or 9-cis retinoic acid are metabolites used in the mechanism of action of vitamin A. Thus, high expression of human CRBP(I) [hCRBP(I)] in transgenic mice might be expected to increase the production of retinoic acid in tissues, thereby inducing a phenotype resembling vitamin A toxicity. Alternatively, a vitamin A-deficient phenotype could also be envisioned as a result of an increased accumulation of vitamin A in storage cells induced by a high hCRBP(I) level. Signs of vitamin A toxicity or deficiency were therefore examined in tissues from transgenic mice with ectopic expression of hCRBP(I). Testis and intestine, the tissues with the highest expression of the transgene, showed normal gross morphology. Similarly, no abnormalities were observed in other tissues known to be sensitive to vitamin A status such as cornea and retina, and the epithelia in the cervix, trachea and skin. Furthermore, hematologic variables known to be influenced by vitamin A status such as the hemoglobin concentration, hematocrits and the number of red blood cells were within normal ranges in the transgenic mice. In conclusion, these transgenic mice have normal function of vitamin A despite high expression of hCRBP(I) in several tissues.

Method of transfection affects the cAMP-mediated induction of the RIIbeta subunit of protein kinase A in Sertoli cells: inhibition of response by increase in intracellullar calcium

mRNA for the regulatory subunit RIIbeta of cAMP-dependent protein kinase is stimulated more than 50-fold by cAMP in primary cultures of rat Sertoli cells. We have previously shown that this induction involves regulation of transcriptional activation as well as mRNA stabilization. The rat RIIbeta gene contains no cAMP response element (CRE), and the induction of RIIbeta mRNA is slow and requires on-going protein synthesis. When a construct containing the 5'-flanking region of the RIIbeta gene upstream of a CAT reporter was transfected into Sertoli cells by the calcium phosphate method, low and variable responses to cAMP (three- to fivefold) were observed, whereas a 15- to 20-fold increase in reporter activity by cAMP was observed after lipofectamine transfection. Interestingly, when a vector containing CRE elements upstream of a reporter gene was transfected into Sertoli cells, the responses to cAMP were similar regardless of the transfection method used. We have also demonstrated that increased intracellular levels of calcium by A23187 and thapsigargin dramatically inhibit cAMP-mediated induction of RIIbeta mRNA, but not the mRNA for the CRE-containing RIalpha gene. Furthermore, decreased cAMP responsiveness of endogenous RIIbetamRNA (but not RIalpha) was also observed in calcium phosphate-transfected Sertoli cells but not in lipofectamine-transfected cells. Thus, calcium-mediated reduction in cAMP response appears to be a gene-specific phenomenon.

Calcium/phospholipid-dependent protein kinases in rat Sertoli cells: regulation of androgen receptor messenger ribonucleic acid

The possibility that Sertoli cell responses to testosterone are modulated by the calcium/phospholipid-dependent protein kinase (protein kinase C; PKC) was examined in rat Sertoli cells in culture. Both soluble and particulate cell fractions showed low constitutive phosphotransferase activity. Incubation with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA; 10(-7) M) was associated with a transient induction in both cell fractions of calcium/phosphatidylserine-dependent PKC activity, which was elevated from 15 min to 1 h. Consistent with this, mRNAs for the calcium/phospholipid-dependent isomeric forms of PKC (alpha, beta, and gamma) were detected. The expression levels of mRNAs for PKCalpha and PKCbeta were also up-regulated (2.5- to 3-fold) by TPA (10(-7) M), but these effects were much slower (peaking after 12 h) than those on phosphotransferase activity. In the presence of TPA (10(-7) M), expression of androgen receptor (AR) mRNA showed a transient time-dependent down-regulation ( approximately 70%), in which the nadir was reached after 6 h and baseline expression was again obtained after 12 h. The regulatory effect of PKC activation on AR mRNA was confirmed by the absence of response to a biologically inactive phorbol ester. A concentration-dependent decrease (half-maximal effect at approximately 10(-8) M TPA) of AR mRNA was also observed. These data suggest that Sertoli cell responses to testosterone may be inhibited by a transiently active PKC with a wide intracellular distribution.

The rat growth hormone and human cellular retinol binding protein 1 genes share homologous NF1-like binding sites that exert either positive or negative influences on gene expression in vitro

High levels of expression for the rat growth hormone (rGH) gene are restricted to the somatotroph cells of the anterior pituitary. Previously, we have shown that rGH cell-specific repression results in part from the recognition of negatively acting silencers by a number of nuclear proteins that repress basal promoter activity. Examination of these silencers revealed the presence of binding sites for proteins that belong to the NF1 family of transcription factors. Indeed, proteins from this family were shown to bind the rGH proximal silencer (designated silencer-1) in in vitro assays. Furthermore, this silencer site is capable of repressing chloramphenicol acetyltransferase (CAT) gene expression driven by an heterologous promoter (that of the mouse p12 gene), even in pituitary cells. Recently, we identified in the 5' untranslated region of the gene encoding human cellular retinol binding protein 1 (hCRBP1) a negative regulatory element (Fp1) that also bears an NF1 binding site very similar to that of rGH silencer-1. However, although deletion of Fp1 in the hCRBP1 gene yielded increased CAT activity, pointing toward a negative regulatory function exerted by this element, its insertion upstream of the p12 basal promoter results in an impressive positive stimulation of CAT gene expression. By exploiting NaDodSO4 gel protein fractionation and renaturation, we identified a 40-kD nuclear protein (designated Bp1) present in GH4C1 cells that binds very strongly to rGH silencer-1 but only weakly to hCRBP1 Fp1. Similarly, we also detected a 29-kD nuclear factor (designated Bp2) that recognizes exclusively the Fp1 element as its target site, therefore suggesting that different, but likely related, proteins bind these homologous elements to either activate or repress gene transcription. Although they bind DNA through the recognition of the NF1-like target sequence contained on these elements, competition and supershift experiments in electrophoretic mobility shift assays provided evidence that neither of these proteins belong to the NF1 family.

