Retinoids in health and disease: A role for hepatic stellate cells in affecting retinoid levels

Vitamin A (retinol) is important for normal growth, vision and reproduction. It has a role in the immune response and the development of metabolic syndrome. Most of the retinol present in the body is stored as retinyl esters within lipid droplets in hepatic stellate cells (HSCs). In case of liver damage, HSCs release large amounts of stored retinol, which is partially converted to retinoic acid (RA). This surge of RA can mediate the immune response and enhance the regeneration of the liver. If the damage persists activated HSCs change into myofibroblast-like cells producing extracellular matrix, which increases the chance of tumorigenesis to occur. RA has been shown to decrease proliferation and metastasis of hepatocellular carcinoma. The levels of RA and RA signaling are influenced by the possibility to esterify retinol towards retinyl esters. This suggests a complex regulation between different retinoids, with an important regulatory role for HSCs.

Identification of potential drugs for treatment of hepatic lipidosis in cats using an in vitro feline liver organoid system

Background

Hepatic lipidosis is increasing in incidence in the Western world, with cats being particularly sensitive. When cats stop eating and start utilizing their fat reserves, free fatty acids (FFAs) increase in blood, causing an accumulation of triacylglycerol (TAG) in the liver.

Objective

Identifying potential new drugs that can be used to treat hepatic lipidosis in cats using a feline hepatic organoid system.

Animals

Liver organoids obtained from 6 cats.

Methods

Eight different drugs were tested, and the 2 most promising were further studied using a quantitative TAG assay, lipid droplet staining, and qPCR.

Results

Both T863 (a diacylglycerol O‐acyltransferase 1 [DGAT1] inhibitor) and 5‐aminoimidazole‐4‐carboxamide 1‐β‐D‐ribofuranoside (AICAR; an adenosine monophosphate kinase activator) decreased TAG accumulation by 55% (P < .0001) and 46% (P = .0003), respectively. Gene expression of perilipin 2 (PLIN2) increased upon the addition of FFAs to the medium and decreased upon treatment with AICAR but not significantly after treatment with T863.

Conclusions and Clinical Importance

Two potential drugs useful in the treatment of hepatic lipidosis in cats were identified. The drug T863 inhibits DGAT1, indicating that DGAT1 is the primary enzyme responsible for TAG synthesis from external fatty acids in cat organoids. The drug AICAR may act as a lipid‐lowering compound via decreasing PLIN2 mRNA. Liver organoids can be used as an in vitro tool for drug testing in a species‐specific system and provide the basis for further clinical testing of drugs to treat steatosis.

Some Lipid Droplets Are More Equal Than Others: Different Metabolic Lipid Droplet Pools in Hepatic Stellate Cells

Hepatic stellate cells (HSCs) are professional lipid-storing cells and are unique in their property to store most of the retinol (vitamin A) as retinyl esters in large-sized lipid droplets. Hepatic stellate cell activation is a critical step in the development of chronic liver disease, as activated HSCs cause fibrosis. During activation, HSCs lose their lipid droplets containing triacylglycerols, cholesteryl esters, and retinyl esters. Lipidomic analysis revealed that the dynamics of disappearance of these different classes of neutral lipids are, however, very different from each other. Although retinyl esters steadily decrease during HSC activation, triacylglycerols have multiple pools one of which becomes transiently enriched in polyunsaturated fatty acids before disappearing. These observations are consistent with the existence of preexisting “original” lipid droplets with relatively slow turnover and rapidly recycling lipid droplets that transiently appear during activation of HSCs. Elucidation of the molecular machinery involved in the regulation of these distinct lipid droplet pools may open new avenues for the treatment of liver fibrosis.

Sex specific differences in hepatic and plasma lipid profiles in healthy cats pre and post spaying and neutering: relationship with feline hepatic lipidosis

BACKGROUND

A link between lipid metabolism and disease has been recognized in cats. Since hepatic lipidosis is a frequent disorder in cats, the aim of the current study was to evaluate liver and plasma lipid dimorphism in healthy cats and the effects of gonadectomy on lipid profiling. From six female and six male cats plasma and liver lipid profiles before and after spaying/neutering were assessed and compared to five cats (three neutered male and two spayed female) diagnosed with hepatic lipidosis.

RESULTS

Intact female cats had a significantly lower level of plasma triacylglycerides (TAG) and a higher liver level of the long chain polyunsaturated fatty acid arachidonic acid (AA) compared to their neutered state. Both male and female cats with lipidosis had a higher liver, but not plasma TAG level and an increased level of plasma and liver sphingomyelin compared to the healthy cats.

CONCLUSION

Although lipid dimorphism in healthy cats resembles that of other species, intact female cats show differences in metabolic configuration that could predispose them to develop hepatic lipidosis. The increased sphingomyelin levels in cats with lipidosis could suggest a potential role in the pathogenesis of hepatic lipidosis in cats.

Lysosome-mediated degradation of a distinct pool of lipid droplets during hepatic stellate cell activation

Activation of hepatic stellate cells (HSCs) is a critical step in the development of liver fibrosis. During activation, HSCs lose their lipid droplets (LDs) containing triacylglycerols (TAGs), cholesteryl esters, and retinyl esters (REs). We previously provided evidence for the presence of two distinct LD pools, a preexisting and a dynamic LD pool. Here we investigate the mechanisms of neutral lipid metabolism in the preexisting LD pool. To investigate the involvement of lysosomal degradation of neutral lipids, we studied the effect of lalistat, a specific lysosomal acid lipase (LAL/Lipa) inhibitor on LD degradation in HSCs during activation in vitro The LAL inhibitor increased the levels of TAG, cholesteryl ester, and RE in both rat and mouse HSCs. Lalistat was less potent in inhibiting the degradation of newly synthesized TAG species as compared with a more general lipase inhibitor orlistat. Lalistat also induced the presence of RE-containing LDs in an acidic compartment. However, targeted deletion of the Lipa gene in mice decreased the liver levels of RE, most likely as the result of a gradual disappearance of HSCs in livers of Lipa^-/- mice. Lalistat partially inhibited the induction of activation marker α-smooth muscle actin (α-SMA) in rat and mouse HSCs. Our data suggest that LAL/Lipa is involved in the degradation of a specific preexisting pool of LDs and that inhibition of this pathway attenuates HSC activation.

Hepatic stellate cells retain the capacity to synthesize retinyl esters and to store neutral lipids in small lipid droplets in the absence of LRAT

Hepatic stellate cells (HSCs) play an important role in liver physiology and under healthy conditions they have a quiescent and lipid-storing phenotype. Upon liver injury, HSCs are activated and rapidly lose their retinyl ester-containing lipid droplets. To investigate the role of lecithin:retinol acyltransferase (LRAT) and acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) in retinyl ester synthesis and lipid droplet dynamics, we modified LC-MS/MS procedures by including multiple reaction monitoring allowing unambiguous identification and quantification of all major retinyl ester species. Quiescent primary HSCs contain predominantly retinyl palmitate. Exogenous fatty acids are a major determinant in the retinyl ester species synthesized by activated HSCs and LX-2 cells, indicating that HSCs shift their retinyl ester synthesizing capacity from LRAT to DGAT1 during activation. Quiescent LRAT^-/- HSCs retain the capacity to synthesize retinyl esters and to store neutral lipids in lipid droplets ex vivo. The median lipid droplet size in LRAT^-/- HSCs (1080nm) is significantly smaller than in wild type HSCs (1618nm). This is a consequence of an altered lipid droplet size distribution with 50.5±9.0% small (≤700nm) lipid droplets in LRAT^-/- HSCs and 25.6±1.4% large (1400-2100nm) lipid droplets in wild type HSC cells. Upon prolonged (24h) incubation, the amounts of small (≤700nm) lipid droplets strongly increased both in wild type and in LRAT^-/- HSCs, indicating a dynamic behavior in both cell types. The absence of retinyl esters and reduced number of lipid droplets in LRAT-deficient HSCs in vivo will be discussed.

