Fecal Ammonium in mice with CKD: gastrointestinal sequestration by SZC

Background: Urinary Ammonium (NH₄⁺) excretion is decreased in CKD but very little is known about fecal NH₄⁺excretion. Sodium Zirconium Cyclosilicate (SZC) is a cation-exchanger that selectively captures K⁺ in the gastrointestinal tract. We investigated if SZC can sequester NH₄⁺ in vivo and evaluated the effect of SZC on fecal NH₄⁺ in a mouse model of CKD. Methods: Mice with CKD induced by 5/6 kidney ablation were fed either a regular diet or a diet containing SZC (4g/Kg) and followed for 7 days. Feces NH₄⁺ was measured before and after addition of 50 mEq KCl/L to release NH₄⁺ from SZC. NH₄⁺ sequestered in SZC in the gastrointestinal tract was estimated from the delta fecal NH₄⁺observed when KCl was added to liberate the sequestered NH₄⁺. Results: In mice with CKD fecal NH₄⁺ excretion was higher than in normal mice and also higher than urine NH₄⁺ excretion measured concurrently. Using data pooled from the SZC diet, the delta NH₄⁺ was 6.5 ± 0.6 as compared to 0.6 ± 0.6 µmol/g on the normal diet p<0.0001. Conclusions:Fecal NH₄⁺ excretion in CKD is increased and about 6-fold higher than urine NH₄⁺ excretion revealing an important route of elimination of NH₄⁺ present in the GI tract. SZC administration sequesters a substantial portion of NH₄⁺ in the GI tract suggesting that the binding of NH₄⁺ offers therapeutic potential beyond its known primary action as a specific K⁺ binder.

Effect of AZD9977 and spironolactone on serum potassium in heart failure with preserved or mildly reduced ejection fraction, and renal impairment: A randomized trial

This phase Ib study compared the effects of AZD9977, a selective mineralocorticoid receptor modulator with predicted low hyperkalemia risk, with spironolactone on serum potassium (sK⁺ ) in patients with heart failure (HF) with preserved or mildly reduced ejection fraction (EF; ≥40%), and renal impairment. Patients with HF with EF greater than or equal to 40% and estimated glomerular filtration rate of 40-70 ml/min/1.73 m² were randomized to once-daily AZD9977 100 mg or spironolactone 25 mg for 14 days, up-titrated to AZD9977 200 mg or spironolactone 50 mg for another 14 days. The primary end point was relative change (%) in sK⁺ for AZD9977 versus spironolactone (baseline to day 28). Serum/urinary electrolytes, fractional excretion (FE) of Na⁺ /K⁺ , plasma aldosterone, cortisol, and renin, and safety were also assessed. Sixty-eight patients were randomized (AZD9977, n = 33; spironolactone, n = 35). Mean (SD) age was 73.0 (8.5) years, 51.5% men. Mean sK⁺ change from baseline to day 28 was 5.7% (AZD9977) and 4.2% (spironolactone), and 1.5% and 4.2% at day 14. Relative change (95% confidence interval) in sK⁺ with AZD9977 versus spironolactone was -0.3% (-5.3% to 4.4%; day 28), and 3.4% (-0.8% to 7.5%; day 14). Median increase from baseline in plasma aldosterone at day 28 was 89.8 pmol/L for AZD9977 and 67.4 pmol/L for spironolactone. Median FE of K⁺ was 12.9% (AZD9977) and 10.1% (spironolactone). AZD9977 was well-tolerated. No discontinuations due to hyperkalemia occurred with either treatment. Evidence of target engagement for AZD9977 with a favorable safety profile, supports further evaluation of AZD9977 in patients with HF and renal impairment.

Bilirubin levels and kidney function decline: An analysis of clinical trial and real world data

Objective

To evaluate if previously found associations between low serum bilirubin concentration and kidney function decline is independent of hemoglobin and other key confounders.

Research design and methods

Clinical trial data from the SAVOR-TIMI 53 trial as well as the UK primary care electronic healthcare records, Clinical Practice Research Datalink (CPRD), were used to construct three cohorts of patients at risk of chronic kidney disease (CKD). The randomized clinical trial (RCT) cohort from the subset of SAVOR-TIMI 53 trial consisted of 10,555 type-2 diabetic patients with increased risk of cardiovascular disease. The two observational data cohorts from CPRD consisted of 71,104 newly diagnosed type-2 diabetes (CPRD-DM2) and 82,065 newly diagnosed hypertensive (CPRD-HT) patients without diabetes. Cohorts were stratified according to baseline circulating total bilirubin levels to determine association on the primary end point of a 30% reduction from baseline in estimated glomerular filtration rate (eGFR) and the secondary end point of albuminuria.

Results

The confounder adjusted hazard ratios of the subpopulation with lower than median bilirubin levels compared to above median bilirubin levels for the primary end point were 1.18 (1.02–1.37), 1.12 (1.05–1.19) and 1.09 (1.01–1.17), for the secondary end point were 1.26 (1.06–1.52), 1.11 (1.01–1.21) and 1.18 (1.01–1.39) for SAVOR-TIMI 53, CPRD-DM2, CPRD-HT, respectively.

Conclusion

Our findings are consistent across all cohorts and endpoints: lower serum bilirubin levels are associated with a greater kidney function decline independent of hemoglobin and other key confounders. This suggests that increased monitoring of kidney health in patients with lower bilirubin levels may be considered, especially for diabetic patients.

Discovery by Virtual Screening of an Inhibitor of CDK5-Mediated PPARγ Phosphorylation

Thiazolidinedione PPARγ agonists such as rosiglitazone and pioglitazone are effective antidiabetic drugs, but side effects have limited their use. It has been posited that their positive antidiabetic effects are mainly mediated by the inhibition of the CDK5-mediated Ser273 phosphorylation of PPARγ, whereas the side effects are linked to classical PPARγ agonism. Thus compounds that inhibit PPARγ Ser273 phosphorylation but lack classical PPARγ agonism have been sought as safer antidiabetic therapies. Herein we report the discovery by virtual screening of 10, which is a potent PPARγ binder and in vitro inhibitor of the CDK5-mediated phosphorylation of PPARγ Ser273 and displays negligible PPARγ agonism in a reporter gene assay. The pharmacokinetic properties of 10 are compatible with oral dosing, enabling preclinical in vivo testing, and a 7 day treatment demonstrated an improvement in insulin sensitivity in the ob/ob diabetic mouse model.

Mineralocorticoid Receptor Antagonism Prevents the Synergistic Effect of Metabolic Challenge and Chronic Kidney Disease on Renal Fibrosis and Inflammation in Mice

Obesity and/or metabolic diseases are frequently associated with chronic kidney disease and several factors associated with obesity may contribute to proteinuria and extracellular matrix production. Mineralocorticoid receptor antagonists have proven their clinical efficacy in diabetic kidney disease with preclinical data suggesting that they may also be efficient in non-diabetic chronic kidney disease associated to metabolic diseases. In the present study we developed a novel mouse model combining severe nephron reduction and High Fat Diet challenge that led to chronic kidney disease with metabolic alterations. We showed that the Mineralocorticoid Receptor antagonist canrenoate improved metabolic function, reduced albuminuria and prevented the synergistic effect of high fat diet on renal fibrosis and inflammation in chronic kidney disease mice.

Electrolyte handling in the isolated perfused rat kidney: demonstration of vasopressin V2-receptor-dependent calcium reabsorption

BACKGROUND

The most profound effect of vasopressin on the kidney is to increase water reabsorption through V₂-receptor (V₂R) stimulation, but there are also data suggesting effects on calcium transport. To address this issue, we have established an isolated perfused kidney model with accurate pressure control, to directly study the effects of V₂R stimulation on kidney function, isolated from systemic effects.