Characterization of the 5'-flanking region of the gene for the cAMP-inducible protein kinase A subunit, RIIbeta, in Sertoli cells

Activation of cyclic AMP-dependent protein kinases (protein kinase A, PKA) by gonadotropins and cyclic AMP (cAMP) plays an important role in the regulation of testicular functions. A regulatory subunit, RIIbeta, of PKA is transcriptionally induced in rat Sertoli cells in response to treatment with cAMP. The present study addresses regulatory mechanisms leading to increased transcription of the rat RIIbeta gene. We have localized a footprint which overlaps one of the major transcription initiation sites in the basal promoter (-293 to -123). One of the proteins binding this sequence belongs to the NF-1 family of transcription factors. We also observed binding to a basic helix-loop-helix (bHLH) response element. Furthermore, transfection studies of various 5'-deletions of the rat RIIbeta gene in primary cultures of rat Sertoli cells and in peritubular cells revealed the presence of an upstream region (-723 to -395, cAMP-responsive region) inhibiting basal expression from the rat RIIbeta gene only in Sertoli cells. This region was found to enhance cAMP responsiveness in Sertoli cells but not in peritubular cells. Interactions with downstream elements seemed to be important for the function of the cAMP-responsive region. Although some short stretches reveal homology to the cAMP-responsive regions of other slowly cAMP-responding genes, and an AP-1-like element is present, no strong resemblance to any known regulatory element responsive to cAMP is found.

Retinyl ester storage is normal in transgenic mice with enhanced expression of cellular retinol-binding protein type I

This report describes the production and characterization of transgenic mice with high expression of human cellular retinol-binding protein type I [hCRBP(I)]. In initial experiments, overexpression of hCRBP(I) was driven by the strong promoter SR(alpha), but no transgenic offspring were produced. When we used the less efficient mouse metallothionein I promoter fused to the hCRBP(I) cDNA for microinjection, we obtained 12% transgenic offspring. Two of these transgenic mice (409/1 and 401/2) expressed mRNA and immunoreactive hCRBP(I) in several organs. Both lines had relatively high contents of hCRBP(I) in intestine, testis and epididymis. On the other hand, only 401/2 transgenic mice had high contents of hCRBP(I) in kidney. Effects on storage of vitamin A were studied by measuring the concentration of retinyl esters in different organs. The concentrations of retinyl esters in liver, lung and kidney did not significantly differ between transgenic and control mice, and the concentration of total retinol in plasma was within the normal range in transgenic mice. Furthermore, feeding mice a diet with high or low concentrations of vitamin A for 2 wks resulted in no marked differences in the concentrations of retinyl esters in liver, kidney, lung, intestine and testis in transgenic mice compared with control mice. Therefore, in spite of high expression of hCRBP(I) in several organs, the transgenic mice had normal storage of retinyl esters in all organs studied. The present in vivo study indicates that the CRBP(I) content alone does not control retinyl ester storage.

Regulation of protein kinase A subunits by cyclic adenosine 3',5'-monophosphate in a mouse Sertoli cell line (MSC-1): induction of RII beta messenger ribonucleic acid is independent of continuous protein synthesis

We report the basal and cAMP-regulated expression of protein kinase A (PKA) subunits in a mouse Sertoli cell line (MSC-1). Of the PKA subunits expressed by these cells (RI alpha, RII alpha, RII beta, C alpha, C beta), only RII beta was regulated by cAMP. An approximately 8-fold induction of RII beta mRNA and a 3-fold induction of RII beta protein was observed during 48 h of cAMP-stimulation. This cAMP-mediated RII beta mRNA induction, reaching maximal levels after approximately 12 h, did not require ongoing protein synthesis. Fairly rapid decay of maximally induced RII beta mRNA was observed after removal of cAMP (t1/2 approximately 5 h). Further, ongoing transcription and translation were necessary for rapid degradation of RII beta mRNA. Thus, the MSC-1 cells expressed all the PKA subunits present in primary cultures of Sertoli cells and responded to cAMP with increased levels of RII beta at both mRNA and protein levels. Although the nature of some of these responses distinguished the observations in MSC-1 cells from previously described responses in primary cultures, these cells may prove to be useful in future studies addressing cAMP-mediated gene regulation.

Tissue distribution of the receptor for plasma retinol-binding protein

The tissue distribution of the retinol-binding-protein receptor has been studied by using a cell-free binding assay. High binding activity was found in placenta, retina pigment epithelial cells, bone marrow and kidneys. Specific binding activity was also found in the small intestines, spleen and liver, and to a lesser extent in lung. Scatchard analysis revealed that the difference in binding activity was due to variations in receptor level and not affinity changes. When the kidneys were separated into cortex and medulla we found that almost all the specific binding activity present in kidneys was recovered in the cortex. The choroid plexus, an important site in the delivery of nutrients to the cerebrospinal fluid, expressed very high binding activity. The pineal gland, which has been shown to store vitamin A, also showed high binding activity. Testes from immature animals showed higher binding activity than testes from mature rabbits. Cultured undifferentiated kidney keratinocytes showed about 40 times higher binding activity than differentiated cells. Skin fibroblasts demonstrated no binding activity. In conclusion, the data presented in this report show that the level of the retinol-binding-protein receptor varies considerably between cell types. The observed tissue distribution of the receptor agrees well with the present knowledge on retinol function and metabolism by various cells.