ATGL and DGAT1 are involved in the turnover of newly synthesized triacylglycerols in hepatic stellate cells

Hepatic stellate cell (HSC) activation is a critical step in the development of chronic liver disease. During activation, HSCs lose their lipid droplets (LDs) containing triacylglycerol (TAG), cholesteryl esters (CEs), and retinyl esters (REs). Here we aimed to investigate which enzymes are involved in LD turnover in HSCs during activation in vitro. Targeted deletion of the Atgl gene in mice HSCs had little effect on the decrease of the overall TAG, CE, and RE levels during activation. However, ATGL-deficient HSCs specifically accumulated TAG species enriched in PUFAs and degraded new TAG species more slowly. TAG synthesis and levels of PUFA-TAGs were lowered by the diacylglycerol acyltransferase (DGAT)1 inhibitor, T863. The lipase inhibitor, Atglistatin, increased the levels of TAG in both WT and ATGL-deficient mouse HSCs. Both Atglistatin and T863 inhibited the induction of activation marker, α-smooth muscle actin, in rat HSCs, but not in mouse HSCs. Compared with mouse HSCs, rat HSCs have a higher turnover of new TAGs, and Atglistatin and the DGAT1 inhibitor, T863, were more effective. Our data suggest that ATGL preferentially degrades newly synthesized TAGs, synthesized by DGAT1, and is less involved in the breakdown of preexisting TAGs and REs in HSCs. Furthermore a large change in TAG levels has modest effect on rat HSC activation.

No up-regulation of the phosphatidylethanolamine N-methyltransferase pathway and choline production by sex hormones in cats

BACKGROUND

Feline hepatic lipidosis (FHL) is a common cholestatic disease affecting cats of any breed, age and sex. Both choline deficiency and low hepatic phosphatidylethanolamine N-methyltransferase (PEMT) activity are associated with hepatic lipidosis (HL) in humans, mice and rats. The PEMT expression is known to be upregulated by oestrogens, protecting the females in these species from the development of HL when exposed to choline deficient diets. The aim of the present study was to evaluate the influence of sex hormones on choline synthesis via the PEMT pathway in healthy male and female cats before and after spaying/neutering, when fed a diet with recommended dietary choline content.

RESULTS

From six female and six male cats PEMT activity was assayed directly in liver biopsies taken before and after spaying/neutering, and assessed indirectly by analyses of PEMT-specific hepatic phosphatidylcholine (PC) species and plasma choline levels. Hepatic PEMT activity did not differ between intact female and male cats and no changes upon spaying/neutering were observed. Likewise, no significant differences in liver PC content and PEMT-specific polyunsaturated PC species were found between the sexes and before or after spaying/neutering.

CONCLUSION

These results suggest that choline synthesis in cats differs from what is observed in humans, mice and rats. The lack of evident influence of sex hormones on the PEMT pathway makes it unlikely that spaying/neutering predisposes cats for HL by causing PC deficiency as suggested in other species.

Role of long-chain acyl-CoA synthetase 4 in formation of polyunsaturated lipid species in hepatic stellate cells

Hepatic stellate cell (HSC) activation is a critical step in the development of chronic liver disease. We previously observed that the levels of triacylglycerol (TAG) species containing long polyunsaturated fatty acids (PUFAs) are increased in in vitro activated HSCs. Here we investigated the cause and consequences of the rise in PUFA-TAGs by profiling enzymes involved in PUFA incorporation. We report that acyl CoA synthetase (ACSL) type 4, which has a preference for PUFAs, is the only upregulated ACSL family member in activated HSCs. Inhibition of the activity of ACSL4 by siRNA-mediated knockdown or addition of rosiglitazone specifically inhibited the incorporation of deuterated arachidonic acid (AA-d8) into TAG in HSCs. In agreement with this, ACSL4 was found to be partially localized around lipid droplets (LDs) in HSCs. Inhibition of ACSL4 also prevented the large increase in PUFA-TAGs in HSCs upon activation and to a lesser extent the increase of arachidonate-containing phosphatidylcholine species. Inhibition of ACSL4 by rosiglitazone was associated with an inhibition of HSC activation and prostaglandin secretion. Our combined data show that upregulation of ACSL4 is responsible for the increase in PUFA-TAG species during activation of HSCs, which may serve to protect cells against a shortage of PUFAs required for eicosanoid secretion.

Bovine cumulus cells protect maturing oocytes from increased fatty acid levels by massive intracellular lipid storage

Metabolic conditions characterized by elevated free fatty acid concentrations in blood and follicular fluid are often associated with impaired female fertility. Especially elevated saturated fatty acid levels can be lipotoxic for several somatic cell types. The aim of this study was to determine the impact of elevated free fatty acid concentrations in follicular fluid on neutral lipids (fatty acids stored in lipid droplets) inside cumulus cells and oocytes and their developmental competence. To this end, cows were exposed to a short-term fasting period during final oocyte maturation. This resulted in elevated, but distinct, free fatty acid concentrations in blood and follicular fluid and a rise in the concentrations of in particular fatty acids with a chain length of 14-18 carbon atoms. Interestingly, elevated free fatty acid concentrations in follicular fluid resulted in a massive increase in the level of neutral lipids in cumulus cells, whereas the level of neutral lipid in oocytes was hardly affected. Furthermore, competence of oocytes to develop to the blastocyst stage after fertilization and culture of cumulus-oocyte-complexes of the experimental and control group was not different. In conclusion these data suggest that short-term elevated free fatty acid concentrations in follicular fluid do not harm oocyte developmental competence. We propose that the involvement of high levels of mobilized oleic acid in follicular fluid in combination with the induced lipid storage in cumulus cells serves to prevent harmful saturated fatty acid exposure to the oocyte.

Replacement of retinyl esters by polyunsaturated triacylglycerol species in lipid droplets of hepatic stellate cells during activation

Activation of hepatic stellate cells has been recognized as one of the first steps in liver injury and repair. During activation, hepatic stellate cells transform into myofibroblasts with concomitant loss of their lipid droplets (LDs) and production of excessive extracellular matrix. Here we aimed to obtain more insight in the dynamics and mechanism of LD loss. We have investigated the LD degradation processes in rat hepatic stellate cells in vitro with a combined approach of confocal Raman microspectroscopy and mass spectrometric analysis of lipids (lipidomics). Upon activation of the hepatic stellate cells, LDs reduce in size, but increase in number during the first 7 days, but the total volume of neutral lipids did not decrease. The LDs also migrate to cellular extensions in the first 7 days, before they disappear. In individual hepatic stellate cells. all LDs have a similar Raman spectrum, suggesting a similar lipid profile. However, Raman studies also showed that the retinyl esters are degraded more rapidly than the triacylglycerols upon activation. Lipidomic analyses confirmed that after 7 days in culture hepatic stellate cells have lost most of their retinyl esters, but not their triacylglycerols and cholesterol esters. Furthermore, we specifically observed a large increase in triacylglycerol-species containing polyunsaturated fatty acids, partly caused by an enhanced incorporation of exogenous arachidonic acid. These results reveal that lipid droplet degradation in activated hepatic stellate cells is a highly dynamic and regulated process. The rapid replacement of retinyl esters by polyunsaturated fatty acids in LDs suggests a role for both lipids or their derivatives like eicosanoids during hepatic stellate cell activation.

Oleic acid prevents detrimental effects of saturated fatty acids on bovine oocyte developmental competence

Mobilization of fatty acids from adipose tissue during metabolic stress will increase the amount of free fatty acids in blood and follicular fluid and, thus, may affect oocyte quality. In this in vitro study, the three predominant fatty acids in follicular fluid (saturated palmitic and stearic acid and unsaturated oleic acid) were presented to maturing oocytes to test whether fatty acids can affect lipid storage of the oocyte and developmental competence postfertilization. Palmitic and stearic acid had a dose-dependent inhibitory effect on the amount of fat stored in lipid droplets and a concomitant detrimental effect on oocyte developmental competence. Oleic acid, in contrast, had the opposite effect, causing an increase of lipid storage in lipid droplets and an improvement of oocyte developmental competence. Remarkably, the adverse effects of palmitic and stearic acid could be counteracted by oleic acid. These results suggest that the ratio and amount of saturated and unsaturated fatty acid is relevant for lipid storage in the maturing oocyte and that this relates to the developmental competence of maturing oocytes.