METHODS

The role of V₂R in renal calcium handling was studied in isolated rat kidneys using a new pressure control system that uses a calibration curve to compensate for the internal pressure drop up to the tip of the perfusion cannula.

RESULTS

Kidneys subjected to V₂R stimulation using desmopressin (DDAVP) displayed stable osmolality and calcium reabsorption throughout the experiment, whereas kidneys not administered DDAVP exhibited a simultaneous fall in urine osmolality and calcium reabsorption. Epithelial sodium channel (ENaC) inhibition using amiloride resulted in a marked increase in potassium reabsorption along with decreased sodium reabsorption.

CONCLUSIONS

A stable isolated perfused kidney model with computer-controlled pressure regulation was developed, which retained key physiological functions. The preparation responds to pharmacological inhibition of ENaC channels and activation of V₂R. Using the model, the dynamic effects of V₂R stimulation on calcium handling and urine osmolality could be visualised. The study thereby provides evidence for a stimulatory role of V₂R in renal calcium reabsorption.

P4420Is exercise-induced cardiac troponin release caused by skeletal muscle injury?

Background

Cardiac troponins (cTn) are highly sensitive and specific markers for cardiac injury and a key element in the diagnosis of acute coronary syndrome. Strenuous exercise is known to induce increases in cTn, but the causative factors remain ambiguous. It is also equivocal whether exercise induced skeletal muscle injury is associated with cTn elevation.

Purpose

The aim of this study was to identify independent predictors for the rise in cardiac troponin T (cTnT) and I (cTnI) concentration and to focus on the relationship between skeletal muscle injury measured by skeletal troponin I (skTnI) and cTn elevations after a marathon race in a large group of male recreational runners.

Methods

A total of 40 recreational runners participating in the marathon in our city were recruited. The study included baseline visit (prerace) and immediate post-race sampling.

Results

The post-marathon cTnT concentration rose above the reference limit in 38 (95%) participants and the detection limit for cTnI was exceeded in 34 (85%) participants. Similarly, a 10-fold increase in skTnI concentration was observed and elevated post-race values were seen in all participants. There was no significant correlation between the post-race cTnT or cTnT change and post-race skTnI (Spearman's rho = 0.249, p=0.122, rho = 0.285, p=0.074). However, post-race cTnI and change in cTnI were associated with post-race skTnI (rho = 0.404, p=0.01, rho = 0.460, p=0.003) and creatine kinase (r=0.368, p=0.019) concentration. Subjective exertion or self-reported muscle symptoms did not correlate with post-race cTnT, cTnI or skTnI levels.

Post-Race cTnT <40 Post-Race cTnT ≥40 p-value n=18 n=22 Age, years 53.3±12.2 44.0±11.9 0.002 Active training, years 12.0 (9.3) 17.0 (15.8) 0.190 Muscle symptoms 7 (38.9) 11 (52.4) 0.523 Creatinine kinase, ug/l 406 (137) 399 (319) 0.163 N-terminal proBNP ng/l 137±168 158±277 0.783 Skeletal Troponin I, ng/ml 28.6 (41) 56.7 (143) 0.199

Figure 1

Conclusions

Cardiac troponin became abnormal in almost all runners after marathon race. The exercise-induced rise in cardiac troponin I is related to simultaneous release of skeletal troponin I. The mechanism of this association remains uncertain, but clinicians should be cautious when interpreting post-exercise troponin levels without clinical symptoms and signs of myocardial ischemia.

The selective mineralocorticoid receptor modulator AZD9977 reveals differences in mineralocorticoid effects of aldosterone and fludrocortisone

Introduction:

AZD9977 is a novel mineralocorticoid receptor (MR) modulator, which in preclinical studies demonstrated organ protection without affecting aldosterone-regulated urinary electrolyte excretion. However, when tested in humans, using fludrocortisone as an MR agonist, AZD9977 exhibited similar effects on urinary Na⁺/K⁺ ratio as eplerenone. The aim of this study is to understand whether the contradictory results seen in rats and humans are due to the mineralocorticoid used.

Materials and methods:

Rats were treated with single doses of AZD9977 or eplerenone in combination with either aldosterone or fludrocortisone. Urine was collected for five to six hours and total amounts excreted Na⁺ and K⁺ were assessed.

Results:

AZD9977 dose-dependently increased urinary Na⁺/K⁺ ratio in rats when tested against fludrocortisone, but not when tested against aldosterone. Eplerenone dose-dependently increased urinary Na⁺/K⁺ ratio when tested against fludrocortisone as well as aldosterone.

Conclusions:

The data suggest that the contrasting effects of AZD9977 on urinary electrolyte excretion observed in rats and humans are due to the use of the synthetic mineralocorticoid fludrocortisone. Future clinical studies are required to confirm the reduced electrolyte effects of AZD9977 and the subsequent lower predicted hyperkalemia risk.

Molecular tuning of farnesoid X receptor partial agonism

The bile acid-sensing transcription factor farnesoid X receptor (FXR) regulates multiple metabolic processes. Modulation of FXR is desired to overcome several metabolic pathologies but pharmacological administration of full FXR agonists has been plagued by mechanism-based side effects. We have developed a modulator that partially activates FXR in vitro and in mice. Here we report the elucidation of the molecular mechanism that drives partial FXR activation by crystallography- and NMR-based structural biology. Natural and synthetic FXR agonists stabilize formation of an extended helix α11 and the α11-α12 loop upon binding. This strengthens a network of hydrogen bonds, repositions helix α12 and enables co-activator recruitment. Partial agonism in contrast is conferred by a kink in helix α11 that destabilizes the α11-α12 loop, a critical determinant for helix α12 orientation. Thereby, the synthetic partial agonist induces conformational states, capable of recruiting both co-repressors and co-activators leading to an equilibrium of co-activator and co-repressor binding.

Na restriction activates epithelial Na channels in rat kidney through two mechanisms and decreases distal Na+ delivery

KEY POINTS

Dietary Na restriction, through the mineralocorticoid aldosterone, acts on epithelial Na channels via both fast (24 h) and slow (5-7 days) mechanisms in the kidney. The fast effect entails increased proteolytic processing and trafficking of channel protein to the apical membrane. It is rapidly reversible by the mineralocorticoid receptor antagonist eplerenone and is largely lost when tubules are studied ex vivo. The slow effect does not require increased processing or surface expression, is refractory to acute eplerenone treatment, and is preserved ex vivo. Both slow and fast effects contribute to Na retention in vivo. Increased Na⁺ reabsorption in the proximal tubule also promotes Na conservation under conditions of chronic dietary Na restriction, reducing Na⁺ delivery to the distal nephron.

ABSTRACT

Changes in the activity of the epithelial Na channel (ENaC) help to conserve extracellular fluid volume. In rats fed a low-salt diet, proteolytic processing of ENaC increased within 1 day, and was almost maximal after 3 days. The rapid increase in the abundance of cleaved αENaC and γENaC correlated with decreased urinary Na⁺ excretion and with increased ENaC surface expression. By contrast, ENaC activity, measured ex vivo in isolated cortical collecting ducts, increased modestly after 3 days and required 5 days to reach maximal levels. The mineralocorticoid receptor antagonist eplerenone reversed the increase in cleaved γENaC and induced natriuresis after 1 or 3 days but failed to alter either ENaC currents or Na⁺ excretion after 7 days of Na restriction. We conclude that Na depletion, through aldosterone, stimulates ENaC via independent fast and slow mechanisms. In vivo, amiloride-induced natriuresis increased after 1 day of Na depletion. By contrast, hydrochlorothiazide (HCTZ)-induced natriuresis decreased gradually over 7 days, consistent with increased ability of ENaC activity to compensate for decreased Na⁺ reabsorption in the distal convoluted tubule. Administration of amiloride and HCTZ together increased Na⁺ excretion less in Na-depleted compared to control animals, indicating decreased delivery of Na⁺ to the distal nephron when dietary Na is restricted. Measurements of creatinine and Li⁺ clearances indicated that increased Na reabsorption by the proximal tubules is responsible for the decreased delivery. Thus, Na conservation during chronic dietary salt restriction entails enhanced transport by both proximal and distal nephron segments.

