Phytosterols are present in Pneumocystis carinii

Pneumocystis jirovecii in solid organ transplant recipients: updates in epidemiology, diagnosis, treatment, and prevention

PURPOSE OF REVIEW

This review highlights the epidemiology of Pneumocystis jirovecii pneumonia in solid organ transplant recipients, advancements in the diagnostic landscape, and updates in treatment and prevention.

RECENT FINDINGS

The increasing use of immune-depleting agents in the context of solid organ transplantation has given rise to P. jirovecii pneumonia in this population. The use of prophylaxis has dramatically reduced risk of infection; however, late-onset infections occur after cessation of prophylaxis and in the setting of lymphopenia, advancing patient age, acute allograft rejection, and cytomegalovirus infection. Diagnosis requires respiratory specimens, with PCR detection of Pneumocystis replacing traditional staining methods. Quantitative PCR may be a useful adjunct to differentiate between infection and colonization. Metagenomic next-generation sequencing is gaining attention as a noninvasive diagnostic tool. Trimethoprim-sulfamethoxazole remains the drug of choice for treatment and prevention of Pneumocystis pneumonia. Novel antifungal agents are under investigation.

SUMMARY

P. jirovecii is a fungal opportunistic pathogen that remains a cause of significant morbidity and mortality in solid organ transplant recipients. Early detection and timely treatment remain the pillars of management.

Current Concepts in the Diagnosis and Management of Pneumocystis Pneumonia in Solid Organ Transplantation

Pneumocystis infection manifests predominantly as an interstitial pneumonia in immunocompromised patients. Diagnostic testing in the appropriate clinical context can be highly sensitive and specific and involves radiographic imaging, fungal biomarkers, nucleic acid amplification, histopathology, and lung fluid or tissue sampling. Trimethoprim-sulfamethoxazole remains the first-choice agent for treatment and prophylaxis. Investigation continues to promote a deeper understanding of the pathogen's ecology, epidemiology, host susceptibility, and optimal treatment and prevention strategies in solid organ transplant recipients.

The Pathogenesis and Diagnosis of Pneumocystis jiroveci Pneumonia

Pneumocystis jiroveci remains an important fungal pathogen in immunocompromised hosts. The environmental reservoir remains unknown. Pneumonia (PJP) results from airborne transmission, including in nosocomial clusters, or with reactivation after an inadequately treated infection. Pneumocystis pneumonia most often occurs within 6 months of organ transplantation, with intensified or prolonged immunosuppression, notably with corticosteroids and following cytomegalovirus (CMV) infections. Infection may be recognized during recovery from neutropenia and lymphopenia. Invasive procedures may be required for early diagnosis and therapy. Despite being a well-established entity, aspects of the pathogenesis of PJP remain poorly understood. The goal of this review is to summarize the data on the pathogenesis of PJP, review the strengths and weaknesses of the pertinent diagnostic modalities, and discuss areas for future research.

Efficacy of Trimethoprim–Sulfamethoxazole in Combination with an Echinocandin as a First-Line Treatment Option for Pneumocystis Pneumonia: A Systematic Review and Meta-Analysis

Although combination therapy using trimethoprim–sulfamethoxazole (TMP–SMX) plus echinocandins has been reported to reduce the mortality of patients with pneumocystis pneumonia (PCP), it remains unclear whether it is more effective than TMP–SMX monotherapy, the current first-line treatment for this disease. Hence, we performed a systematic review and meta-analysis to compare the efficacies of these treatment options for PCP. The Scopus, EMBASE, PubMed, CINAHL, and Ichushi databases were searched for studies (up to January 2022) reporting the mortality and positive response rates (fewer clinical symptoms, improved partial pressure of arterial oxygen, and resolution of pneumonitis on chest imaging) of PCP patients receiving monotherapy or combination therapy. Four studies met the inclusion criteria. All four presented mortality data and one had positive response rates. Compared with the monotherapy, the combination therapy resulted in significantly lower mortality and higher positive response rates (mortality: odds ratio (OR) 2.20, 95% confidence interval (CI) 1.46–3.31; positive response rate: OR 2.13, 95%CI 1.41–3.23), suggesting it to be an effective and promising first-line therapy for PCP. However, further safety evaluations are needed to establish this as a fact.

Pneumocystis jiroveci in solid organ transplantation: Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice

These updated guidelines from the Infectious Diseases Community of Practice of the American Society of Transplantation review the diagnosis, prevention, and management of Pneumocystis jiroveci fungal infection transplant recipients. Pneumonia (PJP) may develop via airborne transmission or reactivation of prior infection. Nosocomial clusters of infection have been described among transplant recipients. PJP should not occur during prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMX). Without prophylaxis, PJP risk is greatest in the first 6 months after organ transplantation but may develop later. Risk factors include low lymphocyte counts, cytomegalovirus infection (CMV), hypogammaglobulinemia, treated graft rejection or corticosteroids, and advancing patient age (>65). Presentation typically includes fever, dyspnea with hypoxemia, and cough. Chest radiographic patterns generally reveal diffuse interstitial processes best seen by CT scans. Patients generally have PO₂  < 60 mm Hg, elevated serum lactic dehydrogenase (LDH), and elevated serum (1 → 3) β-d-glucan assay. Specific diagnosis uses respiratory specimens with direct immunofluorescent staining; invasive procedures may be required. Quantitative PCR is a useful adjunct to diagnosis. TMP-SMX is the drug of choice for therapy; drug allergy should be documented before resorting to alternative therapies. Adjunctive corticosteroids may be useful early. Routine PJP prophylaxis is recommended for at least 6-12 months post-transplant, preferably with TMP-SMX.

Microbial Sterolomics as a Chemical Biology Tool

Metabolomics has become a powerful tool in chemical biology. Profiling the human sterolome has resulted in the discovery of noncanonical sterols, including oxysterols and meiosis-activating sterols. They are important to immune responses and development, and have been reviewed extensively. The triterpenoid metabolite fusidic acid has developed clinical relevance, and many steroidal metabolites from microbial sources possess varying bioactivities. Beyond the prospect of pharmacognostical agents, the profiling of minor metabolites can provide insight into an organism's biosynthesis and phylogeny, as well as inform drug discovery about infectious diseases. This review aims to highlight recent discoveries from detailed sterolomic profiling in microorganisms and their phylogenic and pharmacological implications.