Depletion of phosphatidylcholine affects endoplasmic reticulum morphology and protein traffic at the Golgi complex

The mutant Chinese hamster ovary cell line MT58 contains a thermosensitive mutation in CTP:phosphocholine cytidylyltransferase, the regulatory enzyme in the CDP-choline pathway. As a result, MT58 cells have a 50% decrease in their phosphatidylcholine (PC) level within 24 h when cultured at the nonpermissive temperature (40 degrees C). This is due to a relative rapid breakdown of PC that is not compensated for by the inhibition of de novo PC synthesis. Despite this drastic decrease in cellular PC content, cells are viable and can proliferate by addition of lysophosphatidylcholine. By [(3)H]oleate labeling, we found that the FA moiety of the degraded PC is recovered in triacylglycerol. In accordance with this finding, an accumulation of lipid droplets is seen in MT58 cells. Analysis of PC-depleted MT58 cells by electron and fluorescence microscopy revealed a partial dilation of the rough endoplasmic reticulum, resulting in spherical structures on both sites of the nucleus, whereas the morphology of the plasma membrane, mitochondria, and Golgi complex was unaffected. In contrast to these morphological observations, protein transport from the ER remains intact. Surprisingly, protein transport at the level of the Golgi complex is impaired. Our data suggest that the transport processes at the Golgi complex are regulated by distal changes in lipid metabolism.

The CDP-ethanolamine pathway and phosphatidylserine decarboxylation generate different phosphatidylethanolamine molecular species

In mammalian cells, phosphatidylethanolamine (PtdEtn) is mainly synthesized via the CDP-ethanolamine (Kennedy) pathway and by decarboxylation of phosphatidylserine (PtdSer). However, the extent to which these two pathways contribute to overall PtdEtn synthesis both quantitatively and qualitatively is still not clear. To assess their contributions, PtdEtn species synthesized by the two routes were labeled with pathway-specific stable isotope precursors, d(3)-serine and d(4)-ethanolamine, and analyzed by high performance liquid chromatography-mass spectrometry. The major conclusions from this study are that (i) in both McA-RH7777 and Chinese hamster ovary K1 cells, the CDP-ethanolamine pathway was favored over PtdSer decarboxylation, and (ii) both pathways for PtdEtn synthesis are able to produce all diacyl-PtdEtn species, but most of these species were preferentially made by one pathway. For example, the CDP-ethanolamine pathway preferentially synthesized phospholipids with mono- or di-unsaturated fatty acids on the sn-2 position (e.g. (16:0-18:2)PtdEtn and (18:1-18:2)PtdEtn), whereas PtdSer decarboxylation generated species with mainly polyunsaturated fatty acids on the sn-2 position (e.g. (18:0-20:4)PtdEtn and (18:0-20:5)PtdEtn in McArdle and (18: 0-20:4)PtdEtn and (18:0-22:6)PtdEtn in Chinese hamster ovary K1 cells). (iii) The main PtdEtn species newly synthesized from the Kennedy pathway in the microsomal fraction appeared to equilibrate rapidly between the endoplasmic reticulum and mitochondria. (iv) Newly synthesized PtdEtn species preferably formed in the mitochondria, which is at least in part due to the substrate specificity of the phosphatidylserine decarboxylase, seemed to be retained in this organelle. Our data suggest a potentially essential role of the PtdSer decarboxylation pathway in mitochondrial functioning.

Equine biochemical multiple acyl-CoA dehydrogenase deficiency (MADD) as a cause of rhabdomyolysis

Two horses (a 7-year-old Groninger warmblood gelding and a six-month-old Trakehner mare) with pathologically confirmed rhabdomyolysis were diagnosed as suffering from multiple acyl-CoA dehydrogenase deficiency (MADD). This disorder has not been recognised in animals before. Clinical signs of both horses were a stiff, insecure gait, myoglobinuria, and finally recumbency. Urine, plasma, and muscle tissues were investigated. Analysis of plasma showed hyperglycemia, lactic acidemia, increased activity of muscle enzymes (ASAT, LDH, CK), and impaired kidney function (increased urea and creatinine). The most remarkable findings of organic acids in urine of both horses were increased lactic acid, ethylmalonic acid (EMA), 2-methylsuccinic acid, butyrylglycine (iso)valerylglycine, and hexanoylglycine. EMA was also increased in plasma of both animals. Furthermore, the profile of acylcarnitines in plasma from both animals showed a substantial elevation of C4-, C5-, C6-, C8-, and C5-DC-carnitine. Concentrations of acylcarnitines in urine of both animals revealed increased excretions of C2-, C3-, C4-, C5-, C6-, C5-OH-, C8-, C10:1-, C10-, and C5-DC-carnitine. In addition, concentrations of free carnitine were also increased. Quantitative biochemical measurement of enzyme activities in muscle tissue showed deficiencies of short-chain acyl-CoA dehydrogenase (SCAD), medium-chain acyl-CoA dehydrogenase (MCAD), and isovaleryl-CoA dehydrogenase (IVD) also indicating MADD. Histology revealed extensive rhabdomyolysis with microvesicular lipidosis predominantly in type 1 muscle fibers and mitochondrial damage. However, the ETF and ETF-QO activities were within normal limits indicating the metabolic disorder to be acquired rather than inherited. To our knowledge, these are the first cases of biochemical MADD reported in equine medicine.

Soluble factors released by ATDC5 cells affect the formation of calcium phosphate crystals

During biomineralization the organism controls the nature, orientation, size and shape of the mineral phase. The aim of this study was to investigate whether proteins or vesicles that are constitutively released by growing ATDC5 cells have the ability to affect the formation of the calcium phosphate crystal. Therefore, subconfluent cultured ATDC5 cells were incubated for 1 h in medium without serum. Subsequently, medium was harvested and incubated for 24 h in the presence of additional Pi. This resulted in the formation of flat mineralizing structures (FMS), consisting of complex irregularly shaped flat crystals, which occasionally contained fiber-like structures ( approximately 40 microm in size). Without pre-incubation of medium with cells, only small punctate (dot like) calcium phosphate precipitates were observed. The formation of FMS was shown to be caused by soluble factors released by subconfluent ATDC5 cells. Proteomic analysis by mass spectrometry showed that FMS contained a specific set intracellular proteins, serum proteins, and extracellular matrix proteins. Bulk cytosolic proteins derived from homogenized cells or serum proteins did, however, not induce the formation of FMS. Conditioned medium from HeLa, CHO K1, RAW 264.7 and MDCK cells was also capable to form FMS under our experimental conditions. Therefore the formation of FMS seems to be caused by specific soluble factors constitutively released by ADTC5 and other cells. This in vitro model system can be used as a tool to identify factors that affect the shape of the biomineral phase.

Partial purification and characterization of ferritin from the liver and intestinal mucosa of chickens, turtledoves and mynahs

Ferritin is the iron-storage protein responsible for sequestering excess iron, to be stored in a safe way in the liver or to be shed with the intestinal epithelial cells. The properties of ferritin in iron-overload-susceptible birds have not been elucidated. Furthermore, there is only scarce information on mucosal ferritin, with no information at all in avian species. Here we have studied the liver and proximal intestine ferritins of iron-overload-susceptible (Indian hill mynahs, common mynahs) and non-susceptible (turtledoves, chicken) bird species. A brief purification process preceded native polyacrylamide gel electrophoresis and staining the gels for protein and iron. Protein amounts and iron-binding characteristics of ferritin were measured and ferritin saturation levels were calculated. Although ferritin protein amounts did not differ significantly, liver and mucosal ferritins of sensitive bird species incorporated much more iron, leading to high saturation levels. Significantly higher ferritin iron content and saturation were observed in the liver of both mynah species and in the intestinal ferritin of Indian hill mynahs when compared with the non-susceptible species. Ferritin appears not to play a major role in the regulation of iron absorption, implicating other phases in iron transport to be more important in the onset and process of iron overload in birds.