Clinical safety, tolerability, pharmacokinetics and effects on urinary electrolyte excretion of AZD9977, a novel, selective mineralocorticoid receptor modulator

AIMS

AZD9977 is the first mineralocorticoid receptor modulator in clinical development exerting similar organ protection as eplerenone with minimal urinary electrolyte effects in preclinical studies. The aim was to perform the initial clinical assessment of AZD9977.

METHODS

A first-in-human trial explored doses from 5 to 1200 mg. To study effects on urinary electrolyte excretion an additional randomized placebo controlled cross-over four-period clinical trial was performed. Twenty-three healthy volunteers were administered fludrocortisone alone or in combination with AZD9977, eplerenone or both. AZD9977/eplerenone combination was given to assess if AZD9977 can attenuate eplerenone induced natriuresis.

RESULTS

AZD9977 at doses from 5 to 1200 mg was safe and well tolerated and pharmacokinetics were compatible with further development. AZD9977 exhibited similar effects on urinary ln [Na+ ]/[K+ ] as eplerenone when using fludrocortisone as mineralocorticoid receptor agonist, and the combination had an additive effect on ln [Na+ K+ ].

CONCLUSIONS

The results in man contradict the results in rodent models driven by aldosterone, in which AZD9977 has minimal electrolyte effects. Future clinical studies with AZD9977 should be performed in presence of endogenous or exogenous aldosterone to assess potential benefit of AZD9977 in patients.

Preclinical pharmacology of AZD9977: A novel mineralocorticoid receptor modulator separating organ protection from effects on electrolyte excretion

Excess mineralocorticoid receptor (MR) activation promotes target organ dysfunction, vascular injury and fibrosis. MR antagonists like eplerenone are used for treating heart failure, but their use is limited due to the compound class-inherent hyperkalemia risk. Here we present evidence that AZD9977, a first-in-class MR modulator shows cardio-renal protection despite a mechanism-based reduced liability to cause hyperkalemia. AZD9977 in vitro potency and binding mode to MR were characterized using reporter gene, binding, cofactor recruitment assays and X-ray crystallopgraphy. Organ protection was studied in uni-nephrectomised db/db mice and uni-nephrectomised rats administered aldosterone and high salt. Acute effects of single compound doses on urinary electrolyte excretion were tested in rats on a low salt diet. AZD9977 and eplerenone showed similar human MR in vitro potencies. Unlike eplerenone, AZD9977 is a partial MR antagonist due to its unique interaction pattern with MR, which results in a distinct recruitment of co-factor peptides when compared to eplerenone. AZD9977 dose dependently reduced albuminuria and improved kidney histopathology similar to eplerenone in db/db uni-nephrectomised mice and uni-nephrectomised rats. In acute testing, AZD9977 did not affect urinary Na+/K+ ratio, while eplerenone increased the Na+/K+ ratio dose dependently. AZD9977 is a selective MR modulator, retaining organ protection without acute effect on urinary electrolyte excretion. This predicts a reduced hyperkalemia risk and AZD9977 therefore has the potential to deliver a safe, efficacious treatment to patients prone to hyperkalemia.

Nephropathy in Pparg-null mice highlights PPARγ systemic activities in metabolism and in the immune system

Peroxisome proliferator-activated receptor γ (PPARγ) is a ligand-dependent transcription factor involved in many aspects of metabolism, immune response, and development. Total-body deletion of the two Pparg alleles provoked generalized lipoatrophy along with severe type 2 diabetes. Herein, we explore the appearance and development of structural and functional alterations of the kidney, comparing Pparg null-mice to their littermate controls (carrying Pparg floxed alleles). We show that renal hypertrophy and functional alterations with increased glucosuria and albuminuria are already present in 3 weeks-old Pparg null-mice. Renal insufficiency with decreased creatinine clearance progress at 7 weeks of age, with the advance of the type 2 diabetes. At 52 weeks of age, these alterations are accompanied by signs of fibrosis and mesangial expansion. More intriguingly, aged Pparg null-mice concomitantly present an anti-phospholipid syndrome (APS), characterized by the late appearance of microthrombi and a mesangioproliferative pattern of glomerular injury, associated with significant plasmatic levels of anti-β2- glycoprotein1 antibodies and renal deposition of IgG, IgM, and C3. Thus, in line with the role of PPARγ in metabolic homeostasis, Pparg null-mice first represent a potent model for studying the initiation and the development of diabetic nephropathy. Second, and in relation with the important PPARγ activity in inflammation and in immune system, these mice also highlight a new role for PPARγ signaling in the promotion of APS, a syndrome whose pathogenesis is poorly known and whose current treatment is limited to prevention of thrombosis events.

The peroxisome proliferator-activated receptor α agonist, AZD4619, induces alanine aminotransferase-1 gene and protein expression in human, but not in rat hepatocytes: Correlation with serum ALT levels

Alanine aminotransferase (ALT) in serum is the standard biomarker for liver injury. We have previously described a clinical trial with a novel selective peroxisome proliferator-activated receptor α (PPARα) agonist (AZD4619), which unexpectedly caused increased serum levels of ALT in treated individuals without any other evidence of liver injury. We pinpointed a plausible mechanism through which AZD4619 could increase serum ALT levels; namely through the PPARα-specific activation of the human ALT1 gene at the transcriptional level. In the present study, we present data from the preceding rat toxicity study, demonstrating that AZD4619 had no effect on rat serum ALT activity levels, and further experiments were performed to elucidate the mechanisms responsible for this species-related difference. Our results revealed that AZD4619 increased ALT1 protein expression in a dose-dependent manner in human, but not in rat primary hepatocytes. Cloning of the human and rat ALT1 promoters into luciferase vectors confirmed that AZD4619 induced only the human, but not the rat ALT1 gene promoter in a dose-dependent manner. In PPARα-GAL4 reporter gene assays, AZD4619 was >100-fold more potent on the human vs. rat PPARα levels, explaining the differences in induction of the ALT1 gene between the species at the concentration range tested. These data demonstrate the usefulness of the human and rat ALT1 reporter gene assays for testing future drug candidates at the preclinical stage. In drug discovery projects, these assays elucidate whether elevations in ALT levels observed in vivo or in the clinic are due to metabolic effects rather than a toxic event in the liver.

An additive effect of eplerenone to ACE inhibitor on slowing the progression of diabetic nephropathy in the db/db mice

Although blockade of the renin-angiotensin-system (RAS) has become standard therapy for diabetic nephropathy (DN), decline in kidney function towards end-stage renal disease is seen in many patients. Elevated plasma aldosterone often accompanies RAS blockade by a phenomenon known as "aldosterone escape" and activates the mineralocorticoid receptor (MR). We therefore examined whether addition of the MR antagonist eplerenone to an ACEI would enhance the efficacy in slowing the progression of DN. Untreated uninephrectomized diabetic db/db mice developed progressive albuminuria and glomerulosclerosis between weeks 18 and 22, associated with decreased number of podocytes and increased renal expression of fibrotic markers. The therapeutic effect of eplerenone at 100 mg/kg BW/d on albuminuria, podocyte injury and renal fibrosis was similar to that of enalapril given alone at maximally effective doses. Adding eplerenone to enalapril resulted in further reduction in these measurements. Renal expressions of TNF-α, MCP-1, Nox2 and p47phox and renal TBARS levels, markers of inflammation and oxidative stress, were increased during disease progression in diabetic mice, which were reduced by eplerenone or enalapril given alone and further reduced by the two drugs given in combination. However, there were no treatment related effects on plasma K+. Our results suggest that eplerenone is effective in slowing the progression of DN in db/db mice and that the effect is additive to an ACEI. The addition of an MR antagonist void of effects on plasma K+ to an ACEI may offer additional renoprotection in progressive DN via blocking the effects of aldosterone due to escape or diabetes-induction.