Pneumocystis Carinii Pneumonia in Adult Non-HIV Disorders

Pneumocystis carinii pneumonia (PCP) remains a serious infection in the immunocompromised host (in the absence of HIV infection) and presents significant management and diagnostic challenges to ICU physicians. Non-HIV PCP is generally abrupt in onset, and follows a fulminate course with high rates of hospitalization, ICT admission, respiratory failure, and requirement for intubation. Mortality is generally high, especially if mechanical ventilation is required. Non-invasive ventilatory support may be considered, although the rapid progression to respiratory failure often necessitates intubation at the time of presentation. Bronchoscopy is often required to establish the diagnosis, and empirical antimicrobial treatment specifically targeted to P. carinii should be initiated while awaiting confirmation. Adjunctive corticosteroids may accelerate recovery, although their use has not yet been established in non-HIV PCP. For the ICU physicians to diagnose PCP, the non-specific presentation of an acute febrile illness and respiratory distress with diffuse pulmonary infiltrates requires a high clinical index of suspician, familiarity with clinical conditions associated with increased risk for PCP, and a low threshold for bronchoscopy to establish the diagnosis.

Pneumocystis carinii: a fungus resistant to antifungal therapies - mechanisms of action of antipneumocystis drugs

Pneumocystis carinii is a pathogen that causes a potentially lethal pneumonia in patients with AIDS and other immunodeficiency states. This review discusses the mechanisms of action of four classes of antipneumocystis agents: inhibitors of ergosterol synthesis and function, 1,3-beta-glucan synthase inhibitors, antifolates and DNA binding agents. Investigations of P. carinii's biologic pathways affected by the antipneumocystis actions of each of these classes of agents has generated important insights into the organism's basic biology and supports the organism's classification as a fungus. In addition, this review discusses some recent P. carinii research and its potential impact on drug development.

Transcriptome of Pneumocystis carinii during fulminate infection: carbohydrate metabolism and the concept of a compatible parasite

Members of the genus Pneumocystis are fungal pathogens that cause pneumonia in a wide variety of mammals with debilitated immune systems. Little is known about their basic biological functions, including life cycle, since no species can be cultured continuously outside the mammalian lung. To better understand the pathological process, about 4500 ESTS derived from sequencing of the poly(A) tail ends of P. carinii mRNAs during fulminate infection were annotated and functionally characterized as unassembled reads, and then clustered and reduced to a unigene set with 1042 members. Because of the presence of sequences from other microbial genomes and the rat host, the analysis and compression to a unigene set was necessarily an iterative process. BLASTx analysis of the unassembled reads (UR) vs. the Uni-Prot and TREMBL databases revealed 56% had similarities to existing polypeptides at E values of<or=10(-6), with the remainder lacking any significant homology. The most abundant transcripts in the UR were associated with stress responses, energy production, transcription and translation. Most (70%) of the UR had similarities to proteins from filamentous fungi (e.g., Aspergillus, Neurospora) and existing P. carinii gene products. In contrast, similarities to proteins of the yeast-like fungi, Schizosaccharomyces pombe and Saccharomyces cerevisiae, predominated in the unigene set. Gene Ontology analysis using BLAST2GO revealed P. carinii dedicated most of its transcripts to cellular and physiological processes ( approximately 80%), molecular binding and catalytic activities (approximately 70%), and were primarily derived from cell and organellar compartments (approximately 80%). KEGG Pathway mapping showed the putative P. carinii genes represented most standard metabolic pathways and cellular processes, including the tricarboxylic acid cycle, glycolysis, amino acid biosynthesis, cell cycle and mitochondrial function. Several gene homologs associated with mating, meiosis, and sterol biosynthesis in fungi were identified. Genes encoding the major surface glycoprotein family (MSG), heat shock (HSP70), and proteases (PROT/KEX) were the most abundantly expressed of known P. carinii genes. The apparent presence of many metabolic pathways in P. carinii, sexual reproduction within the host, and lack of an invasive infection process in the immunologically intact host suggest members of the genus Pneumocystis may be adapted parasites and have a compatible relationship with their mammalian hosts. This study represents the first characterization of the expressed genes of a non-culturable fungal pathogen of mammals during the infective process.

Nonribosomal cyclic peptides: specific markers of fungal infections

Some cyclic peptides and depsipeptides are synthesized in microorganisms by large multienzymes called nonribosomal peptide synthetases. The structures of peptide products originating in this way are complex and diverse and are microorganism-specific. This work proposes the use of fungal cyclic peptides and depsipeptides as extremely specific markers of fungal infections. Since a reliable molecular tool for diagnosing fungal infections at an early stage is still missing, we present mass spectrometry as a new, modern, broadband (with respect to fungal strain) and specific tool for clinical mycologists. More than 40 different fungal species can be rapidly characterized according to specific families of cyclic peptides, and in some cases, a particular fungal strain can be identified on the basis of its cyclopeptide profile. This paper is also aimed at initiating discussion on the biological role of these secondary metabolites, especially of those synthesized by medically important strains. Proven cytotoxic, anti-inflammatory or immunosuppressive activities of some cyclic peptides indicate that these molecules may contribute to the synergistic array of fungal virulence factors and support microbial invasion during fungal infection. In addition to an overview on recent mass spectrometric protocols for cyclic peptide sequencing, the structures of new peptides from Paecilomyces and Pseudallescheria are presented.