What triggers cell-mediated mineralization?

Mineralization is an essential requirement for normal skeletal development, but under certain pathological conditions organs like articular cartilage and cardiovascular tissue are prone to unwanted mineralization. Recent findings suggest that the mechanisms regulating skeletal mineralization may be similar to those regulating pathological mineralization. In general, three forms of cell-mediated mineralization are recognized in an organism: intramembranous ossification, endochondral ossification and pathological mineralization. This review summarizes recent work that tried to elucidate how cell-mediated mineralization is initiated and regulated. To explain mineralization, several theories have been proposed. One theory proposes that mineralization is initiated within matrix vesicles (MVs). A second, not mutually exclusive, theory proposes that phosphate induces apoptosis, and that apoptotic bodies nucleate crystals composed of calcium and phosphate. A third theory suggests that mineralization is mediated by certain non-collagenous proteins, which associate with the extracellular matrix. Regardless of the way mineralization is initiated, the organism also actively inhibits mineralization by specific proteins and removal of an inhibitor may also induce mineralization. Although many studies greatly contributed to a better understanding of the mechanisms regulating cell-mediated mineralization, many questions remain about the mechanisms that trigger cell-mediated mineralization and how this process is regulated. Further investigation is necessary to develop in the future novel therapeutic strategies to prevent pathological mineralization.

Iron ions derived from the nitric oxide donor sodium nitroprusside inhibit mineralization

Sodium nitroprusside (SNP) is a nitric oxide (NO) donor drug, which is therapeutically used as a vasodilating drug in heart transplantations. In our previous study it was found that SNP at a concentration of 100 microM inhibited mineralization in a cell culture system, indicating that the beneficial effects of this drug may also include inhibition of vascular calcification. The aim of this study was to investigate which bioactive compounds generated from SNP inhibit mineralization. ATDC5 cells were grown for 14 days and mineralization was induced by addition of 5 mM phosphate for 24 h. Mineralization was determined by staining precipitated calcium with an alizarin red stain. It was found that the NO donors S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine were not able to inhibit mineralization and NO scavengers could not antagonize the inhibiting effect of SNP on mineralization. The iron chelator deferoxamine (200 microM) antagonized the inhibiting effect on mineralization mediated by SNP and ammonium iron sulfate inhibited mineralization in a dose-dependent manner (10-100 microM). Furthermore, iron ions (30 microM) were detected to be released from SNP in the cell culture. These data show that the iron moiety of sodium nitroprusside, rather than nitric oxide inhibits mineralization.

The nitric oxide donor sodium nitroprusside inhibits mineralization in ATDC5 cells

The aim of this study was to test whether the nitric oxide (NO) donor sodium nitroprusside (SNP) has an effect on mineralization in ATDC5 cells. Mineralization in ATDC5 cell culture was induced by addition of beta-glycerophosphate or inorganic phosphate, visualized by staining precipitated calcium with an alizarin red stain, and quantified using atomic absorption spectrometry. SNP was shown to inhibit the mineralization of ADTC5 cells. This inhibition was not affected by inhibitors of guanylyl cyclase nor mimicked by a cyclic guanosine monophosphate (cGMP) analog. Furthermore, SNP did not inhibit phosphate uptake or inhibit apoptosis in ATDC5 cells. These findings indicate that SNP can specifically inhibit matrix mineralization via a cGMP-independent pathway and that the effect is not mediated by inhibition of phosphate transport or apoptosis. These results suggest a preventive role of NO in premature or pathological mineralization.

Luteinizing hormone inhibits Fas-induced apoptosis in ovarian surface epithelial cell lines

Gonadotrophins including LH have been suggested to play an important role in the etiology of epithelial ovarian cancers. The goal of the present study was to obtain more insight in the mechanism of gonadotrophin action on ovarian surface epithelium (OSE) cells. As the Fas system is known to be a major player in the regulation of the process of apoptosis in the ovary, we investigated whether LH interfered with Fas-induced apoptosis in the human OSE cancer cell lines HEY and Caov-3. Activation of Fas receptor by an agonistic anti-Fas receptor antibody induced apoptosis, as was evaluated by caspase-3 activation, poly(ADP-ribose) polymerase fragmentation, phosphatidylserine externalization and morphological changes characteristic of apoptosis. Co-treatment with LH reduced the number of apoptotic cells following activation of Fas in a transient manner, while LH by itself did not affect apoptosis or cell proliferation. The anti-apoptotic effect of LH could be mimicked by the membrane-permeable cAMP analog 8-(4-chlorophenylthio) cAMP (8-CPT-cAMP), and blocked by H89, a specific inhibitor of protein kinase A (PKA). In conclusion, these findings suggest that LH protects HEY cells against Fas-induced apoptosis through a signaling cascade involving PKA. Although it is plausible that in vivo LH might also enhance OSE tumor growth through inhibition of apoptosis, further research is necessary to confirm this hypothesis.

Molecular properties and biological functions of cGMP-dependent protein kinase II

Type II cGMP-dependent protein kinase (cGK II) is the protein product of one of two genes coding for cGKs in mammalian genomes. cGK II has a domain structure similar to cGK I (alpha or beta) consisting of an N-terminal regulatory domain, which contains a dimerization and an autoinhibitory region, two cGMP-binding domains and a C-terminal catalytic domain. However, the position of the high and low affinity cGMP-binding-domains in cGK II are reversed in comparison to cGK I. Moreover, the isoenzymes exhibit a different affinity towards various membrane permeable cGMP-analogs, allowing differentiation between the cGKs. Type II cGK is bound to the membrane by a myristoyl moiety. It has a distinct function and an expression pattern distinct from that of cGKI, being expressed predominantly in intestine, brain, and kidney. It is involved in regulating electrolyte and water secretion by epithelial tissues in response to the luminocrinic hormones guanylin and uroguanylin and in the secretory diarrhea provoked by heat-stable enterotoxins. Type II cGK also plays a role in the regulation of endochondral ossification by C-type natriuretic peptide, in renin secretion by the kidney, aldosterone secretion by the adrenal, and in the adjustment of the biological clock.

Induction of CCAAT/Enhancer-binding Protein (C/EBP)-homologous Protein/Growth Arrest and DNA Damage-inducible Protein 153 Expression during Inhibition of Phosphatidylcholine Synthesis Is Mediated via Activation of a C/EBP-activating Transcription Factor-responsive Element

The gene for the proapoptotic transcription factor CCAAT/enhancer-binding protein (C/EBP)-homologous protein/growth arrest and DNA damage-inducible protein 153 (CHOP/GADD153) is induced by various cellular stresses. Previously, we described that inhibition of phosphatidylcholine (PC) synthesis in MT58 cells, which contain a temperature-sensitive mutation in CTP:phosphocholine cytidylyltransferase (CT), results in apoptosis preceded by the induction of CHOP. Here we report that prevention of CHOP induction, by expression of antisense CHOP, delays the PC depletion-induced apoptotic process. By mutational analysis of the conserved region in the promoter of the CHOP gene, we provide evidence that the C/EBP-ATF composite site, but not the ER stress-responsive element or the activator protein-1 site, is required for the increased expression of CHOP during PC depletion. Inhibition of PC synthesis in MT58 cells also led to an increase in phosphorylation of the stress-related transcription factor ATF2 and the stress kinase JNK after 8 and 16 h, respectively. In contrast, no phosphorylation of p38 MAPK was observed in MT58 cultured at the nonpermissive temperature. Treatment of MT58 cells with the JNK inhibitor SP600125 could rescue the cells from apoptosis but did not inhibit the phosphorylation of ATF2 or the induction of CHOP. Taken together, our results suggest that increased expression of CHOP during PC depletion depends on a C/EBP-ATF element in its promoter and might be mediated by binding of ATF2 to this element.