LT175 is a novel PPARα/γ ligand with potent insulin-sensitizing effects and reduced adipogenic properties

Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent transcription factors regulating lipid and glucose metabolism. Ongoing drug discovery programs aim to develop dual PPARα/γ agonists devoid of the side effects of the marketed antidiabetic agents thiazolidinediones and the dual agonists glitazars. Recently, we described a new dual PPARα/γ ligand, LT175, with a partial agonist profile against PPARγ and interacting with a newly identified region of the PPARγ-ligand binding domain (1). Here we show that LT175 differentially activated PPARγ target genes involved in fatty acid esterification and storage in 3T3-L1-derived adipocytes. This resulted in a less severe lipid accumulation compared with that triggered by rosiglitazone, suggesting that LT175 may have a lower adipogenic activity. Consistent with this hypothesis, in vivo administration of LT175 to mice fed a high-fat diet decreased body weight, adipocyte size, and white adipose tissue mass, as assessed by magnetic resonance imaging. Furthermore, LT175 significantly reduced plasma glucose, insulin, non-esterified fatty acids, triglycerides, and cholesterol and increased circulating adiponectin and fibroblast growth factor 21 levels. Oral glucose and insulin tolerance tests showed that the compound improves glucose homeostasis and insulin sensitivity. Moreover, we demonstrate that the peculiar interaction of LT175 with PPARγ affected the recruitment of the coregulators cyclic-AMP response element-binding protein-binding protein and nuclear corepressor 1 (NCoR1), fundamentals for the PPARγ-mediated adipogenic program. In conclusion, our results describe a new PPAR ligand, modulating lipid and glucose metabolism with reduced adipogenic activity, that may be used as a model for a series of novel molecules with an improved pharmacological profile for the treatment of dyslipidemia and type 2 diabetes.

Ageing Fxr deficient mice develop increased energy expenditure, improved glucose control and liver damage resembling NASH

Nuclear receptor subfamily 1, group H, member 4 (Nr1h4, FXR) is a bile acid activated nuclear receptor mainly expressed in the liver, intestine, kidney and adrenal glands. Upon activation, the primary function is to suppress cholesterol 7 alpha-hydroxylase (Cyp7a1), the rate-limiting enzyme in the classic or neutral bile acid synthesis pathway. In the present study, a novel Fxr deficient mouse line was created and studied with respect to metabolism and liver function in ageing mice fed chow diet. The Fxr deficient mice were similar to wild type mice in terms of body weight, body composition, energy intake and expenditure as well as behaviours at a young age. However, from 15 weeks of age and onwards, the Fxr deficient mice had almost no body weight increase up to 39 weeks of age mainly because of lower body fat mass. The lower body weight gain was associated with increased energy expenditure that was not compensated by increased food intake. Fasting levels of glucose and insulin were lower and glucose tolerance was improved in old and lean Fxr deficient mice. However, the Fxr deficient mice displayed significantly increased liver weight, steatosis, hepatocyte ballooning degeneration and lobular inflammation together with elevated plasma levels of ALT, bilirubin and bile acids, findings compatible with non-alcoholic steatohepatitis (NASH) and cholestasis. In conclusion, ageing Fxr deficient mice display late onset leanness associated with elevated energy expenditure and improved glucose control but develop severe NASH-like liver pathology.

Gut microbiota regulates bile acid metabolism by reducing the levels of tauro-beta-muricholic acid, a naturally occurring FXR antagonist

Bile acids are synthesized from cholesterol in the liver and further metabolized by the gut microbiota into secondary bile acids. Bile acid synthesis is under negative feedback control through activation of the nuclear receptor farnesoid X receptor (FXR) in the ileum and liver. Here we profiled the bile acid composition throughout the enterohepatic system in germ-free (GF) and conventionally raised (CONV-R) mice. We confirmed a dramatic reduction in muricholic acid, but not cholic acid, levels in CONV-R mice. Rederivation of Fxr-deficient mice as GF demonstrated that the gut microbiota regulated expression of fibroblast growth factor 15 in the ileum and cholesterol 7α-hydroxylase (CYP7A1) in the liver by FXR-dependent mechanisms. Importantly, we identified tauro-conjugated beta- and alpha-muricholic acids as FXR antagonists. These studies suggest that the gut microbiota not only regulates secondary bile acid metabolism but also inhibits bile acid synthesis in the liver by alleviating FXR inhibition in the ileum.

Tesaglitazar, a PPARα/γ Agonist, Induces Interstitial Mesenchymal Cell DNA Synthesis and Fibrosarcomas in Subcutaneous Tissues in Rats

The development of the dual peroxisome proliferator-activated receptor (PPAR) alpha/gamma agonist tesaglitazar as an oral antidiabetic was recently discontinued. Here we present tumor data from a 2-year carcinogenicity study in rats given 0.3, 1, 3, and 10 micromol/kg tesaglitazar is presented with focus on the findings of subcutaneous fibrosarcomas. To investigate the mechanism for induction of fibrosarcomas, replicative DNA synthesis (immunohistochemical detection of BrdU-labeled cells) and expression of PPARgamma (immunohistochemistry and reverse transcription-polymerase chain reaction) in subcutaneous adipose tissues was assessed in rats administered 1 or 10 micromol/kg for 2 weeks or 3 months. Poorly differentiated subcutaneous mesenchymal sarcomas with a predominant spindle cell appearance occurred at the highest dose level of 10 micromol/kg in both sexes, and these tumors were diagnosed as fibrosarcomas. The 10-micromol/kg dose was at or above the maximum tolerated dose and caused considerable cardiovascular mortality. Tesaglitazar stimulated DNA synthesis mainly in subcutaneous interstitial mesenchymal cells. The percentage of BrdU-labeled interstitial cells was increased at 1 and 10 micromol/kg after 2 weeks. The increase in DNA synthesis was still significant at the end of the 12-week treatment at 10 mumol/kg, the dose producing fibrosarcoma. However, at 1 micromol/kg, a dose below the no-observed-effect level for fibrosarcoma, the level of DNA synthesis was similar to control levels at 12 weeks. Immunohistochemical analyses showed no detectable PPARgamma protein in the majority of BrdU-labeled interstitial mesenchymal cells in white and brown fat. This indicates that stimulation of DNA synthesis is not mediated via direct activation of PPARgamma in these cells. The results suggest that the induction of rat fibrosarcoma by tesaglitazar, at exposures 100-fold above the human therapeutic exposure, may involve proliferation of undifferentiated mesenchymal cells in subcutaneous tissues.

Gene expression analysis suggests that EBF-1 and PPARγ2 induce adipogenesis of NIH-3T3 cells with similar efficiency and kinetics

Differentiation of multipotent mesenchymal stem cells into lipid-accumulating adipocytes is a physiological process induced by transcription factors in combination with hormonal stimulation. We have used Affymetrix microarrays to compare the adipogenic differentiation pathways of NIH-3T3 fibroblasts induced to undergo in vitro differentiation by ectopic expression of early B cell factor (EBF)-1 or peroxisome proliferator-activated receptor (PPAR)γ2. These experiments revealed that commitment to the adipogenic pathway in the NIH-3T3 cells was not reflected in gene expression until 4 days after induction of differentiation. Furthermore, gene expression patterns at the earlier time points after stimulation indicated that EBF-1 and PPARγ2 induced different sets of genes, while the similarities increased upon differentiation, and that several genes linked to adipocyte differentiation were also transiently induced in the vector-transduced cells. These data suggest that the initial activation of genes associated with adipocyte development is independent of commitment to the adipogenic pathway and that EBF-1 and PPARγ2 induce adipocyte differentiation with comparable kinetics and efficiency.