Binding of a member of the NF1 family of transcription factors to two distinct cis-acting elements in the promoter and 5'-flanking region of the human cellular retinol binding protein 1 gene

We studied the interaction of nuclear proteins with the 5'-flanking and promoter region of the human cellular retinol binding protein 1 (hCRBP1) gene and identified seven specific sequences that interacted with nuclear proteins from liver and prostate. Two of these sequences, footprint 1 (Fp1) and footprint 5 (Fp5), were highly homologous, sharing the core sequence GGCCAAC, which has a certain similarity to the consensus sequence for the NF1 binding site. Competition experiments in gel mobility shift assays and DNasel footprinting indicated that a common protein interacted with both elements. Immunological and biochemical data indicated that this protein belongs to the nuclear factor 1 (NF1) family of transcription factors. The ability of the Fp1 and Fp5 sequences to control gene expression was studied by transient transfections of several cell types. In the wild type promoter, both Fp1 and Fp5 acted as repressors of human (h) CRBP1 gene transcription. Once inserted upstream of the basal promoter from the heterologous p12 gene, the function of both Fp1 and Fp5 was reverted to that of transcriptional activators, although Fp5 exerted only moderate transcriptional activation of the chloramphenicol acetyl transferase (CAT) reporter gene. Hence, the position of these NF1 binding sites and the nature of the flanking sequences appear to direct their effect on transcription. Despite close sequence homology, a common core sequence, and a similar ability to bind nuclear proteins in vitro, these results indicate that Fp1 and Fp5 exert similar regulatory functions but to different levels in vivo. In conclusion, these results indicate that a member of the NF1 family plays a significant role in regulating CRBP1 gene expression.

Protein kinase C activation and positive and negative agonist regulation of 3',5'-cyclic adenosine monophosphate levels in cultured rat Sertoli cells

The present study examines the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) on agonist-regulated 3',5'-cyclic adenosine monophosphate (cAMP) formation and cAMP-mediated effects in cultured Sertoli cells from immature rats. Concentration-dependent stimulation of cAMP levels by follicle-stimulating hormone (FSH) was inhibited dramatically by the coaddition of 100 nmol/l TPA, which exerted a similar inhibition of glucagon- and isoproterenol-stimulated cAMP production. These results show that protein kinase C (PKC) activation by TPA attenuates Gs-protein-mediated agonist activation of cAMP production. (-)-N6(R)-Phenylisopropyladenosine (L-PIA), an A1-adenosine receptor agonist, inhibited cAMP stimulation by FSH in a concentration-dependent manner. When L-PIA was added in increasing concentrations simultaneously with 100 nmol/l TPA, the L-PIA still inhibited FSH-stimulated cAMP production in a concentration-dependent manner. In the presence of TPA, the half-inhibitory concentration (IC50) for L-PIA inhibition of cAMP formation was reduced by more than one order of magnitude, indicating that PKC activation by TPA increases the sensitivity of Sertoli cells to Gi-protein-mediated agonist inhibition of cAMP production. The inhibitory effects of TPA on FSH-stimulated cAMP production were still observed when cAMP phosphodiesterase activity was inhibited by 1 mmol/l methylisobutylxanthine or when the activity of G alpha i-protein was eliminated by pretreatment with 100 micrograms/l pertussis toxin. Taken together, the results indicate that PKC activation inhibits agonist-dependent stimulation of cAMP production by phosphorylation of components common to all the activating agonists used, and not via stimulation of G(i)-protein activity or degradation of cAMP by cAMP phosphodiesterase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum factors induce messenger ribonucleic acid levels for cellular retinol-binding protein in rat Sertoli cells

This report shows that serum factors dramatically increase the levels of mRNA for cellular retinol-binding protein (CRBP) in cultured rat Sertoli cells. Incubation of rat Sertoli cells (0-24 h) with 10% fetal calf serum (FCS) was associated with a time-dependent increase in CRBP mRNA levels. A significant increase (6-fold) was observed after 3 h of incubation. Maximal levels (15- to 50-fold) were reached after 9-12 h and were maintained for as long as serum was present. The effect was concentration dependent, with maximal induction at 10% FCS. Removal of FCS resulted in a decline in the CRBP mRNA levels, with a t1/2 of approximately 7 h. The CRBP mRNA stimulating activity (CMSA) was completely removed from FCS by precipitation with 5% trichloroacetic acid, but was only partly (50%) inhibited by heating at 100 C or trypsin treatment. Removal of retinol from FCS by repeated ether extractions did not alter the CMSA of FCS. Both the induction and degradation of CRBP mRNA were inhibited by the protein synthesis inhibitor cycloheximide. A nuclear protein binding to the 5'-flanking region of the CRBP gene was detected in nuclear extracts from untreated Sertoli cells, but not in nuclear extracts from Sertoli cells treated with 10% FCS for 3 h. Thus, serum factors, different from retinoids, dramatically stimulate the levels of CRBP mRNA in rat Sertoli cells. This is associated with the loss of protein binding to the 5'-flanking region of the CRBP gene.

Down-regulation of messenger ribonucleic acid (mRNA) for the estrogen receptor (ER) by phorbol ester requires ongoing RNA synthesis but not protein synthesis. Is hormonal control of ER mRNA degradation mediated by an RNA molecule?

Treatment of MCF-7 cells, a human mammary adenocarcinoma cell line, with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) (10-7 M) was associated with a time-dependent reduction in the level of estrogen receptor (ER) mRNA (half-life approximately 3 h). In the presence of the RNA synthesis inhibitor actinomycin D [5.0 micrograms/ml (4.0 microM)], half-life of ER mRNA was much longer (approximately 12 h). Furthermore, the TPA-dependent down-regulation of ER mRNA was abolished by actinomycin D. Similar effects were observed with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (150 microM), an inhibitor of RNA polymerase. Inhibition of protein synthesis by cycloheximide (50 microM) or puromycin [50 micrograms/ml (92 microM)] did not alter the steady state level of ER mRNA during a period of 10-12 h. Furthermore, these inhibitors of protein synthesis did not prevent the down-regulation of ER mRNA in the presence of TPA. Our studies show that degradation of ER mRNA by TPA in MCF-7 cells is dependent on ongoing RNA synthesis but not on protein synthesis. This indicates that an RNA molecule with rapid turnover, which does not require translation, might be involved in the TPA-dependent ER mRNA decay.