Sterol metabolism in the opportunistic pathogen Pneumocystis: advances and new insights

Pneumocystis can transiently colonize healthy individuals without causing adverse symptoms, and most people test positive for exposure to this organism early in life. However, it can cause Pneumocystis pneumonia (PcP) in people with impaired immune systems and is a major cause of death in HIV/AIDS. Although it has close affinities to the Ascomycetes, Pneumocystis has features unlike those of any single group of fungi. For example, Pneumocystis does not synthesize ergosterol, which is consistent with the inefficacy of amphotericin B and some triazoles in clearing PcP. Pneumocystis sterols include distinct delta7 24-alkylsterols. Metabolic radiolabeling experiments demonstrated that P. carinii synthesizes sterols de novo. Cholesterol is the most abundant sterol in Pneumocystis; most, if not all, is scavenged from the mammalian host lung by the pathogen. The P. carinii erg7, erg6, and erg11 genes have been cloned, sequenced, and expressed in heterologous systems. The recombinant P. carinii S-adenosyl-L-methionine:C-24 sterol methyl transferase (SAM:SMT) has a preference for lanosterol over zymosterol as substrate, and the enzyme can catalyze the transfer of either one or two methyl groups to the C-24 position of the sterol side chain. Two different sterol compositions were detected among human-derived P. jirovecii; one was dominated by C28 and C29 sterols, and the other had high proportions of higher molecular mass components, notably the C32 sterol pneumocysterol. The latter phenotype apparently represents organisms blocked at 14alpha-demethylation of the sterol nucleus. These studies suggest that SAM:SMT is an attractive drug target for developing new chemotherapy for PcP.

Immunity against the opportunistic fungal pathogen Pneumocystis

Species of the genus Pneumocystis exist as opportunistic fungal pathogens and are associated with severe pneumonia and pulmonary complications in immunocompromised individuals. Although prophylactic therapy for Pneumocystis has significantly decreased the overall incidence of infection, more than 80% of cases in current patient populations are considered breakthrough cases. In the HIV-infected population, in the years following the initiation of highly active antiretroviral therapy (HAART), significant reductions in the incidence of Pneumocystis infection were observed, although trends over the last several years suggest that the incidence of Pneumocystis has plateaued rather than decreased. Furthermore, with the more prominent usage of immunosuppressive therapies, the frequency of Pneumocystis infection in the HIV-negative population, such as those with hematologic malignancies and those who have undergone transplantation, has risen significantly. Investigating host defense mechanisms against P. carinii has historically been problematic due to the difficulty in achieving continuous in vitro propagation of proliferating Pneumocytis organisms. Nevertheless, clinical and experimental studies have documented that host defense against Pneumocystis involves a concerted effort between innate, cell-mediated (T lymphocyte) and humoral (B lymphocyte) responses. However, the pulmonary environment is a tissue site where heightened inflammatory responses can often lead to inflammation-mediated injury, thereby contributing to the pathogenesis of Pneumocystis infection. Accordingly, clearance of Pneumocystis from the pulmonary environment is dependent on a delicate equilibrium between the inflammatory response and immune-mediated clearance of the organism. Furthermore, innate and adaptive responses against Pneumocystis are strikingly similar to those against other medically-important fungi, thus providing additional evidence that Pneumocystis exists as a fungal organism.

Sterol composition of Pneumocystis jirovecii with blocked 14alpha-demethylase activity

Several drugs that interact with membrane sterols or inhibit their syntheses are effective in clearing a number of fungal infections. The AIDS-associated lung infection caused by Pneumocystis jirovecii is not cleared by many of these therapies. Pneumocystis normally synthesizes distinct C28 and C29 24-alkylsterols, but ergosterol, the major fungal sterol, is not among them. Two distinct sterol compositional phenotypes were previously observed in P. jirovecii. One was characterized by delta7 C28 and C29 24-alkylsterols with only low proportions of higher molecular mass components. In contrast, the other type was dominated by high C31 and C32 24-alkylsterols, especially pneumocysterol. In the present study, 28 molecular species were elucidated by nuclear magnetic resonance analysis of a human lung specimen containing P. jirovecii representing the latter sterol profile phenotype. Fifteen of the 28 had the methyl group at C-14 of the sterol nucleus and these represented 96% of the total sterol mass in the specimen (excluding cholesterol). These results strongly suggest that sterol 14alpha-demethylase was blocked in these organisms. Twenty-four of the 28 were 24-alkylsterols, indicating that methylation of the C-24 position of the sterol side chain by S-adenosyl-L-methionine:sterol C-24 methyl transferase was fully functional.

Sterol biosynthesis in Pneumocystis: unique steps that define unique targets

Pneumocystis lacks ergosterol, and several antimycotics that bind ergosterol in fungal membranes or inhibit its synthesis are ineffective against Pneumocystis pneumonia. The organism synthesizes C(28) and C(29) Delta(7) 24-alkylsterols, 24-alkyllanosterol derivatives, and Delta(5) 24-alkylsterols, which may be produced by modifying scavenged Delta(5) sterols. Mammals cannot desaturate C-22 and alkylate C-24 of sterols, thus, these processes are particularly attractive targets for antifungal drug development. Recent data indicate that C-22 desaturation is not, but C-24 alkylation is an attractive target in P. carinii. The P. carinii S-adenosyl-L-methionine:sterol C-24 methyl transferase (SAM:SMT) has unique properties; it prefers lanosterol as its sterol substrate.