Regulation of hepatic metabolic pathways by the orphan nuclear receptor SHP

SHP (small heterodimer partner) is an important component of the feedback regulatory cascade, which controls the conversion of cholesterol to bile acids. In order to identify the bona fide molecular targets of SHP, we performed global gene expression profiling combined with chromatin immunoprecipitation assays in transgenic mice constitutively expressing SHP in the liver. We demonstrate that SHP affects genes involved in diverse biological pathways, and in particular, several key genes involved in consecutive steps of cholesterol degradation, bile acid conjugation, transport and lipogenic pathways. Sustained expression of SHP leads to the depletion of hepatic bile acid pool and a concomitant accumulation of triglycerides in the liver. The mechanism responsible for this phenotype includes SHP-mediated direct repression of downstream target genes and the bile acid sensor FXRalpha, and an indirect activation of PPARgamma and SREBP-1c genes. We present evidence for the role of altered chromatin configurations in defining distinct gene-specific mechanisms by which SHP mediates differential transcriptional repression. The multiplicity of genes under its control suggests that SHP is a pleiotropic regulator of diverse metabolic pathways.

Identification of the human ApoAV gene as a novel RORalpha target gene

Retinoic acid receptor-related orphan receptor-alpha (RORalpha) (NR1F1) is an orphan nuclear receptor with a potential role in metabolism. Previous studies have shown that RORalpha regulates transcription of the murine Apolipoprotein AI gene and human Apolipoprotein CIII genes. In the present study, we present evidence that RORalpha also induces transcription of the human Apolipoprotein AV gene, a recently identified apolipoprotein associated with triglyceride levels. Adenovirus-mediated overexpression of RORalpha increased the endogenous expression of ApoAV in HepG2 cells and RORalpha also enhanced the activity of an ApoAV promoter construct in transiently transfected HepG2 cells. Deletion and mutation studies identified three AGGTCA motifs in the ApoAV promoter that mediate RORalpha transactivation, one of which overlaps with a previously identified binding site for PPARalpha. Together, these results suggest a novel mechanism whereby RORalpha modulates lipid metabolism and implies RORalpha as a potential target for the treatment of dyslipidemia and atherosclerosis.

Differential regulation of cytosolic and peroxisomal bile acid amidation by PPARα activation favors the formation of unconjugated bile acids

In human liver, unconjugated bile acids can be formed by the action of bile acid-CoA thioesterases (BACTEs), whereas bile acid conjugation with taurine or glycine (amidation) is catalyzed by bile acid-CoA:amino acid N-acyltransferases (BACATs). Both pathways exist in peroxisomes and cytosol. Bile acid amidation facilitates biliary excretion, whereas the accumulation of unconjugated bile acids may become hepatotoxic. We hypothesized that the formation of unconjugated and conjugated bile acids from their common substrate bile acid-CoA thioesters by BACTE and BACAT is regulated via the peroxisome proliferator-activated receptor alpha (PPARalpha). Livers from wild-type and PPARalpha-null mice either untreated or treated with the PPARalpha activator WY-14,643 were analyzed for BACTE and BACAT expression. The total liver capacity of taurochenodeoxycholate and taurocholate formation was decreased in WY-14,643-treated wild-type mice by 60% and 40%, respectively, but not in PPARalpha-null mice. Suppression of the peroxisomal BACAT activity was responsible for the decrease in liver capacity, whereas cytosolic BACAT activity was essentially unchanged by the treatment. In both cytosol and peroxisomes, the BACTE activities and protein levels were upregulated 5- to 10-fold by the treatment. These effects caused by WY-14,643 treatment were abolished in PPARalpha-null mice. The results from this study suggest that an increased formation of unconjugated bile acids occurs during PPARalpha activation.

Expression of the insulin-responsive glucose transporter GLUT4 in adipocytes is dependent on liver X receptor alpha

The insulin-responsive glucose transporter GLUT4 plays a crucial role in insulin-mediated facilitated glucose uptake into adipose tissue and muscle, and impaired expression of GLUT4 has been linked to obesity and diabetes. In this study, we demonstrate that liver X receptors (LXRs) regulate the expression of GLUT4 through direct interaction with a conserved LXR response element in the GLUT4 promoter. The expression of GLUT4 in WAT is induced by a potent LXR agonist in wild type, LXR alpha-/-, and LXR beta-/- mice but not in LXR alpha-/-beta-/- mice, demonstrating that both LXRs are able to mediate ligand activated transcription of the GLUT4 gene. However, basal and insulin stimulated expression of GLUT4 in epididymal WAT is reduced only in mice carrying ablation of the LXR alpha isoform. The expression of GLUT4 is furthermore correlated to the induction of LXR alpha during mouse and human adipocyte differentiation. LXR beta is thus apparently not able to rescue basal expression of GLUT4 in the absence of LXR alpha. We have previously demonstrated that LXR alpha is down-regulated in animal models of obesity and diabetes, thus revealing a striking correlation between GLUT4 and LXR alpha expression in insulin-resistant conditions. This suggests that the LXR alpha isoform has a unique role in adipose expression of GLUT4 and suggests that alteration of adipose tissue expression of LXR alpha might be a novel tool to normalize the expression of a gene that is dysregulated in diabetic and insulin-resistant conditions.

On the role of liver X receptors in lipid accumulation in adipocytes

The pivotal role of liver X receptors (LXRs) in the metabolic conversion of cholesterol to bile acids in mice is well established. More recently, the LXRalpha promoter has been shown to be under tight regulation by peroxisome proliferator-activated receptors (PPARs), implying a role for LXRalpha in mediating the interplay between cholesterol and fatty acid metabolism. We have studied the role of LXR in fat cells and demonstrate that LXR is regulated during adipogenesis and augments fat accumulation in mature adipocytes. LXRalpha expression in murine 3T3-L1 adipocytes as well as in human adipocytes was up-regulated in response to PPARgamma agonists. Administration of a PPARgamma agonist to obese Zucker rats also led to increased LXRalpha mRNA expression in adipose tissue in vivo. LXR agonist treatment of differentiating adipocytes led to increased lipid accumulation. An increase of the expression of the LXR target genes, sterol regulatory binding protein-1 and fatty acid synthase, was observed both in vivo and in vitro after treatment with LXR agonists for 24 h. Finally, we demonstrate that fat depots in LXRalpha/beta-deficient mice are smaller than in age-matched wild-type littermates. These findings imply a role for LXR in controlling lipid storage capacity in mature adipocytes and point to an intriguing physiological interplay between LXR and PPARgamma in controlling pathways in lipid handling.

Early B-cell factor (O/E-1) is a promoter of adipogenesis and involved in control of genes important for terminal adipocyte differentiation

Olf-1/early B-cell factor (O/E-1) is a transcription factor important for B-lymphocyte and neuronal gene regulation. Here we report that all three known O/E genes (O/E-1, -2, and -3) are expressed in mouse adipose tissue and are upregulated during adipocyte differentiation. Forced expression of O/E-1 in either the preadipocyte cell line 3T3-L1 or mouse embryonic fibroblasts augmented adipogenesis, and constitutive expression of O/E-1 in uncommitted NIH 3T3 fibroblasts led to initiation of adipocyte differentiation. Furthermore, a dominant negative form of O/E-1 partially suppressed 3T3-L1 adipogenesis, indicating that expression from endogenous O/E target genes is required for 3T3-L1 terminal differentiation. Thus, our data point to the importance of O/E target genes for adipocyte differentiation and suggest a novel role for O/E-1 as an initiator and stimulator of adipogenesis.