Half-lives of different sized mRNAs for the PKA subunit RI alpha are regulated differently in response to inhibition of transcription and translation

The RI alpha mRNA level is induced 3-5 times by FSH or cAMP analogs in primary cultures of rat Sertoli cells. In rat tissues, the RI alpha gene gives rise to three different mRNAs of different size: 3.2, 2.9 and 1.7 kb. In the present study we report that the 1.7 kb transcript has a shorter half-life than the two other mRNAs. In cells which had been pre-stimulated with a cAMP analog, inhibition of transcription stabilizes the two larger, but not the smaller sized RI alpha mRNA. However, in contrast, inhibition of protein synthesis stabilizes all the RI alpha mRNAs. Thus, degradation of various mRNAs coding for the same protein reveals different dependencies on transcription and translation.

Inhibitors of RNA and protein synthesis stabilize messenger RNA for the RII beta subunit of protein kinase A in different cellular compartments

Messenger RNA for RII beta is transiently induced (greater than 50-fold) by cAMP analogs in primary cultures of rat Sertoli cells. The induction is dependent on protein synthesis. We have previously shown that mRNA for RII beta is stabilized by cAMP, as well as inhibitors of transcription and translation. This indicated that rapid degradation of RII beta mRNA involved a protein with a rapid turnover and its corresponding mRNA. The two RNA synthesis inhibitors used in the present study stabilized both nuclear and cytoplasmic RII beta mRNA, whereas inhibition of protein synthesis stabilized RII beta mRNA in the cytoplasm only. These results indicate that only cytoplasmic degradation of RII beta mRNA is dependent on a protein with high turnover. In contrast, nuclear degradation appears to be dependent on an RNA with a short half-life, not involving protein synthesis.

Protein kinase C activation by 12-O-tetradecanoylphorbol 13-acetate modulates messenger ribonucleic acid levels for two of the regulatory subunits of 3',5'-cyclic adenosine monophosphate-dependent protein kinases (RII beta and RI alpha) via multiple and distinct mechanisms

Messenger RNAs (mRNA) for two of the regulatory subunits of cAMP-dependent protein kinases (PKA), RII beta and RI alpha, are transiently (maximal levels at 6 h) stimulated by 12-O-tetradecanoylphorbol 13-acetate (TPA) in cultured rat Sertoli cells in a time- and concentration-dependent manner. Whereas TPA (10(-7) M) stimulated RII beta mRNA 11 +/- 2.8 fold (mean +/- SEM), mRNA levels for RI alpha increased only 2.5 +/- 0.6-fold (mean +/- SEM). No effects of TPA on the other subunits of PKA (RII alpha, C alpha) were observed. TPA-dependent accumulation of mRNAs for RII beta and RI alpha was observed to the same extent in nucleus and cytoplasm. We have previously shown that mRNA levels for all the PKA subunits are increased by cAMP, particularly that of RII beta (greater than 50-fold). TPA modulated the stimulatory effects of cAMP on RII beta and RI alpha mRNAs in opposite directions. Whereas treatment with both 8-CPTcAMP and TPA gave an additive effect on RI alpha mRNA, TPA reduced the cAMP-dependent increase in RII beta mRNA. Although the mRNA for RII beta had returned to basal levels after 24 h of incubation with TPA, the presence of TPA still inhibited cAMP-dependent induction of mRNA for RII beta. In contrast, similar TPA treatment did not influence the subsequent cAMP-dependent stimulation of RI alpha mRNA. Preincubation with 8-CPTcAMP did not influence TPA-dependent stimulation of mRNAs for either RII beta or RI alpha. TPA induction of RII beta mRNA was completely blocked by cycloheximide (an inhibitor of protein synthesis), whereas that of RI alpha was not. The inhibitory effect of TPA on cAMP stimulation of RII beta mRNA was independent of ongoing protein synthesis. These results indicate that TPA induction of mRNAs for RI alpha and RII beta involves multiple and distinct mechanisms. The stimulatory effect of TPA on RI alpha mRNA levels and the inhibitory effect of TPA on cAMP-stimulated RII beta mRNA expression are probably mediated through stable factors, whereas proteins with rapid turnover or factors induced by TPA are involved in the stimulatory effect of TPA on RII beta mRNA.

Evidence for a novel splice variant of the alpha subunit of Go in rat male haploid germ cells

Northern analysis shows that Go alpha mRNA is highly expressed in immature germ cells from rat. Whereas alpha o2 mRNA is the major form in pachytene spermatocytes, a message of shorter chain length is present in large amount in haploid germ cells. This mRNA was detected with an oligonucleotide specific for the 3'-coding region of alpha o2, but did not hybridize to oligonucleotides specific for the 5'-untranslated and 5'-coding regions. The results indicate the presence of a novel splice variant of alpha o mRNA, which may code for a Go alpha protein important for germ cell development.

Adenosine 3',5'-monophosphate-dependent stabilization of messenger ribonucleic acids (mRNAs) for protein kinase-A (PKA) subunits in rat Sertoli cells: rapid degradation of mRNAs for PKA subunits is dependent on ongoing RNA and protein synthesis

cAMP treatment of primary cultures of Sertoli cells is associated with a transient stimulatory effect on mRNA levels for various protein kinase-A (PKA) subunits. We have previously shown that the induction of mRNA for regulatory subunit II beta (RII beta) is due at least partly to transcriptional activation. In the present study we investigate possible regulatory effects of (Bu)2cAMP on the degradation of mRNAs for various PKA subunits in rat Sertoli cells. We demonstrate subunit specific differences in the decay of mRNAs for the various PKA subunits. When (Bu)2cAMP was removed from Sertoli cell cultures after 6 h of stimulation, there was a rapid decay of mRNAs for both RII beta and RI alpha (half-lives, approximately 3 h). In contrast, mRNA levels for RII alpha continued to increase. Removal of (Bu)2cAMP after a longer period of treatment revealed a similar decay of mRNAs for all of the PKA subunits, with half-lives of approximately 3 h. Incubation of Sertoli cells for 12 h with (Bu)2cAMP, followed by continued incubation in the absence and presence of (Bu)2cAMP as well as in the presence of actinomycin-D (an inhibitor of RNA synthesis), revealed (Bu)2cAMP mediated stabilization of mRNA for the RII beta subunit. Interestingly, actinomycin-D as such stabilized mRNAs for all PKA subunits. Similar treatment with cycloheximide (an inhibitor of protein synthesis) revealed distinct differences between the RI alpha and C alpha subunits vs. the RII subunits; cycloheximide reduced the decay of both RII beta and RII alpha mRNAs, whereas steady state levels of mRNAs for RI alpha and C alpha actually increased after cycloheximide treatment of previously (Bu)2cAMP-stimulated cultures. Cycloheximide treatment also increased basal levels of mRNAs for RI alpha and C alpha, whereas basal levels of RII beta and RII alpha mRNAs were not influenced. These studies indicate that the degradation of mRNAs for the various PKA subunits is subject to different regulation by (Bu)2cAMP, and that ongoing RNA and protein synthesis is required for rapid degradation of all PKA subunits.