Evidence for multiple sterol methyl transferase pathways in Pneumocystis carinii

The sterol composition of Pneumocystis carinii, an opportunistic pathogen responsible for life-threatening pneumonia in immunocompromised patients, was determined. Our purpose was to identify pathway-specific enzymes to impair using sterol biosynthesis inhibitors. Prior to this study, cholesterol 15 (ca. 80% of total sterols), lanosterol 1, and several phytosterols common to plants (sitosterol 31, 24alpha-ethyl and campesterol, 24alpha-methyl 30) were demonstrated in the fungus. In this investigation, we isolated all the previous sterols and many new compounds from P. carinii by culturing the microorganism in steroid-immunosuppressed rats. Thirty-one sterols were identified from the fungus (total sterol = 100 fg/cell), and seven sterols were identified from rat chow. Unusual sterols in the fungus not present in the diet included, 24(28)-methylenelanosterol 2; 24(28)E-ethylidene lanosterol 3; 24(28)Z-ethylidene lanosterol 4; 24beta-ethyllanosta-25(27)-dienol 5; 24beta-ethylcholest-7-enol 6; 24beta-ethylcholesterol 7; 24beta,-ethylcholesta-5,25(27)-dienol 8; 24-methyllanosta-7-enol 9; 24-methyldesmosterol 10; 24(28)-methylenecholest-7-enol 11; 24beta-methylcholest-7-enol 12; and 24beta-methylcholesterol 13. The structural relationships of the 24-alkyl groups in the sterol side chain were demonstrated chromatographically relative to authentic specimens, by MS and high-resolution 1H NMR. The hypothetical order of these compounds poses multiple phytosterol pathways that diverge from a common intermediate to generate 24beta-methyl sterols: route 1, 1 --> 2 --> 11 --> 12 --> 13; route 2, 1 --> 2 --> 9 --> 10 --> 13; or 24beta-ethyl sterols: route 3, 1 --> 2 --> 4 --> 6 --> 7; route 4, 1 --> 2 --> 5 --> 8 --> 7. Formation of 3 is considered to form an interrupted sterol pathway. Taken together, operation of distinct sterol methyl transferase (SMT) pathways that generate 24beta-alkyl sterols in P. carinii with no counterpart in human biochemistry suggests a close taxonomic affinity with fungi and provides a basis for mechanism-based inactivation of SMT enzyme to treat Pneumocystis pneumonia.

Is Pneumocystis a plant?

Pneumocystis, an AIDS-associated opportunistic pathogen of the lung has some unusual features. This article focuses on work done by my group to understand the organism's distinct sterols. Although Pneumocystis is closely related to fungi, it lacks the major fungal sterol, ergosterol. Several delta(7) 24-alkysterols synthesized by P. carinii are the same as those reported in some basidiomycete rust fungi. The 24-alkylsterols are synthesized by the action of S-adenosyl-L-methionine:C-24 sterol methyl transferase (SAM:SMT). Fungal SAM:SMT enzymes normally transfer only one methyl group to the C-24 position of the sterol side chain and the cells accumulate C28 24-alkylsterols. In contrast, the P. carinii SAM:SMT and those of some plants catalyze one or two methyl transfer reactions producing both C28 and C29 24-alkylsterols. However, unlike most fungi, plants, and the kinetoplastid flagellates Leishmania and Trypanosoma cruzi, P. carinii does not appear to form double bonds at C-5 of the sterol nucleus and C-22 of the sterol side chain. Furthermore, the P. carinii SAM:SMT substrate preference for C30 lanosterol differs from that of homologous enzymes in any other organisms studied. C31 24-Methylenelanosterol and C32 pneumocysterol, products of SAM:SMT activity on lanosterol, can accumulate in high amounts in some, but not all, human-derived Pneumocystis jiroveci populations.

Comprehensive and definitive structural identities of Pneumocystis carinii sterols

Pneumocystis causes a type of pneumonia in immunodeficient mammals, such as AIDS patients. Mammals cannot alkylate the C-24 position of the sterol side chain, nor can they desaturate C-22. Thus, the reactions leading to these sterol modifications are particularly attractive targets for the development of drugs against fungal and protozoan pathogens that make them. In the present study, the definitive structures of 43 sterol molecular species in rat-derived Pneumocystis carinii were elucidated by nuclear magnetic resonance spectroscopy. Ergosterol, Delta(5,7) sterols, trienes, and tetraenes were not among them. Most (32 of the 43) were 24-alkylsterols, products of S-adenosyl-L-methionine:C-24 sterol methyl transferase (SAM:SMT) enzyme activity. Their abundance is consistent with the suggestion that SAM:SMT is highly active in this organism and that the enzyme is an excellent anti-Pneumocystis drug target. In contrast, the comprehensive analysis strongly suggest that P. carinii does not form Delta(22) sterols, thus C-22 desaturation does not appear to be a drug target in this pathogen. The lanosterol derivatives, 24-methylenelanost-8-en-3 beta-ol and (Z)-24-ethylidenelanost-8-en-3 beta-ol (pneumocysterol), previously identified in human-derived Pneumocystis jiroveci, were also detected among the sterols of the rat-derived P. carinii organisms.

The Pneumocystis carinii drug target S-adenosyl-L-methionine:sterol C-24 methyl transferase has a unique substrate preference

Pneumocystis is an opportunistic pathogen that can cause pneumonitis in immunodeficient people such as AIDS patients. Pneumocystis remains difficult to study in the absence of culture methods for luxuriant growth. Recombinant protein technology now makes it possible to avoid some major obstacles. The P. carinii expressed sequence tag (EST) database contains 11 entries of a sequence encoding a protein homologous to S-adenosyl-L-methionine (SAM):C-24 sterol methyl transferase (SMT), suggesting high activity of this enzyme in the organism. We sequenced the erg6 cDNA, identified the putative peptide motifs for the sterol and SAM binding sites in the deduced amino acid sequence and expressed the protein in Escherichia coli. Unlike SAM:SMT from other organisms, the P. carinii enzyme had higher affinities for lanosterol and 24-methylenelanosterol than for zymosterol, the preferred substrate in other fungi. Cycloartenol was not a productive substrate. With lanosterol and 24-methylenelanosterol as substrates, the major reaction products were 24-methylenelanosterol and pneumocysterol respectively. Thus, the P. carinii SAM:SMT catalysed the transfer of both the first and the second methyl groups to the sterol C-24 position, and the substrate preference was found to be a unique property of the P. carinii SAM:SMT. These observations, together with the absence of SAM:SMT among mammals, further support the identification of sterol C-24 alkylation reactions as excellent targets for the development of drugs specifically directed against this pathogen.