Hexadecylphosphocholine causes rapid cell death in canine mammary tumour cells

Hexadecylphosphocholine (HePC, Miltefosine) is an antitumour phospholipid and known inducer of apoptosis in human breast cancer cells. The mechanism underlying the induction of cell death by HePC, however, is not clear yet. In this study, we have investigated the cytotoxic effects of HePC on canine mammary tumour cells (CMTs) in vitro. Upon addition of HePC, CMTs rapidly exhibited several features that resembled apoptotic cell death. Cells showed externalization of phosphatidylserine, a hallmark of apoptosis, within 5 min after addition of HePC at concentrations as low as 10 microM. Furthermore, rapid swelling of mitochondria was observed. Rounding and detachment of cells followed within 30 min. However, fragmentation of nuclear DNA could not be observed. Overall, HePC was shown to induce a type of cell death in CMTs that in some aspects resembles apoptosis, though the process proceeds much more rapidly than reported for other tumour cell lines.

Control of the CDPethanolamine pathway in mammalian cells: effect of CTP:phosphoethanolamine cytidylyltransferase overexpression and the amount of intracellular diacylglycerol

For an insight regarding the control of PtdEtn (phosphatidylethanolamine) synthesis via the CDPethanolamine pathway, rat liver cDNA encoding ECT (CTP:phosphoethanolamine cytidylyltransferase) was transiently or stably transfected in Chinese-hamster ovary cells and a rat liver-derived cell line (McA-RH7777), resulting in a maximum of 26- and 4-fold increase in specific activity of ECT respectively. However, no effect of ECT overexpression on the rate of [3H]ethanolamine incorporation into PtdEtn was detected in both cell lines. This was explored further in cells overexpressing four times ECT activity (McA-ECT1). The rate of PtdEtn breakdown and PtdEtn mass were not changed in McA-ECT1 cells in comparison with control-transfected cells. Instead, an accumulation of CDPethanolamine (label and mass) was observed, suggesting that in McA-ECT1 cells the ethanolaminephosphotransferase-catalysed reaction became rate-limiting. However, overexpression of the human choline/ethanolaminephosphotransferase in McA-ECT1 and control-transfected cells had no effect on PtdEtn synthesis. To investigate whether the availability of DAG (diacylglycerol) limited PtdEtn synthesis in these cells, intracellular DAG levels were increased using PMA or phospholipase C. Exposure of cells to PMA or phospholipase C stimulated PtdEtn synthesis and this effect was much more pronounced in McA-ECT1 than in control-transfected cells. In line with this, the DAG produced after PMA exposure was consumed more rapidly in McA-ECT1 cells and the CDPethanolamine level decreased accordingly. In conclusion, our results suggest that the supply of CDPethanolamine, via the expression level of ECT, is an important factor governing the rate of PtdEtn biosynthesis in mammalian cells, under the condition that the amount of DAG is not limiting.

Surfactant protein D binding to terminal alpha1-3-linked fucose residues and to Schistosoma mansoni

Pulmonary surfactant protein (SP)-D is an important component of the innate immune system of the lung, which is thought to function by binding to specific carbohydrates on the surface of viruses and unicellular pathogens. SP-D has been shown to have a relatively high affinity for the monosaccharides mannose, glucose, and fucose. However, there is limited information on SP-D binding to complex carbohydrate structures, and binding of SP-D to fucose in the context of an oligosaccharide has not yet been investigated. In this study, we used surface plasmon resonance spectroscopy to examine the potential of SP-D to bind to various synthetic fucosylated oligosaccharides, and identified Fucalpha1-3GalNAc and Fucalpha1-3GlcNAc elements as strong ligands. These types of fucosylated glycoconjugates are presented at the surface of Schistosoma mansoni, a parasitic worm that, during development, transiently resides in the lung. In line with the findings by surface plasmon resonance, we found that SP-D can bind to larval stages of S. mansoni, demonstrating for the first time that SP-D interacts with multicellular lung pathogens.

Inhibition of phosphatidylcholine synthesis is not the primary pathway in hexadecylphosphocholine-induced apoptosis

The anticancer drug hexadecylphosphocholine (HePC), an alkyl-lysophospholipid analog (ALP), has been shown to induce apoptosis and inhibit the synthesis of phosphatidylcholine (PC) in a number of cell lines. We investigated whether inhibition of PC synthesis plays a major causative role in the induction of apoptosis by HePC. We therefore directly compared the apoptosis caused by HePC in CHO cells to the apoptotic process in CHO-MT58 cells, which contain a genetic defect in PC synthesis. HePC-provoked apoptosis was found to differ substantially from the apoptosis observed in MT58 cells, since it was (i) not accompanied by a large decrease in the amount of PC and diacylglycerol (DAG), (ii) not preceded by induction of the pro-apoptotic protein GADD153/CHOP, and (iii) not dependent on the synthesis of new proteins. Furthermore, lysoPC as well as lysophosphatidylethanolamine (lysoPE) could antagonize the apoptosis induced by HePC, whereas only lysoPC was able to rescue MT58 cells. HePC also induced a rapid externalisation of phosphatidylserine (PS). These observations suggest that inhibition of PC synthesis is not the primary pathway in HePC-induced apoptosis.

Aggregation of Cryptococcus neoformans by surfactant protein D is inhibited by its capsular component glucuronoxylomannan

Cryptococcus neoformans is an opportunistic pathogen invading the immunocompromised host. Infection starts with the inhalation of acapsular or sparsely encapsulated cells, after which capsule synthesis is initiated. The capsule is the main virulence factor of this yeast-like fungus. Pulmonary surfactant protein D (SP-D) is an important component of the local innate defense system. In the present study, interactions of SP-D with intact C. neoformans cells and their isolated capsular components were investigated. Although encapsulated cryptococci were bound, SP-D showed the highest affinity for acapsular C. neoformans. Only acapsular cryptococci were aggregated by SP-D. Furthermore, the cryptococcal capsular components glucuronoxylomannan (GXM) and mannoprotein 1 (MP1) were bound with relatively high affinity, in contrast to GalXM and MP2. Binding as well as aggregation of acapsular C. neoformans by SP-D could be inhibited by GXM in concentrations that are likely to be present in the lung after infection, suggesting that not only the capsule hampers SP-D function within the innate defense system of the lung but also the secreted capsular component GXM.

Interactions of influenza A virus with sialic acids present on porcine surfactant protein D

Pigs can be infected with both human and avian influenza A virus (IAV) strains and are therefore considered to be important intermediates in the emergence of new IAV strains due to mixing of viral genes derived from human, avian, or porcine influenza viruses. These reassortant strains may have potential to cause pandemic influenza outbreaks in humans. The innate immune response against IAV plays a significant role in containment of IAV in the airways. We studied the interactions of IAV with porcine surfactant protein D (pSP-D), an important component of this first line defense system. Hemagglutination inhibition analysis shows that the distinct interactions of pSP-D with IAV mediated by the N-linked carbohydrate moiety in the carbohydrate recognition domain of pSP-D depend on the terminal sialic acids (SAs) present on this carbohydrate. Analysis by both lectin staining and by cleavage with linkage-specific sialidases shows that the carbohydrate of pSP-D is exclusively sialylated with alpha(2,6)-linked SAs, in contrast to surfactant protein A, which contains both alpha(2,3)- and alpha(2,6)-linked SAs on its N-linked carbohydrate. Enzymatic modification of the SA-linkages present on pSP-D demonstrates that the type of SA-linkage is important for its hemagglutination-inhibitory activity, and correlates with receptor-binding specificity of the IAV strains. The SAs present on pSP-D appear especially important for interactions with poorly glycosylated IAV strains. It remains to be elucidated to what extent the unique sialylation profile of pSP-D is involved in host range control of IAV in pigs, and whether it facilitates adaptation of avian or human IAV strains that can contribute to the production of reassortant strains in pigs.

