Further studies on the increase in cholesterol ester content of the lungs of tuberculous mice

Cholesterol ester oxidation by mycobacterial cytochrome P450

Mycobacteria share a common cholesterol degradation pathway initiated by oxidation of the alkyl side chain by enzymes of cytochrome P450 (CYP) families 125 and 142. Structural and sequence comparisons of the two enzyme families revealed two insertions into the N-terminal region of the CYP125 family (residues 58-67 and 100-109 in the CYP125A1 sequence) that could potentially sterically block the oxidation of the longer cholesterol ester molecules. Catalytic assays revealed that only CYP142 enzymes are able to oxidize cholesteryl propionate, and although CYP125 enzymes could oxidize cholesteryl sulfate, they were much less efficient at doing so than the CYP142 enzymes. The crystal structure of CYP142A2 in complex with cholesteryl sulfate revealed a substrate tightly fit into a smaller active site than was previously observed for the complex of CYP125A1 with 4-cholesten-3-one. We propose that the larger CYP125 active site allows for multiple binding modes of cholesteryl sulfate, the majority of which trigger the P450 catalytic cycle, but in an uncoupled mode rather than one that oxidizes the sterol. In contrast, the more unhindered and compact CYP142 structure enables enzymes of this family to readily oxidize cholesteryl esters, thus providing an additional source of carbon for mycobacterial growth.

A highly conserved mycobacterial cholesterol catabolic pathway

Degradation of the cholesterol side-chain in Mycobacterium tuberculosis is initiated by two cytochromes P450, CYP125A1 and CYP142A1, that sequentially oxidize C26 to the alcohol, aldehyde and acid metabolites. Here we report characterization of the homologous enzymes CYP125A3 and CYP142A2 from Mycobacterium smegmatis mc(2) 155. Heterologously expressed, purified CYP125A3 and CYP142A2 bound cholesterol, 4-cholesten-3-one, and antifungal azole drugs. CYP125A3 or CYP142A2 reconstituted with spinach ferredoxin and ferredoxin reductase efficiently hydroxylated 4-cholesten-3-one to the C-26 alcohol and subsequently to the acid. The X-ray structures of both substrate-free CYP125A3 and CYP142A2 and of cholest-4-en-3-one-bound CYP142A2 reveal significant differences in the substrate binding sites compared with the homologous M. tuberculosis proteins. Deletion only of cyp125A3 causes a reduction of both the alcohol and acid metabolites and a strong induction of cyp142 at the mRNA and protein levels, indicating that CYP142A2 serves as a functionally redundant back up enzyme for CYP125A3. In contrast to M. tuberculosis, the M. smegmatis Δcyp125Δcyp142 double mutant retains its ability to grow on cholesterol albeit with a diminished capacity, indicating an additional level of redundancy within its genome.

The innate immune response in leprosy

Investigation into the innate immune response in leprosy has provided insight into immunoregulation in human infectious disease. Key advances include the role of pattern recognition receptors in recognizing pathogen-associated molecular patterns of Mycobacterium leprae, cytokine release by innate immune cells, macrophage and dendritic cell differentiation, as well as antimicrobial effector pathways. These insights provide targets for therapeutic intervention in modulating the course of leprosy and other chronic infectious diseases.

Host-derived oxidized phospholipids and HDL regulate innate immunity in human leprosy

Intracellular pathogens survive by evading the host immune system and accessing host metabolic pathways to obtain nutrients for their growth. Mycobacterium leprae, the causative agent of leprosy, is thought to be the mycobacterium most dependent on host metabolic pathways, including host-derived lipids. Although fatty acids and phospholipids accumulate in the lesions of individuals with the lepromatous (also known as disseminated) form of human leprosy (L-lep), the origin and significance of these lipids remains unclear. Here we show that in human L-lep lesions, there was preferential expression of host lipid metabolism genes, including a group of phospholipases, and that these genes were virtually absent from the mycobacterial genome. Host-derived oxidized phospholipids were detected in macrophages within L-lep lesions, and 1 specific oxidized phospholipid, 1-palmitoyl-2-(5,6-epoxyisoprostane E2)-sn-glycero-3-phosphorylcholine (PEIPC), accumulated in macrophages infected with live mycobacteria. Mycobacterial infection and host-derived oxidized phospholipids both inhibited innate immune responses, and this inhibition was reversed by the addition of normal HDL, a scavenger of oxidized phospholipids, but not by HDL from patients with L-lep. The accumulation of host-derived oxidized phospholipids in L-lep lesions is strikingly similar to observations in atherosclerosis, which suggests that the link between host lipid metabolism and innate immunity contributes to the pathogenesis of both microbial infection and metabolic disease.