Prevention of infection caused by Pneumocystis carinii in transplant recipients

Pneumocystis carinii remains an important pathogen in patients who undergo solid-organ and hematopoietic transplantation. Infection results from reactivation of latent infection and via de novo acquisition of infection from environmental sources. The risk of infection depends on the intensity and duration of immunosuppression and underlying immune deficits. The risk is greatest after lung transplants, in individuals with invasive cytomegalovirus disease, during intensive immunosuppression for allograft rejection, and during periods of neutropenia. Prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMZ) prevents many opportunistic infections, including infection with P. carinii, Toxoplasma gondii, and community-acquired respiratory, gastrointestinal, and urinary tract pathogens. Intolerance of TMP-SMZ is common; desensitization is useful less often in transplant patients than in patients with AIDS. Alternative agents provide a narrower spectrum of protection than does TMP-SMZ and less adequate protection against Pneumocystis species. Clinically, the diagnosis of breakthrough Pneumocystis pneumonia often requires invasive procedures. Strategies for the prevention of Pneumocystis infection must be individualized on the basis of a stratification of risk for each patient.

Heterogeneity of Pneumocystis sterol profiles of samples from different sites in the same pair of lungs suggests coinfection by distinct organism populations

Sterol profiles of samples taken from different sites of a Pneumocystis-infected human lung showed large variations in pneumocysterol similar to those that occur among samples from different patients. Thus, the influence of diet or drugs on pneumocysterol accumulation was ruled out, suggesting distinct phenotypic populations as the basis for the heterogeneity.

Pneumocystis carinii f. sp. carinii synthesizes de novo four homologs of ubiquinone

Ubiquinone, coenzyme Q, plays a pivotal role in electron transport and is a target for chemotherapy against a number of eukaryotic infectious agents, including Pneumocystis carinii. Coenzyme Q10 was previously identified as the major ubiquinone homolog in P. carinii isolated and purified from rat lungs; CoQ9 was also present. In contrast, CoQ9 and CoQ8 (but not CoQ10) were detected in the lungs of uninfected rat controls. These observations suggested that the pathogen synthesizes CoQ10, and perhaps CoQ9 as well. In the present study, CoQ biosynthesis in P. carinii was examined in greater detail. Radiolabeled mevalonate, a precursor of the CoQ polyprenyl chain, was incorporated in vitro into P. carinii ubiquinones. Incorporation of radiolabeled mevalonate into P. carinii CoQ was not enhanced by treating cells with lovastatin, suggesting that the cells did not transport the drug, or that a lovastatin-insensitive pathway for de novo synthesis of isoprenoids may also function in this organism. Radiolabeled precursors of the ring moiety, including shikimic acid, p-hydroxybenzoic acid, and tyrosine were also incorporated into P. carinii CoQ. Unexpectedly, it was found that not only CoQ9 and CoQ10, but also CoQ7, and CoQ8, were metabolically radiolabeled by all the precursors tested, indicating that the organism synthesizes CoQ7, CoQ8, CoQ9, and CoQ10. Metabolic radiolabeling of ubiquinones in rat lung controls was not detected in experiments using either radioactive mevalonate or p-hydroxybenzoate. Thus the incorporations measured using purified P. carinii preparations were due to the enzymes of the organism.

Inhibitors of sterol biosynthesis and amphotericin B reduce the viability of pneumocystis carinii f. sp. carinii

Pneumocystis carinii synthesizes sterols with a double bond at C-7 of the sterol nucleus and an alkyl group with one or two carbons at C-24 of the side chain. Also, some human-derived Pneumocystis carinii f. sp. hominis strains contain lanosterol derivatives with an alkyl group at C-24. These unique sterols have not been found in other pathogens of mammalian lungs. Thus, P. carinii may have important differences in its susceptibility to drugs known to block reactions in ergosterol biosynthesis in other fungi. In the present study, inhibitors of 3-hydroxy-3-methyglutaryl coenzyme A reductase, squalene synthase, squalene epoxidase, squalene epoxide-lanosterol cyclase, lanosterol demethylase, Delta(8) to Delta(7) isomerase, and S-adenosylmethionine:sterol methyltransferase were tested for their effects on P. carinii viability as determined by quantitation of cellular ATP levels in a population of organisms. Compounds within each category varied in inhibitory effect; the most effective included drugs targeted at squalene synthase, squalene epoxide-lanosterol cyclase, and Delta(8) to Delta(7) isomerase. Some drugs that are potent against ergosterol-synthesizing fungi had little effect against P. carinii, suggesting that substrates and/or enzymes in P. carinii sterol biosynthetic reactions are distinct. Amphotericin B is ineffective in clearing P. carinii infections at clinical doses; however, this drug apparently binds to sterols and causes permeability changes in P. carinii membranes, since it reduced cellular ATP levels in a dose-dependent fashion.

C27 to C32 sterols found in Pneumocystis, an opportunistic pathogen of immunocompromised mammals

Pneumocystis carinii is the paradigm of opportunistic infections in immunocompromised mammals. Prior to the acquired immunodeficiency syndrome (AIDS) pandemic and the use of immunosuppressive therapy in organ transplant and cancer patients, P. carinii was regarded as a curiosity, rarely observed clinically. Interest in this organism exploded when it was identified as the agent of P. carinii pneumonia (PcP), the direct cause of death among many AIDS patients. Aggressive prophylaxis has decreased the number of acute PcP cases, but it remains among the most prevalent opportunistic infections found within this patient population. The taxonomic assignment of P. carinii has long been argued; molecular genetics data now demonstrate that it is a fungus. Several antimycotic drugs are targeted against ergosterol or its biosynthesis, but these are not as effective against PcP as they are against other fungal infections. This can now be explained in part by the identification of the sterols of P. carinii. The organism lacks ergosterol but contains distinct C28 and C29 delta7 24-alkylsterols. Also, 24-methylenelanost-8-en-3beta-ol (C31) and pneumocysterol, (24Z)-ethylidenelanost-8-en-3beta-ol (C32) were recently identified in organisms infecting humans. Together, the delta7 24-alkylsterols and pneumocysterol are regarded as signature lipids of the pathogen that can be useful for the diagnosis of PcP, since no other lung pathogen is known to contain them. Cholesterol (C27), the dominant sterol component in P. carinii, is probably totally scavenged from the host. De novo synthesis of sterols has been demonstrated by the presence of lovastatin-sensitive 3-hydroxy-3-methylglutaryl-CoA reductase activity, the incorporation of radiolabeled mevalonate and squalene into P. carinii sterols, and the reduction in cellular ATP in cells treated with inhibitors of enzymes in sterol biosynthesis.