AZ 242, a novel PPARalpha/gamma agonist with beneficial effects on insulin resistance and carbohydrate and lipid metabolism in ob/ob mice and obese Zucker rats

Abnormalities in fatty acid (FA) metabolism underlie the development of insulin resistance and alterations in glucose metabolism, features characteristic of the metabolic syndrome and type 2 diabetes that can result in an increased risk of cardiovascular disease. We present pharmacodynamic effects of AZ 242, a novel peroxisome proliferator activated receptor (PPAR)alpha/gamma agonist. AZ 242 dose-dependently reduced the hypertriglyceridemia, hyperinsulinemia, and hyperglycemia of ob/ob diabetic mice. Euglycemic hyperinsulinemic clamp studies showed that treatment with AZ 242 (1 micromol/kg/d) restored insulin sensitivity of obese Zucker rats and decreased insulin secretion. In vitro, in reporter gene assays, AZ 242 activated human PPARalpha and PPARgamma with EC(50) in the micro molar range. It also induced differentiation in 3T3-L1 cells, an established PPARgamma effect, and caused up-regulation of liver fatty acid binding protein in HepG-2 cells, a PPARalpha-mediated effect. PPARalpha-mediated effects of AZ 242 in vivo were documented by induction of hepatic cytochrome P 450-4A in mice. The results indicate that the dual PPARalpha/gamma agonism of AZ 242 reduces insulin resistance and has beneficial effects on FA and glucose metabolism. This effect profile could provide a suitable therapeutic approach to the treatment of type 2 diabetes, metabolic syndrome, and associated vascular risk factors.

Structure of the PPARalpha and -gamma ligand binding domain in complex with AZ 242; ligand selectivity and agonist activation in the PPAR family

BACKGROUND

The peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors belonging to the nuclear receptor family. The roles of PPARalpha in fatty acid oxidation and PPARgamma in adipocyte differentiation and lipid storage have been characterized extensively. PPARs are activated by fatty acids and eicosanoids and are also targets for antidyslipidemic drugs, but the molecular interactions governing ligand selectivity for specific subtypes are unclear due to the lack of a PPARalpha ligand binding domain structure.

RESULTS

We have solved the crystal structure of the PPARalpha ligand binding domain (LBD) in complex with the combined PPARalpha and -gamma agonist AZ 242, a novel dihydro cinnamate derivative that is structurally different from thiazolidinediones. In addition, we present the crystal structure of the PPARgamma_LBD/AZ 242 complex and provide a rationale for ligand selectivity toward the PPARalpha and -gamma subtypes. Heteronuclear NMR data on PPARalpha in both the apo form and in complex with AZ 242 shows an overall stabilization of the LBD upon agonist binding. A comparison of the novel PPARalpha/AZ 242 complex with the PPARgamma/AZ 242 complex and previously solved PPARgamma structures reveals a conserved hydrogen bonding network between agonists and the AF2 helix.

CONCLUSIONS

The complex of PPARalpha and PPARgamma with the dual specificity agonist AZ 242 highlights the conserved interactions required for receptor activation. Together with the NMR data, this suggests a general model for ligand activation in the PPAR family. A comparison of the ligand binding sites reveals a molecular explanation for subtype selectivity and provides a basis for rational drug design.

Rosiglitazone (BRL49653), a PPARgamma-selective agonist, causes peroxisome proliferator-like liver effects in obese mice

The PPAR (peroxisome proliferator activated receptor) transcription factors are ligand-activated nuclear receptors that regulate genes involved in lipid metabolism and homeostasis. PPARalpha is preferentially expressed in liver and PPARgamma preferentially in adipose tissue. Activation of PPARalpha leads to peroxisome proliferation and increased beta-oxidation of fatty acids in rodents. PPARgamma-activation leads to adipocyte differentiation and improved insulin signaling of mature adipocytes. Both PPAR receptors are believed to be functional targets for treatment of hyperlipidemia in man. We have treated obese diabetic mice (ob/ob), which have highly elevated levels of plasma triglycerides, glucose and insulin, for 1 week with WY14,643 (180 micromol/kg/day), a selective PPARalpha agonist, or rosiglitazone (BRL49653; 2.5 micromol/kg/day), a selective PPARgamma agonist. The doses used produce a similar therapeutic effect in both treatment groups (lowering of triglycerides and glucose). High resolution two-dimensional gel electrophoresis of livers showed that WY14,643 and rosiglitazone both produced changes in expression pattern of many proteins involved in peroxisomal fatty acid beta-oxidation. However, similar experiments performed in lean mice showed significant up-regulation of these proteins only with WY14,643 treatment. Furthermore, the proteins up-regulated by the drugs in obese mice had a higher basal expression in obese controls compared to the lean littermates. Liver PPARgamma mRNA levels were determined and we observed that PPARgamma2 mRNA levels were elevated in obese mice compared to lean littermates. As PPARalpha and PPARgamma recognize similar DNA response elements, it is likely that the effects of rosiglitazone on PPARalpha responsive genes in livers of the ob/ob mice are mediated by PPARgamma2.

beta-subunit assembly is essential for the correct packing and the stable membrane insertion of the H,K-ATPase alpha-subunit

The alpha-subunits of H,K-ATPase (HKAalpha) and Na,K-ATPase require a beta-subunit for maturation. We investigated the role of the beta-subunit in the membrane insertion and stability of the HKAalpha expressed in Xenopus oocytes. Individual membrane segments M1, M2, M3, M4, and M9 linked to a glycosylation reporter act as signal anchor (SA) motifs, and M10 acts as a partial stop transfer motif. In combined HKAalpha constructs, M2 acts as an efficient stop transfer sequence, and M3 acts as a SA sequence. However, M5 and M9 have only partial SA function, and M7 has no SA function. Consistent with the membrane insertion properties of segments in combined alpha constructs, M1-3 alpha-proteins are resistant to cellular degradation, and M1-5 up to M1-10 alpha-proteins are not resistant to cellular degradation. However, co-expression with beta-subunits increases the membrane insertion of M9 in a M1-9 alpha-protein and completely protects M1-10 alpha-proteins against cellular degradation. Our results indicate that HKAalpha N-terminal (M1-M4) membrane insertion and stabilization are mediated by intrinsic molecular characteristics; however, the C-terminal (M5-M10) membrane insertion and thus the stabilization of the entire alpha-subunit depend on intramolecular and intermolecular beta-subunit interactions that are similar but not identical to data obtained for the Na,K-ATPase alpha-subunit.

