Cholesterol Metabolism Increases the Metabolic Pool of Propionate in Mycobacterium tuberculosis

Abstract

Mycobacterium tuberculosis can metabolize cholesterol to both acetate and propionate. The mass of isolated phthiocerol dimycoserate, a methyl-branched fatty acylated polyketide, was used as a reporter for intracellular propionate metabolic flux. When Mycobacterium tuberculosis is grown using cholesterol as the only source of carbon, a 42 a.m.u increase in average phthiocerol dimycoserate molecular weight is observed, consistent with the cellular pool of propionate and thus, methylmalonyl CoA increasing upon cholesterol metabolism. In contrast, no shift in phthiocerol dimycoserate molecular weight is observed upon supplementation of medium containing glycerol and glucose with cholesterol. We conclude that cholesterol is only a significant source of propionate in the absence of sugar carbon sources.Mycobacterium tuberculosis (M. tb), the etiologic agent of tuberculosis (TB) in humans, is responsible for the majority of deaths caused by bacterial infections. Although it is estimated that one-third of the world's population is infected with the pathogen, only 10% of immunocompetent individuals carrying the active form of M. tb develop the disease (1) and upon suppression of the immune system, M. tb bacilli may become active. Consequently, TB is responsible for the majority of HIV-related deaths worldwide. The emergence of multidrug resistant tuberculosis (MDR-TB) and extensively drug resistant tuberculosis (XDR-TB) has made the successful treatment of the disease cumbersome, often resulting in death of individuals infected with these strains. Therefore, we have undertaken studies to investigate the metabolic state of M. tb that ensues upon switching to a lipid-based metabolism in the host, in order to better develop new therapies.Unlike most pathogenic bacteria, M. tb lacks common virulence factors like capsules, endotoxins and exotoxins. Its success as a pathogen lies in its ability to elude the host's immune response and persist within the harsh milieu of the macrophage (2,3). Several distinct modes for virulence have been proposed to play important roles in the survival of M. tb within the phagosome including inhibition of phagosome-lysosome fusion (4-6), preventing maturation † This work was supported by the NIH (AI065251 (NSS), AI065987 (IS), DK007521, (NMN)), and the NYSTAR Program (FDP C040076, NSS).

Loving the poison: the methylcitrate cycle and bacterial pathogenesis

“…Indeed, deletion of prpC impaired the ability of M. tuberculosis to grow in media containing cholesterol as a primary carbon source, and recent work has established that an M. tuberculosis prpCD double mutant also exhibits a growth defect in murine bone marrow-derived macrophages [60]. These observations are consistent with the notion that M. tuberculosis obtains a significant amount of its carbon requirements from cholesterol [61]. Building on these observations, an unbiased chemical screen was recently used to identify compounds that inhibit propionate catabolism and thereby diminish M. tuberculosis survival within macrophages.…”

Lipidomic analyses of Mycobacterium tuberculosis based on accurate mass measurements and the novel “Mtb LipidDB”

“…TG abundance waned and waxed dramatically as the cultures aged, and TG FA sum compositions also shifted markedly, with the appearance of higher-molecular-weight TG groups in stationary phase. For the purposes of this Supplemental Material can be found at: ( 15,20 ). These mass increases were shown to result from addition of methylene units to the esterifi ed mycocerosic acids.…”

“…The abundance and biological importance of the Mtb lipids has resulted in extensive and elegant studies to elucidate their structures and functions ( 1-3 ). In many cases, the lipids of Mtb are unique to this pathogen or shared only with other members of this genus.Earlier studies demonstrate variability in lipid profi les among different strains of Mtb ( 7-11 ) and that minor variations in the structure of individual lipids can occur with changes in the growth environment (12)(13)(14)(15)(16)(17)(18)(19)(20). However, targeted and nontargeted assays that monitor changes in Mtb lipid profi les are generally performed by traditional TLC-based methods ( 21 ), and global lipidomics analyses in Mtb have been restricted due to limits in the technology to detect and rapidly identify a large number of lipids in a single experiment.…”

Reprogramming the Mycobacterium tuberculosis transcriptome during pathogenesis

“…However, as lipid precursor pools and toxic intermediates fluctuate as carbon metabolism is transformed during infection, mechanisms are required to utilise or remove these moieties. For example, induction of the M.tb methylcitrate cycle inside macrophages may be a response to the accumulation of potentially toxic levels of CoA-propionate [10], such a build up of propionate might result from the metabolism of cholesterol [14]. This changing flux of propionate intracellularly also drives the synthesis of methyl-branched complex lipids such as phthiocerol dimycocersate which incorporate methyl-malonyl precursors derived from propionate [14,15].…”

“…For example, induction of the M.tb methylcitrate cycle inside macrophages may be a response to the accumulation of potentially toxic levels of CoA-propionate [10], such a build up of propionate might result from the metabolism of cholesterol [14]. This changing flux of propionate intracellularly also drives the synthesis of methyl-branched complex lipids such as phthiocerol dimycocersate which incorporate methyl-malonyl precursors derived from propionate [14,15]. Genes involved in the biosynthesis of several complex lipids, postulated to be mediators of the immune system [16], are induced after macrophage infection [17,18].…”