Sterols of Pneumocystis carinii hominis organisms isolated from human lungs

The opportunistic pathogen Pneumocystis carinii causes pneumonia (P. carinii pneumonia, or PCP) in immunocompromised individuals such as AIDS patients. Rat-derived P. carinii carinii organisms have distinct sterols which are not synthesized by mammals and not found in other microbes infecting mammalian lungs. The dominant sterol present in the organism is cholesterol (which is believed to be scavenged from the host), but other sterols in P. carinii carinii have an alkyl group at C-24 of the sterol side chain (C(28) and C(29) 24-alkylsterols) and a double bond at C-7 of the nucleus. Recently, pneumocysterol (C(32)), which is essentially lanosterol with a C-24 ethylidene group, was detected in lipids extracted from a formalin-fixed human P. carinii-infected lung, and its structures were elucidated by gas-liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectrometry in conjunction with analyses of chemically synthesized authentic standards. The sterol composition of isolated P. carinii hominis organisms has yet to be reported. If P. carinii from animal models is to be used for identifying potential drug targets and for developing chemotherapeutic approaches to clear human infections, it is important to determine whether the 24-alkylsterols of organisms found in rats are also present in organisms in humans. In the present study, sterol analyses of P. carinii hominis organisms isolated from cryopreserved human P. carinii-infected lungs and from bronchoalveolar lavage fluid were performed. Several of the same distinct sterols (e.g., fungisterol and methylcholest-7-ene-3beta-ol) previously identified in P. carinii carinii were also present in organisms isolated from human specimens. Pneumocysterol was detected in only some of the samples.

Sterol composition and biosynthesis in Trypanosoma cruzi amastigotes

A detailed analysis of the endogenous sterols present in the clinically relevant intracellular (amastigote) stages of Trypanosoma cruzi, is presented. The parasites were grown in cultured Vero cells in the absence or presence of different sterol biosynthesis inhibitors, including the C14alpha demethylase inhibitor ketoconazole and two inhibitors of delta24(25)-sterol methyl transferase, 20 piperidin-2-yl-5alpha-pregnan-3beta-20-R-diol (22,26-azasterol) and 24-(R,S),25-epiminolanosterol. Amastigotes were isolated and purified from their host cells and neutral lipids were extracted, separated and analyzed by chromatographic and mass spectrometric methods. Control (untreated) amastigotes contained as main endogenous sterols 24-methyl-cholesta-7-en-3beta-ol (ergosta-7-en-3beta-ol) and its 24-ethyl analog, plus smaller amounts of their precursor, ergosta-7,24(28)dien-3beta-ol; these cells also contained cholesterol (up to 80% by weight of total sterols), probably derived from host cells. Amastigotes that proliferated in the presence of 10 nM ketoconazole (minimal inhibitory concentration, MIC) for 24 h had a sharply reduced content of endogenous 4-desmethyl sterols with a concomitant accumulation of 24-methyl-dihydrolanosterol and 24-methylene-dihydrolanosterol. On the other hand, amastigotes incubated during the same period of time with the two inhibitors of 24(25)-SMT at their respective MICs (100-300 nM) accumulated large amounts of C27 sterols whose structure suggested, in the case of 22,26-azasterol, that delta14 sterol reductase was also inhibited. Ketoconazole produced a dose-dependent reduction in the incorporation of [2-(14)C]-acetate into the parasite's endogenous C4-desmethyl sterols with an IC50 of 50 nM, indistinguishable from the value reported previously for the extracellular epimastigote form. Taken together, the results showed that amastigotes have a simpler sterol biosynthetic pathway than that previously described for epimastigotes, lacking both delta5 and delta22 reductases. They also suggest that the 100-fold higher potency of antifungal azoles as antiproliferative agents against amastigotes, when compared with epimastigotes, is most probably due to a smaller pool of endogenous sterols in the intracellular parasites.

Pneumocysterol [(24Z)-ethylidenelanost-8-en-3beta-ol], a rare sterol detected in the opportunistic pathogen Pneumocystis carinii hominis: structural identity and chemical synthesis

Pneumocystis carinii pneumonia (PcP) remains among the most prevalent opportunistic infections among AIDS patients. Currently, drugs used clinically for deep mycosis act by binding ergosterol or disrupting its biosynthesis. Although classified as a fungus, P. carinii lacks ergosterol. Instead, the pathogen synthesizes a number of distinct Delta7, 24-alkylsterols, despite the abundance of cholesterol, which it can scavenge from the lung alveolus. Thus, the pathogen-specific sterols appear vital for organism survival and proliferation. In the present study, high concentrations of a C32 sterol were found in human-derived P. carinii hominis. The definitive structural identities of two C-24 alkylated lanosterol compounds, previously not reported for rat-derived P. carinii carinii, were determined by using GLC, MS, and NMR spectroscopy together with the chemical syntheses of authentic standards. The C31 and C32 sterols were identified as euphorbol (24-methylenelanost-8-en-3beta-ol) and pneumocysterol [(24Z)-ethylidenelanost-8-en-3beta-ol], respectively. The identification of these and other 24-alkylsterols in P. carinii hominis suggests that (i) sterol C-24 methyltransferase activities are extraordinarily high in this organism, (ii) 24-alkylsterols are important components of the pathogen's membranes, because the addition of these side groups onto the sterol side chain requires substantial ATP equivalents, and (iii) the inefficacy of azole drugs against P. carinii can be explained by the ability of this organism to form 24-alkysterols before demethylation of the lanosterol nucleus. Because mammals cannot form 24-alkylsterols, their biosyntheses in P. carinii are attractive targets for the development of chemotherapeutic strategies against this opportunistic infection.

