Mce3R Stress-Resistance Pathway Is Vulnerable to Small-Molecule Targeting That Improves Tuberculosis Drug Activities

ACS Infect Dis. 2019 Jul 12;5(7):1239-1251. doi: 10.1021/acsinfecdis.9b00099. Epub 2019 May 3.


One-third of the world's population carries Mycobacterium tuberculosis (Mtb), the infectious agent that causes tuberculosis (TB), and every 17 s someone dies of TB. After infection, Mtb can live dormant for decades in a granuloma structure arising from the host immune response, and cholesterol is important for this persistence of Mtb. Current treatments require long-duration drug regimens with many associated toxicities, which are compounded by the high doses required. We phenotypically screened 35 6-azasteroid analogues against Mtb and found that, at low micromolar concentrations, a subset of the analogues sensitized Mtb to multiple TB drugs. Two analogues were selected for further study to characterize the bactericidal activity of bedaquiline and isoniazid under normoxic and low-oxygen conditions. These two 6-azasteroids showed strong synergy with bedaquiline (fractional inhibitory concentration index = 0.21, bedaquiline minimal inhibitory concentration = 16 nM at 1 μM 6-azasteroid). The rate at which spontaneous resistance to one of the 6-azasteroids arose in the presence of bedaquiline was approximately 10-9, and the 6-azasteroid-resistant mutants retained their isoniazid and bedaquiline sensitivity. Genes in the cholesterol-regulated Mce3R regulon were required for 6-azasteroid activity, whereas genes in the cholesterol catabolism pathway were not. Expression of a subset of Mce3R genes was down-regulated upon 6-azasteroid treatment. The Mce3R regulon is implicated in stress resistance and is absent in saprophytic mycobacteria. This regulon encodes a cholesterol-regulated stress-resistance pathway that we conclude is important for pathogenesis and contributes to drug tolerance, and this pathway is vulnerable to small-molecule targeting in live mycobacteria.

Keywords: cholesterol; codrug; low oxygen.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antitubercular Agents / chemistry
  • Antitubercular Agents / pharmacology*
  • Azasteroids / chemistry
  • Azasteroids / pharmacology*
  • Bacterial Proteins / drug effects
  • Bacterial Proteins / genetics*
  • Diarylquinolines / chemistry
  • Diarylquinolines / pharmacology
  • Down-Regulation
  • Drug Resistance, Bacterial / drug effects
  • Drug Synergism
  • Gene Expression Regulation, Bacterial / drug effects
  • Isoniazid / chemistry
  • Isoniazid / pharmacology
  • Molecular Structure
  • Mycobacterium tuberculosis / drug effects*
  • Mycobacterium tuberculosis / genetics
  • Regulon
  • Small Molecule Libraries / chemistry
  • Small Molecule Libraries / pharmacology
  • Structure-Activity Relationship


  • Antitubercular Agents
  • Azasteroids
  • Bacterial Proteins
  • Diarylquinolines
  • Small Molecule Libraries
  • bedaquiline
  • Isoniazid