Identification, characterization, and hormonal regulation of 3', 5'-cyclic adenosine monophosphate-dependent protein kinases in rat Sertoli cells

Recent studies have disclosed multiple isoforms of regulatory (R) and catalytic (C) subunits of cAMP-dependent protein kinase (PKA) at the protein and messenger RNA (mRNA) levels. The purpose of the present study was to identify, characterize, and quantify individual R subunits in rat Sertoli cells both at the mRNA and protein levels. Unstimulated Sertoli cells contain high levels of R (approximately 9.2 +/- 0.8 pmol/mg protein) and C (approximately 7.3 +/- 0.7 pmol/mg protein). Stimulation with (Bt)2cAMP (0.1 mM) for 24 and 48 h revealed a time-dependent increase in [3H]cAMP-binding activity. During the same time period the catalytic activity remained relatively constant, resulting in an increase in the R/C ratio from approximately 1.3 to 3.0. Using diethylaminoethyl cellulose chromatography, 8-N3-[32P]cAMP photoaffinity labeling, autophosphorylation by gamma-[32P]ATP, and specific antibodies, we show that unstimulated Sertoli cells contain approximately 75% RI alpha, 25% RII alpha, and very low levels of RII beta. Stimulation of Sertoli cells with (Bt)2cAMP (0.1 mM, 48 h) was associated with a 2.1-fold increase in RI alpha (6.6-14 pmol/mg) and a 10- to 20-fold increase in RII beta (less than 0.1-1.1 pmol/mg), with little or no change in RII alpha (1.9-2.3 pmol/mg). Treatment with cAMP was associated with a slight increase in RI/RII ratio (3.3-4.1). mRNA levels for RII beta increased 30- to 50-fold after (Bt)2cAMP stimulation, whereas only minor changes in mRNA levels for RI alpha, RII alpha, and C alpha were observed (1.5- to 2.0-fold). mRNA levels for RI beta, C beta, and C gamma were not detected in either unstimulated or in cAMP-stimulated Sertoli cells. It is concluded that chronic treatment with cAMP changes the relative proportion of R subunits of PKA in a manner reflecting the changing levels in respective mRNAs. Furthermore, such treatment is associated with the appearance of a new PKA R subunit (RII beta), which is absent in untreated Sertoli cells.

Down-regulation of messenger ribonucleic acid and protein levels for estrogen receptors by phorbol ester and calcium in MCF-7 cells

Treatment of MCF-7 cells, a human mammary carcinoma cell line, with phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA)] or calcium ionophore (A23187) was associated with striking effects on levels of estrogen receptor (ER) mRNA, specific binding of 17 beta-[3H]estradiol [( 3H]E2), and immunoreactive ER. TPA (10(-7) M) caused a time-dependent reduction of ER mRNA which was below the level of detection after 9 h. Similar effects of TPA appeared at levels of specific binding of [3H]E2 as well as immunoreactive ER. In contrast, TPA induced an increase in mRNA for beta-actin. Incubation of MCF-7 cells with increasing concentrations of TPA (10(-11)-10(-7) M) was associated with biphasic effects on ER mRNA and proteins. Levels of immunoreactive progesterone receptors (PR) were induced by E2 (10(-9) M) in a time-dependent manner. In the presence of TPA (10(-7) M), where ER levels were suppressed, no induction of PR was observed. Removal of TPA (10(-7) M) after 10 h (ER mRNA) or 22 h (ER proteins) of treatment was associated with a continued suppression of both mRNA and protein levels during the entire incubation period (48 h). Treatment with A23187 (2 x 10(-7) M) also caused a time-dependent down-regulation of levels of ER mRNA and proteins. These effects occurred somewhat more slowly than those of TPA. Levels of beta-actin mRNA were not changed by this treatment. These results indicate that changes in estrogen sensitivity are mediated by calcium-dependent protein kinases in human mammary carcinoma MCF-7 cells.

Cellular localization of mRNAs for retinoic acid receptor-alpha, cellular retinol-binding protein, and cellular retinoic acid-binding protein in rat testis: evidence for germ cell-specific mRNAs

In the present study we have examined the cellular localization and developmental changes of mRNAs for retinoid-binding proteins in rat testis. We demonstrate that mRNA (0.7 kb) for cellular retinol-binding protein (CRBP) is expressed only in Sertoli cells and peritubular cells. The mRNA for CRBP could not be detected in other testicular cells. In contrast, mRNA for cellular retinoic acid-binding protein (CRABP) was detected primarily in germ cells and to a small extent in tumor Leydig cells. The mRNA for CRABP in germ cells revealed distinct size heterogeneity and three distinct mRNA species were observed (1.0, 1.8, and 1.9 kb), in contrast to previous data for somatic cells where only the 1.0-kb mRNA has been reported. Messenger RNAs for retinoic acid receptor-alpha (RAR alpha) were detected in both somatic and haploid germ cells. The highest level of RAR alpha was seen in Sertoli cells, round spermatids, and tumor Leydig cells. Lower, but distinct, levels were observed in peritubular cells. Furthermore, we observed germ cell-specific species of RAR alpha mRNA (4 kb and approximately 7 kb). The smallest mRNA for RAR alpha (2.7 kb) in somatic cells was absent in germ cells. The levels of mRNAs for the various retinoid-binding proteins in whole testis obtained from rats of various ages confirmed this cellular localization. The mRNAs for CRBP, the small molecular size (2.7 kb) mRNA for RAR alpha (localized to somatic cells), and the 1-kb mRNA for CRABP showed an age-dependent decrease.(ABSTRACT TRUNCATED AT 250 WORDS)