The lipids of Pneumocystis carinii

Information about a number of Pneumocystis carinii lipids obtained by the analyses of organisms isolated and purified from infected lungs of corticosteroid-immunosuppressed rats has been reported in recent years. Of the common opportunistic protists associated with AIDS (Cryptosporidium, Toxoplasma, and the microsporidia), more is currently known about the lipids of P. carinii than the others. Lipids that are synthesized by the organism but not by humans are attractive targets for drug development. Thus, the elucidation of delta 7C-24-alykylated sterol and cis-9,10-epoxystearic acid biosyntheses in P. carinii is currently being examined in detail, since these have been identified as P. carinii-specific lipids. The development of low-toxicity drugs that prevent sterol C-24 alkylation and the specific inhibition of the lipoxygenase that forms cis-9,10-epoxystearic acid might prove fruitful. Although humans can synthesize coenzyme Q10, the anti-P. carinii activity and low toxicity of ubiquinone analogs such as atovaquone suggest that the electron transport chain in the pathogen may differ importantly from that in the host. Although resistance to atovaquone has been observed, development of other naphthoquinone drugs would provide a broader armamentarium of drugs to treat patients with P. carinii pneumonia. Studies of bronchoalveolar lavage fluid and of infected lungs have demonstrated that the infection causes a number of chemical abnormalities. Bronchoalveolar lavage fluid obtained after the removal of lung cellular material and the organisms has been shown to contain larger amounts of surfactant proteins and smaller amounts of phospholipids than do comparable samples from P. carinii-free lungs. Increased phospholipase activity, inhibition of surfactant secretion by type II cells, and uptake and catabolism of lipids by the pathogen may explain this phenomenon related to P. carinii pneumonia. Although not yet thoroughly examined, initial studies on the uptake and metabolism of lipids by P. carinii suggest that the organism relies heavily on exogenous lipid nutrients.

Pneumocystis carinii pneumonia: the status of Pneumocystis biochemistry

Pneumocystis carinii pneumonia remains a prevalent opportunistic disease among immunocompromised individuals. Although aggressive prophylaxis has decreased the number of acute P. carinii pneumonia cases, many patients cannot tolerate the available drugs, and experience recurrence of the infection, which can be fatal. It is now generally agreed that the organism should be placed with the fungi, but the identification of extant fungal species representing its closest kins, remains debated. Most recent data indicate that P. carinii represents a diverse group of organisms. Since the lack of methods for the continuous subcultivation of this organism hampered P. carinii research, molecular cloning and nucleotide sequencing approaches led the way for understanding the biochemical nature of this pathogen. However, within the last 5 years, the development of improved protocols for isolating and purifying viable organisms from infected mammalian host lungs has enabled direct biochemical and metabolism studies on the organism. The protein moiety of the major high mol. wt surface antigen, represented by numerous isoforms, is encoded by different genes. These proteins are post-transcriptionally modified by carbohydrates and lipids. The organism has the shikimic acid pathway that leads to the formation of compounds which mammals cannot synthesise (e.g., folic acid), hence drugs that inhibit these pathways are effective against the pathogen. Ornithine decarboxylase has now been detected; rapid and complete depletion of polyamines occurs in response to difluoromethylornithine (DFMO). Instead of ergosterol (the major sterol of higher fungi), P. carinii synthesises distinct delta7, C-24-alkylated sterols. An unusual C32 sterol, pneumocysterol, has been identified in human-derived P. carinii. Another signature lipid discovered is cis-9,10-epoxy stearic acid. CoQ10, identified as the major ubiquinone homologue, is synthesised de novo by P. carinii. Atovaquone and other hydroxynaphthoquinone drugs with anti-P. carinii activity probably inhibit pathogen respiration as CoQ analogues. Unlike its effects on Plasmodium, atovaquone does not inhibit the P. carinii dihydroorotate dehydrogenase and pyrimidine metabolism.

Inhibitors of delta24(25) sterol methyltransferase block sterol synthesis and cell proliferation in Pneumocystis carinii

Detailed analysis of the endogenous sterol content of purified Pneumocystis carinii preparations by gas-liquid chromatography coupled to mass spectrometry suggested that this parasite can both synthesize de novo steroid skeletons (to produce delta7 sterols) and take them from the infected host (leading to delta5 sterols). In both cases the final products are 24-alkyl sterols, resulting from the action of delta24(25) and delta24(24') sterol methyltransferases, enzymes not present in vertebrates. To investigate the physiological significance of these sterols, cultures of P. carinii in embryonic lung cells were exposed to 22,26-azasterol (20-piperidin-2-yl-5alpha-pregnan-3beta-20(R)-diol), a compound previously shown to inhibit both enzymes and to halt cell proliferation in fungi and protozoa. This compound produced a dose-dependent reduction in the parasite proliferation, with a 50% inhibitory concentration of 0.3 microM and 80% reduction of growth after 96 h at 10 microM. Correspondingly, parasites treated with the azasterol at 10 microM for 48 h accumulated 24-desalkyl sterols such as zymosterol (cholesta-8,24-dien-3beta-ol) and cholesta-8,14,24-trien-3beta-ol to ca. 40% of the total mass of endogenous sterols. This is the first report on the antiproliferative effects of a sterol biosynthesis inhibitor on P. carinii and indicate that sterol methyltransferase inhibitors could be the basis of a novel and specific chemotherapeutic approach to the treatment of P. carinii infections.