Porcine pulmonary collectins show distinct interactions with influenza A viruses: role of the N-linked oligosaccharides in the carbohydrate recognition domain

Influenza A virus (IAV) infections are a major cause of respiratory disease of humans and animals. Pigs can serve as important intermediate hosts for transmission of avian IAV strains to humans, and for the generation of reassortant strains; this may result in the appearance of new pandemic IAV strains in humans. We have studied the role of the porcine lung collectins surfactant proteins D and A (pSP-D and pSP-A), two important components of the innate immune response against IAV. Hemagglutination inhibition assays revealed that both pSP-D and pSP-A display substantially greater inhibitory activity against IAV strains isolated from human, swine, and horse, than lung collectins from other animal species. The more potent activity of pSP-D results from interactions mediated by the asparagine-linked oligosaccharide located in the carbohydrate recognition domain of pSP-D, which is absent in SP-Ds from other species characterized to date. Presence of this sialylated oligosaccharide moiety enhances the anti-influenza activity of pSP-D, as demonstrated by assays of viral aggregation, inhibition of infectivity, and neutrophil response to IAV. The greater hemagglutination inhibitory activity of pSP-A is due to porcine-specific structural features of the conserved asparagine-linked oligosaccharide in the carbohydrate recognition domain of SP-A. A more efficient lung collectin-mediated immune response against IAV in pigs may play a role in providing conditions by which pigs can act as "mixing vessel" hosts that can lead to the production of reassortant, pandemic strains of IAV.

Autophosphorylation of cGMP-dependent protein kinase type II

Cyclic nucleotides are shown to stimulate the autophosphorylation of type II cGMP-dependent protein kinase (cGK) on multiple sites. Mass spectrometric based analyses, using a quadrupole time-of-flight-mass spectrometry instrument revealed that cGMP stimulated the in vitro phosphorylation of residues Ser110 and Ser114, and, at a slow rate, of Ser126 and Thr109 or Ser117, all located in the autoinhibitory region. In addition Ser445 was found to be phosphorylated in a cGMP-dependent manner, whereas Ser110 and Ser97 were already prephosphorylated to a large extent in Sf9 cells. cGMP-dependent phosphorylation of cGK II was also demonstrated in intact COS-1 cells and intestinal epithelium. Substitution of most of the potentially autophosphorylated residues for alanines largely abolished the cGMP stimulation of the autophosphorylation. Prolonged autophosphorylation of purified recombinant cGK II in vitro resulted in a 40-50% increase in basal kinase activity, but its maximal cGMP-stimulated activity and the EC50 for cGMP remained unaltered. Mutation of the major phosphorylatable serines 110, 114, and 445 into "phosphorylation-mimicking" glutamates had no effect on the kinetic parameters of cGK II. However, replacing the slowly autophosphorylated residue Ser126 by Glu rendered cGK II constitutively active. These results show that the fast phase of cyclic nucleotide-stimulated autophosphorylation of cGK II has a relatively small feed forward effect on its activity, whereas the secondary phase, presumably involving Ser126 phosphorylation, may generate a constitutively active form of the enzyme.

The transmembrane domain of surfactant protein C precursor determines the morphology of the induced membrane compartment in CHO cells

Surfactant protein C (SP-C) is a small lipopeptide of which the main part consists of a typical valyl-rich transmembrane domain. The protein is expressed as a propeptide (proSP-C) which is processed and sorted via the regulated secretory pathway to the lamellar body, where mature SP-C is stored before secretion into the alveolar space. In this study we investigated the identity of the compartment to which proSP-C is sorted in cells that do not have a regulated secretory pathway, such as CHO cells. By electron microscopy we determined that proSP-C was localized in an uncommon membrane compartment with very regular morphology, which was not present in control cells. This membrane compartment is not influenced by the palmitoylation of proSP-C and is probably derived from the endoplasmic reticulum. However, proSP-C chimeras with artificial transmembrane domains induced a membrane compartment with a different morphology. Therefore we propose that the typical amino acid sequence of the transmembrane domain of proSP-C plays a role in membrane formation and morphology, which may be relevant under physiological conditions.

Inhibition of phosphatidylcholine synthesis induces expression of the endoplasmic reticulum stress and apoptosis-related protein CCAAT/enhancer-binding protein-homologous protein (CHOP/GADD153)

Inhibition of de novo synthesis of phosphatidylcholine (PC) by some anti-cancer drugs such as hexadecylphosphocholine leads to apoptosis in various cell lines. Likewise, in MT58, a mutant Chinese hamster ovary (CHO) cell line containing a thermo-sensitive mutation in CTP:phosphocholine cytidylyltransferase (CT), an important regulatory enzyme in the CDP-choline pathway, inhibition of PC synthesis causes PC depletion. Cellular perturbations like metabolic insults and unfolded proteins can be registered by the endoplasmic reticulum (ER) and result in ER stress responses, which can lead eventually to apoptosis. In this study we investigated the effect of PC depletion on the ER stress response and ER-related proteins. Shifting MT58 cells to the non-permissive temperature of 40 degrees C resulted in PC depletion via an inhibition of CT within 24 h. Early apoptotic features appeared in several cells around 30 h, and most cells were apoptotic within 48 h. The temperature shift in MT58 led to an increase of pro-apoptotic CCAAT/enhancer-binding protein-homologous protein (CHOP; also known as GADD153) after 16 h, to a maximum at 24 h. Incubation of wild-type CHO-K1 or CT-expressing MT58 cells at 40 degrees C did not induce differences in CHOP protein levels in time. In contrast, expression of the ER chaperone BiP/GRP78, induced by an increase in misfolded/unfolded proteins, and caspase 12, a protease specifically involved in apoptosis that results from stress in the ER, did not differ between MT58 and CHO-K1 cells in time when cultured at 40 degrees C. Furthermore, heat-shock protein 70, a protein that is stimulated by accumulation of abnormal proteins and heat stress, displayed similar expression patterns in MT58 and K1 cells. These results suggest that PC depletion in MT58 induces the ER-stress-related protein CHOP, without raising a general ER stress response.

Porcine surfactant protein D is N-glycosylated in its carbohydrate recognition domain and is assembled into differently charged oligomers

Surfactant protein D (SP-D) belongs to a subgroup of mammalian collagenous Ca(2+)-dependent lectins known as the collectins. It is thought to play a significant role in the innate immune response against microorganisms within the lungs and at other mucosal surfaces. This report documents the isolation and characterization of SP-D purified from porcine lung lavage using mannan affinity chromatography and gel filtration. Ultrastructural analysis shows both dodecameric and higher order oligomeric complexes of SP-D. The molecular mass of monomeric porcine SP-D (50 kD) is larger than that of SP-D from humans (43 kD). The difference in mass is due to the presence of an Asparagine-linked glycosylation in the carbohydrate recognition domain of porcine SP-D, which is absent in SP-D of other species investigated so far. Analysis of this carbohydrate moiety indicates that it is a highly heterogeneous, complex type oligosaccharide which is sialylated. The heterogeneity of oligosaccharide sialylation results in the existence of many differently charged porcine SP-D isoforms. The removal of the carbohydrate moiety reduces the inhibitory effect of porcine SP-D on influenza A virus haemagglutination. Therefore, the carbohydrate moiety may influence interactions with pathogens.

Structural requirements for palmitoylation of surfactant protein C precursor

Pulmonary surfactant protein C (SP-C) propeptide (proSP-C) is a type II transmembrane protein that is palmitoylated on two cysteines adjacent to its transmembrane domain. To study the structural requirements for palmitoylation of proSP-C, His-tagged human proSP-C and mutant forms were expressed in Chinese hamster ovary cells and analysed by metabolic labelling with [3H]palmitate. Mutations were made in the amino acid sequence representing mature SP-C, as deletion of the N- and C-terminal propeptide parts showed that this sequence by itself could already be palmitoylated. Substitution of the transmembrane domain by an artificial transmembrane domain had no effect on palmitoylation. However, an inverse correlation was found between palmitoylation of proSP-C and the number of amino acids present between the cysteines and the transmembrane domain. Moreover, substitution by alanines of amino acids localized on the N-terminal side of the cysteines had drastic effects on palmitoylation, probably as a result of the removal of hydrophobic amino acids. These data, together with the observation that substitution by alanines of the amino acids localized between the cysteines and the transmembrane domain had no effect on palmitoylation, suggest that the palmitoylation of proSP-C depends not on specific sequence motifs, but more on the probability that the cysteine is in the vicinity of the membrane surface. This is probably determined not only by the number of amino acids between the cysteines and the transmembrane domain, but also by the hydrophobic interaction of the N-terminus with the membrane. This may also be the case for the palmitoylation of other transmembrane proteins.