Different mechanisms are involved in cAMP-mediated induction of mRNAs for subunits of cAMP-dependent protein kinases

The present study addresses possible mechanisms through which cAMP mediates its effects on mRNA levels for the subunits of protein kinase A (PKA) and the cellular protooncogene, c-fos. Messenger RNAs for the PKA subunits (RI alpha, RII alpha, RII beta, and C alpha) were regulated by cAMP with similar kinetics in Sertoli cells. However, effects of cAMP on the PKA mRNAs were slow compared to a well characterized cAMP responsive gene, c-fos. The magnitude of stimulation was dramatically different between the various PKA subunits, in that RII beta mRNA increased more than 50-fold while the mRNAs for the other subunits were induced only two to four times. Separation of nuclear and cytoplasmic RNA demonstrated that mRNAs for PKA subunits were stimulated to the same extent in these two cellular compartments. The more rapid induction of c-fos mRNA by cAMP, compared to the mRNA for RII beta, was also seen at the level of transcription. Maximal transcription rate for c-fos, RI alpha, and C alpha were observed after 30 min, whereas that for RII beta was increasing during the 2-h period examined. Transcriptional activation of the RI alpha gene also appeared faster than that for RII beta. When Sertoli cells were incubated with 8-(4-chlorophenylthio) cAMP and cycloheximide, a potent inhibitor of protein synthesis, we observed a super-induction of the mRNAs for c-fos (10-fold) and RI alpha (2-fold).(ABSTRACT TRUNCATED AT 250 WORDS)

Biphasic regulation of the messenger ribonucleic acid coding for the estrogen receptor by cyclic adenosine 3':5'-monophosphate in tumor Leydig cells

In this report we show that the mRNA level for the estrogen receptor (ER) is regulated by 8-bromo cyclic AMP (8-Br-cAMP) and human chorionic gonadotropin in a mouse tumor Leydig cell line (MA-10 cells). When the MA-10 cells were cultured in the presence of the cAMP analogue for varying time periods, a transient increase in the level of ER mRNA was observed. Short time incubation (0-2 h) with 8-Br-cAMP enhanced the expression of ER mRNA (2-fold), whereas longer times of incubation (6 h) had the opposite effect (the level of ER mRNA was reduced by 60-70%). The inhibitory effect of 8-Br-cAMP on ER mRNA was not counteracted by aminoglutethimide, an inhibitor of steroidogenic enzymes, indicating that this effect is not mediated via steroids (progesterone). Treatment of 8-Br-cAMP for 6 h caused a concentration-dependent inhibition of ER mRNA with a half-maximal effect of approximately 150 microM. Increasing concentrations of human chorionic gonadotropin for 6 h was also associated with a biphasic effect on the ER mRNA level. Low concentrations (0.20-0.40 ng/ml) increased ER mRNA in the MA-10 cells whereas the highest concentration (20 ng/ml) caused a suppression of this mRNA. In contrast to the biphasic effects observed for the ER mRNA, the level of the regulatory subunit type II beta of the cAMP-dependent protein kinase (protein kinase A) was enhanced in a concentration-dependent manner by human chorionic gonadotropin. Furthermore, 8-Br-cAMP stimulated the mRNA for regulatory subunit type II beta (10- to 20-fold) by all concentrations examined (50-1000 microM). The observations reported here indicate that the expression of ER mRNA is regulated both by endogenously formed and exogenously added cAMP and that there may exist regulatory loops between the steroid and the cAMP/protein kinase A systems.

Cellular localization and age dependent changes in mRNA for glutathione S-transferase-P in rat testicular cells

Using Northern blotting techniques we report that mRNA for Glutathione S-transferase-P (GST-P or GST 7-7) is present in rat testis. GST-P mRNA was detected in cultured Sertoli cells, cultured peritubular cells, as well as in transplantable Leydig cell tumor. However, no GST-P mRNA was detected in rat germ cell fractions. There was a marked increase in mRNA for GST-P from day 5 to day 20 in rats, after which a decrease was seen. The decreased level of mRNA for GST-P in the testis after 20 days of age, coincided in time with the exponential increase in germ cells, and accompanying relative decrease in somatic cells. The results show that mRNA for GST-P is primarily present in somatic cells of the rat testis.

Autologous down-regulation of messenger ribonucleic acid and protein levels for estrogen receptors in MCF-7 cells: an inverse correlation to progesterone receptor levels

In the present study we have examined the effects of estradiol on mRNA levels for estrogen (ER) and progesterone receptors (PR) in the estrogen-dependent mammary carcinoma MCF-7 cell line. The changes in ER immunoactivity and specific binding of [3H]R5020 were also assessed. Estradiol (10(-7) M) caused a transient and time-dependent reduction of the level of mRNA for ER, with a maximal effect (30-40% of control; n = 3) after 72 h. This was associated with a similar decrease in ER immunoactivity. Further treatment (96 and 120 h) revealed a return of ER mRNA to control values, whereas the ER immunoactivity remained depressed. The effect on the mRNA level for PR gave almost the inverse curve. Initially (24-72 h), we observed a pronounced increase in this mRNA, with a maximal effect (6-7 times the control value; n = 3) after 72 h. Treatment beyond 72 h was associated with a gradual return of mRNA for PR toward the control level. The variation in specific binding of [3H]R5020 revealed similar changes, except that changes in specific receptor binding were delayed 24 h compared to the levels of mRNA. Incubation with low concentrations (10(-11) and 10(-10) M) of estradiol for 72 h was associated with slightly elevated levels of mRNA for ER, whereas higher concentrations gave a dose-dependent decrease. The mRNA for PR was biphasically stimulated, with a maximal effect at 10(-10)-10(-8) M, where a 10- to 13-fold stimulation was observed. The highest concentration (10(-7) M) gave a lower response. Assessment of concentration-induced variations in protein receptor levels of ER and PR reflected the effects of estradiol on their mRNAs. Low concentrations of estradiol slightly enhanced the ER level, whereas high concentrations clearly reduced ER immunoactivity. The PR level was stimulated by all concentrations used, and 10(-8) M estradiol raised the PR level more than 11-fold. Our results indicate autologous regulation of estrogen receptor gene transcripts and proteins and a clear induction of PR mRNA and receptor proteins by estradiol.