Pathology of postprimary tuberculosis in humans and mice: contradiction of long-held beliefs

Tuberculosis remains one of the world's leading infectious causes of death. Approximately 80% of all disease is due to postprimary (secondary) tuberculosis in the lung. Unfortunately, tissues of developing lesions are seldom available and there are no recognized models of postprimary tuberculosis. In the preantibiotic era when tissues were more abundant, several investigators described early postprimary tuberculosis as a lipid pneumonia quite different from the caseating granulomas commonly described today. We used histopathologic, immunohistochemical and acid fast stains to examine tissues from several people with untreated primary and postprimary tuberculosis and compared the findings with those of mice with reactivation tuberculosis. The results confirmed that developing postprimary tuberculosis begins as a lipid pneumonia accompanied by bronchial obstruction in which infection is restricted to foamy alveolar macrophages. Cavities result from a combination of caseation of tuberculous pneumonia and microvascular occlusion characteristic of delayed type hypersensitivity (DTH). Reactivation tuberculosis in the mouse begins as a similar tuberculous lipid pneumonia with bronchial obstruction and evidence for participation of DTH. Developing necrosis in both species is associated with localization of organisms within lipid droplets. These results suggest that reactivation tuberculosis in mice is a valuable model of developing human postprimary tuberculosis.

Biological autoxidation. II. Cholesterol esters as inert barrier antioxidants. Self-assembly of porous membrane sacs. An hypothesis

The antioxidation defenses recognized thus far appear too weak. Needed are inert barriers to encapsulate foci of activated oxygen (FAOs) and contain their spreading. These capsules must: 1. self-assemble nonenzymatically and spontaneously in face of adversity; 2. resist oxidation and dissolution in water; and 3. be moderately fluid and elastic enough to withstand flexing by tissues. Evidence shows activated oxygen: a. is produced by common cholesterolester (CE)-raising agents; b. boosts accumulation of CEs; and c. splits low-density lipoproteins (LDL), thus releasing CE-rich coalescence-prone lipid micelles. I am proposing that CEs, combined with polar lipids, are uniquely suited to form inert-lipid antioxidation barriers (ILABs). Porous ILAB capsules self-assemble from lipid micelles released by oxidatively degraded LDL. The capsules are thermodynamically unstable but elastic, durable and capable of self-repair through oxidation of ambient LDL. All capsules tend to contract into spheres. Enclosed needle-like "foreign bodies", such as asbestos, puncture the contracting capsules. Hence the odd bulbous architecture of asbestos bodies. ILABs protect from--and their failure initiates and promotes--carcinogenesis and atherosclerosis. ILABs may be mediators of membrane biogenesis. The loss of arterial flexibility in atherosclerosis protects ILAB capsules from breakage.

A suggested role of a host-parasite lipid complex in mycobacterial infection

On the basis of our previous observations and related literatures, was assumed tht cholesterol esters of host origin and phthiocerol dimycocerosate of bacterial origin are located as a lipid mixture around the periphery of pathogenic mycobacteria growing in vivo, probably within the phagocytic vacuole of macrophages. To examine the role of such a postulated lipid complex in mycobacterial infection, a model experiment was made in which tubercle bacilli grown in vitro were "coated" with both lipids and then suspended homogenously in water to serve as an inoculum to infect mice intravenously. Their fate in mouse tissue was compared with that of untreated control bacilli. The results indicated that the lipid "coating" had an infection-promoting effect as revealed by the longer persistence of the treated avirulent bacilli at higher levels of viable counts. When virulent tubercle bacilli were "coated" with the lipid mixture, they became less sensitive to the protective mechanism of BCG-immunized mice.

Accumulation of cholesterol esters in macrophages incubated with mycobacteria in vitro

Macrophages separated from the granulomatous lungs of tuberculous mice had a high amount of cholesterol esters. Resident peritoneal macrophages of normal mice were very low in the ester content. However, when the cells were incubated with mycobacteria in Hanks' solution the ester content of the mixture increased greatly. Peritoneal macrophages harvested by induction with casein had a much larger amount of cholesterol esters than unstimulated resident cells. When such stimulated macrophages were incubated alone in Hanks' solution, the ester content went down probably due to hydrolysis into free form. This reduction was markedly inhibited by incubation with mycobacteria. These observations at a macrophage level presented a cytological explanation for our previous finding that cholesterol ester content increased in the mouse lungs with the development of granulomatous lesions.