Siderophores in Iron Metabolism: From Mechanism to Therapy Potential

Abstract

Iron is an essential nutrient for life. During infection, a fierce battle of iron acquisition occurs between the host and bacterial pathogens. Bacteria acquire iron by secreting siderophores, small ferric iron-binding molecules. In response, host immune cells secrete lipocalin 2 (also known as siderocalin), a siderophore-binding protein, to prevent bacterial reuptake of iron-loaded siderophores. To counter this threat, some bacteria can produce lipocalin 2-resistant siderophores. This review discusses the recently described molecular mechanisms of siderophore iron trafficking between host and bacteria, highlighting the therapeutic potential of exploiting pathogen siderophore machinery for the treatment of antibiotic-resistant bacterial infections. Because the latter reflect a persistent problem in hospital settings, siderophore-targeting or siderophore-based compounds represent a promising avenue to combat such infections.

Keywords: Lcn2; hypoxia; iron; mitophagy; siderocalin; siderophore.

Figure 1. Siderophore Structure

A. Four moieties confer iron-binding capacity to siderophores: carboxylate (orange), phenolate (green), catecholate (red), and hydroxamate (blue). B. Siderophore structures from selected nosocomial pathogens are shown (see Table 1): Mixed-type siderophores contain more than one type of iron-binding moiety.

Key Figure, Figure 2. Host and Pathogens Alter Many Iron-regulated Pathways to Fight for Survival

A. 1) In response to intracellular pathogens, host macrophages upregulate inducible nitric oxide synthase (iNOS) to stimulate ferroportin (Fpn) expression. Fpn is an iron exporter, denying the pathogen access to the intracellular iron pool. 2) The lowered intracellular iron concentration also activates interferon-γ (IFN-γ), a cytokine that stimulates expression of a suite of antipathogenic, immune-related genes. 3) Intracellular pathogens induce IL-6 in host macrophages, leading to Fpn inhibition and resulting in increased intracellular iron. 4) Intracellular pathogens activate Toll-like Receptor 4 (TLR4) to induce the host macrophage to phagocytose erythrocytes (red blood cells; RBCs) in an effort to increase the intracellular iron pool. B. 1) Extracellular pathogens activate TLR4 on the host macrophage, resulting in downregulation of Fpn and lessened extracellular iron access to the pathogen. 2) Extracellular pathogens secrete siderophores to acquire as much iron from the environment as possible.

Figure 3. Novel Antimicrobials utilizing the Siderophore Import System

Both gallium (Ga) salts (Gallium maltolate and Gallium nitrate) and Ga-conjugated siderophores (Ga-DFO and Ga-pyochelin) are used to deliver Ga to bacterial cells through siderophore import machinery. Ga interrupts important metabolic processes by competing with iron in critical redox reactions. Antibiotics can also be conjugated to siderophores (MC-1, BAL30072, S-649266, and GSK3342830), allowing enhanced uptake utilizing siderophore import machinery to boost anti-bacterial effects.

Box 1 Figure I. Iron: A Tug-Of-War

Host cells secrete iron-binding proteins, such as lactoferrin (stage 1), to prevent pathogens from acquiring iron. Pathogens respond by “stealing” iron from host proteins using high affinity siderophores (stage 2). Host cells secrete the siderophore-binding protein lipocalin 2 (Lcn2) to neutralize the siderophore and prevent pathogen reuptake (stage 3). Pathogens can also produce “stealth siderophores” that cannot be sequestered by Lcn2 (stage 4).

Box 2 Figure II. Siderophore Import and Secretion Mechanisms

Top, left. Siderophore import in Gram-negative bacteria is a multistep process, involving recognition of the ferric-siderophore by a specific outer membrane receptor (OMR), followed by TonB-dependent uptake into the periplasm. The ferric-siderophore is then trafficked by a periplasmic binding protein (PBP) to the inner membrane, where an ATP-binding cassette (ABC) transporter pumps it into the cytoplasm. Top, right. Siderophore import in Gram-positive bacteria involves recognition by a siderophore binding protein (SBP) located on the cell membrane. An associated permease is responsible for ferric-siderophore transport across the membrane. Bottom. Enterobactin secretion in Gram-negative bacteria involves transport from the cytoplasm to the periplasmic space through Major Facilitator Subtype (MFS) proteins. Transport across the outer membrane involves a TolC complex and an associated Resistance-Nodulated-Cell Division (RND) efflux pump.