Structural aspects of the gastric H,K ATPase

The gastric H,K ATPase is an alpha beta heterodimeric member of the eukaryotic alkali-cation P-type ion-motive ATPase family. The alpha subunit is composed of 1033 amino acids and the beta subunit of 291 amino acids with 6 or 7 potential N-linked glycosylation sites. Much effort has been expended to define the membrane domain of P-type ATPases. A membrane domain of the large subunit consisting of 10 membrane-spanning sequences is suggested by a combination of methods such as (1) tryptic digestion, separation, and sequencing of membrane peptides, (2) labeling with extracytoplasmic reagents, and (3) in vitro translation of hydrophobic segments. The beta subunit has a single transmembrane segment with strong hydrophobic interactions with the alpha subunit. Blue native gel electrophoresis shows that the enzyme is an (alpha-beta)2 dimer. Cross-linking with Cu-phenanthroline provides evidence that association is between the alpha subunits, and the potential SH groups that are Cu sensitive are at cysteine 565 and cysteine 615, in the region of the large cytoplasmic loop between the fourth and fifth transmembrane segments. No cross-linking is observed in the membrane domain. ATP prevents cross-linking because of a conformational change at the surface of the protein induced by ATP or by direct binding of the nucleotide at the site of cross-linking. The WGA binding properties of the beta subunit allow investigation of the region of interaction with the alpha subunit. Thus, digestion of the enzyme by trypsin followed by SDS solubilization and selective elution from a WGA column resulted in coelution of the membrane fragment containing TM7 and TM8. This result demonstrates major hydrophobic interaction between the seventh and eighth transmembrane segments and the beta subunit. An antibody generated against rat parietal cells also recognized shared epitopes in the same region of both the alpha and beta subunits. Biochemical investigation of the arrangement of the transmembrane segments has been hindered by the lack of effective cross-linking reagents probably because of the compact arrangement of this domain, preventing even Cu access. A series of antiulcer drugs has been developed that have a unique chemistry related to their inhibition of the gastric H,K ATPase. They are 2-(substituted pyridyl methylsulfinyl) benzimidazoles, weak bases with a pKa of 4.0. After accumulation in the acidic space generated by the H,K ATPase either in vivo or in vitro, they undergo acid-catalyzed conversion to a tetracyclic sulfenamide which reacts with luminally accessible SH residues to form stable disulfide derivatives. In the particular case of pantoprazole, 2-(3,4-dimethoxy-2-pyridyl-methylsulfinyl)-5-difluoromethoxy benzimidazole, reaction is confined largely to cysteine 813, placed between the fifth and sixth transmembrane segments. The 5 azido analog of pantoprazole provided acid transport-dependent inhibition of the isolated transporting ATPase by this photoactivatable covalent SH reagent. The inhibited enzyme was then photolyzed, cleaved with trypsin, and the membrane fragments compared before and after photolysis. Disappearance of the segment corresponding to TM3,4 and a relative loss of the segment corresponding to TM7,8 suggests close proximity of these two membrane pairs to the loop joining the fifth and sixth transmembrane segments, in particular TM3,4. Use of this type of covalent, photoactivatable site-specific reagent to determine loop proximity can be extended to other acid transporters.

In vitro translation analysis of integral membrane proteins

A method of in vitro translation scanning was applied to a variety of polytopic integral membrane proteins, a transition metal P type ATPase from Helicobacter pylori, the SERCA 2 ATPase, the gastric H+,K+ ATPase, the CCK-A receptor and the human ileal bile acid transporter. This method used vectors containing the N terminal region of the gastric H+,K+ ATPase or the N terminal region of the CCK-A receptor, coupled via a linker region to the last 177 amino acids of the beta-subunit of the gastric H+,K+ ATPase. The latter contains 5 potential N-linked glycosylation sites. Translation of vectors containing the cDNA encoding one, two or more putative transmembrane domains in the absence or presence of microsomes allowed determination of signal anchor or stop transfer properties of the putative transmembrane domains by the molecular weight shift on SDS PAGE. The P type ATPase from Helicobacter pylori showed the presence of 8 transmembrane segments with this method. The SERCA 2 Ca2+ ATPase with this method had 9 transmembrane co-translational insertion domains and coupled with other evidence these data resulted in a 11 transmembrane segment model. Translation of segments of the gastric H+,K+ ATPase provided evidence for only 7 transmembrane segments but coupled with other data established a 10 membrane segment model. The G7 protein, the CCK-A receptor showed the presence of 6 of the 7 transmembrane segments postulated for this protein. Translation of segments of the human ileal bile acid transporter showed the presence of 8 membrane insertion domains. However, translation of the intact protein provided evidence for an odd number of transmembrane segments, resulting in a tentative model containing 7 or 9 transmembrane segments. Neither G7 type protein appeared to have an arrangement of sequential topogenic signals consistent with the final assembled protein. This method provides a useful addition to methods of determining membrane domains of integral membrane proteins but must in general be utilized with other methods to establish the number of transmembrane alpha-helices.

Localization of mRNA for the muscarinic M1 receptor in rat stomach

Cholinergic stimulation of receptors in the oxyntic mucosa results in secretion of mucus, pepsinogen and hydrochloric acid. There has been speculation as to the cellular localization of these receptors in the mucosa and as to which subtype is present in the different cell types. In the present study, utilizing radioactive riboprobes for the M1 muscarinic receptor subtype, we carried out in situ hybridization to determine which cells of the gastric corpus transcribe mRNA for this receptor. The antisense M1 probe hybridized strongly on the zymogen cells and, to a lesser extent, on the surface mucous cells and the muscle layers. Control sections from brain also displayed specific hybridization.

The expression of pepsinogen c mRNA in normal gastroduodenal mucosa and the gastric ulcer margin of the rat

During the healing of experimental gastric ulcers in the oxyntic mucosa, there is a dedifferentiation of the glands in the ulcer margin: previous studies have shown that the parietal cells lose their capacity to produce HCl, and mucous cells replace the zymogen cells. Primarily, we wished to investigate whether or not the glands of the ulcer margin transcribe mRNA for pepsinogen; secondly we also wanted to locate such transcription in other parts of the gastroduodenal epithelium. For this purpose, we first established the baseline for distribution of pepsinogen mRNA in normal rats. We then studied its location in the margin of ulcers in the corpus region after 1-15 days of healing. Formaldehyde-fixed paraffin sections were used for in situ hybridization of mRNA for pepsinogen C, utilizing radioactive riboprobes. The normal gastroduodenal mucosa showed widespread hybridization: the signal was particularly strong in the zymogen cells; weaker signals were obtained from the mucous neck cells, and the cells of the cardiac, antral, and Brunner glands. Specific hybridization was weak or absent in the ulcer margin during the entire period studied. It is concluded that the capacity to produce pepsinogen C is significantly reduced or absent in the gastric ulcer margin during the first 15 days of healing; this should reduce the risks of peptic attack on the delicate scar and margin tissues during ulcer healing.

Topological analysis of H+,K(+)-ATPase using in vitro translation

The membrane topology of the alpha subunit of the H+,K(+)-ATPase was investigated by using in vitro transcription/translation of DNA sequences encoding fusion proteins that contained possible membrane-spanning segments. The vectors consisted of DNA sequences encoding (a) either the first 101 (M0 vectors) or the first 139 (M1 vectors) amino acids of the N-terminal region of the alpha subunit of the ATPase, (b) a variable region, and then (c) the C-terminal 177 amino acids of the C-terminal region of the beta subunit, with five N-linked glycosylation sites. The variable region of the fusion protein contained the cDNA sequences representing the possible eight or 10 membrane-spanning segments either alone or in various combinations. Transcription/translation was performed in the presence of [35S]methionine using a coupled reticulocyte lysate in the absence and presence of microsomes. The fusion protein was identified by autoradiography following separation using SDS-polyacrylamide gel electrophoresis. Glycosylation of a translated sequence corresponded to membrane insertion and translocation of the C-terminal beta sequence. This method allowed analysis of signal anchor sequences using the M0 vector. The presence of a stop transfer sequence in the variable segment of the M1 vector resulted in inhibition of translocation of the C-terminal beta sequence. The sequences for the first four membrane segments could act as either signal anchor or stop transfer sequences. Therefore, this region of the alpha subunit has four membrane-spanning segments that are co-inserted with translation. The sequence corresponding to membrane segment M8 acted as a stop transfer sequence. The sequence corresponding to membrane segment M9 acted as a signal anchor sequence, and that corresponding to membrane segment M10 acted as a stop transfer sequence. The sequences representing the fifth, sixth, and seventh (M5, M6, and M7) membrane segments were unable to co-insert into the membrane. These data verify the first four and the eight membrane-spanning segments of the alpha subunit of the gastric H+,K(+)-ATPase and provide evidence for translational insertion of an additional pair of membrane-spanning segments, M9 and M10. It appears that insertion of membrane segments M5, M6, and M7 is determined differently from the other membrane-spanning segments. In combination with other methods, this in vitro transcription/translation method is useful for defining the membrane topology of the P type ATPases.