Intracellular transport of formaldehyde-treated serum albumin in liver endothelial cells after uptake via scavenger receptors

Endocytosis of formaldehyde-treated serum albumin (FSA) mediated by the scavenger receptor was studied in rat liver endothelial cells. Suspended cells had about 8000 receptors/cell, whereas cultured cells had about 19,000 receptors/cell. Kd was 10(-8) M in both systems. Cell-surface scavenger receptors were found exclusively in coated pits by electron microscopy, by using ligand labelled with colloidal gold. Cell-surface-bound FSA could be released by decreasing the pH to 6.0; it was therefore possible to assess the rate of internalization of surface-bound ligand. This rate was very high: t1/2 for internalization of ligand prebound at 4 degrees C was 24 s. The endocytic rate constant at 37 degrees C, Ke, measured as described by Wiley & Cunningham [(1982) J. Biol. Chem. 257, 4222-4229], was 2.44 min-1, corresponding to t1/2 = 12 s. Uptake of FSA at 37 degrees C after destruction of one cell-surface pool of receptors by Pronase was decreased to 60%. This finding is compatible with a relatively large intracellular pool of receptors. The intracellular handling of 125I-tyramine-cellobiose-labelled FSA (125I-TC-FSA) was studied by subcellular fractionation in sucrose gradients, Nycodenz gradients or by differential centrifugation. The density distributions of degraded and undegraded 125I-TC-FSA after fractionation of isolated non-parenchymal cells and whole liver were similar, when studied in Nycodenz and sucrose gradients, suggesting that the subcellular distribution of the ligand was not influenced by the huge excess of non-endothelial material in a whole liver homogenate. Fractionation in sucrose gradients showed that the ligand was sequentially associated with organelles banding at 1.14, 1.17 and 1.21 g/ml. At 9-12 min after intravenous injection the ligand was in a degradative compartment, as indicated by the accumulation of acid-soluble radioactivity at 1.21 g/ml. A rapid transfer of ligand to the lysosomes was also indicated by the finding that a substantial proportion of the ligand could be degraded by incubating mitochondrial fractions prepared 12 min after intravenous injection of the ligand. The results indicate that FSA is very rapidly internalized and transferred through an endosomal compartment to the lysosomes. The endosomes are gradually converted into lysosomes between 9 and 12 min after injection of FSA. The rate-limiting step in the intracellular handling of 125I-TC-FSA is the degradation in the lysosomes.

Investigation of detoxification capacity of rat testicular germ cells and Sertoli cells

Isolated pachytene spermatocytes liver longer than round spermatids in vitro. Indigenous formation of oxygen-derived free radicals and hydrogen peroxide can cause damage to germ cells. The germ cell antioxidant capacity may play an important role in this respect. In view of this, we have examined the activity and cellular localization of superoxide dismutase (SOD) and glutathione S-transferases (GST) in rat testicular cells. We have found significant differences in the distribution of these enzymatic activities in the germ cells. In addition, this study shows that alpha-tocopherol is found in various amounts in rat testicular cells in the order of: Sertoli cells greater than pachytene spermatocytes greater than round spermatids, with a factor of 4 in the alpha-tocopherol content between Sertoli cells and round spermatids.

Molecular cloning, complementary deoxyribonucleic acid structure and predicted full-length amino acid sequence of the hormone-inducible regulatory subunit of 3'-5'-cyclic adenosine monophosphate-dependent protein kinase from human testis

In this study, we report the isolation and characterization of a full-length cDNA clone for the hormone-inducible regulatory subunit RII beta (formerly called RII51) of type II cAMP-dependent protein kinase from a human testis cDNA library. The cloned cDNA demonstrated tissue-specific expression of RII beta mRNA in human tissues, with the highest mRNA levels in testis and ovary. The isolated human cDNA clone was 3.3 kilobases (kb) in length and contained 166 base pairs (bp) of G/C-rich 5'-noncoding sequence, an open reading frame of 1254 bp and an A/T-rich 3'-nontranslated region containing 1836 bp followed by an 89 nucleotide long poly(A)-tail. The predicted protein contains 418 amino acids including the start methionine, and the estimated mol wt of human RII beta is 53,856. The nucleotide sequence within the open reading frame and the predicted amino acid sequence of human RII beta are highly conserved compared with partial rat RII beta sequences, displaying 91% and 97% similarity, respectively. Codon preference analysis of the cloned cDNA sequence indicated that the two cAMP-binding domains and the hinge region are highly conserved through evolution, whereas the dimerization domain displayed a codon preference pattern indicative of appearance at a later stage of evolution. The isolated human cDNA detected an FSH- and cAMP-inducible mRNA of 3.2 kb in rat Sertoli cells, thus confirming that the cloned cDNA represents the hormone-inducible regulatory subunit of cAMP-dependent protein kinase. This is the first report documenting the isolation of a full-length cDNA clone for the RII beta of cAMP-dependent protein kinase.