Effects of steroidal allenic phosphonic acid derivatives on the parasitic protists Leishmania donovani, Leishmania mexicana mexicana, and Pneumocystis carinii carinii

Several pathogenic fungi and protozoa are known to have sterols distinct from those of their mammalian hosts. Of particular interest as targets for drug development are the biosyntheses of the sterols of important parasites such as the kinetoplastid flagellates and the AIDS-associated opportunistic protist Pneumocystis carinii. These pathogens synthesize sterols with an alkyl group at C-24, and some have a double bond at C-22 of the side chain. Humans and other mammalian hosts are incapable of C-24 alkylation and C-22 desaturation. In the present study, three steroidal compounds with side chains substituted by phosphonyl-linked groups were synthesized and tested for their effects on Leishmania donovani and L. mexicana mexicana culture growth. The compounds inhibited organism proliferation at concentrations in micrograms per milliliter. The most potent inhibitors of this group of compounds were characterized by two ethyl groups at the phosphate function. Leishmania organisms treated with 17-[2-(diethylphosphonato) ethylidienyl]3-methoxy-19-norpregna-1,3,5-triene exhibited reduced growth after transfer into inhibitor-free medium. Because there are currently no axenic methods available for the continuous subcultivation of P. carinii, the effects of these drugs on this organism were evaluated by two alternative screening methods. The same two diethyl phosphonosteroid compounds that inhibited Leishmania proliferation were also the most active against P. carinii as determined by the potent effect they had on reducing cellular ATP content. Cystic as well as trophic forms responded to the drug treatments, as evaluated by a dual fluorescent staining live-dead assay. Other modifications of steroidal phosphonates may lead to the development of related drugs with increased activity and specificity for the pathogens.

Characterizations of neutral lipid fatty acids and cis-9,10-epoxy octadecanoic acid in Pneumocystis carinii carinii

Pneumocystis carinii causes pneumonitis in immunodeficient individuals and is a prevalent opportunistic infection of patients with AIDS. This pathogen resides extracellularly in the hypophase lining the alveolar epithelium, which is highly enriched in lung surfactant lipids. Procedures yielding highly pure organism preparations that enable reliable biochemical analyses of organisms isolated from the lungs of infected laboratory animals have been developed. The results of the present study revealed that the fatty acid profiles of total lipids, the neutral lipid traction, and individual neutral lipid classes of lungs from normal and immunosuppressed rats as well as P. carinii were grossly similar, although some quantitative differences were detected. One qualitative exception found was the detection in P. carinii of the rare fatty acid cis-9,10-epoxy stearic acid, which was not detected in the lipids of rat lungs. The detection of this fatty acid in P. carinii may also have important taxonomic implications. Unlike phospholipids, many of the fatty acids of nonmembrane neutral lipids may be utilized by P. carinii for other cellular functions, such as stored reserves for energy production and precursors for organism-specific membrane lipids. The present study represents the first report of detailed fatty acid analyses of individual neutral lipid classes of this important opportunistic pathogen.

Identification of C31 and C32 sterols in Pneumocystis carinii hominis-infected human lungs

Two sterols in autopsied whole lung specimens obtained from Pneumocystis carinii pneumonia patients were detected by gas-liquid chromatography and their structures were elucidated by mass spectrometry and nuclear magnetic resonance spectrometry. Both were in the lanosterol series; the C31 sterol, with a methyl group at C-24, was identified as euphorbol, and the more abundant C32 sterol, with an ethyl group at C-24, is given the trivial name pneumocysterol.

Composition of Pneumocystis carinii neutral lipids and identification of coenzyme Q10 as the major ubiquinone homolog

The lipids of purified preparations of Pneumocystis carinii carinii freshly isolated from infected rats were analyzed and compared with those of whole lungs from normal and methylprednisolone-immunosuppressed uninfected rats. In this study, the neutral lipid fraction was examined in detail; the relative concentrations of individual classes making up this fraction were quantified. Of particular interest was the nature of the organism's ubiquinone (coenzyme Q, CoQ) fraction because atovaquone, a hydroxynaphtho-quinone (566C80) analog of ubiquinone, is efficacious in the treatment of P. carinii pneumonia. The ubiquinone concentration in both P. carinii and lung tissues was relatively low compared to that present in rat heart and liver tissues. Two homologs were identified in the organism: CoQ10 was the predominant homolog with lesser amounts of CoQ9 present. In contrast, the lungs of normal and immunosuppressed uninfected rats had CoQ9 and lesser amounts of CoQ8, but no detectable CoQ10. Furthermore, radiolabeled mevalonic acid was incorporated in vitro into the ubiquinone fraction of P. carinii indicating that the organism has the de novo branch of the isoprenoid biosynthetic pathway leading to polyprenyl formation. Hence, it was concluded that CoQ10 (if not both CoQ10 and CoQ9) in P. carinii was not scavenged from the host but was synthesized by the organism. Although lung tissues contained substantial free fatty acids, the organism was enriched in these lipids. The high concentration of free fatty acids and relatively low level of triglycerides in P. carinii suggest that fatty acids may represent major carbon sources for ATP production by the organism.

Fatty acid composition of the major phospholipids of Pneumocystic carinii: comparison with those in the lungs of normal and methylprednisolone-immunosuppressed rats

Large numbers of viable organisms can be isolated from the corticosteroid-immunosuppressed rat model of Pneumocystis carinii pneumonia. With the development of purification protocols that provide organism preparations of high purity, meaningful lipid biochemical analyses of this important opportunistic pathogen can now be conducted. The phospholipid class composition of the pathogen was reported earlier, together with observations of changes that occur in the rat lungs in response to methylprednisolone immunosuppression treatment. In this report, analyses of the effects of corticosteroids on the fatty acid compositions of the major lung phospholipids, individually isolated and purified by thin-layer chromatography, were elucidated and quantified by gas-liquid chromatography. In response to methylprednisolone, there was a relative increase in palmitate and there were decreases in several unsaturated fatty acids of the rat whole-lung total polar lipids leading to a doubling of the saturation index. Reciprocal changes in the relative concentrations of palmitate and stearate in phosphatidylethanolamine, phosphatidylinositol, lysophosphatidylcholine, and cardiolipin were observed, suggesting that there is tight control of acylation of these phospholipids in the lung. Detailed phospholipid fatty acid analyses were also performed with mixed life cycle stages of P. carinii organisms. The most abundant phospholipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol, had much higher concentrations of oleic acid and lower concentrations of palmitate in P. carinii than in lung tissue. Sphingomyelin in lung tissue and P. carinii differed from the glycerophospholipids by the presence of high levels of saturated C(22) and C(24) fatty acids. This study represents the most comprehensive fatty acid analysis of rat lung phospholipids and the changes that occur in response to corticosteroid treatment. It is the first report about the fatty acids of individual phospholipids of the opportunistic protist P. carinii carinii.