In situ hybridization of mRNA for the gastric H+,K(+)-ATPase in rat oxyntic mucosa

The H+,K(+)-ATPase member of the phosphorylating ion motive ATPases is composed of two subunits, a large alpha-subunit composed of about 1030 amino acids and a smaller beta-subunit consisting of about 290 amino acids. By biochemical and immunological methods both subunits have been found in high abundance in the gastric parietal cell. In the present study in situ hybridization was used for localizing and comparing concentrations of the mRNA for the two subunits in the gastric epithelium. For this purpose 3H-labelled probes were preferred. Hybridization was detected only in the parietal cells. The older parietal cells in the bottom of the mucosa gave a weaker hybridization signal than the younger parietal cells closer to the surface. The margin of experimental ulcers, where the parietal cells are of low differentiation, showed very weak, if any, hybridization. The differences observed in hybridization densities may reflect differences in mRNA synthesis or stability. It is conceivable that older parietal cells, as well as parietal cells of low differentiation, produce relatively small amounts of H+,K(+)-ATPase.

Gastric acid secretion: activation and inhibition

Peripheral regulation of gastric acid secretion is initiated by the release of gastrin from the G cell. Gastrin then stimulates the cholecystokinin-B receptor on the enterochromaffin-like cell beginning a calcium signaling cascade. An exocytotic release of histamine follows with concomitant activation of a C1- current. The released histamine begins the H2-receptor mediated sequence of events in the parietal cell, which results in activation of the gastric H+/K+ - ATPase. This enzyme is the final common pathway of acid secretion. The H+/K+ - ATPase is composed of two subunits: the larger alpha-subunit couples ion transport to hydrolysis of ATP, the smaller beta-subunit is required for appropriate assembly of the holoenzyme. Both the membrane and extracytoplasmic domain contain the ion transport pathway, and therefore, this region is the target for the antisecretory drugs of the post-H2 era. The 100 kDa alpha-subunit has probably 10 membrane spanning segments with, therefore, five extracytoplasmic loops. The 35 kDA beta-subunit has a single membrane spanning segment, and most of this protein is extracytoplasmic with the six or seven N glycosylation consensus sequences occupied. Omeprazole is an acid-accumulated, acid-activated, prodrug that binds covalently to two cysteine residues at positions 813 (or 822) and 892, accessible from the acidic face of the pump. Lansoprazole binds to cys321, 813 (or 822) and 892; pantoprazole binds to cys813 and 822. The common binding site for these drugs (cys813 or 822) is responsible for the inhibition of acid transport. Covalent inhibition of the acid pump improves control of acid secretion, but since the effective half life of the inhibition in man is about 48 hr, full inhibition of acid secretion, perhaps necessary for eradication of Helicobacter pylori in combination with a single antibiotic, will require prolongation of the effect of this class of drug.

Location of the cytoplasmic epitope for a K(+)-competitive antibody of the (H+,K+)-ATPase

The monoclonal antibody (mAb) 95-111 binds the alpha subunit of (H+,K+)-ATPase and inhibits the K(+)-ATPase activity. To map the epitope, all of the partial sequences of the alpha subunit were expressed in Escherichia coli HB101 using rabbit alpha subunit cDNA restriction fragments ligated into PuEx vector. Bacterial recombinant lysates were separated by sodium dodecyl sulfate-gel electrophoresis, and the epitope was detected by Western blotting. The antibody site was mapped between Cys529 and Glu561. This is close to the Lys517 that binds fluorescein isothiocyanate (FITC) and is considered to be between M4 and M5 close to the ATP binding domain. However, the mAb inhibition of ATPase is not ATP-competitive but is K(+)-competitive with a KI of 2 x 10(-9) M. The mAb also inhibits K+ quench of FITC fluorescence competitively with a KI of 8 x 10(-9) M. The K+ activation of ATPase activity and quench of FITC fluorescence are dependent on K+ binding to an E2 form of the enzyme from the extracytoplasmic surface. The mAb epitope is cytoplasmic since the K(+)-ATPase activity of ion-tight gastric vesicles is inhibited. The 125I-mAb 95-111 binds to a single class of sites with an apparent KD of 2.3 +/- 0.8 x 10(-9) M and K+ does not displace bound mAb. Hence, antibody binding to a cytoplasmic Cys529-Glu561 epitope allosterically competes with K(+)-dependent reactions at extracytoplasmic sites.

cDNA cloning and membrane topology of the rabbit gastric H+/K(+)-ATPase alpha-subunit

We have cloned and sequenced a cDNA for the rabbit gastric proton-potassium pump (H+/K(+)-ATPase) alpha-subunit. The deduced peptide contains 1035 amino acids (Mr 114,201) and shows 97% sequence identity with the respective rat and hog proteins. A monoclonal antibody 146-14 has been shown previously to react with the extracytoplasmic side of the catalytic H+/K(+)-ATPase subunit and here we show that the epitope is in the region between amino acids 855 and 902 (the numbering of the H+/K(+)-ATPase catalytic subunit throughout the paper refers to the rabbit sequence). The localization of this epitope in conjunction with previously observed trypsin cleavage sites in the C-terminal one third of the enzyme and the hydrophobicity plot of the deduced peptide sequence are evidence for a structural model for the alpha-subunit of the H+/K(+)-ATPase which contains at least ten membrane spanning segments, similar to that deduced for the Ca(2+)-ATPase of sarcoplasmic reticulum.

Acid secretion and the H,K ATPase of stomach

The regulation of acid secretion was clarified by the development of H2-receptor antagonists in the 1970s. It appears that gastrin and acetylcholine exert their effects on acid secretion mainly by stimulation of histamine release from the enterochromaffin-like (ECL) cell of the fundic gastric mucosa. The isolated ECL cell of rat gastric mucosa responds to gastrin/cholecystokinin (CCK), acetylcholine, and epinephrine with histamine release and to somatostatin and R-alpha-methyl histamine by inhibition of histamine release. Histamine and acetylcholine stimulate the parietal cell by elevation of cAMP or [Ca]i by activation of H2 or M3 receptors, respectively. These independent pathways converge to activate the gastric acid pump, the H+,K+ ATPase. Activation is a function of the association of the ATPase with a potassium chloride transport pathway that occurs in the membrane of the secretory canaliculus of the parietal cell. Hence the secretory canaliculus is the site of acid secretion, the acid being pumped into the lumen of the canaliculus. The pump is composed of two subunits, a large catalytic and a smaller glycosylated protein. This final step of acid secretion has become the target of drugs also designed to inhibit acid secretion. The target domain of the benzimidazole class of acid pump inhibitors is the extracytoplasmic domain of the pump that is secreting acid, and the target amino acids are the cysteines present in this domain. The secondary structure of the pump can be analyzed by determining trypsin-sensitive bonds in intact, cytoplasmic-side-out vesicles of the ATPase, and it has been shown that the alpha subunit has at least eight membrane-spanning segments. Omeprazole, the first acid pump inhibitor, forms a disulfide bond with cysteines in the extracytoplasmic loop between the fifth and sixth membrane-spanning segment and to a cysteine in the extracytoplasmic loop between the seventh and eight segments, preventing phosphorylation of the pump by ATP. As a result of the effective and long-lasting inhibition of acid secretion by the acid pump inhibitor, superior clinical results have been found in all forms of acid-related disease.