Differential effect of brefeldin A on the palmitoylation of surfactant protein C proprotein mutants

The surfactant protein C precursor (proSP-C) is palmitoylated on two cysteines adjacent to its transmembrane domain. We showed previously that palmitoylation of proSP-C occurs in a postendoplasmic reticulum compartment and is not affected by the Golgi-disturbing agent brefeldin A (BFA). In contrast, the investigations presented here showed that BFA almost completely abolished palmitoylation of proSP-C mutants that contained alterations in the region between the palmitoylated cysteines and the transmembrane domain, including a Pro 30 to Leu mutant associated with interstitial lung disease. This differential effect of BFA was not caused by differences in the palmitoylation kinetics between wild-type proSP-C and the mutants and was not mimicked by nocodazole and monensin. However, differences between the mutants and wild-type proSP-C in the relative degree of processing suggest that BFA may unmask a difference in routing. This would imply that the amino acids just N-terminal of the transmembrane domain may be important for a proper sorting of proSP-C.

Structure and function of the heat-stable enterotoxin receptor/guanylyl cyclase C

Guanylyl cyclase C (GC-C) was found to function as the principal receptor for heat-stable enterotoxins (STa), major causative factors in E. coli-induced secretory diarrhea. GC-C is enriched in intestinal epithelium, but was also detected in other epithelial tissues. The enzyme belongs to the family of receptor guanylyl cyclases, and consists of an extracellular receptor domain, a single transmembrane domain, a kinase homology domain, and a catalytic domain. GC-C is modified by N-linked glycosylation and, at least in the small intestine, by proteolysis, resulting in a STa receptor that is coupled non-covalently to the intracellular domain. So far two endogenous ligands of mammalian GC-C have been identified i.e. the small cysteine-rich peptides guanylin and uroguanylin. The guanylins are released in an auto- or paracrine fashion into the intestinal lumen but may also function as endocrine hormones in gut-kidney communication and as regulators of ion transport in extra-intestinal epithelia. They are thought to activate GC-C by inducing a conformational change in the extracellular portion of the homotrimeric GC-C complex, which allows two of the three intracellular catalytic domains to dimerize and form two active catalytic clefts. In the intestine, activation of GC-C results in a dual action: stimulation of Cl and HCO3 secretion, through the opening of apical CFTR Cl channels; and inhibition of Na absorption, through blockade of an apical Na/H exchanger. The principal effector of the GC-C effect on ion transport is cGMP dependent protein kinase type II, which together with GC-C and the ion transporters, may form a supramolecular complex at the apical border of epithelial cells.

The juxtamembrane lysine and arginine residues of surfactant protein C precursor influence palmitoylation via effects on trafficking

Surfactant protein (SP)-C propeptide (proSP-C) becomes palmitoylated on cysteines 5 and 6 before mature SP-C is formed by several proteolytic steps. To study the structural requirements for the palmitoylation of proSP-C, his-tagged human proSP-C (his-proSP-C) and his-proSP-C mutants were expressed in Chinese hamster ovary cells and analyzed by metabolic labeling with [(3)H]palmitate and immunocytochemistry. Substitution of cysteines 5 and 6 by serines showed that these were the only two cysteine residues palmitoylated in his-proSP-C. Substitution of the juxtamembrane basic residues lysine and arginine by uncharged glutamines led to a large decrease in palmitoylation level of proSP-C. The addition of brefeldin A nearly abolished this decrease for the lysine and double mutant; the palmitoylation of the arginine mutant increased also, but not to wild-type (WT) levels. Fluorescence immunocytochemistry showed that WT proSP-C was localized in punctate vesicles throughout the cell, whereas the mutant lacking the juxtamembrane positive charges was found more perinuclear, probably in the endoplasmic reticulum (ER). This indicates that the two basic juxtamembrane residues influence palmitoylation of proSP-C by preventing the transport of proSP-C out of the ER, implying that proSP-C becomes palmitoylated normally in a compartment distal to the ER.

Lung surfactant proteins A and D in innate immune defense

The lung surfactant proteins (SP) A and D are large multimeric proteins and belong to a family of collagenous C-type lectins designated collectins. Both SP-A and SP-D are believed to play a role in the innate immunity of the lung. SP-A and SP-D bind to a broad spectrum of pathogens, including bacteria, viruses, fungi and yeasts but also lipopolysaccharides and allergens. Furthermore, SP-A and SP-D enhance the clearing of various pathogens by neutrophils and macrophages in vitro. Recent in vivo studies on SP-A deficient mice also support a role of SP-A in host defense.

Differential role of cyclic GMP-dependent protein kinase II in ion transport in murine small intestine and colon

BACKGROUND & AIMS

The aim of this study was to determine the role of guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase (cGK) type II in intestinal fluid homeostasis under basal conditions and following exposure to cGMP-linked secretagogues, e.g., Escherichia coli heat-stable enterotoxin (STa) and guanylin.

METHODS

Fluid and ion transport was determined in different segments of the intestine of wild-type and cGK II-deficient mice by ligated loop assays in vivo, and by short-circuit current and isotope flux measurements in vitro.

RESULTS

Small intestinal fluid absorption in vivo was enhanced in cGK II-deficient mice under basal conditions and in the presence of STa. Furthermore, STa, guanylin, and 8-pCPT-cGMP stimulation of electrogenic anion secretion and inhibition of Na(+) absorption in vitro were markedly reduced in the small intestine from cGK II -/- mice but not in proximal colon. The type III phosphodiesterase inhibitor amrinone mimicked STa action in cGK II -/- mice, and also stimulated ion secretion in humans.

CONCLUSIONS

This study shows that the cGMP/cGK II pathway regulates fluid homeostasis in the small intestine under basal conditions and mediates STa effects by both increasing anion secretion and inhibiting Na(+) absorption. It also demonstrates the presence of a cGK II-independent pathway for STa/cGMP-provoked secretion predominantly in the colon, which possibly involves a cGMP-inhibitable phosphodiesterase and/or activation of the cAMP-dependent protein kinase pathway.

Nitric oxide and cGMP regulate gene expression in neuronal and glial cells by activating type II cGMP-dependent protein kinase

Nitric oxide (NO) and cGMP have been implicated in many neuronal functions, including regulation of gene expression, but little is known about the downstream targets of NO/cGMP in the nervous system. We found that type II cGMP-dependent protein kinase (G-kinase), which is widely expressed in the brain, mediated NO- and cGMP-induced activation of the fos promoter in cells of neuronal and glial origin; the enzyme was ineffective in regulating gene expression in fibroblast-like cells. The effect of G-kinase II on gene expression did not require calcium uptake but was synergistically enhanced by calcium. G-kinase II was membrane associated and did not translocate to the nucleus; however, a soluble G-kinase II mutant translocated to the nucleus and regulated gene expression in fibroblast-like cells. Soluble G-kinase I also regulates fos promoter activity, but membrane targeting of G-kinase I prevented the enzyme from translocating to the nucleus and regulating transcription in multiple cell types, including glioma cells; this suggests that cell type-specific factor(s) that mediate the transcriptional effects of extranuclear G-kinase II are not regulated by G-kinase I. Our results suggest that G-kinase I and II control gene expression by different mechanisms and that NO effects on neuronal plasticity may involve G-kinase II regulation of gene expression.-Gudi, T., Hong, G. K.-P., Vaandrager, A. B., Lohmann, S. M., Pilz, R. B. Nitric oxide and cGMP regulate gene expression in neuronal and glial cells by activating type II cGMP-dependent protein kinase.

