Cholesterol catabolism as a therapeutic target in Mycobacterium tuberculosis

Abstract

Mycobacterium tuberculosis (Mtb) is an intracellular pathogen that infects 10 million people worldwide and kills 2 million people every year. The uptake and utilization of nutrients by Mtb within the host cell is still poorly understood, although lipids play an important role in Mtb persistence. The recent identification of a large regulon of cholesterol catabolic genes suggests that Mtb can use host sterol for infection and persistence. In this review, we report on recent progress in elucidation of the Mtb cholesterol catabolic reactions and their potential utility as targets for tuberculosis therapeutic agents.

Figure 1. Schematic representation of the cell envelope of Mtb with lipidomic analysis of the surface lipids and PDIM

The cell envelope of Mtb cells, from the inside to the outside, consists of a plasma membrane; a cell wall composed of three covalently linked macromolecules (peptidoglycan, arabinogalactan and mycolic acids) and non-covalently linked lipids and proteins; and a capsule consisting of polysaccharides, proteins and lipids. For the mass spectrometric analysis, total lipids were extracted from cells grown in the presence of glycerol, cholesterol or heptadeuterated cholesterol. As shown, cholesterol metabolism in Mtb cells increases the average mass of the methyl-branched lipid virulence factor PDIM, presumably due to a higher metabolic flux of propionate derived from cholesterol side-chain catabolism [22]. Abbreviations: PIM, phosphatidylinositol mannosides; PDIM, phthiocerol dimycoserate; PAT, polyacyltrehaloses; DAT, diacyltrehaloses; SL-1, sulfolipid-1s; TDM, trehalose-6,6′-dimycolates; TMM, trehalose-6-monomycolates.

Figure 2. The cholesterol catabolic genes of Mtb

The transcriptional repressor KstR controls the expression of the majority of the genes clustered in two chromosomal segments (Rv3494c -Rv3547 and Rv3566c - Rv3574). Many other KstR-regulated genes are scattered throughout the Mtb genome, including Rv1106c. KstR2 controls the expression of a second cholesterol regulon in M. smegmatis [28], with the corresponding orthologs in Mtb genome (Rv3548c - Rv3565). Genes in the map are color-coded according to their assigned or proposed catalytic activity: red, uptake; side-chain degradation, magenta; degradation of rings A and B, green; β-oxidation, blue; transcriptional regulator, orange; unassigned function, black; not regulated by KstR or KstR2, grey. The genes encoding for enzymes involved specifically in the degradation of rings C and D are not unambiguously assigned yet. The catalytic functions of HsaEFG (Rv3534c-Rv3536c) in Mtb have not been investigated yet. Genes predicted [30, 31] or proven [18, 24, 27, 46, 57] to be essential for survival in macrophage cells and in the mouse model of infection are indicated by * and #, respectively. Abbreviations: ABC-transporter, ATP-binding cassette transporter; PE, protein containing Pro-Glu motifs; PE-PGRS, protein consisting of the PE domain followed by a C-terminal extension with multiple tandem repetitions of Gly-Gly-Ala or Gly-Gly-Asn; CHP, conserved hypothetical protein; HP, hypothetical protein; PPE, protein containing Pro-Pro-Glu motifs.

Figure 3. Proposed degradation pathways and flux of metabolites derived from catabolism of the cholesterol aliphatic side-chain and ring nucleus

All the enzymes for which a catalytic activity has been identified are labeled in red, whereas the putative β-oxidation enzymes are labeled in magenta. The carbon atoms derived from the ring nucleus are converted to CO₂ via the tricarboxylic acid cycle (TCA), whereas the propionyl-CoA produced from the degradation of the side-chain is assimilated into mycobacterial lipids, including the virulence factor PDIM. The enzymes and the cholesterol metabolites are described in the text. The methylmalonyl pathway enzymes include a propionyl-CoA carboxylase (PCC) and a vitamin B₁₂-dependent methylmalonyl-CoA mutase (MCM). The fate of DOHNAA is still unknown.