Biphasic response to 3',5'-cyclic adenosine monophosphate (cAMP) at the messenger ribonucleic acid level for a regulatory subunit of cAMP-dependent protein kinase

In the present study we have examined the effect of long-term stimulation with (Bu)2cAMP on mRNA levels for the hormone responsive regulatory subunit (RII beta) of cAMP-dependent protein kinase in cultured rat Sertoli cells. The effects of the same treatment on two other mRNAs [androgen binding protein (ABP) and cellular retinol binding protein (cRBP)], shown to be regulated by cAMP, were examined simultaneously. The addition of (Bu)2cAMP (0.1 mM) to primary Sertoli cell cultures, for 14 and 24 h, caused a 50- to 60-fold stimulation in the steady state levels of mRNA for RII beta. During the same period of stimulation, we also observed a significant increase (2- to 3-fold) in the mRNA levels for ABP, and a 80% decrease in the mRNA levels for cRBP. Continued stimulation for 36 and 48 h was associated with a significant time-dependent decrease in the mRNA level for RII beta, in spite of the continuous presence of (Bu)2cAMP (0.1 mM) in the medium. This reduced response by long term stimulation with (Bu)2cAMP appears to be specific for RII beta, since mRNA for ABP remained elevated and mRNA for cRBP remained depressed during the entire period of cAMP stimulation. Our data demonstrate the presence of a biphasic type of regulation at the mRNA level, specific for the regulatory subunit RII beta of cAMP-dependent protein kinase. This response may be analogous to the desensitization mechanisms observed at other levels of the cAMP signalling pathway. For proteins constituting part of the signal transduction pathway this type of biphasic regulation, may be particularly important in maintaining homeostasis in the cell.

Elimination kinetics of circulating antigens and immune complexes. II. Hepatic and extrahepatic elimination of chemically modified albumin preparations in relation to their non-specific physicochemical properties

Formaldehyde-treated human serum albumin (fHSA) was analysed and compared with dinitrophenylated human serum albumin (DNP35HSA) with respect to net charge, hydrophobicity, and state of aggregation. The blood clearance and tissue distribution of intravenously injected fHSA was studied in mice. Similarly to DNP-HSA, fHSA was rapidly cleared from the blood stream owing to hepatic uptake and to extrahepatic deposition. We found no evidence supporting that fHSA was eliminated from the blood by hydrophobic interaction with cell or tissue surfaces. The blood clearance rate and the hepatic uptake of fHSA was not influenced by the presence of formylated peptides, indicating that receptors for formylated peptides were not involved in the handling of circulating fHSA.

Differential regulation of messenger ribonucleic acids for specific subunits of cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase by cAMP in rat Sertoli cells

In the present study we have examined the effects of FSH, forskolin, and (Bu)2cAMP on messenger RNA (mRNA) levels for all known subunits of cAMP-dependent protein kinase in rat Sertoli cells, using newly developed complementary DNA (cDNA) probes. mRNAs for the three regulatory subunits [RI alpha, RII51, (RII beta), and RII54 (RII alpha)] and the catalytic subunit C alpha were shown to be present in cultured rat Sertoli cells, whereas mRNAs for the subunits designated RI beta and C beta were below the level of detection. A high-levelled, concentration-dependent increase in a 3.2 kilobase mRNA for RII51 was observed when cultured immature Sertoli cells were incubated with increasing concentrations of (Bu)2cAMP (10(-6) to 5 X 10(-3) M) for 16 h. Densitometric scanning indicated a maximal stimulation by (Bu)2cAMP of 30- to 40-fold. Incubation with forskolin (100 microM) and FSH (200 ng/ml) gave rise to a smaller but significant increase in mRNA for RII51. When cultured Sertoli cells were incubated in the presence of 10(-4) M (Bu)2cAMP for varying time periods, there was a biphasic regulation of mRNA for RII51. (Bu)2cAMP caused an initial increase in mRNA for RII51 with maximal levels obtained after 10-16 h, after which a time-dependent decrease was observed. For the other three subunits present in Sertoli cells (RI alpha, RII54, and C alpha) a smaller but significant stimulation by (Bu)2cAMP and forskolin (2-4 fold) was seen. The functional implications of these changes in mRNA levels for the different subunits of cAMP-dependent protein kinase have not yet been revealed. However, our data clearly demonstrate differential regulation of the various subunits of cAMP-dependent protein kinase in Sertoli cells. Furthermore, these results document the presence of distinct adaptational changes taking place at the level of cAMP-dependent protein kinase in response to long term elevation of cAMP.

Regulation of mRNA levels for cellular retinol binding protein in rat Sertoli cells by cyclic AMP and retinol

The levels of mRNA for cellular retinol binding protein (CRBP) were studied in primary rat Sertoli cell cultures treated with cAMP analogues and retinol. In the presence of cyclic AMP analogues a dose- and time-dependent reduction (70-90%) of the levels of mRNA for CRBP was observed. Retinol concentrations above 10 nM induced a dose- and time-dependent increase (2-3 fold) in mRNA levels for CRBP. Assuming that CRBP is important for vitamin A action, our data indicate that both cAMP and retinol itself modulate the sensitivity of the Sertoli cells for retinol.

High sensitivity towards monensin of receptor-mediated endocytosis of formaldehyde treated albumin by liver endothelial cells

Endocytosis of formaldehyde-treated serum albumin (f-albumin) in isolated liver sinusoidal endothelial cells was studied. Uptake occurs via the scavenger receptor and was found to be very sensitive to the ionophore monensin. Binding at 4 degrees C of f-albumin was reduced to 50% of control values by preincubation for 2 min with 2 microM monensin. Both uptake and degradation of f-albumin were more sensitive to monensin. No lag-phase in the inhibitory effect on uptake and degradation was detected. A concentration of 0.1 microM monensin reduced uptake of f-albumin by 50%. Degradation of internalized f-albumin was reduced by 50% in the presence of 0.2 microM monensin. Since uptake and degradation of f-albumin were very sensitive to monensin, the effect of introducing the drug during endocytosis of the ligand was tested. All processing of f-albumin stopped instantly upon addition of monensin; hence, there seems to be no step in the endocytic process beyond which monensin is ineffective. The data suggest that the scavenger receptor of liver endothelial cells is internalized and recycled very rapidly.