Atrial natriuretic peptide-stimulated Ca2+ reabsorption in rabbit kidney requires membrane-targeted, cGMP-dependent protein kinase type II

Atrial natriuretic peptide (ANP) and nitric oxide (NO) are key regulators of ion and water transport in the kidney. Here, we report that these cGMP-elevating hormones stimulate Ca2+ reabsorption via a novel mechanism specifically involving type II cGMP-dependent protein kinase (cGK II). ANP and the NO donor, sodium nitroprusside (SNP), markedly increased Ca2+ uptake in freshly immunodissected rabbit connecting tubules (CNT) and cortical collecting ducts (CCD). Although readily increasing cGMP, ANP and SNP did not affect Ca2+ and Na+ reabsorption in primary cultures of these segments. Immunoblot analysis demonstrated that cGK II, and not cGK I, was present in freshly isolated CNT and CCD but underwent a complete down-regulation during the primary cell culture. However, upon adenoviral reexpression of cGK II in primary cultures, ANP, SNP, and 8-Br-cGMP readily increased Ca2+ reabsorption. In contrast, no cGMP-dependent effect on electrogenic Na+ transport was observed. The membrane localization of cGK II proved to be crucial for its action, because a nonmyristoylated cGK II mutant that was shown to be localized in the cytosol failed to mediate ANP-stimulated Ca2+ transport. The Ca2+-regulatory function of cGK II appeared isotype-specific because no cGMP-mediated increase in Ca2+ transport was observed after expression of the cytosolic cGK Ibeta or a membrane-bound cGK II/Ibeta chimer. These results demonstrate that ANP- and NO-stimulated Ca2+ reabsorption requires membrane-targeted cGK II.

Membrane targeting of cGMP-dependent protein kinase is required for cystic fibrosis transmembrane conductance regulator Cl- channel activation

A recently cloned isoform of cGMP-dependent protein kinase (cGK), designated type II, was implicated as the mediator of cGMP-provoked intestinal Cl- secretion based on its localization in the apical membrane of enterocytes and on its capacity to activate cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels. In contrast, the soluble type I cGK was unable to activate CFTR in intact cells, although both cGK I and cGK II could phosphorylate CFTR in vitro. To investigate the molecular basis for the cGK II isotype specificity of CFTR channel gating, we expressed cGK II or cGK I mutants possessing different membrane binding properties by using adenoviral vectors in a CFTR-transfected intestinal cell line, and we examined the ability of cGMP to phosphorylate and activate the Cl- channel. Mutation of the cGK II N-terminal myristoylation site (Gly2 --> Ala) reduced cGK II membrane binding and severely impaired cGK II activation of CFTR. Conversely, a chimeric protein, in which the N-terminal membrane-anchoring domain of cGK II was fused to the N terminus of cGK Ibeta, acquired the ability to associate with the membrane and activate the CFTR Cl- channel. The potency order of cGK constructs for activation of CFTR (cGK II > membrane-bound cGK I chimer >> nonmyristoylated cGK II > cGK Ibeta) correlated with the extent of 32P incorporation into CFTR observed in parallel measurements. These results strongly support the concept that membrane targeting of cGK is a major determinant of CFTR Cl- channel activation in intact cells.

Distinct and specific functions of cGMP-dependent protein kinases

cGMP-dependent protein kinases I and II conduct signals from widespread signaling systems. Whereas the type I kinase mediates numerous effects of natriuretic peptides and nitric oxide in cardiovascular cells, the type II kinase transduces signals from the Escherichia coli heat-stable enterotoxin, STa, and from the endogenous intestinal peptide, guanylin, stimulating Cl- conductance of the cystic fibrosis transmembrane conductance regulator (CFTR). Although the two kinases may be interchangeable for several functions, CFTR regulation specifically requires the type II kinase.

Endogenous type II cGMP-dependent protein kinase exists as a dimer in membranes and can Be functionally distinguished from the type I isoforms

In mammalian tissues two types of cGMP-dependent protein kinase (cGK) have been identified. In contrast to the dimeric cGK I, cGK II purified from pig intestine was shown previously to behave as a monomer. However, recombinant rat cGK II was found to have hydrodynamic parameters indicative of a homodimer. Chemical cross-linking studies showed that pig cGK II in intestinal membranes has a dimeric structure as well. However, after purification, cGK II was found to be partly proteolyzed into C-terminal monomeric fragments. Phosphorylation studies in rat intestinal brush borders revealed that the potency of cGMP analogs to stimulate or inhibit native cGK II in vitro (i.e. 8-(4-chlorophenylthio)-cGMP > cGMP > beta-phenyl-1,N2-etheno-8-bromo-cGMP > beta-phenyl-1,N2-etheno-cGMP and Rp-8-(4-chlorophenylthio)-cGMPs > Rp-beta-phenyl-1, N2-etheno-8-bromo-cGMPs, respectively) correlated well with their potency to stimulate or inhibit cGK II-mediated Cl- secretion across intestinal epithelium but differed strikingly from their potency to affect cGK I activity. These data show that the N terminus of cGK II is involved in dimerization and that endogenous cGK II displays a distinct activation/inhibition profile with respect to cGMP analogs, which permits a pharmacological dissection between cGK II- and cGK I-mediated physiological processes.

cGMP stimulation of cystic fibrosis transmembrane conductance regulator Cl- channels co-expressed with cGMP-dependent protein kinase type II but not type Ibeta

In order to investigate the involvement of cGMP-dependent protein kinase (cGK) type II in cGMP-provoked intestinal Cl- secretion, cGMP-dependent activation and phosphorylation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels was analyzed after expression of cGK II or cGK Ibeta in intact cells. An intestinal cell line which stably expresses CFTR (IEC-CF7) but contains no detectable endogenous cGK II was infected with a recombinant adenoviral vector containing the cGK II coding region (Ad-cGK II) resulting in co-expression of active cGK II. In these cells, CFTR was activated by membrane-permeant analogs of cGMP or by the cGMP-elevating hormone atrial natriuretic peptide as measured by 125I- efflux assays and whole-cell patch clamp analysis. In contrast, infection with recombinant adenoviruses expressing cGK Ibeta or luciferase did not convey cGMP sensitivity to CFTR in IEC-CF7 cells. Concordant with the activation of CFTR by only cGK II, infection with Ad-cGK II but not Ad-cGK Ibeta enabled cGMP analogs to increase CFTR phosphorylation in intact cells. These and other data provide evidence that endogenous cGK II is a key mediator of cGMP-provoked activation of CFTR in cells where both proteins are co-localized, e. g. intestinal epithelial cells. Furthermore, they demonstrate that neither the soluble cGK Ibeta nor cAMP-dependent protein kinase are able to substitute for cGK II in this cGMP-regulated function.

Guanosine 3',5'-cyclic monophosphate-dependent protein kinase II mediates heat-stable enterotoxin-provoked chloride secretion in rat intestine

BACKGROUND & AIMS

Escherichia coli heat-stable enterotoxins (STa) provoke electrogenic Cl- secretion in the intestine through a guanosine 3',5'-cyclic monophosphate (cGMP)-dependent signal transduction pathway. The cGMP receptor involved in the activation of the Cl- channel is not known with certainty but may comprise either adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (cAK) or cGMP-dependent protein kinase (cGK) type II. The aim of this study was to discriminate between these possibilities using specific kinase inhibitors.

METHODS

Intestinal electrogenic Cl- secretion was determined by measuring short-circuit current (Isc) in a Ussing chamber.

RESULTS

The general protein kinase inhibitors staurosporine and H-8 inhibited rat cGK II activity in vitro with 50% inhibitory concentration values of 4 nmol/L and 3 mumol/L, respectively, which are lower than those reported for cAK. Both staurosporine and H-8, when added to rat proximal colon at concentrations that did not affect the Isc response to 8-bromo-cAMPS, inhibited the STa- and 8-bromo-cGMP-provoked Isc response for more than 80%. Furthermore, the relative specific cGK inhibitor Rp isomer of 8-(chlorophenylthio)-cGMP, but not the cAK inhibitor RP isomer of (Rp) 8-bromo-cAMPS, inhibited the Isc response to submaximal levels of STa in rat proximal colon.

CONCLUSIONS

These data provide further evidence for an important role of cGK II in STa-mediated Cl- secretion in native rat intestinal epithelium.